Fluorine Containing Diazines. Synthesis and Properties

Volochnyuk, Dmitriy M.; Grygorenko, Oleksandr O.; Gorlova, Alina O.

doi:10.1007/978-3-319-04435-4_6

Dmitriy M. Volochnyuk²,
Oleksandr O. Grygorenko³ &
Alina O. Gorlova²

1528 Accesses
3 Citations

Abstract

This review deals with general and significant developments in the area of chemistry of fluorinated pyrimidine, pyrazine and pyridazine. Diazines bearing fluoro or α-fluoroalkyl substituent at carbon atoms of the heterocyclic ring, as well as their fused derivatives are discussed. The literature data are divided into two parts, which describe synthesis and chemical behavior of ring- and chain-fluorinated diazines (RFD and CFD respectively).

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Keywords

1 Introduction

Diazines, especially pyrimidines, are among most widespread six-membered heterocycles including both synthetic and natural compounds [1–3]. It is not surprising therefore that introduction of fluorine into the diazine core or side chain has been used extensively in various areas of chemistry. The first representative of the fluorinated diazines refer to late 1940s when Miller and co-workers described synthesis of 6-trifluoromethyl-2-thiouracil (1) (Fig. 1) [4]. In the next few years, several compounds of general formula 2 were prepared using Biginelli reaction [5]. In 1957, first representatives of ring-fluorinated diazines (e.g. 5-Fluorouracil (3) [6]), as well as fluorinated quinoxaline derivative 4 [7] were described.

These pioneering works initiated an avalanche of publications on chemistry of fluorinated diazines, which have been intensified in recent years (Fig. 2).

It should be noted that different types of the fluorinated diazines are represented unequally among the known compounds (Table 1). In particular, nearly a half of literature references deals with 5-fluoropyrimidines (49.1 %). Other popular structural motifs include 2- and 4-fluoropyrimidines (2.9 and 4.3 %), 2-, 4(6)-, and 5-trifluoromethylpyrimidines (6.1, 16.2 and 5.0 %, respectively), and trifluoromethyl-substituted pyrazines (2.9 % of the literature references, the works on fused derivatives are included into numbers in all the above cases).

Table 1 Distribution of the fluorinated diazines in the literature citations^a

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In this chapter, diazines bearing fluoro or α-fluoroalkyl substituent at any carbon atoms of the heterocyclic ring, as well as their fused derivatives are discussed. The literature data are divided into two parts, which describe synthesis and chemical behavior of ring- and chain-fluorinated diazines (RFD and CFD respectively). It should be noted that only chain-fluorinated diazines having the fluorine atom at α position of the alkyl substituent are discussed, since more distant fluorine atoms have lesser effect on the chemistry of the corresponding heterocycles. Chain-fluorinated diazines with fluorine atoms or fluorine-containing substituents linked to the aromatic ring via heteroatoms are also beyond the scope of this literature survey.

Both ring- and chain-fluorinated diazines are widely used in medicinal chemistry and agrochemistry (see Chap. 7). In fact, fluorinated diazines were used in drug discovery since the very first works on their synthesis. Other areas of application include dyes and liquid crystals. The 5-chloro-2,4-difluoropyrimidinyl radical acts as the reactive group in reactive dyes for cellulose and cotton fibers such as Levafix EA (Bayer) and Drimarene K (Sandoz) and for wool, e.g., Verofix (Bayer) and Drimalene (Sandoz) [8]. Both 2- and 4- fluoropyrimidine derivatives were used in liquid crystals engineering [9–11]. Also the ring fluorinated diazines were actively used as model compounds under investigation of different chemical transformation.

2 Ring Fluorinated Diazines

Ring-fluorinated diazines constitute an important family of organic compounds with a wide array of applications ranging from drugs to multi-ton industrial intermediates [12, 13]. The first representatives of RFDs, were synthesized in the 1960–1970s. Developments in this field were made during all this time. The main purpose of this review is not only to explore the field of chemistry of the ring-fluorinated diazines but also to identify the remaining gaps as opportunities for the future research effort.

3 Synthesis

3.1 Substitution Reactions

3.1.1 Electrophilic Fluorination. Replacement of H by F

Electrophilic fluorination is one of the most direct methods for selective introduction of fluorine into organic compounds. Historically first electrophilic fluorination of diazine derivatives was accomplished in 1960. Silver difluoride has been used in the final stage of an earlier synthesis of tetrafluoropyrimidine 6 from trifluoropyrimidine 5 [14]. Later the similar transformation was carried out using ClF₅ in 15 % yield and was found that side chlorination occurs in 9 % yield [15] (Scheme 1).

The most famous direct fluorination of diazine derivatives is fluorination of uracil by elemental fluorine affording 5-fluorouracil 9 (5-FU) [16] (Fig. 1). This is rare example of the use of fluorine gas in a successful commercial process developed by PCR Inc. in 1976. In spite of low yield of the process (~35 %), the original multistep synthesis of 5-FU was more expensive [17] (Scheme 2). The success of industrial fluorination of uracil was fixed in 1978 by Daikin Kogyo Co., Ltd. which increased the yield up to 85 % [18].

Uracil gave 5-fluorouracil when fluorine was passed into an aqueous suspension of the uracil. It is suspected, that fluorinating species of fluorine formed in water are HOF and/or F₂O, which reacted with uracil [19]. Besides elemental fluorine another reagents were used in the reaction. Among successful reagents are CF₃OF (ca. 90 %) [20], graphite intercalate, C₁₉XeF₆ (90 %) [21], AcOF (80–90 %) [22], CsSO₄F (54 %) [23] and Selectfluore (82 %) [24]. Small-scale preparations involving direct fluorination of uracil with fluorine or trifluoroacetyl hypofluorite gave yields in the region 76–92 %, but scaling-up considerably reduced the efficiency [25]. Problems arising from difluorination of highly activated substrates [16, 26] have been overcome by incorporating an electron-withdrawing group in the ring. Direct fluorination of isoorotic esters, amides, or nitriles 10 in the presence of water, methanol, or acetic acid, followed by mild hydrolysis and decarboxylation of intermediate products gave up to 92 % yields of 5-fluorouracil [25] (Scheme 3). Also the fluorination of the orotic acid was investigated; the initially obtained fluoroorotic acid 13 was subjected to decarboxylation. The use of two-step reaction sequence was claimed to be advantageous due to simplified product isolation and purification [27] (Scheme 3).

All of the direct fluorinations reported appear to be addition-elimination processes with solvent involvement (Scheme 4). A study of the mechanism and stereochemistry of uracil fluorination using F₂ and AcOF has implicated a radical-cation mechanism [28]. The effect of acetate ion on the products proved to be important. In its absence both cis- 16 and trans-isomers 15 were observed in the reaction mixture, but only trans- 15 in its presence. NMR studies have revealed that acetate originated from the solution containing acetate ion, rather than the residue from acetyl hypofluorite, binds to the 6-position of uracil to form the intermediates 15 and 16 (Solv=OAc). Acetate is a sufficiently strong base to induce trans-elimination of acetic acid from the cis-isomer 16 [29, 30].

Due to the high importance of the 5-FU derivatives as anti cancer drugs a lot of different fluorinating agents were tested in the fluorination of the derivatives. The most important examples of the fluorination used in drug synthesis are listed in the next chapter of the book.

In a course of fluorination of uracil derivatives, the fluorination of cytosine derivatives 17 leading to 5-fluorocytosine 18 was investigated [28, 30]. In contrast to uracil some side process were disclosed. 1-Substituted cytosine intermediate adducts 19 rapidly deaminated in water to yield uracil analogues 21 [30] (Scheme 5). The corresponding NF₂-derivatives were detected during fluorination of cytosine in water by fluorine [31]. It should be noted, that occurrence only one electron-donating hydroxy(keto) group in pyrimidinone-2 is sufficient for direct fluorination. The corresponding pyrimidinone-2 and its N1-substituted derivatives give under fluorination by fluorine in HF or AcOH the corresponding 5-fluorinated derivatives in 38–61 % yields. In this case the fluorination proceeds also as addition-elimination process [32].

The intensive development of electrophilic fluorination reagents in last two decades leads to it’s using both in academician and industrial investigation. Recent patents and papers directed to the early stage drug discovery are illustrated this trend. Some examples of such fluorinations based on electron rich pyrimidines are listed in Table 2.

Table 2 Electrophilic fluorination of pyrimidine derivatives

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As seen from the table for direct fluorination of monocyclic pyrimidine ring needs activation at least by one amino group. Activation of the ring by alkoxy groups is not sufficient. In this case of preliminary lithiation is used with subsequent fluorination with NFSI (Table 2, Entry 6). Besides fluorination of lithium derivatives, fluorinations of other organomethallic derivatives of pyrimidines are known. In a series of purines XeF₂ mediated fluorination of 8-tributylstannyl derivatives 22 was developed leading to 8-fluorinated derivatives 23 in high preparative yield (Scheme 6) [39, 40].

Unusual approach to the 5-fluoropyrimidides through organomercury derivatives was elaborated by Polish scientists. 2,4-Dimethoxypyrimidine 24 was readily mercurated with a boiling aq. Hg(OAc)₂ solution acidified with AcOH for 2 h, and this hot solution containing 25 was applied at once in subsequent reactions with saturated aq. KI solution affording 26 in 79 % preparative yield. By analogous way uracil derivatives were synthesised. By successively reacting (at ca. −60 °C) these symmetric organomercurials with excess of neat liquid SF₄ (b.p. −40.4 °C) the corresponding monofluorinated products 28 were obtained in ca. 30 % yield. In this reaction SF₄ formally plays unusual role as F+ source (Scheme 7) [41].

Excluding pyrimidine derivatives electrophilic fluorination of other diazines was almost not studied. To the best of our knowledge there is no examples of electrophilic fluorination of pyridazines and only 1 paper and 1 patent devoted to fluorination of pyrazines and quinoxaline. Chambers and co-workers described fluorination of quinoxalines 29 in good yields using elemental fluorine–iodine mixtures at room temperature (Scheme 8). Mono- (30) and difluorinated products 31 were formed in different ratio depending on amount of fluorine used in the reaction. It should be noted that pyrazine, pyrimidine and pyridazine were recovered unchanged using similar condition [42].

Fluorination of pyrazine 32 activated by amino group using Selectfluor afforded fluoropyrazine 33in 63 % preparative yield (Scheme 9) [43].

3.1.2 Nucleophilic Fluorination. Halogen Exchange Reactions

The most practicable and versatile laboratory and industrial route to ring-fluorinated diazines involves nucleophilic displacement of chloride by fluoride from systems activated towards nucleophilic attack. This is often referred to as the ‘Halex’ (halogen exchange) process [44]. Historically first electrophilic fluorination of diazine derivatives was accomplished in 1960 by silver fluoride [14]. Later different sources of fluoride ion included hydrogen, sodium, potassium, cesium, antimony, silver tetralkylammonium fluorides, and sulfur tetrafluoride have been used. Reactivity of the alkali metal fluorides decreases in the series CsF > KF >> NaF (i.e., with increasing lattice energy), and because the reactivity of fluoride as a nucleophile decreases sharply on solvation, dipolar aprotic solvents are often use. A lot of diverse ring fluorinated diazines were prepared by the manner. The rate determining step in nucleophilic aromatic fluorination by substitution, including the Halex process, is the addition of fluoride to form a Meisenheimer complex. Therefore, aryl chlorides are more suitable substrates in the Halex process than the corresponding aryl bromides and iodides, because chlorine is more electronegative than bromine and iodine.

In the last decade phase-transfer catalysis and ionic liquid using become popular nucleophilic fluorination. The representative set of the reaction illustrated the methodology are listed in the Table 3.

Table 3 Nucleophilic fluorination of diazine derivatives

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The chlorine/fluorine exchange reaction is an equilibrium reaction and can be influenced by altering the pressure, time, temperature and the ratio of the reactants. Usually high-temperature/high-pressure autoclave technique was used for shifted the equilibrium to fluorinated product. Anhydrous potassium fluoride in tetraglyme with a catalytic amount of dicyclohexano-l8-crown-6 at 15–16 °C converted 2,4-dichloropyrimidine into 2,4-difluoropyrimidine. This process solved the problem of having to use an autoclave or dimethylformamide as solvent, because in tetraglyme (bp 275–276 °C) the more volatile fluoro products could be distilled directly from the reaction mixture uncontaminated by solvent. Under similar conditions 2-chloro-5-methoxypyrimidine was converted into the 2-fluoro analogue [65].

Contact time very much controls the degree of conversion of polychlorinated pyrimidines heated in sealed tubes with solid potassium fluoride (Entry 6) [66], and selectivity can also be achieved by careful control of reaction conditions and reagents. With 2,4,5-trichloropyrimidines, substituted at C–6 by chloro, methyl, chloromethyl, di- or tri-chloromethyl, sodium or potassium fluoride use only resulted in nuclear fluorination. Hydrogen fluoride can displace chlorines on either side chain or nucleus (especially 2-chloro), and antimony fluoride is specific for all chlorinated methyl groups. Sodium fluoride initially replaces a 4-chloro group [67]. Fluorination reactions on tetrachloropyridazine using sodium fluoride and potassium fluoride proceeds in 4 steps and produce mixtures of the various fluorinating stages (from 1 to 4) in each case, it being possible to separate the compounds from each other using distillation. The second and third fluorination stages are composed of the difluorinated trifluorinated isomers. In contrast to this, the chlorine/fluorine exchange using hydrogen fluoride proceeds selectively as this reaction only yields one isomer for each fluorination stage. In last case the 4(5)-positions were found to be less active than the 3(6)-positions [68]. (proceeding from)In going from tetrachloropyazine to trichloropyrazine 4-position becomes more active towards Halex process [63]. Some of these processes have been subjected to kinetic investigation, which demonstrated that in polar, aprotic solvents fluorine-chlorine exchange is a pseudo first-order, consecutive reaction [69]. Recently a few papers devoted to the selectivity in clorine-fluorine exchange in polychlorocompounds were published. Solvent-free PTC conditions (KF/18-crown-6) with MW activation or protone sponge (PS) hydrogen fluorides using leads to complete or selective fluorinations of certain dichloro(benzo)diazines in satisfactory yields. In some cases, the selectivity can be explained based on the difference between thermodynamic stability of the Meisenheimer complexes [48, 62].

Among different conditions for Halex process one of the most effective for low activated substrates is (N,N′-dimethylimidazolidino)tetramethylguanidinium chloride 37 (CNC) using as phase-transfer catalyst. The synthesis and using of the catalyst were developed in 2006 by LANXESS Deutschland GmbH in a course of Fluoxastrobin intermediate 39 development [70] (Scheme 10). It should be noted, that traditional phase-transfer catalysts does not work well in the transformation and in original Bayer synthesis stepwise fluorination was used [71].

Besides chlorine, another living group can be involved into nucleophilic fluorination. Preliminary transformation of chloropyrimidines 41 to trimethylammonium salts 42 facilitate further fluorination. In this case the reaction proceeds in very mild conditions – under 5 °C (Scheme 11) [72, 73]. This approach allows to fluorinate pyrimidines deactivated by electron-donated groups. When heated with potassium fluoride in ethylene glycol 2,6-dimethoxy-4-trimethylammoniopyrimidine salts were converted into the 4-fluoroderivatives in 42 % yield [74] Analogously fluorination can be accomplished in 2-d position, which was illustrated by preparation of 2-fluoro-4-phenyl-pyrimidine [75].

3.1.3 Nucleophilic Fluorination. The Balz–Schiemann Reaction

In this classical reaction the leaving group, molecular nitrogen, is lost on pyrolysis and the mechanism appears to involve formation of an aryl cation which then abstracts fluoride ion. In comparison with halogen exchange the Balz–Schiemann reaction is not widespread in diazine chemistry. But from early 1970 to recent times the method is actively used in laboratory scale (Table 4). Generally procedure includes the treatment of aminodiazine solution in aq. HBF₄ by NaNO₂ at ca −10 °C to −15 °C followed by neutralization with NaOH. Another procedure is based on NaNO₂ treatment in HF-Py media with subsequent heating. The last method gives better yields, especially for 4-fluoripyrimidines. The Balz–Schiemann approach allow to synthesized fluoropyrimidines bearing active chlorine atom, which are unacceptable via Halex process (Entry 5 and 6).

Table 4 The Balz–Schiemann reaction of diazine derivatives

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3.2 Cyclization Processes

3.2.1 “Principal Synthesis” of Pyrimidines

The condensation of two acyclic reagents (with any preattached substituents), one to supply N¹C²N³ fragment and the other to supply C⁴C⁵C⁶ fragment to form the resulting ring, is the most used procedure and is known accordingly as the “principal synthesis” of pyrimidines. The approach is important for the synthesis of C⁵-F pyrimidine derivatives.

First synthesis of 5-FU was accomplished by Heidelberger in 1957 according the methodology [6, 81]. Ethyl fluoroacetate 44 was subjected to Claisen condensation with ethyl formate to give 45. The salt 45 was introduced into reaction with S-alkylisothiourea derivatives 46 to give fluoropyrimidines 47, which were hydrolysed to give 5-FU (Scheme 12).

The cyclization of 2-fluoro-3-ketoesters derivatives with 1,3-NCN-bisnucleophiles is general approach to fluorinated pyrimidines. There are a lot of examples of such transformations in the literature. A representative set of the cyclization is listed in Table 5. Besides usual 1,3-NCN-bisnucleophiles, such as amidine, guanidine and urea derivatives in the reaction a set of appropriate aminoheterocycles (Entry 4–6) was used. In this case fused derivatives of 5-fluorineted pyrimidines were synthesised. It should be noted, that synthesis of fused heterocyclic pyrimidines could not be accomplished in basic conditions, in a case of aminopyrazole and aminopyrimidine neutral conditions were used, in a case of aminothiadiazole acidic catalysis works well. Fluorinated malonic acid derivatives were subjected also to the “Principal synthesis” of pyrimidines affording 4,6-dihydroxypyrimidine derivatives (Entry 7–9). Basic conditions in this case give final products in 5°–9°% yields.

Table 5 “Principal synthesis” of pyrimidines

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Latent dicarbonyl compounds, especially fluorinated “push-pull” enaminones also used as 1,3-CCC-bielectrophiles in the cyclization. In earlier examples based on 3-dimethylamino-2-fluoroacrolein both thermal and basic conditions were used for the synthesis of the corresponding pyrimidines in 26–57 % yields (Entry 10–12). Also 3-dimethylamino-2-fluoroacrolein gives parent 5-fluoropyrimidine in 52 % yield under heating at 190 °C in formamide. Recent works dealt with sophisticated fluorinated “push-pull” enaminones also referred basic cyclization conditions as well as thermal.

In many cases synthesis of the fluorinated 1,3-CCC-bielectrophile precursors is the most difficult part of the synthetic sequence and using “classical” methods is usually accomplished by the use of highly toxic fluoroacetic acid derivatives in Claisen condensation with ethyl formiate [81], ethyl chloroformate [95, 96], diethyloxalate [82, 97], acetyl, benzoyl chlorides [82] or Vilsmeier-type formylation [98, 99] (Scheme 13). The product of Vilsmeier-type formylation is 3-dimethylamino-2-fluoroacrolein 54 which reacts with triethyloxonium tetrafluoroborate and dimethylamine to give the vinamidinium salt 55 [91], which also can be used as 1,3-bielectrophile (see Scheme 19). Also Reformatsky-type synthesis of ethyl α-fluoroacetoacetate 52 starting from ethyl chlorofluoroacetate 56 was described in 20 % yield [82].

Wittig and Wittig-Horner reactions were used to prepare α-fluoro-β-keto esters from halofluoroacetates (Scheme 14). Triethyl phosphite and ethyl bromofluoroacetate 57 gave under thermal conditions (145 °C, 5 h) 150 g of fluorinated phosphonate 58 in one run in 61 % yield [100]. Tri-n-butylphosphine (Ph₃P also entered into the reaction, but further transformation described using n-Bu₃P) reacts with ethyl bromofluoroacetate in THF solution at rt during 4°h affording phosphonium salt in 9°% yield, which was converted into ylide 59 by BuLi treatment in THF at −78 °C and used in the solution for further transformation [101]. Both substrates are applicable for the synthesis of α-fluoro-β-keto esters using alkyl and aryl substituted acyl chlorides. Acylation of ylide 59 with perfluorinated and partially fluorinated acyl chlorides did not proceed cleanly, however the anion derived from phosphonate 58 undergo acylation with further hydrolysis affording desired products in good yields [102]. Further some modification of the procedure was reported using phosphonate 58 [103], which is now commercially available.

The further work devoted to the synthesis of fluorinated 1,3-CCC-bielectrophiles tries to avoid the use of fluoroacetic acid derivatives. These three general approaches to synthones gain commercial value:

electrophilic fluorination of the corresponding dicarbonyl compounds (Tosoh F-Tech, Inc., F₂/N₂ [104]; Air Products and Chemicals, Inc. CF₂(OF)₂ [105];)
nucleophilic fluorination of the corresponding chloro-derivatives (Bayer, TEA*3HF [106, 107])
ethanolysis of hexafluoropropene 61 (E.R. SQUIBB and SONS, INC., Scheme 15 [108, 109])
Scheme 15
Synthesis of monofluoromalonate from perfluoropropene
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Lab scale synthesis of fluorinated 1,3-CCC-bielectrophiles based on electrophilic fluorination has disadvantage in difluoroproducts formation. This problem was solved recently by application of iodotoluene difluoride or iodosylbenzene – HF [110, 111]. A possible mechanism for a fluorination reaction of 1,3-dicarbonyl compounds is shown in Scheme 16. First, PhIF₂ should be formed in situ by reaction of PhIO with HF. The reaction of PhIF₂ with 64 is considered to proceed effectively after enolization of 64. The resulting 2-iodanyl-1,3-dicarbonyl compound 68 readily undergoes displacement by a fluoride ion due to the high leaving ability of the phenyliodonium group, to give the fluorine-containing product 65. Also the fluorinated product was formed through the C-protonation of the iodonium ylide, followed by displacement with fluoride ion [112].

Another way to avoid the difluorination can be achieved by using of “push-pull” enamines and Selectfluor or NFSI as fluorine source (Scheme 17) [92, 94].

Electrophilic fluorination of formal formylacetic acid synthon was carried out through 5,6-unsubstituted 1,3-dioxin-4-ones 74 by fluorine followed by treatment with triethylamine. The fluorination proceeds via cis-addition – elimination in 84 % overall yield affording 5-fluoro-1,3-dioxin-4-one 76. The compound could be transformed to 5-FU by analogy with Heidelberger synthesis of 5-FU but original paper doesn’t refer yields for the transformations [113] (Scheme 18).

Alternative approaches to some fluorinated synthons were developed in 1990s by Yamanaka with co-workers starting from commercially available polyfluorinated alcohol. The corresponding quaternary ammonium salt 80 was prepared in four step synthesis in 69 % total yield. 80 under treatment by secondary amines gave vinamidinium salts 81 similar to Vilsmeier-type sequence (see Scheme 13) [114]. The salts appear appropriate synthons for pyrimidine synthesis, for example the reaction with formamidine hydrochloride affords parent 5-fluoropyrimidine 82 in 82 % yield [115]. Same synthetic equivalent of fluoromalonic dialdehyde – 3-dialkylamino-2-fluoroacroleins 85 also acceptable from tetrafluoropropanol via 3-step sequence including tosylation [116], elimination of HF via lithiation [117, 118] and dialkylamino treatment. The overall yield of the sequence is 55 % [117]. Tosylate 84 directly can be subjected to cyclization with amidines and can be used for synthesis of 1-substituted sulfonates through additional lithiation – alkylation/Pd-catalyzed coupling with ArI [119] (Scheme 19).

Another Japanese group of chemists in 1988 described the synthesis of fluorinated pyrimidines starting fluorinated ketones. The treatment of fluoroalkyl ketones 89 with sodium diethyl phosphate in THF at −10 °C gave 1-substituted fluoro-1-alkenyl phosphates 90, which readily reacted with amidine derivative at room temperature afford corresponding pyrimidines 91 bearing fluorine at 5-th position as well as perfluoroalkyl fragment [120]. Another synthon able to incorporate both ring fluorine atom as well as perfluoroalkyl group was described by Sloop in 2002 via fluorination of silyl enol esters. This diketone 94 was converted to pyrimidinol 91 in acidic conditions in 61 % yield [121] (Scheme 20).

3-Substituted-trans-2,3-difluoro-2-acrylates 98 can be used as synthons for pyrimidine synthesis. Trifluorovinyltrimethylsilane 96, prepared from trimethylsilyl chloride, chlorotrifluoroethylene 95 and n-butyl lithium in THF, reacted with a variety of lithium reagents to afford the corresponding addition–elimination products 97. Ethyl chloroformate reacted with trans-(2-alkyl or 2-aryl-1,2-difluoroethenyl)trimethylsilanes 97 in presence of dry potassium fluoride (1.5–2°equiv.) in DMF at 80 °C to afford the corresponding esters 98 stereoselectively in good yields. Treatment of ethyl 3-substituted-trans-2,3-difluoro-2-acrylates with acetamidine hydrochloride and benzamidine hydrochloride, respectively, in presence of K₂CO₃ in 1,4-dioxane gave the corresponding 5-fluoropyrimidine derivatives 99 in good yield [122] (Scheme 21).

Similar approach (trough difluorinated vinyls) was developed by Sizov with co-workers in early 2000-th starting from commercially available tetrafluoropropionitrile 100. The nitrile reacts with PhCH₂SH in presence of 2 eq. of the BF₃*NEt₃ complex affording vinyl sulfide 101 in a preparative yield. Difluorobenzylthioacrylonitrile reacted with amidines to produce the corresponding 4-amino-5-fluoropyrimidines 102 [123, 124] (Scheme 22).

3.2.2 Miscellaneous Cyclization

5-Fluoropyrimidines 106 can be also synthesized using Diels-Alder reaction of fluorinated 2-aza-1,3-diene 105 with tosyl cyanide. The corresponding 2-aza-1,3-diene was easy synthesized from N-acylimldates 104 through silylation with t-butyldimethylsilyl triflate in a presence of triethylamine [125] (Scheme 23).

Unusual approach to 4-fluoriopyrimidines was developed by de Nanteuil where CF₃-group plays a role of fluorine source. The corresponding α-chloro-α′-trifluoromethyl ketones 111 were synthesised in 5 steps starting from 107, which reacted with formamidine affording 5-substituted 4-fluoro-6-chloromethyl pyrimidines 112 in 23–35 % yield [126] (Scheme24).

Very recently unusual approach to 4-fluoropyrimidine N-oxides from alkenes was elaborated in Moscow State University. The method based on three-component heterocyclization involving gem-bromofluorocyclopropanes 113 or 116, nitrosyl tetrafluoroborate, and a molecule of the solvent (nitrile) yielding previously unknown fluorinated pyrimidine N-oxides 114 or 117 (Scheme 25) [127].

The first step of cyclization involves the electrophilic attack of NO⁺ and the opening of the three-membered ring resulting in the formation of the intermediates type 119. Than intermediate 119 is trapped by solvent, resulting in the formation of nitrilium species 120, which undergo intramolecular cyclization into final pyrimidine N-oxides 122 (Scheme 26).

Ichikawa and co-workers described in 2007 intramolecular cyclization leading to 3-fluoroccinnolines. o-Amino-β,β-difluorostyrenes 123, prepared from CF₃CH₂OTs and o-iodoaniline, were treated with isoamyl nitrite (i-AmONO) for diazotization, and then reduced with n-Bu₃SnH. The expected intramolecular substitution of the terminal diazenyl nitrogen (HN=N–) proceeded smoothly, to give 3-fluorocinnoline 126 (R=n-Bu) in 58 % yield. Then several other reducing reagents were tested, and it was found that benzenethiol raised the yield of 126 (R=n-Bu and sec-Bu) to 88 and 87 %, respectively (Scheme 27). In the reaction of 124, diphenyl disulfide (PhSSPh) was obtained in 90 % yield based on PhSH, which implies that PhSH definitely acted as a reducing agent [128].

4 Properties and Chemical Transformation

4.1 General

RFDs without additional chromophores are either colorless liquids or white solids and, apart from having relatively high volatilities, no special handling procedures are required for their use. The boiling points of the perfluorinated diazines somewhat lower than those for the corresponding parent hydrocarbons in contrast to perfluorobenzenoid compounds which have boiling points that parallel those of the corresponding hydrocarbons (Table 6). This is attributed to the much lower intermolecular forces and the very low basicities of the fluorocarbon systems that compensate for the increase in mass upon replacing hydrogen by fluorine [129].

Table 6 Comparison of boiling points of perfluoroheteroaromatics systems with the corresponding hydrocarbon systems

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All the perfluoroheteroaromatic systems are very weak bases and, for instance, superacids are required to protonate pentafluoropyridine. Relative base strengths of the perfluorinated heteroaromatic systems have been determined by NMR competition experiments and the major influence is that of the fluorine atoms ortho to ring nitrogen that significantly decrease the basicity of the system (Fig. 3) [130]. Despite the fact that perfluoropyrimidine did no participate in experimental NMR competition CNDO/2 SCF-MO calculations of energy release on portonation in gaseous phase (ΔE _g) predict that basicity of the compound are between perfluoropyridine and pefluoropyrazine [131].

The common method of securing information about electronic structure is photoelectron spectroscopy (PES), which permits a direct comparison with experiment of various quantum-chemical models used for the calculation of phischem characteristics. The fluorinated diazines have been investigated by the method [132]. By means of fluorine substitution the analysis of the HR HeI line PES of the parent compounds was made in details in Twente University, made it possible to refine the assignment of the bands in the PE spectrums of diazines. Especially in a case of aza- and diazaaromatics, where nitrogen “lone-pair” bands and π-bands lie in the same region of the spectrum, the use of the perfluoro effect is indispensable for a thorough analysis of the spectra. By the same scientists the electrochemical reduction of RFDs was studied [133]. The electrochemical reduction process of fluorinated aza-aromatics can well be described by the pattern which is normally postulated for aryl halogenides, i. g. fission of the carbon-halogen bond. However, the stability of the intermediate mono-negative ions is generally higher than for the comparable fluoro-substituted arenes. The half-wave reduction potentials of the first reduction wave can be related to the electron affinities of the molecules. These electron affinities have been correlated with those obtained by quantum-chemical calculation. Also recently the theoretical MP2 study was performed for the structural investigation of anion-binding involving π-acidic RFDs [134].

The hydrophobicity of molecules plays an important role in structure–activity relationship studies for various bioactive compounds. The introducing of the fluorine atom into diazine core increased hydrophobicity and selected experimental data is presented on Fig. 4 [135–137]

Despite of a lot of NMR data of RFDs the limited data available for simple RFDs allow one to see how the position of the fluorine substituent on a heterocycle can significantly affect its chemical shift [138]. To the best of our knowledge there are not literature data dealing with general analysis of NMR data of RFDs. Only a few reviewed papers just summarized the ¹⁹F NMR data of RFDs described in 1968–1981 [139–142]. For the synthetic chemists one of the most important is ¹⁹F NMR data, because the knowledge allows simple monitoring of the reaction mixtures by ¹⁹F NMR of the reaction mixtures. This data also give possibilities to registrate of non-isolable intermediates in solutions, which significant simplify the mechanistic interpretation of the processes. Besides practical application the ¹⁹F NMR data for RFDs, has been used to verify previously published statistical substituent chemical shift (SSCS) values for fluoroarenes. The data was allowed generation of a set of structure factors for aromatic nitrogen heterocycles which allows the signals for these compounds to be predicted from the same set of SSCS values as fluoroarenes [143].

In case of pyrimidines, large differences in chemical shift are observed for fluorines at the 2-, 4- (6-), and 5- positions with fluorines at the 2-position of pyrimidines being the most deshielded, and those at the 5-position being the most shielded. The chemical shifts for fluoropyrimidines and 5-fluorouracil are provided on Fig. 5. ¹³C and ¹H NMR chemical shift and coupling constant data for some ring fluorinated pyrimidines are also given on Figs. 6 and 7. It should be noted, that in fluorinated pyrimidines, unlike with fluorinated benzenes, the values of coupling ³ J _FH constants are significant small (0.8–2.7 Hz), less then values of ⁵ J _FH constants in 5-fluorinated pyrimidines (~3.3 Hz) and than values of ⁴ J _FH constants in 4-fluorinated pyrimidines (~10 Hz).

In pyridazine series like in pyrimidines large differences in chemical shift are observed for fluorines in 3-(6-)- and in 4-(5-) positions. The fluorines at the 3-(6-)- positions of pyridazine being more deshielded. The values of of fluorine-fluorine coupling constants one can easily find from NMR data of series fluorochloropyridazines [68] (Fig. 8). As in a case of pyrimidine a large value (ca. 30 Hz) of the ⁵ J _FF constant is noteworthy. Also on Fig. 9 ¹³C and ¹H NMR chemical shifts and coupling constant of model 6-chloro-3-fuoropyridazine are provided. The values of coupling ³ J _FH constants are significantly small (~2 Hz) than values of ⁴ J _FH constants (~6–7 Hz).

¹⁹F, ¹³C and ¹H NMR chemical shift and coupling constant data for some ring fluorinated pyrazines are provided on Fig. 10.

The determination of substitution patterns in diazine compounds is particular important. One of the approaches to solve the problem is 2D ¹⁵N NMR spectroscopy. Therefore in the literature there are some ¹⁵N NMR data of ring fluorinated diazines. The ¹⁵N NMR of perfluorinated diazines are summarized in Table 7 [144]. Also recently few works devoted to theoretical calculations of coupling constants in fluorinated azines were published [145, 146].

Table 7 Comparison of ¹⁵N chemical shifts of perfluoroheteroaromatic systems with the corresponding hydrocarbons analogues (shifts relative to liq. NH₃)

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As an example of RFD structure determination based on long-range ¹H–¹⁵N GHMBC spectra one can refer determination of structure of 2-fluoro-3-phenyl-5-iodopyrazine based on comparison of GHMBC spectra of fluorinated pyrazines (Fig. 11) [147].

4.2 Nucleophilic Aromatic Substitution

4.2.1 General Remarks: Orientation and Reactivity

A considerable number of ring-fluorinated diazines undergoes various nucleophilic aromatic substitution reactions. Nucleophilic aromatic substitution reactions follow the well-established two-step addition–elimination mechanism via a Meisenheimer intermediate. The destabilization of sp ²-C bound fluorine by p–π repulsion activates fluorinated aromatic compounds toward nucleophilic attack and subsequent substitution. The susceptibility of the carbon center toward nucleophiles is also enhanced by the negative inductive (−I _σ) effect of fluorine. Therefore the ease of nucleophilic halogen replacement – F > Cl > Br > I – is in the opposite order to that for aliphatic nucleophilic substitution. The kinetic data of pyperidinolysis in diazine series proof the assumption and k_F/k_Cl ratios are listed in Fig. 12 [72, 148]. Also the acid-promoted hydrolysis of the 2-fluoro derivatives of pyrimidine, 4-methylpyrimidine, and 4,6-dimethylpyrimidine have been studied in hydrochloric acid. The mechanism for hydrolysis of the pyrimidines as distinct from that of the less activated 2-fluoroquinoline and the 2-fluoropyridines by suggesting that nucleophilic attack takes place without proton transfer to a second water molecule in the reactions of the former compounds and with transfer in reaction of the latter [149]. Kinetic studies of basic hydrolysis of halogenopyrazines in aqueous NaOH also show that 2-fluoropyrazine in 640 times more active than 2-chloropyrazine [150] (Fig. 12).

The difference in fluorine/chlorine mobility was also practically illustrated by Amgen using bis-pyrimidine 127 for libraries construction [151]. In this case only the fluorine displacement is observed (Scheme 28).

Another example of fluorine/chlorine exchange selectivity one can find in Novartis patent were chlorofluoropyrimidine 132 react with cyclopentylamine leading to compound 133 (Scheme 29) [152]. It should be noted, that the synthesis of starting compound 131 is similar to approach shown on Scheme 24, where CF₃ group is a source of fluorine in pyrimidine nuclear.

High reactivity of 2-fluoropyrimidine was used for the amines, anilines and aminoacids decoration on solid support. In this case among halopyrimidines only 2-fluoropyrimidine is appropriate as reagent and in a case of aminoacids gave corresponding in high preparative yields (Scheme 30) [76, 153, 154]

The increasing of number of ring fluorine atoms in diazines leads to increasing of activity towards various nucleophiles. The results of comparative kinetic studies of various polifluorinated diazines in the reaction with ammonia in dioxane are shown on Fig. 13. Among perfluorinated azines the activities towards nucleophiles of pyrimidine and pyridazine are in the region between pyridine and triazine. Tetrafluoropyrazine is less active than pentafluoropyridine. The reduced reactivity of tetrafluoropyrazine compared to the other perfluorinated azines reflects the absence of highly activated sites in para-position to ring nitrogen. The increasing of activity in pyrimidine (trifluoro substituted to tetrafluoro substituted) series are in agreement with data obtained in pyridine series. Activating influences of fluorine in the pyrimidine ring system are k(ortho-F)/k(H) = 41 and k(meta-F)/k(H) = 3 respectively. This such big influence for ortho fluorine atom explains the loss of the regioselectivity in fluorine displacement in 2,4,6-trifluoropyrimidine [155, 156].

Besides fluorine the influences of another substituents such as Cl, CF₃, NO₂, CN on fluorine displacement in 2,4,6-trifluoropyrimidine were studied. The results (Table 8) can be satisfactory rationalized in terms of bimolecular additional-elimination S_NAr mechanism through Meisenheimer type complexes [157].

Table 8 Nucleophilic substitution in 2,4,6-trifluoropyrimidine

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4.2.2 Application in Organic and Combinatorial Synthesis

The high selectivity in sequential nucleophilic substitution in perfluorodiazines made them attractive scaffolds for the synthesis of a diverse array of polysubstituted diazines. These approaches were recently developed by Stanford group. Thus, tetrafluoropyrimidine may be used as a scaffold for the synthesis of a range of 2,4,6-trisubstituted pyrimidine derivatives upon sequential displacement of the fluorine atoms attached to the strongly activated 4-, 6- and 2-positions (Table 9) [158]. The first two substitutions proceed in very mild conditioned (0 °C or room temperature). The last nucleophilic substitution of fluorine at 2-position proceeds in harsh conditions and needs MW heating.

Table 9 Tetrafluoropyrimidine as a core scaffold

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Similarly, trifluoropyridazinone 140, readily synthesised by reaction of tetrafluoropyridazine 139 with sulfuric acid, may be used as the starting material for the synthesis of a variety of 4,5-diamino-fluoropyridazinone systems 144. Reaction of trifluoropyridazinone gives a mixture of products 141 and 142 arising from displacement of fluorine from either 4- or 5-positions, both positions are activated by para ring nitrogen, but these isomers can be separated by column chromatography and used in subsequent S_NAr processes for the synthesis of a range of aminated pyridazinone derivatives. The predominant product formed in the reaction is product of 4-F substitution 141. The first substitution with amines proceeds in room temperature whereas the next substitution, as in a case of tetrafluoropyrimidine, needs MW heating (Scheme 31) [159].

Reactions of trifluoropyridazinone with highly basic sodium methoxide or phenoxide gave complex mixtures of products and tar derived, presumably, from deprotonation of the pyridazinone ring NH and subsequent polymerisation. Protection of the ring NH group as a tetrahydropyran derivative, however, allows the functionalization of the pyridazinone core scaffold by oxygen-centred nucleophiles extending the range of functional pyridazinone systems which may be accessed by this general strategy [160]. Also the reaction of polyfluorinated pyridazines with binucleophilic compounds leads firstly to intermolecular nucleophilic substitutions followed by cyclization to afford fused systems. This enhanced reactivity is reflected in the relative reactivity found in intramolecular nucleophilic substitution reactions compared to corresponding intermolecular processes. Among binucleophiles N,N′-dimethylethylene diamine, catechol and actoacetic ester derivatives were tested (Scheme 32). Besides tetratrifluoropyridazine derivatives, tetratrifluoropyrazine also entered into annelation reaction. In a case of acetoacetic ester furo[2,3-b]pyrazine derivative formed bearing two active fluorine atoms. But the additional nucleophile treatment showed regioselective displacement only at C-3 position (Scheme 33) [159–162].

In general, tetratrifluoropyrazine is less studied as core scaffold. Reactions of tetrafluoropyrazine with nucleophiles occur readily and, of course, there are no issues regarding regioselectivity of the first nucleophilic substitution process due to the symmetry of this system. The reduced reactivity of tetrafluoropyrazine compared to the other perfluorinated diazines reflects the absence of highly activated sites in para position to ring nitrogen. The regiochemistry of the reaction of trifluoropyrazine derivatives with nucleophiles is influenced by the nature of the substituent as well as the presence of the remaining fluorine atoms. If the substituent is either an alkoxy or amino group, the site of attack is generally ortho to the substituent, although steric effects can also influence the outcome of this reaction. In contrast, when the substituent is a hydrogen or alkyl group or chlorine, the site of attack is para – position to the substituent. (Fig. 14) [57, 163, 164].

Of course in the academic and patent literature there are a lot of examples of nucleophilic substitution of fluorine by different N, S, O – nucleophiles. Also another halogens in appropriate positions are able to entered in substitution reaction. A recent example of using of nucleophilic substitution was described by Lexicon Pharmaceuticals in course of development of deoxycytidine kinase inhibitors. Key intermediate for this investigation was 5-fluoro-2-(piperidin-4-yloxy)pyrimidin-4-amine 157. The medicinal chemistry group used a synthesis based on the Mitsunobu reaction of commercially available 5-fluorocytosine 154 and N-Boc-4-piperidinol 155 (Scheme 34). The resulting iminophosphorane 156 is then treated with HCl to give the 5-fluoro-2-(piperidin-4-yloxy)pyrimidin-4-amine dihydrochloride in about 60 % yield. While this synthesis worked well on small scale and provided rapid access to gram quantities of 157 for early investigations of SAR, it gave inconsistent yields on scale-up [165].

The R@D route to 5-fluoro-2-(piperidin-4-yloxy)pyrimidin-4-amine 157 was developed starting from readily available 2,4-dichloro-5-fluoropyrimidine 158. This dichloroderivative, is one of the most frequently used building block among fluorinated diazines (198 reactions from 164 references in Reaxys® database). It was subjected to the reaction with bisallylamine and than with N-Boc-4-piperidinol affording compound 161. The deprotection of amino group was carried out by isomerization using lithium tert-butoxide in DMSO/tert-butanol with subsequent hydrolysis leading to desired 5-fluoro-2-(piperidin-4-yloxy)pyrimidin-4-amine dihydrochloride 157 (Scheme 35).

Such unusual chemical route to the target compound was developed due to the number of side process disclosed during investigation: fluorine substitution by dimsyl sodium of defluorination by catalytic hydrogenation (Scheme 36).

In this review we have no possibility to give comprehensive information about all types of these diverse transformations. Some of these transformations used for drug synthesis will be discussed in next chapter of this book. Another part of transformations has been partially reviewed early [166–169]. But in the next part of the section we would like to draw the reader’s attention to less known nucleophilic substitutions such as with C-, P- and formally hydride nucleophiles and substitution of fluorine atom in 5-position of pyrimidine.

There are a few examples of fluorine substitution by C-nucleophiles. The reaction of fluorinated pyrimidines with stabilized carbanions affording the corresponding functionalized pyrimidines in low to moderate yields was described [170–176]. In all this cases the reactions do not have preparative value and was used for synthesis of model objects (Table 10, Entry 1–3). Recently such arylation was carried out in asymmetric manner using an O–benzoylated cinchona alkaloid derivative as organocatalyst (Table 10, Entry 4). Besides arylation of stabilized carboanions the non catalytic hetarylation of π-electron reach indole was described by tetrafluoropyrimidine [174]. Also at the early step of pefluorinated diazines studies the reaction with Grignard reagents and alkyl lithium compounds were discovered by Banks and Chambers (Table 10, Entry 6–8). It should be noted, that the regiochemistry of the reaction of fluorinated diazines with C-nucleophiles are in accordance with N- and O– nucleophiles regiochemistry.

Table 10 Fluorine substitution with by C-nucleophile

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The reactions with P-nucleophiles are even more rare than with C-nucleophiles. Recently such reaction was used for the synthesis of polysubstituted pyrimidinylphosphonic acid 172. Microwave-assisted Michaelis-Arbuzov reaction of triisopropyl phosphite with the corresponding 2-fluoropyrimidine 170, followed by deprotection of the phosphonate group using TMSBr in acetonitrile gave the desired acid 172 in 66 % total yield. The derivative 172 exhibits anti-influenza virus A activity in the middle micromolar range (Scheme 37) [177].

Another reaction discussed in this section is reduction of fluorinated pyrimidines by LAH. In this case LAH can be considered as strong hydride donor, which undergo nucleophilic substitution of fluorine. In a case of tetrafluoropyrimidine 6 the defluorination by LAH predominantly proceeds at 4-position. Double defluorination and defluorination at 2-position were detected as byprocess. Analogously allyloxy derivative 176 also undergo defluorination by LAH and major defluorination proceeds at 4-position (Scheme 38) [178, 179].

But this simple defluorination methodology was found to be ineffective in a case of another diazines, therefore another approaches were elaborated. For example, tetrafluoropyrazine 150 was converted to trifluoropyrazine 180 in two steps – using hydrazine hydrate substitution – copper oxidation sequence (Scheme 39) [163]. Another defluorination strategy will be discussed in next sections.

The most problematic nucleophilic substitution in fluorinated diazine series is substitution of fluorine at 5-position of pyrimidine due to the absence of highly activated sites para or ortho to ring nitrogen. The first example of fluorine exchange in 5-position was described simultaneously with tetrafluoropyrimidine. Heating at 220 °C in di-n-buthylamine leads to exhaustive fluorine exchange giving tetracis-di-n-butylaminopyrimidine [14]. But further publication showed that fluorine substitution in 5-position of pyrimidine needed activation by electron withdrawing groups or facilitating by intramolecular cyclization. Ester and CF₃ groups in 2-and 4-position of pyrimidine ring were used as electron withdrawing groups (Table 11). Interesting fact was disclosed recently that fluorine at 5-position activated by ester group is more reactive than chlorine atom at 2-position of pyrimidine (Table 11, Entry 3).

Table 11 Fluorine substitution in 5-th pyrimidine position activated by electron withdrawing group

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The first example of fluorine substitution in 5-position accomplished by intramolecular cyclization was described by Ukrainian chemist in 1991. The reaction of 4-chloro-5-fluoropyrimidines 181 with carbanions generated from 2-tosylmethylazahetarenes 182 in presence of K₂CO₃ in refluxing DMF does not stop at the step involving replacement of the chlorine atom (intermediate 183) but concludes by cyclization to give triazafluorene 184 (Scheme 40) [183].

Further annelation to pyrimidine ring based on substitution of fluorine at 5-position was used in patent literature. Alantos Pharmaceuticals in a course of matrix metalloprotease inhibitors development described an efficient pyrazole annelation to pyrimidine. The synthesis of key intermediate – pyrimidine dicarboxylic acid 188 was accomplished in 3 step bromination – carbonylation – oxidation sequence from pyrimidinone 185. The acid was converted to the corresponding ester 189 and nitrile 190. In both cases cyclization with hydrazine hydrate proceeds in mild conditions affording pyrazolo[4,3-d]pyrimidines 191 and 192 in nearly quantitative yields (Scheme 41) [184].

Another example of annelation was demonstrated by Hoffmann-La Roche at the thiazolo[4,5-d]pyrimidine scaffold synthesis and decoration. Under thionation with P₂S₅, the acylated 6-aminopyrimidine 195 gives the corresponding thioamide, which cyclised spontaneously into thiazolo[4,5-d]pyrimidine thione 196. Thione 196 was used further transformation for synthesis of key building block 197 applied for the preparation of the library 198 (Scheme 42) [185].

The most interesting annelation example was recently described by Takeda during oxa-2,4,4b,10-tetraazaphenanthren-9-one scaffold 204 synthesis. Unlike above mentioned examples, in this case annelation does not proceed with aromatization. In compound 201 fluorine atom is formally deactivated by electron donating dialkylamino residue at 4-position. Despite both this factors the intramolecular fluorine substitution proceed in sufficiently mild conditions (DMF, Cs₂CO₃, 60 °C) affording fused compound 202 in moderate yield (Scheme 43) [186].

4.2.3 Reactive Dyes

One of the most important application of fluorine nucleophilic substitution in diazines was found in reactive dyes industry. In a reactive dye a chromophore contains a substituent that is activated and allowed to directly react to the surface of the substrate. Reactive dyes have good fastness properties owing to the bonding that occurs during dyeing. Reactive dyes are most commonly used in dyeing of cellulose like cotton or flax, but also wool is dyeable with reactive dyes.

Detailed reviews of this subject are now available [187–189]. In reactive dyes with halogen as a leaving group, these two partial structures nearly always linked by an amino function in the chromophore, which makes an important contribution to the chromophore conjugated π-electron system. In practical terms, this means that the reactive component must have at least two reactive groups, one of them reacts with dye base affording reactive dyestuff and another one reacts with cellulosic fiber (Fig. 15).

The development of reactive components based on azines started in 1956 with the launch of chlorotriazine dyes by ICI. The immediate success of the triazine based reactive dyes led to an intensive search for alternative reactive systems by the various dyestuff firms. Much efforts has been expended on the synthesis and evaluation of several related fluoropyrimidinyl derivatives. Out of all the patented components only 5-chloro-2,4,6-trifluoropyrimidine and 5-chloro-2,4-difluoro-6-methylpyrimidine have attained notable technical and economic significants. Figure 16 shows the relevant dyestuffs and their manufacturers.

4.2.4 Acid-Induced Processes

Although ring-fluorinated compounds are only weak bases, nucleophilic substitution can be induced by proton or Lewis acids and interesting contrasts in orientation can sometimes be achieved because attack to ortho-position to nitrogen is often preferred under these conditions. Among perfluorinated diazines pyridazine 139 is the most basic and and protonation of ring nitrogen by strong acids or alkylation is possible if a strong alkylating agent is used. It is clear from the striking tendency for the protonated systems, as shown in Scheme 44, to give ortho-attack to nitrogen that, again, polar influences are extremely important in governing the reactivity of a C-F bond, at least with hard nucleophiles. In both of the examples contained in Scheme 44, the orientation of entry of the nucleophile is changed in comparison with reaction with the neutral system [56, 190].

Another important acid-induced process in fluorinated diazine chemistry is fluorine-halogen exchange promoted by Lewis acids. In some cases the switching of regiochemistry also observed. Thus in a case of dimethyl-(3,5,6-trifluoro-pyrazin-2-yl)-amine 300 treatment with AlCl₃ leads to exchange of fluorine at 6-th position unlike with Me₂NH substitution (compare with Fig. 14). Subsequently dialkylamino – AlCl₃ treatment leads to dichloropyrazine 303, which unavailable by selective manner from tetrachloropyrazine (Scheme 45) [164].

Like with AlCl₃, AlBr₃ promote bromine-fluorine exchange. In a case of perfluoropyrazine 150 AlBr₃ treatment leads to exhaustive bromination affording perbromopyrazine 305. For single fluorine exchange to bromine atom another approaches were used (Scheme 46, compare with Scheme 39) [57]

These bromine-fluorine exchanges are important processes because introduction of bromine by these simple procedures allows access to the powerful range of palladium chemistry that is now available (see next section). Also bromine introduction – Pd catalyzed hydrogenation gives an excess to fluorinated nitrogen heterocycles with unusual substitution patterns (Schemes 47 and 48) [191].

4.2.5 Fluoride-Ion-Induced Reactions

Reactions of perfluorinated alkenes, such as hexafluoropropene, with fluoride ion give perfluoroalkylcarbanions which can act as nucleophiles in S_NAr reactions with perfluoroheteroaromatic systems (Fig. 17). These reactions are example of “mirror-image” chemistry and reflect well-known Friedel–Crafts reactions of hydrocarbon systems that proceed by reaction of the corresponding electrophile and carbocationic intermediates. Reactions involving chlorotrifluoroethene and bromotrifluoroethene introduce further complexities. Direct substitution may occur giving halofluorosubstituent, but this is frequently accompanied by loss of Cl or Br from the side chain to give a pentafluoroethyl derivative. The almost complete list of the reaction with polifluorinated heterocycles was earlier reviewed by Brooke in 1997 [166].

As an example, the discovering of kinetic and thermodynamic control in nucleophilic substitution in fluorinated diazines can be shown. The use of MeO⁻/MeOH is routinely used to test for possible nucleophilic substitution of fluorine in polyfluoroheteroaromatic compounds and identifies sites for kinetically controlled reactions because of the irreversibility of the reaction. Tetrafluoropyridazine 139 forms only the 4,5-dimethoxy isomer 209 (Scheme 44) whereas the variability in the orientations of dipolyfluoroalkylations of tetrafluoropyridazine and other systems is a manifestation of the interplay between kinetic and thermodynamic control of the reaction products (Scheme 49) [192]. In a case of octafluoroisobutene the reaction with tetrafluoropyridazine 139 in a presence of CsF in sulpholan at 20 °C leads to perfluoro-4-t-butylpyridazine 313. Heating the reaction mixture to 40 °C showed formation of perfluoro-3,5-di-t-butypyridazine 314, which under heating to 80 °C intermolecular rearrange to the least crowded perfluoro-3,6-di-t-butypyridazine 316 through perfluoro-3-t-butylpyridazine 315. Meanwhile, less hindered pentafluoroethyl anion in similar conditions gives the products arise exclusively from kinetic control (Scheme 50) [193, 194]. Hexafluoropropene in this investigation occupies an intermediate position and gives more complicate number of products. Therefore the variation in the observed products is consistent with the ease of formation as well as the steric requirements increasing in the series CF₃-CF₂ ⁻ < (CF₃)₂CF⁻ < (CF₃)₃C⁻ and these results provide a striking example of the interplay of kinetic and thermodynamic control of reaction products in nucleophilic aromatic substitution.

Besides tetrafluoropyridazine 139, tetrafluoropyrimidine 6 was investigated in the fluoride induced reactions. These reactions also have specificity. For example, the reaction of tetrafluoropyrimidine 6 with hexafluoropropene 320 induced by CsF in sulpholan gives mixture of perfluoro-2,4,6-triisopropylpyrimidine 322, perfluoro-4,6-diisopropylpyrimidine 321 and perfluoro-2,4,5,6-traisopropylpyrimidine 323 were detected (Scheme 51) [195]. This is unusual pattern of nucleophilic attack on tetrafluoropyrimidine where formation of the highly hindered 323 (with displacement of inactivated fluorine in 5-position) occurs when the reaction mixture still contained some of the disubstituted compound 321, which offers an unhindered fluorine atom at the 2-position. It may be interpreted by formation of possible intermediates 324 and 325 and reversibility of the reaction.

An extension of the idea for generating other anionic nucleophiles by F⁻ addition to unsaturated precursors has been realised for nitrogen, sulphur and more recently for oxygen (Scheme 52). High reaction ability of tetrafluoropyridazine and tetrafluoropyrimidine towards nucleophiles made it useful in “trapping experiments” for anions 327, 329 and 331 [196–199].

Moreover investigation of nitrogen anion 327 leads to another fundamental result. It was found, that 327 generated by KF at room temperature gives kinetically controlled products 332 whereas 327 generated by CsF at higher temperatures produced thermodynamically controlled products 333, which was confirmed by experiments in pyridazine series (Scheme 53). Applying the experiment to tetrafluoropyrimidine leads to discovery of kinetic and thermodynamic control under nucleophilic substitution in pyrimidine (Scheme 54) [196].

4.3 Metalation Reaction

Moving from benzene via azines to the diazines, a decrease of aromaticity can be observed, this being attributed to the weaker overlap of the p orbitals in the rings. In consequence, the acidity of hydrogens is increased in the same order. A selection of calculated pKa values of nitrogen-containing heterocycles of interest is given in Fig. 18 [200]. Introducing into heterocycle of inductively electron-withdrawing fluorine atom increase the thermodynamic acidity of the heteroaromatic hydrogen atoms. These hydrogen atoms can be abstracted by strong bases leading to metalated compounds. The metal atom – usually lithium – is also stabilized by favorable electrostatic and electron-donating interactions with the lone electron pairs of fluorine. The observed ortho selectivity of the metalation of suitably substituted heteroaromatic compounds is, therefore, usually kinetically induced. Fluorine is highly effective as a strongly ortho-directing, acidity-enhancing substituent. Whereas many aryl lithium species are stable up to room temperature and above, ortho-fluoro lithio hetarenes are stable at low temperatures only [201, 202]. Based of this general consideration we can conclude that ring fluorinated diazines bearingortho-unsubstituted position are good objects for direct ortho-methalation (DoM) reaction.

4.3.1 Fluoropyrimidines

First lithiated fluorodiazines were described in early 1980 by Tanaka. These derivatives were prepared by deprotonation of fluorouracils with excess of LDA. The reaction proceeds though dilithiated species of type 339, which after reaction with an electrophile and subsequent acidic treatment gave substituted uracils 340 in high yields (Scheme 55). The approach was used in synthesis of antileukemic nucleosides [204] and anti HIV agents [204–206]. (Table 12)

Table 12 DoM reaction of fluorouracile derivatives

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Recently this transformation was applied for synthesis of orotidine-5′-monophosphate decarboxylase inhibitors [207]. Unexpected results was obtained during the methylation of lithium species by MeI. Earlier Tanaka has observed that lithiation of uridines at C-6 followed by methylation can be accompanied with undesired α-methylation of the newly attached substituent [208]. Bello et al. in 2009 [207] turned the fact to good account and smoothly ethylated substrate 341 via a two-stage methylation. In absence of fluorine, the second methylation is a bit more tricky and under similar conditions (2.5 equiv. LDA, followed by 3.3 equiv. MeI at −78 °C) a mixture of 6-methyluridine 344 (44 %) and 6-ethyluridine 345 (<5 %) was isolated (Scheme 56) [209]. Therefore, assistance of the neighboring fluorine (342) facilitates alkylation.

In 1990 Tanaka obtained lithiated 1,3-dialkyl-5-fluorouracils 347 [210], which were iodinated with ICl to afford 348. Later lithiated 1,3-dimethyl-5-fluorouracils 347 were reacted with hexafluoropropene 320 [211] to form intermediate carbanion 349 giving after fluoride elimination vinylated products 350 as a mixture of E and Z isomers (Scheme 57).

4-Fluoropyrimidines 351a,b were also metallated with LDA affording the functionalized compounds 353a,b in moderate to good yields (Scheme 58) [212]. When LTMP was used as a metalating agent under the same conditions a loss of regioselectivity in the ortho-position of the fluorine atom is observed. Besides compounds 352a,b 7–8 % of metalation at the C-6 position were observed.

When iodine was used as an electrophile the results were quite dependent on the experimental conditions (amounts of metalating agents and iodine) (Scheme 59, Table 13). Metalation of 351b with 1.1 eq. of LDA followed by reaction with iodine in excess led to the C(5) iodo derivative 354 (Table 13, Entry 1). The diiododerivative 356 was the major product when the metalating agent and iodine were in excess (entry 2). It can be assumed that the reaction of 351b with 1.1 eq. of LDA led to the C(5) lithioderivative which reacted with iodine affording compound 354. In a presence of an excess of metalating agent, compound 354 underwent a further lithiation at the C(6) position and after the reaction with another equivalent of iodine afforded the diiodo derivative 356. The unexpected C(6) iodo derivative 355 was observed when LDA was in excess (2.3 eq.) and when iodine was in stochiometric amounts (entry 3). Such as unusual regioselectivity can be explained by formation of diiodo derivative 356, which underwent halogen-lithium exchange affording C(5) lithium derivative 358, which gave after hydrolysis the C(6) monoiodo derivative 355. The formation of the C(5) lithio derivative 358 from the reaction of 356 with LDA was proved by trapping of the lithio derivative with acetic aldehyde affording 359 in 84 % yield.

Table 13 Lithiation/iodination of 4-fluoropyrimidine 351

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Boc-protected aminofluoropyrimidine 360 was subjected to lithiation with LTMP followed by acetone addition to form 361 in 89 % yield. It was used in synthesis of as gaba-A alpha 2/3 ligands for depression [213]. The ethyl ester 363 was synthesized by similar way by lithiation of pyrimidine 362 [182] (Scheme 60).

4.3.2 Fluoropyrazines

Metalation of fluoropyrazine 364 has been achieved in 1998 by Queguiner with LTMP as metalating agent at −75 °C with a short reaction time (5 min) leading to a wide range of 2,3-disubstituted fluoropyrazines 366. Pyrazine derivatives are well known to be good electrophiles, when they are substituted by a good leaving group such as fluorine, a further nucleophilic substitution is observed by the released species coming from the electrophile used during the trapping step. For example, during functionalization of fluoropyrazine, formation of 2,3-diphenylthiopyrazine 367 (89 % yield) and the 2-acetyl and 2-benzoyl-3-dimethylamino pyrazines 368 have been observed besides the expected disubstituted fluoropyrazines when diphenylsulfide or N,N-dimethylacetamide or N,N-dimethylbenzamide have been used as electrophiles (Scheme 61) [214].

Starting from fluoropyrazine, a regioselective synthesis of iodo- and tributylstannyl substituted fluoropyrazines has been elaborated. Lithiation of fluoropyrazine with stoichiometric amounts of LTMP and iodine afforded the 2-fluoro-3-iodopyrazine 366 (E=I) as sole product otherwise a mixture of mono-, di-, and triiodo derivatives were formed (Scheme 62, Table 14) [147]. In a similar manner, use of tributyltin chloride as electrophile led to mono and di-stannylpyrazines [215]. Formation of compounds 369, 370 and 371 is a result of metalation at the position adjacent to the nitrogen atom without assistance of the fluorine atom as DMG. Such a metalation without a DMG has been previously reported during direct metalation of bare pyrazine by use of an excess of LTMP (4 equiv.) with very short reaction time (5 min) at low temperature −78 °C [216].

Table 14 Lithiation of 2-fluoropyrazine

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Lithiation of 2-fluoro-1-hydroxymethylpyrazines 372 with LTMP (3 equiv.) in THF at −78 °C occurred at the C(6) position (Scheme 63). With iodine as the electrophile, the 6-iodo derivative has been obtained. This iodide was used in Negishi and Suzuki cross couplings to access a natural products analogues and liquid crystals [147, 217]. When alkylamides such as LTMP are used as metalating agent, the deprotonation is considered as thermodynamically controlled. The heats of formation of the lithio compound could be examined as a simple approach to estimate the regioselectivity. Considering that the hydrogen of the hydroxyl group is first abstracted by LTMP, a lithium atom could form a chelate between the oxygen of the alcoholate and the neighboring nitrogen. The lithium at the C(6) position may coordinate with the adjacent free nitrogen N(1) whereas such coordination cannot be observed when the lithium is at the C(5) position since the nitrogen N(4) is already chelated. This assumption is in agreement with the calculation of heats of formation of two lithio derivatives (Hal=Cl) by Li/PM3. C(6)-lithioderivative 374 is more stable than 375 with a difference of Δ(ΔH_f) = 6.8 kcal/mol [217].

A regioselective functionalization at the C(6) position has also been achieved when the 2-fluoro-3-phenylpyrazine 376 reacted with an excess of LTMP (3 equiv.) followed by reaction with various electrophiles leading to compounds 377 (Scheme 64) [147]. To explain this regioselectivity which occurs exclusively at the C(6) position, theoretical calculations using Li/PM3 semiempirical method have been performed [147]. A complexation between the lithium of LTMP and the two nitrogen atoms of the pyrazine moiety, which behaves as a complexing agent, has been taken into account (Scheme 64). The values calculated indicated that the C(6)-lithio derivative 378 is clearly more stable than the 5 one 379. This result could explain the complete regioselectivity at C(6) position.

However, when compound 376 was reacted with 3 eq. of LTMP at −78 °C for 5 min followed by reaction with 1 eq. of iodine, a C(5) monoiodo derivative 381 was formed in 64 % yield beside traces of diiododerivative 382. In this case, as in a case of 5-fluoropyrimidine (Scheme 59), the lithiation of 376 followed by reaction with iodine as an electrophile was investigated under various experimental conditions (Scheme 65, Table 15) [147]. In the reaction with 1.1 eq. of LTMP and excess of iodine the C(6) derivative was obtained in good yield (entry 1). An excess of LTPM and of iodine leads to diiodo derivative 382 (entries 3,5). When the metallating agent was in excess in relation to iodine the C(5) iodo derivative 381 was obtained as a sole product (entries 2,4 and 6). These results are in agreement with results obtained in pyrimidine series (see Scheme 59) and could be assumed by the similar way. The reaction of 376 with 1–4 eq. of LTMP led first to C(6)-lithio derivative which reacted with iodine to give C(6) iodo derivative. This compound underwent a further isomerisation involving iodine atom migration leading to C(5) iodo derivative 381. Such isomerisation resulting from a halogen migration is known as a “halogen-dance” reaction.

Table 15 Lithiation / iodination of 2-fluoro-3-phenylpyrazine

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Besides iodine migration in fluoropyrazine serie the migration of the tributylstannyl group is known. When metalation of 2-fluro-3-tributylstannylpyrazine 383 was performed with 2.1 equiv. of LTMP with a short reaction time and was followed by protonation of the lithio derivative, compound 386, resulting from intramolecular tin/lithium exchange, was isolated in 63 % yield (Scheme 66) [215].

To establish a comparison between the ortho directing power of fluoro, chloro, and methoxy groups, the lithiation of 2-halo-6-methoxypyrazines 387a,b has been investigated using various alkylamides as metalating agent at −78 °C with a short reaction time (5 min) (Scheme 67, Table 16) [218]. The main isomer has the substituent at the ortho position relative to the fluorine atom as in 387a, contrary to what is observed with the chlorine atom as in 387b. When LDA is used as metalating agent, the metalation is more regioselective than with more bulky bases such as LTMP or LB (lithium tertbutyl-(1-isopropylpentyl)amide). These results allowed to estimate the relative ortho directing power as F > OMe > Cl.

Table 16 Lithiation of 2-halo-6-methoxypyrazines

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4.3.3 Fluoropyridazines

To the best of our knowledge only one paper deals with metalation of ring fluorinated pyridazines [218]. In order to compare directly the ortho directing power of two halogens Quenguiner et al. in 2003 tried to synthesise 3-chloro-6-fluoro-pyridazine by nucleophilic fluorination of 3,6-dichloropyridazine. However the reaction of 3,6-dichloropyridazine 390 with TEA*3 HF in a presence of proton sponge gave a mixture of products. The monofluoro compound 391 was the main product beside 28 % of starting material and a small amount (5 %) of difluoro derivative 392 (Scheme 68). Subsequent metalation of this mixture was performed with three electrophiles (Table 17). The proportions of the functionalized products 393–393 were constant with the electrophiles, the metalating agent and the time and reflected the proportion of the starting material; this showed a similar behavior of the three compounds (393–395) with regard to the metalation reaction. The most important result was that the metalation of 391 was regioselective in ortho position relative to the fluorine atom, leading to the conclusion that the fluorine atom was a much better ortho-directing group than the chlorine.

Table 17 Lithiation of 2-fluoropyrazine

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4.4 Organometallic Compounds and Transition Metals Catalyzed Process

The most obvious feature of the chemistry of highly fluorinated aromatic compounds which can be exploited is their susceptibility to nucleophilic attack. Therefore, reactions with anionic species containing metals can be useful and the most significant examples of this type involve transition-metal carbonyl anions. Francis Gordon Albert Stone in 1968 described the reaction of carbonyl metal anions derived from Fe, Mn, Re and Mo with tetrafluoropyridazine leading to organometallic complexes wherein 4-trifluropyridazine group is σ-ligand to transition metals (Scheme 69) [219]. Compound 396 with [M] = π-C₅H₅Mo(CO)₃ was one of the first molibdenium complex obtained by nucleophilic displacement with [π-C₅H₅Mo(CO)₃]⁻, and their isolation further illustrates the reactivity of perfluoropyridazine, since the anion is a relatively weak nucleophile tolerated towards hexafluorobenzene and pentafluoropyridine.

But due to the extraordinary strength of the carbon–fluorine bond, transition metal-mediated activation of fluoroalkanes and arenes is not easy to achieve. Nevertheless, Braun disclosed in 1999 activation of the C–F bond in highly electron-deficient compounds such as 2,4,6-trifluoropyrimidine 397a (R=H), with stoichiometric amounts of bis(triethylphosphano) nickel(0) giving activated complex 398 (Scheme 70) [220]. Later it was found that the transition from 2,4,6-trifluoropyrimidine to 5-chloro-2,4,6-trifluoropyrimidine 397b (R=Cl) in similar conditions (Ni(COD)₂, PEt₃) leads to C-Cl activation. But using PCy₃ instead of PEt₃ exclusive activation of the C–F bond takes place affording trans-[NiF(4-C₄N₂ClF₂)(PCy₃)₂] 400. The treatment of the complex 400 by HCl led to 5-chloro-2,4-difluoropyrimidine 402 meanwhile reaction with iodine gives 5-chloro-2,6-difluoro-4-iodopyrimidine 401 (Scheme 70) [221].

In 2005 the same scientist showed that treatment of Ni(COD)₂ with 5-chloro-2,4,6-trifluoropyrimidine 397b in presence of Pi-Pr₃ or PPh₃ effects the formation of the fluoro complexes trans-[NiF(4-C₄N₂ClF₂)(PR₃)₂] 404. In contrast, a reaction of 397b with Pd(PPh₃)₄ leads to the insertion of a {Pd(PPh₃)₂} unit into the C-Cl bond yielding trans-[PdCl(5-C₄N₂F₃)(PPh3)₂] 403. Treatment of 404 with an excess of TolB(OH)₂ results in the slow formation of trans-[NiF(4-C₄N₂TolClF)(PPh₃)₂] 405 and subsequently 5-chloro-2-fluoro-4,6-ditolylpyrimidine 406 (Scheme 71) [222]. In catalytic experiments 397b is converted with the set of boronic acids into 407 when 10 % of 404b is employed as catalyst (Scheme 72) [222].

Meanwhile Queguiner with co-workers in 2002 disclosed Kharasch cross-coupling reactions of phenylmagnesium halides with fluorodiazines. The nickel-catalyzed cross-coupling reactions between aryl Grignard reagents and fluorodiazines 364, 408 and 409 occurred in THF at rt using commercially available 1,2-bis(diphenylphosphino)ethane, 1,3-bis(diphenylphosphino)propane, or 1,1′-bis(diphenylphosphino)ferrocene as ligands (Scheme 73) [223].

Earlier in 1983 Norwegian chemists tried to applied the similar cross-coupling conditions to 2,4-dichloro-5-fluoropyrimidine 158. In a case of EtMgBr the diethylated product 415 formed in 53 % yield with small amount of C(6) addition product 416 [224]. But when PhMgBr was used the dihydropyrimidine 417 formed as a sole product (Scheme 74) [225].

Recently such type of transformation was used for the scalable process to the GSK3β Inhibitor AZD8926 422 (Scheme 75). The process include a lithiation of 1-(pyran-4-yl)-2-trifluoromethyl-imidazole 418, a Ziegler-type coupling of lithiated 419 with commercially available 2-chloro-5-fluoropyrimidine 362 via 1,2-addition over the 3,4-C–N bond and a copper-catalyzed dehydrogenative aromatization using oxygen as the stoichiometric oxidant giving imidazopyrimidine 421 in 60 % total yield [226].

Although aryl fluorides are very unreactive toward oxidative addition of palladium, whether the electron-deficient pyrimidine ring coupled with the strong electron-withdrawing effect of fluorine would allow 2,4,6-trifluoropyrimidine to function as a suitable partner in a Suzuki coupling process. Unfortunately, when 2,4,6-trifluoropyrimidine was treated in a manner similar to the other halogenated pyrimidines no arylation was observed. The major reaction appeared to be hydrolysis of one or more of the fluorine substituents [227]. But this tolerance of fluorine atom towards Pd-catalyzed coupling reactions can be used in organic synthesis. In can be illustrated by reactivity comparison of 2-fluoro-5-bromopyrimidine and 2,5-dibromopyrimidine (Fig. 19).

In a case of 2,5-dibromopyrimidine Suzuki coupling with boronic acids proceeds selectively at 2-position leaving 5-bromine atom intact [228, 229] meanwhile Sonogashira coupling with terminal alkynes proceeds unselectively by both position [230]. But in a case of 2-fluoro-5-bromopyrimidine the selectivity pattern of Pd-catalyzed couplings is switched off. Thus 2-fluoro-5-bromopyrimidine 425 entered in Suzuki coupling with boronic species 424 exclusively in 5-th brominated position leaving fluorine intact giving compound 426 – BACE inhibitor, potential drug for the prevention of treatment of neurodegeneration [231] (Scheme 76). The selectivity pattern is general for fluoro-bromo diazines. In course of diacylglycerol acyltransferase 1 (DGAT1) inhibitors 431 design and synthesis this assumption was proved. All fluoro-bromo diazine derivatives 427 undergo Suzuki coupling only with bromine participation. Moreover, after the transformations the fluorine atom in compounds 430 remains active towards nucleophilic substitution [232] (Scheme 77).

Another example of such differentiation of activity using was described during phosphoinositide 3-kinase (PI3K) inhibitors 435 development. In this case the core compound for the library construction was 7-bromo-2-fluoroquinoxaline 432. First the quinoxaline 432 was subjected to coupling reaction with boronic acid pinacol ester 433 in MW condition giving corresponding key building block 434 in 64 % yield [233]. Than the 434 was used in parallel synthesis based on fluorine substitution in DMSO solution with a list of 14 aliphatic amines with subsequent mass-directed purification [234] (Scheme 78).

The similar situation was observed with fluoro-iodo diazines. Fluoro-iodo pyrimidine 354 (see Scheme 59) was subjected to Stille coupling followed by intramolecular cyclization into targeted azacarboline 438 [212] (Scheme 79).

In 2003 the same group of scientists shown wide applicability of Pd-catalyzed coupling – metalation sequences in various pyrazine synthesis. Starting from 2-fluoro-6-tributhylstannanyl-pyrazine 386 (Scheme 66) as building block a general synthetic route to access to various alkylaryl or diaryl pyrazines with multiple fluorosubstituents in strategic lateral position to generate a wide range of molecules was elaborated. The Stille, Suzuki, Sonogashira and Negishi couplings gave yields in range 50–80 % [215] (Scheme 80).

Later the different iodo/fluoro activity was used in nicotinic acetylcholine receptor (nAChR) PET ligand 447. On the early step of the synthesis starting from 2-fluoro-5-iodopyrimidine 444 the corresponding trialkyltin heteroaromatic intermediate 445 was obtained based on “stanno-Stille” coupling in 90 % yield. Further the fluorinated stannate was entered in reaction with iodopyridine 446 leading to cross-coupled pyridine-pyrimidine in 50 % yield. The final step of the synthesis was Boc-deprotection by TFA affording 447 [235] (Scheme 81).

In 1995 selective Sonogashira reaction with 3-fluoro-6-iodopyridazine 449 was described leading to selective substitution of iodine by propargyl alcohol in 64 % yield [236] (Scheme 82).

A lot of cross couplings were described for 2,4-dichloro-5-fluoropyrimidine 158. In a case of this substrate the exclusive substitution of chlorine in 4-position occurs leaving chlorine in 2-position intact. Iron-catalyzed coupling with Grignard reagent was used for introduction of alkyl group instead chlorine atom in 2,4-dichloro-5-fluoropyrimidine leading to 2-chloro-5-fluoro-4-methylpyrimidine in 48 % yield [237]. Alternative approaches to 2-chloro-5-fluoro-4-alkylpyrimidines include Negishi coupling with organozinc derivatives and Suzuki coupling with alkyl boronic acids [238, 239] (Scheme 83).

Different variants of Suzuki reaction were used for introduction of aryl and hetaryl group into 5-fluoropyrimidine core. In majority cases the yield of reaction remains high and some functionality such as protected amino group or free alcohol function could be introduced together with benzene ring [240–243]. Also the reaction allows introduction of different heterocyclic moieties [244, 245] via Stille reaction [246] and Negishi coupling [247] using 158 (Scheme 84).

More unusual transformation based on 2,4-dichloro-5-fluoropyrimidine were described during development of inhibitors of the insulin-like growth factor-1 receptor tyrosine kinase [248]. In a course of the project selective β-arylation of vinyl ethers by 4-chloropyrimidines could be achieved using a phosphine-free Heck reaction in polyethyleneglycol. The reaction of 158 with butyl vinyl ether (3 equiv.) in the presence of Et₃N (1 equiv.) and Pd(OAc)₂ (7 mol %) using PEG-400 as a solvent provided vinyl ether 458 in 51 % yield. Bromination of the vinyl ether 458 with NBS in dioxane–water followed by a cyclocondensation with 2-aminopyridine provided the imidazopyridine 460 in 7 % yield, presumably via the α-bromo hemiacetal 459. With the aim of the replacement of the imidazopyridine ring by a pyrazolo[1,5-a]pyridine the 1,3-dipolar cycloaddition between vinyl ethers 458 and the azomethine imine formed by deprotonation of a 1-amino-pyridinium ion, followed by an oxidative aromatization was carried out affording 461 in 49 % yield [249]. Alternative stereoselective approach to β-arylatated vinyl ethers 462 was described by Banyu Pharmaceutical starting from cis-l-ethoxy-2-tri-n-butylstanylethylene [250] (Scheme 85).

Another useful Stille coupling was performed based on tributyl(1-ethoxyvinyl)stannane 463 which allowed to obtain additional function in pyrimidine ring. The reaction of stannane 463 with 158 in presence of bis(triphenylphosphine) PdCl₂ in DMF leads to corresponding 4-vinyl ether 464 in 84 % yield. The amino group was introduced at position 2 upon treatment with aqueous concentrated ammonia in ethanol under heating with microwaves, and bromination of the resulting vinyl ether 465 to α-bromo-ketone 466 was accomplished with N-bromosuccinimide in aqueous tetrahydrofuran [251]. Also the 2-ethoxyvinyl moiety of 464 was subsequently converted into ethyl ester 466 by oxidative cleavage using NaIO₄-KMnO₄ system [182] (Scheme 86).

4.5 Miscellaneous Cyclizations and Rearrangements

In the literature there are a lot of cyclization reaction were fluorine in diazine core does not play significant role. As an example the cyclizations of 6-fluoropyridazin-3-ylamine 468 were shown [252–254] (Scheme 87).

But in the section we would like to draw the reader’s attention to the more rare reaction, which results are not always clear from general considerations. For example the reaction of 2,5,6-trifluoropyrimidin-4-ol 471 with DMSO in presence of TFAA proceeds with 2,3-sigmatropic rearrangement leading to pyrimidine 472, which was hydrolyzed to 5-fluorouracil derivative 473 [255] The same scientists described the Claisen rearrangement of 4-allyloxy fluoropyrimidines 474 in which N(3) is the migration terminus [181] (Scheme 88). 5-Fluoro-4,6-dimethoxypyrimidine in reaction with activated DMSO also gave 2,3-sigmatropic rearrangement leading to 5-fluoro-4,6-dimethoxymethylthiomethylpyrimidin-2(1H)-one in 59 % yield.

Recently pyrimidine substituted alkynes 476 were subjected to intramolecular inverse-electron-demand hetero-Diels-Alder reaction with extrusion of HCN affording fused fluorinated pyridines 478. The reaction proceeds at high temperatures in sealed tubes for small amount of the starting materials [256] or as scalable flow process [257] (Scheme 89)

Unusual tert-amino reaction was found under investigation of nucleophilic substitution in 3,5-bisheptafluoroisopropyl-4,6-difluoropyridazine 479 with dimethylamine. The reaction gives bis(dimethy1amino) derivative 480 which then undergoes spontaneous cyclisation by a displacement of fluorine from a perfluoroisopropyl group. This process is accelerated by water in DMF to give the colourless compound 481 in 90 % yield. The cyclization proceeds trough the loss of ‘tertiary’ fluorine and formation of purple intermediate compound 482, which was proved by isolation of a purple solid 484 (87 % yield) by adding boron trifluoride-dietliyl ether to 480 [258] (Scheme 90).

Historical developments of the structure of benzene is important part of history of chemistry. Structures considered were Dewar benzene 486, Ladenberg’s prismane 487 and benzvalene 488 (Fig. 20). Photochemistry of fluorinated aromatic systems has made an important contribution to the study of valence isomers because it has been possible to isolate and characterize some species on which there had previously only been speculation.

The fluorinated diazines plays important role as model object for the investigation. Tetrafluoropyridazine has been converted photochemitally into tetrafluoropyrazine via formal 1,3-shift of nitrogen [259] (Scheme 91).

The use of polyfluoroalkyl substituents in positions 4 and 5 (compound 489), however, enabled a mechanistic pathway to pyrazine 490 substituted at positions 2 and 5, to be suggested (Scheme 92) Individual para-bonded species 491 and 492 have been isolated in this and other cases, and converted into the next component along the reaction pathway by photo or thermal reactions [260]. In a case of 4,6-disubstituted pyridazine 493 only pyrazine substituted at positions 2 and 6 494 was observed. A very unusual mechanistic pathway may be drawn from the structures of the isolated and characterised valence isomers (Scheme 92). This appears to be the first case where substituent labelling has allowed each stage in a photochemical aromatic rearrangement to be identified through various intermediate valence isomers.

Also during the investigation of photolysis of 4,6-disubstituted pyridazine 493 have been established that by-products are formed in the reaction through the intermediacy of an azacyclobutadiene derivative 496 [261] (Scheme 93). The azacyclobutadiene 496 was not isolated but the products of its dimerization 498–500 were isolated and characterized. From all characterized dimers the 1,5-diazocine derivative 500 appears most thermodynamically stable (Scheme 94).

A benzodiazabenzvalene derivative 502 has been proposed to account for the photochemical rearrangement of perfluorocinnoline 501 to the quinazoline 503 [59] (Scheme 95).

A remarkable series of transformations has been discovered with fluorinated pyridazines, giving pyrimidines and small amounts of pyrazines on pyrolysis. The pyrolysis of tetrafluoropyridazine 139 gave the isomer tetrafluoropyrimidine 6 as the major product (30 %) (Scheme 95) [262], though at 815 °C tetrafluoropyrazine was among the products.

Once again, the use of 4,5-di( polyfluoroalkyl) derivatives 489 and the orientation of the substituents in the products enabled the reaction pathway to be rationalised on the basis of the formation and rearrangement of three intermediate diazabenzvalenes 504, 506 and 507. For Rf=C₂F₅, the major component 505 was accompanied by a small amount of the 2,6-substituted pyrazine 494, whereas for Rf=(CF₃)₂CF, is formed with a smaller amount of the 2,5-substituted pyrazine 490 [263] (Scheme 96). Despite of no valence isomers have actually been isolated. Cycloaddition processes have been ruled out by N-15 labelling experiments. Furthermore, rearrangement is encouraged by free-radical promotors, leading to the conclusion that these processes involve free-radicals formation [264].

After investigation of Chambers group made in late 70-th in 1984 Clark and co-worker investigated plasma polymerization of the isomeric perfluorodiazines. The studies showed that plasma polymers are produced by rearrangement mechanisms. A comparison of rates of formation of plasma polymer films reveals distinctive differences between the isomeric diazines that suggest that equilibration of valence isomers occurs on a substantially slower time scale than for isomeric fluorinated benzenes [265]. Also extensive MNDO SCF MO calculations were made to determine the heats of formation of the ground state of geometry-optimized perfluorodiazabenzenes (pyridazine, pyrimidine, and pyrazine) and some of their structural isomers (Dewar benzene, benzvalene, prismane, fulvene and hexadienyne) [266]. From these calculations it is readily apparent that perfluoropyridazine could eliminate nitrogen without further rearrangement with a heat of reaction of 27 kcal/mol. Despite of known pyrolyses of perfluoroalkylpyridazines leads to rearrangements to pyrimidines and pyrazines, rather than loss of nitrogen, the pyridazines bearing perfluoroaryl substituents loss the nitrogen under termolysis. Thermal elimination presents a route to fluorinated alkyne derivatives [267] (Scheme 97).

4.6 Fluorination Reactions

Chambers and coworkers studied direct fluorination of perfluoropyrimidine by elemental fluoride. The formation of the dimeric compound 511 in the reaction indicated the radical nature of the process [267]. Addition of highly electrophilic fluorine atom at a position meta to nitrogen in followed by dimerisation of 510 is easily understandable. Meanwhile the fluorination of perfluoropyrimidine by cobalt (III) fluoride with calcium fluoride leads to another product. In this case the reaction proceeds trough cation radical 512, which after fluoride anione additional formed radical 513. Dimerization of the radical 513 followed by fluorination leads to bispyrimidine 514 in 25 % isolated yield [268] (Scheme 98). Whereas perfluoropyrazine under CoF₃-CaF₂ fluorination gives 1,3-diene 515 in 58 % yield with ~50 % conversion of starting materials (Scheme 99). Bulky perfluoroisopropyl group in 4- and 6-positions of pyrimidine prevent dimerisation under CoF₃-CaF₂ fluorination and reaction results in formation of 1,4-dienes 519 in high yield. Similar situation is observed with fluorination of perfluoro-2,5-diisopropylpyrazine 490 (Scheme 100). But the pyridazine derivatives 489 and 493 each lost nitrogen on fluorination. However, this provides a novel synthetic approach to some unusual fluorinated alkenes (Scheme 101).

5 Side Chain Fluorinated Diazines

Chain-fluorinated diazines is another family of organofluorine compounds which is of great importance for synthetic, medicinal and agricultural chemistry [12, 13]. The first representatives of this class, namely, chain-fluorinated pyrimidines and quinoxalines were obtained in late 1950s. Since then, over a thousand papers dealing with synthesis and chemistry of chain-fluorinated diazines were published. Due to a huge number of the data, this chapter gives a general review of synthetic methods and chemical properties of chain-fluorinated diazines. Only selected (sometimes deliberately) literature examples are given to illustrate them.

Known approaches to the synthesis of chain-fluorinated diazines are subdivided into two categories: the methods, which rely on introduction of fluorine or fluorine-containing substituents into diazine core (i.e. direct (per)fluoroalkylation and nucleophilic substitution with fluoride), and construction of the diazine core starting from fluorine containing building blocks. Chemical properties of the chain-fluorinated diazines are discussed in a separate section.

6 Introduction of Fluorine-Containing Substituents into Diazine Core

6.1 Direct (Per)Fluoroalkylation

6.1.1 Perfluoroalkylcopper Reagents

Perfluoroalkylation with perfluoroalkyl copper species is one of the most known “direct” method for introducing CF₃ group and other fluorinated alkyl substituents into aromatic cores. One of the first reports in this area was made in 1977 [269] and later in 1980 – as a full paper [270]. In these works, Kobayashi and co-workers used CF₃I–Cu – HMPA system to generate CF₃Cu solution, which reacted with 5-bromo- 526 and 5-iodouracil derivatives 528 to give 5-trifluoromethylpyrimidines 527 and 529 (Scheme 102). The method was used for the synthesis of Trifluridine and its analogues (see Chap. 20) [269–271].

A related procedure employed less expensive but less reactive CF₃Br instead of CF₃I as the source of the trifluoromethyl group [272]. In this case, CF₃Cu was generated by heating CF₃Br and copper powder in DMF – pyridine at 115 °C in a sealed tube. It was found that addition of 4-dimethylaminopyridine (DMAP) substantially accelerated formation of the complex. The preformed CF₃Cu reacted with pyrimidine 530 at to give 531. This procedure was used for the preparation of FTC-092, an investigational anti-cancer drug (Scheme 103, see Chap. 20).

The method was extended to other perfluoroalkyl iodides, i.e. n-C₈F₁₇I (532) or (CF₃)₂CFO(CF₂)₄I (533) (Table 18) [273]. In this case, the reaction was performed in C₆F₆ as the solvent at ca. 85 °C; DMSO and 2,2′-bipyridyl were used as additives to accelerate formation of perfluoroalkyl copper reagents. The method was effective only for the primary perfluoroalkyl iodides.

Table 18 Perfluoroalkylation of diazines with iodides 3 and 4. Conditions: 532 or 533, Cu, 2,2′-bipy, DMSO, C₆F₆, ca. 85 °C

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Analogous reaction of ethyl dibromofluoroacetate with 2-bromopyrimidine 534 gave the corresponding product 535 in low yield (12 %) (Scheme 104) [274].

Burton’s conditions (i.e. Cu–CF₂Br₂ – DMA [275]) were also checked for trifluoromethylation of diazines (namely, 2-chloropyrimidine). Despite high conversion of the substrate, the method gave nearly equimolar mixture of 2-perfluoroalkylpyrimidines 536 and 537 was obtained (Scheme 105) [276, 277]. The mechanism of the reaction included reduction of CF₂Br₂ with copper to give difluorocarbene, which reacted with dimethylacetamide to give the adduct 538. The latter acted as a source of fluoride ion and reacted with difluorocarbene to give CF₃Cu species. In case of low activated substrates (i.e. 2-chloropyrimidine), CF₃Cu slowly reacted with difluorocarbene to give C₂F₅Cu, which also took part in the transformation.

Recently, several novel methods for the generation of perfluoroalkyl copper species were used for direct perfluoroalkylation of diazines, e.g. transmetallation reactions involving (CF₃)₂Hg [278] and CF₃ZnBr · 2DMF [279]. In both cases, the procedure did not require special laboratory equipment such as autoclaves or steel tubes, which were necessary in the case of using CF₃Br or CF₃I as the source of trifluoromethyl group. The first method ((CF₃)₂Hg–Cu – dimethylacetamide, 110–140 °C) was successfully applied for trifluoromethylation of 5′-iodouridine derivatives; the corresponding products were obtained in more than 90 % yields. In the second procedure, C₂F₅Cu species were generated selectively (from CF₃ZnBr · 2DMF and CuBr), which reacted with 2-iodopyrimidine to give the product 537 in 69 % yield (Scheme 106).

An alternative source for the generation of trifluoromethyl copper species, which gained momentum in the last years, is Ruppert – Prakash reagent (CF₃SiMe₃). Hartwig and co-workers used this reagent to obtain stable complex CF₃Cu · phen (539) [280]. The latter was obtained in 96 % yield on a gram scale by reaction of copper (I) tert-butoxide with 1,10-phenantroline (phen) and then – with CF₃SiMe₃ in benzene at rt (Scheme 107). The complex 539 is called Trifluoromethylator™_, which is an easily handled, thermally stable, single-component reagent for the trifluoromethylation of aryl iodides and now is available from Aldrich. Reaction of 539 with uracil derivative 540 gave the product 541 in 78 % yield.

One more method relied on reaction of CF₃SiMe₃ with AgF in DMF, which led to CF₃Ag species [281]. The latter was treated with copper to obtain the trifluoromethyl copper reagent, which was rather effective for the trifluoromethylation of 2-bromopyrimidine (75 % yield) (Scheme 108). In case of 5-bromopyrimidine, a mixture of perfluoroalkylation products 542–544 was formed at 56:41:3 ratio, respectively.

In a recent paper, S-(trifluoromethyl)diphenylsulfonium triflate (545) in the presence of copper was proposed as an efficient reagent for trifluoromethylation of heteroaromatic compounds [282]. In particular, 3-chloro-6-iodopyridazine smoothly reacted with this reagent to give the product of the iodine selective substitution (546) in 98 % yield (Scheme 109). The proposed mechanism for the formation of active species included reduction of 545 leading to trifluoromethyl radicals, which in turn reacted with copper to give CF₃Cu.

It was found that FSO₂CF₂CO₂Me readily eliminates CO₂ and SO₂ in the presence of CuI in DMF at 60–80 °C to produce CuCF₃ species that can be used for aromatic trifluoromethylation [283]. The method was used for trifluoromethylation of pyrimidines 547 [284] and 548 [285] (Scheme 110).

An analogous idea was implied in an older method which used CF₃COONa–CuI – N-methylpyrrolidone system for trifluoromethylation of 2-bromopyrimidine (Scheme 111) [286]. The corresponding product 546 was obtained in 34 % yield. The authors proposed [CF₃CuI]^– species as the active trifluoromethylation agent.

The most recent methodology for CF₃Cu generation based on CF₃H was elaborated by Grushin [287]. The method is based on a novel ate complex reagent, [K(DMF)][(t-BuO)₂Cu], that is formed quantitatively upon treatment of CuCl with 2 equiv. of t-BuOK. This dialkoxycuprate, generated in situ or preisolated, reacts with CHF₃ at room temperature and atmospheric pressure within minutes to give rise to CuCF₃ in >90 % yield. Stabilization of thus produced trifluoromethyl copper(I) with a source of HF such as Et₃N*3HF furnishes the reagent that is stable at room temperature for days (Scheme 112). Prior to the stabilization, fluoroform-derived CuCF₃ reacted with haloarenes to give the corresponding arenetrifluorides. The conditions were also checked for trifluoromethylation of diazines [288]. Due to the cupration occurs within seconds at room temperature and is not mediated by CF₃ ⁻ or CF₂, which accounts for its remarkably high selectivity, the reaction leads only to trifluoromethyl derivatives without C₂F₅ derivatives side-formation, unlike to above-mentioned procedures (Table 19).

Table 19 Trifluoromethylation of diazine bromides with fluoroform-derived CuCF₃

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6.1.2 Perfluorocarboxylic Acids or Their Derivatives

Perfluorocarboxylic acids, in particular CF₃COOH, are probably the most accessible sources of perfluoroalkyl fragments. It is not surprisingly, therefore, that some efforts were put to develop the methods for direct perfluoroalkylation of diazines using perfluorocarboxylic acids or their derivatives. One of such methods was already mentioned in the previous section, namely, decarboxylative trifluoromethylation using CF₃COONa–CuI system reported in 1988 [286]. An alternative procedure used XeF₂ to generate active species from perfluorocarboxylic acids [289]. In particular, intermediate xenon (II) perfluocarboxylate 551 decomposed to give perfluoroacyl radical. The latter eliminated CO₂ to form the corresponding perfluoroalkyl radicals (i.e. CF₃ or C₂F₅). These active species reacted with aromatics (in particular, pyrimidine derivative 552) (Scheme 113) at rt. The procedure was used for the synthesis of antiviral drug Trifluridine (see Chap. 20). The reagent was also applied for trifluoromethylation of 2-mercaptopyrimidines; in this case, mixtures of products (554–558) was obtained (Scheme 114) [290].

Electrochemical generation of trifluoroacetyl (and hence trifluoromethyl) radicals in the presence of uracil derivatives was studied [291]. Electrolysis of CF₃COOK/CF₃COOH solutions of N-1- and N-3-methylated uracils provided mixtures of 5-trifluoromethyl derivatives 559 and N–C uracil dimers 560 (Scheme 115). In case of 1,3-dimethyluracil, N-1 demethylathion was also observed.

The reaction of bis(perfluoroacyl)peroxides and various O–protected uracils is a valuable method for the introduction of perfluoroalkyl group at C-5 atom of uracil [292]. The corresponding products were obtained in 20–56 % yields (Scheme 116). Substitution at C-6 of uracil did not interfere with reaction. The method could be extended for unprotected uracils and uridine derivatives (26–42 %).

6.1.3 (Per)Fluoroalkylsulfinates

Although perfluorocarboxylic acids are readily available sources for of perfluoroalkyl groups, their use for direct perfluoroalkylation often requires higher temperatures, transition-metal additives, or strongly oxidizing conditions. (Per)fluoroalkylsulfinic acids are alternative reagents, which can also deliver (per)fluoroalkyl radicals. In particular, a method for trifluoromethylation of heterocycles (including diazines) reported by Baran and co-workers in 2011 used CF₃SO₂Na–tBuOOH system (Table 20) [293]. A putative mechanism of the transformation included reaction of tert-butoxy radical, generated from tBuOOH and trace metal or another initiator, with CF₃SO₂ ^– to produce CF₃SO₂ · radical (Scheme 117). This intermediate decomposed to release SO₂ and CF₃·. The trifluoromethyl radical was then trapped with heterocyclic substrate; the intermediate formed was oxidized to the final product with tBuOOH, concomitantly generating another molecule of tBuO·.

Table 20 Trifluoromethylation of diazines with CF₃SO₂Na–tBuOOH. Conditions: CF₃SO₂Na (3 eq.), tBuOOH (5 eq.), CH₂Cl₂–H₂O, rt

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Later, it was found that zinc sulfinates are superior reagents for the (per)fluoroalkylation of heterocycles in terms of both stability and reactivity [294]. A toolkit of zinc sulfinates (Baran reagents), most of which are now commercially available from Sigma-Aldrich, was developed [295], including (CF₃SO₂)₂Zn (TFMS), (CHF₂SO₂)₂Zn (DFMS), and (CH₂FSO₂)₂Zn (Table 21). Although in many cases the yields in the transformations using zinc (per)fluoroalkylsulfinate – tert-butyl hydroperoxide were moderate, these reactions can be conducted open to the air on unprotected.

Table 21 (Per)fluoroalkylation of diazines with (R_fSO₂)₂Zn–tBuOOH. Conditions: (R_fSO₂)₂Zn, tBuOOH, organic solvent – H₂O, rt – 50 °C

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Site-selectivity of (per)fluoroalkylation depends on combined electronic properties of the reacting π-system and incoming radical species. This point is illustrated through a comparison between CF₃ and CF₂H radical additions to Varenicline (marketed in the U.S. as the prescription medication Chantix by Pfizer). In this case, high levels of selectivity are observed for CF₃ and CF₂H radical addition, in spite of the multiple potentially reactive sites (Scheme 118). For Varenicline, innate radical C–H trifluoromethylation takes place at the most electron rich position within the arene rings (C5, giving 566). Conversely, difluoromethylation occurs exclusively at electron-poor sites adjacent to heteroatoms within the heteroarene rings (C2, giving 567).

Taking the advantage of photoredox catalysis, Nagib and MacMillan developed in 2011 a Ru(phen)₃Cl₂-catalyzed trifluoromethylation reaction of arenes and heteroarenes by the use of trifluoromethanesulfonyl chloride (CF₃SO₂Cl) as trifluoromethyl group source (Scheme 119) [296]. The relatively low cost and ease of handing of CF₃SO₂Cl as well as the mild reaction conditions led this method to become particular interesting. The absorption of one photon by the photocatalyst Ru(phen)₃ ²⁺ will generate a high energy excited species *Ru(phen)₃ ²⁺. The reaction is initiated by the reduction of triflyl chloride with *Ru(phen)₃ ²⁺ (called oxidative quench) via one-electron transfer. The triflyl chloride rapidly collapses to trifluoromethyl radical when it ensues an electron from *Ru(phen)₃ ²⁺.

The addition of the trifluoromethyl radical to (het)arenes would form a new cyclohexadienyl like radical species, which would give trifluoromethylate (het)aryl compounds by the oxidation of Ru(phen)₃ ³⁺ followed by deprotonation. The reaction has wide substrate scope. Different types of diazines gave good to excellent yields of trifluoromethylated products under treatment by 1–4 eq. of CF₃SO₂Cl in MeCN in a presents of 1–2 % of Ru(phen)₃Cl₂ and K₂HPO₄ as a base with irradiation by 26 W light source (Fig. 21).

6.1.4 Other Methods

There are several other methods for generation of (per)fluoroalkyl radicals in reactions with diazines. In particular, photochemical decomposition of perfluoroalkyl mercury derivatives was used for perfluoroalkylation of uracils 568 (Scheme 120) [297]. Whereas for the parent uracil the method gave satisfactory results (30–54 % yields), the procedure was unfruitful for the sugar-modified derivatives (6–11 % yields).

Photochemical reaction of uracil 7 with CF₃Br also gave the corresponding 5-trifluoromethyl derivative, although in this case, the yield was unsatisfactory (11 %, 56 % conversion) (Scheme 121) [298].

Yamakawa with co-workers reported in 2010 Fe-catalyzed trifluoromethylation of various aromatics (including diazines) with CF₃I (Table 22). The method used FeSO₄–H₂O₂ system to generate active species from CF₃I. Since the reaction was of electrophilic nature, only diazines with electron-donating substituents were used as the substrates [299].

Table 22 Fe(II)-catalyzed trifluoromethylation of diazines with CF₃I

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Several methods for the preparation of chain-fluorinated pyridazines relied on the so-called “anionic Friedel – Crafts” reactions (i.e. aromatic nucleophilic substitution with perfluoroalkyl anions or their synthetic equivalents) with tetrafluoropyridazine, which was discussed in the corresponding section on chemistry of ring-fluorinated diazines.

6.2 Nucleophilic Substitution with Fluoride

6.2.1 Substitution of Other Halogens

Nucleophilic substitution of halogen atoms with fluoride ion was relatively rarely used for the preparation of chain-fluorinated diazines. The method was applied for the preparation of monofluoroalkyl and trifluoromethyl diazines, and various reaction conditions were used in these two cases. Monofluoroalkyl diazines were obtained by reaction of the corresponding benzyl-type halides with CsF in DMF [300–304] or HMPA – DMSO [307] (Table 23). This approach was successfully used for the preparation 2-(fluoroalkyl)pyrimidines; the only literature example with pyrazine derivative reported low yield (9 %) [305].

Table 23 Preparation of monofluoroalkyl diazines using nucleophilic substitution

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For the synthesis of trifluoromethyl substituted diazines, the corresponding tricloromethyl derivatives were treated with HF [306], SbF₃ – cat. SbCl₅ [67, 307], or SbF₅ [308] at elevated temperatures (Table 24). Notably, the latter two reagents allowed selective fluorination of the side chain in the presence of chlorine substituents in the heteroaromatic ring [67, 308]. Contrary, in case of fluorination using HF the chlorine atoms in the diazine core were substituted first [67].

Table 24 Fluorination of trichloromethyl diazines

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6.2.2 Deoxofluorination

Reaction of alcohols, aldehydes and ketones with fluorinating agents (i.e. deoxofluorination) is a well-established method for the synthesis of chain-fluorinated diazines. Mono- and difluoroalkyl substituted diazines were obtained in moderate to good yields using this method (Tables 25 and 26). A common reagent used to achieve this type of transformations is diethylaminosulfur trifluoride (DAST) (571). Bis(2-methoxyethyl)aminosulfur trifluoride (Deoxo-Fluor^TM) (572), which is of higher thermal stability and therefore more amenable to large-scale use [309], is an alternative reagent for deoxofluorination. Both reagents work under mild conditions and are tolerant to a number of functional groups; they were successfully used for the fluorination of many functionalized diazines. In the case of chiral alcohols (Table 25, Entry 12–15), the reaction proceeded with inversion of the configuration (Fig. 22).

Table 25 Deoxofluorination of diazine-derived alcohols

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Table 26 Deoxofluorination of diazine-derived aldehydes and ketones

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Exhaustive deoxofluorination of carboxylic group in diazines was studied scarcely: only a few examples included reaction of 2-aminopyrimidine-5-carboxylic acid (45), pyrazine mono- (575) and tetracarboxylic acids (577) with HF–SF₄ at 120–150 °C, giving the corresponding trifluoromethyl derivatives 547, 576 and 578 respectively in 20–68 % yield (Scheme 122) [329–331]. Under milder reaction conditions (e.g. with DAST), deoxofluorination of carboxylic acids stops at the formation of the corresponding acyl fluorides.

6.3 Electrophilic Fluorination

In principle, fluorine atom can be introduced into the side chain of heterocycles by reaction of the corresponding α-hetarylcarbanions with electrophilic fluorinating reagents. Nevertheless, this approach was rarely used for diazines. In particular, 6-fluoromethylpurines 580 were obtained in 48–58 % yields by deprotonation of purine derivatives 579 with NaHMDS followed by reaction with N-fluorobenzenesulfonimide (NFSI, 51) (Scheme 123) [332]. A phthalazine derivative 583 was prepared by an analogous approach, using N-fluoro-2,α-cumenesultam (584) as the fluorinating agent [333].

7 Construction of Diazine Core Using Fluorine-Containing Building Blocks

7.1 Synthesis from α-Fluorocarboxylic Acids and Their Derivatives

7.1.1 CCC+NCN Approach to Pyrimidines (Principal Synthesis)

Because of its wide applicability, the method that involves the reaction of a binucleophile to supply the three-atom fragment (NCN) with a bis-electrophile to provide the three-carbon fragment (CCC), i.e. CCC+NCN or [3+3] approach to pyrimidines, is known as the principal synthesis [334]. Among the NCN binucleophiles, only amidines provide possibility to introduce the fluorinated side chain into the pyrimidine core.

Trifluoroacetamidine (585) is most widely used for the principal synthesis of pyrimidines. Compound 585 can be prepared from ethyl trifluoroacetate by ammonolysis, followed by dehydration with P₂O₅ and reaction with ammonia (Scheme 124) [335, 336]. Amidine 585 has been introduced into reaction with various β-dicarbonyl compounds and their synthetic equivalents (Table 27), including β-ketoesters (Entries 1–6), in particular β-ketopyruvates (Entry 3) and α-alkoxymethylene-β-ketoesters (Entries 4–6), β-enaminocarbonyl compounds (Entries 7–9), malonic acid derivatives (Entry 10), fluorinated β-diketones (Entry 11), vinamidinium salts (Entry 12), α,β-unsaturated nitriles with leaving group at β position (Entries 13–15) and other bis-electrophiles (Entries 16, 17). Usually, the reaction gives moderate yields of the target 2-CF₃-pyrimidines (ca. 50 %).

Table 27 Syntheses of fluorinated diazines using trifluoroacetamidine (585)

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A three-component one-pot reaction of 585, methyl formate, methyl methoxyacetate in presence of sodium methoxide was also studied (Scheme 125) [353]. Obviously, in this case Claisen condensation occurred first, followed by heterocyclization with 585 to give pyrimidine derivative 586.

Reaction of amidine 585 with perfluorinated alkene 587 led to the formation of amidine 588, which upon heating with alkali gave a mixture of pyrimidines 590 (7 %) and 589 (34 %) (Scheme 126) [354]. Pyrimidine 589 was obtained in 78 % yield when the second step of this sequence was performed using DABCO.

Apart from 585, another fluorinated amidines were used for the principal synthesis of pyrimidines including compounds 591–594 (Fig. 23).

Since the corresponding nitrile 595 does not react with ammonia directly, preparation of 591 starting from 595 used several steps (Scheme 127), including isolation of ethyl 2-fluoroacetimidate (as hydrochloride 596) [355, 356] or ethyl 2-fluorothioacetimidate [357]. In some literature sources the step including reaction of 592 with ammonia was omitted [358]. Compound 592 was prepared in 48 % yield from methyl difluoroacetate 597 by reaction with NH₄Cl–Me₃Al in toluene at 80 °C [359]. Synthesis of 59 is analogous to that of trifluoroacetamidine 585 [360], and 594 – of fluoroacetamidine 591 [361].

Only a few examples of principal synthesis of pyrimidines involving amidines 591–594 were described to date (Table 28), including reaction of 591 and 592 with malonic acid derivatives (Entries 1 and 2), 593 – with β-diketone, enamino ketone and ethyxymethylene derivative of a β-ketoester (Entries 3–5), and 594 – with enamino ketone (Entry 6).

Table 28 Syntheses of fluorinated diazines using amidines 591–594

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7.1.2 Other Approaches to Pyrimidines

Apart from the principal synthesis of pyrimidines (i.e. CCC+NCN or [3+3] approach), several other methods involve the use of fluorinated carboxylic acid derivatives:

NCCCN+C or [5+1] approach;
CC+2CN or [2+2+2] approach;
NCCCC + N or [5 + 1] approach;

One of them (NCCCN+C approach) is a common method for the preparation of quinazolines and their hetero-analogues (see the next section). Nevertheless, malonamide 602 was shown to react with various fluorinated esters 603 to give pyrimidine derivatives 604 in good yields (70–75 %) (Scheme 128) [365] (in some patents, lower yield of the product was reported, e.g. 29 % for the difluoromethyl derivative [356]). Excess of sodium alcoxide was used in the reaction, therefore, sodium salt of malonamide was likely an active species, which allowed retaining nitrogen atoms of the amide fragment in the final product. In the case of ethyl trifluoroacetate, the reaction was performed on a kilogram scale (conditions: NaH, nBuOH, toluene, 100 °C, then 23–25 °C, 44 %) [366].

One more method relying on NCCCN+C approach was used for the synthesis of perfluorinated pyrimidine derivatives. It relied on acylation of β-diimine 606 – a product of reaction of alkene 605 (an analogue of 587 mentioned in the previous section) with ammonia – with perfluorinated anhydrides or acyl chlorides (Scheme 129) [367]. Pyrimidines 607–609 were obtained in 71–88 % yields.

Several methods for the preparation of chain-fluorinated pyrimidines involve reactions of fluorinated nitriles. One of such methods relies on reaction of two trifluoroacetonitrile molecules with one molecule of a substrate, i.e. CC+2CN or [2+2+2] approach. This approach was used for several types of substrates (Table 29), including ynamines (Entry 1), enamines (Entries 2–7), imines and methylene active compounds (Entries 8, 9).

Table 29 CC+2CN approach to chain-fluorinated pyrimidines

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Stepwise mechanism was proposed for the reaction of CF₃CN with enamines including formation of tautomeric 1:1 adducts 610–612 (Scheme 130) [369]. Each of these adducts can react further with CF₃CN to give 1:2 adducts 613–615, which can undergo cyclization to dihydropyrimidines 616 and 617, either directly or via tautomerization. Both 616 and 617 give the final product 618 upon elimination of a secondary amine. In case of enamines lacking β′-hydrogen atom, only one of the pathways mentioned above is possible, namely, via adducts of the type 610 and 613.

Ketimines (e.g. 619) were also introduced into reaction with fluorinated nitriles (CF₃CN and CHF₂CF₂CN) to give 2:1 adducts (Table 29, Entries 10 and 11) [374]. In case of 619, pyrimidines 620 and 621 were obtained in 78–90 % yields, whereas for the 2-thienyl analogue of 619, the yields of the corresponding products were moderate (26–45 %). Notably, the method allowed to use two different electron-deficient nitriles, if CCl₃CN was used in the first step of the reaction (Scheme 131). In this case, intermediate 82 could be isolated.

Recently, an interesting approach to chain-fluorinated pyrimidines was described (CF₃COOH as the source of CF₃ group). The method relied on CCCCN+N ([5+1]) cyclization of enamides 628 and 629 with NH₄OAc to give pyrimidines 630 and 631 in 66 and 31 % yields respectively (Scheme 132) [373]. Compounds 628 and 629 were prepared by generation of the corresponding lithiated allene derivatives 626 and 627, followed by reaction with nitrile and subsequent acylation with CF₃COOH. In case of 628, the synthesis was complicated by partial removal of the trimethylsilylethyl protecting group (TMSE) to result in low yield of the product (14 %).

7.1.3 Construction of Pyrimidine Ring of Quinazolines and Their Hetero-analogues

A classical method for the synthesis of quinazolines is reaction of anthranilamides (i.e. NCCCN-binucleophiles) with carboxylic acids or their derivatives (i.e. C1-electrophiles) [374]. The reaction proceeds in two steps: acylation of aromatic amino group followed by heterocyclization. For example, reaction of anthranilamide 632 or its derivatives with trifluoroacetic anhydride afforded amide 633, which underwent cyclization upon action of aqueous NaOH [375], AcOH [376] or Ac₂O [377] to give quinazolone 634 (56–99 % from 632) (Scheme 133). The reaction sequence was also performed in one-pot manner by heating of anthranilamides and CF₃COOH at 300 °C upon MW irradiation; in this case moderate to good yields of the products were obtained (29–75 %) [378]. Alternatively, 634 was obtained by heating of 632 and ethyl trifluoroacetate [379]. Several modifications of the method were applied for the synthesis of fused quinalozolones and hetero-analogues, e.g. pyrazolo[3,4-d]pyrimidine (Table 30, Entry 1), isoxazolo[5,4-d]pyrimidine (Entry 2), pteridine (Entry 3), or benzoquinazoline (Entry 4) derivatives.

Table 30 Synthesis of hetero-analogues of chain-fluorinated quinalozolones

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The above mentioned approach was used extensively for the preparation of 2-(aryldifluoromethyl)quinazolin-4-ones and their hetero-analogues. In particular, anthranilamides, as well as their thiophene or pyrazole analogues 635–642 were coupled with (het)aryldifluoroacetic acids 643–645 to give amides 646 (Table 31). For the activation of the carboxylic acid, a number of common reagents were used, including (COCl)₂, HATU, and pentafluorophenyl trifluoroacetate. Cyclization of 103 was performed by heating with Me₃SiCl/Et₃N in 1,2-dichloroethane (DCE) at 80–85 °C (Entries 1–5), as well as by refluxing in AcOH or Ac₂O/AcOH (Entries 6 and 7). One-pot reaction of 637–642 and 643–645 was also developed by using of trimethylsilyl polyphosphate (TMSPP) at 115–130 °C (Table 32).

Table 31 Two-step synthesis of 2-(aryldifluoromethyl)quinazolin-4-ones and their hetero-analogues

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Table 32 One-pot synthesis of 2-(aryldifluoromethyl)quinazolin-4-ones and their hetero-analogues

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A convenient precursor of quinazoline derivatives is anthranilic acid nitrile 646. It should be noted that in order to introduce the substituent at C4 position of the quinazoline core starting from 646 or its derivatives, a nucleophile is necessary for the cyclization – a feature which can be advantageous since additional diversity point appears in the synthesis. In particular, reaction of N-trifluoroacetyl derivative of 646 – compound 647 – with PCl₅ in sulfolane gave 4-chloro derivative 648 in 80 % yield (Scheme 134) [388]. Upon heating of 646 with H₂O₂/NaOH at 35–45 °C, quinazolone 634 was obtained; in this case, amide 633 was an intermediate in the reaction [393]. Reaction of 646 with Grignard reagents gave intermediates 649 and 650, which upon acylation with TFAA or CClF₂C(O)Cl gave quinazolines 651–653 in 42–90 % yields [390]. Indium-promoted version of the latter method was also developed for the synthesis of 4-allylquinazolines, which could be illustrated by reaction of compound 47 with allyl bromide in a presence of indium [391].

Compound 646 and the corresponding heterocyclic nitriles were used in the synthesis of other perfluorinated quinazolines (Table 33, Entry 1) and their hetero-analogues, e.g. pyrazolo[3,4-d]pyrimidine (Entry 2), 7-deazahypoxanthine (Entry 3), pyrido[2,3-d]pyrimidine (Entry 4), or thieno[2,3-d]pyrimidine derivatives (Entry 5). In all these cases, the corresponding amides (e.g. 633) can be proposed as the intermediates in the reaction.

Table 33 Synthesis of chain-fluorinated quinalozolones and their hetero-analogues of quinalozolones from the corresponding nitriles

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An unusual (although scarcely preparative) variation of using nitrile 647 for the synthesis of quinazolines was described in late 1970s [396]. Compound 647 reacted with organoboron derivatives to give bora-heterocycles 655, which upon hydrolysis rearranged to quinazoline derivatives 656 (Scheme 135).

Certain peri-substituted aromatic diamines can act as NCCCN binucleophiles in reaction with fluorinated acid derivatives to give pyrimidines. In particular, fused pyrimidine derivatives 658, 659 and 661 were obtained from amines 657 [397] and 660 [398] by reaction with fluorinated anhydrides (Scheme 136).

An unusual reaction sequence, which also falls into category of NCCCN+C approaches, was described in 1980 [399]. In particular, oxime 662 reacted with chlorofluoroacetyl chloride to give acyl derivative 663, which underwent cyclization to quinazoline N-oxide 664 upon action of BF₃⋅Et₂O (Scheme 137).

One more example of an uncommon NCCCN binucleophile, compound 122, was used recently in the synthesis of human A₁ adenosine receptor ligands (Scheme 138) [400]. In this case, a pyrazole nitrogen atom was one of the nucleophilic centers in the reaction, which led to the formation of tricyclic fused aromatic ring system (compound 666).

NCCCN+C approach was also used for the preparation of fused dihydropyrimidines. In particular, reaction of imines 668 with trifluoroacetaldehyde ethyl hemiacetal in the presence of acidic catalyst gave pyrimido[4,5-d]pyrimidine derivatives 669, although in low yields (14–28 %) (Scheme 139) [401].

Apart from NCCCN+C ([5+1]) approach discussed in all syntheses of chain-fluorinated quinazolines and their hetero-analogues described above, other methods were also developed, in particular:

CCCN+CN ([4+2]) approach;
intramolecular cyclizations of alkynes;
other heterocyclizations.

An example of using CCCN+CN ([4+2]) approach includes preparation of chain-fluorinated pyrimido[4,5-d]pyrimidine derivative 671 (Scheme 140) [402]. In this method, enamine 670 reacted with trifluoroacetamide at 120 °C to give 671 in 64 % yield. Analogously, reaction of trifluoroacetamide and pyridine derivative 672 led to the formation of pyrido[3,4-d]pyrimidine 673 [403].

A different example of using [4+2] approach described formation of tricyclic trifluoromethyl-substituted pyrimidine derivatives 677 via direct C–H bond functionalization in azoles (Scheme 141) [408]. In this case the trifluoromethyl group arrived to the final products from CCNC reactants.

Several methods for the preparation of chain-fluorinated quinazolines relied on using properly functionalized alkynes. In particular, upon treatment with a Lewis acis (i.e. ZnCl₂, BF₃⋅Et₂O, InCl₃⋅3H₂O, CuSO₄ or Cu(OTf)₂), amidine 679 (prepared in two steps from aromatic amine 678) gave quinazoline derivative 681 in 68–82 % yield (Scheme 142) [405]. Furthermore, amidines 682 underwent analogous reaction upon treatment with K₂CO₃ in CH₃CN at 80 °C. Products 683 (obtained as mixtures of E/Z isomers) were subjected to ozonolysis to give quinazolones 684 in 64–82 % overall yield.

Palladium-catalyzed reaction of bis(o-trifluoroacetamidophenyl) acetylene 685 with various aryl and vinyl halides and triflates led to the formation of indolo[1,2-c]quinazolines 686 (Scheme 143) [406]. If the reaction was performed in presence of CO, the corresponding acyl derivatives 687 were obtained [407]. In both cases, the indole heterocyclic system was formed first; the subsequent pyrimidine ring closure resulted in construction of the tetracyclic ring system.

3,4-Dihydroquinazolines 690 were obtained by intramolecular Staudinger – aza-Wittig tandem sequence from azides 689, which in turn were synthesized from aldehyde 688 using a four-component Ugi reaction (Scheme 144) [408].

Tautomeric trifluoromethyl-substituted spirocyclic quinazolines 693 and 694 were formed when p-benzoquinone imines 692 (synthesized by electrochemical oxidation of the corresponding p-anisidine derivatives 691) were heated in DMSO at 120 °C (Scheme 145) [409]. The reaction was affected by solvent (DMSO giving the highest yields) and nature of the substituents in 692. In the case of naphthalene derivatives (e.g. 695), the cyclization gave single tautomers (e.g. 696). A synchronous mechanism was proposed for this transformation.

An interesting approach to chain-fluorinated quinazolines relied on photochemical recyclization of 1,2,4-oxadiazole derivatives 697 (Scheme 146) [410]. The corresponding quinazolones 698 were obtained in 40–75 % yields when pyrene was used as a sensitizer. This is contrary to the data obtained for simple alkyl-substituted oxadiazoles, which gave highest yields of the products upon irradiation in the presence of triethylamine.

7.2 Principal Synthesis from Fluorinated β-Dicarbonyl Compounds and Their Analogues

In the previous section, synthesis of pyrimidine derivatives bearing fluorinated alkyl substituent at C-2 atom was discussed. Derivatives of fluorinated carboxylic acids and related compounds were used as the fluorine sources. The most important method for the preparation of other chain-fluorinated pyrimidines is the principal synthesis from fluoroalkyl-substituted three-carbon bis-electrophiles (e.g. β-dicarbonyl compounds). A huge number of fluorinated bis-electrophiles were introduced in the principal synthesis of pyrimidines bearing fluoroalkyl substituent at C-4 atom of the heterocyclic ring (Fig. 24), including fluorine-containing:

β-dicarbonyl compounds 699, i.e. β-diketones, β-ketoesters, β-ketoamides (385 reaction hits in Reaxys®);
β-alkoxy-substituted enones 700 (202 hits), β-enaminones 701 (32 hits), (thio)acetals 702 (9 hits), as well as other enones 703 (52 hits);
chromone derivatives and hetero-analogues 704 (23 hits) and 705 (22 hits);
β-halosubstituted α,β-unsaturated carbonyl compounds 706 (33 hits), as well as corresponding nitriles 707 (16 hits);
β,β-dihalosubstitutes carbonyl compounds 708 (35 hits) and the corresponding vinyl iodides 709 (9 hits);
perfluorinated alkenes 710 (14 hits);
ynones 711 (39 hits);
α,β-unsaturated sulfones 712 (28 hits);
methylenemalonodinitrile derivatives 713 (46 hits).

An overview of these reactions is given in the further sections; due to the huge number of data, only selected examples are provided. A separate section is related to principal synthesis of pyrimidines bearing fluorinated substituent at C-5 atom.

7.2.1 Pyrimidines with Fluorinated Alkyl at C-4 from β-Dicarbonyl Compounds

More than a third part of all the described principal syntheses of pyrimidines bearing fluorinated alkyl at C-4 atom commences from fluorinated β-dicarbonyl compounds 699. The chemistry of these bis-electrophiles was reviewed recently [411, 412]; therefore, their preparation is not discussed herein. This synthesis of pyrimidines is fairly general (Table 34); it allows for introducing aliphatic, alicyclic and aromatic β-diketones (Entries 1–10), β-ketoesters (Entries 11–16), and cyclic β-ketoamides (Entry 17). Presence of some functional groups, such as additional ester moiety (Entry 15), is more or less tolerated, whereas increasing steric hindrance results in lowered yields of the products (Entry 10). A scope of common NCN binucleophiles include amidines (Entries 1, 11, 12, 17), (thio)urea and its derivatives (Entries 2–4), guanidines (Entries 5, 16) and biguanides (Entry 6). Electron-rich amino heterocycles (e.g. aminoazoles and even 2,6-diaminopyridine) are excellent NCN binucleophiles for the principal synthesis of fused pyrimidine derivatives (Entries 7–10, 13–15).

Table 34 Principal synthesis of pyrimidines with fluorinated β-dicarbonyl compounds 699

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Although there are examples of uncatalyzed principal synthesis of pyrimidines using 699 as the starting material, the reaction usually requires acid or base as a promoter. Typical reaction conditions are reflux of the starting materials in AcOH or in alcohol in presence of alcoxide. Whereas AcOH is a common solvent for the reactions of (thio)ureas and amino heterocycles, the latter conditions are preferable if amidine or guanidine salts are used as the source of NCN binucleophiles, since the active species are liberated as the free bases in this case. Other reaction promoters include H₂SO₄, polyphosphoric acid (PPA), TsOH, BF₃⋅Et₂O, AcONa and K₂CO₃ (see Table 34).

In case of non-symmetrical binucleophiles, the reaction with 699 is regioselective (although not always 100 %). Normally, it should start with attack of more nucleophilic nitrogen atom of the nucleophile at the fluoroalkyl-substituted carbonyl group of the electrophile (see, for example, Table 34, Entries 4 and 12). Nevertheless, the available data, reported mainly for the reactions of aminoazoles, are somewhat controversial. For example, reaction of 1,1,1-trifluoro-2,4-pentanedione 714 and pyrazole 715 afforded a single product, assigned to structures 716 [429] and 717 [419] by two groups of authors (Scheme 147). Although different reaction conditions were used in these two works (piperidine – EtOH and AcOH, respectively), a more thorough NMR study confirmed the second structure, 717, for both cases [430]. Further evidences for such regioselectivity, including X-Ray crystallographic data, were obtained for analogous substrates [431] (see Table 34, Entries 7–9 for additional examples).

It should be noted that the reaction outcome depended strongly on the solvent. In particular, a mixture of 716 and 717 was obtained by heating the starting materials in EtOH instead of AcOH, 717 still being the major isomer (717:716 = 70:30) [419]. These data show that the structures of the products in the reactions of fluorinated β-diketones 699 with amino azoles should be checked carefully in each particular case, especially for the early reports in this area. It is interesting to note, that opposite regioselectivity (confirmed by X-Ray) was observed in the case of fluorinated β-ketoesters (Table 34, Entries 13–15) [432].

Recently, intermediates 718 were isolated in the reaction of 1,1,1-trifluoro-2,4-pentanedione and aminopyrazoles 719 (Scheme 148) [433]. Compounds 718 were formed in CH₂Cl₂ below 10 °C in several minutes. Upon heating to 50 °C or standing at ambient temperature, they underwent dehydration to form the expected aromatic products 720. An analogous intermediate 722 was isolated in the reaction of ethyl trifluoroacetoacetate and aminopyrazole 721 [434]. Intermediates 723 and 724, which correspond to isomerization and partial dehydration of an analogue of 718 – compound 725 – were also detected by NMR [431]. Obviously, formation of these intermediates is responsible for the diminished regioselectivity of the process, which is observed at elevated temperatures or upon change of the solvent.

7.2.2 Pyrimidines with Fluorinated Alkyl at C-4 from Enones

Fluoroalkyl-substituted enones represent another important class of CCC bis-eletrophiles widely used for the preparation of pyrimidines with fluorinated alkyl at C-4. Their chemistry has been reviewed recently [435, 436], therefore, preparation of these bis-electrophiles is not discussed herein. Several subtypes of β-enones are amendable for the synthesis of chain-fluorinated pyrimidines. The most common group include compounds with a leaving group at β position (700–702 and 706). (Fig. 24). A special case of these bis-electrophiles is chromone derivatives and their hetero-analogues (704, 705). Other β-enones 703 (i.e. non-functionalized) and 712 (β-sulfonyl, hydrates) are also used.

β-Alkoxy-substituted enones 700 were used as the starting materials in nearly quarter of all the principal syntheses of pyrimidines with fluorinated alkyl at C-4. The structures of the compounds of general formula 700 are summarized in Fig. 25. Quite expectedly, most of them contain trifluoromethyl substituent, although compounds with difluoromethyl (727 [239], 749 [437, 438]), chlorodifluoromethyl (728 [413]), 1,1,2,2-tetrafluoroethyl (750 [437, 439, 4406]), pentafluoroethyl (729 [441]), heptafluoropropyl (751 [432, 433, 440]), and difluoro(fluorosulfonyl)methyl (730 [441]) groups were also involved. Apart from the parent trifluoromethyl-substituted β-alkoxyenones 726 (Table 35), compounds with alkyl (731 [442–448], 732 [443], 733 [449], 734 [450]), aryl (735 [442, 444, 447, 451–455], 736 [454], 739 [449, 451]) and hetaryl (737 and 738 [442, 449, 455]) substituents were used in the synthesis of 4(6)-trifluoromethylpyrimidines. Cyclic enones 743 [449, 454–456], 744 [445–448], and 745 [445, 446, 452] can be outlined. Among functionalized β-alkoxy-substituted enones, the derivatives containing an ester moiety at α-position (748–751) are most important (Table 35); other examples include allyl bromides 740 [457, 458], 741 [457, 458], and 742 [458], β-diketones 746 [459] and 747 [447, 454], and ζ-ketoester 752 [442].

Table 35 Principal synthesis of pyrimidines with β-alkoxy-substituted enones

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A range of NCN binucleophiles introduced into reaction with 700 is similar to that for fluorinated β-dicarbonyl compounds discussed in the previous section and includes amidines (Table 35, Entries 1, 2), (thio)urea and its derivatives (Entries 3–5), guanidines (Entry 6), semicarbazide derivatives (Entry 7), and electron-rich amino heterocycles (Entries 8–12).

As in case of fluorinated β-diketones, in most cases reactions of 700 with NCN binucleophiles were promoted by either acids (e.g. HCl, AcOH, BF₃⋅Et₂O, Ti(OiPr)₄) or bases (RONa, Et₃N, AcONa) (see Table 35). Again, basic conditions were preferred when the binucleophile was used in a salt form; in cases of urea derivatives and amino heterocycles the reaction was either promoted by acids or non-catalyzed.

The reaction of 700 with unsymmetrical NCN binucleophiles demonstrated regioselectivity, which was influenced by the nature of binucleophile, substituents in 700, and even catalyst loading. In particular, reaction of N-alkylureas with β-alkoxyenones 726 and 731 in the presence of conc. aq HCl in refluxing MeOH led to the formation of pyrimidines 753 and 754 in 65–90 % yields [443] (Scheme 149). In the case of 732, analogous products 755 were obtained when high concentration of HCl were used; upon lower acidity of the reaction medium, the products of alternative regioselectivity – compounds 756 – were obtained. Presumably, the reaction starts with the Michael addition of the amino groups of the N-alkylurea at the β-carbon atom of the enone, followed by elimination of an alcohol molecule to give enaminones 757 or 758. Formation of 758 is faster due to higher steric accessibility of the primary amino group of the N-alkylurea. Cyclization of 758 furnishes compounds of the type 756. In the case of R2=Me and low concentration of acid, 756 is stable, probably due to the steric effect of the methyl substituent on the trifluoromethyl and hydroxyl groups. When R2=H or the reaction is carried out at high concentration of acid, compounds of type 756 can equilibrate back to the starting compounds and then – to enaminone 757. Cyclization of 757 followed by dehydration leads to the formation of pyrimidinones 753–755. It should be noted that an intermediate 758 was isolated in the reaction of 726 with N-methylthiourea [462]. Again, upon prolonged reaction time this kinetic product rearranged to give enaminone of the type 757, which underwent heterocyclization to give 760 (Table 35, Entry 4).

Most reaction of enones 700 with amino azoles have analogous mechanism, i.e. formation of enaminones 761, followed by their cyclization and subsequent dehydration of intermediates 762 (Scheme 150) (Table 35, Entries 9–12). In many cases, hydrates 762 and/or enones 761 are reasonably stable and can be isolated [437–439, 453]. For example, reaction of 751 and 3-amino-1H-[1, 2, 4]triazole in dioxane gives hydrate 763, which exists in equilibrium with its open forms 764 and 765 in solution (DMSO, 763:764:765 = 80:11:9; acetone, 763:764:765 = 28:41:31) [438]. Dehydration of these species to obtain pyrimidine 766 requires prolonged reflux (~60 h) in AcOH. It should be noted that for all examples mentioned in the above paragraph, the reaction started with substitution of alkoxy group in the enone molecule with amino group of the amino azole (via addition – elimination mechanism). Therefore, the perfluoroalkyl group was in the neighboring position to the fusion nitrogen atom in the final product. Opposite regioselectivity was reported for the reaction of enones 700 with 2-aminobenzimidazole (Table 35, Entry 8). It was postulated that in this case, the reaction started with attack of endocyclic nitrogen at β-carbon of the enone [452].

Interesting intermediates of the type 767 were obtained in the reaction of enones 700 with amidines (Scheme 151) [445]. Formation of these intermediates cannot be rationalized using the mechanistic schemes discussed above.

Enaminones 701 demonstrated similar behavior in the reactions with NCN binucleophiles compared with β-alkoxy-substituted enones 700 (Table 36). It should be noted, however, that rather unusual substituents were introduced into the molecules of the target pyrimidines using reagents 701 (Entries 3–9). In particular, the enaminone fragment of 701 can be a part of aminouracil moiety (Entry 9), although in this case, CCCN+CN mechanism for the pyrimidine ring formation is possible.

Table 36 Principal synthesis of pyrimidines with fluorinated β-enaminones

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Two enones of general formula 702 were introduced into reaction with NCN binucleophiles, namely, 768 [453] and 769 [475] (Scheme 152). In case of 768, one of the ethoxy groups can be retained in the final structure. Reactions with 769 were accompanied with the dithiane ring opening to give thiols 771.

Chromone derivatives and their analogues 704 and 705 were used for synthesis of 4-fluoroalkylpyrimidines. In both cases, reaction with NCN binucleophiles was accompanied with recyclization of the γ-(thia)pyrone ring to give (2-(thio)hydroxyphenyl)-substituted pyrimidines or their analogues (Table 37).

Table 37 Synthesis of pyrimidines with fluorinated chromones and their hetero-analogues

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The method was extended to 2,6-bis(perfluoroalkyl)-substituted γ-pyrones 772; in this case, equilibrium mixtures of products 773 and the corresponding hydrates 774 were obtained (Scheme 153) [480].

Apart from α,β-unsaturated carbonyl compounds having O–, N- and S-leaving groups at the β-position, compounds 706 with halogen nucleofuges (i.e. Cl, F) were also used in the synthesis of 4-fluoroalkylpyrimidines. In particular, reaction of aldehyde 775 [481], ketone 776 [482], or ester 777 [483] with amidines or guanidines gave pyrimidines 778–780 in 39–98 % yields (Scheme 154).

Reaction of fluorinated α,β-unsaturated esters 781 with 2-aminobenzothiazole derivatives resulted in a regioselective pyrimidine ring fusion and led to the tricyclic compounds 782 in 67–83 % yields (Scheme 155) [484]. On the contrary, reaction of 781 with 2-aminopyridines was not regioselective and led to the mixtures of isomers 784 and 785. Similar results were obtained with 2-aminothiazole. In the case of 6-methyl-2-aminopyridine, only one regioisomer 783 was formed, presumably due to the steric effect of the methyl group which prevented attack of the β carbon of 781 at the endocyclic nitrogen atom.

Aromatic (786) and xylose-derived (787, 788) fluorinated enones were successfully introduced into reaction with amidines or analogous NCN binucleophiles to give pyrimidines 789 in 58–80 % yields (Scheme 156) [485].

In principle, enones without a leaving group in β-position 703 can also react with NCN binucleophiles; in this case, partially hydrogenated pyrimidine derivatives are obtained. In particular, reaction of β-trifluoroacetylstyrene (790) with urea and thiourea gave tetrahydropyrimidines 791 with more than 30:1 dr (Scheme 157) [486]. Analogous products 792 were obtained in case of acetamidine and guanidine. In case of aminotriazole and aminotetrazole, fused pyrimidine derivatives 793 were obtained as single diastereomers, whereas 2-aminobenzimidazole gave a 3:1 mixture of diastereomers 794 [487]. It should be noted that in both cases, the reactions were regioselective. Tetrahydropyrimidine derivatives 791 and 793 were subjected to dehydration to give 795 and 796, respectively. Similar results were obtained in the reactions of urea and thiourea [488], as well as amino azoles [489, 490] with enones 797.

Analogously, reaction of enone 798 with thiourea gave tetrahydropyrimidines 799, whereas in the case of enone 800, dihydropyrimidine 801 was obtained (Scheme 158) [486].

One-pot procedure for the synthesis of aromatic pyrimidine derivatives 803 from β-aryl-enones 802 was developed (Scheme 159) [491]. It included reaction of 802 with amidines, followed by dehydration with POCl₃ and oxidation with MnO₂.

Reaction of β-sulfonyl-enone hydrates 804 with aminotriazoles in acetonitrile at room temperature led to the formation the 5-CF₃ isomer of tetrahydropyrimidines 805, which were transformed to their aromatic counterparts (e.g. 806) by reflux in AcOH (Scheme 160). The reaction of 804 with 805 in other conditions (heating in water or acetic acid) lead to losing of the regioselectivity [492].

7.2.3 Other CCC Bis-electrophiles

Apart from fluorinated β-dicarbonyl compounds and β-enones, CCC bis-electrophiles used for the synthesis of 4-(per)fluoroalkylpyrimidines include alkynes 711, α,β-unsaturated nitriles 707 and 713, as well as compounds in which the fluorine atoms of the perfluoroalkyl substituent act as leaving group (708–710). Reactions of alkyne-derived aldehydes, ketones and esters of general formula 711 were analogous to that of β-enones discussed in the previous section (Table 38). Due to presence of triple bond, neither leaving group at β position nor using the oxidizing reagents were necessary to obtain aromatic derivatives. The method was used successfully for amidines (Entry 1), guanidines (Entries 2 and 3), aminopyridines and their fused analogues (Entry 4), and 2-aminooxazolidines (Entry 5) as NCN binucleophiles.

Table 38 Synthesis of pyrimidines with alkyne derivatives 711

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Malonodinitrile derivative 807, as well as compounds with phosphonate (808) and trifluoromethylthio (809) groups were used in reactions with NCN binucleophiles, including N-alkylamidines, 3-aminopyrazoles and 2-aminopyridines to form 707 (Table 39). Activated alkenes 807–809, unlike the compound 777 containing COOMe group, gave amino or imino derivatives of pyrimidines, which arose from attack of the nucleophile at the nitrile group. Analogous reaction was observed in case of 810; as a result dihydropyrimidine derivatives 811 or 812 were formed (Scheme 161) [497–500].

Table 39 Principal synthesis of pyrimidines with nitriles of general formula 707

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Aldehydes, ketones and esters of general formula 708 reacted with amidines to give pyrimidine derivatives (Table 40, Entries 1–4). Analogous methods were developed for (per)fluorinated vinyl halides 709 (Entry 5) and 710 (Entries 6, 7). Analogous reaction was successful with enol phosphate 814, obtained from ketone 813 and sodium diethyl phosphite (Scheme 162) [120]. In all these cases, nucleophilic substitution of two fluorine atoms at α-carbon of the perfluoroalkyl group occurred.

Table 40 Synthesis with compounds type 708–710 (see Fig. 24)

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7.2.4 Pyrimidines with Fluorinated Group at C-5 Position

Unlike their C-2 and C-4-substituted counterparts, pyrimidines with fluorinated alkyl at C-5 were rarely prepared using reaction of NCN binucleophiles and CCC bis-electrophiles. Several examples of such transformations were already mentioned in previous section (reactions with 710 leading to 4,5-bis-perfluoroalkylpyrimidines, see Table 40, Entries 6,7). Analogous reactions with alkene 817, as well as its precursor 816 (a stable adduct of methanol and 2-(trifluoromethyl)-1,1,3,3,3-pentafluoropropene), led to the formation of 5-trifluoromethylpyrimidines 818 (Scheme 163) [505, 506]. Pyrimidines 822 and 825 were also prepared from 816. Reaction of 816 with triethylamine resulted in formation of enolate 819 (Scheme 163) [507]. When in situ generated 819 was treated with HCl, acyl fluoride 820 was formed. Reaction of 820 with N,N′-dimethyl(thio)urea resulted in the formation of adduct 821, which underwent cyclization to 822 upon treatment with triethylamine. Reaction of 819 with S-alkyl isothiouronium salts resulted in generation of 820, which reacted with S-alkyl isothiourea base to give adduct 2823 [508]. Cyclization of 823 led to formation of pyrimidine 824. Synthesis of 824 from 817 was also performed in one-pot manner.

2-Trifluoromethylmalonic acid derivatives were used in the synthesis of 5-trifluoromethyl-substituted pyrimidines. In particular, acyl fluorides 827 were obtained by alkylation of trifluoromethylmalonyl fluoride (826), in turn prepared from acyl fluoride 825 (Scheme 164) [509]. Reaction of 827 with urea led to the formation of fluorinated barbiturates 828.

α-Trifluoromethylacrylic acid (829), as well as and its derivatives 830 and 831 are valuable building blocks which can be used for preparation of 5-trifluoromethylpyrimidines. In particular, reaction of 829 with ureas in acetic anhydride led to the formation of 5-trifluoromethyl-5,6-dihydrouracils (832) in 67–84 % yields (Scheme 165) [510]. In case of unsymmetrical ureas, the reaction was regioselective (except N-methylurea, which gave a mixture of regioisomers). An alternative method was more effective for thioureas (DCC, DMF, 90 °C, 50–55 % yields), since S-acetylation occurred when Ac₂O was used.

The reaction of 5-iodo-1,3-dioxin-4-one 833 with trifluoromethyl iodide in the presence of copper powder in HMPA led to formation of 1,3-dioxine derivative 830 (Scheme 166) [511]. Compound 830 reacted with N,N'-dimethylurea in refluxing toluene to give pyrimidine 834 in 84 % yield.

α,β-Unsaturated esters 831 were prepared by reaction of bis(trifluoromethyl) acetates 835 with various thiols in presence of BF₃⋅Et₃N complex (Scheme 167) [124]. Reaction of 831 with amidines led to pyrimidines 836 in 47–71 % yield.

Vinamidinium salt 840 is a promising reagent for the synthesis of 5-trifluoromethylpyrimidines 841, unsubstituted at positions C-4 and C-6. Compound 840 was prepared from 2,2,2-trifluoropropanoic acid (839). Acid 839 was obtained via radical addition of trifluoromethyl iodide to TBS-enolate 838 of tert-butyl acetate 837, followed by acidic hydrolysis (Scheme 168) [512]. Reaction of 840 with amidines and their analogues led to formation of the corresponding pyrimidines 841 in 54–85 % yields. Additional examples of such transformations were described [347, 513], including also reaction with aminopyrazole 843 (Scheme 169) [514]

7.3 CNC+CCN Approach to Fluoroalkyl-Substituted Pyrimidines

A group of methods for the preparation of fluoroalkyl-substituted pyrimidines relied on CNC+CCN connection of the aromatic ring. A variety of fluorinated 1,3-dielectrophiles was used for the annulations of CNC triade to CCN binucleophiles (i.e. anilines, enamines and electron-rich amino heterocycles). The most widely used among such 1,3-CNC-dielectrophiles are functionalized heterocumulenes 845–847 (Fig. 26); trifluoromethyl substituted imine derivatives 848, 849 and 850 can be also mentioned.

Most of these building blocks are not commercially available, since their preparation requires quite expensive fluorine-containing starting compounds, and the procedures leading to their formation cannot be classified as easy-to-perform. This may be the reason why their chemical behaviour has not been documented to a full extent. Only few research groups (mainly at the Institute of Organic Chemistry NAS Ukraine) deal with these CNC fluorine-containing bis-electrophiles [515].

Isocyanates 854 were first synthesized in Kiev by Samarai and co-workers in 1975 using the synthetic pathway described in the Scheme 170 [516, 517]. The starting aryl Grignard reagent was coupled with trifluoroacetonitrile to give of 2,2,2-trifluoro-1-arylethanimine 851. The subsequent treatment of 851 with trichloromethyl isocyanate, which exists predominantly in the iminocarbonyl chlonde form 853 [518] led to the formation of the isocyanate 854. The reaction proceeds through intermediate 854, which undergo thermal extrusion of cyanogen chloride. Heterocumulenes 846 can be prepared in satisfactory yields from isocyanate 845 using aza-Wittig protocol with arylphosphinimines [519].

The above mentioned approach to isocyanates 854 has considerable disadvantages: the necessity of using highly toxic and difficultly accessible trichloromethyl isocyanate and excretion of highly toxic cyanogen chloride. In 2008 Vovk and co-workers propose a more convenient approach to 1-aryl-1-chloro-2,2,2-trifluoroethyl isocyanates. Acylation of imines 852 with alkyl chloroformates gives the corresponding carbamates 855 (Scheme 171) [520]. The subsequent reaction of 855 with phosphorus pentachloride in boiling phosphoryl chloride leads to the formation of target isocyanates 845 in 65–86 % yield. But in spite of visible benefits, the Vovk approach to isocyanates 854 has not been scaled up jet and Samarai method is still in use for multigram synthesis.

1,1-Dichloro-2,2,2-trifluoro-1-isocyanatoethane 286 [521] was also first synthesized in Kiev by Boiko and co-workers via chlorination of N-trifluoroacetylcarbamate 856 with phosphorus pentachloride at 170 °C. The method gives no more than 34 % of the target product. The same scientists optimized in 2002 the synthesis of trifluoroacetylisocyanate 857 [522], which is available from tris(trimethysilyl)cyanurate 858 [523]. Compound 847 readily reacts with alcohols in presence of triethylamine to give alkyl 1-chloro-2,2,2-trifluoroethylidenecarbamates 848 [524]. Alternatively, 848 were prepared by Osipov using the reaction of 856 with SOCl₂ in presence of Et₃N [525], this method was effective only for benzyl derivative (Scheme 172).

Finally, acyl imines of hexafluoroacetone (e.g. 850, Scheme 173) can be prepared using aza-Wittig reaction [526].

Reactions of fluorinated alkylheterocumulenes 845–847 with CCN binucleophiles were studied for more than 30 years by Vovk laboratory. Initial reports in this area, however, were limited to reactions with (1-cyclohexenyl)dialkylamines [527, 528] and ethyl β-N-methylaminocrotonate (859) [529]. For example, reaction of 859 with isocyanates 845 led to the formation of dihydropyrimidines 860 (Scheme 174). Remarkably, the regioselectivity observed was opposite to that for the reaction of 859 with isocyanates 861 lacking trifluoromethyl group [530, 531].

Tertiary enamines 863 also undergo reaction with isocyanates 283 in presence of triethylamine as a base, but the reaction products are dihydropyrimidones (Scheme 175) [532]. The reaction gives the corresponding carbamoyl derivatives 864. Treatment of the adducts 864 with strong bases like DBU or DBN results in cyclization to pyrrolidin-1-yl-6-(trifluoromethyl)-5,6-dihydropyridin-2(1H)-ones 300. The reaction is faster if enaminoesters rather than enaminonitriles were used as the starting compounds of the type 863; on the other hand, donor substituents in the aryl fragment of 845 reduced the reaction rate. Other enamines studied in the reaction with 845 are shown in Table 41. It should be noted that in all these cases, the regioselectivity of the reaction with 845 was opposite to that observed for isocyanates 861 lacking the trifluoromethyl group.

Table 41 Reaction of 845 with enamines

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It was found that benzothiazole derived acetonitriles 866 react with 845 in benzene in presence of triethylamine at room temperature to give adducts 868, whereas reaction in toluene without a base at reflux led to the formation of isomeric compounds 870 via 869 (Scheme 176) [536]. These results were explained by reversible formation of intermediate 867. Analogous results were obtained in reaction of 845 with 2-pyridylacetonitrile 871 as CCN binucleophile (Scheme 177) [537].

Detailed investigation of the reaction of 845 with benzimidazole derivative 874 demonstrated that the process carried out both in presence of a base at room temperature or without base at heating resulted in a single type of compounds 876.

The result is obviously due to the enhanced basicity of the benzimidazole ring compared to those of benzimidazole and pyridine. Therefore the adduct 875 is more stable than its pyridine and benzothiazole analogues and does not dissociate into starting reagents under heating. Also when benzimidazolyl acetate with R=H, EWG=CO₂Me was used, the corresponding compounds 876 was found unstable to give decarboxylated compounds 877 (Scheme 178) [538].

Besides cyclizations of hetaryl acetonitriles, the reactions of isocyanate 845 with anilines were studied. In presence of base the reaction leads to N-alkylidene-N-arylureas 879. The compounds 879 bearing the C=N bond activated by trifluoromethyl group undergo thermal intramolecular cyclization to give 4-trifluoromethyl-1,2,3,4-tetrahydroquinazolin-2-ones 880 in good yields. A wide range of anilines bearing EDG as well as EWG enters into cyclization. This fact is indirect proof that the reaction proceeds as synchronic process through 6-membered transition state 881. Aminonaphthalenes afforded angular tricyclic compounds 882 and 883 (Scheme 179) [539].

In order to establish scope and limitation of the method the set of electron-rich aminoheterocycles [540, 541] were reacted with isocyanates 845. Unexpectedly it was found that the reaction resulted in complex mixture of products. For increasing of reaction selectivity the less electrophilic acyl imine 855 was used. But in this case the reaction proceeds at nitrogen atom of aminoheterocycle and further thermal treatment resulted in trifluoromethyl-containing hetarylimines (Scheme 180) [542].

Only aminothiazoles generated in situ from 887 and aminoimidazoles 888 were appropriate CCN binucleophiles to provide thiazolo[4,5-d]pyrimidones 889 and fluorinated dihydropyrines 890 (Scheme 181) [543]. The optimal reaction conditions were: Et₃N, CH₂Cl₂, ambient temperature.

In many cases, reactions of carbodiimide 846 with binucleophiles were analogous to that of 845 in terms of regioselectivity. In particular, 846 reacted with enamine 859 to provide adduct 891, which was transformed to pyrimidine derivative 892 upon heating (Scheme 182) [544].

On the contrary, reaction of 846 with 2-pyridylacetonitrile (871) showed some differences compared to 845. Unlike 846, the same products 893 were formed either upon action of tertiary amines or heating without bases (Scheme 183) [553]. The reaction of 846 with benzimidazole derivatives 874 proceeds in a similar way to afford tricyclic compounds 894 (Scheme 183) [538].

The feature of chemical behavior of carbodiimides 846 is ability to undergo intramolecular cyclization into dihydroquinazolidenes 897 in a presence of TEA. In absence of proton donating nucleophiles TEA attacks the highly electrophilic carbon of heterocumulene 846 providing intermediate 895. The bond between a chlorine and a α-carbon in the latter is strongly polarized, and therefore these intermediates may exist in equilibrium with diazadiene 896 (compare with 881, Scheme 179).

The presence of triethylamonium moiety considerably increases the electrophilicity of the α-carbon in the 895 as well as the carbon in the N-ethylidene fragment in 896. As a result a ring closure at the ortho-position of the N-tolyl moiety becomes possible (Scheme 184) [545]. The structure of unusual zwitterionic compounds was confirmed by X-ray.

Besides the reactions of 845 and 846 with hetaryl acetonitriles interaction of these compounds with α,α-dichloroisocyanate 847 was studied. In all cases annelation of pyrimidine ring affording compounds 898–900 (Scheme 185) [536–538].

Also the reaction of α,α-dichloroisocyanate 847 with anilines was studied by Samarai and Vovk in presence of equimolar amounts of 2,4,6-trimethylbenzonitrile oxide [546]. The role of nitrile oxide consists in its unique blocking of the initial stage of the reaction to form 904 [547]. Nitrile oxide can be recovered in almost quantitative yield after completion of the process. It was established by IR spectroscopy that the reaction actually proceeds through isocyanates 901, which undergo intramolecular cyclization (Scheme 186).

It should be noted, that isocyanates 901 can be generated by another ways. Samarai in 1977 described thermal nitrogen elimination from α-azidoisocyanates 905 followed by rearrangement to 901 [548]. Uneyama described in 1990 dehydromethoxylation at 250 °C of 907 to form ketenimide 901, which afforded finally 902 in 71 % yield (Scheme 187) [549].

Reaction of 847 with aminothiazole precursors 887 was also studied; although the corresponding products of pyrimidine ring annelation 315 were obtained, their yields (15 %) were too low (Scheme 188; note the different regioselectivity compared to that observed for 283, Scheme 181) [543].

Imines 848 are promising reagents for the synthesis of fluoroalkyl-substituted pyrimidinines and their fused analogues since they lead to functionalized low-molecular-weight compounds which are of special interest for medicinal chemistry. Many CCN binucleophiles discussed in this section were also introduced into reaction with 848. In particular, formation of pyrimidine derivatives 911 in reaction of 848 and enamines 909 (including compounds 859 discussed above) was described recently. The most likely initial step is the imidoylation of the nucleophilic carbon atom in the enamine moiety of 909. The resulting intermediate 910 appears to cyclize to compounds 911 already at room temperature (Scheme 189) [550].

Reaction of 848 with hetaryl acetonitriles in presence of triethylamine also proceeds through first C-imidoalkylation step as it confirmed by isolation of a stable compounds 912. Compounds 912 were quantitatively converted into annelation products 898–900 by heating in boiling o-xylene (Scheme 190) [536–538]. It should be noted that the yields of 898–900 were somewhat lower than in the case of isocyanate 847 (see Scheme 185).

Reaction of 848 with amino heterocycles is a valuable approach to chain-fluorinated purines and their bioisosteres. It was found that cyclocondensation of 848 with electron-rich aromatic and heteroaromatic amines proceeded in two steps (Scheme 191) [543, 551–554]. First, amidines 913 were formed under mild conditions (Et₃N, CH₂Cl₂, ambient temperature) in moderate yields (44–60 %). Cyclization of 913 occurred under harsh conditions (toluene or xylene, reflux); however, the yields of the products 914 in this step were high (83–96 %). It should be noted that in the case of 5-methylisoxazol-3-amine 915, triazine derivative 917 was formed instead of the corresponding pyrimidine [555].

This approach was recently used for the synthesis of 2-trifluoromethyl allopurinol riboside. Aminopyrazole 918 reacts with two equivalents of imine 848 under mild conditions affording the product bearing two methyl 1,1,1-trifluoropropan-2-ylidenecarbamate groups, which was detected by HPLC. During the elution with EtOAc 1,1,1-trifluoropropan-2-ylidenecarbamate group on the 5-th position of the sugar residue was removed to give intermediate 919. Amidine 919 appeared to be stable to the ring cyclization and required heating at 180 °C during 1 h. Removing the protecting group by acidic treatment leads to target 2-trifluoromethyl allopurinol riboside 921 (Scheme 192) [556].

The chemistry of imines of hexafluoroacetone 849 and 850 as CNC bis-electrophiles was elaborated by Chkanikov and than developed by Sokolov group. Dihydroquinazoline derivatives 924 were obtained by reaction of imine 849 and N-alkylanilines (Scheme 193) [557, 558]. Initially, N-alkylation product 922 was formed, which is in equilibrium with starting materials. This intermediate rearranged to the product of C-alkylation to give 924 upon further dehydration. The water liberated in this reaction caused hydrolysis of the imine 849 as a side reaction. The rate of heterocyclization depends on stability of 922, which decreases with increasing steric effect at the nitrogen atom of aniline. Indeed, N-methyl derivative of the type 922 is stable enough under reaction conditions and is converted to dihydroquinazoline only by heating, whereas in the case of N-isopropyl derivative, the heterocyclization product is formed already at 20 °C.

In 2001 Sokolov with co-workers extended the reaction to aminouracil. In a case of benzoyl imine 849 the reaction with uracil 925 gives C-aminoalkylated product 927 cyclized into fused pyrimidine derivative 928 (compare with amopyrazole behaviour, see Scheme 180). It should be noted, that similar sequence with imine of trifluoropyruvate 926 leads instead of 849 to pyrrolo[2,3-d]pyrimidine derivative 929 (Scheme 194) [559].

Puch-pull enamines and more complicated hexafluoroacetone acyl imines 849b afforded pyrimidines 930 (Scheme 195) [560, 561]. Ethyl 2,2,2-trifluoro-1-(trifluoromethyl)ethylidenecarbamate 850 is another CNC bis-electrophile, which has been used for pyrimidine ring assembly. Aminouracil, aminocrotononitrile and 3-amino-5,5-dimethylcyclohex-2-en-1-one react readily with 850 to give pyrimidines 931 (Scheme 195) [562] as a single regioisomer.

The reaction of anilines with perfluorinated imine 932 can be considered also as CNC+CCN approach to fluoroalkyl-substituted pyrimidines. Imine 932 was synthesized by perfluobutane elimination from perfluorotributylamine [563] under ACF (Aluminium chlorofluoride, AlCl_xF_3−x, x ≈ 0.05–0.25) catalyzed thermolysis [564]. This imine have been found to react smoothly with 2 equivalents of anilines in presence of 3 equivalents of TEA in MeCN to give the fused pyrimidines 933 in good yields. In a case of 2,6-dimethylaniline the reaction leads to the dihydroquinazoline derivative 934 (Scheme 196) [565].

7.4 Multicomponent Synthesis of Fluorinated Pyrimidines

Due to their productivity, high yields, convergence and facile execution multicomponent reactions are widely used in the synthesis of heterocycles. Surely, the most known multicomponent reactions in the field of pyrimidines is Biginelli reaction – a three-component condensation of aldehyde, methylene active compound and urea [566]. The use of fluorinated β-dicarbonyl compounds as methylene components in Biginelli reaction was documented first in 1950s [5]. In this work, formation of the structure 935 was reported in the reaction of ethyl trifluoroacetoacetate, benzaldehyde and urea (Scheme 197). Reinvestigation of these results, made in late 1990s, showed that stable hydrate 934 (most thermodynamically stable stereoisomer) was formed at these conditions [567]. Elimination of water from 934 occurred only in presence of a strong acid (TsOH, reflux in toluene).

The method was extended to other classes of fluorinated β-dicarbonyl compounds, including β-ketoesters (Table 42, Entry 1), β-diketones (Entry 2), β-ketosulphones (Entry 3), β-ketosulphamides (Entry 4), and β-ketophosphonates (Entries 5 and 6). It should be noted that in case of some β-diketones (i.e. 1,1,1,5,5,5-hexafluoroacetylacetone), the products of principal pyrimidine synthesis were formed instead of Biginelli adducts under reaction conditions [568]. Apart from urea and thiourea, other classes of NCN binucleophiles were also introduced, including N-alkylureas (Entries 7 and 8, note different stereochemistry of the products), aminotriazoles (Entries 9 and 10), aminotetrazole (Entry 11), and 2-aminobenzimidazole (Entry 12).

A huge number of the reaction promoters were used for the preparation of structures of both types 934 and 935 in a selective manner. Apart from those mentioned in the Table 42, these include:

Table 42 Biginelli reaction with fluorinated β-dicarbonyl compounds

Full size table

for hydrates 934: Yb(OTf)₃ [576], ZrCl₄ [577], ionic liquids [578], LiCl–CuCl₂ [579], LiCl–SnCl₂ [580], SmI₂ [581], K₅CoW₁₂O₄₀⋅3H₂O [582], p-dodecyl benzenesulfonic acid [583], TsOH with grinding [584].
for dihydropyrimidines 935: ytterbium (III) perfluorooctanoate [585], Bi(OTf)₃ [586], TaBr₅ [587], bioglycerol-based sulfonic acid functionalized carbon catalyst [588], cerium ammonium nitrate with ultrasound activation [589], citric acid [590].

Some interesting results were found recently by Shermolovich with co-authors upon investigation of 2-oxo-2-polyfluoroalkylethane-1-sulfones and -sulfamides in Biginelli reaction (Table 42, Entry 3). The corresponding Biginelli compounds undergo ‘retro-Biginelli’ reaction by treatment with hexamethylenetetramine (HMTA) under thermal conditions involving replacement of 6-aryl substituent of the pyrimidinone cycle with a hydrogen atom donated by HMTA (Scheme 198) [570]. The formation of 937 proceeds through acyclic intermediate 940, generated via thermal cleavage of 936. In this intermediate the carbanion adjacent to the sulfonyl substituent is stabilized strong electron-withdrawing group, while the iminium cation is postulated as an intermediate in the condensation of aldehydes with urea in the classical Biginelli reaction. The arylidene group is transferred to HMTA releasing the methylidene moiety to afford the intermediate 942, subsequent cyclization of which results in the 6-unsubstituted tetrahydropyrimidinone 937. When ketones 944 were directly subjected to the Biginelli reaction with urea and HMTA using TMSCl as promoter the expected tetrahydropyrimidinones 937 were obtained in ca. 40 % yield (Scheme 198).

In another paper dealing with TMSCl promoted by Biginelli reaction unusual results were obtained in a case of trifluoroacetyl acetone and N-substituted (thio)ureas. In this case the cyclization leads to compounds 945 bearing CF₃CO group at 5th positions. To the best of our knowledge this reaction is the only example of Biginelli reaction with fluorinated unsymmetrical β-dicarbonyl compounds were CF₃CO groups leaves intact and compounds 945 are the only representatives of Biginelly compounds bearing CF₃CO groups at the 5th positions (Scheme 199) [572].

Apart from the Biginelli reaction, several additional three-component condensations are worth mentioning in this section. In particular, a transformation closely related to Biginelli reaction was described by Shermolovich and coworkers, namely, reaction of β-ketophosphonates 946, urea and orthoformates, which leads to adducts 947 (Scheme 200) [571]. The corresponding aromatic compounds 948 were not isolated, presumably due to high electrophilicity of C=N bond in their molecules. Evidently, the reaction occurs via intermediate formation of 946, which has been reported by same scientists in the reaction of 3-arylsulfonyl-1,1,1-trifluoropropan-2-ones 944 with orthoformates [591].

Reaction of fluorinated β-enaminones 951 with formaldehyde and primary amines resulted in tetrahydropyrimidines 952 in 78–85 % yields (Scheme 201) [592].

Dihydropyrimidines 954 or 955 were the products of the reaction of 2,2-dihydropolyfluoroalkylaldehydes 953 with ammonia and aldehydes, ketones or enol ethers (Scheme 202) [593].

One more three-component condensation leading to formation of chain-fluorinated pyrimidines 956 was microwave-assisted reaction of malononitrile, 2,2,2-trifluoro-1-phenylethanone and amidine in water (Scheme 203) [594]. Attempts to perform this reaction under thermal conditions were unsuccessful.

7.5 Miscellaneous Methods for the Preparation of Pyrimidines

Several methods for construction of diazine core of chain-fluorinated pyrimidines do not fall into any of the mentioned above categories. One of such approaches is discussed in Sect. 7.8, namely, inverse-electron-demand Diels-Alder reactions with fluorinated sym-triazines. Other methods that fall into category “miscellaneous” are too different to discuss them systematically; therefore, selected examples of them are listed in this section.

An unusual method for the preparation of fluorinated uracil derivatives 962 relied by Shermolovich with co-workers on reaction of fluorinated sulphones 957 with sodium cyanate in presence of triethylamine, followed by acidification (Scheme 204) [595]. The mechanism of the reaction included base-catalyzed elimination of HF, followed by addition of two cyanate ions to the formed alkene 958.

Another cyanate-based method was used in the synthesis of reverse transcriptase inhibitors, namely, reaction of trifluoroacetophenones 963 with sodium cyanate, affording hydrate 964, followed by dehydration in xylene (Scheme 205) [596].

One more method relied on reaction of chiral fluorinated amino ketones 966 and aryl isothiocyanates to give pyrimidine derivatives 968 in 58–87 % yields and optical purity of 88–96 % (Scheme 206) [597].

A method which relied on the use of benzoyl isocyanate was developed for the synthesis of 5-fluoro-6-fluoromethylcytosine (971) (Scheme 207) [598]. Thorpe condensation of fluoroacetonitrile generated unstable enamine 969, which reacted with benzoyl isocyanate in presence of pyridine to give urea derivative 970. Cyclization of 970 upon action of NaOEt led to the formation of 971.

Tricyclic pyrimidine derivative 976 was obtained in the reaction of bis(pyrrolyl)methane (974), enaminone 972 and Tf₂O (Scheme 208) [599]. This is an example of quite unusual reactivity of enaminones (as C₁ synthons) in the synthesis of pyrimidines. This reactivity can be explained by preferential formation of the six-membered ring over eight-membered.

Another example of fluorinated pyrimidine synthesis via [5+1] approach using fluorinated C₁ synthon was described by Burger in 1980. Amidines 976 react with hexafluoroacetone (HFA) affording adduct 977, which under treatment with POCl₃-Py generating diazabuta-1,3-dienes 978 spontaneously cyclized into 3,4-dihydro-quinazolines 979 (compare with Schemes 179, 184, 193 and 194). If the firs step of the cyclization proceeds in ether with an excess of HFA the chemistry becomes more complicated. In this case 5,6-dihydro-2H-1,3,5-oxadiazines 980 are formed, which undergo retro Diels Alder reaction on thermolysis. Elimination of hexafluoroacetone leads to diazabuta-1,3-dienes 978 transformed into final 3,4-dihydro-quinazolines 979 (Scheme 209) [600]

Recently Chinese chemists elaborated unusual cyclization based on alkynylimines 981. Reaction of 981 (prepared by CuI-catalyzed coupling of terminal alkynes with fluoroalkylimidoyl chlorides) and primary amines led to formation of enamines 982, which were not isolated but treated with t-BuOK at −40 °C to give dihydropyrimidines 984 (Scheme 210) [601].

On the contrary, reaction in presence of Cs₂CO₃ at 80 °C provided pyridines 986. A possible explanation of these results included formation of 982, followed by generation of anion 983 upon action of base. When the reaction was carried out at a low temperature with a soluble base (t-BuOK), intramolecular N-nucleophilic cyclization of 983 was observed immediately to form dihydropyrimidine 984 through a kinetically controlled pathway. However, C-nucleophilic addition became an option upon elevated temperatures and/or with the use of an insoluble base (Cs₂CO₃), providing 1,2-dihydropyridine ring under thermodynamic control. The subsequent proton migration, β-fluoro elimination and final aromatization to form the pyridine ring of 986 also provided a driving force for this pathway.

7.6 Synthesis of Chain-Fluorinated Pyridazines

7.6.1 Synthesis from Fluorinated 1,4-Dicarbonyl Compounds and Their Analogues

The reaction of 1,4-dicarbonyl compounds or their synthetic equivalents with hydrazines is one of the most significant methods for the construction of the diazine ring of chain-fluorinated pyridazines. Two subtypes of fluorinated 1,4-dicarbonyl compounds can be used for this aim (987 and 988) (Fig. 27), leading to formation of 3(6)- and 4(5)-fluoroalkyl-substituted pyridazines, respectively.

Formation of pyridazine ring by reaction of 1,4-dicarbonyl compounds of the type 987 was reported first in 1960, when characterization of 5,5,5-trifluorolevulinic acid (991) and its lactone 992 was performed (Scheme 211) [602]. Upon their reaction with 2,4-dinitrophenyl hydrazine (2,4-DNPG) in ethanol, a product 993 was obtained instead of 2,4-dinitrophenyl hydrazones. The starting compound 991 was obtained by hydrolysis of a Claisen adduct 990, prepared from ethyl trifluoroacetate and diethyl succinate 989.

Recently, a modification of this approach was developed by chemists from China [603]. In this method, esters 995 were prepared from 994 by heating with H₃BO₃ (Scheme 212). Compound 995 reacted with various hydrazines in presence of TsOH to produce dihydropyridazines 996 (although in the case of aryl hydrazines with EWG (e.g. 2,4-DNPG), the corresponding hydrazones 998 did not or only partially underwent cyclization to 996). Aromatization of 996 could be performed by action of bromine in acetic acid to give products of the type 997.

Reaction of functionalized enamines 1001 with hydrazines assumed a slightly anomalous course leading to pyridazine derivatives 1002 (Scheme 213) [604]. The starting compounds 1001 were obtained in two steps from dimethyl acetylenedicarboxylate 999.

Fluorinated 1,4-dicarbonyl compounds of the type 987, which carbonyl groups are mounted on an aromatic core, are common starting materials for the preparation of chain-fluorinated phthalazines and their analogues. Apart from benzene derivatives 1003 [605] and 1005 [606] (Scheme 214), derivatives of azulene [607], indene [608] and furane [609] were introduced into these transformations.

Thioesters 1009 are examples of fluorinated compounds of the type 987, which give 4(5)-fluoroalkyl-substituted pyridazines upon reaction with hydrazine. Compounds 1009 were prepared by reaction of ketene dithioacetal 1007 with the corresponding enolate anion, followed by hydrolysis (Scheme 215) [610]. Reaction of 1009 with hydrazines led to the formation of dihydropyridazines 1010 in a regioselective manner. Aromatization of 1010 was achieved by heating with CuCl₂ in acetonitrile. Analogous reaction sequence with thioester 1009 led to formation of dihydropyridazines 1013, which underwent easily elimination of HF upon action of a base affording pyridazine 1014. In case of hydrazine hydrate, compound of the type 1013 was not isolated; HF elimination occurred upon reaction conditions. The drawback of the approach is poor availability of commercially unavailable ketene dithioacetals 1007. As in a case of above mentioned CNC-biselectrophiles, preparation of these compounds requires quite expensive fluorine-containing starting compounds, and the procedures leading to their formation cannot be classified as easy to perform [611].

A convenient two-step methodology for the preparation of 4-trifluoromethyl-(2H)-pyridazin-3-ones starting from methyl trifluoropyruvate MeTFP (1015) was developed (Scheme 216) [612]. The approach relied on aldol condensation of 1015 with various ketones, followed by reaction of the adducts 1016 with hydrazine. Recently, this methodology was used for the preparation of γ-secretase modulators [613].

An unusual synthetic equivalent of trifluoromethyl substituted 1,4-dicarbonyl compounds is furanone 1020, which can be obtained by condensation of acetophenone and methyl 2-methoxytetrafluoropropionate 1018 (synthetic equivalent of MeTFP in Claisen condensation), followed by acidic dehydration (Scheme 217) [614]. Reaction of 1020 with hydrazine led to the formation of pyridazine derivative 1021 in 81 % yield.

7.6.2 Diaza-Wittig Reactions

An interesting approach to pyridazine derivatives was described in 1998 by Guillaume and others [615] and studied later by Nikolaev and co-workers [616, 617]. The key idea of the method is implemented in the final step of sequence – intramolecular cyclization of intermediates 1026 – the so-called “diaza-Wittig” reaction (Scheme 218). Two alternative pathways for the generation of 1026 were used, both commencing from diazo-β-dicarbonyl compounds 1022, in turn obtained by diazo transfer reaction. Compounds 1022 were subjected to Wittig reaction with stabilized ylides 1023; the reaction proceeded in a stereoselective manner, so that E-isomer of product 1024 was formed. Reaction of 1024 with triphenylphosphine resulted in generation of 1026, which underwent cyclization to give pyridazines 1027 in good yields. In an alternative scheme, compound 1022 was treated with triphenylphosphine to give Staudinger adducts 1025. Wittig reaction of 1023 and 1025 resulted in generation of 1026, which spontaneously underwent cyclization to give 1027, although in moderate yields.

A closer look into stereochemical aspects of the diaza-Wittig reaction discussed above revealed that only one isomer of 1026 (E or cis) undergoes spontaneous cyclization to give pyridazine derivatives 1027 [618, 619]. The other isomer (Z or trans) can be isolated and in some cases even characterized by X-Ray data.

It was found that compounds 1025 (i.e. 1028) react with β-dicarbonyl compounds to give pyridazine derivatives 1029 (Scheme 219) [620]. In this case, intermolecular diaza-Wittig reaction occurred, followed by intramolecular heterocyclization. The reaction was sensitive to steric factors: in case of β-diketone possessing bulky tert-butyl group, 1029 was isolated in low yield (9 %).

7.6.3 NNCC+CC Approaches

3-Hydrazono-1,1,1-trifluoroalkan-2-ones 1030 are NNCC units that have found use in synthesis of pyridazine derivatives. In particular, they undergo dimerization upon treatment with trifluoroacetic acid to give pyridazines 1031 (Scheme 220) [621]. Mechanistic study of this transformation showed that a key step of the reaction is concerted [4+2] cycloaddition of protonated 1030 [622]. Pyridazines (i.e. 1032 and 1033) were the products in other reactions of hydrazones 1030 with acetylene dicarboxylates [621] and β-dicarbonyl compounds [623].

Another fluorinated NNCC building blocks which give pyridazines through [4+2] cycloadditions are dichlorohydrazones 1035 (Scheme 221) [624, 625]. In this case, the fluoroalkyl substituent arrives from the CC partner of the reaction. Upon treatment with Hünig’s base, 4-chloroazodienes 1036 are generated from 1035, which undergo reaction with fluorinated enamines 1037 to form a mixture of diastereomers 1038 and 1039. This mixture can be transformed to their aromatic counterparts 1040 by action of a strong base.

Hydrazone 1042 (prepared from 2-amino-1,1,1-trifluoro-3-phenylsulfonyl-2-propanol) is one more NNCC binucleophile for the synthesis of chain-fluorinated pyridazines (Scheme 222) [626]. In this case, two-step reaction of 1042 with α-diketones is used, including acid-catalyzed hydrazone formation and base-promoted heterocyclization.

In another two-step NNCC+CC strategy, acylation of hydrazones 1045 with perfluoropropionic anhydride led to the formation of 1046, which underwent cyclization to pyridazines 1047 upon heating with silica gel (Scheme 223) [627].

Heteroaromatic hydrazines were used as NNCC building blocks For the preparation of fused pyridazines. In particular, hydrazine 1048 reacted with hydrate of fluorinated α-diketone 1049 to give 4-trifluoromethylpyrimido[4,5-c]pyridazine derivative 1050 (Scheme 224) [628]. Furthermore, reaction of hydrazines 1051 with ethyl trifluoroacetoacetate led to the formation of pyridazino[3,4-b]quinoxaline derivatives 1052 [629].

7.6.4 Other Methods

Reaction of 4-aminotriazole (1053) with fluorinated β-diketones is a method for the preparation of triazolopyridazines 1054 (Scheme 225) [630]. The method can be considered as NNC+CCC approach to the construction of the pyridazine ring.

[4+2] cycloaddition of fluorinated diene 1055 and azo compound 1056 provided tetrahydropyridazine derivative 1057 – an example of CCCC+NN disconnection of chain-fluorinated pyridazine ring (Scheme 226) [631].

Analogous pyridazine derivatives 1059 were prepared from diene precursors 1058 using metathesis reaction (Grubbs II catalyst, toluene, 100 °C). The corresponding trifluoromethyl-substituted cyclic hydrazines 1059 were obtained in reasonable to good yields. In almost all cases, 20 mol% of catalyst had to be added over a period of approximately 1 h in order to reach full conversion. (Scheme 227) [632].

7.7 Synthesis of Chain-Fluorinated Pyrazines

7.7.1 Synthesis from 1,2-Diamines and Fluorinated 1,2-Bis-Electrophiles

A common method for the preparation of chain-fluorinated pyrazines relies on reaction of NCCN binucleophiles (i.e. 1,2-diamines) and fluorinated CC bis-electrophiles. This approach is especially valuable for the synthesis of quinoxalines and their hetero-analogues, since aromatic system is formed directly under reaction conditions. Therefore, most of the literature data concern heterocyclization with o-phenylenediamines, as well as the corresponding heterocyclic 1,2-diamines. Aliphatic diamines (mostly 1,2-ethylenediamine) are much less studied and often give poor results in the reaction with fluorinated 1,2-bis-electrophiles; possibly the only exception is diaminomaleonitrile (394), which also gives aromatic systems in these transformations.

The range of fluorinated 1,2-bis-electrophiles (some of these reagents (e.g. trifluoropyruvic acid) are available as hydrates) used for the construction of pyrazine core is vast (Fig. 28) and includes:

trifluoropyruvic acid, its esters and higher homologues (1061);
hexafluorobiacetyl, its derivatives and its higher homologues (1062);
perfluoroalkyl-substituted α-diketones and their derivatives 1063;
trifluoromethyl glyoxal and its synthetic equivalents 1064;
fluorinated α-halo-β-dicarbonyl compounds 1065;
perfluorinated epoxides 1066;
other fluorinated 1,2-bis-electrophiles.

Preparations of these 1,2-bis-electrophiles share some common features. In particular, esters of trifluoropyruvic acid (like MeTFP 1015) are available commercially, but they can be prepared in two steps from an epoxide 1067 (namely, hexafluoropropylene oxide, which is available on industrial scale) [633] (Scheme 228). In turn, epoxides 1067 are obtained by oxidation of the corresponding perfluorinated alkenes, e.g. with hypochlorite [634].

Hexafluorobiacetyl can be prepared in a reproducible manner in satisfactory yield (50 %) by oxidation of commercially available alkene 1068 (Scheme 229) [635]. Due to highly inhalation toxicity (LC₅₀ inhalation – rat – 4 h – 16 ppm) the purchase and transport of compound 1068 has some restriction, but the alkene can be prepared in laboratory by SbF₅ fluorination of hexachlorobutadiene [636].

Although generation and reactions of 1,1,1-trifluorobiacetyl was reported as early as in 1957, the compound was not isolated in this work [637]. Trifluoromethyl-substituted derivatives 1071 were obtained via trifluoroacetylation of hydrazones 1070 [638, 639], acylation of (trifluoroacetimidoy1)lithium derivatives 1075 [640], or condensation of trifluoroacetimidoyl chlorides 1077 with aromatic aldehydes in presence of sodium hydride [641] (Scheme 230). These methodоlogies were also used for the synthesis of trifluoromethyl glyoxal equivalents 1064 [640, 642].

Higher perfluoroalkyl homologues of the type 1063 were prepared via oxidation of benzyl-substituted ketones (i.e. 1080) with SeO₂ [643] or hydrolysis of α,α-dichloroketones 1084 [644] (Scheme 231).

Synthesis of fluorinated α-halo-β-dicarbonyl compounds 1065 was straightforward and relied on halogenation of the corresponding fluorinated β-dicarbonyl counterparts (e.g. with NBS), which could be performed even in one-pot manner [645].

Selected examples of reactions of 1,2-bis-electrophiles 1061–1066 with 1,2-diamines are given in Table 43. The method gives good to excellent results when at least one of the starting components is symmetric; otherwise, the reaction is usually regioselective (see Entries 3, 12 and 14). Isolation of hydrates is a common feature in case of aliphatic 1,2-diamines (Entry 2), with a few exceptions (Entry 6), they are not observed in case of aromatic binucleophiles or diaminomaleonitrile 1060. In a number of cases, the reactive 1,2-bis-electrophile is generated in situ or using one-pot procedure (Entries 7, 9, 12 (see also earlier work [646]), 13, 15). Apart from 1061–1066, other fluorinated 1,2-bis-electrophiles were involved into reactions with 1,2-diamines (Table 24, Entries 18–26). Some of these bis-electrophiles can be considered as synthetic equivalents of 1061–1066. In particular, epoxide 1086 can be used instead of trifluoromethyl glyoxal 1064 (Entry 18), whereas oxime 1087 – as a replacement for fluorinated α-halo-β-dicarbonyl compounds 1065 (Entry 19). Other 1,2-bis-electrophiles give an access to rather unusual pyrazine-derived structures. For example, adduct 1089 is obtained upon reaction of malonodinitrile derivative 1088 with o-phenylenediamine (Entry 20) (upon prolonged reaction times, however, malonodinitrile is eliminated from 1089). In the reaction of imine 1090 with ethylenediamine, double formation of the pyrazine rings occurs (Entry 21), whereas in the case of chromone derivative 1091, recyclization is observed (Entry 24). Another recyclization – a variation of Yur’ev reaction – was found in the case of ethylenediamine and furane derivative 1092 (Entry 25).

Table 43 Synthesis of pyrazines from 1,2-diamines and fluorinated 1,2-bis-electrophiles

Full size table

7.7.2 Other Methods

An interesting method for preparation of fluorinated quinoxaline N,N'-dioxides 1094 relies on reaction of benzofuroxanes 1093 with fluorinated β-dicarbonyl compounds – a fluoro version of the so-called Beirut reaction (named after the city where it was discovered) (Scheme 232). The reaction has attracted some attention due to the products 1094 revealed high antitumor and anti-trypanosomatid activity [665–667]. The method gave satisfactory results when at least one of the starting components was symmetric; otherwise, the reaction was not always regioselective. The approach was also used for simple fluorinated ketones; in this case, the corresponding products 1095 were obtained in low to moderate yields (16–48 %) [668].

Pictet-Spengler-type reaction of pyrrole-derived amine 1096 and enamines 1097 was used recently for the preparation of pyrrolo[1,2-a]pyrazine derivatives 1098 – a [5+1] approach to the construction of pyrazine ring (Scheme 233) [669].

A rare example of [3+3] retrosynthetic disconnection of fluorinated pyrazine ring was implemented by dimerization of fluorinated formamidine 1099 (Scheme 234) [670]. Another example is dimerization of azirine derivative 1101 [671].

Fused tricyclic pyrazine derivative 1104 was prepared by intramolecular cyclization of amide 1103 under rather drastic conditions (P₂O₅, POCl₃, 180 °C in autoclave) (Scheme 235) [672]. It should be noted that for the synthesis of trifluoromethyl analogue 1106, direct trifluoromethylation was used instead of this cyclization (see Sect. 6.1.1).

o-Iodoaniline derivatives 1107 and 1109 were the key intermediates for the preparation of fused tricyclic pyrazines 1108 and 1110. To obtain 1108, Friedel-Crafts acylation was used, followed by intramolecular cyclization based on Pd-catalyzed arylation (Scheme 236) [404]. A tandem azide click reaction – Ullman-type intermolecular coupling allowed for the construction of tricyclic system 1110 [673]. Bromo and chloro analogues of 1109 were also used to synthesize 1110, but they were less effective.

7.8 Inverse-Electron-Demand Diels – Alder Reaction with Fluorinated Building Blocks

An unusual approach to the synthesis of chain-fluorinated diazines relies on the inverse-electron-demand hetero-/retro-Diels – Alder (ihDA/rDA) sequence. The background of this method for the preparation of nitrogen-containing heterocycles in general has been reviewed recently [674]. Typical dienes used for the synthesis of chain-fluorinated diazines are given in Fig. 29. Since electron-deficient dienes are necessary for the first step of the sequence – inverse-electron-demand hetero-Diels – Alder reaction, fluoroalkyl substituents of tri- and tetrazines 1111–1112 are favorable for the process. Typical electron-rich dienophiles for the reactions with 1111–1112 are enamines (including amino heterocycles) and alkynes, although other examples are also known.

3,6-Bis(trifluoromethyl)-1,2,4,5-tetrazine (1111) and its homologue 1112 are extremely reactive hetero-dienes towards Diels – Alder reaction. Compounds 1111 and 1112 were prepared by reaction of oxadiazole 1114 [675] or perfluoropropene 1116 [676], respectively, with hydrazine, followed by oxidation (Scheme 237).

ihDA/rDA sequence with 1111 or 1112 and CC-dienophiles is accompanied with elimination of molecular nitrogen and results in formation of pyridazine derivatives – a process which is known in 1,2,4,5-tetrazine chemistry as Carboni – Lindsey reaction [677]. In fact, this reaction was discovered by Carbony and Lindsey when they studied chemical properties of 1112 (among some other sym-tetrazines) [678]. The method worked effectively with various non-functionalized alkenes (Table 44, Entries 1–3), including strained ones (Entry 4), to give dihydropyridazines. Aromatic pyridazine derivatives were formed in reactions with alkynes (Entry 5); the procedure showed high functional group tolerance (Entry 6) and was used for the preparation of nucleoside analogues (Entry 7). Even benzene underwent [4+2] cycloaddition with 1111, although under harsh conditions (Entry 8); notably, in the case of substituted and fused benzene derivatives, the reaction demonstrated regioselectivity (Entry 9). Analogous results were obtained in the case of heteroaromatic compounds (Entries 10 and 11), although in some cases, ring opening of the aromatic ring occurred. Enol ethers and enamines are especially good dienophiles, which were used in a number of preparative syntheses (Entries 13–15). Unlike usual alkenes, these dienophiles gave aromatic pyridazines due to elimination of the leaving group (alkoxy or dialkylamino) under reaction conditions.

Table 44 Carboni – Lindsey reactions of 426 and 427

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Sym-triazine derivatives of general formula 1113 can be prepared by trimerization of the corresponding perfluorinated nitriles 1118 (Scheme 238) [687]. Compounds 1113 are much less reactive towards cycloaddition reactions than 1111 or 1112; it is not surprising therefore that the first reports on such transformations were made in early 2000s. In particular, reaction of 5-amino-1-phenyl-4-pyrazolecarboxylic acid (1119) with 1111 resulted in a tandem decarboxylation – ihDA/rDA sequence with formal elimination of the perfluorinated nitrile 431 and ammonia to give pyrazolo[3,4-d]pyrimidine derivatives 1120 (Scheme 239) [688].

A wide range of amino heterocycles was introduced into reaction with 1111 (Table 45), including pyrazoles (Entry 1), pyrroles (Entry 2), furans (Entry 3), indoles (Entry 4), thiophenes (Entry 5), imidazoles (Entry 6), push-pull enamines (Entry 7) and even anilines (Entry 8). The method was also used for the synthesis of nucleoside analogues (Entry 9). Moreover, it was shown that amino imidazoles can be generated in situ in the reaction mixture containing 1111 for the preparation of fluorinated purines (Scheme 240) [689]. The latter procedure worked well for aliphatic amines and hydrazines (including those containing additional basic center); by using TMSOTf catalyst, it was also extended to aromatic and heteroaromatic amines.

Table 45 ihDA/rDA sequence with 428

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The mechanism of ihDA/rDA reaction of 1111 with aminoheterocycles was studied extensively in the pyrrole series (Scheme 241) [694, 695]. It was shown that formation of “Diels – Alder” adduct of the type 1125 is nonconcerted; instead, the reaction starts as aromatic nucleophilic substitution to give Meisenheimer complex 1123 as an initial intermediate. Then, cyclization of 1123 leads to the formation of 1125. Decomposition of 1125 occur as retro-[4+2] cycloaddition to form intermediate of the type 1129. Aromatization of 1123 occurs via elimination of perfluorinated amidine, which was detected among the products of the reaction.

8 Properties and Chemical Transformation of CFD

In this part of the chapter, chemical properties of chain-fluorinated diazines are discussed. Since both diazine ring and fluoroalkyl group are electron-withdrawing, it is not surprising that most of the transformations discussed herein are reactions with nucleophiles. Diazine rings, namely, carbon atoms of the C=N double bonds, are common centers of the nucleophilic attack. Depending on the nucleophile and presence of the nucleofuge, the result of the reaction can be nucleophilic addition, or nucleophilic substitution. The reaction can occur not only with common nucleofuges like halogens, but with fluoroalkyl group itself. Fluoroalkyl substituent can also act as electrophilic center; in this case, nucleophilic substitution of fluorine occurs, which is promoted by electron-withdrawing diazine ring. Other reactions to be discussed are electrophilic substitution, metallation, reduction, oxidation and recyclization of the diazine ring, transition metal-catalyzed cross-couplings, photochemical cycloadditions as well as electron-demanding Diels-Alder reactions. Most of the examples will be taken from pyrimidine series since their chemistry is studied more thoroughly; chemical transformation of other side-chain fluorinated diazines will be discussed occasionally.

8.1 Addition of Nucleophiles to C=N Double Bond

8.1.1 Formation of Hydrates and Other Solvates

In the previous sections discussing synthesis of chain-fluorinated diazines by heterocyclizations, there were many examples of formation of di-, tetra- and hexahydropyrimidines – so-called hydrates (or other solvates) – instead of the corresponding aromatic products. This situation was quite common for the preparation of chain-fluorinated pyrimidinones or fused pyrimidines possessing an additional electron-withdrawing group, as well as Biginelli-type adducts (Fig. 30, see also Sect.7 of this chapter).

Moreover, these adducts were often so stable that they underwent water elimination only upon heating and/or action of dehydration agents. Nevertheless, in most cases the stability of these covalent hydrates was kinetic rather than thermodynamic, and the corresponding reverse reaction, i.e. formal addition of water or alcohols to the C=N bonds of the diazine ring are not common. The first observation of this type was made in 1990 by Lee and Sing, who noticed surprisingly high solubility of pyrimidine 1143 in aqueous alkali. More detailed investigations showed that a mixture of two adducts 1144 and 1145 were formed from 1143 in either aqueous sodium hydroxide or methanolic sodium methoxide-d ₃ solutions (Scheme 242) [696]. Whereas with OH^–, a 1:1 mixture was formed, in the case of CD₃O^– ion, regioselectivity of the reaction was observed, presumably due to its higher steric volume.

Another example was also found serendipitously: upon demethylation of pyrimidine derivative 1144 with HBr/AcOH, covalent hydrate 1146 was obtained in 72 % yield (Scheme 243) [468]. The method was extended to some other substrates (1145 and 1148).

Formation of adducts with solvent was observed for the pyrimidine derivatives 1150 upon their recrystallization from methanol (Scheme 244) [697]. Unlike the previous example, in this case the reaction was reversible, since the adducts 1151 gave pyrimidines 1150 upon heating.

8.1.2 Addition of C-Nucleophiles

Addition of carbanions to C=N bond in chain-fluorinated diazines received considerable attention in the quinazoline series, since the products obtained in this reaction are intermediates in the synthesis of HIV 1 reverse transcriptase inhibitors (see Chap. 20). In particular, quinazolines 1152 react with acetylenides in the presence of BF₃⋅Et₂O in THF to give adducts 1153 in moderate to excellent yields (Scheme 245) [698–700].

The method was extended for the preparation of optically pure compounds. In the first strategy, chiral auxiliary approach was used; namely, camphanoyl and α-phenyletylamine auxiliaries were introduced, the latter being more productive (Table 46) [701, 702]. Although quinazoline 1155 had limited stability, they could be generated in situ prior the reaction with the nucleophile. Notably, a wide range of nucleophiles was studied in this reaction, including organolithium, organomagnesium compounds and even methanol; nevertheless, lower chemical yields and/or diastereoselectivities were observed in many cases. The method was amendable to kilogram preparations.

Table 46 Diastereoselective addition of nucleophiles to C=N bond of quinazoline 1155

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Alternative strategy for the preparation of enantiopure quinazolines of the type 9 relied on enantioselective moderation. It was found that in the presence of carene-derived chiral moderator 1157, addition of cyclopropylethynyl lithium to quinazoline 1158 occurs in high yield and good enantioselectivity (Scheme 246) [703]. It should be noted that a number of other amino alcohols were evaluated as chiral additives in this reaction [704]. The mechanism of this transformation includes formation of mixed aggregates of the type 1160–1162 (Fig. 31) [704]. Even more complex aggregates are formed upon reaction of 1160–1162 with lithium salt of the quinazoline substrate [705].

Completely different reaction conditions for the synthesis of enantiopure quinazolines 1153 relied on Lewis acid catalysis. In particular, treatment of quinazoline 1163 with cyclopropyl acetylene and Zn(OTf)₂ in the presence of chiral additive 1164 (Scheme 247) [706] was extended to enantioselective diynylation of quinazolines [707]. An example of using organocatalysis included enantioselective Mannich-type reaction of 1166 or its analogues with ketones in the presence of chiral diamine 1167 and L-dibenzoyltartaric acid (L-DBT) (Scheme 248) [708]. In the latter case, the enantioselectivity was moderate, it might be improved to >99 % by a single recrystallization of the product.

Apart from quinazolines, quinoxalines 1169 were successfully introduced into the reaction with lithium acetylenides to give adducts 1170 (Scheme 249) [709].

8.2 Nucleophilic Substitution at the Diazine Ring

8.2.1 Substitution of Common Nucleofuges

Nucleophilic substitution of common nucleofuges such as halogenes is one of the most well-studied reactions in the chain-fluorinated diazine series. Analysis of the literature data shows that nearly 90 % examples of chain-fluorinated halodiazine reactions with N-, S-, and O–nucleophiles refer to pyrimidine derivatives (Table 47). Only 2- and 4-fluoroalkyl-5-halopyrimidines have received almost no attention in these transformations. Data on nucleophilic substitution of halogens in chain-fluorinated diazines correlates with the accessibility of the corresponding substrates, and to a lesser extent – with their reactivity towards nucleophiles.

Table 47 Reactions of chain-fluorinated halodiazines with nucleophiles (Reaxys®)

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The reactivity of the chain-fluorinated halodiazines towards nucleophiles is to a considerable extent similar to that of the corresponding non-fluorinated analogues. In particular, chain-fluorinated 2(6)- and 4-halopyrimidines are the most reactive substrates for the nucleophilic attack, so that very mild reaction conditions are possible (Table 48, Entries 1–4, 6, 8). In the case of less reactive substrates, very harsh reaction conditions (e.g. heating or MW irradiation at 140–180 °C) still can promote “classical” nucleophilic substitution (Entries 9, 11, 13, 14, 16, 18), although using palladium or copper catalysts might be more convenient (Entries 10, 12).

Table 48 Typical reaction conditions for the nucleophilic substitution in CFD

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Due to their electron-withdrawing inductive effect, the fluoroalkyl substituents activate substitution at α- and γ-positions. Although this activation does not overcome the effect from the nitrogen atoms of the diazine ring, it may define regioselectivity of the reaction in certain cases. In particular, reaction of 3,6-dichloro-4-fluoroalkylpyridazines 1171, 1172 with hydrazine or sodium methoxide results in nucleophilic substitution at C-4 (Scheme 250) [723, 725]. On the contrary, reaction of 3,4-dibromo-6-trifluoromethylpyridazine 1177 with NaOMe leads to the formation of 4-substituted derivative 1178 – a usual regioselectivity observed for the non-fluorinated analogues (Scheme 251) [727].

Several reports deal with nucleophilic substitution in chain-fluorinated 2,4-dichloropyrimidines. It is widely accepted that reaction of 2,4-dichloropyrimidines with nucleophiles occurs first at C-4 atom of the diazine ring, and the corresponding products can be obtained with high regioselectivity. In the case of 2,4-dichloro-5-trifluoromethylpyrimidine 1179, nearly 1/1 mixtures of the corresponding regioisomers are obtained (Scheme 252) [728, 729]. The situation is changed if the reaction is carried out in the presence of a Lewis acid (ZnCl₂); in this case, substitution at C-2 atom occurs regioselectively. These features were addressed to the increased steric demands at C-4 provided by the fluoroalkyl group. Both aromatic and aliphatic amines [728], as well as thiolates were successfully introduced into the latter transformation [730], although in the case of aromatic amines capable of zinc coordination, as well as aliphatic amines, 2 equivalents of ZnCl₂ were necessary to ensure high regioselectivity. However, the latter conditions in most cases led to the diminished yields of the products (28–33 % instead of 72–95 %) and prolonged reaction times.

For 2,4-dichloro-6-trifluoromethylpyrimidines, usual regiselectivity was observed in nucleophilic substitution reactions, namely, preferential attack of the nucleophile at C-4 atom (Scheme 253) [731, 732].

The reactions of the halogenated chain-fluorinated diazines with C-nucleophiles are less studied in comparison with N, S, O– derivatives. The most actively used transformation is chlorine-cyanide exchange in a case of 4-chloro substituted pyrimidines (Table 49). In a case of nucleophilic catalysis by DMAP or DABCO the yields are in region 50-93 %. Without nucleophilic catalysis the yields of the cyanation decreased extremely (Table 49, Entry 11).

Table 49 Chlorine-cyanide exchange in 4-chloro chain-fluorinated pyrimidines

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In a case of 2-chloro substituted chain-fluorinated pyrimidines the cyanation is also described on two examples in “classical” variant as well as palladium catalyzed conditions (Scheme 254) [735, 736].

Among other common C-nucleophiles only malonate and hetarylacetonitriles derivatives were used. 2-Chloro as well as 4-chloro chain-fluorinated pyrimidines 1193 and 1195 gave the corresponding pyrimidinylacetic acid derivatives 1194 and 1196 depicted on Schemes 255 [737, 738] and 256 [738–740].

Recently Vanelle reported the first example of a S_NAr reaction using TDAE-initiated carbanions in fluorinated quinazoline series. The o-nitrobenzyl carbanion 1200, formed by the action of TDAE on o-nitrobenzyl chloride 1197, reacts with 4-chloro-2-trifluoromethylquinazoline 1198 via a S_NAr mechanism affording 4-benzyl-2-trifluoromethylquinazolines 1199. The reaction as electron withdrawing group-dependent and in a case of non-fluorinated analogue of 1198 does no work (Scheme 257) [741].

Another unusual S_NAr reaction of fused trifluoromethylchloropyrimidines was disclosed in a course of human adenosine A_2A receptor antagonists discovery [742]. This is the reaction of aroylation of 2-chloro-4-trifluoromethylthieno[3,2-d]pyrimidine 1201 by aldehyde incorporation catalyzed by N,N-dimethylimidazolium chloride affording the ketoaryl compound 1202. The process based on S_NAr reaction of zwitterionic intermediate 1203 with followed dimethylimidazolium elimination. Unfortunately the exact procedures does not refer in original paper but reported in Vernalis patent (Scheme 258) [743].

Also Friedel–Crafts-type reaction of 2-naphthol with 4-chloro-2-trifluoromethylquinazoline 1198 was recently reported Guiry. The reaction conditions employed 3 equiv. of AlCl₃ at 80 °C in DCE for 3.5 h. In the condition quinazoline 1198 gives 4-(2-hydroxynaphthalen-1-yl)quinazoline 1205 in quantitative yield. The latest compound is useful intermediate for the synthesis of atropisomeric P–N ligand, Quinazolinap, which has been successfully applied to the rhodium-catalyzed hydroboration of vinylarenes and palladium-catalyzed allylic alkylation (Scheme 259) [744].

8.2.2 Addition with Elimination of the Fluoroalkyl Substituent

In principle, addition of a nucleophile to the C=N bond of the diazine ring can be accompanied by elimination of fluoroalkyl substituent. Two-step version of this reaction was used in the synthesis of alkaloid rutaecarpine (1210). In particular, reaction of anhydride 1206 with trifluoroacetic anhydride and then – with tryptamine led to the formation of quinazoline 1208, which was transformed to 1210 with elimination of trifluoromethane upon acid-catalyzed cyclization, followed by alkaline hydrolysis (Scheme 260) [744].

Elimination of trifluoromethane was undesirable reaction which was observed during attempted stereoselective synthesis of HIV 1 reverse transcriptase inhibitors via intermediate 1154; it occurred upon treatment of 1154 with bases (Scheme 261) [702].

The reaction is particularly illustrative in the case of 6,7-bis(trifluoromethyl)-8-ribityllumazines 1212, which were evaluated as potential inhibitors of lumazine synthase. It was found that both diastereomers of 1212 slowly eliminate trifluoromethane in neutral aqueous solutions above 37 °C giving 6-(trifluoromethyl)-7-oxo-8-ribityllumazine 1213; at 60 °C, half-life of 1212 was 15 min (Scheme 262) [745]. Interestingly, the reaction was catalyzed by lumazine synthase, but only for one diastereomer (1212). A mechanistic rationale for this stereoselectivity was proposed from the data obtained by 2D NMR data [746].

8.3 Transformation of Fluoroalkyl Substituent in CFD

Despite the widely accepted opinion that fluoroalkyl substituents in aromatic rings are chemically stable and rarely susceptible towards nucleophilic attack, a number of reactions of chain-fluorinated diazines at α-carbon of the fluoroalkyl moiety can be found. In fact, the first examples of such transformations were reported in 1960s [747]; they concerned an unusually easy hydrolysis of 5-trifluoromethyluracil and its derivatives upon warming in alkaline media (Scheme 263). Mechanism of the reaction was proposed, which included elimination of fluoride from anionic species 1214 and 1215, formed either by deprotonation of the substrate or addition of hydroxide ion [748]. Additional (but similar) reaction pathways were also possible if more acidic NH protons were present in the molecule of the substrate.

It was shown that other nucleophiles can undergo analogous reactions with 5-trifluoromethyluracil or its derivatives, e.g. amines (methoxyamine [749]), bisulfite [750], and NH₄OH (to form 5-cyano-2′-deoxyuridine) [751]. Recently, this reaction was used for the synthesis of various heterocycles containing uracil moiety 1216–1219 (Scheme 264) [752].

Hydrolysis of trifluoromethyl group was accomplished in a different type of substrates, namely, tetrazolyl-substituted pyrimidines 1220 affording pyrimidine carboxylic acids 1221 (Scheme 265) [471]. Although support from the tetrazolyl moiety was stated, no explanation for this effect was proposed by the authors.

Methanolysis of trifluoromethyl group in fused pyrimidine derivative 1222 was reported; in this case, orthoester 1223 was obtained (Scheme 266) [753].

Apart from nucleophilic substitution in the trifluoromethyl-substituted diazines discussed above, transformations related to mono- and difluoromethyl groups were also mentioned in the literature. In particular, hydrolysis of 5-difluoromethyluracyl and its derivative to form 5-formyl uracils was described [754]; the proposed mechanism reaction was analogous for the corresponding trifluoromethyl analogue. Nucleophilic substitution in fluoromethyl derivative 1224 was reported; in this case, N-acetylcysteine was acting as S-nucleophile (Scheme 267) [755].

In another work, fluoromethyl group in pyrimidines 1225 acted as CH-acid in an intramolecular condensation with carbonyl compound, leading to the formation of 8-fluoro-pyrrolo[1,2-a]pyrimid-4-one derivatives 1226 (Scheme 268) [358, 756].

Another type of reactivity was disclosed for difluorochloro group in pyrimidine series. Recently Iaroshenko, Langer and co-workers shown that difluorochloro substituted pyrimidines 1227 can be converted into corresponding difluoromethyl pyrimidines 1228 by radical reduction with tributyltin hydride in the presence of AIBN in moderate yields. In addition, CF₂Cl-substituted pyrimidines 1227 were transformed to the corresponding 1,1-difluorobut-3-enyl pyrimidines 1129 by reaction with allyltributyltin and AIBN. In this case the yields of transformation are lower in comparison with tributyltin hydride reduction (Scheme 269) [412].

1,1-Difluorobut-3-enyl substituent is useful fragment for further transformation. Fustero with co-workers used the fragment for RCM reactions in synthesis of fused fluorinated uracils. Starting uracils in this case were synthesized not by CF₂Cl function transformation but by cyclization based on α,α-difluoro-4-pentenenitrile 1230 served as the starting materials. Nitrile 1230 reacted with ester enolates at −78 °C to afford intermediate β-enaminoesters, which could then be reacted with several isocyanates in the presence of sodium hydride in DMF-THF as solvent. In this way, several intermediate uracils 1231 were prepared in good yields. Than allyl acetate was used as alkylating agent in the presence of Pd(0) as catalyst to provide the N-allyl derivatives 1232. Finally, these dienes reacted with the first generation Grubbs catalyst to afford the bicyclic seven-membered derivatives 1233 in excellent yields (Scheme 270) [757, 758].

A slightly different strategy was used for the synthesis of the C5-C6 fused bicyclic uracils. The reaction of pentenoic and butenoic ester enolates with nitrile 1230 initially provided β-enamino esters 1234, which then reacted with isocyanates to afford C5-C6 disubstituted uracils 1235 in variable yields. These uracils were transformed into the new family of fused bicyclic six- and seven-membered uracils 1236 by means of treatment with first generation Grubbs catalyst under the same conditions as described above, also in good yields (Scheme 271). These new families of uracils 1233 and 1226 were tested on acaricidal activity against Tetranychus urticae. Preliminary results showed that the best results for these compounds were slightly inferior than those for Tehufenpyrad [757, 758].

8.4 Electrophilic Substitution and Metalation at the Diazine Ring

Electrophilic substitution at the aromatic ring of chain-fluorinated diazines is rather unfavourable due to their electron-deficient nature. It is possible however when electron-donating substituents are also present in the diazine ring. For example, Shlösser reported successful bromination of pyrimidone 1237 with molecular bromine (Scheme 272) [715]. Halogenation of chain-fluorinated pyrimidine 1239 with SO₂Cl₂–FeCl₃ was also reported (Scheme 273) [759].

An alternative approach for introducing electrophilic species into diazine ring relies on metalation. The first report about metalation CFD was made in 1997 by Queguiner using 2-thiomethyl-4-trifluoromethylpyrimidine 1241 [760]. In case of alkyllithium as metalating agent in THF at −100 °C, only 6-alkyl derivatives 1242 were obtained as a result of the nucleophilic addition at C6. To avoid the nucleophilic addition, lithium alkyl amides were tested in the reaction. In spite of electron-withdrawing effect of CF₃-group which favors ortho-lithiation, the steric hindrance of the group alters the orientation in this case. With excess of LTMP in THF at −100 °C metalation occurs at the C6, whereas with weaker base LDA in similar conditions only starting material was recovered. In a case of using 1.1 equivalent of LTMP the starting material was recovered with dimeric product 1244. The formation of 1244 assumed that the metalation is slow or incomplete and 1241 underwent the nucleophilic attack from 1245. To prevent the competitive reaction 4-fold excess of LTMP was used, so deuteriated compound 1243 was obtained without starting material, but small amounts of dimer 1244 were always present (Scheme 274, Table 50).

Table 50 Lithiation of 4-trifluoromethylpyrimidine

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To avoid nucleophilic addition, a metalation/in situ trapping was used with a set of electrophiles. The simultaneous introduction of the electrophile and the compound 1241 prevent the dimer formation. Reaction of the lithioderivative 1245 with iodine or hexachloroethane as electrophiles gave 6-halopyrimidines in low yield whereas moderate or good yields observed for carbonyl compounds trimethylsilyl chloride and diphenyl sulphide (Scheme 275, Table 51) [760].

Table 51 Lithiation of 4-trifluoromethylpyrimidine by in situ trapping method

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Nine years after Queguiner paper the Shlösser group have studied lithiation of another trifluoromethyl-substituted pyrimidines [761]. According to in situ trapping method for pyrimidine 1247 with trimethylsilyl chloride no silylated pyrimidine was detected in the reaction mixture, only dimer 1248 was isolated in low yield. But consecutive treatment of 1247 with butyllithium in toluene at −90 °C leads to bromo/lithium permutation. Subsequent reaction with carbon dioxide followed by neutralization and esterification with diazomethane afforded methyl 4-(trifluoromethyl)pyrimidine-2-carboxylate 1250 in 36 % yield (Scheme 276).

On the other hand, excellent results were achieved with 2,4-dihalo-6-(trifluoromethyl)pyrimidines 1251. The corresponding pyrimidine-5-carboxylic acids 1253 were isolated in good preparative yields (Scheme 277).

Another 5-metalated 4-trifluoromethylpyrimidines could be generated by halogen-lithium exchange. The corresponding derivative 1254 was lithiated at 5-th position via iodine-lithium exchange affording intermediate 1255, which was converted to acid 1256, but the yield was low. In contrast, clean reactions were encountered with 5-bromo-4-chloro-6-(trifluoromethyl)pyrimidine 1257a and 4,5-dibromo-6-(trifluoromethyl)pyrimidine 1257b as the substrates when isopropylmagnesium chloride in diethyl ether and, respectively, butyllithium in toluene were employed as the exchange reagents. 4-Chloro and 4-bromo substituted 6-(trifluoromethyl)pyrimidine-5-carboxylic acids were isolated in 73 and 54 % yield respectively. The rigorous discrimination between the two bromine atoms by the Grignard reagent is observed (Scheme 278) [761]. Notably, the halogen atoms in the molecules of 1259 a,b could be removed by catalytic hydrogenation.

The bromine-lithium exchange by butyllithium in CFD began to find industrial application. Kumiai Chemical Industry and Syngenta used the lithiation-formylation sequence for the synthesis of the corresponding aldehydes in herbicides development programs (Scheme 279) [713, 762]. Both methyl formate and DMF were used as formilating agents to afford aldehydes 1262 in good yields.

Also the bromine-lithium exchange in 4-bromo-2-trifluoromethylquinazoline 1263 was described using butyl lithium. The corresponding litho-derivative 1264 was entered into reaction with benzaldehyde to give alcohol 1265 (Scheme 280) [742]

Besides pyrimidine derivative, as to the best of our knowledge, only one example is described for another CFD. Janseen in 2008 described DoM reaction of pyridazine 1266 with LTMP followed by iodination affording iodo-derivative 1268 in 82 % preparative yield (Scheme 281) [724].

8.5 Transition Metal-Catalyzed Cross-Couplings

Unlike nucleophilic substitution with N-, S-, and O–nucleophiles discussed in Sect. 8.2.1 of this chapter, transition metal-catalyzed C–C couplings are not well-documented in chain-fluorinated diazine series. Almost all examples deal with palladium-catalyzed cross-coupling involving chain-fluorinated halodiazines and the corresponding organoelement compounds (e.g. Suzuki, Stille, Negishi or Kumada reactions), alkenes (i.e. Heck reaction), or alkynes (i.e. Sonogashira reaction) (Table 52). The reaction conditions are quite common for the analogous transformations involving aryl halides (Table 53).

Table 52 Reaxys® data for the C–C cross-couplings of chain-fluorinated halodiazines

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Table 53 Typical reaction conditions for the C–C couplings in chain-fluorinated diazines

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So far, the only reported example of organoelement compound derived from chain-fluorinated diazines used in C–C couplings is boronate 1270, prepared from bromo derivative 1269 (Scheme 282) [787]. Compound 1270 was successfully introduced intro Pd-catalyzed coupling with chloride 1272 to give the product 1273 in good yield (95 %).

8.6 Reduction and Oxidation of the Diazine Ring

Usually nuclei of CFD are stable to common reduction agent such as complex metal hydride (NaBH₄, LAH etc.) and metals in low oxidation state (SnCl₂, Fe etc.) which allow to made different transformation of functional groups in these compounds leaving the ring of CFD intact. But in literature there are rare examples of reduction of the CFD nuclear by NaBH₄. Recently Vovk with co-workers shown that due to the alternation of bonds in the ring pyrimidones 911 (see Scheme 189) react with NaBH₄ in methanol at room temperature to give quantitatively a mixture of two stable isomeric tetrahydropyrimidines, 1274 and 1273, with the predominance of the latter product as a result of 1,4-reduction of the endocyclic conjugated double bonds. 2-Oxo-1,2,3,4-tetrahydropyrimidine-5-carboxylates 1273 can be isolated in pure form by double recrystallization from ethanol (Scheme 283) [550].

Also the NaBH₄ reduction of fused pyrazolo[1,5-a]pyrimidines was disclosed in a course of discovery of antitubercular agents and novel structural class of potent calcium-sensing receptor antagonists. The reduction proceeds also in mild condition giving diastereoselectively desired tetrahydropyrazolo[1,5-a]pyrimidines in good preparative yields (Scheme 284) [418, 778].

Besides pyrimidines one example of NaBH₄ reduction described for 4-trifluoromethylpyrimido[4,5-c]pyridazines 1050 (see Scheme 224), which reacted with NaBH₄ giving corresponding dihydroderivatives 1277 in good yields (Scheme 285) [628].

Another agent used for CFD nuclear reduction is triethylsilane in the presence of trifluoroacetic acid. In a case of pyrimidine 1278 (R=t-Bu) the reaction leads to cyclic guanidine 1279 in 30 % yield [789]. The better result in similar transformation gives catalytic hydrogenation over palladium. In this case the preparative yield of guanidine 1280 is near to quantitative [780] (Scheme 286). In should be noted, that formation of cyclic guanidines under aminopyrimidines reduction is typical also for non-fluorinated analogues using triethylsilane as well as catalytic hydrogenation. The latest method also was used for tetrahydropyrazolo[1,5-a]pyrimidine 1281 synthesis. In this case dechlorination and pyrimidine reduction occurs by one step (Scheme 286) [418].

Reduction of fluoroalkylpyrazines could be a promising method for the synthesis of chain-fluorinated piperazines. This method was used for the synthesis of difluoromethyl- and (1,1-difluoroethyl)piperazines 1283; nevertheless, it was obtained in low yield [322, 781]. Therefore the synthesis of “parent” (trifluoromethyl) piperazine 1284 is based on 4 steps synthesis started from methyl trifluoropyruvate and N,N′-dibenzyl ethylenediamine (Scheme 287) [782].

Among oxidation of the diazine ring in CFD the major part of the reports deal with the oxidation of the partially unsaturated diazines. Thus, tetrachloro-1,4-benzoquinone (TCBQ) or 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) [611, 815], copper (II) halides [593, 784] and bromine [603, 785] were used as oxidizing agents. Also ozonolysis of chain-fluorinated diazines was reported. In particular, reaction of 5- and 6-trifluoromethyluracils with ozone led to the formation of hydantoins 1254 (Scheme 288) [786]. It was assumed that oxidative cleavage occurs at C5–C6 bond of the diazine ring, followed by hydrolysis and cyclization of the intermediate formed.

8.7 Recyclizations

Since ANRORC-type processes are characteristic for pyrimidine series, it is not surprising that there are some examples of recyclizations with chain-fluorinated pyrido[1,2-a]pyrimidin-4-ones 1255. In particular, heating of fused pyrimidines 1255 in diphenyl ether resulted in the formation of 1,8-naphtyridine derivatives 1256 (Scheme 289) [787, 788].

Another example includes rearrangement of quinazoline derivative 1257 into benzodiazepine 1258, described in a patent (Scheme 290) [789].

8.8 Inverse-Electron-Demand Diels-Alder Reaction

It was described in Sect. 7.8 of this chapter that chain-fluorinated diazines can be synthesized using inverse-electron-demand Diels-Alder reactions. Some of the fused pyridazines can also undergo analogous reactions with electron-rich alkenes. In particular, Diels-Alder reactions of pyridopyrazine 1259 were studied. It was found that 1259 reacted with enamines to give quinoline derivatives (e.g. 1260) (Scheme 291) [790]. Reaction of 1259 with ketene N,S-acetal 1261 led to a mixture of regioisomers 1262 and 1263, whereas reaction with N-methylindole gave complex mixture of products 1264–1267 (Scheme 292) [791].

Pyridazino[4,5-b]indole 1268 is another example of aza-diene which was successfully introduced into inverse-electron-demand Diels-Alder reactions with enamines. The reaction proceeds upon prolonged refluxing in 1,4-dioxan (Scheme 293) [792]. Notably, reaction of 1268 with acyclic enamine 1270 proceeded in a regioselective manner.

8.9 Photochemical [2+2] Cycloadditions

Chain-fluorinated diazines can undergo photochemical [2+2] cycloaddition with alkenes to give cyclobutane or azetidine derivatives. In particular, 1,3-dimethyl-5-trifluoromethyluracil 1272 reacted with naphthalene under UV-irradiation in the presence of piperylene preferentially underwent 1,2-cycloaddition to give cis-tetrahydronaphthocyclobutapyrimidine 1273 in high stereoselectivity [793]. It should be noted that similar reaction without piperylene pass through 1,4-cycloaddition affording an ethenobenzoquinazoline derivative 1274 as sole product of the reaction [794] (Scheme 294).

Also the reaction of 1,3-dimethyl-5-trifluoromethyluracil 1272 with isobutylene was studied. In this case nearly exclusively the head to-tail adducts 1274 formed, but the yield of transformation was extremely low (Scheme 295) [795].

In 2006 Aitken with co-workers developed a procedure of [2+2] cycloaddition, which in a case of trifluoromethyluracils and ethylene gave excellent preparative yields (Scheme 296). Based on cycloaddition product 1279 the synthesis of cyclobutane derived amino acid 1281 was elaborated [796].

Intramolecular [2+2] photochemical cycloaddition based on 2-trifluoromethylquinazolines was studied. Compounds 1282 afforded the corresponding [2+2] adducts 1283 in 90–95 yields even on irradiation at 350 nm. Treatment of the adducts 1283 by methanol containing borontrifluoride etherate leads to cleavage of the azetidine cycle leading to fused compounds 1284. In a case of treatment of the compound 1284 (n = 1) with DBU elimination of the CF₃-group occurs affording compound 1285 (see Sect. 8.2.2 of this chapter) (Scheme 297) [797].

Also itermolecular photochemical [2+2] cycloaddition based on 2-trifluoromethylquinazoline 1286 was studied. In a similar conditions the reaction with ethylene gives compound 1287 in 17 % yield as a sole product probably due to low solubility of ethylene in methanol. Ene-type product 1288 was isolated in 65 % yield when isobutylene was used in the reaction, showing that biradical intermediate is involved in the transformation. In a case of ethyl vinyl ether acetal 1292 was formed as product of methanolysis of intermediate azetidine 1289. Similarly was used intermediate azetidine 1290 was not isolated when dichloroethylene and its formation was proved by isolation of methanolysis product 1293 in 89 % yield. It should be noted, that treatment of product 1288 with base leads to elimination of CF₃-group as in a case of 1284 (Scheme 298) [797]

The same group of Japanese authors studied the photochemical reaction of fluorinated quinoxalines. Photochemical cycloadditions with quinoxaline derivative 1294 occurred and C=N double of the diazine ring, leading to the formation of azetidine derivatives (Scheme 299). The presence of trifluoromethyl group in the molecule of 1294 activated the substrate towards cycloaddion, so that even electron-deficient methyl methacrylate was introduced in the reaction [797]. In the case of ethyl vinyl ether as the alkene, the adduct 1296 also as in a case with 1289 was not stable and underwent azetidine ring-opening upon action of the solvent. Ketene was also successfully introduced in [2+2] cycloaddition with trifluoromethyl-substituted quinoxaline derivatives [797]

9 Conclusions and Outlook

Since discovery of the first fluorinated diazine – antineoplastic agent 5-fluorouracil more than 20 compounds from the class were introduced into the pharmaceutical and crop protection market. Also these compounds find industrial application as reactive component for the synthesis of reactive dyestaff in textile industry and as a component of liquid crystals. Besides industrial application fluorinated diazines appear excellent objects for theoretical investigations. Starting from Halex process and electrophilic fluorination of uracil fluorinated diazines still attract the attention of chemists working in different industries as interesting objects of study. Undoubtedly the success was achieved due to joint progress of medicinal chemistry, agrochemistry as well as synthetic methods of heterocyclic and fluoroorganic chemistry. But despite really the huge number of articles and patents in this field the chemical space covered by fluorinated diazines remains “white spots”. Thus, diazine scaffold decorated by important for medicinal chemistry and agrochemistry fluorinated fragments such as -CHF₂, -CH₂CF₃, -OCF₃, -SCF₃, -SF₅ were not investigated because the synthetic chemistry of these compounds is still on development phase or not developed at all. For example only in this year Yagupolskii with co-workers developed the first method of synthesis of 5-OCF₃ substituted pyrimidines [798]. Also the chemistry of organoelement (B, Si, Sn) derivatives of fluorinated diazines, able to transition metal catalyzed coupling reaction still remains almost unexplored, especially in a case of CFD. Therefore the comprehensive investigations in the field of fluorinated diazines still are interesting both for academic and industrial scientists.

Abbreviations

acac:: Acetylacetone
AcOH:: Acetic acid
AIBN:: Azobisisobutyronitrile
Amphos:: 2-(2,4,6-i-Pr₃-C₆H₂)–C₆H₄-PCy₂
ANRORC:: Addition of the nucleophile, ring opening, and ring closure in nucleophilic attack on ring systems
aq.:: Aqua is the Latin word for water
Bmim:: 1-Butyl-3-methylimidazolium hexafluorophosphate
Bn:: Benzil
Boc:: tert-Butyloxycarbonyl
(BPin)₂ :: Bis(pinacolato)diboron
Bz:: Benzoyl
CFD:: Chain-fluorinated diazines
CNC:: (N,N′-dimethylimidazolidino)tetramethylguanidinium chloride
COD:: Cyclooctadiene
Cy:: Cyclohexyl
DABCO:: Dimethylbenzylamine
DAST:: Diethylaminosulfur trifluoride
Dba:: Dibenzylideneacetone
DBN:: 1,5-Diazabicyclo[4.3.0]non-5-ene
DBU:: 1,8-Diazabicycloundec-7-ene
DCC:: N,N′-Dicyclohexylcarbodiimide
DCE:: Dichloroethane
DCM:: Dichloromethane
DDQ:: 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
Deoxo-Fluor:: Bis(2-methoxyethyl)aminosulfur trifluoride
DEAD:: Diethyl azodicarboxylate
DIPEA:: Ethyl diisopropyl amine
DMA:: Dimethylacetamide
DMAP:: 4-Dimethylaminopyridine
DME:: Dimethoxyethane
DMG:: Dimethylglyoxime
DFMS:: Zinc difluoromethanesulfinate
DMSO:: Dimethyl sulfoxide
DNPG:: Dinitrophenyl hydrazine
DoM:: Direct ortho-methalation
Dppf:: 1,1′-bis(diphenylphosphino)ferrocene
EDG:: Electron donating group
EWG:: Electron withdrawing group
5-FU:: 5-Fluorouracil
HATU:: 1-[Bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium 3-oxid hexafluorophosphate
HFA:: Hexafluoroacetone
HIV:: Human immunodeficiency virus
HMBC:: Heteronuclear Multiple Bond Correlation
HMPA:: Hexamethylphosphoramide
HMTA:: Hexamethylenetetramine
i-Am:: Isoamyl
ihDA/rDA :: Inverse-electron-demand hetero-/retro-Diels – Alder
KHMDS:: Potassium Hexamethyldisilazane
LAH:: Lithium aluminum hydride
LB:: Lithium tertbutyl-(1-isopropylpentyl)amide
LDA:: Lithium diisopropylamide
L-DBT:: L-dibenzoyltartaric acid
LTMP:: Lithium 2,2,6,6-tetramethylpiperidide
MeTFP:: Methyl trifluoropyruvate
MW:: Microwave
MNDO SCF MO:: Modified Intermediate Neglect of Differential Overlap is a semi-empirical method
NaHMDS:: Sodium Hexamethyldisilazane
NAS Ukraine:: The National Academy of Sciences of Ukraine
NBS:: N-Bromosuccinimide
NFSI:: N-fluorobenzenesulfonimide
NMP:: N-methylpyrrolidone
NOAc:: N4-Octadecylcytosine β-D-arabinofuranoside
O-TBDMS:: O-tert-butyldimethylsilyl
PEG-400:: Polyethylene glycol 400
PES:: Photoelectron spectroscopy
PET:: Polyethylene terephthalate
Phen:: Phenantroline
PhMe:: Methylbenzene
PPA:: Polyphosphoric acid
Ph:: Phenyl
py:: Pyridine
PM3:: Parameterized Model number 3 (a semi-empirical method)
RCM:: Ring-closing metathesis
RFD:: Ring-fluorinated diazines
SSCS:: Statistical substituent chemical shift
TBAF:: Tetra-n-butylammonium fluoride
TBS:: Tert-butyldimethylsilyl
TCBQ:: Tetrachloro-1,4-benzoquinone
TDAE:: Tetrakis(dimethylamino)ethylene
TEA:: Triethylamine
TEBAC:: Benzyltriethylammonium chloride
Tf:: Trifluoromethylsulfonyl
TFA:: Trifluoroacetic acid
TFAA:: Trifluoroacetic anhydride
TFMS:: Zinc trifluoromethanesulfinate
THF:: Tetrahydrofuran
THP:: Tetrahydropyran
TMSPP:: Trimethylsilyl polyphosphate
TMSBr:: Bromo(trimethyl)silane
Ts:: Tosy

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Biologically Active Compound Department, Institute of Organic Chemistry, National Academy of Sciences of Ukraine, Murmanska Street 5, Kyiv, 02660, Ukraine
Dmitriy M. Volochnyuk & Alina O. Gorlova
Department of Chemistry, National Taras Shevchenko University of Kyiv, Volodymyrska Street, 64, Kyiv, 01601, Ukraine
Oleksandr O. Grygorenko

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Dmitriy M. Volochnyuk
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Oleksandr O. Grygorenko
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Alina O. Gorlova
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Correspondence to Dmitriy M. Volochnyuk .

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Department of Chemistry, Moscow State University, Moscow, Russia
Valentine Nenajdenko

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Volochnyuk, D.M., Grygorenko, O.O., Gorlova, A.O. (2014). Fluorine Containing Diazines. Synthesis and Properties. In: Nenajdenko, V. (eds) Fluorine in Heterocyclic Chemistry Volume 2. Springer, Cham. https://doi.org/10.1007/978-3-319-04435-4_6

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DOI: https://doi.org/10.1007/978-3-319-04435-4_6
Published: 13 June 2014
Publisher Name: Springer, Cham
Print ISBN: 978-3-319-04434-7
Online ISBN: 978-3-319-04435-4
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Fluorine Containing Diazines. Synthesis and Properties

Abstract

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Recent advances in the synthesis and applications of oxazolo[5,4-d]pyrimidines (microreview)

Synthesis and Properties of New Fluorine-Containing Thieno[2,3-b]pyridine Derivatives

Fluorinated Triazoles and Tetrazoles

Keywords

1 Introduction

2 Ring Fluorinated Diazines

3 Synthesis

3.1 Substitution Reactions

3.1.1 Electrophilic Fluorination. Replacement of H by F

3.1.2 Nucleophilic Fluorination. Halogen Exchange Reactions

3.1.3 Nucleophilic Fluorination. The Balz–Schiemann Reaction

3.2 Cyclization Processes

3.2.1 “Principal Synthesis” of Pyrimidines

3.2.2 Miscellaneous Cyclization

4 Properties and Chemical Transformation

4.1 General

4.2 Nucleophilic Aromatic Substitution

4.2.1 General Remarks: Orientation and Reactivity

4.2.2 Application in Organic and Combinatorial Synthesis

4.2.3 Reactive Dyes

4.2.4 Acid-Induced Processes

4.2.5 Fluoride-Ion-Induced Reactions

4.3 Metalation Reaction

4.3.1 Fluoropyrimidines

4.3.2 Fluoropyrazines

4.3.3 Fluoropyridazines

4.4 Organometallic Compounds and Transition Metals Catalyzed Process

4.5 Miscellaneous Cyclizations and Rearrangements

4.6 Fluorination Reactions

5 Side Chain Fluorinated Diazines

6 Introduction of Fluorine-Containing Substituents into Diazine Core

6.1 Direct (Per)Fluoroalkylation

6.1.1 Perfluoroalkylcopper Reagents

6.1.2 Perfluorocarboxylic Acids or Their Derivatives

6.1.3 (Per)Fluoroalkylsulfinates

6.1.4 Other Methods

6.2 Nucleophilic Substitution with Fluoride

6.2.1 Substitution of Other Halogens

6.2.2 Deoxofluorination

6.3 Electrophilic Fluorination

7 Construction of Diazine Core Using Fluorine-Containing Building Blocks

7.1 Synthesis from α-Fluorocarboxylic Acids and Their Derivatives

7.1.1 CCC+NCN Approach to Pyrimidines (Principal Synthesis)

7.1.2 Other Approaches to Pyrimidines

7.1.3 Construction of Pyrimidine Ring of Quinazolines and Their Hetero-analogues

7.2 Principal Synthesis from Fluorinated β-Dicarbonyl Compounds and Their Analogues

7.2.1 Pyrimidines with Fluorinated Alkyl at C-4 from β-Dicarbonyl Compounds

7.2.2 Pyrimidines with Fluorinated Alkyl at C-4 from Enones

7.2.3 Other CCC Bis-electrophiles

7.2.4 Pyrimidines with Fluorinated Group at C-5 Position

7.3 CNC+CCN Approach to Fluoroalkyl-Substituted Pyrimidines

7.4 Multicomponent Synthesis of Fluorinated Pyrimidines

7.5 Miscellaneous Methods for the Preparation of Pyrimidines

7.6 Synthesis of Chain-Fluorinated Pyridazines

7.6.1 Synthesis from Fluorinated 1,4-Dicarbonyl Compounds and Their Analogues

7.6.2 Diaza-Wittig Reactions

7.6.3 NNCC+CC Approaches

7.6.4 Other Methods

7.7 Synthesis of Chain-Fluorinated Pyrazines

7.7.1 Synthesis from 1,2-Diamines and Fluorinated 1,2-Bis-Electrophiles

7.7.2 Other Methods

7.8 Inverse-Electron-Demand Diels – Alder Reaction with Fluorinated Building Blocks

8 Properties and Chemical Transformation of CFD

8.1 Addition of Nucleophiles to C=N Double Bond

8.1.1 Formation of Hydrates and Other Solvates

8.1.2 Addition of C-Nucleophiles

8.2 Nucleophilic Substitution at the Diazine Ring

8.2.1 Substitution of Common Nucleofuges

8.2.2 Addition with Elimination of the Fluoroalkyl Substituent

8.3 Transformation of Fluoroalkyl Substituent in CFD

8.4 Electrophilic Substitution and Metalation at the Diazine Ring

8.5 Transition Metal-Catalyzed Cross-Couplings

8.6 Reduction and Oxidation of the Diazine Ring

8.7 Recyclizations

8.8 Inverse-Electron-Demand Diels-Alder Reaction

8.9 Photochemical [2+2] Cycloadditions

9 Conclusions and Outlook