Abstract
The review summarizes the literature data on the synthesis, structure, reactivity, and rearrangements of pentacarboxycyclopentadienes and their derivatives. Their potential for creating new chiral organic catalysts for enantioselective Diels–Alder reactions, cationic polymerization of vinyl ethers, enantioselective protonation of silylenol ethers, aminomethylation, and other processes is described, and aspects of their use as effective carriers of functional groups, new ligand systems for the synthesis of metal complexes and donor-π-acceptor chromophores for organic photovoltaics are considered.
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INTRODUCTION | |
|---|---|
1. | SYNTHESIS AND STRUCTURE OF PCCP |
2. | CHIRAL ACID PCCP CATALYSTS OF ENANTIOSELECTIVE REACTIONS |
3. | PCCP-CATALYZED CONTROLLED CATIONIC POLYMERIZATION OF VINYL ETHERS |
4. | REARRANGEMENT OF ORGANIC AND ORGANOELEMENT GROUPS AND HALOGENS IN THE PCCP SYSTEM |
5. | EFFECTIVE CARRIERS OF FUNCTIONAL GROUPS BASED ON FLUCTUATING PCCP |
6. | TETRACARBOMETOXYCYCLOPENTADENYL LIGAND SYSTEMS AND THEIR METAL COMPLEXES |
7. | PUSH–PULL CHROMOPHORES WITH HYDRAZONE PCCP FRAGMENTS FOR ORGANIC PHOTOVOLTAICS |
CONCLUSIONS | |
INTRODUCTION
Pentasubstituted cyclopentadienes (Cp) with alkyl, aryl, benzyl, halogen-, cyano-, and other substituents in the five-membered ring are popular organic ligands which form various metal complexes widely used as catalysts for C–H bond functionalization [1–3], cycloaddition reactions [4, 5], olefin polymerization [6, 7], and other processes [8] and are also used to create materials for chemosensorics [9], molecular electronics [10–12], and spintronics [13, 14]. Such compounds are more stable than unsubstituted Cp derivatives and have a wide range of useful properties [15–25].
Pentakis(methoxycarbonyl)cyclopentadiene (1) has been attracting close attention since 1942, when it was first synthesized, and already in the 1960s, its main derivatives were obtained [26]. Due to the fact that 5 CO2Me substituents strongly stabilize the Cp anion, compound 1 is a strong organic acid, nearly as strong as HCl, and its stable anion forms salts that are stable in air and soluble in water. Such characteristics make compound 1 attractive for medicine and green chemistry. Its metal complexes with s-, p-, d-, and f-metals coordinated mainly to the carbonyl oxygen, as well as with soft metals (Re, Ru, Rh, and Au) coordinated to the carbon centers of the Cp ring were synthesized. However, in the literature on the synthesis and structure of cyclopentadiene 1 and its metal complexes, we found only one review [27]. Publications on Cp ligands and their metal complexes, including chiral ones, contain no information on cyclopentadiene 1 derivatives [28, 29].
At present, the interest in pentacarboxycyclopentadiene derivatives (PCCP) has increased significantly, which is associated with the discovery of their new properties and possibile use as enantioselective acidic Brønsted organocatalysts [30]. In addition, a wide range of circumambulating PCCP 1 derivatives were obtained and arylazo and thio derivatives of PCCP 1 were shown to act as efficient carriers of functional groups [31]. Chiral amidinyl-Cp ligand systems and their metal complexes were synthesized [32–34], and a great number of push–pull chromophores with an acceptor hydrazone Cp fragment were obtained, which can be used in organic photovoltaics [35]. The use of the PCCP anion in the synthesis of charge-transfer complexes with organic cations and radical cations for the needs of molecular electronics and for the creation of polymer ionic liquids with high ionic conductivity was demonstrated [36–38]. The potential of ruthenium complexes of PCCP 1 in anticarcinogenic drug design was revealed [39].
An attractive feature of PCCP 1 is that its CO2Me substituents are easily functionalized to form a wide variety of analogs. This direction is being actively developed, which has led to the preparation of new PCCP derivatives and the creation of effective organic catalysts for enantioselective Diels–Alder reactions, stereoselective aminomethylation, cationic polymerization, enantioselective synthesis of cyclic amines, and many other processes [40, 41].
1. SYNTHESIS AND STRUCTURE OF PCCP
The condensation of malonic ester 2 with 3 equiv of dimethyl acetylenedixarboxylate (3) in the presence of acetic acid and pyridine forms 2 isomeric cycloheptadienes 4 and 5, which transform into salt 6 under the action of potassium acetate (Scheme 1) [26].
1.
Acidification of an aqueous solution of salt 6 with HCl gives rise to PCCP 1.
Recently Radtke et al. [42] reported a one-stage version of this synthesis at room temperature in the presence of benzyltrimethylammonium chloride (BTMAC) as a catalyst; the yield of the target product was 48% (Scheme 2).
2.
According to the X-ray diffraction analysis (XRD), PCCP 1 exists in the crystalline state as 6-hydroxy-6-methoxy-1,2,3,4-tetrakis(methoxycarbonyl)fulvene [27], as well as other polycarbomethoxycyclopentadienes [43]. In a Cl2CDCDCl2 solution, according to 1H NMR data, PCCP 1 is also present in the fulvic form, which undergoes intramolecular prototropic tautomerism with an activation barrier of ~ 18–19 kcal/mol in the temperature range 60–90°C. At a concentration of PCCP 1 solutions of 0.51 M and higher, the dynamics of its 1H NMR spectra are concentration-dependent, on account of the intermolecular proton transfer with ΔG≠298 14.5 kcal/mol (in CD2Cl2), and in the highly polar DMSO it completely dissociates to form an anion [44].
Quantum-chemical calculations (DFT B3LYP/6-311++G**) showed that the fulvene form of PCCP 1 in the gas phase is energetically more favorable than Cp form 1A by ΔEZPE 7.8 kcal/mol [45]. The calculated energy barrier for thermally forbidden 1,7-O,C shifts 1 → TS (transition state) 7 → 1A (Scheme 3) is fairly high (ΔE≠ZPE 42.1 kcal/mol), while degenerate 1,5-H shifts over the periphery of the Cp ring 1A ←→ TS 8 ←→ 1A' ←→ … should have the ΔE≠ZPE barrier of 26.3 kcal/mol, a typical barrier for substituted cyclopentadienes [46].
3.
By contrast, the 1,9-O,O'-H shifts in fulvene 1 ←→ TS 9 ←→ 1' are extremely fast (ΔG≠298 1.2 kcal/mol), and the subsequent rotation about the C7=C8 bond 1' ←→ TS 10 ←→ 1'', leading to isomeric fulvene 1'', should have the ΔE≠ZPE barriers of 23.5 kcal/mol (gas) and 20.9 (CH2Cl2) kcal/mol (Scheme 4).
4.
Thus, the route involving hydrogen circular migration in fulvene 1 according to Scheme 4 (1 ←→ TS 9 ←→ 1' ←→ TS 10 ←→ 1''...) is energetically more favorable by 18.6 kcal/mol than the route according to scheme 3, and the calculated barrier for the former route is in good agreement with the experimental value [45].
Recently, the syntheses of pentacarboxycyclopentadienes, starting from compound 1, have been developed [30, 40]. By transesterification in one step, the CO2Me groups were functionalized to obtain a variety of aliphatic ester derivatives, and the treatment of PCCP 1 with amines gave the corresponding amides or diamides.
For example, heating l-menthol 11 with PCCP 1 under reflux in the presence of N-methylimidazole (NMI) yielded pentamenthyl derivative 12 in 96% yield (Scheme 5).
5.
The same procedure was used to prepare in high yields PCCPs on the basis of linear alkyl 13, alkenyl 14, and alkynyl 15 alcohols (Scheme 6).
6.
For example, a chiral derivative of terpineol (1S,2S,5S)-(–)-myrtanol 16 was synthesized in 97% yield, and the following derivatives of cyclic alcohols were prepared in good yields: 17 from CyOH, 18 from dicyclohexylmethanol, and 19 and 20 from chiral secondary alcohols. Even though such a large alcohol as cholesterol 21 were found possible to introduce in PCCP in 47% yield.
It was found that PCCP 1 converts into intermediate pentachloroanhydride 22 under the action of SOCl2 and a catalytic amount of DMF (Scheme 7).
7.
The addition of an excess of alcohol to chloroanhydride 22 led to the corresponding PCCP in moderate yield. This procedure was used to success to synthesize other PCCPs with alcohols inactive in transesterification reactions. The reactions with BnOH, (R)-1-phenylethanol, and (S)-benzylmandelate gave PCCPs 23–25. Phenol and ethyl l-lactate were introduced in this reaction to obtain compounds 27 and 28. PCCPs 29–31 were prepared in low yields in the presence of Na2CO3 to prevent decomposition of acid-sensitive esters, using t-BuOH, trifluoroethanol, hexafluoroisopropanol, and pentafluorophenol.
The wide accessibility of amines, especially in the enantio-enriched form, stimulated the synthesis of amido derivatives of PCCP. Thus, boiling PCCP 1 with primary amines produced monoamides in good yields (Scheme 8).
8.
Derivatives 32–34 were prepared on the basis of aliphatic alcohols, PCCPs 35 and 36 were obtained from bulkier amines (t-BuNH2 and 1-AdNH2), and chiral derivatives 37 and 38 were synthesized from (R)-(+)-1-(1-naphthyl)ethylamine, and methyl l-phenylalanine. It was found that the weakly nuclophilic aniline reacted to form PCCP monoamide 39 in 60% yield, and the yield of the reaction with ammonia was as low as 18%. Secondary amines did not react by this procedure, and the reaction with 2 equiv of (R)-(+)-1-(1-naphthyl)ethylamine gave 1,2-diamide derivative 41 in 30% yield (Scheme 9).
9.
The molecular structures of monoamide 37 and diamide 41 were established by XRD analysis, according to which these compounds exist in the hydroxyfulvene form, and, therewith, each of the amido N–H bonds is H-bonded to the neighboring carbonyl group.
2. CHIRAL ACID PCCP CATALYSTS OF ENANTIOSELECTIVE REACTIONS
Chiral Brønsted acid catalysts are actively used for the synthesis of valuable chemical compounds in the enantioenriched form [47]. The most common such chiral catalysts are BINOL-derived ([1,1'-binaphthalene]-2,2'-diol) phosphoric acids [48], whose reactivity and selectivity are controlled by varying 3,3' substituents or the acid component, as well as phosphoric acids with other chiral cores, specifically VAPOL [49] and SPINOL [50]. However, to create the artificial chirality in such structures requires labor-consuming separation into enantiomerically pure compounds during synthesis, complicates their optimization, and limits their widespread use due to the high cost.
Another class of effective chiral Brønsted acid catalysts but not having the above-mentioned disadvantages includes PCCPs 12 and 42, recently synthesized by the derivatization of PCCP 1 with chiral alcohols, as well as its amido derivatives 37 and 43, synthesized using simple chiral amines (Scheme 10) [30].
10.
PCCPs 1, 12, and 42 are strong CH acids existing generally as fulvenes [27, 43, 44], and their deprotonation gives highly stable Cp anions. The aromatic stabilization of these anions, as well as the presence of 5 electron-acceptor alkoxycarbonyl groups significantly increase their acidity, making it comparable with the acidity of strong mineral acids and increasing the efficiency of such catalysts, the enantioselectivity of which is associated with the chiral alkoxy substituents (Scheme 11) [30, 51–53].
11.
Chiral PCCPs 12, 37, 42, and 43 were used to catalyze the Mukaiyama–Mannich reaction [30, 54] for the enantioselective addition to imines 44 of ketene acetal 45 (Scheme 12).
12.
It was found that PCCP 12 not only outperforms in enantioselectivity (ee) (97% vs 89%) BINOL-phosphoric acid commonly used to catalyze this reaction, but also exhibits high activity, and, therefore, in concentration can be safely reduced to 0.01 mol % without sacrificing the enantioselectivity of adduct 46 formation, and monoamides 37 and 43 can also be used in this reaction as enantioselective catalysts, albeit with lower efficiency.
Furthermore, PCCP 12 was used for the enantioselective addition silylketene acetal 45 to oxocarbenium ions 47 (oxocarbenium Mukaiyama aldols [55]) generated in situ from acetals 48, which allowed synthesis of alkoxy esters 49 with good yiedls and high enantioselectivity (Scheme 13).
13.
It was shown that the aromatic Cp anion generated from PCCP catalyst 12 and stabilized by 5 CO2R groups, which increase the acidity of the catalyst, plays a central role. According to XRD data for the Me4N+ salt of PCCP 12 [30], its ester groups have a propeller orientation relative to the planar Cp ring. In this case, the chiral menthyl substituents imparting enantioselectivity to the reaction are directed in one direction, forming a hydrophobic pocket, and the C=O groups are oriented in the opposite direction. The stereochemical non-rigidity of PCCP 12 allows realization, together with the reaction substrates, of suitable transition states to ensure that the catalytic process occurs in the desired direction.
The stereochemical rationale for the catalysis of the Mannich reaction includes the association of the protonated imine with one of the carbonyls of catalyst 12 through 2 H-bonds to form structure 50 (Scheme 14, a).
14.
The carbonyl group is involved in this process, which leads to that the iminium ion turns to be located in the immediate vicinity of the neighboring carboxymenthyl group. The resulting blocking of the front side (reprochiral surface) of the iminium ion explains the observed stereochemistry.
A similar model was also proposed for the addition of the oxocarbenium ion to PCCP 12; however, in this case, the binding is realized only through one H-bond between the o-phenolic group of the oxocarbenium ion and the Cp carbonyl (Scheme 14, b). In structure 51, additional stabilizing interactions occur between the C–H bonds adjacent to the oxocarbenium oxygen and the PCCP 12 carbonyl closest to the substrate and/or the Cp ring. In this case, blocking the π face of the substrate leads to a reaction across the si prochiral surface, which corresponds to the observed stereoselectivity for the R-enantiomer.
According the mechanism of the catalysis of the oxocarbenium aldol reaction of 12, the initial protonation of acetal 48 with 12 leads to intermediate salt 52 (Scheme 15).
15.
The addition of silylketene acetal 45 to the highly electrophilic oxocarbenium ion in salt 52 first leads to structure 53 and then to intermediate 54. The subsequent reaction of the silyl group with the alcohol generated at the initial stage of ionization leads to product 49 and return acid 12 in the catalytic cycle.
The tetrahydroquinoline fragment is contained in many biologically active natural compounds, as well as in a wide range of drugs [56], and this stimulates the development of synthetic approaches to these compounds. The most common method of synthesis involves the regioselective catalytic reduction of quinoline derivatives with hydrogen or its sources [57]. It is known that chiral phosphoric acids derived from BINOL catalyze the hydrogenation of quinolines with asymmetric transfer, when Hantzsch ester (HEH) 55 is used as a source of hydrogen [58].
PCCP acids 1 and 12 were used as effective catalysts for the hydrogenation of 2-aryl(alkyl)quinolines 56 as a result of their activating protonation of the substrate and subsequent hydride transfer of from the Hantzsch ester to it (Scheme 16) [59].
16.
We used the example of the reduction of 2-phenylquinoline (56a) to study the effect of the medium on the yields of product 57a, which varied from moderate (33–46%; MeOH, DMF) to high (86–92%; toluene, CHCl3). In polar media, the reaction yield decreased, probably due to the effect of the competitive reaction of the H-bonds between the solvent and the catalyst.
Taking into account that the highest yields of product 57a were obtained in CHCl3, the effect of PCCP 1 loading on the reaction yield was studied in this solvent. Thus, a decrease in the PCCP 1 loading from 5 to 1 mol % did not decrease the yield, while with a loading of 0.1–0.001 mol %, only traces of product 57a formed. However, after raising the temperature to 60°C, product 57a was isolated in good yield even with 0.001 mol % of PCCP 1, but this reaction did not proceed without catalyst. When this reaction was carried out under the same conditions in the presence of diphenyl phosphate, product 57a was obtained in a yield of only 61%, which indicates a higher activity of PCCP 1 compared to other catalysts.
Under the optimal conditions, we assessed the scope the PCCP 1–catalyzed hydrogenation of 2-substituted quinolines 56b–56k (Scheme 17).
17.
It was found that quinolines 56b–56k all were reduced in good to excellen yields.
In the asymmetric version of this reaction with chiral PCCP catalyst 12 (1 mol %), quinolines 56a, and 56i gave their reduction products with enantiomeric excesses of 34 and 43%, respectively (Scheme 18).
18.
The asymmetric reduction of 3-substituted quinolines [60] was performed using chiral PCCPs 12 and 58 and various Hantzsch esters (HEH). Since chiral 3-substituted tetrahydroquinolines are the structural fragments of a wide range of biologically active compounds, as well as drugs, the development of methods of their synthesis is an urgent task [61].
It was found that the hydrogenation reaction of 3-methylquinoline 59a proceeded with better enantioselectivity and yield with PCCP 12, while acid 58 gave a racemic mixture of products and was excluded from further studies (Scheme 19).
19.
The best reaction parameters were obtained with diethyl Hantzsch ester at 25°C in toluene. Decreasing the loading of PCCP 12 decreased the conversion and enantioselectivity, while increasing the loading (to 10 mol %) increased the yield and slightly increased enantioselectivity.
Under the optimal conditions, was assessed the scope of this PCCP 12–catalyzed reaction for quinolines 59a–59j (Scheme 20).
20.
It was found that substrates 59a–59j all were readily reduced, giving products 60a–60j in moderate to good yields. In 3-alkylquinolines 59a–59f, a steric substituent effect was observed. For example, compound 59e containing a bulky cyclohexyl group reacted with a high enetioselectivity, but the reaction yield was than with quinolines 59a–59d, and 59f.
The catalytic enantioselective desymmetrization of mesoepoxides is an attractive method for the preparation of chiral alcohols [62], since mesoepoxide substrates are readily available compounds, and 1,2-difunctional products with two adjacent chiral centers are useful chemical structural blocks [63]. Various nucleophiles used in these reactions include amines, azides, alcohols, thiols, and halides, and, as a rule, the reactions are catalyzed by Lewis acids [62]. The reaction was successfully accomplished with 2-sulfanylbenzothiazoles and a chiral BINOL-phosphoric acid [64]. Since chiral PCCP acids proved to be a good alternative to BINOL-phosphoric acids in terms of availability and efficiency, they were used for the enantioselective desymmetrization of mesoepoxides with 2-thiobenzothiazoles [65]. The ring opening reaction of cyclohexene oxide 61a with benzothiazole 62a in the presence of PCCP 12 was used as a model reaction to optimize its conditions (Scheme 21).
21.
The reaction in the absence of PCCP 12 (CH2Cl2, 10°C, 12 h) gave a racemic product 63a in 18% yield, while adding 2.5 mol % of PCCP 12 increased the yield to 93% and led to an enantiomeric ratio (er) of 57 : 43. The best result (99% yield, 72 : 28 er) was obtained in CHCl3 at 22°C. The use of other sulfur and nitrogen nucleophiles in this reaction did not met with success, however, the addition of a catalytic amount of amine bases together with PCCP 12, leading to H-bonded adducts or ammonium salts, had an effect on the reaction enantioselectivity and yields. Thus, Et3N and i-Pr2NH reduced yields and enantioselectivity, while derivatives of aniline (i-PrNHPh, Ph2NH), pyridine (2,6-t-Bu2C5H3N, 2,6-Ph2C5H3N), and pyridine itself gave good yields and increased the enantioselectivity of the reaction compared to the reaction with PCCP 12 alone.
Probably, aniline or pyridine bases, when present, interacted with PCCP 12 to form, due to π-π-stacking interactions, stronger adducts than adducts of PCCP 12 with Et3N and i-Pr2NH (such stacking interaction with aliphatic amines are impossible), which increased enantioselectivity. Further increase in enantioselectivity was achieved with a chiral PCCP catalyst 64; in this case, product 63a was obtained in a yield of 99% and an enantioselectivity of 89.5 : 10.5 er. The high steric hindrance in PCCP 64 improved its performance compared to PCCP 12.
By varying the substituents in the aryl ring of benzothiazoles 62a–62j, products 63a–63j were obtained in moderate-to-good yields and enantioselectivity (Scheme 22).
22.
With benzothiazole 62c, product 63c was obtained in a yield of 99% and an enantioselectivity of 90.5 : 9.5 er. Benzothiazoles with electron-donor and electroneutral substituents gave higher yields and enantioselectivities, than substrates with electron-acceptor groups.
The effect of the structure of mesoepoxides 61a–61h on their desymmetrization with benzothiaozoles 62a was studied with the use of PCCP 64 (Scheme 23, Table 1).
23.
The highest enantioselectivity for epoxides 61a–61h was obtained at 15–30°C and decreased with increasing temperature. The epoxide ring opening products 63a, 63c, 63d, and 63h were obtained with a better enantioselectivity, while the results for compounds 63b, 63e, 63f, and 63g were moderate.
Cyclic aminals are often included as structural fragments in various drugs, for example, diuretics such as aquamox, thiabutazide, and bendroflumethiazide, which are widely used to treat hypertension [66]. However, their enantiomers have different biological activities [67], and drugs used in medical practice are still supplied in the form of a racemic mixture because of the lack of effective asymmetric methods for their synthesis.
To optimize the conditions of the enantioselective synthesis of cyclic amines, Sui et al. [68] chose the reaction of sulfonamide 65 with aldehyde 66a (CH2Cl2, –20°C, 24 h) in the presence of 2 mol % PCCP 12 (Scheme 24, Table 2) as a model reaction.
24.
Chiral acid PCCP 12 showed better results in enantioselectivity and reaction yield than chiral phosphoric acids based on BINOLs (S)-PA1 and (S)-PA2. It turned out that toluene gave the highest yield and the best enantioselectivity, while in other solvents, product 67a formed with good yields but with lower enantioselectivity. Decreasing the PCCP 12 loading from 2 to 1 mol % decreased the yield to 73%. An increase in the temperature adversely affected enantioselectivity, and at –45°C it reached 98%. For the optimal conditions, the following parameters were chosen: 2 mol % PCCP 12, –45°C, 70 mg of 4Å molecular sieves.
Using the optimal conditions, a wide variety of aldehydes 66a–66j were introduced into this reaction to assess its substrate scope (Scheme 25).
25.
For exampe, aliphatic aldehydes 66a–66f gave aminals 67a–67f in a yield of 91–95% and an enantioselectivity of 90–98%, and the enantioselectivity of the formation of aminal 67g from acetaldehyde 66g was as low as 85%. It was shown that the cycloalkanecarboxaldehyde ring size appreciably affetcs the reaction enantioselectivity. This, aminal 67i was obtained from cyclopentanecarboxaldehyde 66i with a high enantioselectivity (93%), whereas the respective values for aminals 67j (ee 88%) and 67h (ee 72%) obtained from cyclic aldehydes 66j and 66h are lower.
Scheme 26 depicts the plausible route of the reaction of 2-aminobenzenesuslfonamide 65 with aldehydes 66, catalyzed by chiral PCCP acid 12.
26.
The protonation of aldehyde 66 with PCCP acid 12 gave compound 68, further on the aldehyde condensed with sulfonamide 65 to form the enantiodetermining structure 69, in which the chiral catalyst PCCP 12 combined the sulfonamido and imino groups via H-bonds. Because of this, the sulfonamido attacked the imine mainly from the sterically less loaded surface Re, imparting the R-configuration to product and regenerating PCCP 12 for the next catalytic cycle.
Ketones with α-heteroatomic substituents are of considerable interest due to their widespread use as building blocks in the synthesis of more complex structures. In addition, they are found as key substructures in many biologically active compounds and drugs, for example, in bupropion [69] and brephedrone [70], drugs used in the therapy of psychiatric disorders.
A one-step asymmetric synthesis of α-heterosubstituted ketones via sulfur-mediated bifunctionalization of internal alkynes is shown in Scheme 27 [71].
27.
According to this scheme, alkyne 70 was attached by Tf2O activated with Ph2SO to give vinylsulfonium intermediate 71, whose hydrolysis formed α-sulfonium ketone 72. At the last, enantiodetermining step of this reaction, chiral PCCP catalyst 12 was added, leading to compound 73, after which aniline was added as a nucleophile to the latter at –20°C to obtain α-aminoketone 74 in a yield of 67% and an enantioselectivity of 75 : 25 er. This result points that the use of chiral catalysts makes it possible to prepare enantioenriched α-aminoketones directly from alkynes.
The asymmetric protonation of prochiral enolates is a simple and reliable method for the preparation of optically active α-substituted carbonyl compounds [72]. Stable prochiral silylenol ethers often used for this purpose are protonated with an excess of an achiral proton source in the presence of chiral Lewis or Bronsted acids [73].
Li et al. [74] reported the asymmetric protonation of silyl enol ethers using chiral PCCP catalysts and water (methanol) as a source of protons. To select the most efficient catalyst for this reaction, silyl enol ester 75a and chiral acids PCCP 12, 58, 64, 77 and 78 were tested, and methanol was chosen as the source of protons (Scheme 28).
28.
The best results were obtained with PCCP 77 (yield of 76a 88%, enantioselectivity 28%), and it was used for further research (Table 3). To increase the stereoselectivity of the reaction, optimization of its conditions was performed.
It was shown that phenols cannot be used as sources of protons, because the reaction was nonstereoselective and provided low yields of the target products. Good results were obtained with methanol, but the best result was obtained with H2O (xylene, –10°C, yield 99% and enantioselectivity 74%). A decrease in temperature did not lead to an increase in the selectivity of the reaction.
The aza-Piancatelli reaction is one of the popular methods for the synthesis of nitogen-substitued polyfunctional cyclopentenes 85 from readily available 2-furylcarbinols 79 (Scheme 29) [75].
29.
It was shown that the products of this reaction have a trans bond between the substituents on C4 and C5 [76], and the stage of conrotatory 4π electrocyclization, which converts cation 83 formed during this reaction into product 85, is responsible for controlling the relative diastereoselectivity in this cascade rearrangement (Scheme 30).
30.
To control the absolute stereochemistry of this reaction, Cai et al. [77] used chiral phosphoric acids, which are capable of controlling clockwise or counterclockwise rotation in the key stage of 4π-electrocyclization as an enantioselectivity inducing element.
To expand the range of catalysts for this reaction, chiral PCCP acid 12 was tested to find that the reaction of furylcarbinol 79a with aniline 80a in the presence of this catalyst gave 4-aminocyclopentenone 85a with a yield of 78% and enantioselectivity of 65% (Scheme 31) [78].
31.
At 30°C (ee 73%) and 22°C (ee 78%), the selectivity increased by the yield dropped, but it proved possible to increased by prolonging the reaction time from 48 to 120 h. Of the solvent, CH2Cl2 was found to be the most suitable. The following optimal conditions were chosen for further research: 5 mol % PCCP 12, CH2Cl2, 22°C, 48 h (Scheme 32).
32.
Anilines with electron-acceptor groups in the p-position ensured a balance between the yields of products 85a–85e and the enantioselectivity of the reaction. Therewith, o-aminobenzoic acid with an additional group of H-bonds gave the highest selectivity (85h, ee 84%). Slightly lower selectivities were observed with m-aminobenzoic acid (85f, ee 74%) and methyl o-aminobenzoate (85g, ee 80%).
To the scope of the aza-Piancatelli reaction, the dependence of its yields and enantioselectivity of nature of substitution in furylcarbinols 79b–79e was studied (Scheme 33).
33.
An all cases, o-aminobenzoic acid 80h gave a higher enantioselectivity compared to p-iodoaniline 80a, which provides evidence for the importance of the additional ability of the CO2H group to bind hydrogen. On the contrary, except to compounds 85p, 85q, and 85a, a lower yield was obtained with acid 80h, probably due to an increase in the steric volume of aniline, which slows down the initial nucleophilic attack on the furan ring necessary to initiate the cascade sequence. With tertiary furylcarbinol 79e, a noticeable difference in the selectivity of the formation of products 85r and 85s was observed. In this case, methyl benzoate 80g led to a product with good enantioselectivity (72%), while aniline 80a gave only 41% enantioselectivity for compound 85r.
Silicon Lewis acids are widely used in various catalytic processes [79]. One of the methods of their activation is the binding of the silicon center to a stabilized conjugated base, whose role can be played by electron-deficient pentacyanocyclopentadienide anions, the silyl complexes of which showed good results [80]. However, no attempts have been made to modify them to increase the catalytic activity. Silicon Lewis acids were obtained from cyclopentadienes with ester groups, the presence of which made it possible to modify these acids in order to increase their catalytic activity [81].
Treatment of silver salts 86a, 86b, and 87a–87c with Ph3CCl followed by the reaction with i-Pr3SiH gave silyl derivatives 88a, 88b, and 89a–89c (Scheme 34).
34.
To compare the catalytic activities of silicon Lewis acids 88a, 88b, and 89a–89c, the allylation of 4-(trifluoromethyl)benzaldehyde 90 in the presence of these acids was studied (Scheme 35).
35.
Catalyst 88a did not show any activity, and, by-contrast, Lewis acid 88b containing the electron-acceptor CF3CH2 groups catalyzed allylation to result in a 90% conversion less than within 6 h, and silicon complexes 89a–89c exhibited the highest activity. The most active of them, complex 89a, catalyzed complete conversion of compound 90 to 91 less than within 5 min.
In view of the high electrophilicity of complex 89a, its ability to split off halides to form phenylethyl cations that are trapped by the silane was studied (Scheme 36).
36.
Ether 92a and fluoride 92b proved to be the most active, which meets the expectation of their silyl-enhanced nucleofugacity. Bromide 92d showed moderate activity, whereas chloride 92c and iodide 92e gave low yields.
The mechanism of catalytic allylation is shown in Scheme 37.
37.
The reaction of PCCP catalyst 89a with bromide 92d led to ionization due to the elimination of triisopropylsilyl bromide. The resulting carbenium Cp salt 94 was then reacted allylsilane to form intermediate 95, and its desilylation provided allylated adduct 93 and regenerated silyl catalyst 89a, thereby completing the catalytic cycle.
The high electrophilicy of the resulting carbocationic compounds allowed this reaction to be used for other nucleophilic substitutions (Scheme 38).
38.
Radtkea and Lambert [81] showed that N-benzyl- and N-allylindoles reacted with high yields, whereas the yield with indole intelf was moderate. The alkylation of 1,3-dimethoxybenzene led to product 97f with a yield of 80%. Even though furan itself failed to react, 2-methylfural gave product 97g with a yield of 97%. The reaction with N-phenylpyrrole gave product 97h with good yield as a 4 : 1 mixture of 2- and 3-substituted pyrroles. It was found that substrates with electron-acceptor substituents (Ac, Ts) on nitrogen did not enter this reaction.
The introduction of an aminomethyl group into organic compounds is an urgent task, because this group is widely present in natural products and in pharmaceuticals [82], as well as because aminomethylated products are used as universal building blocks in the synthesis of biologically active compounds. Although many aminomethylation methods have been developed [83], most of them are racemic versions, and only a few asymmetric aminomethylation reactions have been described so far [84].
Kang et al. [85] developed a new three-component stereoselective aminomethylation reaction, involving a diazo compound, an alcohol, and an α-aminomethyl ether and initiated by asymmetric counteranion directed catalysis (ACDC), was developed [85], which opened up a convenient synthetic approach to optically active α-hydroxyl-β-amino acids (Scheme 39).
39.
In this reaction, enolate 102a formed in situ from α-diazo ester 98a and alcohol 99a under the action of a Pd catalyst, entered into stereoselective addition with a tight ion pair of methyleneimine cation 103a generated in situ from α-aminomethyl ether 100a with chiral counter-anion of PCCP 12, which ensured trapping intermediate 102a and led to enantioselective aminomethylation [86].
Chiral PCCPs 12 and 77 gave with good yield (88–92%) and high enantioselectivity (84–96%) product 101a, in contrast to BINOL phosphoric acids, which gave moderate enantioselectivity with good yields. Control experiments and DFT calculations showed that the chiral Cp anion is responsible for asymmetric induction due to its electrostatic interaction with intermediate compounds and the formation of H-bonds with them.
Chiral PCCP catalysts were used in the enantioselective inverse electron demand Diels–Alder cycloaddition of ethyl vinyl ethers 104 to oxocarbenium ions 47 (Scheme 40) [41].
40.
The importance of this reaction from a practical point of view is that it leads to 2,4-dialkoxychromanes 105 contained as structural units in biologically active natural substances, including the antitumor agent berkelic acid [87], antibiotic pecylospiron [88], and anti-HIV agent calanolide A [89].
To select the optimal catalyst for the reaction of diethyl acetal 48a with ethyl vinyl ether 104a, the effect of chiral PCCP catalysts 12, 20, 24, 42, and 106–116 with various alkoxy substituents on its parameters was studied (Table 4).
The reaction with 5 mol % PCCP in C6H6 at 22°C gave cycloadduct 105a with a diastereoselectivity higher than 20 : 1 in all cases, and catalysts 114 and 115 containing respectively the p-CH3O and p-CH3S substituents in the benzene ring turned out to be the most effective in terms of both yield and enantioselectivity. Based on its activity and availability, PCCP catalyst 114 was chosen for further work.
Using PCCP 114, the effect of structural changes in the starting compounds on the reaction products was studied (Scheme 41).
41.
It turned out that the replacement of Et by i-Pr in the acetal moiety led to products 105a and 105b with almost equal enantioselectivity. Methyl substitution in aryl did not decrease enantioselectivity unless it was orthogonal to the phenolic group 105d–105f. The introduction of halogen into the aryl ring had a significant effect on the enantioselectivity of the reaction. Thus, fluorine-substituted diethyl acetal 105g and its fluorine-unsubstituted analog 105a showed the same enantioselectivity, and the transition to chlorine 105h and especially to bromine 105i resulted to a sharp decrease in enantioselectivity. Varying the structure of the vinyl ester showed that its β-substitution leads to compounds 105j and 105k with 3 stereogenic centers in each, and β-substitution reduces the yield of product 105j and the enantioselectivity of its formation.
To establish the structure and mechanism of action of PCCP 114, an X-ray diffraction analysis of its NMe4+ salt was carried out, according to which all CO2R* groups are strongly linked to each other and formed a chiral helical structure. Each carboxyl is directed at an angle from 28° to 57° to the Cp ring plane, and the C=O atoms are arranged on one side and 2-arylcyclohexyl groups on the other. Such arrangement is not related to crystal packing, since the lowest energy B3LYP/6-31G* transition structures 117 and 118 for the reaction between the oxacarbenium cation–PCCP anion complex and vinyl ether had exactly the same helical conformation of the C=O groups (Scheme 42).
42.
In view of the lack of difference between steric interactions in the compounds 117 and 118, the fact that the TS(S,S) structure is energetically preferred TS(R,R) can be explained by better stabililization of the transition state by noncovalent interactions in TS(S,S) than in TS(R,R). From this it follows that the helical conformation of the anion of PCCP 114 decreases its energy, thereby minimizing conformational interactions between chiral CO2R* groups. Therewith, the point chirality of the CO2R* substituents induces the helical chirality of the PCCP anion as a whole, which leads to a decrease in the energy of the enantiodetermining TS. This conclusion made it possible to explain the observed structure–activity relationships (SAR) for PCCP 12, 20, 24, 42, and 106–116 (Table 4). Thus, chiral groups that enhance helical organization lead to high enantioselectivity (PCCP 114), while helix-destabilizing substituents are less effective either due to steric volume (PCCP 109, 111, and 112) or length (PCCP 116).
The mechanism of the enantioselective inverse electron demand Diels–Alder cycloaddition of ethyl vinyl ether 104a to oxocarbenium ion 47a is shown in Scheme 43.
43.
According to Scheme 43, PCCP 114 induces ionization of acetal 48a to form salt 119 (oxocarbenium and PCCP anion), after which the latter takes up ether 104a through TS 117, where the absolute stereochemistry of the product is dictated by the helical chirality of the Cp anion transmitted through a network of H-bonds, C–H···O interactions, and aryl CH interactions. Subsequently, the deprotonation of resulting intermediate 120 with the PCCP anion affords final chroman product 105a and regenerates catalyst 114.
3. PCCP-CATALYZED CONTROLLED CATIONIC POLYMERIZATION OF VINYL ETHERS
“Live” ionic polymerizations is an abundant class of reactions that allow the synthesis of macromolecules with a high level of control [90]. However, the field of application of these processes is limited due to their sensitivity to impurities and harsh conditions. Thus, controlled cationic polymerization should be carried out at low temperatures in an inert atmosphere using thoroughly purified monomers and catalysts [91], which limits its widespread use [92].
Kottisch et al. [93] undertook a search for catalysts that would allow cationic polymerizations to be performed under milder conditions. The close interaction of the end of the cationic chain with a well-chosen counter anion will presumably allow cationic polymerizations at room temperature and to selectively add a monomer to the reaction mixture, even if it contains nucleophilic impurities, without causing chain termination or transfer. The choice of PCCP 1 as a catalyst for this reaction was made taking into account the fact that it forms a stable anion upon dissociation [30], and its complexes with oxocarbenium ions react with vinyl ethers through TSs, in which noncovalent interactions arise between key reacting C–H bonds, both with the Cp ring and with the carbonyls of the anion [41]. In this case, the mechanism of PCCP 1 catalysis of cationic polymerization would involve the reaction of vinyl ether 121 with the catalyst, resulting in covalent compound 122, which exists in equilibrium with salt 123 containing compound 122, and the end of the growing chain (Scheme 44).
44.
In this case, the addition of monomers to salt 123 would take place through TS 124. Due to the high reactivity of the oxocarbenium ion, the end of the chain would be mainly in the covalent form 122, which would provide controlled polymerization at ambient temperature. This mechanism would eliminate the need for highly purified reagents, an inert atmosphere, and low temperatures. To test this mechanism, the authors of the cited work studied the polymerization of isobutyl vinyl ether (IBVE) with PCCP 1 at 22°C in the absence of an inert atmosphere, solvent, and additional purification of IBVE. As a result, the polymerization with 50 equiv of IBVE led to complete consumption of the monomer after 16 h and gave a polymer with Mnexp 5.1 kg/mol and Đ 1.1 (Table 5, experiment 1).
Therewith, the experimental molar mass (Mnexp) fitted well the theoretical value (Mntheor), indicating that the polymer chain was initiated by each PCCP 1 molecule, whereas the low dispersities Đ indicated an effective protonation of IBVE by PCCP 1. From this it followed that the events of chain termination and transfer did not play an important role in this reaction. Moreover, the relatively low polymerization rate observed in this case was associated with a strong interaction between the Cp anion and the chain end of the oxocarbenium ion, arising from the formation of a strong ion pair between these two species or due to the presence of a dynamic covalent bond between them. By varying the ratio of PCCP 1 to IBVE ratio and the target polymers with a higher molar mass (Table 5, runs 2 and 3), polymers with narrow dispersities Đ were obtained in all cases, and the Mnexp values were slightly lower than Mntheor, but nevertheless agreed well with each other. This indicated a minimal effect of chain transfer on the polymerization process if transfer took place.
When these reactions were performed in an inert atmosphere with well purified IBVE, almost the same results were obtained, which made it possible to carry out the reactions catalyzed by PCCP 1 without thorough purification and dehydration of the reagents. Dihydrofuran monomer 121d, the starting material for poly(DHF) with a high glass temperature (126°C), polymerized under standard conditions, but the Mnexp of the resulting polymer was lower than expected, and the dispersity Đ was broad (test 6).
The activity and chain-end fidelity of this reaction was investigated in the synthesis of diblock copolymers (Scheme 45).
45.
For the first block, poly(EVE) with Mnexp 4.0 kg/mol was obtained from purified monomers in an N2 atmosphere, and then, after 95% conversion, IBVE was added to obtain poly(EVE-b-IBVE) diblock polymer with Mnexp 8.2 kg/mol. The SEC plot of the polymer after chain elongation showed a shift toward higher Mn while maintaining a narrow dispersity (Đ = 1.2), which indicated the formation of a diblock polymer. When the reaction was let to proceed to complete conversion before the second monomer was added, the chain was terminated by nucleophilic impurities, and chain termination became competitive at a high degree of monomer conversion.
To extend the range of application of the developed polymerization methods, polymers were functionalized at the end of the chain by quenching the propagating oxocarbenium ion with alcohols after complete conversion of the monomer. Thus, the addition of 5 equiv of various alcohols and Et3N to poly(IBVE) gave polymers with the desired acetal group at the end of the chain with a yield of more than 95% (Scheme 46, a).
46.
In addition, the ends of the oxocarbenium chains were efficiently trapped by the dithiocarbamate salt to form macroinitiator poly(127a), which opens access to the multiblock material through chain elongation (Scheme 46, b). The use of FcBF4 as a chemical mediator for poly(127a) effectively lengthened the chain by cationic reversible addition-fragmentation-chain transfer polymerization (RAFT) to obtain poly(IBVE-b-IBVE), which indicates the possibility of efficient control of chain ends after polymerization, as well as extending the chain using other methods.
Attempts to obtain polymers with a degree of polymerization greater than 100 from monomers 121 using PCCP 1 gave products with lower Mn and broadened dispersity Đ. The loss of control over this process was associated with a high level of elimination at the end of the chain and its subsequent transfer through the PCCP anion (Scheme 47) [94].
47.
The close location of the PCCP 1 anion to the last-by-one C–H bond of the chain in compound 128 promoted intramolecular deprotonation to form a strong acid PCCP 1, which protonated an additional monomer and initiated growth of a new poly(121) chain, the transfer of which led to low Mn values and broad dispersities Đ (Scheme 47).
To solve this problem, the basicity of the PCCP 1 anion was lowered by adding H-bond donors (HBD) to it, and this inhibited chain transfer through interaction with one or more carbonyl groups of the Cp anion (Scheme 48).
48.
When used as effective polymerization cocatalysts, HBD must firmly bind to the PCCP 1 anion, reducing its basicity, and remain close to the end of the polymer chain throughout the reaction to prevent its termination by nucleophiles. Based on these requirements, a number of different HBDs 130–135 were selected, and their effect on IBVE polymerization studied (Scheme 49).
49.
Among H-bond donors (HBD 130–135), thiophosphoramide 133 turned out to be the best in the polymerization of IBVE. When it was used in the PCCP 1-catalyzed cationic polymerization of vinyl ethers, it was possible to obtain polyvinyl ethers with a molar mass of up to 60 kg/mol even in the surrounding atmosphere, thereby maintaining control over Mn and narrow dispersities Đ, which indicates an improvement in the previously described method [93].
The interactions between HBD 133, the PCCP 1 anion, and the oxocarbenium ion chain end were simulated by DFT calculations (Scheme 50).
50.
The resulting geometry-optimized transition structure 136 revealed several important interactions that facilitate polymerization. In structure 136 hydrogen bond donor HBD 133 located above the PCCP 1 anion in structure 136 is slightly displaced relative to the center and forms 2 H-bonds with neighboring carbonyls of the anion, and the third N–H bond is coordinated with the π system of this anion. The growing polymer chain is located in a pocket between HBD 133 and the PCCP 1 anion, protruding from it, while the monomer moves inward, forming the key C–H···O interaction between the vinyl α-C–H bond and another PCCP 1 carbonyl. This model also determines the resistance of polymerization to external nucleophiles, because the gap between HBD 133 and the anion can protect the chain end from external nucleophiles, minimizing chain transfer.
4. REARRANGEMENT OF ORGANIC AND ORGANOELEMENT GROUPS AND HALOGENS IN THE PCCP SYSTEM
Wilkinson and Piper were the first to describe in 1956 [95] an extremely rapid migration of the Fe(η5-C5H5)(CO)2 and Hg(η1-C5H5) groups along the Cp ring in the unsubstituted Cp. To date, rapid circular migrations along the perimeter of the Cp ring have been found for many metal-centered groups formed by transition and non-transition metals [96, 97], as well as for σ-derivatives of Cp with migrants liked with the Cp ring through Group 13–17 elements of the Periodic Table [31, 98–100]. Since organometallic derivatives and PCCP 1 complexes are mainly characterized by the coordination of the metals to the carbonyl groups, only circular migrations of element-centered substituents were observed in them, and such migrations are mainly carried out as 1,5-sigmatropic shifts. However, due to the stability of its Cp anion, this system in some alkyl and arylazo derivatives features migrations that occur as a result of dissociation–recombination through the formation of tight ion pairs.
The displacement of alkyl groups in the unsubstituted or permethylated Cp ring occurs at t > 300°C [31]. The electron-acceptor CO2Me groups into the Cp ring decreased the energy barrier for the shift of the alkyl substituents in the Cp ring and significantly accelerated the shifts of the 4-nitrobenzyl group in compounds 137a–137c (Scheme 51) [101].
51.
Isomers 137a–137c were preparatively isolated, and from the dynamics of their 1H NMR spectra, intramolecular 1,5-shifts of the 4-nitrobenzyl group along the Cp ring were revealed, with ΔG≠120°C: 137a → 137b 29.5 kcal/mol, 137b → 137a 30.0 kcal/mol, and 137b → 137c 33.6 kcal/mol.
Allyl group migrations along the Cp ring were studied in allyl derivatives 138a–138c. For preparatively isolated isomers 138a–138c, analysis of the time dependence of their 1H NMR spectra at 80–130°C showed that they interconverted due to 3,3-sigmatropic shifts of the allyl group along the perimeter of the Cp ring with ΔG≠25°C: 138a → 138b 28.5 kcal/mol, 138b → 138a 29.7 kcal/mol, 138b → 138c 30.2 kcal/ mol, and 138c → 138b 30.3 kcal/mol (Scheme 52) [102].
52.
The mechanism of 3,3-sigmatropic shifts was confirmed by the DFT B3LYP/6-311++G(d,p) method on the example of allyl derivative 139, in which degenerate migrations of the allyl group along the perimeter of the Cp ring also proceeded along the Cope rearrangement pathway through transition states with a chair (TS 140) or a boat (TS 141) six-membered ring conformations with close barriers (ΔG≠25°C 27.4 and 27.7 kcal/mol, respectively) (Scheme 53).
53.
At the same time, the alternative 1,5-shifts of the allyl group in the PCCP 139 ring should proceed with a higher barrier (ΔG≠25°C 30.8 kcal/mol).
Jefferson and Warkentin [103] showed that, in polar media, the rearrangement of alkyl derivatives 142 proceeded by the ionization–recombination mechanism through the heterolysis of the R–Cp bond (Scheme 54).
54.
Thus, in a methanol solution, compound 142 dissociate to give ion pairs 143, whose collapse forms covalent isomers 142. The latter undergo degenerate circular rearrangement competing with an irreversible reaction with the solvent.
In cyclopentadienes 144–146, the energy barrier for the 1,5-sigmatropic shifts of the methoxycarbonyl group was estimated at ΔG≠100°C 24.1–29.9 kcal/mol (Scheme 55) [104].
55.
Substituents in the Cp ring appreciably affected the rate of the shift of the methoxycarbonyl group. The rearrangement 144 → 145 occurred at room temperature at R = NMe2 (ΔG≠ 24.1 kcal/mol), while R = Cl migration occurred at 150–190°C (ΔG≠ 25.7 kcal/mol).
The dynamics of the NMR spectra (–30 ÷ 30°C) of aryl derivatives 147a–147l provided evidence showing that the arylazo groups migrate along the Cp ring due to randomization and forms a tight ion pair 148 with a barrier of ΔG≠25°C 12.9–16.0 kcal/mol (Scheme 56) [31, 105].
56.
Electron-acceptor substituents in the aryl ring decreased the migration rate due to destabilization of cations 148a–148l, while electron-donor substituents increased. Thus, in going from compound 148d with one NO2 group in the aryl ring to compound 147j with 3 NO2 groups in the aryl ring, the migration marrier ΔG≠25°C increased by 2.6 kcal/mol.
In was established by IR spectroscopy and XRD analysis that, it was found that compounds with one NO2 or CF3SO2 substituent in the aryl ring exist exclusively as aryldiazonium salts 148a–148g, and 148k, while compounds with two NO2 substituents in the aryl ring exist in 2 forms: covalent azo compounds 147h and aryldiazonium salts 148h and 148i, while compounds with three NO2 substituents (147j) or two CF3SO2 substituents (147l) are stable only in the form of azo compounds. The molecular and crystal structures of compounds 148c and 147h were determined by XRD analysis [105]. Crystals of compound 148c are built of ion pairs lying in almost parallel planes, and the molecular structure of 147h is a covalent 2,4-dinitrophenylazo derivative.
Compounds 147 decompose on heating (Scheme 57) [106].
57.
The reaction proceeds along 2 channels: the first involves the elimination of the CO2Me group to form hydrazones 149 and the second involves the elimination of nitrogen to form arylcyclopentadienes 150, which then undergoes the 1,5-shift of the CO2Me group, leading to products 151. In arylhydrazones 149, hydrogen is bonded to the hydrazone nitrogen and coordinated to the carbonyl of the neighboring CO2Me group. We earlier studied the syn–anti isomerization with respect to the double C=N bond in hydrazones 149 (ΔG≠25°C 20.7–25.1 kcal/mol) by dynamic 1H and 13C NMR spectroscopy [106].
In nitrocyclopentadienes 152a–152d, nondegenerate 1,5-sigmatropic shifts of the nitro group along the perimeter of the Cp ring were found and investigated (Scheme 58) [31, 107].
58.
Isomers 152a–152d and 153a–153d were obtained as an equilibrium mixture and isolated preparatively. The structure of nitro derivatives 152a and 153a was confirmed by XRD. According to 1H NMR, isomers 152a–152d and 153a–153d interconvert with ΔG≠25°C 23.9–26.0 kcal/mol.
The interconversion of isomers 152a and 153a was studied by quantum-chemical calculations at the B3LYP/6-311++G** level (Scheme 59) [108].
59.
It was shown that these interconversions occur by the way of 1,3-shift of the nitro group along the perimeter of the Cp ring, which is impossible to detect on the NMR scale, and, according to the principle of the conservation of orbital symmetry, via consecutive 1,5-sigmatropic shifts of the nitro group and formation of an unstable isomer 154a. According to the gas-phase calculations, isomer 154a is less stable than 152a by ΔEZPE 3.6 kcal/mol, and the barrier for the step process 152a → 153a is 24.5 kcal/mol, which agrees with the NMR data [107].
Dynamic 1H NMR was used to study fast and reversible migrations of the arylthio groups along the perimeter of the Cp ring in arylthiocyclopentadienes 157a–157g (Scheme 60) [31, 109, 110].
60.
According to the XRD analysis, compound 157d exists in a conformation, in which the aryl ring is arranged over the the Cp moiety, forming with the former a dihedral angle of 50.0°. The dynamics of the NMR spectra of compounds 157a–157g in the range 30–100°C are indicative of 1,5-sigmatropic shifts of the arythio group along the perimeter of the Cp ring (ΔG≠25°C 16.0–20.9 kcal/mol). In going from compound 157a with an electron-donor substituent in the aryl ring to compounds 157c–157g with electron-acceptor substituents, the barrier for migration of the arylthio group increased substantially, which gave evidence for the formation of betaine structures 158a–158g, in which the positive charge is localized on the sulfur atom, and the negative charge, on the Cp ring.
The reaction of potassium cyclopentadienides 159a and 159b with selenyl chloride formed selenium derivatives 160a–160c (Scheme 61) [31, 110].
61.
Since selenyl chloride can also act as halogenative agents, chlorine derivatives 161a–161c, diselenides 162, and dimers 163 were isolated. The dynamic 1H NMR spectra of compounds 160a–160c revealed fast intramolecular degenerate and nondegenerate 1,5-sigmatripic shifts of the seleno groups along the Cp ring with a barrier of ΔG≠25°C 16.3–17.1 kcal/mol, which is 3–5 kcal/mol lower than for the respective thio groups (Scheme 62).
62.
The reaction of PCCP–Ag+ with aryltellurenyl bromides was used to prepare air-resistant aryltellurenyl cyclopentadienes 164a–164c, evidence for the η1 structure of which, both in crystal and in solution, as well as the intramolecular Te···O and Te··N coordination in them was obtained by IR and NMR spectroscopy (Scheme 63) [111, 112].
63.
The 1H and 13C NMR spectra of compound 164a (Y = O) at low temperatures display diastereotopic splitting of signals from the sp2-C atoms of the Cp ring and substituents on them, due to the asymmetric arrangement of the tellurium substituent relative to this ring. The dynamics of the 1H and 13C NMR spectra of compound 164a in the temperature range 25–65°C indicates fast intramolecular 1,5-sigmatropic shifts of the aryltellurenyl group along the Cp ring (ΔG≠25°C 13.4 kcal/mol). In azomethine derivatives 164b and 164b (Y = NAr), the migrations of the tellurenyl group occur much faster and are too fast to be detectable on the 1H and 13C NMR time scales even at –90°C; the energy barrier for these migrations is 6.0 ± 2.0 kcal/mol. Thus, the migratory capacity of thio, seleno, and telluro groups in the PCCP 1 system increases in the order S < Se <Te, which is associated with a decrease in the energy of the C5–chalcogen bond with an increase in the atomic number of the central atom of the migrating group.
Halocyclopentadienes are an important class of precursors of Cp compounds and their metal complexes [15, 16, 113–116]. According to DFT calculations, the iodocyclopentadiene molecule in a rotating electric field is a prototype of a molecular rotary motor with a fast (k25°C 630 s–1) unidirectional movement of iodine along the Cp ring [117].
Dynamic 1H and 13C NMR spectroscopy revealed reversible intramolecular 1,5-shifts of Cl and Br along the perimeter of the penta(alkyltetra)methoxycarbonylcyclopentadiene ring (Alk = Me, i-Pr, CH2Ph, CH2CO2CH3), with proceeding with ΔG≠25°C 25.7–27.3 (Cl) and 16.2–22.9 (Br) kcal/mol [118, 119]. Chlorocyclopentadienes 165a–165c and 167a–167c were obtained as mixtures of isomers by the reaction of the corresponding cyclopentadienes with N-chlorosuccinimide (Scheme 64).
64.
Upon heating o-dichlorobenzene solutions of preparatively isolated isomers 165a–165c at 70–100°C for 0.5–2.5 h resulted in the formation of the following equilibrium mixtures: 165a : 166a = 0.11 : 0.89, 165b : 166b = 0.35 : 0.65, and 165c : 166c = 0.25 : 0.75.
A similar reaction with NBS led to an equilibrium mixture of bromocyclopentadienes 168a–168c and 170a–179c, the dynamics of the 1H and 13C NMR spectra of which at 25–140°C indicated 1,5-shifts of bromine along the Cp ring with ΔG≠25°C 16.2–22.9 kcal/mol (Scheme 65) [120].
65.
Quantum-chemical calculations at the (B3LYP/6-311++G(d,p) and B3LYP/Gen, 6-311++G(d,p)/SDD) levels showed that compounds 171a–171d prefer 1,5-sigmatropic shifts of halogens along the perimeter of the Cp ring to 1,3-shifts and estimated the effect of the halogens on the activation barriers for their migrations (Scheme 66) [119].
66.
The calculated barriers for symmetry-allowed 1,5-shifts of halogens through TS 172 are ΔE≠ZPE 42.9(F), 26.9(Cl), 19.8(Br), and 15.4(I) kcal/mol (Scheme 66, a), and they are lower those for 1.3-shifts through TS 173 [ΔE≠ZPE 65.4(F), 47.6(Cl), 38.1(Br), and 34.2(I) kcal/mol] (Scheme 66, b). The calculated barrier fit well the experimental values. The migration ability of halogens in the corresponding derivatives of PCCP 1 strongly increases in the series F < Cl < Br < I, with increasing atomic number of the halogen.
5. EFFECTIVE CARRIERS OF FUNCTIONAL GROUPS BASED ON FLUCTUATING PCCP
It was shown that arylazo derivatives 147 are convenient reagent for introducing the aryl azo group in compounds containing N- and C-nuclephilic centers, because compounds readily enter the azo coupling reaction with such centers in organic media at 22°C (Scheme 67).
67.
For example, arylazo derivatives 147 react with N,N'-diarylbenzamidines 174a and 174b [121], react with 2-methylindole, N,N-diethylaniline, and malononitrile, as well as with aniline and secondary amines to form C–azo coupling products (176, 178, 179) and triazenes (175, 177) [122, 123].
The reaction of compound 147a, which contains one NO2 substituent in the aryl ring, with Cp* leads to isomer 180 with the arylazo substituent in the Cp* ring, whereas 2,4-dinitophenyl azo derivative 147b the diazonium cation generated from 2,4-dinitophenyl azo derivative 147b attacks both the methyl substituent and the Cp* ring (Scheme 68) [124].
68.
The reaction of arylzocyclopentadiene 147a with compound 183 involves the azo coupling product at the carbon atom of the Cp ring with the substitution of the CO2Me group (Scheme 69) [125].
69.
In this cascade reaction, the CO2Me group undergoes a 1,5-shift to the nitrogen of the amidine triad, followed by hydrolysis of the imido group to give product 184.
Air oxygen- and moisture-resistant arylthiocyclopentadienes 157a and 157b are convenient sulfenylating reagents, giving arylthio derivatives in reactions (22°C, 2–48 h) with amidines (185), primary and secondary amines (186, 187), and compounds with activated carbon centers (188, 189) (Scheme 70) [124, 126, 127].
70.
6. TETRACARBOMETOXYCYCLOPENTADENYL LIGAND SYSTEMS AND THEIR METAL COMPLEXES
The nucleophilic substitution of the nitro group in compound 191 by the amine nitrogen atom of amidines 190 allowed the synthesis of a wide range of bidentate amidinylcyclopentadienyl ligands 194a–194f (Scheme 71) [32–34, 128–131].
71.
This reaction involves the 1,4-sigmatropic shift of the CO2Me group to the amidine imino nitrogen to give Cp ylides 193a–193f, and HNO2 released during reaction nitrosylated amidine 191 (double excess), yielding N-nitrosoamidene 192. Treatmeny of ylides 193a–193f with methanolic NaOH resulted in the elimination of the N–CO2Me group, and subsequent acidification with HCl led to ligands 194a–194f.
Potassium and Tl(I) complexes complexes 195a–195f and 196a–196c, respectively, were prepared by treatment of ylides 193 with methanolic KOH and TlOH, respectively (Scheme 72) [33, 34, 132].
72.
The reaction of compounds 195a, 195b, and 195f with (Ph3P)AuCl in MeCN gave Au(I) complexes 197a, 197b, and 197f (Scheme 73) [31, 32].
73.
The reaction of ligands 194a and 194b with 4-Me2NC6H4HgOCOMe in methanol resulted in the synthesis of arylmercury complexes 198a and 198b (Scheme 74) [32].
74.
It was shown that ligands 194 and metal complexes 195–198 have a zwitter ionic structure, with the positive charge localized in the amidine triade and the negative charge in the Cp ring. The metals in complexes 195–198 are coordinated to the terminal nitrogen and additionally coordinated to the π-system of the Cp. Ligands or K, Tl(I), Au(I), and Hg(II) complexes with axially asymmetric α-naphthyl or o-substituted Ar' groups in the amidine triad characteristically have a chiral structure, which is associated with high barriers for rotation of the aryl groups about the C–C bond and of the amidinium fragment about the CCp–N bond.
The cascade reactions of potassium cycloheptatrienyl 199 with organic azides were used to prepare similar β-aminovinyl Cp anions 200 (Scheme 75) [133].
75.
Therewith, the formation of the Cp fragment involved C–C bond cleavage and CO2Me migration. The structure of N–Ms complex 200 was confirmed by XRD analysis, which showed that the K+ or Na+ ions are coordinated to two CO2Me carbonyl groups on the Cp ring and vinyl group. Complex 200 gave stable salts with organic cations [134].
7. PUSH−PULL CHROMOPHORES WITH HYDRAZONE PCCP FRAGMENTS FOR ORGANIC PHOTOVOLTAICS
It was shown that the reaction between N-mesyl complex 200 and aryldiazonium followed by hydrolysis and decarboxylation of one CO2Me leads to isomeric 3- and 4-aminovinyl-5-hydrazonocyclopentadienes 201 and 202, which undergo cyclization on silica gel to form pyridazine 203 (Scheme 76) [135].
76.
Hydrazines 201 and 202 strongly absorb in the range 400–600 nm and are of interest for dye-sensitized solar cells. X-ray diffraction analysis of hydrazone 201c revealed a short N–N in it, which indicates strong conjugation between the donor and acceptor parts of the molecules. The polarized Cp–hydrazone fragment is of interest as an acceptor substituent for chromophores of the D–π–A type. The short N–N bond characteristic of these structures, the length of which corresponds to a greater extent to the bond length in azo compounds than in hydrazones, indicates polarization of hydrazones 201 and 202, similar to fulvenes. The developed synthetic approach of the azafulvene fragment can be in demand in the preparation of D–π–A chromophores for photovoltaics and nonlinear optics and for the creation of organic light-emitting diodes and field-effect transistors.
The reaction of cyclopentadienone 204 with arylhydrazine hydrochlorides gave stable arylhydrazones 205 (Scheme 77) [35].
77.
The range of D–A chromophores with hydrazinylidene acceptors 205 was expanded by the action of organolithium reagents on cyclic diazo compounds 206, which made it possible to introduce thiophene fragments into the chromophore to create planar molecules (Scheme 78) [136].
78.
Arylhydrazones 205 intensely absorbed visible light in the 373–562 nm range with extinction coefficients (ε) up to 36500 M−1 cm−1, while thiophene derivatives had the longest absorption compared to other hydrazones.
Compounds with an acceptor hydrazone Cp fragment, a π-conjugated phenylene spacer, and an electron-donor triphenylamine substituent 207a and 207b were also obtained and exhibited optical, electrochemical, and photoelectric properties, due to which these compounds have a high potential for use in organic photovoltaics (Scheme 79) [137].
79.
Despite their non-coplanar structure, hydrazones 207a and 207b showed semiconducting properties and were capable of intramolecular charge separation. Hydrazone 207a as donor components of photovoltaic layers of organic solar cells were found to have the highest efficiency (up to 1.85%), which opens up the prospects for the use of hydrazone Cp fragments in the design of polarized chromophores for photovoltaics.
A new A–D–A chromophore 209 with the indacenodithienothiophene core linked to 2 terminal acceptor hydrazone Cp groups was obtained by the reaction of dilithium indacenodithienothiophene 208 with diazo derivative 206 (Scheme 80) [138].
80.
Chromophore 209 intensely absorbed in the range 500–800 nm, with a maximum ε value of 86700 M−1·cm–1 at 669 nm. Its absorption spectrum in the film had a similar maximum, which indicated the absence of significant molecular organization and amorphous structure in it. The optical band gap for chromophore 209 in the film was estimated from the absorption edge (800 nm) as 1.55 eV. The energies of the boundary orbitals were estimated at –5.42 (HOMO) and –3.80 (LUMO) eV. These data allowed the conclusion that compound 209 is suitable for use as an acceptor material in organic solar cells (OSC).
Based on the synthetically available dimethyldibromosuccinate 210, a new method for the synthesis of pyridinium cyclopentadienolate 211 (Scheme 81) was developed [139].
81.
It was shown that salt 211 is as reactive toward arylhydrazines as cyclopentadienones, giving arylhydrazonecyclopentadienes 212. Thus, compounds 212 were introduced in intramolecular cyclization reactions, which made it possible to obtain chromophores 213–215 (Scheme 82).
82.
CONCLUSIONS
Despite the fact that (pentamethoxycarbonyl)cyclopentadiene (1) and a number of its derivatives were obtained in the middle of the last century, the chemistry of these compounds continues to evolve at present, which is due to the synthetic availability and unique properties of these compounds. In particular, the extremely high acidity of PCCP makes it possible to use these compounds as highly efficient enantioselective Bronsted catalysts, due to their fulvic structure favors the formation of a wide range of metal complexes, and the ester groups in their composition allows functionalization, which leads to a wide range of derivatives. Of particular note is the unusual propeller orientation of the chiral ester groups in these compounds relative to the Cp ring, which allows enantioselective control of reactions with their participation.
Together the ability of these groups to act as H-bond acceptors and of the Cp anion to participate in intermolecular interactions with substrates favors the formation of suitable transition states for the chemical reactions to take the desired direction. A high potential of PCCP derivatives as enantioselective catalysts for a wide range of reactions and as effective carriers of arylazo and arylthio groups, in the preparation of push-pull chromophores for organic photovoltaics and in the creation of new pharmaceuticals, including anticarcinogenic ones was revealed. This review aims to draw further attention to this interesting and promising class of compounds.
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The work was financially supported by the Ministry for the Education and Science of the Russian Federation (State order for Scientific Research, project no. 0852-2020-0031).
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Translated from Zhurnal Organicheskoi Khimii, 2021, Vol. 57, No. 11, pp. 1505–1559 https://doi.org/10.31857/S051474922111001X.
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Mikhailov, I.E., Dushenko, G.A. & Minkin, V.I. Pentacarboxycyclopentadienes in Organic Synthesis. Russ J Org Chem 57, 1757–1808 (2021). https://doi.org/10.1134/S1070428021110014
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DOI: https://doi.org/10.1134/S1070428021110014