Degradation Pathways for Porphyrinoids

Abstract

Porphyrin, a tetrapyrrolic aromatic macrocycle, is relatively resistant to degradation. However, certain strong oxidants (e.g. chromic acid) cause its decomposition to monopyrrolic units. More often, ring opening caused by attack of oxidant on a meso-position has been observed. Such degradation by metal salts (thallium(III), cerium(IV)), nitric acid, and other reagents has been studied. Light-driven macrocycle opening by dioxygen has also been noted. Coupled oxidation of metalloporphyrins has been investigated mainly as a mimics of heme degradation observed in vivo.

Modifications of parent porphyrin macrocycle can cause a prominent change of its reactivity toward oxidants. In particular, inversion of one of the pyrrole rings (in N-confused porphyrin) or removal of one of the methine bridges (in corrole) increases macrocycle susceptibility to oxidative ring opening.

1 Scope and Limitations

This review is focused on degradation of tetrapyrrolic macrocycles: porphyrins, their N-confused isomers, and corroles (1–3, Fig. 1). “Degradation” is understood here as a disruption of a macrocyclic system. For this reason, reactions leading only to the lowering of number of rings of the starting pentacyclic system are not included, although formation of secochlorins 4 [1–3] or vacataporphyrins 5 [4, 5] (Fig. 2) also results in a qualitative change of the macrocycle properties. Similarly, processes connected with the loss of the macrocyclic aromaticity without ring opening (e.g. formation of phlorins) will not be discussed unless they serve as a preliminary stage of the actual degradation. Ring-opening reactions of phthalocyanines, porphyrazines, and similar macrocycles as well as systems containing less or more than four pyrrolic rings are not presented. The emphasis is laid on the literature published in the years 2000–2012, but for the sake of comparison, older achievements are also briefly described.

Fig. 1

Porphyrin, its N-confused isomer and corrole (meso-aryl derivatives are shown)

Fig. 2

Examples of secochlorins (4) and vacataporphyrins (5) [1–5]

The porphyrin macrocycle containing a conjugated 18 π-electron system is known to be highly stable toward destruction. This fact inspired search for methods of ring opening. The interest in degradation of cyclic tetrapyrroles is connected with several aspects: analytical (structure determination), biochemical (heme and chlorophyll metabolism, formation of algae biliproteins), catalytic (stability of porphyrin derivatives used as catalysts and photosensitizers), and synthetic (preparation of linear oligopyrrolic systems exhibiting interesting properties: helical chirality [6–9], conformational flexibility connected with possible E–Z isomerization [10], specific and sometimes unpredictable coordination modes [11–14]).

A direct opening of porphyrin macroring is achieved when one of the C(α)–C(meso) bonds is cleft. Reactions at the macrocycle periphery occur preferentially on meso positions unless sterical reasons preclude access to this part of molecule [15]. In general, degradation is caused by various oxidants (reduction with hydriodic acid in acetic acid being a notable exception) and is thus preceded by their attack on one of the methine bridges. On the other hand, numerous examples of pyrrole- and metal-centered oxidations have been also described, which can also constitute a preliminary step of further macrocycle decomposition.

This chapter is divided into eight sections. Section 2 is devoted to traditional methods of structure determination based on destruction of tetrapyrrolic systems. In Sect. 3, macrocycle opening by oxidants is discussed, excluding light-driven reactions with dioxygen (Sect. 4) and coupled oxidation of metalloporphyrins (Sect. 5). Biodegradation is shortly presented in Sect. 6, followed by concluding remarks (Sect. 7) and reference list.

2 Degradation Used as Analytical Tool

Classical methods used for structure elucidation of tetrapyrrolic compounds (both cyclic ones and linear derivatives) utilized oxidative degradation with chromic acid, potassium permanganate [16, 17], and ozone [18] or hydriodic acid reduction [19]. Analysis of the resulting monopyrrolic units (maleimides, succinimides) which could be identified, allowed recognition of β-substitution pattern, and in certain cases also meso substituents [15]. Among those methods, chromic acid (CrO₃/H₂SO₄) oxidation used in combination with gas chromatography and mass spectrometry has been most widely applied, particularly for identification of chlorophyll derivatives, bilins, and geoporphyrins [20–27]. More recently, this method was used in the analysis of hematoporphyrin derivative used in photodynamic therapy [28, 29]. A new method was described allowing quantitative determination of chlorophyll derivatives by analysis of amount of ethylmethylmaleimide formed during degradation with chromic acid [30].

Formally, part of analytical methods commonly used for the characterization of newly synthesized tetrapyrrolic macrocycles also involves destruction of the molecule. Elementary (combustion) analysis is widely performed, though the results are sometimes not quite satisfactory due to the ease of incorporation of various guest molecules, including solvents, in the crystal lattice of porphyrins [31]. Also a conventional method of carbon isotopic composition of geoporphyrins relies on combustion to CO₂ which is examined by mass spectrometry [32, 33]. Fragmentation observed in certain techniques of mass spectrometry serves as a source of a valuable structural information [34–36]. Analytical data based on other methods involving sample decomposition, such as combustion calorimetry experiments [37, 38], differential scanning calorimetry, and thermogravimetry [39–41] are less frequently reported.

3 Ring Opening by Oxidants

Ring-centered reactions of porphyrin derivatives with various oxidants can lead to opening of the macrocycle without its complete disintegration. Systematic research on oxidation of tetrapyrrolic macrocycles was performed in the 1960–1970s; in most of the recent contributions specifically modified systems or reactions conducted under modified conditions have been discussed.

In Sect. 3.1, reactions of porphyrins and their complexes with redox innocent metals are described. Degradation of iron and manganese porphyrin complexes by reagents which are typically used in metalloporphyrin-catalyzed oxidations is discussed in Sect. 3.2. The section is concluded by description of reactivity of N-confused porphyrins and corroles.

3.1 Oxidation of Porphyrins and Their Complexes

Reactions of porphyrins and their complexes with oxidants were extensively studied by Bonnett and coworkers [42–46] and Smith et al. [47–53]. Special attention was devoted to meso oxidation leading to oxophlorin (5-hydroxyporphyrin) derivatives due to importance of iron oxophlorins as intermediates in the process of heme degradation. Octaethyloxophlorin (OEPOH₃, 8) was obtained from the reaction of 2,3,7,8,12,13,17,18-octaephylporphyrin (OEPH₂, 6) with benzoyl peroxide [43, 44, 46]. A radical attack at meso position gave 5-benzoyloxyporphyrin 7 at ca. 30% yield, and its hydrolysis led to the desired product 8 (Scheme 1). This compound was also prepared by ring synthesis and by coupled oxidation (see 5.1) [43, 44].

Scheme 1

Synthesis of octaethyloxophlorin [43, 44, 46]

Bonnett et al. prepared octaethyloxophlorin 8 by treatment of (OEP)Fe^II(py)₂ dissolved in pyridine with hydrogen peroxide [43, 45]. Later it was found that reaction did not occur with zinc(II), nickel(II), copper(II), iron(III), and cobalt(III) complexes, while oxophlorins were obtained for Fe(II), Co(II) and Mn(II) or Mn(III) (i.e. metal ions with an easily accessible higher oxidation state) [45]. Conversion of iron(III) oxophlorin into verdoheme analog and its further conversion to biliverdin 9 (Fig. 3) was also described [45].

Fig. 3

Octaethylbilindione – a synthetic biliverdin analogue

Kalish et al. demonstrated that treatment of deuteroheme, mesoheme, or protoheme with hydrogen peroxide in pyridine solution yielded all four isomeric oxophlorin complexes in comparable yields [54]. In contrast, oxidation of iron(II) 5-substituted-octaethylporphyrins (5-R-OEP)Fe^III(py)₂ (R = NO₂, CHO, CN, Cl, OMe, Ph, n-Bu) exhibited a strong dependence on the nature of the substituent: yields of (OEPO)Fe(py)₂, a product of replacement of R group with oxygen function, varied from 0% (R = Ph, n-Bu) to 100% (R = NO₂), while ratio of cis to trans-oxygenated products (12 and 13, Scheme 2) changed from 5.0 (R = CN) to 1.4 (R = Ph) [55].

Scheme 2

Oxidation of 5-substituted iron(II) octaethylporphyrins [55]

Treatment of zinc or magnesium complexes of octaethylporphyrin 14 with thallium(III) trifluoroacetate (TTFA) followed by demetallation gave high yields (55–79%) of oxophlorin 8 [49, 50]. 5-Trifluoroacetoxyporphyrins 15 were isolated as stable intermediates of this process (Scheme 3). Similar reactivity was observed when lead(IV) or mercury(II) trifluoroacetates were used, but yields of oxophlorins were significantly lower (19–37%) [50]. Iron(III), copper(II), and nickel(II) complexes of OEP were found resistant to the TTFA attack. An analogous reaction of zinc(II) methyl pyropheophorbide a 16 with TTFA, followed by hydrolysis in the presence of ascorbic acid and air proceeded regioselectively to give dihydrobiliverdin 18 (Scheme 4) [53].

Scheme 3

Oxidation of OEP complexes with TTFA leading to OEOPOH₃ [49, 50]

Scheme 4

TTFA oxidation of zinc(II) methyl pyropheophorbide a [53]

In contrast to OEP complexes, zinc tetraphenylporphyrin ((TPP)Zn^II 19) was converted by TTFA, thallium(III) nitrate (TTN) or cerium(IV) ammonium nitrate (CAN) into a ring-opened tetrapyrrole 20 along with 5,15-disubstituted products 21, 22 (Scheme 5) [51, 52]. These compounds were obtained after acidic workup and chromatography on alumina column. The proper structure of compound 20, formed by addition of water molecule to the demetallated primary product, was established in the course of studies on photooxidation of TPP complexes (Sect. 4.1).

Scheme 5

TTFA oxidation of (TPP)Zn^II [51, 52]

Interestingly, when zinc(II) 5,10,15-triarylporphyrins were reacted with thallium(III) trifluoroacetate, an oxidative dimerization was observed leading to meso–meso linked diporphyrins (Scheme 6) [56]. A similar reactivity of zinc di- and triarylporphyrins with silver(I) salts was reported by Osuka and coworkers [57–59].

Scheme 6

Dehydrodimerization of zinc(II) triphenylporphyrin [56]

In case of TTN and CAN oxidation of (TPP)Zn^II, β-nitrated product 25 (Fig. 4) was also isolated [51, 52]. Meso-nitration of octaethylporphyrin was reported by Bonnett and Dimsdale, who used fuming nitric acid–acetic acid mixture for this reaction; ring opening was not observed under these conditions [42]. Catalano et al. established the dependence of the site of reaction with nitrogen dioxide on the metal coordinated to tetraphenylporphyrin [60]. Nickel(II), copper(II), and palladium(II) complexes were exclusively converted to 2-nitro derivatives, while for more electropositive zinc(II) and magnesium(II) ions ring opening resulting from the reaction at meso position was noted. This observation was rationalized by a different symmetry of π-cation radicals formed by oxidation of metalloporphyrin with NO₂. Also reaction of (TPP^+•)Zn^II(ClO₄) with various nucleophiles yielded mainly 2-substituted derivatives, but in the particular case of nitrite anion, β-nitrated porphyrin product was accompanied with an open-chain compound 20 [61]. More recently, Sarkar et al. described a formation of meso-hydroxylated isoporphyrin 26 upon treatment of meso-tetrakis(3,4,5-trimethoxyphenyl)porphyrin iron(III) or zinc complex with NO₂ (O₂ and NO, Scheme 7) [62, 63]. Further degradation of iron isoporphyrin in solution was observed, and formation of verdoheme- and biliverdin-type products was postulated on the basis of UV–vis spectra. In contrast, zinc derivative remained stable in presence of air and light.

Fig. 4

Zinc 2-nitro-5,10,15,20-tetraphenylporphyrin

Scheme 7

Formation of zinc(II) isoporphyrin [63]

Oxidation of macrocycle can be facilitated by an appropriate modification of the porphyrin ring (both sterical aspects and generation of specific reactivity by substitution are of importance). Ring opening of sterically hindered, dodecasubstituted porphyrins 27 via NaNO₂ treatment in the presence of trifluoroacetic acid and air was studied by Ongayi et al. [64–66]. Authors attributed the ease of degradation of porphyrinic substrates 27 to the tendency to relieve steric strain. The proposed reaction pathway involved oxidation of macrocycle by NO⁺ to a π-cation radical followed by ring opening by dioxygen. A primary bilitrienone product 29 was isolated in 70% yield (Scheme 8), but only for nonyl-substituted system, while in case of meso-tetraphenyl derivative the unstable compound 29 was converted to a biladienone 31 by addition of water. Two isomers of hydrated benzoylbiliverdin 31 were separated, presumably differing in the configuration of C(4)–C(5) bond. Hydration of nonyl derivative 29 was observed as well, but it could be inverted by heating the product 31 above 40°C [66].

Scheme 8

Degradation of dodecasubstituted porphyrins [64–66]

Metallation of 31 with Ni(II), Cu(II), and Zn(II) ions led to formation of 4N chelates 30 in which a dehydrated form of tetrapyrrole was found [65]. Nickel(II) and copper(II) complexes were also prepared by an alternative route from the corresponding metalloporphyrins 28 which were oxidized using meta-chloroperoxybenzoic acid in pyridine in the presence of air (Scheme 8) [65].

Yashunsky, Morozova, and Ponomarev described a conversion of nickel complexes of 5-formylporphyrin oximes 32 in a mixed water-organic solvent system into brown-yellow products [67, 68]. These products were identified as open-chain tripyrroryloxazoles 33 and were isolated by column chromatography in ca. 50% yield (Scheme 9) [68]. A mechanism was proposed involving conversion of oxime substituent into 1,2-oxazine ring and oxidation of formed intermediates by dioxygen leading to fission of pyrrolic β,β′ bond and elimination of α-carbon.

Scheme 9

Conversion of 5-formylporphyrin oximes to tripyrroryloxazoles [67, 68]

A remarkable ease of ring opening was observed for meso-amino-substituted octaethylporphyrin complexes, (H₂N-OEP)Fe^II(py)₂ 34 and (H₂N-OEP)Fe^IIICl 36 [69, 70]. The exposure of their pyridine solutions to dioxygen resulted in its regioselective attack at the substituted carbon; ring opening was followed by a second oxidation step introducing another meso-oxygen atom; at the same time the terminal amide fragment was dehydrated to cyano group (Scheme 10). A resulting (3N + O) complex 37 and its analog with an axial ethanol ligand were characterized by X-ray crystallography. In the case of 34 oxidation, a green intermediate was detected [69]. Its ¹H NMR spectrum indicated a significant degree of ligand radical character and symmetry lowering with respect to the starting iron(II) complex 34, which was attributed to the formation of dioxygen adduct or iron biliverdin derivative 35. A prolonged contact with dioxygen resulted in a slow conversion of compound 37 to a mixture of tripyrrole complex 38 and small amounts of another unidentified product [70].

Scheme 10

Ring opening of iron meso-aminoporphyrin complexes [69, 70]

A pyridine solution of nickel(II) complex of 5-aminooctaethylporphyrin 39 remained unchanged upon exposure to dioxygen [71]. A slow reaction was observed, however, when iron(III) chloride was used as oxidant (Scheme 11), yielding a biliverdin derivative 41 as a minor isolated product (10% yield).

Scheme 11

Oxidation of nickel(II) meso-aminoporphyrin

Phillips et al. reported an oxidative ring opening of copper oxophlorin complex 42 yielding an ester-linked, dinuclear copper complex 43 (Scheme 12) [14]. A proposed mechanism included oxidation of macrocycle by dioxygen leading to (OEPO^•)Cu^II complex, its reaction with the starting (OEPOH)Cu^II to produce a C–O link, ring opening by addition of dioxygen and termination of the process by superoxide anion.

Scheme 12

Formation of dinuclear copper complex [14]

A formation of verdoheme analog 45, which was further hydrolyzed to octaalkylbiliverdin 46, was observed by Chang et al. upon oxygenation of cobalt(II) porphyrin substituted with naphthoic acid 44 (Scheme 13) [72]. The substituent was believed to support the activation of molecular oxygen by the metal center and was finally cleft as 8-formyl-1-naphthalenecarboxylic acid. A helical cobalt(III) complex of acyclic pentapyrrole 48 was obtained by Yamanishi et al. by treatment of cobalt(II) 5-(2-carbamoylphenyl)-10,15,20-triphenylporphyrin 47 with 1-methylimidazole and air (Scheme 14) [73]. An amide substituent and axial base (imidazole and pyridine derivatives were tested) was found essential for dioxygen activation, which resulted in breaking in C(4)–C(5) bond, followed by formation of oxoisoindole ring and addition of hydroxyl group to a meso position. Chiral HPLC separation of racemic 48 was performed. The application of chiral axial ligands bearing (S) configuration: nicotine, cotinine, or bifonazole led to the preferential formation of (M)-helical form of pentapyrrolic product.

Scheme 13

Oxygenation of cobalt(II) porphyrin substituted with naphthoic acid [72]

Scheme 14

Formation of Co(III) complex of an acyclic penatpyrrole [73]

An unexpected ring opening upon bromination of tetraphenylporphyrin with 20 equivalents of N-bromosuccinimide (NBS) in chloroform–methanol solution was described by Liu et al. [74]. From a mixture of reaction products which was treated with zinc acetate, crystals of compound 50 were isolated (Scheme 15). An X-ray structure of this zinc complex revealed the presence of nine bromo substituents at pyrrole rings and three methoxy groups attached to meso positions. Various para-phenyl-substituted tetraarylporphyrins could also be converted to the corresponding ring-opened products formed in 11–46% yield; also zinc tetraphenylporphyrin underwent a similar reaction, while the use of copper(II) and nickel(II) as central ions resulted only in β-bromination. A mechanism of the transformation was proposed involving MeOBr (formed from NBS and methanol) as an active species responsible for perbromination of pyrrole rings to form a highly congested dodecasubstituted macrocycle. The steric hindrance could be released by addition of another MeOBr molecule to C(meso)–C(α) bond followed by nucleophillic addition of methoxide to the meso positions of ring-opened product.

Scheme 15

Ring opening upon bromination of TPPH₂ [74]

3.2 Degradation of Metalloporphyrin Catalysts

In this part, we shall discuss reactions of iron and manganese complexes with reagents which are typically used in metalloporphyrin-catalyzed oxidations (hydroxylations, epoxydations): peroxides, peroxyacids, and molecular oxygen [75–78]. Since typically an organic substrate is used in an excess in these processes, the problem of catalyst stability under such conditions has been often neglected. If this has been taken into account, methods of increasing metalloporphyrin robustness have been sought, mainly via its appropriate modification [79–81]. It was achieved by a substitution of porphyrin ring increasing catalytical activity and/or providing steric protection not only against formation of μ-oxo dimer PFe^III–O–Fe^IIIP but also against attack of oxidants on meso positions [75]. Possible inter- and intramolecular processes leading to degradation of metalloporphyrin have been addressed [82, 83], though papers devoted to the analysis of catalyst stability have been relatively rare [84, 85].

In the recent years, several groups concentrated their efforts on the analysis of oxidation of porphyrin complexes by different oxidants used for the metalloporphyrin-catalyzed oxidations of organic substrates. Starting from simple, rather qualitative observations of possible decomposition of macrocycle as indicated by intensity lowering of Soret band in the UV–vis spectra, the studies have been typically extended to the analysis of reaction kinetics and attempts of determination of possible reaction mechanisms. However, in most cases the fate of catalyst and structures of degradation products have not been considered.

Stephenson and Bell investigated mechanism and kinetics of iron porphyrin-catalyzed epoxidation of olefins by hydrogen peroxide [86, 87]. Among factors affecting the activity of catalyst, oxidative degradation of porphyrin ring and μ-oxo dimer formation were discussed. The authors attributed the macrocycle decomposition to the attack of hydroxyl radicals (generated from of coordinated hydrogen peroxide). This hypothesis was in agreement with the observation that factors increasing the rate of hydroxyl radical generation contributed also to porphyrin degradation. The efficiency of iron porphyrin epoxidation catalysts was also studied by Cunningham and coworkers [88–90]. They connected the observed bleaching of the catalyst with its direct oxidation in the resting state (Fe(III)) rather than the high-valent intermediates.

Rocha Gonsalves and coworkers analyzed the epoxidation of alkenes by peroxides catalyzed by manganese porphyrins [91]. Two mechanisms of degradation of catalysts were found, depending on their structure and reaction conditions: an intramolecular pathway predominated when a metallo-oxo species was an active intermediate, while a metalloacylperoxo derivative favored an intermolecular one.

Ungvarai-Nagy and coworkers reacted iron(III) complexes of protoporphyrin IX and tetra(4-sulfonatophenyl)porphyrin with bromate and observed macrocycle degradation in acidic solutions [92–94]. Türk et al. investigated the stability of water-soluble porphyrins and their manganese(III) complexes toward peroxides and sodium hypochlorite [95–98]. The degradation rate constants were found dependent on the structure of porphyrin substrate, nature of oxidant, and pH of the solution. However, possible degradation pathways and structures of products formed were not discussed. Lente and Fábián studied kinetics and mechanism of oxidation of water-soluble porphyrin 51 with hydrogen peroxide and peroxomonosulfate anion [99]. The analysis of ESI mass spectra of the reaction mixture revealed the presence of iron complex of biliverdin-type tetrapyrrole 52 and a sulfonated benzoic acid 53 as dominant products of porphyrin decomposition (Scheme 16). Hopefully, this precedent will prompt further works on structural characterization of ring-opened oligopyrroles produced in the course of degradation of metalloporphyrin catalysts.

Scheme 16

Degradation of water-soluble iron porphyrin catalyst [99]

3.3 Oxidation of N-Confused Porphyrins

Though N-confused porphyrins have been known for almost two decades [100, 101], relative little studies have been devoted to their degradation. However, the instability of these macrocycles during metallation performed under aerobic conditions has been frequently observed. This led Furuta et al. to investigate the nature of the degradation product [102]. They found that in the course of reaction with copper(II) acetate in the presence of air N-confused tetraphenylporphyrin 54 underwent an oxidative transformation. Copper(II) complex of a linear tripyrrole 55 was isolated from the reaction mixture in 34% yield (Scheme 17). No other products were identified. Free tripyrrinone 56 and its zinc(II), nickel(II), palladium(II), platinum(II), and cobalt(II) derivatives were obtained [102]; crystal structures of Cu(II) and Pd(II) complexes showed a square-planar, N₃O-coordination mode [103].

Scheme 17

Degradation of N-confused porphyrin [102]

A suggested mechanism of the degradation involved two successive reactions with molecular oxygen, activated by coordinated Cu(II) ion, leading to scission of two C(meso)–C(α) bonds. Further studies on the regioselectivity of the process, performed on 5-(2-pyridyl) derivative, showed that the N-confused pyrrole was cleft together with 5-aryl substituent, which proved the primary attack of dioxygen at C(1)–C(20) bond [102]. In contrast to this observation, Pawlicki et al. found that copper(II) complex of pyrrole-appended O-confused tetraaryloxaporphyrin 57 reacted with dioxygen yielding both possible tripyrrolic degradation products 58, 59 (resulting from breaking of either C(1)–C(20) or C(4)–C(5) bond) formed in 7:3 ratio, along with and the product of oxygen atom insertion into a copper–carbon bond 60 (Scheme 18) [104]. Apparently, meso- and pyrrole substitution can direct the attack of dioxygen molecule; a discussion on the regioselectivity of oxidative ring opening of N-confused porphyrin can be found in the part devoted to photooxidation of tetrapyrroles (Sect. 4.1).

Scheme 18

Oxygenation of copper(II) complex of pyrrole-appended O-confused oxaporphyrin

3.4 Oxidation of Corroles

Despite general similarity to porphyrins, corroles exhibit a specific and sometimes unpredictable reactivity [105]. Both macrocycle families share a common 18-π-electron system, but lack of one meso bridge in corroles leads to increase of electron density and, as a consequence, a susceptibility to oxidative ring opening. Interestingly, all reports on such reactions concern meso-substituted systems [105], though any systematic and comprehensive research on factors influencing corrole stability has not been performed. Most work in the field concentrated on photooxidation of corroles (see Sect. 4.2). Macrocycle opening by certain oxidants has been also described, though typically formation of biliverdin-type compounds only accompanied the reaction of major interest.

A fully brominated open-chain tetrapyrrole 61 (Fig. 5) was identified as a reaction by-product resulting from breaking of C(4)–C(5) bond of germanium(IV) 5,10,15-triphenylcorrole treated with bromine [106]. A linear tetrapyrrole 62 was formed in minor quantities when triarylcorroles were reacted with 4-amino-4H-1,2,4-triazole [107]. This time, C(5)–C(6) bond of the original macrocycle was cut (Fig. 5). Ring opening at C(10) was observed upon conversion of triarylcorrole 63 to a corresponding porphyrin (Scheme 19) [108]. A proposed mechanism of the transformation involved a [2 + 2] cycloaddition of two corroles and cleaving of a spirocyclobutane intermediate by dioxygen connected with an extrusion of meso-carbon bearing para-nitrophenyl substituent.

Fig. 5

Ring-opened products of corrole oxidation [106, 107]

Scheme 19

Conversion of triarylcorrole to poprhyrin and a linear tetrapyrrole [108]

Other pathways of corrole oxidation were reported, including isocorrole formation by 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) treatment [109, 110] or demetallation [111, 112] and oxidative dimerization of 5,10,15-tris(pentafluorophenyl)corrole with formation of β-β′ bond(s) upon heating in 1,2,4-trichlorobenzene [113].

4 Photooxidation of Tetrapyrroles

Photooxidation of tetrapyrrolic macrocycles and their complexes is considered as the most important process responsible for the frequently observed photobleaching of these compounds [114]. This phenomenon is connected with the ability of porphyrin derivatives to activate molecular oxygen in the presence of light. Energy transfer from the excited state of the macrocycle to the ground state of the dioxygen molecule results in the generation of singlet oxygen. As a practical consequence, tetrapyrroles are used as photosensitizers for degradation of various organic substrates [115–117] and in photodynamic therapy (PDT) for treatment of cancer, macular degeneration, chronic skin diseases, and other conditions [118–121]. Under certain conditions, also tetrapyrrole itself can be attacked by singlet oxygen, which may eventually lead to ring opening.

In the context of not only photosensitizer stability but also other applications of tetrapyrroles, light-driven reactivity of porphyrin derivatives toward O₂ is of particular interest. Photobleaching of photosensitizers used in photodynamic therapy was thoroughly reviewed by Bonnett and Martínez [114]. Thus, older contributions will be only briefly described in his chapter, and the attention will be focused on recent developments in the field.

4.1 Photooxidation of Porphyrins, N-Confused Porphyrins and Phlorins

Most metal-free porphyrins are not prone to photooxidative degradation due to the relative high value of oxidation potential. However, their deprotonation or conversion to complexes of electropositive metal ions (e.g. with Zn(II), Cd(II) or Mg(II)) lowers redox potential and therefore the robustness of the system toward oxidative degradation is also reduced.

Fuhrhop and Mauzerall reported the photooxidation of magnesium(II) octaethylporphyrin 66 and identified a linear tetrapyrrole 67 as the final product for this transformation (Scheme 20) [122]. This compound was also found by Bonnett et al. as one of the two main products of photooxidation of octaethyloxophlorin 8 in neutral solution (the other being 5,15-dioxoderivative 68, Scheme 20) [123]. Light-driven ring opening of zinc, magnesium, cadmium, thallium(I) complexes of tetraphenylporphyrin 69 as well as the porphyrin dianion (TPP²⁻) was examined by several groups [124–128]. A proper structure of the final product 20 or 70 was finally established by Cavaleiro and coworkers [128]. A bilindione derivative bearing –OR substituent in 15-position resulted from dioxygen attack on the C(meso)–C(α) bond, followed by demetallation and addition of water or alcohol (ROH, Scheme 21). As proved by isotope labeling studies, both carbonyl oxygen atoms are derived from the single molecule of O₂ [125, 126]. Silva et al. studied effects of substitution of tetraarylporphyrin on the degradation of cadmium(II) complexes and showed that the reaction was governed by steric factors rather than electronic ones [129]. The presence of substituents in ortho positions of phenyl rings prevented the macrocycle from the dioxygen attack (Scheme 22). Both cadmium(II) tetra(3,4,5-trimethoxyphenyl)porphyrin 74 and zinc(II) 2,3,12,13-tetrabromoporphyrin 77, however, were converted to the corresponding open-chain products 75, 76 (two forms were observed) and 78, respectively (Schemes 23, 24).

Scheme 20

Photooxidation of (OEP)Mg^II [122]. Compounds 67 and 68 were found among products of light-driven oxidation of oxophlorin [123]

Scheme 21

Photooxidation of TPP complexes [128]

Scheme 22

Zinc(II) and cadmium(II) tetraarylporphyrins not prone to photooxidation [129]

Scheme 23

Photooxidation of cadmium(II) tetra(3,4,5-trimethoxyphenyl)porphyrin [129]

Scheme 24

Photooxidation of zinc(II) β-tetrabromoporphyrin [129]

Zinc(II) complexes of linear tetrapyrrole 20 were obtained [130]. Depending on metallation conditions, 3N + O or 4N coordination was found in these chelates, in the latter the loss of methanol or water led to a fully conjugated structure (Scheme 25). Copper(II) complex, formed by transmetallation of photooxidation product of magnesium(II) tetraphenylporphyrin, heated with excess of copper(II) acetate yielded a dinuclear species 82 (Fig. 6); the additional meso-oxygen bridging two copper ions originated probably from water since compound 82 was obtained also under dioxygen-free conditions [12].

Scheme 25

Metallation of bilindione 20 [130]

Fig. 6

Dinuclear Cu(II) complex

Mixed 3N + O copper(II), nickel(II), and zinc(II) complexes were formed from ligands 84 and 85, obtained by the photooxidation of a meso-substituted phlorin 83 [131]. Two isomers of bilindione and its complexes were described, with a different orientation of the terminal pyrrolone ring (Scheme 26). Their interconversion upon irradiation which caused E–Z isomerization was demonstrated. LeSaulnier et al. investigated photodegradation of phlorins bearing different number of mesityl substituents 86 (Fig. 7) [132]. As expected, the incorporation of bulky mesityl substituents enhanced phlorin stability.

Scheme 26

Photooxidation of meso-substituted phlorin [131]

Fig. 7

Mesityl-substituted phlorins

Photobleaching of certain metal-free porphyrins was also observed, not necessarily connected with ring-opening reactions. Water-soluble, cationic 5,10,15,20-tetrakis(1-pentyl-4-pyridyl)porphyrin underwent fast photodegradation in aqueous media [133]. Niziolek and coworkers observed that lipid peroxidation in membranes, mediated by protoporphyrin IX as a singlet oxygen photosensitizer, can be prolonged in the presence of nitric oxide [134]. NO was found to protect the macrocycle against oxidative destruction. Cavaleiro et al. carried out photochemical studies on stability of porphyrins and their copper(II) complexes and showed that the latter had shorter triplet lifetimes and were more stable with respect to photodegradation than the respective free bases [135]. Similarly, perfluorination of phenyl substituents of tetraphenylporphyrin had a beneficial effect on the macrocycle robustness.

When 2-aza-21-carba-5,10,15,20-tetraphenylporphyrin (inverted porphyrin 54) was dissolved in dichloromethane and irradiated with visible light in the presence of air, only traces of degradation products could be detected. Instead, photooxidation of the dianion of N-confused tetraphenylporphyrin 87 was performed which led to a mixture of linear oligopyrroles within 1 h [136]. Chromatographic separation yielded fractions containing tripyrrinone 56 (33% of reacted substrate), its dimethyl acetal 88 (24%) and N-confused tetrapyrrole 89 (31%, Scheme 27). Upon metallation with palladium(II), compound 89 converted into complex 90 containing a conjugated N-confused biliverdin analog acting as a binucleating ligand with two types of coordination surroundings: (NNNO) and (CNOO) (Scheme 27) [136]. Further exploration of photooxidation products led to detection of the additional, unexpected tetrapyrrolic compound 70 (present in ca. 6–9% yield), typically formed in the course of TPP²⁻ degradation [137]. This observation led to a conclusion that two different mechanisms operate in one molecule. Apart from 1,2-dioxygen addition, which is common for tetrapyrrolic macrocycles, the rare 1,3-addition was also found (Scheme 28).

Scheme 27

Photooxiadtion of dianion of N-confused porphyrin and formation of dinuclear palladium complex of N-confused biliverdin derivative [136]

Scheme 28

Mechanism of 1,2- and 1,3-dioxygen addition to dianion 87 [137]

Compound 89, the major isolated tetrapyrrolic product of photooxidation of N-confused porphyrin dianion resulted from cleavage of C(10)–C(11) bond of the original macrocycle. However, changing of reaction conditions (metallation with zinc or replacing of methoxide with ethoxide for the conversion of N-confused porphyrin to its dianion) allowed us to detect other tetrapyrrolic degradation products (Wojaczyński J, Popiel M, Gońka E, Latos-Grażyński L, unpublished results). DFT calculations performed on inverted porphyrin dianion did not show any significant differences among meso positions which could be responsible for any preference of dioxygen attack. Apparently, the observed product distribution reflects not only the regioselectivity of O₂ addition but also the relative stability of degradation products under given conditions since part of them can undergo further reactions (as proved by the observation of tripyrrinone products 56, 88 which could be formed from primary ring opening at C(5) or C(20) followed by loss of inverted pyrrole in the second oxidation step).

4.2 Photooxidation of Corroles

The question of photochemical stability of corroles is particularly important in context of their possible application in photoactive devices, chromophores for light energy conversion and singlet oxygen generation [138–140]. Early observations indicated a stepwise degradation of corroles in solution in the presence of light and air. The process was monitored by UV–vis spectroscopy since a systematic lowering of Soret band intensity was observed [141, 142]. The presence of electron-withdrawing substituents in corrole ring or complexation with metal ion was shown to increase the macrocycle robustness. The first proposal of a structure of degradation product was made by Guilard and coworkers who investigated photooxidation of 2,3,17,18-tetraethyl-7,8,12,13-tetramethyl-10-phenylcorrole 95 (Scheme 29) [143]. A biliverdin derivative 97 was obtained in 24% yield and characterized by ¹H NMR, IR, MS and elemental analysis which were in general agreement with an intuitive assumption that pyrrole–pyrrole (C(1)–C(19)) bond was attacked by dioxygen molecule. No other reaction products were isolated. Opening of corrole ring by breaking of C(α)–C(α) bond was also postulated by Paolesse et al. for photooxidation of β-octaalkylcorrole with a porphyrin attached to a 10-position 96 [144]. In both cases the symmetry of resulting ¹H NMR spectrum was lower than expected for the proposed structure (an analogous triarylbilindione obtained by Yamauchi et al. by coupled oxidation of iron porphyrin exhibited a simple ¹H NMR pattern [145]). The difference was attributed to isomerization of biliverdin moiety to (E,Z,Z) configuration; however, certain spectral features (e.g. a doublet at ca. 8 ppm which could be assigned to ortho-aryl protons) suggest that a structure resulting from opening at aryl-substituted meso position 99 could be considered as well. On the other hand, the observation that 2,3,17,18-tetraphenyl analog 100 (meso-unsubstituted!) was found far more stable than 95 and a similar behavior of corresponding cofacial bis(corroles) connected with a 10-anthracene bridge suggested efficiency of a steric protection of bipyrrole fragment limiting the access of dioxygen molecule to C(1)–C(19) bond [146].

Scheme 29

Photooxidation of meso-aryl-substituted corroles 95, 96 [143, 144]. An alternative structure of degradation product 99 and a tetraphenyl analogue of compound 95 are also shown

Degradation of meso-triarylcorroles has received a considerable attention [141, 142], but only a systematic mass spectrometry study on decomposition pathways of these compounds by Świder et al. led to identification of isocorroles and biliverdin derivatives as photooxidation products [36]. Preparative degradation experiment was conducted with corrole 101 with 5 and 15 positions protected by bulky substituents, which was dissolved in acetonitrile and exposed to sunlight for 60 h. Three major compounds 102–104 were isolated from the reaction mixture (Scheme 30), indicating dioxygen attack on meso-C(10) carbon atom. In our studies on photooxidation of triphenylcorrole and tris(p-methoxyphenyl)corrole, scission of C(9)–C(10), but also of C(4)–C(5) bond of symmetrical, non-hindered substrate was noted [147]. As can be seen, any product resulting from breaking of a direct pyrrole–pyrrole bond has not been detected from photodegradation of triarylcorroles. One couldn’t exclude, however, that the presence of β-alkyl substituents in compounds 95, 96 directs dioxygen attack to the C(1)–C(19) bond. A strong dependence of reaction outcome on substitution of macrocycle is illustrated by reactivity of 5,10,15-tris(pentafluorophenyl)corrole which stirred at room temperature under ambient light and air slowly converted to 3,3′-linked dimer and 3,3′,17′,3″-trimer [148].

Scheme 30

Photooxidation of triarylcorrole [36]

5 Coupled Oxidation

Heme oxygenase, responsible for the oxidative destruction of unwanted heme, requires molecular oxygen but also the source of electrons for its function (see Sect. 6 of this contribution). Oxidation of iron porphyrin in pyridine in the presence of reducing agent (hydrazine or ascorbic acid) has been used as a model for the enzymatic reaction [149, 150]. Pioneering studies by Lemberg (who described coupled oxidation of iron protoporphyrin IX with H₂O₂-ascorbic acid), Fischer and Libowitzky were performed on natural heme derivatives [151, 152]. Later on, higher symmetry synthetic model compounds such as complexes of octaethylporphyrin or ethioporphyrins have been used. A thorough analysis of coupled oxidation process was presented in a series of papers published in the years 1992–2008 by Balch, Latos-Grażyński, and coworkers. They isolated and characterized two main products of degradation of (OEP)Fe^II(py)₂ 105 caused by air in the presence of ascorbic acid: a diamagnetic verdoheme 106 (50%) and a paramagnetic dimeric iron biliverdin complex 107 (38%, Scheme 31) [153, 154]. In situ monitoring of the degradation of (OEP)Fe^II(py)₂ by dioxygen with hydrazine as sacrificial reductant identified iron oxophlorin, (OEPO)Fe(py)₂ 11 as a key intermediate of the process [155].

Scheme 31

Coupled oxidation of (OEP)Fe^II(py)₂ [153–155]

Oxidation of (OEP)Fe^IIICl under pyridine-free conditions, but in the presence of cyanide ions as axial ligands, was also demonstrated [156]. Depending on cyanide concentration, iron oxophlorin or 5-oxaporphyrin complex (verdoheme) was formed. Coupled oxidation of Co(II) octaethylporphyrin leading to cobalt verdoheme and biliverdin analogs was also described [157]. In the recent years, degradation of iron complexes of β-unsubstituted, meso-arylporporphyrins under coupled oxidation conditions was investigated as well [145, 158–160].

5.1 Oxophlorins

The question of structure and reactivity of oxophlorins (hydroxyporphyrins) has been considered in numerous contributions. In addition to tautomeric equilibrium (Scheme 32), ocatethyloxophlorin was shown to undergo a facile one- and two-electron oxidation [161]. In consequence, it can serve as a trianionic, dianionic, and monoanionic ligand, and various electron distributions between metal ion and ligand are possible. Not surprisingly then, a rich coordination chemistry was observed for octaalkyloxophlorins: zinc(II), nickel(II), cobalt(II), copper(II), iron(III), and manganese(III) monomeric complexes with meso-hydroxyl groups [14, 162–166], dimeric complexes linked by meso-oxygen bridges with Fe(III), Mn(III), and In(III) [162, 165, 167–171], coordinated oxophlorin trianions [165, 166], coordinated radicals [163, 164, 168], and complexes of oxidized monoanionic form [168, 170] were reported. Variety of structures and their mutual interconversion is exemplified by iron(III) complexes shown in Scheme 33 [161]. The thorough overview of coordination chemistry of oxophlorins/meso-hydroxyporphyrins was published by Balch in 2000 [161].

Scheme 32

Keto-enol tautomeric equilibrium of oxophlorin

Scheme 33

Iron complexes of octaethyloxophlorin [161]

Electronic structure of iron oxophlorin (OEPO)Fe(py)₂ and its analogs was a subject of a long-lasting debate [162, 167, 172–175]. Three possible electron distributions have been taken into account (Fig. 8). Patterns of paramagnetically shifted ¹H NMR signals observed for (OEPO)Fe(py)₂ and related species suggested a significant contribution of a ligand radical form (OEPO^•)–Fe^II [162, 172, 173]. A similar alteration of isotropic shifts was found for iron triphenyloxophlorin complexes [158]. DFT calculated spin density maps for oxophlorin radicals allowed to reproduce the major observed spectroscopic features [176]. Later on, Rath et al. showed the dependence of electronic structure on the nature of axial ligands, with 2,6-xylyl isocyanide stabilizing the radical resonance structure [(OEPO^•)Fe^II(CNR)₂] [177]. Recent crystallographic, magnetic, and spectroscopic measurements indicated the importance of Fe(III)/oxophlorin trianion form for bis-pyridine and bis-imidazole complexes [178]. DFT calculations of electronic structure of (OEPO)FeL₂ complexes performed by Gheidi et al. confirmed the dependence of electron distribution and iron spin state on the nature of axial ligands [179].

Fig. 8

Resonance forms of iron oxophlorin

Reactivity of iron oxophlorin (OEPO)Fe(py)₂ (11) was extensively explored. Apart from coordination chemistry depicted in Scheme 33, interaction with small molecules was investigated [180, 181]. A reversible binding of NO to (OEPO)Fe(py)₂ connected with the formation of dimeric species 115 was reported (Fig. 9) [180]. A reduced form of oxophlorin, (OEPOH)Fe^II(py)₂, was converted to (OEPOH)Fe^II(CO)(py) (116) upon treatment with carbon monoxide, and pyridine could be replaced with hydrazine to form (OEPOH)Fe^II(CO)(N₂H₄) (117); both diamagnetic complexes were found extremely air sensitive and in the presence of dioxygen an immediate reaction leading to (OEPO)Fe(py)₂ 11 was observed [181].

Fig. 9

Iron oxophlorin NO and CO complexes

Both redox processes preserving a basic skeleton of oxophlorin [168, 170] and coupled oxidation leading to verdoheme and biliverdin have been reported [155, 156]. Under certain conditions, oxidative degradation is not limited to macrocycle opening. Rath et al. observed that in the absence of reducing agent, addition of dioxygen to a pyridine solution of oxophlorin complex (OEPO)Fe(py)₂ (11) caused stepwise changes, resulting in formation of iron biliverdin 118, and, finally, oxidative removal of pyrrole unit yielding a linear tripyrrole complex 38 (Scheme 34) [182]. This compound was also formed when compound 118 or verdoheme 106 was exposed to O₂.

Scheme 34

Conversion of iron oxophlorin to a linear tripyrrole complex [182]

5.2 Verdohemes

A green iron complex of 5-oxaporphyrin, called verdoheme, is an important intermediate in the process of heme oxidative cleavage by heme oxygenase [183]. It is also formed in the course of coupled oxidation of iron porphyrins but can be also obtained by dehydration of biliverdin in the presence or iron salts [184, 185].

Metal-centered reactions have been reported, including changes of axial ligation, and metal oxidation and spin state, as demonstrated for iron (Scheme 35) and cobalt 5-oxaporphyrin complexes [153, 172, 186–192]. Coordination chemistry of verdohemes and biliverdin derivatives has been recently reviewed by Balch and Bowles [193].

Scheme 35

Interconversion of iron 5-oxaporphyrin complexes [193]

Ligand transformations are particularly important for the study of macrocycle degradation since they can lead to linear tetrapyrrolic products. Two mechanisms of verdoheme ring opening leading to biliverdin have been described: an oxidative pathway [194, 195], resulting in release of Fe³⁺, and a hydrolytic route (Scheme 36). The latter is generally believed to begin with addition of hydroxide to the macrocycle. To characterize this kind of reactivity of 5-oxapophyrin complexes, their conversions by anionic nucleophiles have been investigated [196–200]. Helical, ring-opened products resulting from the addition of alkoxide, thiolate, and amide ions to zinc(II) (125) and cobalt(II) verdoheme (126) were isolated and structurally characterized (Scheme 37) [197, 201]. More complex process was observed when cyanide ion was added to zinc 5-oxaporphyrin 125, as macrocycle cleavage was accompanied with substitution at one or two meso positions (Scheme 38) [199]. A dimeric complex [(OEBOMe)Fe^II]₂ 130 was isolated from the reaction of iron(II) verdoheme with OMe⁻ ion (Scheme 39) [198]. Ring opening of Fe^II and Fe^III verdohemes with methoxide or hydroxide was monitored by ¹H NMR spectroscopy [200]. Characteristic alternating shift patterns indicating radical character of the particular intermediates and remarkable paramagnetic shifts of meso resonances of certain species were noted.

Scheme 36

Two pathways of verdoheme to biliverdin coversion

Scheme 37

Opening of zinc(II) and cobalt(II) verdohemes by nucleophiles [197, 201]

Scheme 38

Zinc(II) verdoheme opening by cyanide [199]

Scheme 39

Reaction of iron(II) verdoheme with methoxide [198]

Utilizing O₂ as oxidant, Rath et al. demonstrated a conversion of Fe(II) verdoheme into a highly oxidized (Fe(IV) bound to bilindione ligand or Fe(III) coordinated to oxidized form of ligand) biliverdin complex (Scheme 40) [195]. Its reduction with zinc amalgam resulted in previously characterized dimeric [(OEB)Fe^III]₂ (107). Earlier, Saito and Itano reported that prolonged (1 month) exposure to air of verdoheme dissolved in ethylene glycol – pyridine solution led to several iron-free ring-opened products, including tripyrrolic ones [202]. Most of the starting material was recovered from the reaction.

Scheme 40

Oxidation of iron(II) verdoheme [195]

Theoretical study on factors determining verdoheme conversion to biliverdin was performed by Safari and coworkers. The role of axial ligands as well as coordinated metal ion was taken into account [203–206].

5.3 Biliverdins

A dimeric helical iron(III) complex 107 of octaethylbilindione, a biliverdin analog, was obtained by Balch et al. along with verdoheme from the coupled oxidation of (OEP)Fe^II(py)₂ [154]. Its treatment with pyridine resulted in cleavage of Fe–O bonds and formation of monomeric (OEB)Fe^III(py)₂ 132 (Scheme 41). An easy demetallation of [(OEB)Fe^III]₂ with hydrochloric acid released the blue bilindione OEBH₃ (9) [154]. Its complexes with other metal ions were investigated by Bonnett and coworkers [207, 208] and by Balch group [13, 209–215]. Interestingly, remetallation of OEBH₃ with iron has not been successful [193], while manganese, cobalt, nickel, copper, zinc, palladium, and boron complexes have been obtained. For Mn(III), a dimeric complex with oxygen bridges [(OEB)Mn^III]₂, which was cleft by pyridine to monomeric (OEB)Mn^III(py)₂ (in a full analogy with Fe(III) complexes) was described [210]. Spectroscopic investigations of monomeric, four-coordinate complexes of OEBH₃ with cobalt, nickel, copper, and palladium suggested their electronic structure consistent with the presence of M(II) ion and oxidized ligand radical (OEB^•)M^II [208–210, 214] A significant degree of radical character was also postulated for iron complexes obtained by verdoheme ring opening [200]. Cobalt biliverdins were alternatively obtained by a coupled oxidation of Co(II) octaethylporphyrin [157]. Oxidation with iodine converted (OEB^•)M^II complexes (M = Co, Ni, Pd) into ones containing an oxidized form of bilindione ligand [211, 214], while aerial oxidation of copper and cobalt complexes 133 resulted in cleavage of tetrapyrroles yielding complexes with two coordinated dipyrrolic units 134 (Scheme 42) [209, 213].

Scheme 41

Splitting of dimeric iron(III) octaethylbiliverdin complex [154]

Scheme 42

Oxidation of cobalt and copper biliverdin complexes [209, 213]

A unprecedented tetranuclear complex 135 consisting of two helical (OEB)Pd^II units bridged by (Pd₂ ^I)²⁺ fragment was isolated along with monomeric (OEB)Pd from the insertion of palladium into OEB ligand [13, 214, 215]. Reaction of this compound with iodine resulted in formation of rearranged monomeric complex 136: an incorporation of oxidized meso-carbon into a terminal pyrrolone unit was observed (Scheme 43)[215].

Scheme 43

Oxidation of tetranuclear palladium biliverdin complex by I₂ [215]

Oxidative cyclization of biliverdin complexes leading to metalloverdohemes was also studied [216]. Nickel(II), cobalt(II), and copper(II) octaethylformylbiliverdins were converted to verdoheme analogues by treatment with hydrogen peroxide or (in case of Cu(II) species) by heating with trifuoroacetic acid under dioxygen (Scheme 44) [9]. Formation of carbon monooxide and dioxide was detected in the course of the reaction. Addition of trifluoroacetic acid to the dichloromethane solution of palladium octaethylbilindione also resulted in ring closure. Only 5 min of stirring at room temperature was found sufficient to cause the transformation [215] .

Scheme 44

Conversion of copper(II) formylbiliverdin to verdoheme [9]

Formation of biliverdin derivatives in a process of coupled oxidation of iron porphyrins is not limited to β-octaalkyl derivatives. Mizutani’s group worked out a high-yielding method of preparation of tetraphenylbiladienone 20 (a major product of degradation of TPP complexes by Tl(III), Ce(IV) or photooxidation, see Sects. 3.1 and 4.1) [145, 159, 160]. Iron meso-tetraphenylporphyrin subjected to coupled oxidation procedure in a chloroform solution yielded a mixture of isomeric biladienones 20 (63%) and 140 (15%; Scheme 45) [145]. Compound 140 could be photoisomerized to 20, while the reverse transformation did not proceed. The additional bilindione products 141, 142 were obtained when the reaction was carried out in refluxing chloroform; both compounds were converted to each other with visible light illumination. An X-ray structure of isomer 141 proved its ZZZ configuration and a helicoidal conformation. The procedure could be extended to other tetraarylporphyrins substituted in para positions with OCH₃, COOCH₃, CN, OC₁₂H₂₅, and COOC₁₂H₂₅ groups [159, 160]. The reaction was accelerated by electron-withdrawing substituents, which also favored the formation of triarylbilindiones (maximum yield of 19% was noted for p-COOC₁₂H₂₅ derivative) while electron-donating ones increased the amounts of biladienones (85% yield for methoxy-substituted substrate was found). Interestingly, the presence of one methoxy substituent in ortho position of each of phenyl groups did not prevent the macrocycle from oxidative degradation: both biladienone and bilindione were formed in 14% and 10% yield, respectively. Cyclization of bilindiones 141 was also described yielding the corresponding zinc triarylverdohemes, which were isolated as trfiluoroacetates [217].

Scheme 45

Coupled oxidation of iron(III) tetraphenylporphyrin [145]

Theoretical studies on biliverdin and its complexes involved such aspects as molecular and electronic structure of its isomeric forms [218] and biliverdin-based metalloradicals [219], spin density distribution in metallobiliverdin radicals [220], energetics and dynamics of dimer formation by oxidized species [221], and mechanism of reduction to bilirubin [222].

5.4 Regioselectivity of Coupled Oxidation

Studies on regioselectivity of coupled oxidation of iron porphyrins were aimed to establish the influence of factors connected with a structure of macrocyclic substrate on the outcome of degradation process. Four isomeric biliverdins were isolated in comparable yields from coupled oxidation of iron(III) protoporphyrin IX, thus regioselectivity observed in natural systems (see Sect. 6) was lost [223, 224]. Later studies showed that replacement of 3-methyl group of mesoheme with CF₃ substituent had a great influence on product distribution: ring-opening occurred mainly at C(20) yielding δ isomer as a major product [225].

Coupled oxidation of 5- or 15-phenyl-substituted iron(III) protoporphyrin IX in pyridine solution yielded biliverdins opened only at three unsubstituted meso positions (as illustrated in Scheme 46 for 5-phenyl derivative) [35, 226]. Similarly, 5-aryl-mesohemes III were cleft at C(10), C(15) or C(20) yielding (due to symmetry of the starting complex) only two isomeric products [227]. The character of aryl ring substituent influenced the reaction yield, but its impact on the product distribution was rather negligible. The identified biliverdin isomers served as references for studies on the regioselectivity of heme oxygenase (see Sect. 6 of this contribution) [228].

Scheme 46

Coupled oxidation of 5-phenylprotoheme [35]

6 Biodegradation of Tetrapyrrolic Macrocycles

Degradation of tetrapyrrolic macrocycles is used by living organisms both as a method of removal of unwanted (redundant) heme or chlorophyll and as a way of synthesis of linear systems (bilins) which can also fulfill important physiological functions [229–232]. Mechanism of transformation of macrocycles to acyclic oligopyrroles has now become much more clear and better understood in the result of numerous studies on model reactions and determination of active intermediates and structures of key enzymes.

6.1 Heme Oxygenase

Heme oxygenase (HO), an enzyme responsible for the oxidative conversion of heme to biliverdin, was discovered by Tenhunen et al. in 1968 [233]. Since that report, numerous studies have been devoted to understanding the mechanism of the enzymatic action [183, 234–239]. HO is unique among heme enzymes in that activation of dioxygen by prosthetic group is utilized for its own degradation. A regiospecific conversion of heme to biliverdin IXα, carbon monooxide and Fe²⁺ ions requires three molecules of O₂ and the total uptake of seven electrons, and proceeds in three successive steps (Scheme 47): meso-hydroxylation, followed by release of CO and verdoheme formation and ring opening connected with iron loss yielding free biliverdin. Formation of such metabolites implies other functions of heme oxygenase, involving iron homeostasis, cytoprotection against oxidative injury and cellular stress, and postulated role in cellular signaling.

Scheme 47

Heme degradation catalyzed by heme oxygenase

In mammals three isoforms of HO have been identified; heme degradation enzymes can also be found in plants and some pathogenic bacteria [183, 240–242]. Many of these proteins have been structurally characterized, including cofactor-free enzymes and their complexes with heme and subsequent intermediates of its enzymatic conversion [240, 241, 243–248]. Since the structural aspects and mechanism of heme oxygenase have been thoroughly reviewed [183], only chosen aspects of recent investigations in the field will be presented in this contribution.

Several groups concentrated their efforts on detailed analysis of mechanism of heme degradation. A theoretical study on meso-hydroxylation step by Shaik and coworkers indicated a preference for homolytic dissociation of O–O bond in Fe–OOH intermediate and the crucial role of hydrogen bonding network of distal heme pocket in trapping of ^•OH radical, in full agreement with the experimental data [249–251]. Verdoheme opening, the less understood third step of degradation process, was investigated by Ikeda-Saito and coworkers [239]. They prepared verdoheme complexes with various heme oxygenases and characterized them by various techniques [245, 252, 253]. A similarity of the final stage of heme oxidation to the first one was observed, including the participation of water cluster in the radical intermediate binding. Verdoheme-heme oxygenase complexes were also characterized by other groups [246, 254, 255].

Factors influencing regioselectivity of heme degradation have been also studied. The exclusive formation of α isomer of meso-hydroxyheme and, finally, of biliverdin α was substantiated by specific seating of heme in the protein and the construction of distal pocket limiting the access of coordinated dioxygen molecule to other meso positions [183, 236]. Mutant heme oxygenases were prepared with an altered regioselectivity which was attributed to various possible orientations of heme moiety [256, 257]; mutations can even change the typical function of enzyme to peroxidase activity [258]. Bacterial heme oxygenases were characterized exhibiting different preference of heme oxidation site as a result of specific seating of the heme [241, 259–261]. Part of regioselectivity studies utilized modified hemes to explore the impact of porphyrin ring substitution on the degradation process. Heme oxygenase was shown to accept various iron porphyrins as substrates, though the presence of propionate chains at C(13) and C(17) seemed to be an important feature required for enzymatic action [183, 234, 262]. Ikeda-Saito and coworkers showed that HO is capable of oxidizing of all isomers of meso-hydroxyhemin to the corresponding verdohemes, but only verdoheme α was further converted to biliverdin [263]. Meso-substitution effects were particularly important for the analysis of ring-opening mechanisms. Oxidation of mesoheme with methylated meso-position by human HO-1 was investigated by Torpey and Ortiz de Montellano [264]. Surprisingly, α-CH₃-derivative was converted to biliverdin α, while γ-CH₃-mesoheme yielded exclusively γ isomer (Scheme 48; in both cases the fate of extruded meso substituent remained unknown); β and δ-substitution resulted in a mixture of products (both methylated and meso-unsubstituted).

Scheme 48

Oxidation of methylated mesoheme by human HO-1 [264]

When protoheme substituted with 5- or 15-phenyl group was used as a substrate, biliverdin α was formed (Scheme 49; benzoic acid by-product was isolated in the first case) [228]. Mesobiliverdin α was identified as the major degradation product of various 5-aryl-mesohemes; isoporphyrin intermediate was detected in this reaction [265]. In contrast, 5-formylmesohemes were exclusively oxidized by heme oxygenase at non-substituted carbons (C(10) or C(20)) to give a formylated biliverdin derivative [266]. Generally, product distribution was found dependent mainly on the possible orientations of modified heme in the protein crevice, but electronic effects of substituents were also of importance.

Scheme 49

Oxidation of phenyl-substituted protoheme by HO-1 [228]

The observation that under certain conditions various heme proteins also can exhibit oxygenase activity led to elaboration of protocol of coupled oxidation, which was used as a model of enzymatic heme degradation [267]. Though the detailed mechanism of meso-hydroxylation step is slightly different [237, 268], both processes share common intermediates: hydroxyheme, verdoheme, and, finally, iron- and metal-free biliverdin. In a typical experiment, these compounds are produced upon treatment of heme protein with an excess of ascorbate and dioxygen or H₂O₂; sometimes also the addition of pyridine was necessary to replace the protein axial ligands [231]. Coupled oxidation of hemoglobin (Hb) and myoglobin (Mb) has been most widely studied, leading mainly to α isomer of biliverdin, but in case of Hb a significant amount of β isomer is also produced [194, 269, 270]. This regioselectivity is changed for abnormal or mutant hemoglobins [270, 271] as well as for cobalt(II) porphyrins used as substrates [272]. Coupled oxidation of heme covalently attached to a variant of Escherichia coli cytochrome b ₅₆₂ yielded a verdoheme protein complex which could be converted with formic acid to protein-attached α-biliverdin [273]. One of axial ligand mutants of mitochondrial cytochrome b ₅, H63V, also stopped at the verdoheme stage while H39V variant allowed to oxidize heme to biliverdin [274]. This different behavior was attributed to the presence of polar amino acid residues in H39V mutant able to interact with heme-bound iron.

6.2 Chlorophyll Degradation

The principal transformations and main intermediates of chlorophyll breakdown have been identified [275–278]. Chlorophyll a 160 and chlorophyll b 161 lose phytol side chain and magnesium ion and pheophorbide a 162 is formed (Scheme 50). Ring opening occurring exclusively at C(5) meso position yields a tetrapyrrole called red chlorophyll catabolite (RCC, 163) which is further converted to fluorescent and nonfluorescent chlorophyll catabolites (FCCs and NCCs, respectively). A key ring-opening step is catalyzed by a specific enzyme, pheophorbide a oxygenase [279, 280]. Isotope labeling experiments showed that only one of newly introduced oxygen atoms is derived from O₂ molecule, while the second one probably originates from water.

Scheme 50

Chlorophyll degradation

Studies on photooxygenation of chlorophyll and bacteriochlorophyll derivatives were conducted in context of the catabolism of these compounds occurring in vivo. Typically, ring-opening reactions occurred by dioxygen attack on C(1)–C(20) bond [114, 281]. However, Iturraspe and Gossauer demonstrated the regioselectivity change by metal coordination: zinc(II) pyropheophorbide a methyl ester 16 led to C(20)-opened product 165 while cadmium complex 164 underwent cleavage of C(4)–C(5) bond yielding compound 166 (Scheme 51) [282]. Recent studies on the degradation of zinc chlorophyll derivatives substituted at 3- and 13-positions showed a systematic change of electronic absorption maxima (up to 919 nm) of the ring-opened products with the electron-withdrawing character of the substituent, demonstrating their attractiveness as near-infrared light absorbing pigments [283].

Scheme 51

Photooxidation of pyropheophorbide a derivatives [282]

7 Summary: Future Directions

The word “degradation” is commonly associated with the loss of quality, with a conversion of an object or a person to less attractive and less valuable state or form. These negative connotations, however, should not come to mind when porphyrin degradation is considered. Certainly, formed products lack many of properties of a parent compound, but at the same time they gained certain unique features, such as a conformational flexibility or an interesting coordination behavior. Ring opening of cyclic tetrapyrroles can be applied as the easiest method of preparation of these linear oligopyrroles.

On the other hand, many of degradation processes are not selective and are frequently accompanied by subsequent reactions (demetallation, Z-E isomerization, water/alcohol addition) which further increase the number of possible products. In many classical papers on porphyrin degradation, only major products were isolated and characterized, and the fate of the rest of starting material remains unknown. Perhaps the use of modern analytical techniques could lead to identification of minor decomposition products.

In general, a great progress has been made in deciphering of degradation processes of tetrapyrrolic macrocycles in nature and of their synthetic models. Still, some fields remain underexplored, including pathways of inactivation of metalloporphyrin catalysts. Since the ways of porphyrin ring modification are unlimited, new developments in the field can be expected because a specific reactivity can be generated connected with the particular substitution or/and metal ion insertion.

One can also imagine that wider synthetic availability of such members of porphyrinoid family, as expanded porphyrins, contracted ones, porphyrin isomers (N-confused, fused, porphycenes,…), and heteroporphyrins could result in investigations on their oxidative degradation. Ring opening of octaphyrins upon metallation with Cu(II) and interesting oxidative conversions of dithiaethyneporphyrin and dioxaporphyrin which were described quite recently show a potential hidden in these porphyrin analogs [284–286].