Oxidation of porphyrins

Oxidation of porphyrins and chlorins with osmium(VIII) oxide is an equally valuable method to produce bacteriochlorins, although it also leads to problems of regio- and stereoselectivity. 

Fig. 11 Correlation between electrochemical potentials and OMTS bands for more than ten compounds including polyacenes, phthalo-cyanines, and porphyrins. OMTS data were acquired both from tunnel junctions and STM measurements. The standard potential relative to the normal hydrogen electrode associated with the half reaction M(solution) + e-(vac) —> M-(solution) is the y axis. The three outliers are assigned to the ring oxidation of porphyrins. (Reprinted with permission from [26])...

Permanganate oxidation of porphyrins yields pyrrole-2,5-dicarboxylic acids (Scheme 5), which can be readily identified by paper chromatography. This method for structure... 

Only a few Fe porphyrin jr-cation radicals have been characterized structurally,i53 i i since they are quite reactive species. They have been characterized mainly by NMR spectroscopy, magnetic susceptibility measurements, and a diagnostic band in the IR spectrum in the region of 1270-1295cm i.15 13 Their main use has been as chemical oxidants of porphyrins, metalloporphyrins, and so on. ... 

Jurgen Fuhrhop was born in Berlin in 1940 and grew up in a post-war cinema. Thereafter he studied chemistry in Berlin and Braunschweig, and gained his Dr.rer.nat. by researching the oxidation of porphyrins under the guidance of Hans Herloff Inhoffen. 

Usually the chemical oxidation of porphyrins and their deprotonated anions and protonated cations is not reversible. Poor reductants and oxidants do not induce any reaction, whereas the action of powerful reagents usually leads to irreversible decomposition. There are, however, some reports on unresolved, low-percentage radical esr signals in solutions of free-base porphyrins and phthalocyanines [George (81), Gibson (52)], and there are reports of polarographic redox potentials of some non-metallic porphyrin derivatives, which correspond to reversible reactions (Tables 2 and 3). 

Bilirubinoids are coloured compounds occurring in vertebrates, in some invertebrates and even in algae. They are formed by the biological oxidation of porphyrins. The most important representative is the orange-coloured bilirubin. It occurs in the bile and in gallstones and is excreted in the faeces and in urine. Bilirubin was first isolated by Staedeler (1864) and can be purified via its crystalline ammonium salt. It is oxidized to blue-green biliverdine by iron(III) chloride ... 

Oxidation of porphyrins was used to elucidate its structure. It leads (e.g. with KMn04) the formation of pyrrole-2,5-dicarboxylic acids with retention of the methine C-atoms, or (e.g. with Cr03) to maleimides with loss of the methine C-atoms ... 

Giraudeau, H.J. Callot, and M. Gross (1979). Effects of electron-withdrawing substituents on the electrochemical oxidation of porphyrins. Inorg. Chem. 18, 201-206. 

The actual site of electron transfer upon reduction or oxidation of porphyrins with bound NO groups has not been well established except for compounds with Fe(II) or Co(II) central metals, where an oxidation of the former complex involves the Fe(II)/Fe(III) redox couple and an oxidation of the latter involves electrogeneration of a Co(II) porphyrin rr-cation radical (7j. 

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