Porphyrins, organocobalt derivatives

Porphyrins and chlorophylls are the most widespread natural pigments. They are associated with the energy-converting processes of respiration and photosynthesis in living organisms, and the synthesis of specific porphyrin derivatives is often motivated by the desire to perform similar processes in the test tube. The structurally and biosynthetically related corrins (e.g. vitamin B,j) catalyze alkylations and rearrangements of carbon skeletons via organocobalt intermediates. The biosyntheses of these chromophores are also of topical interest. 

Direct aerial oxidation of cobalt(ii) in the presence of acetone, acetonitrile, and malononitrile has been used for the first time in the porphjrrin series to generate organocobalt(m) porphyrin derivatives. The acetonyl species [Co-(TPP)CHaCOCH3] was characterized by an JlT-ray crystal structure and found to have (Co—C)=2.028 A. The complexes are stable in solution and apparently fairly stable to light. A further novel alkylation of prophyrins involves electro-... 

Organocobalt porphyrin complexes have also yielded useful mechanistic information. The reactions of (tetrakis(p-methoxyphenyl)porphyrinato)cobalt(II) ((TAP)Co(II)) with radicals derived from dialkylazo thermal initiators with acrylic monomers provide evidence for the intermediacy of Co(III)-H species in CCTP. Reaction of (TAP)Co (II) with tertiary alkyl radicals, for example, as derived from AIBN in the presence of monomers that form stable Co-alkyl complexes, such as methyl acrylate, results in quantitative formation of Co(III)-alkyl. Whereas with monomers leading to tertiary C-Co bonds, such as MMA, the Co (II) is very much a spectator as normal polymerization ensues. Thermodynamic and activation parameters have been measured for the dissociation of (TAP)Co(III)-C(CH3)2CN to Co(II) and organic radical in solution as a probe into CCTP mechanism by low-spin Co(II). ... 

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