PHOTO-DECARBOXYLATION OF SUBSTITUTED ALKYLCARBOXYLIC ACIDS BROUGHT ABOUT BY VISIBLE-LIGHT AND IRON(III) TETRA(2-N-METHYLPYRIDYL)PORPHYRIN IN AQUEOUS-SOLUTION

The complexes of iron(III) tetra(2-N-methylpyridyl)porphyrin with viny l acetic acid and a selection of alpha-halo and alpha-hydroxycarboxyli c acids, alpha-, beta- and gamma-ketoacids, beta-dicarboxylic acids an d polyacrylic acid have been photolysed (lambda > 390 nm) in anaerobic aqueous solution, UV-VIS spectroscopy shows that the primary reaction is photoreduction of the iron(III) atom by the axial carboxylate liga nd to give a solvent-caged carboxyl radical and iron(II) porphyrin. Th e rate of iron(II) formation is determined by the competitive reaction s of the carboxyl radical in the solvent cage: recombination with the iron(II) porphyrin to regenerate the starting complex versus decarboxy lation. EPR spin-trapping methods have been used to detect the carbon radicals arising from decarboxylation. Vinylacetic and alpha-hydroxyca rboxylic acids lead to a rapid rate of iron(II) porphyrin formation; w ith the latter this is partly due to the one-electron reduction of the iron(III) porphyrin by the alpha-hydroxyalkyl radicals from the photo -decarboxylation. By contrast the build-up of iron(II) porphyrin from the photoreactions of the alpha-haloacids is slow and does not go to c ompletion since the iron(III) porphyrin is regenerated by reaction of the halosubstrate with the iron(II) compound. Ketoacids show an unusua l variety of reactions, depending on structure. The alpha-ketoacids gi ve acyl and, by decarbonylation, alkyl radicals and the iron(II) porph yrin CO complex, whereas with beta-ketoacids the photoreaction is cata lytic, since, following decarboxylation of the carboxyl radical, the i ron(III) porphyrin is rapidly regenerated by oxidation of the iron(II) porphyrin by the alpha-keto radical, gamma-Ketoacids behave in a simi lar fashion to alkylcarboxylic acids. The photoreactions of beta-dicar boxylic acids resemble those of their monocarboxylic analogues. With p olyacrylic acid the rate of iron(II) formation is slow and this is att ributed to the polyacid encouraging recombination of iron(II) porphyri n and carboxyl radical.