Photo-decarboxylation of substituted alkylcarboxylic acids brought about by visible light and iron(III) tetra(2-N-methylpyridyl)porphyrin in aqueous solution

Abstract

The complexes of iron(III) tetra(2-N-methylpyridyl)porphyrin with vinyl acetic acid and a selection of α-halo and α-hydroxycarboxylic acids, α-,β- and γ-ketoacids, β-dicarboxylic acids and polyacrylic acid have been photolysed (λ > 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 ligand to give a solvent-caged carboxyl radical and iron(II) porphyrin.

The rate of iron(II) formation is determined by the competitive reactions of the carboxyl radical in the solvent cage: recombination with the iron(II) porphyrin to regenerate the starting complex versus decarboxylation. EPR spin-trapping methods have been used to detect the carbon radicals arising from decarboxylation.

Vinylacetic and α-hydroxycarboxylic acids lead to a rapid rate of iron(II) porphyrin formation; with the latter this is partly due to the one-electron reduction of the iron(III) porphyrin by the α-hydroxyalkyl radicals from the photo-decarboxylation. By contrast the build-up of iron(II) porphyrin from the photoreactions of the α-haloacids is slow and does not go to completion since the iron(III) porphyrin is regenerated by reaction of the halosubstrate with the iron(II) compound.

Ketoacids show an unusual variety of reactions, depending on structure. The α-ketoacids give acyl and, by decarboxylation, alkyl radicals and the iron(II) porphyrin CO complex, whereas with β-ketoacids the photoreaction is catalytic, since, following decarboxylation of the carboxyl radical, the iron(III) porphyrin is rapidly regenerated by oxidation of the iron(II) porphyrin by the α-keto radical. γ-Ketoacids behave in a similar fashion to alkylcarboxylic acids.

The photoreactions of β-dicarboxylic acids resemble those of their monocarboxylic analogues. With polyacrylic acid the rate of iron(II) formation is slow and this is attributed to the polyacid encouraging recombination of iron(II) porphyrin and carboxyl radical.