The mechanism of the photo-decarboxylation of alkyl- and arylmethyl-carboxylates using visible light and iron(III) tetra(2-N-methylpyridyl)porphyrin pentachloride in aqueous solution

Abstract

The photo-decarboxylations of a selection of alkyl-, arylmethyl- and cycloalkyl-carboxylate complexes of iron(III) tetra(2-N-methylpyridyl)porphyrin pentachloride in aqueous solution have been studied. An analysis of the influence of substituent electronic effects, σ* for alkyl- and σ for arylmethyl-carboxylic acids, and α,α-deuteration on the rates of reaction indicates that the reactions involve the discrete formation of a carboxyl radical. Thus after the initial photo-excitation, there is a rate-determining homolytic cleavage of the iron–carboxylate bond followed by rapid decarboxylation of the resultant carboxyl radical. Support for this two-step mechanism, over an alternative synchronous process, comes from a study of the influence of ring size on the reactions of five cycloalkylcarboxylic acids.