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

The photo-decarboxylations of a selection of alkyl-, arylmethyl- and cycloa lkyl-carboxylate complexes of iron(III) tetra(2-N-methylpyridyl)porphyrin p entachloride in aqueous solution have been studied. An analysis of the infl uence of substituent electronic effects, sigma* for alkyl- and sigma for ar ylmethyl-carboxylic acids, and alpha,alpha-deuteration on the rates of reac tion indicates that the reactions involve the discrete formation of a carbo xyl radical. Thus after the initial photo-excitation, there is a rate-deter mining homolytic cleavage of the iron-carboxylate bond followed by rapid de carboxylation of the resultant carboxyl radical. Support for this two-step mechanism, over an alternative synchronous process, comes from a study of t he influence of ring size on the reactions of five cycloalkylcarboxylic aci ds.