Oxidative decarboxylation of 4-methoxyphenylacetic acid induced by potassium 12-tungstocobalt(III)ate. The role of intramolecular electron transfer

A kinetic study of the oxidation of 4-methoxyphenylacetic acid (AnCH(2)CO(2)H: An = 4-MeOC6H4) with potassium 12-tungstocobalt(III)ate (K-5[(CoW12O40)-W-III]=(CoW)-W-III) has been carried out in aqueous solution at different pH values (between 2.15 and 4.98). The reaction proceeds via a rate determining electron transfer, followed by a fast decarboxylation step leading to a 4-methoxybenzyl radical. From the analysis of the kinetic data the rate constants for reaction of (CoW)-W-III with AnCH(2)CO(2)H and AnCH(2)CO(2)(-) respectively, k(1) and k(2), have been derived as k(1) = 0.109 M-1 s(-1) and k(2) = 2.948 M-1 s(-1), indicating that in the oxidation of 4-methoxyphenylacetic acid by (CoW)-W-III, ionization of the carboxylic group results in an almost 30-fold acceleration of the decarboxylation rate. In order to explain this behavior, it is proposed that no aromatic radical cation is formed as a reaction intermediate, electron removal from the aromatic ring being instead concerted with an intramolecular side-chain to nucleus electron transfer, directly leading to a carboxyl radical which then undergoes rapid decarboxylation to give the 4-methoxybenzyl radical.