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

Abstract

A kinetic study of the oxidation of 4-methoxyphenylacetic acid (AnCH₂CO₂H: An = 4-MeOC₆H₄) with potassium 12-tungstocobalt(III)ate (K₅[Co^IIIW₁₂O₄₀] ≡ Co^IIIW) 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 Co^IIIW with AnCH₂CO₂H and AnCH₂CO₂⁻ respectively, k₁ and k₂, have been derived as k₁ = 0.109 M⁻¹ s⁻¹ and k₂ = 2.948 M⁻¹ s⁻¹, indicating that in the oxidation of 4-methoxyphenylacetic acid by Co^IIIW, 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.