Paper
Oxidative decarboxylation of 4-methoxyphenylacetic acid induced by potassium 12-tungstocobalt(III)ate. The role of intramolecular electron transfer
Published Mar 25, 2002 · E. Baciocchi*, M. Bietti
Journal of The Chemical Society-perkin Transactions 1
3
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Abstract
A kinetic study of the oxidation of 4-methoxyphenylacetic acid (AnCH2CO2H: An = 4-MeOC6H4) with potassium 12-tungstocobalt(III)ate (K5[CoIIIW12O40] ≡ CoIIIW) 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 CoIIIW 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 CoIIIW, 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.
The oxidation of 4-methoxyphenylacetic acid by potassium 12-tungstocobalt(III)ate leads to a rapid decarboxylation of the carboxyl radical, with no intermediate aromatic radical cation formed.
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