Homolytic and heterolytic radical cleavage in the Kolbe reaction - Electrochemical oxidation of arylmethyl carboxylate ions

The mechanism of oxidative decarboxylation of arylmethyl carboxylate ions i s derived from the analysis of the cyclic voltammetric responses of an exte nded series of compounds, and, in a few selected cases, of product distribu tion. In all cases, the removal of the first electron and the cleavage of t he bond that results in the formation of CO2, are successive rather than co ncerted processes. In a majority of cases, the unpaired electron is located on carboxyl oxygen of the acyloxy radical which undergoes a fast homolytic cleavage. The reaction is kinetically controlled by the first electron ste p. The resulting alkyl radical is formed so close to the electrode surface that it is oxidized before having time to dimerize thus yielding exclusivel y carbocation-derived products (non-Kolbe reaction). When the aromatic port ion of the carboxylate ion is easier to oxidize, as with the 4-dimethylamin o-benzyl and 9-anthracenyl-methyl derivatives, the acyloxy radical has a zw itterionic character, the cleavage is slower and follows a heterolytic mech anism (involving an intramolecular dissociative electron transfer). This is the reason that the kinetic control passes progressively to the cleavage s tep. The slower cleavage is also the cause of the formation of a substantia l amount of dimer together with carbocation-derived products. (C) 2001 Else vier Science B.V. All rights reserved.