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

The mechanism of oxidative decarboxylation of arylmethyl carboxylate ions is derived from the analysis of the cyclic voltammetric responses of an extended series of compounds, and, in a few selected cases, of product distribution. In all cases, the removal of the first electron and the cleavage of the bond that results in the formation of CO2, are successive rather than concerted 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 step. The resulting alkyl radical is formed so close to the electrode surface that it is oxidized before having time to dimerize thus yielding exclusively carbocation-derived products (non-Kolbe reaction). When the aromatic portion of the carboxylate ion is easier to oxidize, as with the 4-dimethylamino-benzyl and 9-anthracenyl-methyl derivatives, the acyloxy radical has a zwitterionic character, the cleavage is slower and follows a heterolytic mechanism (involving an intramolecular dissociative electron transfer). This is the reason that the kinetic control passes progressively to the cleavage step. The slower cleavage is also the cause of the formation of a substantial amount of dimer together with carbocation-derived products.