Publication
528
J. Electroanal.
Chem ., 498 (1-2), 171-180, 2001
DOI: 10.1016/S0022-0728(00)00365-X |
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Homolytic and heterolytic radical cleavage in the Kolbe reaction Electrochemical oxidation of arylmethyl carboxylate ions
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C. P. Andrieux, F. Gonzalez and J.-M Savéant
Laboratoire d'Electrochimie Moléculaire,
Unité Mixte de Recherche Université-CNRS No 7591, Université de Paris 7-Denis
Diderot, Case Courrier 7107, 2 place Jussieu, 75251 Paris Cedex 05, France
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. |