Publication
529
Chem.
Eur. J ., 7 (8), 1712-1719, 2001
DOI: 10.1002/1521-3765(20010417)7:8<1712 |
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Electrochemical Oxidation of -Complex-Type Intermediates in Aromatic Nucleophilic Substitutions
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Gilles Moutiers, Jean Pinson, François Terrier, and Régis Goumont
SIRCOB, UPRES-A CNRS 8086 Université de Versailles, Bâtiment Lavoisier 45 Avenue des Etats-Unis, 78035 Versailles Cedex (France) Fax: (+33) 1-39-25-44-52,
Laboratoire d'Electrochimie Moléculaire UMR CNRS no 7591. Université Paris7-Denis Diderot 2 Place Jussieu, 75251, Paris Cedex 05 (France) Fax: (+33) 1-44-27-76-25
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. |