Reactive intermediates such as carbocations, carbanions, radicals
and radical ions can be electrochemically generated from various
electroactive species. Those intermediates may react chemically (C) or
electrochemically (E) according to EC, ECE mechanisms. Anodic oxidations
produce acidic or electrophilic species, which can react with nucleophiles or
(and) eliminate protons or electrophiles. Cathodic reductions afford basic or
nucleophilic species, which can react with protons or electrophiles or (and)
eliminate nucleophiles. In this way, functional group conversion,
substitution reactions, addition reactions, cleavage reactions and coupling
reactions can be selectively performed by using direct electrolyses.
Activation by transition-metal catalysts is required when the organic
substrate is not electroactive or leads to non desired reactions. The metalcatalysed
electrosynthesis proceeds by a double activation: i) chemical
activation of the organic substrate by the electrogenerated active form of a
transition metal catalyst that generates an organometallic species more easily
reduced than the organic substrate, ii) followed by activation by electron
transfer of the organometallic species formed in the previous chemical
activation step. This double chemical and electrochemical activation causes
new reactions to proceed, which involve either the classical organic reactive
species, produced in any electrochemical steps (carbanions) or
organometallic complexes (anionic or neutral) as the basis of new reactivity.
Universite de Rennes 1, UMR CNRS 6509, Institut de Chimie de Rennes, Campus de Beaulieu 35042 Rennes Cedex, France.