Oxidative Dimerisationof Some Stabilised Carbanions: a Mechanistic Investigation

The electrochemical oxidation of carbanions (p-CH3-C6H4)Z1Z2C- (1: Z1 = Z2 = CN; 2: Z1 = CN, Z2 = COOEt; 3: Z1 = Z2 = COOEt) was studied in CH3CN-n- Bu4NBF4 by experimental and simulated voltammetry, macroscale electrolyses and product analyses, with the aim of investigating the mechanism of the dimerisation process. The carbanions, generated by cathodic reduction of the conjugated acids, show a chemically irreversible oxidation peak whose position reflects the relative basicity of the anions (3 > 2 > 1). Kinetic data deriving from peak potential analysis indicate that neutral radicals are generated at the electrode, and that a fast radical-radical coupling (DIM 1 mechanism) is by far the most effective dimerisation process for all the substrates. Accordingly, the corresponding homodimers are formed quantitatively after exhaustive, one-electron macroscale electrolyses, also when the oxidation is performed in the presence of a second anion, oxidable at more positive potentials. However, at high anion to radical ratio, a significant contribution of a radical-anion coupling (DIM 2) can be involved when the anion is a good nucleophile. The position of the voltammetric oxidation peak of 3 is strongly affected by the presence by alkaline cations, which cause a shift of the wave towards more positive potentials (Li+ > Na+ > K+ > NR4+) and also a decrease of the rate of the heterogeneous electron transfer in the same order. Both these effects can be explained by ion pairing.