Effects of ion-pairing and adsorption on the stereochemistry of the cathodic pinacolisation of acetophenone

Abstract

The racemic : meso ratio for the pinacol formed from the reduction of acetophenone at a mercury cathode has been determined as a function of the water content of the dimethylformamide solvent and of electrolyte type. The results show that the stereochemistry of the coupling process is strongly influenced by ion-bridging and a ratio of 19 : 1 was found using a mixed base electrolyte to maximise ion-bridging and minimise protonation and hydrogen bonding of the intermediates. On the basis of the stereochemical data and of mechanistic measurements using linear sweep voltammetry it is suggested that adsorbed neutral ion-pairs are involved in the coupling reaction.