Electro-organic reactions. Part 26. The cathodic reduction of dicyanoethylene derivatives; competition between hydrogenation and hydrodimerisation

Abstract

Dicyanoethylene derivatives, and related compounds, undergo smooth cathodic reduction to give the products of hydrogenation (2 F mol^–1), hydrodimerisation (1 F mol^–1), or a mixture of both. The product distribution depends crucially upon steric factors and upon the stability–basicity of the intermediate radical-anions. The latter factor is reflected in the significant influence exerted by electrolyte acidity. Twelve activated ethylene compounds of considerable structural diversity have been included in the study and by consideration of substituent effects and the results of cyclic voltammetric experiments a detailed mechanistic rationalisation of their reduction is presented. In particular, the hydrodimerisation reactions are shown to be under kinetic control (giving cyclic hydrodimers through subsequent intramolecular reaction); one highly strained cyclic hydrodimer (20) rearranges thermally to the more stable linear isomer (19).