Reductive coupling and isomerization of electrogenerated radical ions of cis- and trans-isomers

Abstract

The electrochemical behaviour of the cis-trans-pairs: diethyl maleate (DEM), diethyl fumarate (DEF) and cis- and trans-dibenzoylethylene (CDBE and TDBE) in N,N-dimethylformamide solutions was investigated using cyclic and rotating ring-disc electrode (RRDE) voltammetry and bulk electrolysis. In both cases the cis-isomer is reduced at more negative potentials than the trans-isomer. Although the radical anion formed from the trans-isomer is fairly stable, that of the cis-isomer undergoes a rapid isomerization to form the trans-radical anion and also reacts more rapidly in either self- or cross-coupling reactions. The electron spin resonance (e.s.r.) spectra obtained by electrolytic reduction of either DEM or DEF has been re-interpreted as being caused by different conformations of the trans-radical anion. Identical e.s.r. spectra are also obtained from reduction of CDBE and TDBE; this is attributed to a single conformation of TDBE radical anion.