Reductive elimination in vicinal dibromides. Electrochemical reduction of 1,2-dibromo-3-(4-substituted)-phenylpropanes and induction of double-bond migration in the resulting olefins.

Abstract

Cyclic voltammetry reveals that the electrochemical reduction of 1,2-dibromo-3(4-substituted)-phenylpropanes, with 4-cyano and 4-carbethoxy as sustituents leads to the reductive cleavage of the 2-carbon-bromine bond. The ensuing 1-bromo-2-radical is then immediately reduced to the 1,2-olefin. It appears upon investigation of authentic samples that the reduction of the olefin triggers, by means of a catalytic self-protonation mechanism, its conversion to the more stable isomer where the double bond and the phenyl ring are conjugated. The dissociation energy of the 2-carbon-bromine bond, which can be derived from the peak potential of the reductive cleavage wave, is of the same order as that obtained for previously investigated aliphatic vicinal dibromides, showing that the 1-bromo-2-radical is stabilized by delocalization of the unpaired electron over the CCBr framework.