Cathodic elimination reactions of acyclic vicinal dibromides

Abstract

Electrochemical reductions of both meso- and DL-isomers of 3,4-dibromohexane and 2,5-dimethyl-3,4-dibromohexane have been carried out on various cathodes in ammonia and in DMF solvent systems in order to measure reduction potentials and to determine product distributions. The cis/trans ratios of the products hex-3-ene or 2,5-dimethylhex-3-ene varied with electrode potential in most cases. anti-Elimination of two bromide ions by a concerted mechanism occured preferentially except in the icase of DL-2,5-dimethyl-3,4-dibromohexane which was unable to attain the anti-configuration. At sufficiently negative potentials all rotamers were reduced under diffusion control such that the product distribution reflected the distribution of conformers in the reactant. In liquid ammonia the product yields at negative potentials were similar for meso- and DL-reactants as reduction by solvated electrons proceeded stepwise through an intermediate which can undergo free rotation. Appendices are included which describe calculations of (1) the intramolecular van der Waals contributions to the conformational energies of the reactant dibromides and (2) distances of electron transfer from cathodes to diffusion-controlled de-polarisers when the tunnelling barrier is rectangular.