Carbanion and Radical Intermediacy in the Electrochemical Reduction of Benzyl Halides in Acetonitrile

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Abstract
The electrochemical reduction of benzyl iodide, benzyl bromide, and benzyl chloride was studied in acetonitrile on platinum and glassy carbon electrodes. The reductions of benzyl chloride, benzyl bromide, and benzyl iodide all occur as a single wave at , respectively, on freshly polished platinum and at , respectively, on glassy carbon. Chronoamperometric and coulometric measurements indicate an overall electron stoichiometry of unity for the wave. Controlled‐potential electrolysis of benzyl iodide at potentials sufficiently negative (−1.7V) to form the benzyl anion results in the formation of significant yields of hydrocinnamonitrile (40%) in addition to toluene. Reduction of benzyl iodide in the presence of either diethyl malonate or carbon dioxide at −1.7V produces >40% yields of the diethyl ester of benzyl malonate and benzyl phenylacetate, respectively. Reduction of benzyl iodide at −1.8V in results in 97% deuterium incorporation in major‐product toluene, whereas reduction in the presence of produces less than 10% toluene‐d1. Reduction of benzyl iodide at potentials where significant reduction of the benzyl radical to the anion does not occur (positive of −1.5V) produces only toluene and bibenzyl as the major products.

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