Electrochemical Reduction of Cinnamyl Bromide at Carbon Cathodes in Acetonitrile: A Further Mechanistic Study

Abstract

Cyclic voltammetry and controlled-potential electrolysis have been employed to examine the electrochemical reduction of cinnamyl bromide at glassy carbon cathodes in acetonitrile containing tetramethylammonium tetrafluoroborate. A cyclic voltammogram for the reduction of cinnamyl bromide exhibits one irreversible wave, which is due to the cleavage of the carbon–bromine bond. Bulk electrolyses of the substrates have been carried out at two different potentials (−1.35 and −1.85 V vs saturated calomel electrode) in the absence and in the presence of diethyl malonate as a proton donor. Coulometric values and product distributions indicate that the reduction of cinnamyl bromide at −1.35 V mainly gives radical intermediates, whereas that at −1.85 V forms cinnamyl anions. A direct reduction of cinnamyl bromide in the presence of nitric oxide (NO) at −1.35 V reveals that the cinnamyl radical can couple with NO to generate cinnamaldehyde oxime and cinnamonitrile. More interestingly, both 5-phenylisoxazoline and 5-phenylisoxazole, arising from the cyclization of cinnamaldehyde oxime, have been found among the products. Detailed mechanisms for the reductions are proposed and discussed.