Direct electrochemical reduction of organic halide droplets dispersed in water

Abstract

The direct electroreductive homocoupling of benzyl bromide has been efficiently achieved using water as the solvent. This process does not involve any organic co-solvent, transition metal catalyst, oil, surfactants, but only a cheap and non-toxic supporting electrolyte (KCl). Benzyl bromide droplets dispersed in water were reduced at a low current density, in an undivided cell fitted with a sacrificial aluminum anode. Various cathode materials have been tested (Ni, Pt, C, Ag) to favor organic halide reduction versus hydrogen evolution. Moreover, it was shown that the presence of aluminum cations generated by the oxidation of the anode played a crucial role in the efficiency of the electrochemical reduction step. Surprisingly, this property was exalted in acidic solutions (pH = 1). Under such acidic conditions, both bibenzyl proportion and faradaic yields were considerably improved. The electrochemical activation of energetically stronger C–X bonds such as those encountered in benzyl chloride (C^sp3–Cl) or ethyl 4-iodobenzoate (C^sp2–I) could be also achieved in water though resulted in lower faradaic yields. From a mechanistic point of view, both the faradaic yields and the product distribution obtained under various conditions suggested the occurrence of a radical coupling pathway occurring within the organic droplets or at their surface.