Reductive Coupling of Halomethylbenzene Derivatives by Nickel and Cobalt

Reduction of benzyl bromide, benzal chloride and benzotrichloride with a commercial nickel powder at 150°C gave bibenzyl, trans-stilbene, and trans-1,2-dichlorostilbene. The reaction intermediates 1,2-dichloro-1,2-diphenylethane for benzal chloride and 1,1,2,2-tetrachloro-1,2-diphenylethane for benzotrichloride were also isolated or detected by NMR spectroscopy. Dehalogenation of these intermediates by the nickel metal or by KI yielded the corresponding trans isomer as the main product. A commercial cobalt powder also reacted with these organic halides leading to similar results. Mechanisms for these reactions are proposed.