Mechanism of the nickel-catalyzed electrosynthesis of ketones by heterocoupling of aryl and benzyl halides

The mechanism of the nickel-catalyzed electrosynthesis of ketones by heterocoupling of phenacyl chloride and benzyl bromide has been investigated by fast scan rate cyclic voltammetry with [Ni(bpy)(3)(2+)](BF4-)(2) as the catalytic precursor (bpy = 2,2'-bipyridine). The key step is an oxidative addition of Nio(bpy) (electrogenerated by reduction of the Ni(II) precursor) to PhCH2Br whose rate constant is found to be 10 times higher thad that of PhCH2COCl. The complex PhCH2Ni(II)Br(bpy) formed in the oxidative addition is reduced at the potential of the N-II/Ni-0 reduction by a two-electron process which affords an anionic complex PhCH2Ni0(bpy)- able to react with PhCH2COCl to generate eventually the homocoupling product PhCH2COCH2Ph. The formation of the homocoupling product PhCH2COCOCH2Ph is prevented because of the too slow oxidative addition of Nio(bpy) to PhCH2COCl compared to PhCH2Br. The formation of the homocoupling product PhCH2CH2Ph is also prevented because PhCH2Ni0(bpy)- does not react with PhCH2Br. This explains why the electrosynthesis of the ketone can be performed selectively in a one-pot procedure, starting from an equal mixture of PhCH2COCl and PhCH2Br and a nickel catalyst ligated by the bpy ligand.