Nickel-catalyzed electrochemical homocoupling of alkenyl halides: Rates and mechanisms

We describe the electroanalytical determination of the successive electron transfer/chemical steps involved in the nickel-catalyzed electrosynthesis of conjugated dienes from alkenyl halides in DMF. Two catalytic sequences are proposed, depending on the potential applied. The first mechanism is shown to proceed through oxidative addition of the vinyl halide to electrogenerated Ni-0 and vinyl-Ni-I species. The diene is obtained by reductive elimination from a Ni-III species. An alternative coupling mechanism is proposed which involves both oxidative addition of RX on Ni-0 and electron transfer from Ni-0 to RNiX. The influence of various parameters on the rates are discussed, such as the nature of both the halide and the stereoisomer used, as well as the coordination environment of the nickel complex.