Nickel(0)-induced geminal bond cleavages leading to carbon-carbon double bonds: Intermediacy of nickel(0)-carbene complexes as a unifying hypothesis

The possibility of effecting geminal bond cleavages of organic monomers with nickel(0) complexes to achieve olefinic dimers has been investigated both in stoichiometric reactions between monomers of the type R(2)CE(2) and R(2)C=E and the complexes (Cod)(2)Ni, (Et(3)P)(4)Ni and (Bpy)(Cod)Ni and in nickel(0)-catalytic alpha-elimination reactions of alpha-lithioorganic sulfones, RR'C(Li)(SO(2)R ''). Substrates for the stoichiometric geminal bond cleavages comprised geminal dihalides, aminomethyl sulfides, cyclopropenes, thioketones and ketones, Reductive dimerization to the olefin or its hydroxy derivatives was found to occur to a varying degree in most cases, including the catalytic decomposition of the alpha-lithioorganic sulfones, Accordingly, this method offers considerable promise in organic synthesis. A gamut of known nickel(0) chemistry, results of chemical-trapping and spectroscopic evidence are marshaled in support of the proposal that such geminal bond cleavages and reductive coupling processes most likely proceed via nickel(0)-carbene intermediates of the type R(2)C=Ni . L(n).