MECHANISTIC ASPECTS OF THE KHARASCH ADDITION-REACTION CATALYZED BY ORGANONICKEL(II) COMPLEXES CONTAINING THE MONOANIONIC TERDENTATE ARLYDIAMINE LIGAND SYSTEM [C6H2(CH2NME2)(2)-2,6-R-4]

The addition reaction of polyhalogenated alkanes to alkenes (Kharasch addition reaction) is homogeneously catalyzed in the absence of Oz und er mild reaction conditions (25 degrees C) by the arylnickel complexes [Ni-II{C6H2(CH2NMe2)(2)-2,6-R-4}Br] (R = H, MeC(O), Cl, MeO, NH2) and shows a high selectivity for the 1:1 adduct. Kinetic data on the cata lytic system with [Ni{C6H3(CH2NMe2)(2)-2,6}Br] (R = H; abbreviated as [Ni(NCN)Br]), methyl methacrylate, and CCl4 reveal a rate of reaction that is first order in nickel complex and in alkene. In our series of para-substituted arylnickel catalysts, the rate of catalysis increases with the electron donating character of the para substituents on the aryl Ligand and this rate correlates directly with the Ni-II/Ni-III re dox potential. These data, together with separate spectroscopic studie s and results from individual experiments employing other solvents, ot her polyhalogenated alkanes such as CBr4 and CF3CCl3 and other alkene substrates such as styrene, 1-octene, and cyclohexene, lead to the pro posal of a catalytic cycle based on a nonchain mechanism with a mononu clear nickel species. Before or in the rate-determining step oxidation of the Ni(II) center to a d(7) arylnickel(III) species occurs by a si ngle electron transfer and halide transfer from the polyhalogenated al kane in an inner-sphere activated complex [Ni(NCN)(mu-Cl)CCl4]. This s tep generates an organic radical intermediate which is proposed to sta y in the coordination sphere of the metal where it reacts with the alk ene. The reaction rate decreases with an increase in the steric conges tion at the N-donor centers in derivatives of the [Ni(NCN)Br] catalyst (i.e., NMe2 > NEt2 > NMe(i-Pr) > NMe(t-Bu)). This behavior is consist ent with the characteristics for an inner-sphere electron-transfer pro cess. Selective 1:1 Kharasch product formation then results from a cha in transfer in the Ni(III) coordination sphere by the reverse process, i.e., single electron transfer with concomitant halide transfer. Impo rtant conclusions of this study are that the initially active site of the catalyst is the Ni-X unit (X = halide) and that activation of CCl4 occurs in the absence of a free coordination site.