Bimetallic Oxidative Addition Involving Radical Intermediates in Nickel-Catalyzed Alkyl-Alkyl Kumada Coupling Reactions
Many nickel-based catalysts have been reported for cross-coupling reactions of nonactivated alkyl halides. The mechanistic understanding of these reactions is still primitive. Here we report a mechanistic study of alkyl-alkyl Kumada coupling catalyzed by a preformed nickel(II) pincer complex ([(N2N)Ni-Cl]). The coupling proceeds through a radical process, involving two nickel centers for the oxidative addition of alkyl halide. The catalysis is second-order in Grignard reagent, first-order in catalyst, and zero-order in alkyl halide. A transient species, [(N2N)Ni-alkyl(2)] (alkyl(2)-MgCl), is identified as the key intermediate responsible for the activation of alkyl halide, the formation of which is the turnover-determining step of the catalysis.