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Heteroleptic Nickel Complexes for the Markovnikov-Selective Hydroboration of Styrenes

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posted on 10.10.2016, 15:51 by Eric E. Touney, Ryan Van Hoveln, Carl T. Buttke, Michael D. Freidberg, Ilia A. Guzei, Jennifer M. Schomaker
First-row transition metal catalysis offers a cheaper, more environmentally sustainable alternative to second- and third-row transition metal catalysts. Nickel has shown great promise as a tool for the borylation of unsaturated compounds to yield boronic esters, but Markovnikov-selective hydroborations of simple styrenes have not been well-explored. Herein, we report the synthesis of benzyl boronic esters via nickel-catalyzed hydroboration of styrenes using a heteroleptic N-heterocyclic carbene (NHC)–phosphine nickel complex, IMes­(Cy3P)­NiCl2. The IMes­(Cy3P)­NiCl2 complex displays a broad substrate scope and maintains the integrity of yield and regioselectivity when challenged with substrates bearing increased steric hindrance. The heteroleptic complexes also tolerate both electron-withdrawing and -donating groups, in contrast to traditional bis-phosphine and Ni(0) complexes.