Jason Martin - Ligand redox effects in the synthesis, electronic structure, and reactivity of an alkyl-alkyl cross-coupling catalyst

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      Publication Details (including relevant citation   information):

      Gavin D. Jones,  Jason L. Martin,  Chris McFarland,Olivia   R. Allen,  Ryan E. Hall,  Aireal D. Haley,  R. Jacob Brandon,  Tatyana Konovalova,  Patrick J. Desrochers,§  Peter Pulay,  and David A. Vicic*   J. Am. Chem. Soc., 2006, 128 (40), pp 13175–13183

      Abstract:

      The ability of the terpyridine ligand to stabilize alkyl   complexes of nickel has been central in obtaining a fundamental   understanding of the key processes involved in alkyl−alkyl   cross-coupling reactions. Here, mechanistic studies using   isotopically labeled (TMEDA)NiMe2(TMEDA =   N,N,N‘,N‘-tetramethylethylenediamine)   have shown that an important catalyst in alkyl−alkyl   cross-coupling reactions, (tpy‘)NiMe (2b, tpy‘ =   4,4‘,4‘‘-tri-tert-butylterpyridine),   is not produced via a mechanism that involves the formation of   methyl radicals. Instead, it is proposed that (terpyridine)NiMe   complexes arise via a comproportionation reaction between a   Ni(II)−dimethyl species and a Ni(0) fragment in solution upon   addition of a terpyridine ligand to (TMEDA)NiMe2. EPR   and DFT studies on the paramagnetic (terpyridine)NiMe   (2a) both suggest that the unpaired electron   resides heavily on the terpyridine ligand and that the proper   electronic description of this nickel complex is a Ni(II)−methyl   cation bound to a reduced terpyridine ligand. Thus, an important   consequence of these results is that alkyl halide reduction by   (terpyridine)NiRalkyl complexes appears to be   substantially ligand based. A comprehensive survey investigating   the catalytic reactivity of related ligand derivatives suggests   that electronic factors only moderately influence reactivity in   the terpyridine-based catalysis and that the most dramatic   effects arise from steric and solubility factors.

      Address (URL): http://pubs.acs.org/doi/abs/10.1021/ja063334i