Department of Chemistry, New York University , 100 Washington Square East, New York, New York 10003, United States.
J Am Chem Soc. 2016 Apr 13;138(14):4779-86. doi: 10.1021/jacs.6b00016. Epub 2016 Apr 4.
Ni-catalyzed cross-coupling reactions have found important applications in organic synthesis. The fundamental characterization of the key steps in cross-coupling reactions, including C-C bond-forming reductive elimination, represents a significant challenge. Bimolecular pathways were invoked in early proposals, but the experimental evidence was limited. We present the preparation of well-defined (pyridine-pyrrolyl)Ni monomethyl and monophenyl complexes that allow the direct observation of bimolecular reductive elimination to generate ethane and biphenyl, respectively. The sp(3)-sp(3) and sp(2)-sp(2) couplings proceed via two distinct pathways. Oxidants promote the fast formation of Ni(III) from (pyridine-pyrrolyl)Ni-methyl, which dimerizes to afford a bimetallic Ni(III) intermediate. Our data are most consistent with the subsequent methyl coupling from the bimetallic Ni(III) to generate ethane as the rate-determining step. In contrast, the formation of biphenyl is facilitated by the coordination of a bidentate donor ligand.
镍催化的交叉偶联反应在有机合成中有着重要的应用。交叉偶联反应中关键步骤的基本特征，包括 C-C 键形成的还原消除，代表了一个重大的挑战。在早期的提议中引入了双分子途径，但实验证据有限。我们制备了结构明确的（吡啶-吡咯基）Ni 单甲基和单苯基配合物，允许直接观察双分子还原消除分别生成乙烷和联苯。sp(3)-sp(3)和 sp(2)-sp(2)偶联通过两条不同的途径进行。氧化剂促进（吡啶-吡咯基）Ni-甲基中 Ni(III)的快速形成，其自组装形成双金属 Ni(III)中间体。我们的数据与随后从双金属 Ni(III)进行甲基偶联生成乙烷作为速控步骤最为一致。相比之下，双齿供体配体的配位促进了联苯的形成。
