Bimetallic C–C Bond-Forming Reductive Elimination from Nickel

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 sp3–sp3 and sp2–sp2 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.

镍催化交叉偶联反应已在有机合成领域获得重要应用。对交叉偶联反应关键步骤——包括碳-碳键形成型还原消除过程——的基础表征，是一项极具挑战性的工作。早期的机理假说曾提出双分子路径，但相关实验证据十分有限。本工作制备了结构明确的（吡啶-吡咯基）一甲基镍与一苯基镍配合物，可直接观测到双分子还原消除过程分别生成乙烷与联苯。sp³-sp³与sp²-sp²偶联反应通过两条截然不同的路径进行。氧化剂可促进（吡啶-吡咯基）甲基镍快速生成三价镍物种，该物种会发生二聚形成双金属三价镍中间体。我们的实验数据最支持如下结论：后续从双金属三价镍中间体上发生的甲基偶联生成乙烷的步骤为决速步。与之相反，联苯的生成则通过双齿给体配体的配位作用得以促进。