Ni-catalyzed cross-electrophile coupling (XEC) reactions have gained prominence for the construction of C-C bonds. Prior studies of XEC routes to biaryls have invoked several different mechanisms for the formation of key Ni(Ar)₂ intermediates. Here, we provide evidence for a previously unrecognized pathway involving reductively induced transmetalation between Ni^I(Ar) and Ni^II(Ar)X species. Chemical and electrochemical reduction of (^tBubpy)Ni^II(2-tolyl)Br (^tBubpy = 4,4'-di-tert-butyl-2,2'-bipyridine) to (^tBubpy)Ni^I(2-tolyl) is shown to initiate rapid transmetalation of the 2-tolyl ligand to a second equivalent of (^tBubpy)Ni^II(2-tolyl)Br, affording (^tBubpy)Ni^II(2-tolyl)₂ and (^tBubpy)Ni^IBr as well-defined products. Experimental and computational data show that the Ni^I-to-Ni^II transmetalation mechanism is much more favorable than Ni^II-to-Ni^II transmetalation. Oxidation of (^tBubpy)Ni^II(2-tolyl)Br results in rapid reductive elimination of 2-tolyl-Br, rather than promoting the analogous oxidatively induced Ni^II/Ni^III transmetalation. The Ni^II(2-tolyl)₂ product of Ni^I-to-Ni^II transmetalation is stable at room temperature, while sterically less encumbered Ni^II(Ar)₂ species undergo rapid reductive elimination to afford biaryl and a Ni⁰ byproduct. The latter species can serve as a source of electrons to promote further transmetalation and biaryl formation. The unhindered complex (^tBubpy)Ni^II(4-CF₃-phenyl)Br undergoes biaryl formation in the absence of added reductant; however, kinetic analysis reveals an induction period and autocatalytic time course. Addition of catalytic quantities of a cobaltocene-based reductant eliminates the induction period and accelerates biaryl formation, consistent with the Ni^I-to-Ni^II transmetalation pathway. The results of this study provide a new rationale for previously reported results in the literature and introduce an alternative pathway to consider in the development of Ni-catalyzed biaryl coupling reactions.

镍催化交叉偶联（XEC）反应在构建碳-碳键方面得到了广泛应用。之前关于生物芳烃的XEC途径的研究提出了几种不同机制来形成关键的Ni(Ar)₂中间体。在这里，我们提供了证据证明了一种以前未被认识的路径：涉及Niᴵ(Ar)和Niᴵᴵ(Ar)X物种之间的还原诱导转金属化过程。通过化学和电化学方法将(^tBubpy)Niᴵᴵ(2-甲苯基)Br (^tBubpy = 4,4'-二叔丁基-2,2'-联吡啶) 还原为 (^tBubpy)Niᴵ(2-甲苯基)，证明了此过程可以启动快速的转金属化，即将2-甲苯基配体转移到第二个等量的(^tBubpy)Niᴵᴵ(2-甲苯基)Br中，生成(^tBubpy)Niᴵᴵ(2-甲苯基)_₂和(^tBubpy)NiᴵBr作为明确的产品。实验数据和计算表明，从Niᴵ到Niᴵᴵ的转金属化机制比从Niᴵᴵ到Niᴵᴵ的转金属化更有利于进行。氧化 (^tBubpy)Niᴵᴵ(2-甲苯基)Br 会导致快速的还原消除生成2-甲苯基-溴，而不是促进类似的氧化诱导的Niᴵᴵ/Niᴵᴵ转金属化过程。由Niᴵ到Niᴵᴵ转金属化产生的Niᴵᴵ(2-甲苯基)_₂产物在室温下是稳定的，而空间位阻较小的Niᴵᴵ(Ar)_₂物种会快速经历还原消除生成联芳烃和一个镍零副产品。后者可以作为电子来源以促进进一步转金属化及联芳烃形成。未受阻碍的配体(^tBubpy)Niᴵᴵ(4-CF₃-苯基)Br在没有添加还原剂的情况下会发生联芳烃生成；然而，动力学分析显示存在诱导期和自催化时间过程。加入适量的钴ocene类还原剂可以消除此诱导期并加快联芳烃的生成，这与从Niᴵ到Niᴵᴵ转金属化路径一致。本研究的结果为文献中先前报告的研究提供了一个新的解释，并引入了在开发镍催化的生物芳烃偶联合成反应中的一个替代路径来考虑。