Reductively Induced Aryl Transmetalation: An Alternative Catalytically Relevant Ni-Catalyzed Biaryl Coupling Mechanism

Abstract

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 (^<i>t</i>Bubpy)Ni^II(2-tolyl)Br (^<i>t</i>Bubpy = 4,4'-di-<i>tert</i>-butyl-2,2'-bipyridine) to (^<i>t</i>Bubpy)Ni^I(2-tolyl) is shown to initiate rapid transmetalation of the 2-tolyl ligand to a second equivalent of (^<i>t</i>Bubpy)Ni^II(2-tolyl)Br, affording (^<i>t</i>Bubpy)Ni^II(2-tolyl)₂ and (^<i>t</i>Bubpy)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 (^<i>t</i>Bubpy)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 (^<i>t</i>Bubpy)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.