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

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) intermediates. Here, we provide evidence for a previously unrecognized pathway involving reductively induced transmetalation between Ni(Ar) and Ni(Ar)X species. Chemical and electrochemical reduction of (bpy)Ni(2-tolyl)Br (bpy = 4,4'-di--butyl-2,2'-bipyridine) to (bpy)Ni(2-tolyl) is shown to initiate rapid transmetalation of the 2-tolyl ligand to a second equivalent of (bpy)Ni(2-tolyl)Br, affording (bpy)Ni(2-tolyl) and (bpy)NiBr as well-defined products. Experimental and computational data show that the Ni-to-Ni transmetalation mechanism is much more favorable than Ni-to-Ni transmetalation. Oxidation of (bpy)Ni(2-tolyl)Br results in rapid reductive elimination of 2-tolyl-Br, rather than promoting the analogous oxidatively induced Ni/Ni transmetalation. The Ni(2-tolyl) product of Ni-to-Ni transmetalation is stable at room temperature, while sterically less encumbered Ni(Ar) species undergo rapid reductive elimination to afford biaryl and a Ni byproduct. The latter species can serve as a source of electrons to promote further transmetalation and biaryl formation. The unhindered complex (bpy)Ni(4-CF-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-to-Ni 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.