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

Ni-catalyzed cross-electrophile coupling reactions have gained increasing prominence for the construction of C–C bonds. Most prior studies of biaryl coupling invoke one of two possible mechanisms to form the key Ni(Ar)2 intermediate: (1) sequential Ni reduction/Ar–X oxidative addition with two Ar–X substrates, or (2) parallel formation of two NiII–Ar intermediates that undergo aryl transmetalation between two NiII centers. Herein, we provide evidence for a reductively induced transmetalation pathway leading to biaryl coupling from NiII–Ar species. Chemical or electrochemical reduction of (bpy)NiII(2-tolyl)Br (bpy = 2,2’-bipyridine) to a (bpy)NiI(2-tolyl) species initiates rapid transmetalation of the 2-tolyl ligand to a second equivalent of (bpy)NiII(2-tolyl)Br to afford (bpy)NiII(2-tolyl)2, which undergoes reductive elimination to afford 2,2’-bitolyl. Cyclic voltammetry studies, analysis of reactions under stoichiometric and catalytic conditions, and computational data provide valuable insights into this NiI-to-NiII transmetalation mechanism, which has important implications for Ni-catalyzed biaryl coupling reactions.