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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 NiI(Ar) and NiII(Ar)X species. Chemical and electrochemical reduction of (tBubpy)NiII(2-tolyl)Br (tBubpy = 4,4’-di-tert-butyl-2,2’-bipyridine) to (tBubpy)NiI(2-tolyl) is shown to initiate rapid transmetalation of the 2-tolyl ligand to a second equivalent of (tBubpy)NiII(2-tolyl)Br, affording (tBubpy)NiII(2-tolyl)2 and (tBubpy)NiIBr as well defined products. Experimental and computational data show that the NiI-to-NiII transmetalation mechanism is much more favorable than NiII-to-NiII transmetalation. Oxidation of (tBubpy)NiII(2-tolyl)Br results in rapid reductive elimination of 2-tolyl–Br, rather than promoting the analogous oxidatively induced NiII/NiIII transmetalation. The NiII(2-tolyl)2 product of NiI-to-NiII transmetalation is stable at room temperature, while sterically less encumbered NiII(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)NiII(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 NiI-to-NiII 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. 

The Paternò–Büchi reaction is the [2+2] photocycloaddition of a carbonyl with an alkene to afford oxetane products. Enantioselective catalysis of this classical photoreaction, however, has proven to be a long-standing challenge. Many of the best-developed strategies for asymmetric photochemistry are not suitable to address this problem because the interaction of carbonyls with Brønsted or Lewis acidic catalysts can alter the electronic structure of their excited state and divert their reactivity towards alternate photoproducts. We show herein that an alternative triplet rebound strategy enables the stereocontrolled reaction of an excited-state carbonyl compound in its native, unbound state. These studies have resulted in the development of the first highly enantioselective catalytic Paternò–Büchi reaction, cata-lyzed by a novel hydrogen-bonding chiral Ir photocatalyst. 

The Paternò−Büchi reaction is the [2 + 2] photocycloaddition of a carbonyl with an alkene to afford an oxetane. Enantioselective catalysis of this classical photoreaction, however, has proven to be a long-standing challenge. Many of the best-developed strategies for asymmetric photochemistry are not suitable to address this problem because the interaction of carbonyls with Brønsted or Lewis acidic catalysts can alter the electronic structure of their excited state and divert their reactivity toward alternate hotoproducts. We show herein that a triplet rebound strategy enables the stereocontrolled reaction of an excited-state carbonyl compound in its native, unbound state. These studies have resulted in the development of the first highly enantioselective catalytic Paternò−Büchi reaction, catalyzed by a novel hydrogen-bonding chiral Ir photocatalyst.