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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. 

Sacrificial anodes composed of inexpensive metals such as Zn, Fe and Mg are widely used to support electrochemical nickel-catalyzed cross-electrophile coupling (XEC) reactions, in addition to other reductive electrochemical transformations. Such anodes are appealing because they provide a stable counter-electrode potential and typically avoid interference with the reductive chemistry. The present study outlines development of an electrochemical Ni-catalyzed XEC reaction that streamlines access to a key pharmaceutical intermediate. Metal ions derived from sacrificial anode oxidation, however, directly contribute to homocoupling and proto-dehalogenation side products that are commonly formed in chemical and electrochemical Ni-catalyzed XEC reactions. Use of a divided cell limits interference by the anode-derived metal ions and supports high product yield with negligible side product formation, introducing a strategy to overcome one of the main limitations of Ni-catalyzed XEC. 

Nickel-catalyzed cross-electrophile coupling (XEC) reactions of (hetero)aryl electrophiles represent appealing alternatives to palladium-catalyzed methods for biaryl synthesis, but they often generate significant quantities of homocoupling and/or proto-dehalogenation side products. In this study, an informer library of heteroaryl chloride and aryl bromide coupling partners is used to identify Ni-catalyzed XEC conditions that access high selectivity for the cross-product when using equimolar quantities of the two substrates. Two different catalyst systems are identified that show complementary scope and broad functional-group tolerance, and time-course data suggest the two methods follow different mechanisms. A NiBr2/terpyridine catalyst system with Zn as the reductant converts the aryl bromide into an aryl-zinc intermediate that undergoes in situ coupling with 2-chloropyridines, while a NiBr2/bipyridine catalyst system with tetrakis(dimethylamino)ethylene as the reductant uses FeBr2 and NaI as additives to achieve selective cross-coupling. 

Although screening technology has heavily impacted the fields of metal catalysis and drug discovery, its application to the discovery of new catalyst classes has been limited. The diversity of on-and off-cycle pathways, combined with incomplete mechanistic understanding, means that screens of potential new ligands have thus far been guided by intuitive analysis of the metal binding potential. This has resulted in the discovery of new classes of ligands, but the low hit rates have limited the use of this strategy because large screens require considerable cost and effort. Here, we demonstrate a method to identify promising screening directions via simple and scalable computational and linear regression tools that leads to a substantial improvement in hit rate, enabling the use of smaller screens to find new ligands. The application of this approach to a particular example of Ni-catalyzed cross-electrophile coupling of aryl halides with alkyl halides revealed a previously overlooked trend: reactions with more electron-poor amidine ligands result in a higher yield. Focused screens utilizing this trend were more successful than serendipity-based screening and led to the discovery of two new types of ligands, pyridyl oxadiazoles and pyridyl oximes. These ligands are especially effective for couplings of bromo- and chloroquinolines and isoquinolines, where they are now the state of the art. The simplicity of these models with parameters derived from metal-free ligand structures should make this approach scalable and widely accessible. 

Cross-electrophile coupling has emerged as an attractive and efficient method for the synthesis of C(sp2)–C(sp3) bonds. These reactions are most often catalyzed by nickel complexes of nitrogenous ligands, especially 2,2’-bipyridines. Precise prediction, selection, and design of optimal ligands remains challenging, despite significant increases in reaction scope and mechanistic understanding. Molecular parame-terization and statistical modeling provide a path to the development of improved bipyridine ligands that will enhance the selectivity of existing reactions and broaden the scope of electrophiles that can be coupled. Herein, we describe the generation of a computational lig-and library, correlation of observed reaction outcomes with features of the ligands, and in silico design of improved bipyridine ligands for Ni-catalyzed cross-electrophile coupling. The new nitrogen-substituted ligands display a fivefold increase in selectivity for product formation versus homodimerization when compared to the current state of the art. This increase in selectivity and yield was general for several cross-electrophile couplings, including the challenging coupling of an aryl chloride with an N-alkylpyridinium salt. 

Abstract Strained rings are increasingly important for the design of pharmaceutical candidates, but cross‐coupling of strained rings remains challenging. An attractive, but underdeveloped, approach to diverse functionalized carbocyclic and heterocyclic frameworks containing all‐carbon quaternary centers is the coupling of abundant strained‐ring carboxylic acids with abundant aryl halides. Herein we disclose the development of a nickel‐catalyzed cross‐electrophile approach that couples a variety of strained ringN‐hydroxyphthalimide (NHP) esters, derived from the carboxylic acid in one step, with various aryl and heteroaryl halides under reductive conditions. The chemistry is enabled by the discovery of methods to control NHP ester reactivity, by tuning the solvent or using modified NHP esters, and the discovery that^t‐BuBpyCam^CN, an L2X ligand, avoids problematic side reactions. This method can be run in flow and in 96‐well plates.