Achieving Nickel-Catalyzed Reductive C(sp2)–B Coupling of Bromoboranes via Reversing the Activation Sequence

Transition metal-catalyzed reductive cross-couplings to build C–C/Si bonds have been developed, but the reductive cross-coupling to create the C(sp2)–B bond has not been explored. Herein, we describe a nickel-catalyzed reductive cross-coupling between aryl halides and bromoboranes to construct a C(sp2)–B bond. This protocol offers a convenient approach for the synthesis of a wide range of aryl boronate esters, using readily available starting materials. Mechanistic studies indicate that the key to the success of the reaction is the activation of the B–Br bond of bromoboranes with a Lewis base such as 2-MeO-py. The activation ensures that bromoboranes will react with the active nickel(I) catalyst prior to aryl halides, which is different from the sequence of the general nickel-catalyzed reductive C(sp2)–C/Si cross-coupling, where the oxidative addition of an aryl halide proceeds first. Notably, this approach minimizes the production of undesired homocoupling byproduct without the requirement of excessive quantities of either substrate.