Atroposelective Access to π-Conjugated 1,2-Azaborepines Enabled by Palladium-Catalyzed Cyclization of N-Heterobiaryls...

π-Conjugated organoboron compounds that feature N→B coordination bonds exhibit attractive photophysical and electronic properties. However, the efficient catalytic asymmetric synthesis of medium-sized BN-heterocycles remains underdeveloped. Herein, we report a palladium-catalyzed atroposelective cyclization of racemic N-heterobiaryl triflates with alkynyl boronates, yielding a variety of 1,2-BN-bridged seven-membered biaryls in high yields (up to 99%) and excellent enantioselectivity (>99% ee). The reaction occurs via a palladium-catalyzed dynamic kinetic asymmetric transformation (DyKAT) of N-heterobiaryl triflates to generate a palladium species, which then undergoes cis-carbopalladation with the alkyne moiety of alkynylboronates, followed by a 1,2-migration to afford the chiral π-conjugated 1,2-azaborepines. The reaction shows broad substrate scope and excellent functional group tolerance. And the resulting boron-containing products are amenable to further transformations, including direct oxidation and Suzuki-Miyaura cross-couplings.