Diazoborane-Promoted Boron Insertion into Olefinic C-H Bonds

The discovery of new borylation strategies remains a major focus in synthetic chemistry, as organoboron compounds serve as highly versatile intermediates that enable a wide range of transformations, while also playing increasingly important roles in small-molecule activation and advanced functional materials. Herein, we report a novel borylation reaction mediated by diazoboranes (RLBN2). In contrast to conventional borylation processes, which proceed through addition to unsaturated bonds or substitution of leaving groups, diazoboranes, behaving as dicoordinate borylenes, undergo net insertion into olefinic C-H bonds. This diazoborane-mediated borylation displays a broad substrate scope, encompassing monosubstituted and gem-disubstituted olefins, as well as electron-poor and conjugated systems. Most substrates afford the E isomer with excellent selectivity (>99%). In contrast, less sterically hindered olefins exhibit partial formation of the Z isomer. Notably, acrylonitrile delivers the Z-configured product with >99% selectivity. Detailed mechanistic studies reveal that this stereochemical outcome arises from a subtle interplay between orbital interactions and steric effects during the conversion of a cyclic intermediate to the C-H activation product.