Pd-Catalyzed C─C Bond Borylation of Biphenylenes Leading to Tri-Ortho-Substituted Biaryls

Ring-opening diborylation of carbon─carbon (C─C) single bonds is a powerful strategy for installing two versatile functional groups at nonadjacent carbon atoms, enabling skeletal editing of strained ring systems. However, such transformations remain rare for rings larger than cyclopropanes due to kinetic and thermodynamic challenges. Herein, we describe a palladium-catalyzed diborylation of 1-substituted biphenylenes enabled by a highly electron-rich and sterically demanding N-heterocyclic carbene (NHC) ligand. The reaction proceeds via selective cleavage of the least sterically hindered C─C bond and affords ortho-diborylated biphenyls in 39%-89% isolated yields across a broad range of 1-substituted biphenylenes with diverse steric and electronic properties. High regioselectivities (up to >20:1) are observed for cleavage of the least sterically hindered C─C bond. Regioselectivity is modulated by both electronic and steric effects: electron-donating aryl substituents enhance selectivity, as indicated by a Hammett correlation, whereas spherical substituents favour higher selectivity than planar aryl groups. Supporting stoichiometric experiments indicate a pathway involving initial C─C bond activation. The resulting sterically hindered tri-ortho-substituted biaryls may serve as valuable synthetic intermediates, as demonstrated by selective sequential orthogonal postfunctionalization of a representative example.