Phenylpyridine-Based Borenium Salts as Lewis Acid Catalysts for Homocoupling of Diaryldiazomethanes

Cationic borenium salts have emerged as powerful Lewis acids, but their potential in carbon-carbon bond formation remains largely unexplored. Here, we report the synthesis of a family of sterically and electronically differentiated arylpyridine-based borenium species prepared by controlled hydride elimination and subsequent heteroatom or aryl substitution at boron. These compounds act as highly active catalysts for the homocoupling of diaryldiazomethanes to give tetraarylethylenes under remarkably mild conditions (0.1 mol % catalyst loading at room temperature). The majority of the prepared cationic Lewis acids outperformed neutral tris(pentafluorophenyl)borane, highlighting the advantage of cationic species in this transformation, with the sterically crowded mesityl-substituted borenium catalyst proving to be the most active. Control experiments and DFT calculations support a mechanism in which the borenium catalyst coordinates to the diazo substrate, promotes stepwise nitrogen extrusion and carbocation formation, and releases the tetraarylethylene product in a thermodynamically favorable pathway. This work reveals an unexploited avenue for the main-group catalysis of C-C bond-forming reactions and broadens the synthetic utility of borenium Lewis acids beyond their established reactivity profiles.