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.