Synthesis of Less-Substituted Borirenes Enabled by a Crystalline Arylhalohydroborane Reagent.

As the smallest boron-containing Hückel 2π-aromatic rings, borirenes (i.e., boracyclopropene) are isoelectronic with the all-carbon cyclopropenyl cation. However, their structural diversity has long been confined to fully substituted scaffolds due to the stringent requirement for kinetic stabilization and limited synthetic methods, which hampers the exploration of their chemical space. Herein, we report an efficient synthetic route to a variety of less-substituted borirenes, enabled by a crystalline arylchlorohydroborane reagent featuring dual B-H and B-Cl functionalities (aryl = 2,6-(CF3)2phenyl, XylF). The protocol comprises a selective 1,1-hydroboration of alkynes, followed by cooperative NHC- and base-promoted ring-closure. The enhanced "push-pull" effect induced by the π-donating group on carbon and the electron-withdrawing XylF group on boron stabilizes these less-sterically protected aromatic borirenes, facilitating their isolation and structural characterization. Consequently, these borirenes readily undergo ring expansion into five- and six-membered boracycles via multiple isocyanide insertions into the polar C-B σ bond.