Synthesis, Characterization, and Trapping of a Cyclic Diborylcarbene, an Electrophilic Carbene.

A carbene bearing two geminal boryl substituents, called diborylcarbene (DBC), has been predicted to be highly Lewis acidic in sharp contrast to the well-studied persistent carbenes stabilized by π-donating substituents. Studies on DBC have been limited to either the base-trapping or theoretical calculations. Herein, we developed chemical equivalents for DBC, namely, K/X-diborylcarbenoids (X = F or Cl). Treatment of with Al(CF) yielded [AlF(CF)]-stabilized DBC , which showed a significant low-field shift of the carbenoid carbon from 169 ppm (doublet, coupling with F) to 242 ppm (singlet). The loss of halogen was also detected through electrospray ionization time-of-flight mass spectrometry analysis of only in the presence of Al(CF). Generated DBC from or was successfully trapped with excess amounts of trialkylphosphines (PR, R = Me or Et), which afforded the corresponding DBC-PR adducts. In addition, the Lewis acidity of DBC was evaluated both experimentally and theoretically to reveal that is one of the most Lewis acidic species among neutral molecules.