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 2 X (X = F or Cl). Treatment of 2 F with Al(C 6 F 5 ) 3 yielded [AlF(C 6 F 5 ) 3 ] - -stabilized DBC 1-FAl , which showed a significant low-field shift of the carbenoid carbon from 169 ppm (doublet, coupling with 19 F) to 242 ppm (singlet). The loss of halogen was also detected through electrospray ionization time-of-flight mass spectrometry analysis of 2 X only in the presence of Al(C 6 F 5 ) 3 . Generated DBC 1 from 1-FAl or 2 Cl was successfully trapped with excess amounts of trialkylphosphines (PR 3 , R = Me or Et), which afforded the corresponding DBC-PR 3 adducts. In addition, the Lewis acidity of DBC 1 was evaluated both experimentally and theoretically to reveal that 1 is one of the most Lewis acidic species among neutral molecules.