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 2X (X
= F or Cl). Treatment of 2F with Al(C6F5)3 yielded [AlF(C6F5)3]−-stabilized DBC 1-FAl, which showed a significant low-field shift of the carbenoid carbon
from 169 ppm (doublet, coupling with 19F) to 242 ppm (singlet).
The loss of halogen was also detected through electrospray ionization
time-of-flight mass spectrometry analysis of 2X only in the presence of Al(C6F5)3. Generated DBC 1 from 1-FAl or 2Cl was successfully trapped with excess amounts of trialkylphosphines
(PR3, R = Me or Et), which afforded the corresponding DBC-PR3 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.