Mechanism of Asymmetric Homologation of Alkenylboronic Acids with CF₃-Diazomethane via Borotropic Rearrangement

Density functional theory calculations have been performed to investigate the mechanism for the BINOL-catalyzed asymmetric homologation of alkenylboronic acids with CF3-diazomethane. The reaction proceeds via a chiral BINOL ester of the alkenylboronic acid substrate. The calculations reveal a complex scenario for the formation of the chiral BINOL-alkenylboronate species, which is the key intermediate in the catalytic process. The aliphatic alcohol additive plays an important role in the reaction. This study provides a rationalization of the stereoinduction step of the reaction, and the enantioselectivity is mainly attributed to the steric repulsion between the CF3 group of the diazomethane reagent and the gamma-substituent of the BINOL catalyst. The complex potential energy surface obtained by the calculations is analyzed by means of microkinetic simulations.