Can steric effects induce the mechanism switch in the rhodium-catalyzed imine boration reaction? A density functional and ONIOM study

Combined density functional and ONIOM studies have been performed to investigate the mechanism of rhodium-catalyzed boration of imines. Catalytic imine boration has been found to proceed via the following stages: (1) oxidative addition of B-B to the Rh complex, (2) imine coordination, (3) migratory insertion of the imine into the rhodium-boron (Rh-B) bond, and (4) beta-hydrogen elimination to give a monoboration product or carbon-boron (CB) bond formation to yield a diboration product. The choice of the final stage depends on the structure of the imine and boration reagent. Bulky substrate molecules facilitate C-H bond activation and retard C-B bond formation, while in the absence of sterical hindrance C-B bond formation is preferred over C-H bond activation. The present study is the first that outlines the mechanistic differences in C=C and C=N bond boration and rationalizes the effect of bulky substituents on the mechanism of imine boration reaction. The expected difference in regioselectivity between imine and alkene boration is also discussed.