An umpolung mechanism of B(pin)-mediated Cu/B rearrangement and origin of regioselectivity for NHC-Cu-catalyzed allylation of imines

Abstract

The mechanism of N-heterocyclic carbene (NHC)-Cu-catalyzed selective allylation of N-benzylideneaniline with three allyl bromides was investigated using density functional theory (DFT) calculations. The calculations revealed a unique B(pin)-mediated Cu/B rearrangement mechanism. The results demonstrate that B(pin) played a key role not only in producing an active catalyst, but also, as a mediator, in inducing and relaying [Cu] rearrangement to achieve the 1,2-migration of the boryl group from carbon to nitrogen. Moreover, the calculations also disclosed the origin of regioselectivity of allyl bromides. For example, the selective coupling site for allyl bromide is in the alkenyl end C₁(sp²), while for cinnamyl bromide, it is in C₁(sp²) for the Z isomer and C₃(sp³) for the E isomer. These mechanisms, wherein B(pin) plays a prominent role, are important for the rational design of reaction systems.