Mechanistic Study on Palladium-Catalyzed Cycloaddition of Vinylethylene Carbonates with α,β-Unsaturated Imines

Abstract

Vinylethylene carbonates (VECs) have been a versatile synthon for the construction of cyclic oxygenates with a medium-sized ring structure. Zhao’s group first demonstrated the potential of VECs as 1,5-dipoles in medium-sized ring synthesis. In their study, (5 + 4) cyclization between VECs (R1 = Ph) and N-tosyl azadienes occurs predominantly under the catalysis of Pd2(dba)3·CHCl3/dppbz. By contrast, when unsubstituted VECs (R1 = H) were used, the five-membered ring product of (3 + 2) cycloaddition was yielded dominantly. We elucidated the key role of the regioselectivity via the mechanistic study of VEC-participated (3 + 2) and (5 + 4) cycloadditions with DFT calculations. The results indicate that the VEC moiety is close to the α,β-unsaturated imine moiety in the (3 + 2) cycloaddition transition state, and the phenyl group deviates from the conjugation plane and thus results in the higher distortion energy. In the presence of the unsubstituted VEC substrate, the steric hindrance in the (3 + 2) transition state is greatly reduced. Meanwhile, the formation of the five-membered ring is facilitated by the entropic effect, and thus, (3 + 2) dominates over the (5 + 4) cycloaddition in this case.