Insights into N-heterocyclic carbene-catalyzed [3 + 4] annulation reactions of 2-bromoenals with N-Ts hydrazones

Abstract

The possible mechanisms and the origin of the stereoselectivity of the N-heterocyclic carbene (NHC)-catalyzed [3 + 4] annulation reaction of 2-bromoenal with N-Ts hydrazone have been investigated using density functional theory (DFT). The general mechanism of NHC-catalyzed debromination of 2-bromoenals has been uncovered in theory for the first time. Our calculated results indicate that the C–C bond formation involved in the stepwise [3 + 4] process is the rate- and stereoselectivity-determining step, and the SR-isomer is the predominant product, which is consistent with the experiment. Non-covalent interaction (NCI) analysis discovers that the strong C–H⋯O and C–H⋯π interactions should be the key for controlling the stereoselectivity in the reaction. The local reactivity index analysis shows that the reaction prefers to proceed via the [3 + 4] annulation reaction rather than the [3 + 2] annulation reaction. It is noteworthy that further analysis indicates that the main role of the NHC in this kind of reaction is to activate the C–Br bond, which is different from the umpolung role in the NHC-catalyzed annulation reactions of enals.