Mechanism and Origins of Regio- and Stereoselectivities of NHC-Catalyzed Dearomative Annulation of Benzoazoles and Cinnamaldehydes from DFT.

A theoretical study on the mechanism, regioselectivity, and enantioselectivity of NHC-catalyzed dearomatizing annulation of benzoxazoles with enals has been conducted using density functional theory calculations. Our calculated results indicate that the favored mechanism occurs through eight reaction steps: initial binding of the NHC to enals, followed by formation of the Breslow intermediate via proton transfer. Subsequent oxidation generates the α,β-unsaturated acylazolium intermediate, which can undergo Michael addition with benzoxazoles. Sequential protonation/deprotonation/cyclization produces the six-membered cyclic intermediate that undergoes catalyst elimination, leading to the final product. DABCO·H was found to play important roles in proton transfer and cyclization. Without DABCO·H, the energy barrier up to 44.2 kcal/mol for step 2 is too high to be accessible. With DABCO·H, the corresponding value is lowered to 18.6 kcal/mol. The energy barrier for cyclization can be lowered by 7.4 kcal/mol by using DABCO·H. The Michael addition step determines both the enantioselectivity and the regioselectivity. According to NCI analysis, the enantioselectivity is controlled by the strong interactions (such as C-H···O, C-H···N, and π···π) between the α,β-unsaturated acylazolium intermediate and benzoxazoles. We also discuss the solvent and substituent effects on the enantioselectivity and the role of the NHC. The mechanistic insights obtained in the present study would help improving current reaction systems or designing new synthetic routes.