Theoretical investigations toward the [4 + 2] cycloaddition of ketenes with N-benzoyldiazenes catalyzed by N-heterocyclic carbenes: mechanism and enantioselectivity.

Density functional theory (DFT) calculations have been performed to provide the first detailed computational study on the mechanism and enantioselectivity for the [4 + 2] cycloaddition reaction of ketenes with N-benzoyldiazenes catalyzed by N-heterocyclic carbenes (NHCs). Two possible mechanisms have been studied: first is the "ketene-first" mechanism (mechanism A), and second is the novel "diazene-first" mechanism (mechanism B). The calculated results reveal that mechanism B is more favorable than mechanism A because it is not only of lower energy barrier but also more consistent with the provided general experimental procedure (Huang, X.-L.; He, L.; Shao, P.-L.; Ye, S. Angew. Chem., Int. Ed.2009, 48, 192-195). The enantioselectivity-determining step is demonstrated to present during the first process of cycloaddition, and the main product configuration is verified to agree with the experimental ee values very well. This study should be of some worth on forecasting how different substituent groups of catalysts and/or reactants affect the enantioselectivity of products. The obtained novel mechanistic insights should be valuable for not only rational design of more efficient NHC catalysts but also understanding the general reaction mechanism of [4 + 2] cycloaddition of ketenes.