Theoretical Investigations towards the Staudinger Reaction Catalyzed by N-Heterocyclic Carbene: Mechanism and Stereoselectivity

N-Heterocyclic carbenes (NHCs) have experimentally proved to be powerful catalysts for the Staudinger reaction ([2+2] cycloaddition of a ketene with an imine) but without giving a clear catalytic mechanism. According to different experimental results, the ketene-first and the imine-first mechanisms, arguing which reactant should be initially activated by the NHC catalyst, have been proposed. Our theoretical investigation by employing density functional theory (DFT) reveals that the reaction mechanism of the NHC-catalyzed Staudinger reaction is exclusively the ketene-first mechanism, but the competitive reactions of NHC catalysts with ketenes or imines will lead to different experimental observations. On the basis of this conclusion, we found that the NHC-catalyzed Staudinger reaction would exhibit different stereoselectivities by appropriate choice of the nitrogen substituent of the imines. Furthermore, these results are supposed to be applicable for other nucleophile-catalyzed Staudinger reactions.