Cooperative Asymmetric Catalysis by N-Heterocyclic Carbenes and Bronsted Acid in γ-Lactam Formation: Insights into Mechanism and Stereoselectivity

Current developments in the burgeoning area of cooperative asymmetric catalysis indicate the use of N-heterocyclic carbenes (NHCs) in conjunction with other catalysts such as a Bronsted acid. Herein, mechanistic insights derived through a comprehensive DFT (M06-2X) computational study on a dual catalytic reaction between an enal and an imine leading to trans-gamma-lactams, catalyzed by a chiral NHC and benzoic acid, is presented. In the most preferred pathway, we note that the NHC catalyst activates one of the reactants (enal) in the form of a Breslow intermediate, whereas the electrophilic partner (imine) is activated by the benzoic acid through protonation of the imino nitrogen. In this article, we focus on the origin of cooperative action of both catalysts as well as on the stereoselectivity by identifying the stereocontrolling transition states. The explicit and cooperative participation of the Bronsted acid and NHC lowers the energetic barrier both in the Breslow intermediate formation and in the stereocontrolling step through a number of C-H center dot center dot center dot pi, N-H center dot center dot center dot O, and pi center dot center dot center dot pi noncovalent interactions. The enantio- and diastereoselectivities computed using the transition state models with an explicit benzoic acid are in good agreement with the earlier experimental reports.