A mechanism exploration of stereodivergent coupling of aldehydes and alkynes catalyzed synergistically by rhodium and amine
Using metal complexes and organic species together to synergistically catalyze reactions is of great interest to the chemical community. However, the mechanism of stereodivergent dual catalysis, especially the role of each catalyst, is elusive. In this paper, mechanistic details for stereodivergent coupling of aldehydes and alkynes via synergistic catalysis of rhodium and amine were explored using density functional theory calculations. The synergistic roles of each catalyst, and the origin of the regioselective and enantioselective stereodivergent coupling for the reaction are disclosed. The results show that two substrates are activated independently at first by rhodium and amine so that a Rh–π-allyl complex and enamine are generated. The combination of two activated intermediates provides access to the divergent stereocenters through an outer sphere mechanism. Rhodium catalyst predominates, not only in the activation of substrates, but also in the control of regioselectivity and enantioselectivity. The Rh–π-allyl complex regulates the molecular skeleton and each transition state is controlled by the outer sphere via an enamine–phosphate hydrogen bond. Thus, the energy barriers of the reaction are significantly reduced to 11.1 kcal mol−1 via synergistic catalysis of metal complexes and organic amines. The calculated ee and de values are in very good agreement with the experimental results.