Study on the Catalytic Behavior of Bifunctional Hydrogen-Bonding Catalysts Guided by Free Energy Relationship Analysis of Steric Parameters

Free energy relationship (FER) studies to correlate steric parameters with the enantiocatalytic performance of bifunctional tertiary-amine hydrogen-bonding catalysts, including (S,S)-cyclohexane-1,2-diamine-derived thioureas, Cinchona alkaloid derived thioureas, and (S,S)-cyclohexane-1,2-diamine-derived squaramides, in Michael reactions revealed that the reactions are much favored by catalysts with less bulky N-substituents. The observed FERs are independent of the chiral scaffold and hydrogen-bond donor, and deepen the understanding of current bifunctional hydrogen-bonding catalysts. Moreover, DFT calculations were performed to interpret the observed high reactivities of thioureas with less bulky substituents. In particular, the computations demonstrated the advantage of a benzyl thiourea catalyst, in which an extra CH⋅⋅⋅π interaction between catalyst and substrate is the key factor.