Sequential catalytic role of bifunctional bicyclic guanidine in asymmetric phospha–Michael reaction

The catalytic mechanism and origin of enantioselectivity of bicyclic guanidine-catalyzed phospha–Michael reaction between diphenyl phosphine oxide and β-nitrostyrene were investigated by DFT calculations at M06-2X/cc-pVTZ//M06-2X/cc-pVDZ level in conjunction with the implicit SMD solvation method. The catalyst is found to be involved in all 3 steps of the proposed catalytic cycle, namely (1) tautomerization of phosphine oxide, (2) C–P bond formation and (3) concerted hydrogen transfer. The bifunctional role of the guanidine catalyst is clearly demonstrated in all 3 key steps. Due to the geometry of the bicyclic guanidine catalyst, the preferred orientation of the reactants in the transition state of enantioselective C–P bond forming step favours the R enantiomer, in excellent accord with the observed enantioselectivity. Analysis of various transition states suggests that the asymmetric C–P bond formation is controlled by the hydrogen bonding interaction and steric effect between the catalyst and substrate. Various weaker C–HX (X = N, O and π) interactions also play a role in stabilizing the key transition states.