Mechanistic Basis for the High Enantioselectivity and Activity in the Multichiral Bimetallic Complex-Mediated Enantioselective Copolymerization of meso-Epoxides

Abstract

The desymmetric ring-opening copolymerization of meso-epoxides with cyclic anhydrides or carbon dioxide (CO2) constitutes a straightforward route to diverse isotactic polymers with two contiguous stereogenic centers. Both high enantioselectivity and excellent activity have been observed in catalyst systems based on biaryl-linked bimetallic complexes with multiple chiralities. In this study, we analyzed the basis for the high enantioselectivity and activity of multichiral bimetallic catalysts by combining experimental observations and theoretical calculations. More specifically, density functional theory calculations of the entire catalytic cycles for the copolymerization of cyclohexene oxide (CHO) and phthalic anhydride (PA) or CO2 demonstrated that ligand exchange was required for CHO coordination, while the ring-opening stage was the rate-determining step. Notably, the M···M separation and the endo dihedral angle of the bimetallic complex changed continuously during copolymer propagation, wherein the cleft opened and closed alternately, demonstrating that a flexible phenol–phenol (biphenol) axis is essential. The carboxylate anion coordinated simultaneously with both metal ions in the dinuclear catalyst reduces the reaction energy barrier during the ligand exchange process significantly. Calculation of the optimal routes for the coordination of CHO to Al(III) or Co(III) ions indicated three possible states. The energy barriers for CHO ring-opening mediated by the (R,R,Ra,R,R)- and (R,R,Sa,R,R)-isomers were calculated in each state, and it was found that epoxide ring-opening was sensitive to the absolute stereochemistries of the biphenol linker and the cyclohexyl diamine skeletons, as well as the phenolate ortho-substituents. The two diastereoisomers have opposite stereoselectivities in the epoxide ring-opening stage, wherein the (R,R,Ra,R,R)-isomer-mediated CHO ring-opening process at the (S)-C–O bond in the most stable CHO coordination state exhibited the lowest energy barrier, therefore determining the enantiomer preference and the copolymerization rate.