Development of Highly Enantioselective Catalysts for Asymmetric Copolymerization of meso-Epoxides and Cyclic Anhydrides: Subtle Modification Resulting in Superior Enantioselectivity

Abstract

The asymmetric alternating copolymerization of meso-epoxides with cyclic anhydrides promoted by chiral catalysts or reagents is a powerful strategy for the synthesis of optically active polyesters with main-chain chirality. Herein, we show that, in conjunction with a nucleophilic cocatalyst, enantiopure dinuclear Al­(III) complexes efficiently catalyze this asymmetric copolymerization, exhibiting high activity and achieving enantioselectivities up to 99% ee under mild conditions. Copolymer enantioselectivity and catalytic activity are revealed to be strongly affected by the axial linker, chiral diamine structure, and phenolate ortho-substituents. Density functional theory calculations confirm that the reactions corresponding to ring opening at (R)-C–O and (S)-C–O bonds of the Al-coordinated meso-epoxide during copolymerization exhibit significantly different Gibbs free energies of activation (Δ‡ G). Enantiopure dinuclear Al­(III) complex 3 bearing a hydrogenated binaphthol linker with the matched configuration is demonstrated to be the most active and enantioselective catalyst, featuring a broad substrate scope and allowing one to obtain a wide range of isotactic polyesters with a completely alternating structure and low polydispersity. Notably, most of the produced isotactic polyesters are typical semicrystalline materials with melting temperatures between 120 and 240 °C. Additionally, the mixing of selected isotactic (R)- and (S)-polyesters in a 1:1 mass ratio afforded two crystalline stereocomplexes that exhibited enhanced thermal stability as well as new crystallization behavior and therefore significantly differed from the parent enantiopure polymers.

Enantioselective terpolymerization of racemic and meso -epoxides with anhydrides for preparation of chiral polyesters

“…As a result, the preparation of isotactic-enriched polyesters from the aforementioned enantioselective copolymerization reactions triggered just by one kind of catalyst is extremely challenging. In previous studies, various dinuclear catalysts based on the bimetallic synergistic effect enabled the preparation of stereoregular polymers through enantioselective ring-opening (co)polymerization of epoxides ( 16 – 20 ). The chirality of both the axial linker and the diamine backbones of the ligand were responsible for the chiral induction of these polymerizations.…”

“…Also, a subtle modification of the phenolate ortho -substituents on the ligand resulted in marked changes for both the catalytic activity and enantioselectivity. As far as epoxides/anhydrides copolymerization, we have discovered that enantiopure binaphthol-linked dinuclear aluminum complexes 1a and 1b were more efficient in the enantioselective resolution copolymerization of racemic terminal epoxides and anhydrides to produce isotactic polyesters with extraordinary levels of enantioselection ( 18 ), while hydrogenated binaphthol-linked bimetallic aluminum complexes 2a and 2b could effectively catalyze the asymmetric copolymerization of meso -epoxides with anhydrides to afford polyesters with ee (enantiomeric excess) values of up to 99% ( 19 ). Moreover, for the two types of enantioselective copolymerization reactions, the enantioselectivity and polymerization rate were found to be significantly affected by the axial linker, the chiral diamine structure, and the phenolate ortho -substituents present on the ligand.…”

“…PA and NA were purified by sublimation before the use. Complexes 1 and 2 were synthesized according to the literature methods (19).…”

Triblock polyester thermoplastic elastomers with semi-aromatic polymer end blocks by ring-opening copolymerization

“…This catalyst performance is at the upper end in ROCOP, particularly given that most catalysts operate effectively only when exposed to <200 equivalents of anhydride. 10,41,42,[75][76][77] To assess the polymerization kinetics, a reaction was conducted with regular removal of aliquots which were analyzed by 1 H NMR spectroscopy to determine conversion with time.…”

Heterodinuclear complexes featuring Zn(ii) and M = Al(iii), Ga(iii) or In(iii) for cyclohexene oxide and CO2 copolymerisation

“…30 Al-catalyst systems, with co-catalysts, are also widely investigated in the ring-opening copolymerisation of epoxides and cyclic anhydrides. [31][32][33][34][35][36] Group 13 complexes also show good activity for mechanistically related reactions such as in the ringopening polymerisation of lactide, lactones or cyclic carbonates. [37][38][39][40][41][42] For the target macrocyclic catalysts, it was envisaged that combining the labile Zn(II) site with Group 13 elements could allow modification of metal Lewis acidity and alkoxide lability.…”