Enantioselective Copolymerization Of Disubstituted Internal Cis-Epoxides

As a kind of valuable chemical raw material,the chiral epoxides have been widely used in the synthesis of chiral pharmaceuticals and natural products.Through the enantioselective resolution polymerization of terminal epoxides or the desymmetric ring-opening polymerization of meso-epoxides,the polymers with main-chain chirality could be readily produced.Compared to the widely investigated racemic terminal epoxides and meso-epoxides,the study on the polymerization of asymmetric disubstituted epoxides still remained in unveiled,largely due to the existance of multiple isomers,the low reactivity,as well as the complicated regioselective and stereoselective ring-opening issues.As a consequence,this thesis majorly focused on the asymmetric copolymerization or terpolymerization of disubstituted internal cis-epoxides and carried out as follows:1)Utilizing the multichiral binuclear aluminum complexes as catalyst system,the enantioconvergent copolymerization of disubstituted internal cis-epoxides and phthalic anhydrides（PA）was achieved:in which both stereoisomers of epoxide afforded the same configured ester-units in the polymer,manifested in calculated s-factor values apparently surpassed that of k_rel in the reaction(s-factor>k_rel).The reaction allows not only highly effective kinetic resolution of epoxides(k_rel=118),but also a concomitant access to highly stereoregular crystalline polyesters with up to 99%enantiomeric excess（ee）（s-factor>400）.The density functional theory（DFT）calculations suggested that both the enantiomorphic site of the ligand set and the propagating chain-end control mechanisms promoted this enantioconvergent copolymerization process.2)The binaphthyl-linked binuclear chromium catalysts were designed and synthesized for the highly efficient kinetic resolution copolymerization of diverse disubstituted internal cis-epoxides with anhydrides.A variety of epoxides,including alicyclic epoxides,aliphatic epoxides,aromatic epoxides and ether-contained epoxides,could be effectively resolved,with k_rel surpassed 100 in most cases.The reaction activity,enantioselectivity and the thermal property of corresponding polymers in the reaction of different epoxides were highly relevant to their structure characteristics.While the inherent electronic effect or steric hindrance effect of epoxides dominated the regioselectivity in the ring-opening process.3)The enantioconvergent copolymerization of aliphatic internal cis-epoxides and carbon dioxide（CO₂）was effectively achieved by applying the biphenyl-linked multichiral binuclear[Salen Co（III）X]₂ catalyst,producing polycarbonate with>99%enantioselectivity.Apparent enantioconvergence was observed in this copolymerization,with the polymer enantioselectivity of surpassing 90%ee could still be obtained at 70%epoxide conversion.Besides,through the aminolysis reaction of the stereoregular polycarbonates,two kinds of isomers of carbamate with high enantioselectivity could be simultaneously obtained in the one-pot reaction.The configuration of chiral centers was maintained in the aminolysis process,with the absolute configuration and ee%of afforded carbamates remained the same to the original polycarbonate.4)A facile asymmetric copolymerization strategy for effectively resolving the racemic internal cis-epoxides was developed.In the copolymerization of cis-β-methylstyrene oxide（OMSy）and CO₂ mediated by multichiral biphenyl-linked binuclear[Salen Co（III）X]₂ catalyst,enhanced activity,product selectivity an enantioselectivity of OMSy could be realized(k_rel>211)by using oxide cyclopentene（CPO）as comonomer participated in the reaction.The reactivity ratio determination and monitoring experiment in the reaction of OMSy,CPO and CO₂indicated the statistical distribution of two kinds of carbonates in the terpolymer,with both units exhibited excellent enantioselectivity in the terpolymer（ee>98%）.The DFT calculation suggested that the insertion of CPO changed the polymeric chain-end in the reaction,thus decreasing the Gibbs free energy barriers in the ligand exchange process and asymmetric ring-opening process of racemic epoxides.