Enhanced Asymmetric Induction for the Copolymerization of CO₂ and Cyclohexene Oxide with Unsymmetric Enantiopure SalenCo(III) Complexes: Synthesis of Crystalline CO₂-Based Polycarbonate

Abstract

Enantiopure metal-complex catalyzed asymmetric alternating copolymerization of CO(2) and meso-epoxides is a powerful synthetic strategy for preparing optically active polycarbonates with main-chain chirality. The previous studies regarding chiral zinc catalysts provided amorphous polycarbonates with moderate enantioselectivity, and thus, developing highly stereoregular catalysts for this enantioselective polymerization is highly desirable. Herein, we report the synthesis of highly isotactic poly(cyclohexene carbonate)s from meso-cyclohexene oxide using dissymmetrical enantiopure salenCo(III) complexes in conjunction with bis(triphenylphosphine)iminium chloride (PPNCl) as catalyst. The presence of a chiral induction agent such as (S)-propylene oxide or (S)-2-methyltetrahydrofuran significantly improved the enantioselectivity regarding (S,S)-salenCo(III) catalyst systems. Up to 98:2 of RR:SS was observed in the resultant polycarbonates obtained from the catalyst system based on (S,S)-salenCo(III) complex 4d bearing an adamantyl group on the phenolate ortho position, in the presence of (S)-2-methyltetrahydrofuran. Primary ONIOM (DFT:UFF) calculations, which were performed to investigate the effect of the competitive coordination of (S)-induction agent versus cyclohexene oxide to Co(III) center on enantioselectivity, suggest that the (S)-C-O bond in cyclohexene oxide is more favorable for cleavage, due to the interaction between oxygen atom of (S)-induction agent and (S)-C-H of the coordinated cyclohexene oxide. The highly isotactic poly(cyclohexene carbonate) is a typical semicrystalline polymer, possessing a melting point of 216 °C and a decomposition temperature of 310 °C.

Flexibly Tethered Dinuclear Zinc Complexes: A Solution to the Entropy Problem in CO2/Epoxide Copolymerization Catalysis?

“…[9][10][11][12] Latest research even led to isotactic poly(cyclohexene carbonates), opening a route to semi-crystalline thermoplastic materials. [13] Despite the fact that aliphatic polycarbonates were already introduced 40 years ago by Inoue et al, the development of an economic production process is still hampered by the availability of efficient catalysts. The main focus of numerous publications in this field was based on a dinuclear catalysts, which accommodate the epoxide ring opening and a CO 2 insertion in a single molecule.…”

Synthesis, molecular and supramolecular structure of a new dinuclear aluminium(III) complex derived from 3-aminopyrazine- 2-carboxylic acid

“…Few Al(III)-based metal organic frameworks with carboxylate linkers, such as those derived from terephthalic acid [24,25] , 2-aminoterephthalic acid [26] , benzene tricarboxylic acid [27,28] and benzene tetracarboxylic acid [29,30] , have been reported. Hydroxo [31,32] and methoxy [33] bridged dinuclear aluminium(III) complexes of Schiff base ligands are also known.…”

Chemical datuments as scientific enablers

“…by addition of one molecule of carbon dioxide monomer was invariably followed by one molecule of cyclohexene epoxide, and then again by CO 2 . By 2012 the complexity and subtle nature of the challenge had increased, the challenge now being an understanding [17] of how asymmetric induction in the resulting polymer can be achieved. Answering such questions involves a detailed and intricate knowledge of the reacting (covalent or ionic) bonds themselves and (as with the DNA project discussed above) the nature of the non-covalent interactions [12-14].…”