Dual Catalyst System for Asymmetric Alternating Copolymerization of Carbon Dioxide and Cyclohexene Oxide with Chiral Aluminum Complexes: Lewis Base as Catalyst Activator and Lewis Acid as Monomer Activator

Abstract

Optically active aluminum complexes such as Schiff base, binuclear β-ketoiminate, and bisprolinol complexes were found to promote asymmetric alternating copolymerizations of carbon dioxide and cyclohexene oxide. The aluminum Schiff base complexes−tetraethylammonium acetate afforded isotactic poly(cyclohexene carbonate)s with low enantioselectivities. Lewis bases having two coordinating sites were utilized to enhance activity and selectivity based on the binuclear structure of the aluminum β-ketoiminate clarified by X-ray crystallography.[2gAlMe] 2 −bulky bisimidazole produced the alternating copolymer with high enantioselectivity (62% ee). The polymerization is considered to preferentially proceed at more crowded, enantioselective site owing to coordination of bulky Lewis bases to aluminums in less enantioselective sites. 3 2 AlMe−2-picoline also exhibited a high enantioselectivity (67% ee). Methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) was applied to perform faster and more enantioselective copolymerizations at low temperature (82% ee). The asymmetric copolymerizations were found to be significantly dependent on size of epoxide, temperature, and kind/amount of activators.

Asymmetric Copolymerization of CO2 with meso‐Epoxides Mediated by Dinuclear Cobalt(III) Complexes: Unprecedented Enantioselectivity and Activity

“…[5] A moderate enantioselectivity (60-80 % ee) was also observed in the copolymerization of CO 2 with meso-epoxides catalyzed by optically active dinuclear aluminum complexes of b-ketoiminate or aminoalkoxide, in conjunction with a bulky Lewis base as catalyst activator. [6] Recently we reported the synthesis of highly isotactic poly(cyclohexene carbonate) composed of 1,2-cyclohexanediol units with 96 % ee using a binary catalyst system that consisted of an unsymmetrical chiral SalenCo III complex and bis(triphenylphosphine)iminium chloride (PPNCl), but the rigorous reaction conditions, such as a low temperature of À25 8C and the use of a large amount of chiral induction agent, were prerequisites for obtaining optically active copolymer with low molecular weight in a very low rate (less than 3 h À1 ). [7] In addition, this binary catalyst system was found to be inactive for the coupling of CO 2 with cyclopentene oxide (CPO; a less reactive epoxide).…”

“…[8] Also, in the copolymerization of epoxides with CO 2 using coordination polymerization catalysis, the bimetallic synergistic effect was frequently observed. [4][5][6][9][10][11] Indeed, our initial study mainly focused on enantiomerically pure mononuclear Co III complexes 2 and 3 with an interor intramolecular nucleophilic cocatalyst for CO 2 /CPO copolymerization, stimulated by the success with these chiral Co III complexes in catalyzing alternating copolymerization of CO 2 with aliphatic epoxides. [12][13][14] To our disappointment, the formation rate of CO 2 copolymer is significantly lower than that of propylene oxide or cyclohexene oxide copolymers when using the same catalyst systems.…”

Aluminium coordination complexes in copolymerization reactions of carbon dioxide and epoxides

“…Very exciting results concerned with asymmetric copolymerization of CHO with carbon dioxide have been reported in the past year. 48 Such results were only possible by building on the mechanistic understanding for these reactions, which has been built up over the past 15 years. There are a wealth of opportunities available in terms of studying carbon dioxide utilization using this cheap metal, for example oxetane has been copolymerized with carbon dioxide using Cr complexes, 57 and as far as we are aware this has not been studied using Al complexes.…”

“…48 Using (Salophen)AlMe (discussed above) relatively low amounts of asymmetric induction were achieved, however, bimetallic aluminium β-ketoiminate complexes could be used, Fig. 5(c) and very good stereoselectivity was obtained.…”

Dinuclear Metal Complex-Mediated Formation of CO2-Based Polycarbonates

“…Complex 9, the most active of the series, affords polycarbonate (M n ¼ 23,000 g/mol, PDI ¼ 4.0) with 92% carbonate linkages, at 12 atm of CO 2 and 70 C. Recently, Sugimoto and co-workers investigated various dimeric β-diketiminate (Fig. 18) and Schiff base aluminium complexes, including 10a-b [149]. Complexes 10a-b, in the presence of Et 4 NOAc, showed good selectivity (carbonate linkages >98%) and moderate activity (TOF < 13 h…”