Aluminum porphyrins with quaternary ammonium halides as catalysts for copolymerization of cyclohexene oxide and CO(2): metal–ligand cooperative catalysis

Bifunctional Al(III) porphyrins with quaternary ammonium halides, 2-Cl and 2-Br, worked as excellent catalysts for the copolymerization of cyclohexene oxide (CHO) and CO(2) at 120 °C. Turnover frequency (TOF) and turnover number (TON) reached 10 000 h(–1) and 55 000, respectively, and poly(cyclohexene carbonate) (PCHC) with molecular weight of up to 281 000 was obtained with a catalyst loading of 0.001 mol%. In contrast, bifunctional Mg(II) and Zn(II) counterparts, 3-Cl and 4-Cl, as well as a binary catalyst system, 1-Cl with bis(triphenylphosphine)iminium chloride (PPNCl), showed poor catalytic performances. Kinetic studies revealed that the reaction rate was first-order in [CHO] and [2-Br] and zero-order in [CO(2)], and the activation parameters were determined: ΔH(‡) = 12.4 kcal mol(–1), ΔS(‡) = –26.1 cal mol(–1) K(–1), and ΔG(‡) = 21.6 kcal mol(–1) at 80 °C. Comparative DFT calculations on two model catalysts, Al(III) complex 2′ and Mg(II) complex 3′, allowed us to extract key factors in the catalytic behavior of the bifunctional Al(III) catalyst. The high polymerization activity and carbonate-linkage selectivity originate from the cooperative actions of the metal center and the quaternary ammonium cation, both of which facilitate the epoxide-ring opening by the carbonate anion to form the carbonate linkage in the key transition state such as TS3b (ΔH(‡) = 13.3 kcal mol(–1), ΔS(‡) = –3.1 cal mol(–1) K(–1), and ΔG(‡) = 14.4 kcal mol(–1) at 80 °C).