Search

The desymmetrization copolymerizationof meso-epoxideswith CO2using chiral catalysts or reagents is regardedas a valuable strategy for the synthesis of optically active polycarbonateswith main-chain chirality. The present study demonstrates that thebiphenol-linked dinuclear Co(III) complexes show unprecedented activity,enantioselectivity, and broad substrate scope for coupling CO2with various meso-epoxides to afford thecorresponding isotactic polycarbonates. This investigation also focuseson the mechanistic understanding on the origin of enantioselectivityand highly catalytic activity in the enantiopure dinuclear Co(III)complex mediated copolymerization process, using cyclohexene oxideas a model monomer of meso-epoxides. The kineticstudy by in situinfrared spectroscopy revealed afirst-order dependence on the catalyst concentration in the systemsof biphenol-linked dinuclear Co(III) complex alone or in the presenceof an ionic cocatalyst. An intramolecular bimetallic cooperation mechanismwas proposed to be predominantly responsible for the copolymerizationprocess, wherein alternating chain growth and dissociation take turnsbetween two Co(III) ions from the inside cleft of the catalyst bythe nucleophilic attack of the growing carboxylate species at onemetal center toward the activated epoxide at the other. Density functionaltheory calculations suggested that in the two diastereoisomers ofthe biphenol-linked dinuclear Co(III) complexes the matched configurationwas (S,S,S,S,S)- rather than (S,S,R,S,S)-conformer for CO2/meso-epoxide copolymerizationto give the corresponding polycarbonate with S,S-configuration.The addition of an ionic cocatalyst with bulky cation significantlyimproves both the catalytic activity and enantioselectivity, whilethe presence of a coordination Lewis base caused dramatically a changein the chiral induction orientation. [ABSTRACT FROM AUTHOR]

Trivalent cobalt complexes of salicylaldimine in thepresence ofan inter- or intramolecular nucleophilic cocatalyst have proven tobe excellent catalysts for the copolymerization of CO2andepoxides to selectively afford the corresponding polycarbonates inperfectly alternating nature. Especially, bifunctional cobalt(III)–salencomplexes bearing an appended quaternary ammonium salt are more efficientin catalyzing this copolymerization even at high temperatures andextremely low catalyst loading. The present study focuses on comparativekinetics of two different catalyst systems (binary catalyst systemof salen(III)X 1/nBu4NX and bifunctional catalyst 2bearing an appendedquaternary ammonium salt, X = 2,4-dinitrophenoxide) for coupling CO2and epoxides (propylene oxide or cyclohexene oxide) by meansof in situinfrared spectroscopy. An induction periodwas readily found in the binary catalyst system, and its length significantlydepends on catalyst loading. Contrarily, no induction period was observedin the bifunctional catalyst 2, in which the overallreaction pathway is consistent with the first-order dependence oncatalyst concentration. A reaction order of 1.61 of catalyst concentrationwas obtained from the binary 1/nBu4NX catalyst system, indicating the complexityof the copolymerization. The energies of activation determined forcyclic carbonate and copolymer formation in the coupling reactionof CO2and propylene oxide catalyzed by the binary 1/nBu4NX system are50.1 and 33.8 kJ/mol, respectively, compared to the correspondingvalues in the bifunctional catalyst 2of 77.0 and 29.5kJ/mol. The big difference in the energies of activation for cycliccarbonate versus copolymer formation accounts for the excellent selectivityfor copolymer formation in the bifunctional catalyst systems evenat elevated temperatures. In the coupling system of CO2and cyclohexene oxide, the energy of activation for copolymer (Ea) formation is 47.9 kJ/mol for the binary 1/nBu4NX catalyst system,higher than 31.7 kJ/mol determined in the bifunctional catalyst 2. [ABSTRACT FROM AUTHOR]