Reaction Of Epoxide And Carbon Dioxide Catalyzed By Amine Bridged Poly(Phenolate) Aluminum And Lanthanide Complexes

A series of aluminum and lanthanide complexes stabilized by amine-bridged poly(phenolato) ligands were synthesized and their catalytic property for the reaction of epoxides and carbon dioxide was explored. The ligand precursors used include N,N-bis(3, 5-di-tert-butyl-2-hydroxybenzyl)-2-hydroxyethylamine, abbreviated as L1H3; N,N-bis(3, 5-di- methyl-2-hydroxybenzyl)-2-hydroxyethylamine, abbreviated as L2H3; N,N-bis(3,5-di-chloro-2-hydroxybenzyl)-2-hydroxyethylamine, abbreviated as L3H3 and N,N-bis(3, 5-di-tert-butyl-2-hydroxybenzyl)-2-hydroxyphenylamine, abbreviated as L4H3 and N,N,N’-tris-(3,5-di-tert-butyl-2-hydroxybenzyl)-1,2-phenylenediamine, abbreviated as L5H3. 1. Synthesis of aluminum complexes stabilized by amine-bridged poly(phenolato) ligands and their catalytic property for the reaction of epoxides with carbon dioxide(1) Reactions of LnH3 with AlMe3(n = 1, 2, 3, 4) in a 1:1 molar ratio in THF gave the corresponding aluminum complexes(LnAl)2(n =1, 1; n =2, 2; n =3, 3; n =4, 4) in good isolated yields.(2) The coupling reaction of epoxides with carbon dioxide to produce cyclic carbonates catalyzed by complexes 1–4 in the presence of quaternary ammonium salt was explored. Through the optimization of reaction conditions, the optimum reaction conditions are 0.3 mol% of 3 as catalyst, 0.9 mol% of NBu4 Br as cocatalyst, 85 °C. A series of monosubstituted terminal epoxides bearing an alkyl, aryl, halogen, alkene, hydroxyl, ether, morpholine or ester substituent were commendably converted to the corresponding cyclic carbonates in good to excellent yields(60 to 97%) under 1 bar CO2, whereas the bulky and internal epoxides, which were generally considered as substrates with low reactivity, can be transformed into the corresponding cyclic carbonates in good to excellent yields at elevated pressure(10 bar CO2) after prolonged reaction time. 2. Synthesis of lanthanide complexes stabilized by amine-bridged poly(phenolato) ligands and their catalytic property for the reaction of epoxides with carbon dioxide(1) Reactions of L1H3 with LnCp3(THF)(Ln = Yb, Nd, La) in a 1:1 molar ratio in THF gave the corresponding lanthanide complexes(L1Yb)3(THF)2(5), L1Nd(THF)2(6), L1La(THF)2(7) in good isolated yields.(2) The catalytic property of the binary system composed of complexes 5–7 and cocatalysts for the coupling reaction of epoxides with carbon dioxide was explored. Through the screening of reaction conditions, the optimal reaction conditions are: 0.2 mol% of complex 7 as catalyst, 0.4 mol% of NBu4 Br as cocatalyst, 100 °C, 18 h. Under these conditions, a series of monosubstituted and disubstituted epoxides bearing various substituents, including halogen, aryl, alkenyl, ether, morpholine, hydroxyl, and ester groups can be converted to the corresponding cyclic carbonates in good to excellent yields(60 to 97%) at atmospheric pressure or 10 bar CO2. 3. Copolymerization of cyclohexene oxide and carbon dioxide catalyzed by lanthanide complexes 8–10(1) Reactions of L5H3 with LnCp3(THF)(Ln = Yb, Sm, La) in a 1:1 molar ratio in THF gave corresponding lanthanide complexes L5Yb(THF)(8), L5Sm(DME)(9), L5La(DME)(10) in good isolated yields.(2) The catalytic property of complexes 8–10 for the copolymerization of cyclohexene oxide with carbon dioxide was explored. It was found that the polymerization conditions have obvious influence on the catalytic activity and the content of polycarbonate. It should also be noted that the up to 94% selectivity for polycarbonate was obtained in most cases. In addition, CO2 pressure and reaction time have significant influence on the molecular weight of the resulting copolymers. High molecular weight copolymers(up to 6.46×104 g/mol) were obtained at 40 °C. The results analyzed by MALDI-TOF mass spectrometry indicated that the initiating group is the phenolate group, and the initial step of the copolymerization should be the ring-opening of cyclohexene oxide.