One-Component Aluminum(heteroscorpionate) Catalysts for the Formation of Cyclic Carbonates from Epoxides and Carbon Dioxide.

New neutral and zwitterionic chiral NNO-donor scorpionate ligands 1 and 2 were designed to obtain new mononuclear and dinuclear NNO-heteroscorpionate aluminum complexes. Reaction of 1 with [AlR3 ] (R=Me, Et) in a 1:1 or 1:2 molar ratio afforded the neutral mononuclear alkyl complexes [AlR2 (κ2 -bpzappe)] {R=Me (3), Et (4); bpzappeH=2,2-bis(3,5-dimethylpyrazol-1-yl)-1-[4-(dimethylamino)phenyl]-1-phenylethanol} and bimetallic complexes [{AlR2 (κ2 -bpzappe)}(μ-O){AlR3 }] [R=Me (5), Et (6)]. By reaction of complexes 3-6 with PhCH2 Br, mononuclear and dinuclear cationic aluminum complexes [AlR2 (κ2 -bbpzappe)]Br {R=Me (7), Et (8); bbpzappeH=N-benzyl-4-[2,2-bis(3,5-dimethyl-1H-pyrazol-1-yl)-1-hydroxy-1-phenylethyl]-N,N-dimethylbenzenaminium bromide} and [{AlR2 (κ2 -bbpzappe)}(μ-O){AlR3 }]Br [R=Me (9), Et (10)] were synthesized. Both neutral aluminum complexes in the presence of Bu4 NBr and cationic aluminum complexes were investigated as catalysts for cyclic carbonate formation from epoxides and carbon dioxide. Amongst them, complex 10 was found to be an efficient one-component catalyst for the synthesis of cyclic carbonates from both monosubstituted and internal epoxides and was shown to have broad substrate scope.