Monomeric or Dimeric Aluminum Complexes as Catalysts for Cycloaddition between CO2 and Epoxides

A monomeric aluminum complex containing aliphatic tetradentate ligand L1 (HOCMe2CH2NMeCH2CH2NMeCH2CMe2OH) was synthesized and used as a catalyst for cycloaddition between CO2 and epoxides in the presence of PPNCl as a cocatalyst. To check the effect of ligand L1, coordinated to the aluminum center, on the activity of cycloaddition, the new ligand HOCMe2CH2NMe2, which corresponds to half of L1, was systematically designed to make monomeric or dimeric aluminum complexes. Comparison of the catalytic properties of the aluminum complex containing the tetradentate ligand with those of the two related aluminum complexes containing the bidentate ligand under the same conditions revealed that the first system showed higher activity than the other two for cycloaddition between CO2 and epoxides in the presence of PPNCl, which was the best cocatalyst out of the six compounds nBu(4)PBr, nBu(4)NCl, nBu(4)NBr, nBu(4)NI, DMAP, and PPNCl.