Synthesis of cyclic carbonates using monometallic, and helical bimetallic, aluminium complexes

The aluminium complexes of a series of bis(pyrazol-1-yl) methane derived ligands were investigated as catalysts for the synthesis of cyclic carbonates from carbon dioxide and epoxides. A bimetallic, helical, heteroscorpionate complex displayed significantly higher catalytic activity than the corresponding monometallic complexes. The optimal reaction conditions were found to involve the use of 5 mol% of the bimetallic complex and 5 mol% of tetrabutylammonium bromide under solvent free conditions in the presence of 10 bar of wet carbon dioxide. Under these conditions, eleven monosubstituted epoxides were converted into the corresponding cyclic carbonates in 51-91% yield. A kinetic study showed that the reactions were first order in epoxide, catalyst and tetrabutylammonium bromide and allowed a catalytic cycle to be proposed.