Comparison of immobilized Box and azaBox–Cu(II) complexes as catalysts for enantioselective Mukaiyama aldol reactions

Abstract

Enantioselective Mukaiyama aldol reactions of different α-ketoesters was tested with copper complexes of chiral Box and azaBox ligands in both homogeneous and heterogeneous systems. Results in the homogeneous reactions were greatly influenced by the nature of the ketoester, the chiral ligand, and the reaction solvent. In the case of supported catalysts, the use of strongly coordinating azaBox ligands prevented the leaching of metal but reduced the Lewis acidity, and thus the catalytic activity, and did not solve the problem of poisoning by strong coordination of products, byproducts, or solvents. The counter-ion effect also was very significant, and electrostatic immobilization was efficient only with Box ligands (up to 86% ee at room temperature), whereas covalent immobilization allowed the use of azaBox ligands (up to 85% ee at room temperature) in the heterogeneous phase. Recovered deactivated solids could be reused in a reaction with a completely different mechanism that does not require acid centers, such as cyclopropanation.