Highly diastereoselective and sustainable synthesis of cyclopropanes via amino acid-Schiff base cop-per(II) catalysis

A series of copper(II) complexes with Schiff bases derived from 2'-hydroxyacetophenone and various L-amino acids (glycine, serine, alanine and phenylalanine) were synthesized using a one-pot procedure. The obtained ligands and their metal complexes were characterized by FT-IR, UV-Vis, NMR, EPR spectroscopy, and thermogravimetric analysis. Spectroscopic data suggested square-planar coordination geometry around the Cu(II) centers, with involvement of imine nitrogen, hydroxyl, and carboxylate oxygen atoms in metal binding. The catalytic activity of the complexes was evaluated in the cyclopropanation reaction of styrene with ethyl diazoacetate under mild conditions. All complexes exhibited excellent yields and high diastereoselectivity (up to 99:1 trans:cis), with the L-alanine-derived complex showing the highest enantiomeric excess (24% trans and 46% cis). These findings highlight the potential of these bio-derived Schiff base complexes as efficient and selective catalysts in stereoselective carbene-transfer reactions.