Stereoselective Formation of Ternary Copper(II) Complexes of (S)‐amino‐acid amides and (R)‐ or (S)‐amino acids in aqueous solution

Formation constants of ternary complexes of CuII with (S)‐amino‐acid amides (phenylalaninamide, prolinamide, and tryptophanamide) and (R)‐ or (S)‐amino acids (valine, phenylalanine, proline, and tryptophan) were determined potentiometrically at 25° and I = 0.1M (KC1). Significant stereoselectivity was found for the systems (S)‐tryptophanamide/proline, (S)‐prolinamide/tryptophan, and (S)‐phenylalaninamide/proline, the diastereoisomeric complexes with ‘homochiral’ (SS) being more stable than with ‘heterochiral’ (SR) ligands. The stereoselectivity observed may be explained on the basis of repulsive interactions between the ligand side‐chain residues. The present data on the stability of mixed complexes in solution allow to draw some conclusions on the mechanism of chiral discrimination of amino acids in HPLC (reversed‐phase) using CuII complexes of (S)‐amino‐acid amides as selectors for ligand‐exchange chromatography (LEC).