Chiral discrimination phenomena in mixed-ligand copper(II) complexes with amino acids and 1,3-dicarbonyl compounds

Circular dichroism (CD) spectra of individual mixed‐ligand copper(II) complexes of 1,3‐dicarbonyl compounds, (1S)‐ or (1R)‐3‐hydroxymethylene camphor, (1S)‐3‐trifluoroacetyl camphor, or (1R)‐2‐hydroxymethylene menthone, and α‐amino acids, alanine, valine, proline, or their N‐alkyl derivatives, were calculated from CD spectra of equilibrium solutions containing the above constituents in methanol or ethylene dichloride. Diastereomeric mixed‐ligand complexes incorporating identical dicarbonyl but enantiomeric N‐alkyl‐α‐amino acid ligands exhibit quasi‐enantiomeric CD spectra. Unsubstituted amino acids, on the contrary, will make no decisive contributions to the net optical activity spectrum of the mixed‐ligand complexes. Formation constants of diastereomeric mixed‐ligand complexes have been calculated from data on disproportionation of the latter into corresponding equally paired complexes. Enantioselectivity was demonstrated to amount to up to 700 cal/mol. Possible steric structures of mixed‐ligand complexes are discussed. © 1993 Wiley‐Liss, Inc.