Reaction of N-acetyldehydrophenylalanyl-S-proline with Ca(II) and Ni(II) ions and its role in asymmetric hydrogenation

N-Ac-ΔPhe-S-Pro forms strong coordination complexes with the CO2H group of the proline residue and weak coordination complexes through the -C(=O)-N groups with Ca(II) and Ni(II) ions in 95% aqueous methanol as indicated by the pKa values, PMR spectroscopy, and paramagnetic broadening of the13C NMR signals in the nickel complex. Complex formation enhances the rigidity of the dehydrodipeptide conformation, leading to a strong increase in the optical yield upon hydrogenation over achiral catalysts.