Asymmetric Synthesis. Metal Complex Mediated Synthesis of Chiral Glycine by Enantioselective Proton Exchange

Article abstract of DOI:10.1021/ja00268a051

The complex (1+), a species containing a chiral tridentate triamine ligand, (S,S)-proam, and a tridentate ligand incorporating a glycine residue, picgly, has been prepared.The α-protons of the coordinated glycine residue exchange at different rates in basic D2O solutions.The difference in rate was found to be 7.8:1 in favor of the pro-S proton at pD 11.2 at 25 deg C with a NaHCO3/Na2CO3 buffer.It is proposed that the origins of this enantiosection arise from both steric and hydrogen-bonding effects as inferred from the determined crystal structure of the complex.A kinetic analysis of the exchange process shows that the system is essentially that of an asymmetric synthesis (CH2 -> CHD) followed by a reinforced kinetic resolution (CHD -> CD2).As such, the optical purity of the chiral glycine (NH2CHDCO2H) continuously increases with the extent of reaction.This was confirmed.It is suggested that the present kinetic relationships are representative of the majority of asymmetric syntheses involving enantiotopic atoms or groups, and it follows that, for such systems, quoting an enantiomeric excess has meaning only when the extent of reaction is specified.