Kinetic Resolution of rac-Phenylalanine by Stereoselective Complexation to a Chiral Cobalt Complex through (pi)-(pi) Stacking Interaction

A cobalt(III) complex with chiral ligand, H2cpel (N-carboxymethyl-N-pyridylethyl-L-leucine), was prepared for chiral recognition of amino acids. Through the competitive coordination of racemic phenylalanine to the chiral cobalt complex, [Co(cpel)(CO3)]^- (1), enantioselective recognition was achieved on the ternary complex, which was determined on the basis of HPLC analysis with a chiral column. The formation rate for the [Co(cpel)(L-phe)] complex (2) was 6-times superior to that of [Co(cpel)(D-phe)] (3). The preferential formation of 2 might be illustrated by the interligand (pi)-(pi) stacking interaction. Crystal structural analysis for 2 and 3 revealed that aromatic rings, pyridine ring of CPEL and phenylalanine sidechain, in 2 were very close each other but those in 3 were far apart. Such interligand aromatic interaction in 2 was also examined by the use of 1H NMR spectra.