Kinetic amplification of the enantioselective cleavage of α-amino acid esters by metallomicelles

The enantioselective cleavages of p-nitrophenyl esters of phenylalanine (PhePNP), phenylglycine (PhgPNP), and leucine (LeuPNP) catalyzed by Cu(II) complexes of homochiral ligands have been investigated in conditions of very fast change of pH ("pH-jump") at the beginning of the reaction. Using lipophilic ligands comicellized with hexadecyltrimethylammonium bromide and decreasing the pH from 9 to 5, a remarkable amplification of the enantioselectivity has been observed when compared to the same reaction performed at constant pH (from 24 to 58 for PhgPNP, as the ratios of the rate constants measured for the faster and slower reacting enantiomers). This has been correlated with the changes in the ligand/Cu(II) complex concentration induced by the change of pH. In fact, it has been established that under the conditions employed the faster enantiomer of the substrate reacts with a higher concentration of catalyst than the slower one: in the first case the reaction occurs before decomplexation takes place and in the second case after decomplexation is virtually complete.