Chiral lipophilic ligands .4. Metallomicelles made of Cu(II) complexes of alpha-amino-amide-type ligands as catalysts of the cleavage of alpha-amino-acid esters

A new series of lipophilic ligands, 1-3, featuring an alpha-amino amido (1) or a dipeptide amido (2) moiety as chelating subunit and a hydroxy function (except for 3) close to the coordination centre, have been synthesized and their copper(II) complexes investigated as catalysts for the cleavage of p-nitrophenylalanine in aqueous comicelles of CTABr. Rate acceleration up to two orders of magnitude have been observed. The most effective complexes are those with ligands 1 that are the best suited for the formation of a ternary complex ligand-Cu(II)-substrate, in which the hydroxyl activated by the metal ion (pK(a)=6.8) may act as a nucleophile in the cleavage of the ester. Using chiral 1b,c, 2b and the enantiomers of the substrate, sizable enantioselectivity (up to 2.4) has been observed only in the case of ligands 1b,c. These effects are suggested to arise from a different partitioning of the two diastereomeric ternary complexes within the aggregate.