CHIRAL LIPOPHILIC LIGANDS .5. ENANTIOSELECTIVE ESTER CLEAVAGE OF ALPHA-AMINO ESTERS BY CU(II) COMPLEXES OF CHIRAL DIAMINO ALCOHOLS IN AQUEOUS SURFACTANTS SOLUTIONS

A series of lipophilic ligands, 1-3, featuring an 1,2-ethylendiamino m oiety as chelating subunit, one (1, 3) or two (2) chiral carbons, and an hydroxy function (except for 3) in the proximity of the coordinatio n center, have been synthesized. Their Cu(II) complexes have been inve stigated as catalysts for the cleavage of p-nitrophenyl esters of phen ylalanine (PhePNP) and phenylglycine (PhgPNP) in the presence of catio nic aggregates formed by cetyltrimethylammonium bromide (CTABr) or dit etradecyldibutylammonium bromide (DMDBAB). Large rate accelerations (u p to two order of magnitude) and quite remarkable enantioselectivities (from 11 to 35, as the ratios of the rate constants measured far me f aster and slower reacting enantiomers) have been observed. In the case of ligands I the S-ligand complex reacts faster with the S-substrate and the enantioselectivity increases with the lipophilicity of the sub stituent of the chiral carbons. Using ligands 2, having two chiral cen ters, the most favoured situation is reached when all the chiral carbo ns of ligands and substrate have the same absolute configuration; in s uch a case, and using DMDBAB as cosurfactant enantioselectivities as h igh as 35 have been observed. The results are explained on the basis o f a different reaction mechanism due to the compartmentalization of th e reacting species (a ternary complex ligand/Cu(II)/substrate) in diff erent loci of the aggregate. It is suggested that, depending on the hy drophobicity of the ternary complex, the effective nucleophile may swi tch from the Cu(II)-bound ligand's hydroxyl to a Cu(II) bound water mo lecule. The first mechanism is faster and prevails for the more lipoph ilic ternary complex. Copyright (C) 1996 Elsevier Science Ltd