Chiral lipophilic ligands. 5. Enantioselective ester cleavage of α-amino esters by Cu(II) complexes of chiral diamino alcohols in aqueous sufactants solutions

A series of lipophilic ligands, 1-3, featuring an 1,2-ethylendiamino moiety as chelating subunit, one (1, 3) or two (2) chiral carbons, and an hydroxy function (except for 3) in the proximity of the coordination center, have been synthesized. Their Cu(II) complexes have been investigated as catalysts for the cleavage of p-nitrophenyl esters of phenylalanine (PhePNP) and phenylglycine (PhgPNP) in the presence of cationic aggregates formed by cetyltrimethylammonium bromide (CTABr) or ditetradecyldibutylammonium bromide (DMDBAB). Large rate accelerations (up to two order of magnitude) and quite remarkable enantioselectivities (from 11 to 35, as the ratios of the rate constants measured far me faster 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 substituent of the chiral carbons. Using ligands 2, having two chiral centers, the most favoured situation is reached when all the chiral carbons of ligands and substrate have the same absolute configuration; in such a case, and using DMDBAB as cosurfactant enantioselectivities as high as 35 have been observed. The results are explained on the basis of a different reaction mechanism due to the compartmentalization of the reacting species (a ternary complex ligand/Cu(II)/substrate) in different loci of the aggregate. It is suggested that, depending on the hydrophobicity of the ternary complex, the effective nucleophile may switch from the Cu(II)-bound ligand's hydroxyl to a Cu(II) bound water molecule. The first mechanism is faster and prevails for the more lipophilic ternary complex.