CHIRAL LIPOPHILIC LIGANDS .3. CONTROL OF ENANTIOSELECTIVITY IN COPPER(II)-CATALYZED CLEAVAGE OF ALPHA-AMINO-ACID ESTERS BY AGGREGATE MORPHOLOGY

The cleavage of the enantiomers of the p-nitrophenyl eaters of phenylg licine (PhgPNP) was studied using chiral Cu(II) complexes of ligands 2 R(1)-N-(1-R(2)-2-hydroxyethyl)aminomethylpyridine] as catalyst. Lipop hilic ligands 2a,b (2a, R(1) = n-dodecyl, R(2) = methyl; 2b, R(1) = n- dodecyl, R(2) = isopropyl) were studied in aggregates of nonfunctional surfactants forming micelles (cationic, anionic, nonionic) or vesicle s. With respect to the nonmicellar complex 2c . Cu(II) (2c, R(1) = met hyl, R(2) = isopropyl), large rate accelerations (up to 400 times) and moderate (up to 11) enantioselectivities (as rate ratio between the f aster and slower enantiomer) were found in cationic micelles. On the c ontrary, large inhibition was observed in anionic micelles, whereas in the nonionic ones the kinetic effects were negligible. In cationic ve sicles the enantioselectivities are strongly influenced by the fluidit y of the aggregate bilayer: remarkably large values (up to 26) were ob served below the main phase transition temperature, T-c. The results w ere explained on the basis of 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 the more lipophilic diastereomeric complex reacts with the subst rate with attack of the Cu(II)-bound alkoxide of the ligand while the more hydrophilic one reacts with attack of a metal-bound hydroxyl of a n exogenous water molecule. The first mechanism is both faster and mor e enantioselective.