Mc. Cleij et al., CHIRAL LIPOPHILIC LIGANDS .3. CONTROL OF ENANTIOSELECTIVITY IN COPPER(II)-CATALYZED CLEAVAGE OF ALPHA-AMINO-ACID ESTERS BY AGGREGATE MORPHOLOGY, Langmuir, 12(12), 1996, pp. 2956-2960
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.