Mc. Cleij et al., CHIRAL LIPOPHILIC LIGANDS .5. ENANTIOSELECTIVE ESTER CLEAVAGE OF ALPHA-AMINO ESTERS BY CU(II) COMPLEXES OF CHIRAL DIAMINO ALCOHOLS IN AQUEOUS SURFACTANTS SOLUTIONS, Tetrahedron, 53(1), 1997, pp. 357-368
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