M. Fanti et al., Ester cleavage catalysis in reversed micelles by Cu(II) complexes of hydroxy-functionalized ligands, LANGMUIR, 16(26), 2000, pp. 10115-10122
The hydrolytic reactivity of ligands featuring a 6-alkylaminomethylpyridine
, 1, or an N-alkylethylenediamine, 2, as chelating subunits, in the presenc
e of Cu(II), has been investigated in AOT/H2O/isooctane reversed micelles.
The substrates of choice were the p-nitrophenyl esters of picolinic acid (P
NPP), of acetic acid (PNPA), and of diphenylphosphoric acid (DPPNPP). In th
e presence of Cu(II) complexes of hydroxy-functionalized ligands, such as l
a or Ib, the cleavage of PNPP is a million-fold faster than in the absence
of Cu(II) and any ligand, the most effective stoichiometry being 1:1. By co
nverse, the rate effects are rather modest using ligands Ic and 2, devoid o
f the hydroxy function. The cleavage of PNPA and DPPNPP is only slightly ac
celerated using all kind of ligands investigated. The high reactivity obser
ved using la and Ib and PNPP accords with the mode of action established in
aqueous micelles. This involves formation of a ternary complex (ligand/Cu(
II)/substrate), pseudointramolecular attack of the (deprotonated) hydroxyl
on the ester carbonyl to give a transesterification intermediate, and metal
ion promoted hydrolysis of such intermediate. The kinetic response of the
reversed micelles is in many ways quite different from that of analogous aq
ueous aggregates previously investigated. Peculiar features include the app
arent insensitivity to relevant variables, such as the pH of added water, t
he w(0) (the H2O/AOT ratio) value, and the lipophilic or hydrophilic charac
ter of the ligand. Clean burst kinetics using 1a.Cu(II) and excess PNPP wer
e obtained but indicate a disappointingly low turnover rate. These and othe
r aspects are discussed, also with reference to the behavior of aqueous mic
elles, and an attempt is made to describe the rather puzzling nature of int
erface and core of the reversed aggregates.