Ester Cleavage Catalysis in Reversed Micelles by Cu(II) Complexes of Hydroxy-Functionalized Ligands

Abstract

The hydrolytic reactivity of ligands featuring a 6-alkylaminomethylpyridine, 1, or an N-alkylethylenediamine, 2, as chelating subunits, in the presence of Cu(II), has been investigated in AOT/H2O/isooctane reversed micelles. The substrates of choice were the p-nitrophenyl esters of picolinic acid (PNPP), of acetic acid (PNPA), and of diphenylphosphoric acid (DPPNPP). In the presence of Cu(II) complexes of hydroxy-functionalized ligands, such as 1a or 1b, 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 converse, the rate effects are rather modest using ligands 1c and 2, devoid of the hydroxy function. The cleavage of PNPA and DPPNPP is only slightly accelerated using all kind of ligands investigated. The high reactivity observed using 1a and 1b 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 aqueous aggregates previously investigated. Peculiar features include the apparent insensitivity to relevant variables, such as the pH of added water, the w0 (the H2O/AOT ratio) value, and the lipophilic or hydrophilic character of the ligand. Clean burst kinetics using 1a·Cu(II) and excess PNPP were obtained but indicate a disappointingly low turnover rate. These and other aspects are discussed, also with reference to the behavior of aqueous micelles, and an attempt is made to describe the rather puzzling nature of interface and core of the reversed aggregates.