The catalytic effect of catiohic amino micelles on the hydrolysis of substituted phenyl esters

The catalytic effects of two aminocationic micelles on the hydrolysis of substituted phenyldecanoate esters and a positively charched benzoate ester (CPNBA) were determined. The micellaric catalysts were of the general structure [CH₃(CH₂)₃N(CH₃)₂(CH₂)_nNH₂]Br where n=2 (micelle 1); n=3 (micelle 2). The kinetics followed the expression: k_obs =k_o+k_cat x K_a/(K_a+H⁺)+k^o_OH[OH^-]. From the comparison of the kc OH rates with specific base catalysis rates deduced from reactions in non catalytic micelles, it was concluded that the kc OH term, is compatible mainly with an aminolysis reaction catalyzed by hydorxide ion. The Hammett and Bronsted correlations (p=2.8; β=1.0), in addition to the very small deuterium isotope effect, suggested that kcat corresponded with a nucleophilic mechanism. The Bronsted plot of log k_cat vs pKa of the phenolate leaving groups in micelles 1 and 2 showed a biphasic behaviour. The break in the curve occured at pK_o=5.89 and pK_o=6.78 respectively. The partition ratio k^±/k_-a of the zwiterionic tetrahedral intermediate was derived from the experimental data and produced the following correlation: log k^±/k_-a=-0.92pK_o+0.43pK_N+2.466. The ester CPNBA exhibited a deuterium isotope effect of 2.1. From product analysis it was concluded that the reaction proceeds via a general base catalysis of aminolysis.