Metal ion promoted transesterifications of carboxylate esters. A structure/activity study of the efficacy of Zn2+ and La3+ to catalyze the methanolysis of some aryl and aliphatic esters

Abstract

The methanolysis of various aryl and aliphatic carboxylate esters promoted by methoxide, 1,5,9-triazacyclododecane : Zn²⁺(⁻OCH₃) and La³⁺(⁻OCH₃), were studied and the derived rate constants (k_OCH3, k_cat^3:Zn(OCH3) and k_cat^La(OCH3)) correlated in various ways. The metal ion catalyzed reactions are very much faster than the background reactions in some cases reaching up to 7 × 10⁶-fold acceleration when present at concentrations of 5 mmol dm⁻³. The data for both metals exhibit non-linear Brønsted correlations with the pK_a of the leaving group which are analyzed in terms of a change in rate limiting step from formation to breakdown of a metal-coordinated tetrahedral intermediate as the pK_a increases above values of ∼14.7. Plots of the log k_OCH3 reaction vs. the log k_cat values for each metal ion indicate low sensitivity for aryl esters and a higher sensitivity for the aliphatic esters. A mechanistic rationale for the observations is presented.