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.
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 (kOCH3, kcat3:Zn(OCH3) and kcatLa(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 x 10(6)-fold acceleration when present at concentrations of 5 mmol dm(-3). The data for both metals exhibit non-linear Bronsted correlations with the pKa 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 pKa increases above values of approximately 14.7. Plots of the log kOCH3 reaction vs. the log kcat 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.
Neverov
Alexei A
AA
Department of Chemistry, Queen's University, Kingston, Ontario, Canada K7L 3N6.
Sunderland
N E
NE
Brown
R Stan
RS