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
Neverov, Alexei A.; Sunderland, N. E.; Brown, R. Stan; Neverov Alexei A.; Department of Chemistry; Sunderland N. E.; Department of Chemistry; Brown R. Stan; Department of Chemistry
Журнал:
Organic & Biomolecular Chemistry
Дата:
2005
Аннотация:
The methanolysis of various aryl and aliphatic carboxylate esters promoted by methoxide, 1,5,9-triazacyclododecane : Zn<sup>2+</sup>(<sup>â </sup>OCH3) and La<sup>3+</sup>(<sup>â </sup>OCH3), were studied and the derived rate constants (kOCH3, kcat<sup>3:Zn(OCH3)</sup> and kcat<sup>La(OCH3)</sup>) correlated in various ways. The metal ion catalyzed reactions are very much faster than the background reactions in some cases reaching up to 7 à 10<sup>6</sup>-fold acceleration when present at concentrations of 5 mmol dm<sup>â 3</sup>. The data for both metals exhibit non-linear Brønsted 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 â ¼14.7. 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.
213.6Кб