Theoretical study on the mechanism of ester hydrolysis in micellar catalysis using model systems

Abstract

A study has been made on the mechanism of ester hydrolysis in micellar catalysis, using model systems with ab initio MO calculations. The study ascertained that neutral imidazole cannot form a tetrahedral (TD) intermediate with esters since its basicity is very weak; the present MO calculations indicate that, immediately after its formation, the TD intermediate from p-nitrophenyl acetate and the imidazolyl anion decompose into p-nitrophenol and N-acetylimidazole. This accords with the experimental finding that p-nitrophenol is released quickly so that measuring the UV spectra of p-nitrophenol enables the reaction to be monitored. The TD intermediate formation step governs the enantioselectivity of micellar catalysis. N-Acetylimidazole reacts with the OH anion taken in from the liquid phase. We examined two paths for the decomposition: one similar to that of ester hydrolysis and the other the direct product formation of the acetate anion and imidazole. MP2/6-31+G//RHF/6-31 G level calculations show that, in the activation energy, the latter mechanism is lower than the former by 7.5 kcal mol^–1.