Intramolecular catalysis by the ionised carboxy-group of the hydrolysis of enol esters, and of the general acid catalysed ketonisation of the enols produced

Abstract

2-Acetoxycyclohexene-1-carboxylate ion is hydrolysed extraordinarily rapidly to the 2-hydroxy-compound. The ketonisation of this enol is slow enough to be measured between pH 7–11. In the case of 2-acetoxycyclopentene-1-carboxylate ion both reactions are too fast to measure by conventional techniques. A mechanism involving intramolecular nucleophilic catalysis leading to a β-keto-keten intermediate is suggested to account for the rapid hydrolysis of the enol esters. The ketonisation of 2-hydroxycyclohexene-1-carboxylate is subject to both general acid and general base catalysis, and attack by general acids is itself general base-catalysed by the neighbouring carboxylate group. Rate constants measured for the general base-catalysed ketonisation of ethyl 2-hydroxy-cyclohexene-1-carboxylate agree closely with those calculated from the known equilibrium and rate constants for enolisation of ethyl 2-oxocyclohexane-1-carboxylate.