Mechanism and catalysis in the hydrolysis of some acetals and esters

Recent advances in the study of acetal hydrolysis and the effect of neighbouring hydroxyl groups on ester hydrolysis are reviewed. Some of the criteria used to elucidate the mechanism of such reactions are discussed. The enhanced rates of intra-molecular reactions over their inter-molecular counterparts are suggested to be due to factors other than just a 'concentration' effect, product stability reflected in the transition state may be important. Intra-molecular and inter-molecular general-acid catalysis have been detected in the hydrolysis of benzyli-dene 2,3-dioxy benzoic acid and benzylidene catechol, respectively. Several kinetic parameters of these reactions have been determined and are discussed. An example of nucleophilic catalysis by a carboxyl group has been found in the hydrolysis of o-carboxy benzylidene catechol. o-carboxy benzylidene 2,3-dioxy benzoic acid, a system which apparently contains suitably disposed carboxyl groups to act as general-acid and nucleophilic catalysts, was synthesised. However, the hydrolysis of this acetal showed no evidence of bifunctional catalysis. Ring opening during the acid catalysed hydrolysis of benzaldehyde acetals of 2,3-exo-norbornanediol is shown to be reversible. An A-2 mechanism is suggested to be the pathway of hydrolysis of these 1,3-dioxolanes. The facilitated rate of hydrolysis of beta-hydroxy esters, in particular beta-hydroxy butyrates and 2-hydroxy cyclopentane carboxylates, has been shown not to be due to any direct interaction between the two functional groups. 'Solvent sorting' is tentatively suggested to account for the observed phenomenon.