Kinetics of the silver ion-promoted hydrolysis of 2-methyl-2-(substituted phenyl)-1,3-dithianes in 10% dioxane–water mixtures. Implications for cyclic acetal hydrolysis catalysed by hydrogen ions

Abstract

The hydrolysis of 1,3-dithianes derived from para-substituted acetophenones is promoted by silver ions. Kinetic study using a 10%(v/v) dioxane–water solvent shows that, when [Ag+]≲ 0.2 mol dm–3, the p-NO2, -Cl, -H and -Me derivatives hydrolyse via rapidly-formed 1 Ag+ : 1 dithiane-complexes, but that the p-MeO derivative forms an unreactive 1 : 1-complex and hydrolyses via a 2 Ag+ : 1 dithiane-complex. Comparison of the kinetic parameters with those available for analogous open-chain S,S-acetals reveals that cyclisation leads to a substantial (> 104-fold) overall loss of reactivity and that this loss arises both from a lowering of acetal basicity towards Ag+ and from a slower rate of hydrolysis of the 1 : 1-complex. The implications for hydrogen ion-catalysed hydrolyses of cyclic acetals are discussed, as are the reasons for the lower reactivity of the cyclic acetals.