A kinetic study of the soft metal ion-promoted hydrolyses of some S-acetals

Abstract

The S-acetals (III)–(VII) hydrolyse readily in aqueous ethanol or in aqueous dioxan solutions containing an excess of Tl³⁺ or Ag⁺ ions to give benzophenone and the appropriate sulphide. The hydrogen-ion catalysed hydrolyses are very slow by comparison and the supposedly soft ions Cd²⁺ and Cu²⁺ have even less effect than H₃O⁺. Kinetic and spectroscopic studies of the hydrolyses, mainly at 25 °C, show that the Tl³⁺-promoted reactions occur via the rapid formation of a 1 Tl³⁺: 1-S-acetal adduct. This adduct is formed stoicheiometrically with (IV), (V), and (VII), but in small amounts with (III) and (VI). The tripositive adduct probably reacts with water in the slow step of the overall hydrolysis. This adduct can also lose charge by proton dissociation from hydroxy groups in the bound S-acetal, or from thallium-bound water. The resulting dipositive adducts, however formed, hydrolyse much less rapidly than the tripositive adducts. The (chelated) dipositive adduct from (V) is sufficiently stable for contributions to hydrolysis from a [2 Tl: 1-S-acetal]⁴⁺ adduct to be detected. The Ag⁺-promoted hydrolyses proceed principally via small amounts of rapidly formed dipositive 2 Ag⁺: 1-S-acetal adducts. For (V), where loss of a carboxy proton can occur, the formation constant of the monopositive [2 Ag: 1-S-acetal]⁺ adduct is very large (K ca. 10⁷ l mol^–1) but that of the dipositive adduct is small, as for the other S-acetals. With (V) and (VI) the 1 Ag⁺: 1-S-acetal adduct is formed stoicheiometrically and with (IV) its formation constant is large (K ca, 900 l mol^–1). For (III), (IV), (VI), and (VII) these 1:1 adducts contribute significantly to the hydrolysis at low silver ion concentrations. For both the Tl³⁺- and the Ag⁺-promoted reactions the effects of ionic strength are consistent: where more charged adduct can form an increase in ionic strength increases the reaction rate; where adduct formation is complete, changes in ionic strength have little effect on the rate. The S-acetals display wide (up to 10⁵-fold) differences in reactivity towards hydrolysis and their sequence of reactivity alters markedly with the promoting ion. With both ions (VI) hydrolyses relatively slowly. For all the substrates Tl³⁺ is a better promoter than Ag⁺. Exact comparisons of reactivity are impossible owing to the differences in mechanistic detail and in solvent. In view of its soft acid character Tl³⁺ is surprisingly more acidic towards the O-containing acetals than towards those having just two S atoms. For (IV) and Ag⁺ the establishment of the pre-equilibrium giving the 1 Ag⁺ : 1-S-acetal adduct is slow enough at 25 °C to be followed by stopped-flow spectroscopy.