Kinetics and mechanism of the metal ion-promoted hydrolysis of 2-phenyl-1,3-dithiane in aqueous dioxane solution

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Abstract
In 1%(v/v) aqueous dioxane the hydrolysis of 2-phenyl-1,3-dithiane (1) is promoted by thallium(III), mercury(II), mercury(I), and silver(I) ions; the qualitative sequence of their efficiencies is TI3+ [graphic omitted] Hg2+ > Hg2 2+ Ag+. Kinetic studies suggest the reaction mechanism with the thallium and mercury ions involves the rapid formation of a 1:1-adduct between the metal ion and the dithiane (formation constant, K 54.5, 3.0 × 104, and 630 dm3 mol–1 for TI3+, Hg2+, and Hg2 2+, respectively at 25 °C) followed by a slow ring-opening step (k2 77, 0.20, and 0.07 s–1, respectively) which may not involve water. Added chloride ions inhibit the mercury(II) ion promotion: HgCl+ and HgCl2 have, respectively, ca. 90% and ca. 14% of the reactivity of Hg2+. An increase in the dioxane content of the solvent to 20%(v/v) has relatively little effect on the reaction. The general kinetic pattern, and level of reactivity, is similar to that observed previously for the TI3+- and Hg2+-promoted hydrolyses of 2-phenyl-1,3-dithiolane (4). Both (1) and (4) are less basic towards TI3+ than towards Hg2+, and the thallium adducts of (1) and (4) both react more rapidly than the corresponding Hg2+ adducts. However, (1) is less basic than (4) towards TI3+, whereas the reverse is true for Hg2+, and the TI3+ adduct of (1) is more reactive than that of (4), whereas the reverse is true of the reactivities of the corresponding Hg2+ adducts. These differences of detail suggest that the (unknown) conformations of the adducts control to some extent the values of K and k2.

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