Kinetics of the hydrolysis of diaryl(acylamino)(acyloxy)spiro-λ4-sulfanes (2a-e, 3-5) leading to sulfoxides have been studied under pseudo-first-order conditions in dioxane-water mixtures or aqueous buffer solutions. A sulfonium-carboxylate-type dipolar structure of the starting spiro-λ4-sulfanes is supported by IR spectroscopic data. Solvent polarity and ionic strength have no significant influence on the rate of hydrolysis of compounds 2a and 3. Electron-withdrawing para-substituents promote the reaction in both neutral (ρ 1.43) and acidic media (ρcat 0.90). Hydrolysis is moderately accelerated by strong acids owing to the protonation of the negatively polarized acyloxy group of the substrate. In 50:50 (v/v) dioxane-H2O(D2O) the primary deuterium isotope effect is kH2O/kD2O 3.68. Spiro-λ4-sulfanes 2a-e and 3 with a five-membered N-containing ring are much more reactive than the six-membered analogues 4 and 5. A mechanism involving the rate-determining nucleophilic attack of water on the positively polarized sulfur atom is proposed which is accompanied by a simultaneous O-H and S-N bond cleavage. Spiro-λ4-sulfanes 4 and 5 with six-membered N-containing spiro-rings, which are slightly reactive towards water, undergo a fast parallel reaction with OH- ions even in neutral solutions. The different reactivities of (acylamino)(acyloxy)spiro-λ4-sulfanes and their diacyloxy analogues are discussed and interpreted.
|Number of pages||8|
|Journal||Journal of the Chemical Society. Perkin Transactions 2|
|Publication status||Published - Oct 1997|
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