Spiro-λ4-sulfanes with a N–SIV–O axial bond system. A kinetic study on the mechanism of hydrolysis

Abstract

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 (ρ_cat 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 k_H2O/k_D2O 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.