SPIRO-LAMBDA(4)-SULFANES WITH A N-S-IV-O AXIAL BOND SYSTEM - A KINETIC-STUDY ON THE MECHANISM OF HYDROLYSIS

Kinetics of the hydrolysis of diaryl(acylamino)(acyloxy)spiro-lambda(4 )-sulfanes (2a-e, ; 3-5) leading to sulfoxides have been studied under pseudo-first-order conditions in dioxane-water mixtures or aqueous bu ffer. solutions. A sulfonium-carboxylate-type dipolar structure of the starting spiro-lambda(4)-sulfanes is supported by IR spectroscopic da ta. 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 (rho 1.43) and acidic media (rho(cat) 0.90). Hydrolysis is moderately accelerated by strong acids owing to the protonation of the negatively polarized acyl oxy group of the substrate. In 50:50 (v/v) dioxane-H2O(D2O) the primar y deuterium isotope effect is k(H2O)/k(D2O) 3.68. Spiro-lambda(4)-sulf anes 2a-e and 3 with a five-membered N-containing ring are much more r eactive than the six-membered analogues 4 and 5. A mechanism involving the rate-determining nucleophilic attack of water on the positively p olarized sulfur atom is proposed which is accompanied by a simultaneou s O-H and S-N bond cleavage. Spiro-lambda(4)-sulfanes 4 and 5 with six -membered N-containing spiro-rings, which are slightly reactive toward s water, undergo a fast parallel reaction with OH-ions even in neutral solutions. The different reactivities of (acylamino)(acyloxy)spiro-14 -sulfanes and their diacyloxy analogues are discussed and interpreted.