Kinetic Study on the Alkaline Hydrolysis of S,S-Diaryl-N-halosulfilimines

Article abstract of DOI:10.1246/bcsj.63.1764

Kinetics for the alkaline hydrolysis of S,S-diaryl-N-bromosulfilimines were carried out in aqueous methanol.The observed pseudo-first-order rate constants were found to give a linear correlation with the concentration of sodium hydroxide, k=k₁+k₂.The first-order rate constants k₁ showed a large negative Hammett ρ value (-2.43) for the substituent effect on the phenyl group, nearly zero activation entropy (-0.9+/-13.1 JK^-1mol^-1) and a relatively large m value (0.638) against the solvent ionizing power Y value suggesting that the reaction process fork₁ close to S_N1.The salt effect, the deuterium solvent isotope effect and the steric effect are also in accord with the S_N1 mechanism.On the other hand, the second order rate constants k₂ revealed a small Hammett ρ value, a negative activation entropy (-44.0+/-4.0 JK^-1mol^-1), a small m value (0.153) and a steric deceleration by ortho substituents showing that the reaction for k₂ is S_N2-like.The salt effect and the solvent isotope effect are also compatible with the S_N2-like mechanism.Meanwhile, the k₁ for S,S-diphenyl-N-halosulfilimines remarkably increased in the order of N-iodo N-bromo N-chloro.This reactivity might be due to the lone pair-lone pair repulsion at the reactant state.From these observations, the alkaline hydrolysis of S,S-diaryl-N-halosulfilimines was confirmed to proceed via concurrent two mechanisms, the S_N1-like mechanism involving nitridosulfonium cation as an intermediate and the S_N2'-like mechanism with the transition state in which the N-X bond cleavage is more progressed than the S-O bond formation with nucleophiles (^-OH, ^-OMe).