Model Reaction of Self-Condensation of Sulfenic Acids. Kinetic Investigation for the Reaction of Methyl Benzenesulfenate with trans-Decalin-9-sulfenic Acid and 2-Methyl-2-propanesulfenic Acid

Abstract

Abstract Reaction of 2-methyl-2-propanesulfenic acid (1) with methyl arenesulfenates (2) gave S-aryl 2-methyl-2-propanethiosulfinates quantitatively, and kinetic investigation was carried out. Second-order rate dependence (first-order in methyl benzenesulfenate (2a) and first-order in the sulfenic acid (1)) was observed, and a small activation enthalpy (ΔH‡ = 28 kJ mol−1) and a large negative activation entropy (ΔS‡ = −130 J K−1 mol−1) were obtained. Kinetics for the reaction of trans-decalin-9-sulfenic acid (3) with the sulfenates 2 under acidic conditions in methanol–water gave the observation of the second-order dependence, a small activation enthalpy (ΔH‡ = 30.5 kJ mol−1), a large negative activation entropy (ΔS‡ = −140 J K−1 mol−1), a large negative Hammett ρ-value (ρ = −1.39) and no solvent isotope effect (kMeOH/kMeOD = 1.10—1.14 in the range of 15.64—29.98 °C). Kinetics for acidic hydrolysis of the sulfenate 2 under the same conditions afforded the similar activation parameters (ΔH‡ = 40.7 kJ mol−1, ΔS‡ = −200 J K−1 mol−1 at [H2O] = 0.80 × 10−3mol dm−3) and a larger negative Hammett ρ-value (ρ = −2.22) than that for the reaction of the sulfenic acid 3 with the sulfenate 2. From these results, structures of the transition state for both the self-condensation of sulfenic acid and the hydrolysis of the sulfenate ester 2 were discussed. The reaction of sulfenate esters 2 with stable sulfenic acids is a good model for studying the self-condensation of sulfenic acids.