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

Reaction of 2-methyl-2-propanesulfenic acid (1) with methyl arenesulfe nates (2) gave S-aryl 2-methyl-2-propanethiosulfinates quantitatively, and kinetic investigation was carried out. Second-order rate dependen ce (first-order in methyl benzenesulfenate (2a) and first-order in the sulfenic acid (1)) was observed, and a small activation enthalpy (Del ta H-double dagger=28 kJ mol(-1)) and a large negative activation entr opy (Delta S-double dagger=-130 JK(-1) mol(-1)), were obtained. Kineti cs for the reaction of trans-decalin-g-sulfenic acid (3) with the sulf enates 2 under acidic conditions in methanol-water gave the observatio n of the second-order dependence, a small activation enthalpy (Delta H -double dagger=30.5 kJ mol(-1)), a large negative activation entropy ( Delta S-double dagger=-140 JK(-1) mol(-1)), a large negative Hammett r ho-value (rho=-1.39) and no solvent isotope effect (k(MeOH)/k(MeOD)=1. 10-1.14 in the range of 15.64-29.98 degrees C). Kinetics for acidic hy drolysis of the sulfenate 2 under the same conditions afforded the sim ilar activation parameters (Delta H-double dagger=40.7 kJ mol(-1), Del ta S-double dagger=-200 JK(-1) mol(-1) at [H2O]=0.80x10(-3) mol dm(-3) ) and a larger negative Hammett rho-value (rho=-2.22) than that for th e reaction of the sulfenic acid 3 with the sulfenate 2. From these res ults, structures of the transition state for both the self-condensatio n of sulfenic acid and the hydrolysis of the sulfenate ester 2 were di scussed. The reaction of sulfenate esters 2 with stable sulfenic acids is a good model for studying the self-condensation of sulfenic acids.