Mechanism of Nucleophilic Substitution Reactions of o - Nitrobenzenesulfenamides : Evidence for a Substitution Proceeding through a Sulfuranide Intermediate '

o-Nitrobenzenesulfenamides (1) undergo acid-catalyzed methanolysis (eq 2) in acetonitrile-methanol, affording methyl o-nitrobenzenesulfenate (2). They also undergo acid-catalyzed reaction with a thiol, giving an alkyl o-nitrophenyl disulfide (eq 3). In MeCN-8.2 M MeOH the thiolysis reaction exhibits a simple fmt-order dependence on both [R'SH] and [H+] for acid concentrations in the range 0.001-0.012 M. In marked contrast the rate of the methanolysis reaction is only slightly dependent on [H+] for the same range of acid concentrations. It is also markedly dependent on [MeOH], being seven times faster in MeCN-8.2 M MeOH than in MeCN-3.0 M MeOH. Measurement of the equilibrium constant for protonation of 1 to 1-H+ in MeCN-8.2 M MeOH shows that under the conditions used for the kinetic studies of eqs 2 and 3 the extent of protonation of 1 to 1-H+ is always small (<6%). Consideration of this fact, and the markedly different kinetic behavior of the two substitution reactions, leads to the following interpretation: (a) for the thiolysis the rate-determining step is the reaction of the thiol with 1-H+; (b) for the methanolysis, on the other hand, the mechanism is that shown in eq 9, where a sulfuranide species 4 is the key intermediate on the reaction coordinate. The lack of dependence of the rate on [H+] is because acid-catalyzed reversion of 4 to 1-H+ and methanol (step k,) is faster than the cleavage of 4 (step k,) to give 2 and R2NH. The methanolysis of 1 therefore represents another substitution at dicoordinate sulfur (similar to ref 10) where the kinetics require a mechanism with a hypervalent sulfur species as an intermediate on the reaction coordinate.