Evidence for Concerted General Acid Catalysis in an SN2-like Transition State for the Reduction of Sulfilimines by Thiols

Article abstract of DOI:10.1021/ja00311a059

N-(Substituted benzyl)-S,S-dimethylsulfilimmonium salts are reduced by thiol anions to give dimethyl sulfide, the benzylamine, and the reductant disulfide.With use of 3-nitro-5-thiobenzoic acid (NTBA) or 3-thiobenzoic acid (TBA) as reductant, the reaction is first order in proton activity below pH 5 and first order in both thiol anion and sulfilimine cation (aqueous solution, 25 deg C, ionic strength 1.0 with KCl).The rate constants for the reduction reaction are a linear function of the concentration of buffer acids, and Broensted α values of 0.50 and 0.63 are observed for the NTBA reduction of N-benzyl- and N-(4-chlorobenzyl)sulfilimines, respectively.The leaving group effects for these compounds are β_lg = -1.1 and +0.8 for the reductions catalyzed by difluoroacetic and acetic acids, respectively.For the reduction of the N-benzylsulfilimine by NTBA and TBA, values of β_nuc of 0.38 and 0.51 are observed for the proton- and acetate-catalyzed reductions, respectively.The data are consistent with a concerted mechanism in which proton transfer is further advanced than either S-N cleavage or S-S bond formation.The direction of the reaction coordinate is oriented toward the formation of a protonated tetracoordinate sulfurane addition intermediate.The thermodynamic restraints on concerted catalysis, however, require that this tetracoordinate intermediate cannot be the final product of the concerted proton transfer.It is concluded that the substitution reaction is concerted and follows a reaction coordinate proceeding near to, but not including, the sulfurane intermediate.