Iodide Reduction of Sulfilimines. Evidence for the Partitioning of Sulfurane Intermediates

Article abstract of DOI:10.1021/ja00311a058

The iodide reduction of N-(substituted phenyl)-S,S-dimethylsulfilimmonium salts (aqueous solution, 25 deg C, μ = 1.0 with KCl) is firts order in proton activity in the pH range 0.5-3.0.The reduction of N-phenyl-S,S-dimethylsulfilimmonium chloride is also catalyzed by general acids with a Broensted α of 0.7.Electron-donating groups on the aniline leaving group accelerate the rate of the reduction with a β_lg = 0.54.Rate constants for the reduction of sulfilimines derived from higher pK_a amines are also linear with proton activity.For N-benzyl-S,S-dimethylsulfilimmonium chloride, no general catalysis is observed.For sulfilimines with unhindered primary amines as leaving group, a small β_lg of about -0.1 is observed.For sulfilimines with benzamide and sulfonamide leaving groups the proton-catalyzed reaction contains both first- and second-order terms in proton activity.The rate of the reduction of the sulfilimine ylide is accelerated by electron-withdrawing substituents with β_lg = -0.5.These data are interpreted in terms of a mechanism involving rate-limiting partitioning of a common tetracoordinate sulfurane intermediate.For aniline leaving groups, proton transfer to the neutral sulfurane is suggested to be rate limiting.For higher pK_a leaving groups, the protonated sulfurane is solvent equilibrated and breakdown of this intermediate becomes rate limiting.For sulfilimines with very low pK_a leaving groups, the predominant pathway is suggested to involve uncatalyzed breakdown of the neutral sulfurane intermediate with expulsion of sulfonamide anion.A parallel pathway involving the general catalyzed breakdown of the neutral sulfurane is also suggested to account for the greater than first-order proton dependence.