Iodide Reduction of Sulfilimines. 2. Evidence for Concurrent Stepwise and Concerted Mechanisms for the Decomposition of Sulfurane Intermediates

Article abstract of DOI:10.1021/ja00240a034

The iodide reduction of N-(substituted ethyl or phenyl)-S,S-dimethylsulfilimmonium salts (aqueous solution, 25 deg C, μ = 1.0 with KCl) is first order in proton activity and iodide concentration in the pH range 0.5-5.The solvent deuterium isotope effects for the reduction reaction vary in the range k_H/k_D = 0.26-0.48 as the nitrogen substituent is changed from ethyl- to trifluoroethylamine.Electron-withdrawing groups in the leaving group decrease the rate of the reaction and give β_l.g. values of ca. 0.7 for cyanoethyl- and trifluoroethylamine leaving groups and ca. 0.1 for the more basic ethylamine derivatives; a β_l.g. of 0.58 is observed for aniline derivatives.General acid catalysis is observed in the reduction of the acidic ethylamine and aniline derivatives with Broensted α values of 0.59 and 0.39 for cyanoethyl- and trifluoroethylamine leaving groups, respectively; for anilines, the Broensted α values decreased from 0.67 to 0.50 as the leaving group is changed from 4-methyl- to 3-nitroaniline.The value of β_l.g. decreases with decreasing strength of the catalyzing acid and the term p_xy = (δβ_l.g./δpK_a^HA) = (δα/δK_a^l.g.) ca. -0.06 to -0.1.The solvent deuterium isotope effect on the general catalyzed reduction reaction increases with increasing acid strength; for the cyanoethylamine derivative, k_BH/k_BD = 1.47-2.32 for acetic and chloroacetic acids, respectively.A mechanism is suggested involving concurrent stepwise and concerted mechanisms for the reduction reaction; the mechanism observed seems to depend on the nature of the catalyzing acid.