Thiolate Reduction of Sulfilimines. 2. Further Evidence for a Highly Coupled Concerted Transition State

Article abstract of DOI:10.1021/jo00389a012

The reduction of S-methyl-S(substituted phenyl)sulfilimmonium salts by 3-nitro-5-thiobenzoic acid (NTBA) and by 3-thiobenzoic acid (TBA) (aqueous solution, 25 deg C, μ 1.0 with KCl) is first order in proton activity and thiolate concentration in the range pH 3.5-6.6.The solvent deuterium isotope effects for the reduction by NTBA are k_H/k_D = 7.62 and 6.50 for the S-phenyl- and S-(4-nitrophenyl)-substituted compounds, respectively; for TBA reduction of the same compounds, k_H/k_D = 2.89 and 1.66, respectively.Plots of third-order rate constants for the proton and acetic acid catalyzed reactions against the ?ⁿ scale give sharply curved Hammet plots; approximate values for Β_nuc vary in the range 1-1.4 for the proton-catalyzed reaction and 0.2-0.7 for the acetic acid catalyzed reaction and are also nonlinear when plotted against the ?ⁿ scale.General-acid catalysis is observed and Broensted α values of 0.61 and 0.49 are obtained for NTBA reduction of S-(4-methoxyphenyl)- and S-(4-nitrophenyl)substituted compounds, respectively; the Broensted α for the TBA reduction of the S-phenyl-subsituted compounds is ca. 0.7; P_xy = δα/δ?ⁿ ca. 0.13.The term P_xy = δΒ_nuc/δpK^HA_a = δpK^RSH_a ca. 0.08.The data are consistent with a mechanism in which S-S bond formation, S-N bond cleavage, and proton transfer all occur in a fully concerted transition state.