Nucleophilic Substitution Reactions of N-Chloramines: Evidence for a Change in Mechanism with Increasing Nucleophile Reactivity

摘要

Third-order rate constants (k_(Nu))_H (M~(-2) s~(-1)) for the hydronium ion catalyzed reactions of a range of nucleophiles with N-chlorotaurine (1) in water at 25℃ and I = 0.5 (NaClO_4) are reported. The solvent deuterium isotope effects on hydronium ion catalysis of the reaction with 1 of bromide and iodide ion are (k_(Br))_H/(k_(Br))_D= 0.30 and (k_I)_H/(K_I)_D = 0.54, respectively. The inverse nature of these isotope effects and the absence of general acid catalysis are consistent with a stepwise mechanism involving protonation of 1 in a fast preequilibrium step. The appearance of strong catalysis by general acids for the reaction of the more nucleophilic SO_3~(2-) and HOCH_2CH_2S~- with the chloramine indicates a change to a concerted mechanism, with protonation of the chloramine at nitrogen and chlorine transfer to the nucleophile occurring in a single step. A rough estimate of the lifetime of the protonated chloramine in the presence of the thiolate anion suggests that the concerted mechanism is enforced by the absence of a significant lifetime of the protonated substrate in contact with the nucleophile. Theoretical calculations provide evidence against an electron-transfer mechanism for chlorination of the nucleophiles by protonated 1.