Elimination Reactions of β-Cyano Thioethers: Internal Return and the Lifetime of the Carbanion Intermediate

Article abstract of DOI:10.1021/ja00223a029

The E1cB elimination reaction of the pentafluorothiophenol adduct of fumaronitrile (3) in water containing 8.3percent Me2SO shows strong buffer catalysis and primary deuterium isotope effects of k_H/k_D = 4-5.In contrast, hydrogen exchange of the methanethiol adduct of 3 shows little or no buffer catalysis.There is no incorporation of deuterium that gives an inverse solvent isotope effect in the buffer-catalyzed elimination of the thionitrobenzoate adduct of 3 in D2O and H2O, although thiol anion expulsion is partly rate limiting for this reaction.These observations are consistent with internal return of the abstracted proton from the protonated buffer base that is competitive with the expulsion of leaving groups with pK_a > 4.The primary deuterium isotope effects for elimination catalyzed by hydroxide ion are k_H/k_D = 4.9 +/- 0.4, 4.2 +/- 0.1, 4.2 +/- 0.3, and 3.1 +/- 0.1 for the pentafluorothiophenol and thionitrobenzoate adducts of 3, the N-methyl-2-mercaptopyridinium ion adduct of acrylonitrile (1), and the pentafluorothiophenol adduct of chloroacrylonitrile (2), respectively.These isotope effects are significantly larger than values of k_H/k_D = 2.6 +/- 0.3, 2.2 +/- 0.1, and 2.1 +/- 0.1 for buffer-catalyzed elimination from the adducts of 1 and 3.They are also larger than (1) the primary isotope effects of k_H/k_D = 2.5 +/- 0.3, 2.3 +/- 0.2, and 2.3 +/- 0.2 for the p-nitrothiophenol adducts of 1 and 3 and the thiophenol adduct of 2 that were obtained from the biphasic kinetics for elimination of these compounds in D2O and (2) the product discrimination isotope effects of ca. 3 for the addition of thiol anions to 1 and 3.These observations are consistent with the formation of an unstable carbanion intermediate that undergoes competitive reprotonation by solvent, k_-1, exchange of the abstracted proton with the bulk solvent, k_s, and elimination, k₂.Ratios of k_-1/k_s and k_-1/k₂ that were obtained from the results give values of k_-1 ca. 1E11 s^-1 and k₂ = 1E10-1E13 s^-1, assuming a value of k_s = 1E11 s^-1.Estimated pK_a values in aqueous solution include 26.8 for NCCH2CH2SPh, 22.0 for NCCH(Cl)CH2SPhNO2, and 23.2 for NCCH2CH(CN)SCH3.