A Change in the Rate-Determining Step in the E1cB Reactions of N-(2-(4-Nitrophenyl)ethyl)pyridinium Cations

Article abstract of DOI:10.1021/ja00018a036

Second-order rate constants have been measured (aqueous solution, I = 1.0, 25 deg C) for the hydroxide ion catalyzed elimination reactions of 12 N-(2-(4-nitrophenyl)ethyl)pyridinium cations (3) bearing a variety of substituents in the pyridine ring.Broensted plots as a function of the basicity of the pyridine leaving group are concave-down, which is consistent with a change in rate-determining step within an E1cB mechanism.These plots are characterized by β_lg = -0.17 for the rate-determining deprotonation for pK_BH < 6.5, and β_lg = -0.39 for the rate-determining expulsion of the pyridine nucleofuge from the carbanionic intermediate for pK_BH > 6.5.Elimination reactions in basic D2O occur without any significant incorporation of deuterium into the 4-nitrostyrene product, and require the presence of a hydrogen-bonded carbanionic intermediate in which nucleofuge expulsion occurs faster than exchange of hydrogen-bonding water molecules.Rate-determining deprotonation in these elimination reactions occurs 50-fold more slowly than for the corresponding reactions of the N-<2-(4-nitrophenyl)ethyl>quinuclidinium cations that have also been reported to have β_lg = -0.17, but which do not show an analogous change in the rate-determining step upon variation of the nucleofuge basicity.The analogous elimination of the 1-methyl-3-<2-(4-nitrophenyl)ethyl>imidazolium cation occurs a further 30-fold more slowly than that predicted for 3 having a pyridine leaving group of the same basicity as 1-methylimidazole.The E1cB reactions of 3 are similar to the analogous reactions of N-(2-cyanoethyl)pyridinium cations (1) in displaying a change in the rate-determining step with nucleofuge basicity; however, theβ_lg values for 1 and 3 are quite different for both k₁ and k₂/k_-1.