Transition-state imbalance in the deprotonation of sulfonyl-activated carbon acids in aqueous solution. A quantitative comparison of the kinetic acidities of sulfones and ketones

Article abstract of DOI:10.1021/ja00094a051

Second-order rate constants for the deprotonation of ten N-methyl 4-((X-phenylsulfonyl)methyl)pyridinium cations (4) and eight N-(Y-benzyl) 4-((phenylsulfonyl)methyl)pyridinium cations (5) by hydroxide ion and carbonate ion have been measured in aqueous solution at 25 °C, ionic strength 0.1. Br?nsted α-values for deprotonation of 4 are 0.59 and 0.45 for hydroxide ion and carbonate ion, respectively. The corresponding Br?nsted α-values for 5 are 0.33 and 0.29, which clearly indicate a pronounced lag in the delocalization of π-electron density into the pyridinium ring in the transition state for deprotonation. Br?nsted β-values of 0.55 and 0.52 are found for the deprotonation of 4 with X = H and 5 with Y = H, respectively, by a series of primary amine bases. Extrapolation of Br?nsted relationships for the deprotonation of sulfones and ketones indicates that the intrinsic kinetic acidities of sulfones are 80-fold greater than those of structurally analogous ketones. The difference in the substituent-independent intrinsic barriers for the deprotonation of sulfones and ketones by hydroxide ion is found to be 2.85(±0.03) kcal/mol, irrespective of whether one analyzes X-substituent effects in 4 or Y-substituent effects in 5.