200864-16-0Relevant articles and documents
Electronic effects on enol acidity and keto-enol equilibrium constants for ring-substituted 2-tetralones
Yao,Pollack
, p. 634 - 638 (2007/10/03)
Equilibrium constants for the ionization of a variety of phenyl-substituted 2-tetralones (pK(a)(K)), for the ionization of their enols (pK(a)(E)), and for keto-enol tautomerization (PK(E)) were determined. Hammett plots of pK(a)(K) and pK(a)(E) vs. σ are linear with slopes (-ρ) of -1.66 ± 0.06 and -0.90 ± 0.03, respectively, except for deviations of the points corresponding to 6-nitro-2-tetralone (1b) and its enol. We have previously attributed the negative deviation of 1b from the correlation for the acidities of the ketones obtained with the more limited set of data to the lack of a free electron pair on C-1 of the free tetralone (Nevy et al.). The negative deviation of the point for 1b from the correlation for the acidities of the enols suggests that charge transfer from the hydroxyl group of the enol to the nitro group is less important than it is for phenols. This study represents the first systematic study of electronic effects on equilibria among ketone, enol, and enolate in aqueous solution.
Transition state imbalance in proton transfer from phenyl ring-substituted 2-tetralones to acetate ion
Yao, Xudong,Gold, Mark A.,Pollack, Ralph M.
, p. 6220 - 6225 (2007/10/03)
Rate constants for the acetate ion-catalyzed ketonization of phenyl-substituted 2-tetralone enols have been determined by stopped-flow UV spectroscopy. From these rate constants and the keto - enol equilibrium constants, the rate constants (k-2) for enolization were calculated. A Bronsted plot of these rate constants (log k-2) vs the acidity of the appropriate 2-tetralone (pKaK) is linear, with a slope ( - αE) of - 0.78 ± 0.03, except for the point corresponding to 6-nitro-2-tetralone (4b). Rate constants for the ionization of 2-tetralone by substituted acetates were determined directly by NMR, giving a corresponding Bronsted βE of 0.54 ± 0.03. Both the negative deviation of the point for 4b from the correlation line for αE and the inequality between αE and βE indicate an imbalanced transition state for the proton abstraction of 2-tetralone by acetate ion. This reaction is impeded by a thermodynamic barrier of 11 kcal/mol, along with an intrinsic kinetic barrier of 14 kcal/mol. A comparison of the transition states for proton abstraction of 2-tetralone by hydroxide ion and by acetate ion shows similar transition state imbalance and intrinsic kinetic barriers for both reactions. The relevance of these results to the mechanism of enzymatic acceleration of enolization is discussed.
Transition state imbalance in the deprotonation of substituted 2- tetralones by hydroxide ion
Nevy, John B.,Hawkinson, David C.,Blotny, Grzegorz,Yao, Xudong,Pollack, Ralph M.
, p. 12722 - 12726 (2007/10/03)
Rate and equilibrium constants for the deprotonation of a series of phenyl-substituted 2-tetralones in aqueous sodium hydroxide have been determined. A Bronsted plot of log k for deprotonation vs pK(a) of the appropriate 2-tetralone is linear with a slope (-α) of -0.60 ± 0.01, except for the point corresponding to 6-nitro-2-tetralone (1b). The negative deviation of 1b from the correlation indicates that the transition state for deprotonation of 2-tetralone is imbalanced, with delocalization of charge into the phenyl ring lagging behind proton transfer. A semiquantitative assessment of the charge distribution in both the fully formed anion and the transition state for deprotonation was calculated from these results and 13C NMR spectra of the 2-tetralone anion in methanol/water mixtures. Although approximately twice as much negative charge is localized on the oxygen than on the enolate carbon in the anion, slightly more charge is on the enolate carbon in the transition state.
