- Proton transfers among oxygen and nitrogen acids and bases in DMSO solution
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Rate constants for the proton-transfer reactions between conjugate acids and bases of several amines, phenols, carboxylic acids, and the solvated proton in DMSO-d6 at 20 °C have been determined by the use of NMR line-shape analysis. Equilibrium constants for the same reactions are obtained from the pKa's of the acids in dimethyl sulfoxide, some of which have been reported in earlier work and the rest obtained in the present work by use of Bordwell's indicator techniques. All of the reactions have rale constants considerably below expected diffusion-controlled limits for the proton transfers in the thermodynamically favorable direction, and several of the reactions, including the identity reactions of carboxylic acids, have kinetic deuterium isotope effects, kH/kD, between 0.8 and 1.3. For reactions of N,N-dimethylbenzylammonium ion with several phenoxides, carboxylates, and solvent, the rate constants for transfers in the unfavorable directions show a reasonable Bronsted correlation with β ≈ 1 and a reasonably constant reverse rate constant of ≈3 × 106 M-1 s-1. The data clearly indicate that the proton-transfer step is not rate-limiting in these reactions. Most likely, desolvation is involved in the rate-limiting steps, but the rate constants are not simple functions of acidities as might have been expected if hydrogen bonding of acid to solvent were the major factor involved in the solvation Other factors, particularly dispersion interactions of solvent with solutes, are discussed. We suggest that the formation of an acid-base complex with proper orientation to allow contact between the proton and the basic site is rate-determining and involves desolvation along with detailed steric interactions of the acid-base pair.
- Ritchie, Calvin D.,Lu, Shanzheng
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p. 7748 - 7756
(2007/10/02)
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- Extended E1cB Mechanism for Ester Hydrolysis: Allylic Substitution via Carbanion in Ester Hydrolysis
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Studies of E1cB hydrolysis of ester have been extended to the investigation of ester hydrolysis via allylic substitution (eq 2).The hydrolysis of p-nitrophenyl 2-cyano-3-(p-methoxyphenyl)propenoate (1) provides p-nitrophenol (p-NP), 2-cyano-3-(p-methoxyphenyl)propenoic acid (2), p-methoxybenzaldehyde (6), and cyanoacetic acid.The percentage yields of the products varied with pH; below pH 8 hydrolysis of 1 occurs predominantly by way of retro-Knoevenagel reaction to yield p-NP and 6, while above pH 8, the ester hydrolysis of 1 provides 2 and by way of retro-Knoevenagel reaction 6.The pH dependence of the two competing processes (ester hydrolysis and retro-Knoevenagel reaction) were provided by a log (infinite/infinite)-pH profile.The rate of hydrolysis of 1 (30 deg C) obeys the first-order rate law, and the carboanion (7) and carbon acid (8) intermediates do not accumulate in the course of hydrolysis.At pH values above 12, two reaction processes, well separated in time, were observed; the initial first-order rate process is associated with the formation of the same products obtained between pH 8 and 12, and the slow reaction is associated with hydrolysis of 2 to 6 cyanoacetic acid (Scheme V).The mechanism of lyate species hydrolysis of 1 was elucidated by use of log kly vs. pH and log (infinite/infinite) vs. pH profiles and by comparison of ΔS(excit.) and ΔS2(excit.) - ΔS6(excit.) with known literature values.The three mechanisms which may be considered for hydrolysis of 1 involve: (i) competitive and parallel first-order decay of 1 by C-C bond scission to provide 6 + p-NP and ester hydrolysis to provide 2 + p-NP (Scheme II); (ii) addition of HO to the C-C double bond of 1 to provide a carboanion 7 which then indergoes parallel first-order decomposition via C-C bond scission to yield p-NP + 6 and ester hydrolysis through a ketene intermediate to provide p-NP + 2 (Scheme III); and (iii) addition of the elements of HO- plus H+ to the C-C double bond of 1 to yield the carbon acid ester 8 which then partitions to p-NP + 2 and p-NP + 6 (Shceme IV).The reactions of Scheme II do not predict the correct pH dependencies for the two hypothetical parallel reaction paths, which may be determined from the pH dependence of kly and infinite/infinite, nor the values of ΔH(excit.) and ΔS(excit.) for ester hydrolysis typical of the BAC2 mechanism required by Scheme II.The sequences of Scheme III predict the observed dependencies of kly, predict ratios infinite/infinite upon pH, and explain the determined values of ΔS(excit.)2 - ΔS(excit.)6.The mathematical forms of equations derived from the sequences of reactions of Scheme IV are sufficient to fit the log kly vs. pH and log infinite/infinite vs. pH profile.However, Scheme IV is strongly disfavored on the basis, among other considerations, that values of the rate constant for the BAC2 hydrolysis of 8 determined by any of the four possible simplifying kinetic assumptions is more than 100-fold ...
- Inoue, Masashi,Bruice, Thomas C.
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p. 1644 - 1653
(2007/10/02)
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