4468-52-4Relevant articles and documents
Generation and stability of a simple thiol ester enolate in aqueous solution
Amyes, Tina L.,Richard, John P.
, p. 10297 - 10302 (1992)
The exchange for deuterium of the α-protons of ethyl thioacetate and of acetone in 3-quinuclidinone buffers in D2O at 25 °C and pD = 7.7-9.3 was followed by 1H NMR spectroscopy. The exchange reactions lead to the appearance of signals due to the α-CH2D and α-CHD2 species that are cleanly resolved from each other and from the signal due to the α-CH3 species. Observed rate constants for the 3-quinuclidinone-catalyzed exchange were determined during exchange of 30-37% of the first α-proton of each methyl group of ethyl thioacetate or acetone. The rate constants for exchange correspond to those for deprotonation of ethyl thioacetate and acetone by 3-quinuclidinone to give the free enolates, with kB = 2.2 × 10-5 and 5.2 × 10-4 M-1 s-1, respectively. These rate constants were combined with the known pKa of acetone to estimate pKa = 20.4-21.5 for ethyl thioacetate and kBH = 1.7 × 108 to 2 × 109 M-1 s-1 for the reaction of the free thiol ester enolate with the 3-quinuclidinone cation. The lifetime of the buffer acid-enolate intimate ion pair BH+·-CH2COSEt with respect to proton transfer to give B-CH3COSEt is estimated to be from 10-9 to 10-10 s. These results provide evidence against the suggestion that enzyme-catalyzed Claisen condensation and related reactions proceed by concerted mechanisms that are enforced by the insignificant lifetime of the thiol ester enolate in the presence of an acidic amino acid residue at the enzyme.
Determination of the p K a of Cyclobutanone: Bronsted correlation of the general base-catalyzed enolization in aqueous solution and the effect of ring strain
Cope, Steven M.,Tailor, Dishant,Nagorski, Richard W.
, p. 380 - 390 (2011)
The induction of strain in carbocycles, thereby increasing the amount of s-character in the C-H bonds and the acidity of these protons, has been probed with regard to its effect on the rate constants for the enolization of cyclobutanone. The second-order rate constants for the general base-catalyzed enolization of cyclobutanone have been determined for a series of 3-substituted quinuclidine buffers in D2O at 25 °C, I = 1.0 M (KCl). The rate constants for enolization were determined by following the extent of deuterium incorporation (up to ～30% of the first α-proton) into the α-position, as a function of time. The observed pseudo-first-order rate constants correlated to the [basic form] of the buffer and yielded the second-order rate constants for the general base-catalyzed enolization of cyclobutanone for four tertiary amine buffers. A Bronsted β-value of 0.59 was determined from the second-order rate constants determined. Comparison of the results for cyclobutanone to those previously reported for acetone and a 1-phenylacetone derivative, under similar conditions, indicated that the ring strain of the carbocycle appeared to have only a small effect on the general base-catalyzed rate constants for enolization. The similarity of the rate constants for the general base-catalyzed enolization of cyclobutanone to those determined for acetone allowed for an estimation of the limits of the rate constant for protonation of the enolate intermediate of cyclobutanone by the conjugate acid of 3-quinuclidinone (kBH = 5 × 108 -2 × 109 M-1 s-1). Combining the rate constants for deprotonation of cyclobutanone (kB) and protonation of the enolate of cyclobutanone (kBH) by 3-quinuclidinone and its conjugate acid, the pKa of the α-protons of cyclobutanone has been estimated to be pKa = 19.7-20.2.
The influence of H/D kinetic isotope effect on radiation-induced transformations of hydroxyl-containing compounds in aqueous solutions
Bekish, Andrei V.,Nepachalovich, Palina S.,Shadyro, Oleg I.,Shmanai, Vadim V.
, p. 732 - 744 (2020/12/28)
Vicinal diols and its derivatives can be exploited as model compounds for the investigation of radiation-induced free-radical transformations of hydroxyl-containing biomolecules such as carbohydrates, phospholipids, ribonucleotides, amino acids, and peptides. In this paper, for the first time, the prospects of isotope reinforcement approach in inhibiting free-radical transformations of hydroxyl-containing compounds in aqueous solutions are investigated on the example of radiolysis of 1,2-propanediol and 1,2-propanediol-2-d1 aqueous solutions. At an absorbed dose rate of 0.110 ± 0.003 Gy·s?1 a profound kinetic isotope effect (KIE) is observed for the non-branched chain formation of acetone, which is a final dehydration product of predominant carbon-centred radicals CH3·C(OH)CH2OH. In 0.1 and 1 M deaerated solutions at pH 7.00 ± 0.01, the values of KIE are 8.9 ± 1.7 and 15.3 ± 3.1, respectively. A rationale for the fact that a strong KIE takes place only in the case of chain processes, which may occur during free-radical transformations of vicinal diols, is also provided herein based on the results of 2-propanol and 2-propanol-2-d1 indirect radiolysis. Lastly, the lack of KIE is shown in the case of 2-butanone formation from 2,3-butanediol or 2,3-butanediol-2,3-d2. This indicates that the type (primary, secondary) of the β-carbonyl radicals formed as a result of CH3·C(OH)CH(OH)R (R = H, CH3) dehydration determines the manifestation of the effect.
