Catalysis of the deprotonation of β-keto esters by cyclodextrins

1993 ◽  
Vol 71 (12) ◽  
pp. 2139-2143
Author(s):  
Oswald S. Tee ◽  
N. Rani Iyengar ◽  
Bryan K. Takasaki
Keyword(s):  
Aqueous Solution ◽  
Rate Constants ◽  
Ethyl Acetoacetate ◽  
Substrate Selectivity ◽  
Hydroxide Ion ◽  
Alkoxyl Group ◽  
Keto Esters ◽  
Saturation Kinetics ◽  
Carbon Acids ◽  
Marked Dependence

The rates of deprotonation of several β-keto esters in basic aqueous solution are elevated by α- and β-cyclodextrin (α- and β-CD). In most cases saturation kinetics are observed that indicate fairly strong binding of the esters to the CDs (Kd = 0.22–11 mM); catalytic ratios (kc/ku) are in the range 1.9–17. For esters of the form RCO-CH2COOEt with both α- and β-CD the values of Kd and kc/ku show little sensitivity to the acyl group, RCO (R = Me, Et, Pr, i-Pr), and for 2-carboethoxycyclopentanone, a cyclic analogue, these parameters are similar. In contrast, for R-COCH2CO-OR′ (R = Me or Et; R′ = Me, Et, allyl) there is a marked dependence of the parameters on the alkoxyl group, OR′. These results suggest that the β-keto esters studied bind to the CDs with their alkoxyl groups in the CD cavity and that the catalysis ensues from the complexes thus formed. Variations in two other kinetic parameters, k2 = kc/Kd (substrate selectivity) and KTS = ku/k2 (apparent constant for dissociation of CD from the transition state) support this interpretation. Apparent second-order rate constants for the reaction of the esters with neutral CD (k2 = kc/Kd) are 1900–360 000 M−1 s−1 (at pH ≈ 10), whereas for hydroxide ion attack on ethyl acetoacetate kOH = 5500 M−1 s−1. Assuming the pKas of the CDs are 12.2 and 12.3, deprotonation of the esters by the CD anions has rate constants of 105 to 5 × 107 M−1 s−1. Thus, binding of the β-keto esters in the CD cavities, adjacent to a basic oxyanion site, enhances the reactivity of the CD anions towards these weak carbon acids by at least 2–4 orders of magnitude.

Tautomerization tendencies of 2-acetylcycloalkanones, 2-acetyl-1,3-cycloalkanediones, and cyclic β-keto esters of five- and six-membered rings

2001 ◽  
Vol 79 (4) ◽  
pp. 448-454 ◽  
Author(s):  
Antonio Rojas ◽  
Alfonso Perez-Encabo ◽  
Ignacio Herraiz-Sierra ◽  
Carlos A Blanco
Keyword(s):  
Aqueous Solution ◽  
Rate Constants ◽  
Nmr Spectra ◽  
Tautomeric Equilibrium ◽  
Concentration Effects ◽  
Keto Esters ◽  
H Nmr ◽  
Kinetics Studies ◽  
Dilute Aqueous Solution ◽  
C Nmr

NMR and kinetics studies of the tautomeric equilibrium of 2-acetylcycloalkanones, 2-acetyl-1,3-cycloalkanediones, and methyl 2-oxocycloalkanecarboxylates of five- and six-membered rings have been carried out. By comparing 1H NMR and 13C NMR spectra recorded in D2O–DOCD3 and CDCl3, with those results derived using a bromination kinetic procedure in dilute aqueous solution, information concerning tautomeric interconversion is provided. Some discussions considering ionisation equilibria and rate constants involved have also been considered. Solvent and concentration effects on keto-enol equilibria have also been discussed.Key words: 2-acetylcycloalkanones, 2-acetyl-1,3-cycloalkanediones, methyl 2-oxocycloalkanecarboxylates, equilibrium, tautomerization, ionization.

Rates and hydrogen isotope effects in the ionization of 1,1-dinitroethane

1974 ◽  
Vol 337 (1611) ◽  
pp. 517-527 ◽  
Keyword(s):  
Aqueous Solution ◽  
Rate Constants ◽  
Hydrogen Isotope ◽  
Isotope Effects ◽  
Acid Base ◽  
Base Strength ◽  
Carbon Acids

The p K of 1,1-dinitroethane has been determined in aqueous solution, and rate constants measured for its reaction with nine bases. Rate constants were also obtained for the reaction of 1,1-dinitroethane- 1- d with three of these bases. The isotope effects and the relation between rate and acid-base strength are compared with those observed for other carbon acids.

Kinetics and mechanism of phenoxide anions addition to 4-nitrobenzofurazan in aqueous solution

2017 ◽  
Vol 95 (7) ◽  
pp. 723-728 ◽  
Author(s):  
S. Ben Salah ◽  
T. Boubaker ◽  
R. Goumont
Keyword(s):  
Aqueous Solution ◽  
Electron Transfer ◽  
Rate Constants ◽  
Second Order ◽  
Kinetics And Mechanism ◽  
Single Electron ◽  
Single Electron Transfer ◽  
Hydroxide Ion ◽  
Order Rate

Second-order rate constants (k1) for the σ-complexation of 4-nitrobenzofurazan 1 with four 4-X-substituted phenoxide anions 2a–2d (X = OCH3, CH3, H and Cl) were measured in aqueous solution at 20 °C. Using this series of phenoxide anions as a reference, the electrophilicity parameter (E) of this electrophile 1 has been evaluated according to Mayr’s approach. With the E value of –9.42, Mayr’s equation was found to correctly predict the rate constants for the reactions of 1 with hydroxide ion in H2O and a 1:1 ratio of H2O to CH3CN. However, the large βnuc value of 1.12 obtained in the present work is clearly consistent with a single electron transfer (SET) mechanism.

Rates of Reaction of the Sulfoxides and Sulfones of Sulfur Mustard and 2-Chloroethyl Ethyl Sulfide With Hydroxide Ion in Water

1995 ◽  
Vol 48 (10) ◽  
pp. 1781 ◽  
Author(s):  
RI Tilley ◽  
DR Leslie
Keyword(s):  
Aqueous Solution ◽  
Rate Constants ◽  
Sulfur Mustard ◽  
Second Order ◽  
Hydroxide Ion ◽  
Order Rate

The reactions of the sulfoxides and sulfones of sulfur mustard and an analogue, 2-chloroethyl ethyl sulfide, with hydroxide ion in aqueous solution at 25° have been studied. In contrast with the behaviour of the parent sulfides, for which exclusive substitution of chlorine is observed under comparable conditions, the oxidized compounds react through base-catalysed elimination of HCl to give the corresponding alkenes. Second-order rate constants for these reactions are reported and implications for the metabolism of sulfur mustard are discussed.

Electrostatic effects of substrate charge on base-catalyzed proton exchange in purines. Implications for the involvement of zwitterions

1980 ◽  
Vol 58 (1) ◽  
pp. 86-91 ◽  
Author(s):  
Spencer E. Taylor
Keyword(s):  
Aqueous Solution ◽  
Transition State ◽  
Exchange Reaction ◽  
Rate Constants ◽  
Electrostatic Interactions ◽  
Equilibrium Constants ◽  
Proton Exchange ◽  
Net Charge ◽  
Hydroxide Ion ◽  
Rate Behaviour

Rates of detritiation from the C-8 position of various ionized purine molecules have been shown to reflect electrostatic interactions occurring within the transition state of the exchange reaction. The purines are capable of existing as protonated, anionic, neutral, and zwitterionic species in aqueous solution, each of which have been shown previously to undergo specific hydroxide ion-catalyzed detritiation. The effects of substrate charge are quantified as the protonactivatingfactor, aniondeactivatingfactor, and zwitterionactivatingfactor. Localized charge and not net charge is shown to govern the reactivities of the various species, especially the zwitterions.Estimation of rate constants for the zwitterionic species (hitherto only postulated to account for the observed rate behaviour) has enabled the calculation of the corresponding zwitterionic equilibrium constants (Kzw). Kzw values calculated in this manner agree with those estimated from suitable literature pKa data.

Pseudobase formation from 9-substituted 10-methylacridinium cations in aqueous solution

1984 ◽  
Vol 62 (2) ◽  
pp. 351-354 ◽  
Author(s):  
John W. Bunting ◽  
Vivian S. F. Chew ◽  
Sudhir B. Abhyankar ◽  
Yukiko Goda
Keyword(s):  
Aqueous Solution ◽  
Rate Constants ◽  
Quantitative Relationship ◽  
Hydroxide Ion ◽  
Resonance Interactions ◽  
The Stability ◽  
Ph Range

Rates of cation–pseudobase equilibration have been measured for a series of C-9 substituted (CH3, CH3CH2, C6H5CH2, (CH3)2CH, C6H5, 4-(CH3)2NC6H4) 10-methylacridinium cations in aqueous solution at 25 °C over the pH range of approximately 9–13. Separation of the rate constants for formation (kOH) and decomposition (k2) of each of these pseudobases allowed the calculation of the [Formula: see text] value for each cation. The presence of a C-9 isopropyl substituent enhances the stability of the pseudobase relative to the cation, whereas all other C-9 substituted cations have [Formula: see text] values greater than the 10-methylacridinium cation. There is no simple quantitative relationship between [Formula: see text] and the size of the C-9 substituent. Rate constants (kOH) for hydroxide ion attack on these C-9 substituted cations are in the order: H > primary alkyl > secondary alkyl > aryl, while rate constants (k2) for pseudobase decomposition are less predictable but generally follow the order: H > primary alkyl > aryl > secondary alkyl. These phenomena are shown to be consistent with a competition between destabilization of the cation by peri interactions between the C-9 substituent and H(1) and H(8) and reduced pseudobase solvation for large C-9 substituents. Resonance interactions of 9-aryl substituents with the acridine moiety of these cations are shown to be quite small. 9-Aryl-10-methylacridinium cations are approximately 30-fold more reactive towards hydroxide ion attack than are their triphenylmethyl carbocation analogues.

The cleavage of aspirin by α- and β-cyclodextrins in basic aqueous solution

1985 ◽  
Vol 63 (12) ◽  
pp. 3540-3544 ◽  
Author(s):  
Oswald S. Tee ◽  
Bryan K. Takasaki
Keyword(s):  
Aqueous Solution ◽  
Rate Constant ◽  
Dissociation Constants ◽  
Hydroxide Ion ◽  
Saturation Kinetics

The deacetylation of aspirin is promoted by α- and β-cyclodextrins (αCD and βCD) in basic aqueous solution. From saturation kinetics the dissociation constants (Kd) for the aspirin anion – CD complexes are 12 mM (αCD) and 20 mM (βCD). At pH 12.25 the limiting rate constant for ester cleavage is 14 times (αCD) and 7 times (βCD) that in the medium alone. For the 4-chloroaspirin anion (Kd = 3.8 mM) the cleavage induced by αCD is 240 times that in the medium, but 5-chloroaspirin shows a very small effect. The results are discussed in terms of a model derived from that of Bender and co-workers (1967) and are compared to earlier work. For the αCD anion, the 4-chloroaspirin anion is almost as good as "substrate" as m-chloro-and m-tert-butylphenyl acetates. Compared to hydroxide ion, the αCD anion is better at cleaving 4-chloroaspirin (2000×) and aspirin (36×).

Ferricyanide Oxidation of Butanol Homogeneously Catalysed by Chlororuthenium Complexes

1979 ◽  
Vol 32 (9) ◽  
pp. 1905 ◽  
Author(s):  
AF Godfrey ◽  
JK Beattie
Keyword(s):  
Aqueous Solution ◽  
Complex Formation ◽  
Linear Dependence ◽  
Rate Constants ◽  
Homogeneous Catalyst ◽  
Basic Solution ◽  
Rate Law ◽  
Kinetics Of ◽  
Solution Estimates ◽  
Butanol Concentration

The oxidation of butan-1-ol by ferricyanide ion in alkaline aqueous solution is catalysed by solutions of ruthenium trichloride hydrate. The kinetics of the reaction has been reinvestigated and the data are consistent with the rate law -d[FeIII]/dt = [Ru](2k1k2 [BuOH] [FeIII])/(2k1 [BuOH]+k2 [FeIII]) This rate law is interpreted by a mechanism involving oxidation of butanol by the catalyst (k1) followed by reoxidation of the catalyst by ferricyanide (k2). The non-linear dependence of the rate on the butanol concentration is ascribed to the rate-determining, butanol-independent reoxidation of the catalyst, rather than to the saturation of complex formation between butanol and the catalyst as previously claimed. Absolute values of the rate constants could not be determined, because some of the ruthenium precipitates from basic solution. With K3RuCl6 as the source of a homogeneous catalyst solution, estimates were obtained at 30�0�C of k1 = 191. mol-1 s-1 and k2 = 1�4 × 103 l. mol-1 s-1.

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