- Unexpected resistance to base-catalyzed hydrolysis of nitrogen pyramidal amides based on the 7-azabicyclic[2.2.1]heptane scaffold
-
Non-planar amides are usually transitional structures, that are involved in amide bond rotation and inversion of the nitrogen atom, but some ground-minimum non-planar amides have been reported. Non-planar amides are generally sensitive to water or other n
- De Velasco, Diego Antonio Ocampo Gutiérrez,Su, Aoze,Zhai, Luhan,Kinoshita, Satowa,Otani, Yuko,Ohwada, Tomohiko
-
-
- On the dual role of N-heterocyclic carbenes as bases and nucleophiles in reactions with organic halides
-
The synthetic consequences of different basicities, nucleophilicities, and sterics of N-heterocyclic carbenes have been studied in reactions of imidazolin-2-ylidenes with organic halides. Highly nucleophilic and less basic carbenes cleanly gave alkyli-deneimidazolines, the deoxy analogues of Breslow-type intermediates. More basic NHCs engaged in unwanted deprotonation or dehydrohalogenation reactions. Georg Thieme Verlag Stuttgart. New York.
- Knappke, Christiane E. I.,Arduengo, Anthony J.,Jiao, Haijun,Neudoerfl, Joerg-Martin,Jacobivonwangelin, Axel
-
p. 3784 - 3795
(2012/01/11)
-
- Using toluates as simple and versatile radical precursors
-
(Chemical Equation Presented) The viability of the toluate moiety as a radical precursor has been examined by studying deoxygenation and cyclization reactions.
- Lam, Kevin,Marko, Istvan E.
-
scheme or table
p. 2773 - 2776
(2009/05/30)
-
- Structure-activity analysis of base and enzyme-catalyzed 4-hydroxybenzoyl coenzyme A hydrolysis
-
In this study, the second-order rate constant k2 of base-catalyzed hydrolysis and the values of kcat, Km and kcat/Km of wild-type Pseudomonas sp. CBS3 4-hydroxybenzoyl coenzyme A (4-HBA-CoA) thioesterase-catalyzed hydrolysis of 4-HBA-CoA and its para-substituted analogs were measured. For the base-catalyzed hydrolysis, the plot of log k2 vs the σ value of the para-substituents was linear with a slope (ρ) of 1.5. In the case of the enzyme-catalyzed hydrolysis, the kcat/Km values measured for the para-substituted analogs defined substrate specificity. Asp32 was shown to play a key role in substrate recognition, and in particular, in the discrimination between the targeted substrate and other cellular benzoyl-CoA thioesters.
- Song, Feng,Zhuang, Zhihao,Dunaway-Mariano, Debra
-
-
- Nucleophilic displacement at the benzoyl centre: A study of the change in geometry at the carbonyl carbon atom
-
The second-order rate constants for the reaction between hydroxide ion and phenoxide ion with 4-nitrophenyl esters of substituted benzoic acids in 10% acetonitrile-water (v/v) solution obey Hammett σ correlations. The values of the Hammett ρ of 1.67 (κArO) and 2.14 (κOH) are consistent with a large change in hybridization at the central carbon by comparison with the ρ value for a standard reaction where a full sp2 to sp3 change occurs. The transition state for the concerted reaction thus has a substantially tetrahedral geometry. The observation of the anti-Hammond effect whereby the ρ value for the hydroxide ion exceeds that of the less reactive phenoxide ion is consistent with a concerted, ANDN, mechanism for these reactions. A stepwise mechanism, AN + DN, is unlikely to yield a measurable break in the Hammett correlation for a change in the benzoyl substituent if the partitioning of the putative tetrahedral intermediate involves forward and reverse reactions with Hammett correlations possessing similar ρ values.
- Colthurst, Matthew J.,Williams, Andrew
-
p. 1493 - 1497
(2007/10/03)
-
- Dissociation Constants of Weak Organic Acids in Protic Solvents Obtained from Their First Hyperpolarizabilities in Solution
-
The first hyperpolarizabilities (β) of some weak aromatic organic acids have been measured in protic solvents by the hyper-Rayleigh scattering (HRS) technique at low concentrations.The measured hyperpolarizability (βm) varies between the two extreme limits: the hyperpolarizability of the acid form (βHA) at the lower side and that of the basic form (βA-) at the higher side.The degree of dissociation (α) of the acid in a solvent is related to the measured hyperpolarizability, βm, by the following relationship: βm2 = (1 - α)βHA2 + αβA-2.The calculated β's including solvent effects in terms of an Onsager field do not reproduce the experimentally measured hyperpolarizabilities.Other solvent-induced effects like hydrogen bonding and van der Waals interactions seem to influence the first hyperpolarizability and, thus, indirectly the extent of dissociation of these weak acids in these protic solvents.
- Ray, Paresh Chandra,Das, Puspendu Kumar
-
p. 17891 - 17895
(2007/10/03)
-
- Cationic Micellar Effect on the Kinetics of the Protolysis of Aromatic Carboxylic Acids studied by the Ultrasonic Absorption Method
-
The protolysis of carboxylic acids has been kinetically studied by the ultrasonic absorption method in the presence of tetradecyltrimethylammonium bromide (TTAB) micelles in aqueous solution.The carboxylic acids studied were classified into two categories, one capable of formation of intramolecular hydrogen bond, namely the salicylic acid derivatives (SAD) and the other which cannot form the bond, namely the benzoic acid derivatives (BAD).The rate constant (γ2kf, kb), the apparent dissociation constant (Ka), and the volume change of the reaction (ΔV) were obtained.Different Ka dependences of the rate constants observed for SAD and BAD are discussed in relation to the effect of intramolecular hydrogen bond. pKa dependences were also observed for ΔV of SAD and BAD.These dependences are larger than those in aqueous solution.This result was attributed to the change of arrangement of water molecules around the solute in micellar solution and aqueous solution.
- Isoda, Teruyo,Yamasaki, Miyuki,Yano, Hiroshige,Sano, Takayuki,Harada, Shoji
-
p. 869 - 874
(2007/10/02)
-
- Hammett Equation and Micellar Effects upon Deacylation
-
Substituent effects upon second order rate constants of reaction of OH- with phenyl p-substituted benzoates at surface of micelles of cetyltrialkylammonium bromide (alkyl = Me, Et, Pr and Bu) and tetradecylquinuclidinium bromide fit the Hammett equation.Values of ρ increase with increasing bulk of the surfactant head group and are considerably larger than in water, corresponding to decreases in micellar surface polarity.The effects of substitution of the propyl group into the phenyl and benzoyl groups show that substrate orientation at micellar surface plays no significant kinetic role.
- Bartoletti, Antonella,Bartolini, Simona,Germani, Raimondo,Savelli, Gianfranco,Bunton, Clifford A.
-
p. 723 - 728
(2007/10/02)
-
- Determination of 18O Exchange Accompanying Basic Hydrolysis of p-Toluamide and N,N-Dimethyl-p-toluamide
-
The base-catalyzed hydrolysis and 18O-exchange rate constants for p-toluamide (IV) and N,N-dimethyl-p-toluamide (V) have been determined under pseudo-first-order conditions as a function of -> at T = 100 deg C, μ = 1.0 M.Both sets of rate constants are linearly dependent on ->, and therefore the ratio of kex/khyd for IV and V does not change as a function of ->.For IV and V the ratios are 3.5 +/- 0.2 and 0.11 +/- 0.007 respectively.The fact that V shows substantial exchange suggests that the original formulation of the Deslongchamps stereoelectronic theory needs to be reassessed with respect to the conformational equilibration of the anionic tetrahedral addition intermediate produced from N,N-dimethylbenzamide and OH-.
- Slebocka-Tilk, H.,Brown, R. S.
-
p. 1153 - 1156
(2007/10/02)
-