Proton activating factors

There is room for further analysis in many traditional areas, as pointed out above during the discussion of enolization. Also, it is noted that the employment of transition state pKf values is very close to the use of the proton activating factors and deprotonating factors, introduced by Stewart (Stewart and Srinivasan, 1978 Stewart, 1985). It is to be hoped that the two approaches can be consolidated in a common view of acid-base catalysis. 

Enolization and ketonization kinetics and equilibrium constants have been reported for phenylacetylpyridines (85a), and their enol tautomers (85b), together with estimates of the stability of a third type of tautomer, the zwitterion (85c). The latter provides a nitrogen protonation route for the keto-enol tautomerization. The two alternative acid-catalysed routes for enolization, i.e. O- versus Af-protonation, are assessed in terms of pK differences, and of equilibrium proton-activating factors which measure the C-H acidifying effects of the binding of a proton catalyst at oxygen or at nitrogen. 

Here (x)t and (x)f denote the mean values of the relative coordinate x over the states of the proton in the first and second potential wells, respectively. Equation (107) shows that the inertia effects lead to a decrease of the activation factor in the transition probability due to an increase of the reorganization energy. The greater the mass, m of the tunneling particle and the frequency of the vibrations of the atom, w0, the greater is this effect. The above result corresponds to the conclusion drawn in Ref. 66. 

Therefore, if the excited-state lifetimes r0 and Tq are known, the plot of ( / 0)/( / ) versus [H30+] yields the rate constants k3 and k i. However, it should be emphasized that corrections have to be made (i) the proton concentration must be replaced by the proton activity (ii) the rate constant k 3 must be multiplied by a correction factor involving the ionic strength (if the reaction takes place between charged particles), because of the screening effect of the ionic atmosphere on the charged reactive species. 

Fig. 1. Schematic diagram showing the different mechanisms of action proposed for the antiulcer action of flavonoids. 1. Blockade of add secretion by decreasing histamine production or inhibiting the proton pump. 2. Bactericidal effect on H. pylori. 3. Antioxidative activity by scavenging free radicals and preventing ROM formation. 4. Potentiation of the mucosal protective factors. PAF platelet activating factor ROM reactive oxygen metabolites H2 histamine receptor 2 M muscarinic receptor G gastrin receptor.

Dalton TP, Li Q, Bittel D, Liang L, Andrews GK (1996) Oxidative stress activates metal-responsive transcription factor-1 binding activity. Occupancy in vivo of metal response elements in the metallothionein-I gene promoter. J Biol Chem 271 26233-26241 Danscher G, Howell G, Perez-Clausell J, Hertel N (1985) The dithizone, Timm s sulphide silver and the selenium methods demonstrate a chelatable pool of zinc in CNS. A proton activation (PIXE) analysis of carbon tetrachloride extracts from rat brains and spinal cords intravitally treated with dithizone. Histochemistry 83 419 22 Danscher G, Jensen KB, Frederickson CJ, Kemp K, Andreasen A, Juhl S, Stoltenberg M, Ravid R (1997) Increased amount of zinc in the hippocampus and amygdala of Alzheimer s diseased brains a proton-induced X-ray emission spectroscopic analysis of cryostat sections from autopsy material. J Neurosci Methods 76 53-59... 

Electrolysis can occur only at the boundary between an electrode and a medium that conducts the electric current, and the nature of the solvent is important for the course of electrolytic reactions. Such factors as proton activity, usable potential range, dielectric constant, ability to dissolve electrolytes and substrates, ion pair formation, accessible temperature range, vapor pressure, viscosity, toxicity, and price must be taken into consideration when the choice of solvent is made. 

The introduction of an acyl group activates the heteroaromatic ring towards further acylation, which however always takes place exclusively at the positions X and y to the heterocyclic nitrogen (the protonated nitrogen is by far the main activating factor, which determines the positional selectivity). Thus, if a heterocyclic compound has two reactive positions, it is easy to obtain diacyl derivatives, but only one isomer (for example 2,4-diacylderivatives in the case of quinoline), whereas the monoacylderivatives prevail only at very low conversions. Due to the nucleophilic character of alkyl and acyl radicals, the behavior of homolytic... 

Catalytic asymmetric arylation of alkenes.4 This arylation can be effected by a Heck-type arylation of 2,3-dihydrofuran with an aryl triflatc catalyzed by Pd(OAc)2/(R)-BINAP in the presence of 1,8-bis(dimelhylamine)naphthalcnc (proton sponge) as base. This reaction was used to prepare an antagonist (4) of platelet activating factor. 

An example of the use of an intermolecular carbopalladation in complex molecule synthesis is the preparation of a PAF (platelet activating factor) antagonist (Scheme 11). In the key step, an intermolecular Heck reaction of 2-naphthyl triflate with 2,3-dihydrofuran 71 yields 2-naphthyl-2,3-dihydrofuran 72 in 52% yield with excellent enantioselectivity. The reaction presumably occurs via the cationic manifold and the alkene is isomerized by a hy-dropalladation/dehydropalladation reaction. The minor product 2,5-dihydrofuran 73 is obtained in 26% yield with modest enantioselectivity favoring the opposite absolute configuration at the key center. Critical to the reaction is the use of the sterically demanding and highly basic proton sponge [l,8-bis(dimethylamino)naphthalene] as the base. It is... 

Due to the importance of electrochemical reduction of oxygen in life processes such as biological respiration and industrial applications including fuel cells or batteries, that reduction process and mechanism have been investigated over the last decades. The mechanism of the oxygen reduction reaction is highly complicated and dependent on a large number of factors, such as the electrode material, catalyst, solvent electrolyte and proton activity [1]. 

Figure A3.8.3 Quantum activation free energy curves calculated for the model A-H-A proton transfer reaction described 45. The frill line is for the classical limit of the proton transfer solute in isolation, while the other curves are for different fully quantized cases. The rigid curves were calculated by keeping the A-A distance fixed. An important feature here is the direct effect of the solvent activation process on both the solvated rigid and flexible solute curves. Another feature is the effect of a fluctuating A-A distance which both lowers the activation free energy and reduces the influence of the solvent. The latter feature enliances the rate by a factor of 20 over the rigid case.

The mobility of the proton in position 2 of a quaternized molecule and the kinetics of exchange with deuterium has been studied extensively (18-20) it is increased in a basic medium (21-23). The rate of exchange is close to that obtained with the base itself, and the protonated form is supposed to be the active intermediate (236, 664). The remarkable lability of 2-H has been ascribed to a number of factors, including a possible stabilizing resonance effect with contributions of both carbene and ylid structure. This latter may result from the interaction of a d orbital at the sulfur atom with the cr orbital out of the ring at C-2 (21). 

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