Buffer pyridine

The homogeneous, anaerobic, oxidation of propargyl alcohol by cupric acetate in buffered pyridine solution is an example of a general reaction... 

Table 2. Competition between C-protonation (formation of nitro compounds 45) and O-proton-ation (formation of the Nef-product 47) in the protonation of nitronate ions 44 (s. Scheme 8). The percentage of C-protonated product 45 for different buffers (pyridine/p-toluenesulfonic acid) and unbuffered p-toluenesulfonic acid (48) is listed...

Pyridine-4-carboxylic acid Britton-Robinson buffer, pH 6.1 -1.14... 

Diazophenols, ie, o-hydroxyaryldiazonium salts, couple to 1-naphthol in weaMy basic solution primarily in the para position, but as the hydroxyl ion concentration is increased, formation of the ortho isomer is favored and is frequentiy the sole product. Pyridine and pyridine derivatives, urea, and acetate, etc, used as buffers can also catalyze azo coupling reactions (28). l-amino-2-naphthol-4-sulfonic acid [116-63-2] (1,2,4-acid) and 1-naphthol yield the important Eriochrome Black A [3564-14-5] (18a, R = H) (Cl Mordant Black 3 Cl 14640) which is reportedly (20) a mixture of ortho and para isomers. 

The positions of substitution, orientation, and configuration of the stable form are determined by a balance between opposing steric and dipole ef-fects. There is less agreement regarding the factors influencing kinetically controlled reaction (see below). Essentially neutral conditions, such as provided by an acetate or pyridine buffer, are required to avoid isomerization. Frequently, however, bromination will not proceed under these conditions, and a compromise has been used in which a small amount of acid is added to start and maintain reaction, while the accumulation of hydrogen bromide is prevented by adding exactly one equivalent of acetate... 

A commonly used alternative to the direct bromination of ketones is the halogenation of enol acetates. This can be carried out under basic conditions if necessary. Sodium acetate, pyridine or an epoxide is usually added to buffer the reaction mixture. The direction of enolization is again dependent upon considerations of thermodynamic and kinetic control therefore, the proportion of enol acetates formed can vary markedly with the reaction conditions. Furthermore, halogenation via enol acetates does not necessarily give the same products as direct halogenation of ketones 3. 23... 

Two surprising observations were made in the course of this work first that the enol acetate (5) is stable under the conditions for formation of (6) from (4) second, that the course of the buffered bromination of (5) depends on the conditions used. Thus, in the presence of epichlorohydrin, (7) is the sole isomer produced, whereas in pyridine-acetic acid approximately equal amounts of (7) and (8) are formed. It was suggested that this difference is inherent in the mechanism and not a result of isomerization of (7) to (8) during the course of the reaction. 

Cationic, hydrophilic, and hydrophobic Chitosan, poly-2-vinyl pyridine 0.3-1.0 M salt/buffer, pH 2-7 with the addition of methanol for more hydrophobic polymers... 

These rate constants are for the hydrolysis of cinnamic anhydride in carbonate buffer, pH 8.45, total buffer concentration 0.024 M, in the presence of the catalysts pyridine, A -methylimidazole (NMIM), or 4-dimethylaminopyridine (DMAP). In the absence of added catalyst, but the presence of buffer, the rate constant was 0.005 24 s . You may assume that only the conjugate base form of each catalyst is catalytically effective. Calculate the catalytic rate constant for the three catalysts. What is the catalytic power of NMIM and of DMAP relative to pyridine ... 

Pyridine-HF, THF, 0-25°, 70% yield. Cyclic acetals and THP derivatives were found to be stable to these conditions. In the following reaction, if excess pyridine was not included as a buffer, some acyl transfer was observed. ... 

The procedure is experimentally simple, and the workup involves only the destruction of the traces of hydrogen peroxide with manganese dioxide and evaporation of the hexamethyldisiloxane. Pyridine additives serve to buffer the highly acidic rhenium species and to shut down the detrimental acid-catalyzed epoxideopening pathways. The scope of this transformation is best appreciated through the examples presented in Table 12.2 [28],... 

When two equivalents of pyridine were added to the nmr sample and the probe heated to 80° C, the enol formate 61 decreased and phenyl cyclopropyl ketone 58 appeared at a rate approximately ten times faster than in the previous buffered system. The observation of intermediate 61 and the kinetic results, together with the observed induction periods, are consistent with the idea that some and perhaps all of the rearranged product ketone in the solvolysis of this system arises via double-bond participation in 61 rather than triple-bond participation and a vinyl cation (80). 

Solvolysis of 1-r-butylvinyl triflate, 184, at 80° in 80% aqueous ethanol buffered with pyridine gave five products, as shown in Scheme XVI (176). 

While asymmetric approaches are certainly important, other synthetically significant epoxidation protocols have also been reported. For example, buffered two-phase MCPBA systems are useful for epoxidations in which the alkenes and/or resultant epoxides are acid-sensitive. Bicarbonate works quite well for cinnamate derivatives (e.g., 55) <96SC2235> however, 2,6-di-t-butyl-pyridine was shown to give superior results in the case of certain allyl acetals (e.g., 57) <96SC2875>. 

The GMT in human serum reacts most rapidly with Y-glutamyl-p-nitroanilide at pH 8.2. The same activity is found in 2-amino-2-methylpropane-l 3 diol, diethanolamine, triethanolamine and tris buffers. Magnesium ions have no effect on the activity but favor the solubilization of the substrate. Bondar and Moss (54) found that free glutamate, due to elevated serum glutamate concentrations or glutamate released by substrate breakdown, increases the apparent GMT activity. They concluded that the assay should be performed in the presence of 1.0 vM/1 glutamate in order to reduce the possibility of falsely elevated results. This was not observed by others. Rowe and co-workers have indicated that certain batches of p-nitroanilide substrate contain impurities which may reduce GMT activity and increase the values ( ). Huesby and Stromme (56) confirmed the presence of such impurities and recommended pyridine extraction for substrate purification. 

To test the quality of some synthetic dyes according to standardized procedures, a screening is recommended based on TLC analysis on silica plates 60 F 254 using elutions with an ethyl acetate pyridine water 25 25 20 (v v v) mixture. To determine purity and secondary dyes (components or by-products of a dye that are not allowed to be present), successive TLC separations are recommended or, for more accurate answers, HPLC-DAD using RP-18 columns and eluents like acetonitrile and phosphate buffer."... 

Kinetic Studies. The pioneering work of Hierl et al. (8) and Delaney et al. (9) had established that hydrolysis of jr-nitro-phenylcarboxylates was an excellent means of observing the nucleophilic catalysis by 4-(dialkylamino) pyridine functionalized polymers. Hydrolysis of p-nitrophenylacetate in a buffer at pH 8.5 showed that the polymer was a slightly better catalyst than the monomeric analog PPY (Table II). However, preliminary results indicate that the polymer bound 4-(dialkylamino) pyridine is more effective as a catalyst than the monomeric analog in the hydrolysis of longer carbon chain p-nitrophenylcarboxylates, such as p-nitrophenylcaproate. 

The demetalating abilities of buffer species depend on both their structures and their acidities. Thus, while pyridine-2-carboxylic (picolinic) acid catalyzes the demetalation even of the rather inert lm, its 3- and 4-isomers (nicotinic and isonicotinic acids) are inactive. The difference is rationalized to result from the ability that only coordinated picolinic acid has to deliver a proton to an amidato nitrogen in an intramolecular manner. The reaction order in picolinic acid equals one for la and two for lm. For lm, inactive pyridine and nicotinic acid speed up the demetalation in the presence of picolinic acid (Fig. 8). 

Scheme 4. Proposed general mechanism of demetalation of 1 by picolinic acid accounting for first (la, in the box) and second (lm) orders in the buffer acid concentration. The charge of the Fem-TAML complex is shown outside the bracket and localized charges are shown for the deprotonated pyridine carboxylates. From Ref. (27).

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