Pyridine catalytic action

In the case of noncyclized derivatives of the aromatic series (nitrotolanes), isato-gen is obtained either by a longer heating of the substrate in pyridine, or by photochemical (sun rays) or catalytic action (69MI2) (Scheme 113). 

In 1967, (3) it was discovered that DMAP catalyzes the benzoylation of m-chloroaniline 10 times faster than pyridine. This enormous increase in reaction rate is unmatched by any other nucleophilic acylation catalyst (3. It was shown that the catalytic action of DMAP and PPY is not primarily due to their larger pKa s with respect to pyridine, but is a result of enhanced nucleophilic catalysis. 

Cyclohexaamylose continued) mono-6-O-tosyl-, preparation of, 23 250 pyridine-2,5-dicarboxylic acid derivative, catalytic action of, 23 251 separation of, by complexing, 23 214 structure, stereochemistry and physical properties of, 23 210-213... 

Jones and Savill ( 6) have described the very rapid catalytic effect upon trimerization of isocyanates by ethylene oxide, propylene oxide, styrene oxide, or epichlorhydrin when a trace of pyridine is present. No reaction occurs in the absence of pyridine. They express the opinion that the function of the tertiary base is to open the epoxide ring and that the catalytic action is believed due to initial formation of... 

While bases, like pyridine and diethylcyclohexylamine, or sulfuric acid and water showed little catalytic action, ureas were found to accelerate the reaction. This action of ureas is reponsible for the autocatalysis observed in the interaction of isocyanates and amines. The initial rate is more dependent on the concentration of the amine than on that of the isocyanate, the order with respect to the amine being close to 2. 

Alkyl cyanoacrylates are used extensively as instant adhesives and are grouped with vinylidene cyanide, methylene malonates, and a-cyanosorbates at the top of the anionic reactivity scale. As well as readily polymerizing in the presence of conventional anionic initiations such as Bu"Li, they also react readily with tertiary amines and phosphines. Pepper and Johnstonhave examined the catalytic action of triethylphosphine and pyridine at —78 °C in tetrahydrofuran. The former initiator appears to add rapidly and irreversibly to monomer to yield a conventional living anionic polymerization. With pyridine, however,... 

Under the catalytic action of trifluoroacetic acid, 2-aininonicotinonitrile and trimethyl orthoformate react to yield 2-[(methoxymethylene)amino]pyridine-3-carbonitrile, the starting material for the cyclization with methylhydrazine. Whereas at 0 °C the reaction provides a 1 1 mixture of the cis/truns-isomers of A,-amino-,V-(3-cyano-2-pyridyl)-A-methylforrnimidamide, the same reaction at room temperature yields 4-(/ -methylhydrazino)pyrido[2,3-d]pyrimidine (17).25... 

As will be shown in Sections 5 and 6 methylviologen can be reduced at the cathode of an electrochemical cell to the mono cation radical MV which transfers electrons by the action of an enzyme type which we call AMAPOR for artificial mediator accepting pyridine nucleotide pxidoreductase. Finally the NADH is used for reducing the substrate by the catalytic action of an NADH dependent reductase. 

A similar conclusion had been obtained earlier by Letai r and Savereide for the solvolysis of several anionic dienyl esters catalyzed by poly-4-vinylp5aidine 7 in 50% ethanol-water (5/). It is noteworthy that a rate maximum is again observed with three-quarters of the pyridine group in the neutral form. A complicating factor in quantitatively estimating the electrostatic contribution is that the reactivity of a ven nucleophilic group may vary with the degree of protonation. This problem will be discussed later in connection with the catalytic action of vinylimidazole copolymers. 

The findings of Swain and Brown 55) support the ternary mechanism proposed by Lowry for the mutarotation of tetra-O-methylglucose. It was found that the mutarotation of tetra-O-methylglucose in benzene in the presence of both an acid (phenol) and a base (pyridine) followed third-order kinetics but was first-order with respect to each component tetra-O-methyl-glucose, pyridine, and phenol 55). 2-Hydroxypyridine was found to be a very effective bifunctional catalyst, and since both acid and base functions were in the same molecule, the mutarotation followed second-order kinetics. Its catalytic action was essentially independent of the other acid and base species present. Although it is a much weaker acid or base than either phenol or pyridine, its catalysis of the mutarotation of tetra-O-methylglucose in benzene was much greater than that of either pyridine or phenol, or a mixture of both 55). 

Isoxazole compounds can be converted into the corresponding isothiazoles by successive catalytic hydrogenation, sulfuration with phosphorus pentasulfide and oxidation with chloranil (72AHC(14)l, 75SST(3)541). 2,1-Benzisoxazoles give the 2,1-benzisothiazoles directly, by the action of phosphorus pentasulfide in either pyridine or molten imidazole (73SST(2)556, 77SST(4)339). (See also Chapter 4.16 for further discussion of these topics.)... 

Since spillover phenomena have been most directly sensed through the use of IR in OH-OD exchange [10] (in addition, in the case of reactions of solids, to phase modification), we used this technique to correlate with the catalytic results. One of the expected results of the action of Hjp is the enhancement of the number of Bronsted sites. FTIR analysis of adsorbed pyridine was then used to determine the relative amounts of the various kinds of acidic sites present. Isotopic exchange (OH-OD) experiments, followed by FTIR measurements, were used to obtain direct evidence of the spillover phenomena. This technique has already been successfully used for this purpose in other systems like Pt mixed or supported on silica, alumina or zeolites [10]. Conner et al. [11] and Roland et al. [12], employed FTIR to follow the deuterium spillover in systems where the source and the acceptor of Hjp were physically distinct phases, separated by a distance of several millimeters. In both cases, a gradient of deuterium concentration as a function of the distance to the source was observed and the zone where deuterium was detected extended with time. If spillover phenomena had not been involved, a gradientless exchange should have been observed. 

The acetonide 151 is available from a 4-substituted azetidinone in three steps and is converted into the 3,4-disubstituted azetidinone 152 by the action of pyridine chlorochromate in the presence of a catalytic amount of Jones reagent. The disubstituted /3-lactam is a suitable starting material for the synthesis of trinems <1996TL2467>. 

The joint action of pyridine and thionyl chloride on aliphatic or aromatic aldehydes RCHO results in 1-chloromethylpyridinium chlorides 272, which react with ethyl jS-aminocrotonate to give 1,4-dihydropyridines 273 (equation 113). The reaction constitutes a modified Hantzsch synthesis it has the advantage over the latter that it proceeds under milder conditions and gives better yields138. The dihydropyridine 273 (R = H) is also readily obtained from ethyl / -aminocrotonate and formaldehyde in the presence of catalytic amounts of 4-(dimethylamino)pyridine139. 

Synthesis of a piperidine compound is commonly carried out by reduction of the corresponding pyridine derivative. An excellent discussion has been presented. " Earlier techniques concerned with reduction by the action of sodium and alcohol have been largely replaced by procedures of catalytic hydrogenation. An example is the reduction of pyridine to piperidine in 83% yield over nickel at 170-200°. Reduction of its homologs occurs in the same way. Platinum oxide is poisoned by pyridine... 

The catalysis of hydrolysis of carboxylic acid derivatives by weak bases has not been carefully studied until relatively recently. Koshland reported in 1952 the catalysis of acetyl phosphate hydrolysis by pyridine Bafna and Gold (1953) reported the pyridine-catalyzed hydrolysis of acetic anhydride. A short time later the catalysis of aromatic ester hydrolysis by imidazole was demonstrated (Bender and Turnquest, 1957 a, b Bruice and Schmir, 1957). Since that time a large amount of work has been devoted to the understanding of catalyzed ester reactions. Much of the work in this area has been carried out with the purpose of inquiry into the mode of action of hydrolytic enzymes. These enzymes contain on their backbone weak potential catalytic bases or acids, such as imidazole in the form of histidine, carboxylate in the form of aspartate and glutamate, etc. As a result of the enormous effort put into the study of nucleophilic displacements at the carbonyl carbon, a fair understanding of these reactions has resulted. An excellent review is available for work up to 1960 (Bender, 1960). In addition, this subject has been... 

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