Pyridinium cation

Pyridinium bromide, N-phenacyl-NMR, 2, 121 reactions, 2, 336 Pyridinium cations metabolism, 1, 234 reactivity, 2, 167 Pyridinium chloride hydrogenation, 2, 284 Pyridinium chloride, N-acyl-as acylating agent, 2, 180 Pyridinium chloride, cetyl-as antiseptic, 2, 519... 

Methyl propiolate and pyridine give a rather unstable 2 1 molar adduct which is the 1,2-dihydropyridine (112). The reaction sequence proposed to account for its formation is identical in principle to a similar scheme proposed earlier in the acridine series (Section II,A,2) and is also supported by the observation that the 1-benzoyl-pyridinium cation with the phenylacetylide anion yields (113). ... 

Tire calculated Ca-N" bond lengths, Table III, of these pyridinium cations [as well as the bond orders, BO, and the reaction energies, zIT/r, for a hypothetical dissociation to give the corresponding carbenium ion 44 and pyridine (Scheme 13)] indicate that these bonds are significantly weakened... 

Tliis interpretation is based only upon the structural and electronic properties of the pyridinium cations. Tire calculation of relative activation Ijarri-ers for the competing substitution reactions will give more reliable results —especially if solvent effects are included in the calculations. In order to assess the reliability of actual theoretical methods as applied to model sys-... 

In the case of ions, the repulsive interaction can be altered to an attractive interaction if an ion of opposite charge is simultaneously adsorbed. In a solution containing inhibitive anions and cations the adsorption of both ions may be enhanced and the inhibitive efficiency greatly increased compared to solutions of the individual ions. Thus, synergistic inhibitive effects occur in such mixtures of anionic and cationic inhibitors . These synergistic effects are particularly well defined in solutions containing halide ions, I. Br , Cl", with other inhibitors such as quaternary ammonium cations , alkyl benzene pyridinium cations , and various types of amines . It seems likely that co-ordinate-bond interactions also play some part in these synergistic effects, particularly in the interaction of the halide ions with the metal surfaces and with some amines . 

Kabanov et al.131) found that a copolymer of 4-vinylpyridine and acroleinoxime, 82 (PPox), is a powerful catalyst for the hydrolyses of PNPA, NABA, and 3-nitro-4-trimethyl-acetoxybenzoic acid 83 (NTBA). The activity of the copolymer was 103 times higher than that of the low-molecular-weight oxime, iso-butyraldoxime. They proposed the cooperative activation of the oxime- and pyridine-groups in the vicinity of pyridinium cation groups of the copolymer. 

This entry to supramolecular chemistry allows one, at the same time, to develop a host-guest chemistry because the space inside the cucurbituril barrel is sufficient to confine small "guesf molecules. This is illustrated with the crystallization of the supramolecular adduct [W3S4(H20)8Cl](pyH ccuc) Cl4a 5.5 H2O with a pyridinium cation inside the cucurbituril cavity [48]. The introduction of guest molecules may vary the net charge on the assembly and consequently the whole packing in the solid state. 

Pyridinium chloride is a salt that generates ions in solution. The major species are the pyridinium cation (C5 H5 NH+), cr, and H2 O. The formula identifies the pyridinium cation as the conjugate acid of the weak base, pyridine (see Table 17-41. There are two acid-base equilibria with major species as... 

As mentioned in Sect. 4.2.2, salt-type associates are the only representatives of the aggregates formed by 7. In these crystals, the pyridinium cations appear surrounded by a rectangle-like environment maintained by 7. 

Reactions of highly electron-rich organometalate salts (organocuprates, orga-noborates, Grignard reagents, etc.) and metal hydrides (trialkyltin hydride, triethylsilane, borohydrides, etc.) with cyano-substituted olefins, enones, ketones, carbocations, pyridinium cations, etc. are conventionally formulated as nucleophilic addition reactions. We illustrate the utility of donor/acceptor association and electron-transfer below. 

The decarboxylation of carboxylate anions is carried out chemically by a variety of one-electron oxidants such as lead tetraacetate, uranyl nitrate, peroxides, quinones, pyridinium cations, etc.199 Importantly, the carboxylate anion (as... 

Tapia, O., Cardenas, R., Andres. J. and Colonna-Cesari, F. Transition structure for hydride transfer to pyridinium cation from methanolate. Modeling of LADH catalyzed reaction, J. Am. Chem. Soc., 110 (1988), 4046-4047... 

Fig. 8.14. Stepwise activation of dihydrotrigonelline-based chemical delivery systems, first by oxidation to a pyridinium cation (Reaction a), and then by hydrolysis to trigonelline and the drug ROH (Reaction b). Direct hydrolysis (Reaction c) is slow in comparison to the Reactions...

The catalytic constants for acids in aqueous solution correlate well with pK, and there is no apparent difference between uncharged and cation acids (carboxylic acids and pyridinium cations), though some steric inhibition is evident for 2,6-substituted pyridinium cations. The catalytic constants for bases behave somewhat more erratically, and it has been suggested by Gruen and MoTigue (1963b) that errors are intro-... 

From Chryseomonas luteola in addition to chrysobactin a derivative (chrysomonin) was isolated where C-6 of the DHB unit is substituted with the N-atom of a pyridinium cation. Chrysomonin could be synthesized from chrysobactin (la). 

However, pyridine and pyrrole are significantly less basic than either of their saturated analogues. The pyridinium cation has pATa 5.2, making pyridine a much weaker base than piperidine, whereas the pyrrolium cation (pATa - 3.8) can be considered a very strong acid, and thus pyrrole is not at all basic. 

Pyridine is a base (pATa pyridinium cation 5.2), but it is a considerably weaker base than a typical non-aromatic heterocyclic amine such as piperidine (pATa piperidinium cation 11.2). This is because the lone pair electrons in pyridine are held in an sp orbital. The increased character of this orbital, compared with the sp orbital in piperidine, means... 

Direct attack at a ring carbon, even C-3, is normally slow (a) because the concentration of free pyridine in equilibrium with the pyridinium salt is extremely low, and (b) attack upon the salt would also require the positive pyridinium cation to bond to a positively charged reactant. 

Rotation barriers have also been looked at (80TL1553) in TV-substituted 2,4,6-trimethyI-pyridinium cations (267). In addition to diastereotopic protons HA and HB, the a-Me and (8-protons are anisochronous as shown by HNMR, as are the a-Me, a-ring and /3-ring carbons, as shown by 13C NMR. This anisochronism appears to be due to hindered rotation about the pyridinium N—C bond (268), the stable conformation being that in which the hydrogen atom on the sp3 C is in the plane of the pyridine ring. Observation of coalescence temperatures in deuterated acetone or pyridine solvent allows calculation of energy barriers to this rotation, and these are as follows (267 R = Me, R = H), 7.1 (267 R = Me, R = COMe) (267 R = Et, R = H), 8.1 kJ mol-1. 

Stepnowski, R, Application of chromatographic and electrophoretic methods for the analysis of imidazolium and pyridinium cations as used in ionic liquids, Int. J. Mol. Sci., 7, 497, 2006. 

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