Pyridinium ions, formation

In vitro metabolic studies with rodent and human liver microsomal prepara- tions have established that MPTP undergoes both oxidative N-demethylation and C-6 (allylic) oxidation in reactions that are -nicotinamide adenine dinucleotide phosphate (NADPH) dependent and therefore likely to be cytochrome P-450 catalyzed (Weissman et al. 1985 Ottoboni et al. 1990). Although the latter transformation can lead to the toxic pyridinium metabolite MPP, the cytochrome P450-catalyzed pathway is unlikely to contribute significantly to the neurotoxicity of MPTP. As mentioned above, liver aldehyde oxidase diverts the inter-mediate dihydropyridinium metabolite away from pyridinium ion formation by catalyzing the conversion of structure 40 to the nontoxic lactim structure 41. Further-more, even if formed in the periphery, the polar pyridinium metabolite would have limited access to the central nervous system (CNS). The low... 

When mixtures of Mn(N03)2 and H-ZSM-5 were thermally treated for 2 h at 870 K, XPS revealed a significant decrease in the surface concentration of manganese [35]. Further evidence for introduction of Mn into H-ZSM-5 was provided by the ESR spectra of MnO/H-ZSM-5 and MnCl2/H-ZSM-5 mixtures after heat-treatment at 870 K and 770 K followed by rehydration at ambient temperature [14,35]. The spectra showed a signal with six hyperfine lines typical of Mn + in cation sites with Oj, symmetry. Also, TPDA, TPE and IR (consumption of acidic OH groups, decreased pyridinium ion formation upon pyridine admission to the heat-treated mixture, increased density of... 

However, an evaluation of the observed (overall) rate constants as a function of the water concentration (5 to 25 % in acetonitrile) does not yield constant values for ki and k2/k i. This result can be tentatively explained as due to changes in the water structure. Arnett et al. (1977) have found that bulk water has an H-bond acceptor capacity towards pyridinium ions about twice that of monomeric water and twice as strong an H-bond donor property towards pyridines. In the present case this should lead to an increase in the N — H stretching frequency in the o-complex (H-acceptor effect) and possibly to increased stabilization of the incipient triazene compound (H-donor effect). Water reduces the ion pairing of the diazonium salt and therefore increases its reactivity (Penton and Zollinger, 1971 Hashida et al., 1974 Juri and Bartsch, 1980), resulting in an increase in the rate of formation of the o-complex (ik ). 

Spectroscopy. In the methods discussed so far, the information obtained is essentially limited to the analysis of mass balances. In that re.spect they are blind methods, since they only yield macroscopic averaged information. It is also possible to study the spectrum of a suitable probe molecule adsorbed on a catalyst surface and to derive information on the type and nature of the surface sites from it. A good illustration is that of pyridine adsorbed on a zeolite containing both Lewis (L) and Brbnsted (B) acid sites. Figure 3.53 shows a typical IR ab.sorption spectrum of adsorbed pyridine. The spectrum exhibits four bands that can be assigned to adsorbed pyridine and pyridinium ions. Pyridine adsorbed on a Bronsted site forms a (protonated) pyridium ion whereas adsorption on a Lewis site only leads to the formation of a co-ordination complex. 

Acyl chlorides are highly reactive acylating agents and react very rapidly with alcohols and other nucleophiles. Preparative procedures often call for use of pyridine as a catalyst. Pyridine catalysis involves initial formation of an acyl pyridinium ion, which then reacts with the alcohol. Pyridine is a better nucleophile than the neutral alcohol, but the acyl pyridinium ion reacts more rapidly with the alcohol than the acyl chloride.103... 

Use of 2,4,6-trichlorobenzoyl chloride, Et3N, and DMAP, known as the Yamaguchi method,128 is frequently used to effect macrolactonization. The reaction is believed to involve formation of the mixed anhydride with the aroyl chloride, which then forms an acyl pyridinium ion on reaction with DMAP.129... 

On co-adsorbing phenol and methanol, the protonation of methanol occurs on the active acid sites as the labile protons released from the phenol reacted with methanol. Thus protonated methanol became electrophilic methyl species, which undergo electrophilic substitution. The ortho position of phenol, which is close to the catalyst surface, has eventually become the substitution reaction center to form the ortho methylated products (Figure 3). This mechanism was also supported by the competitive adsorption of reactants with acidity probe pyridine [79]. A sequential adsorption of phenol and pyridine has shown the formation of phenolate anion and pyridinium ion that indicated the protonation of pyridine. 

Another experiment in which sequential adsorption of phenol and pyridine then followed by methanol shows formation of pyridinium ion and phenolate anion whereas no traces of methanol or electrophilic methyl species or formation of methylated products were identified on the catalysts surface. This result was supposedly confirmed from another experiment in which anisole and methanol were co-adsorbed on the catalyst. The spectra were referred to the molecular species of methanol and anisole without any significant interaction among them and above 200°C they simply desorbed from the catalyst. 

It was necessary to assume that l-(4-pyridyl) pyridinium dichloride was formed without the formation of pyridinium chloride in the chloride reaction mixture, in order to account for the stoichiometries observed. The absence of the characteristic spectra of pyridinium chloride from samples of chloroform-soluble residues supported this assumption. However, for similar reasons it was necessary to conclude that pyridinium bromide was a product of the bromide reaction. The difference between the two reactions in this respect may be explained by the relative solubilities of the two halide salts of the protonated l-(4-pyridyl) pyridinium ion in pyridine. This point, however, was not pursued further in this investigation. 

Reduction of pyridinium ions with sodium dithionite (Na2S204-H20-Na2C03) gives 1,4-dihydro products (80JA1092). The mechanism involves initial formation of a sodium sulfinate intermediate which is stable in alkaline solution, but which decomposes as shown in Scheme 34 in acid or neutral solution. 

Eberly (151) found that exposure of dehydroxylated alkaline earth samples to water vapor resulted in reappearance of the acidic hydroxyl bands at 3650 and 3550 cm 1. Subsequent exposure to pyridine resulted in interaction with the 3650 cm 1 groups and the formation of pyridinium ions. A band at 3585 cm-1 has also been reported (156), which appears simultaneously with the reformation of the acidic hydroxyl groups upon exposure to water. This band does not react with pyridine and is thought to arise from hydroxyl groups associated with the cations that result from dissociation of the added water. 

Adsorption of pyridine on dehydroxylated SMM resulted in formation of a small amount of pyridinium ions and at least two Lewis-bound species. Rehydration occurred upon addition of water to the sample, and the Lewis-bound species were converted to pyridinium ions. The relative amounts of pyridine bound at Lewis and Br0nsted sites was found to be a strong function of the residual water content. Quantitative infrared data... 

One of the simplest demonstrations of the effect incarceration has on a guest s reactivity is the measurement of the basicity of included amine ligands. Solutions of pyridine in CDC13 may be shown by H NMR spectroscopy to be readily protonated by CF3C02D. An analogous reaction of the pyridine hemicarceplex of the open portal hemicarcerand 6.101 results in the pyridine remaining unprotonated. This means that incarcerated pyridine is a much weaker base than the free molecule. This difference is explained most reasonably by the fact that the host has only a very limited ability to solvate the pyridinium ion and will sterically inhibit the formation of pyridinium-trifluoroacetate contact ion pairs. 

Stacey and Turton61 objected to Isbell s mechanism on two counts first, that he did not specify that a proton acceptor must be used to promote the reaction and second, that the orthoacetate intermediate would not be applicable in the conversion which they demonstrated (by absorption spectra data) to take place on treatment with dilute, aqueous sodium hydroxide. (The presence of the proton acceptor seems implicit in Isbell s general description of the process of enolization.) The mechanism of Stacey and Turton is shown in Formulas XXIV to XXVIII it calls for the donation of electrons by pyridine to the incipient, ionic proton at C2 and elimination of acetic acid between C2 and C3 with the formation of the partially acetylated enediol-pyridinium complex. The pyridinium ion is removed by acetic acid. Electronic readjustment results in the elimination of acetic acid from positions 4 and 5. The final step, conversion of XXVII to XXVIII, was not explained. Stacey and Turton considered that with sodium hydroxide the reaction proceeds after deacetylation by a similar mechanism except that hydroxyl groups take the place of acetyl groups. Neither mechanism requires a free hydroxyl group at Cl, a condition considered by Maurer to be essential to kojic acid formation. 

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