Pyridinium cations, reactivity

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... 

N withdraws electrons by induction and destabilizes the R intermediates formed from pyridine. Also, the N atom reacts with electrophiles to form a pyridinium cation, whose + charge decreases reactivity. 

The most effective single criterion of whether free base or conjugate acid nitrates is the comparison of a model compound in which the possibilities of prototropic and tautomeric equilibria have been eliminated. For example, the model compound for a substitued pyridinium cation is its N-methyl cation. If the nitration of the pyridine proceeds via the conjugate acid, the rate profile will have the same shape as for the N-methyl model compound. The individual rates of nitration for the model compound may or may not be exactly the same as for the pyridine itself, depending upon the effect of the methyl group upon the reactivity. If, on the other hand, nitration of the pyridine takes place on the free base, then the N-methyl cation will not be nitrated under the same conditions. 

A 6n electrocyclic closure could be implicated in the photocyclisation reaction reported for the N-amino pyridinium ion (257) and some of its ring substituted derivatives to give (258). This type of reactivity was also seen for compound (233). A summary of the previously published work and of new results for the 67t photocyclisation of photochromic ethylenes substituted by derivatives of cyclopentadiene anion and pyridinium cation has appeared.The basic skeleton involved in the rearrangement is shown in structure (259) which is in photochemical equilibrium with (260). 

These reactivity trends clearly show that polar effects are involved in these radical substitution reactions. The transition state is thought to include a charge transfer 9) from the radical (electron donor) to the pyridinium ion (electron acceptor) [13], Frontier Molecular Orbital Theory (FMO) [14] has been applied to explain the reactivity differences which have been observed upon varying the substituents at the pyridinium ion and upon altering the nucleophilicity of the attacking radical. Moreover, FMO can be used to explain the regioselectivities obtained in these homolytic aromatic substitutions. The LUMO of the substituted pyridinium cation... 

CB[ ] hosts, namely CB[7], CB[8] and CB[10], resulted in selective population of specific confoimers, and controlled the folding and refolding of these oligomers. Sotiriou-Leventis, Leventis et al. showed that inclusion of A -methyl-4-(p-nitrobenzoyl)pyridinium cation 18 CB[7] shows a preference for the keto form, whereas in water solution there is an equilibrium between the ketone and gem-diol forms [106], This is illustrated in Fig. 3.10, and showed that CB[7] can be used to force this compound to be in the ketone form again this would have significant implications for the reactivity of this compound. 

It was shown that the electron-rich oxazolium anions decrease the rate of reaction for NEDDA, and increase the activation barrier however, electron-poor oxazolium cations have low activation barriers for lEDDA and reactivity can be promoted by adding alkyl, Lewis, and Br0nsted acids to the oxazole nitrogen atom. The dehydration of cationic Diels-Alder cycloadducts was also found to be a highly exothermic process, favoring formation of pyridinium cations. Therefore, the best strategy to harness the potential of oxazole as a diene in lEDDA reaction is to use a proton-assisted process (i.e., to exploit the electron-poor oxazolium cation). 

Registration of radical cations by the EPR method in the presence of nitrogen dioxide could provide direct experimental evidence that the initiation proceeds through scheme (36). However, because of high reactivity and fast decomposition [45], these particles are difficult to detect by this mediod. Nevertheless, the formation of radical cations can be revealed indirectly in the act of their decomposition with emission of a proton. Pyridine is known to be capable of accepting protons to yield p5rridinium cations. Hence, if protons are formed in during decomposition of radical cations by Eq. (36), they can be detected easily from IR spectra typical of pyridinium cations. Note that pyridine can be nitrated only imder quite severe conditions. For example, iV-nitropyridinium nitrate was obtained only when pyridine was treated with a N02-ozone mixture in an inert solvent [46], This is evident from the IR spectrum (Fig. 5) of 1 1 mixture... 

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