Pyridinium ions, electrophilic

One reason for the low reactivity of pyridine is that its nitrogen atom, because it is more electronegative than a CH in benzene, causes the tt electrons to be held more tightly and raises the activation energy for attack by an electrophile. Another is that the nitrogen of pyridine is protonated in sulfuric acid and the resulting pyridinium ion is even more deactivated than pyr idine itself. 

Parry (344) determined the infrared spectrum of pyridine adsorbed on rj-alumina dehydrated at 450°. Characteristic differences in the 1400-1700 cm region exist in the spectra of pyridine adsorbed via hydrogen bonds, pyridinium ions, and pyridine coordinately bonded to electrophilic sites. Pyridinium ions are characterized by a strong band at 1540 cm and a very strong band at 1485-1500 cm" coordinately bonded pj ridine has a strong absorption at 1447-1460 cm". No evidence was found for the existence of Bronsted sites on the alumina surface. 

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. 

The pyridinium salts have been shown to have electrophilic positions at the 2-, 4-, and 6-carbon atoms. Of these, the 2- and 6-positions should be the more positive because of the proximity to the quaternary nitrogen. From the ultraviolet absorption spectra of the reaction mixtures during the reduction and of the isolated products, it can be demonstrated that the predominant attack of the hydride ion from sodium borohydride occurs at these two positions.5,6 The 1,6-dihydro-pyridine (such as 5) formed from the reduction of a 1,3-disubstituted pyridinium ion appears to be stable toward further reduction, for a number of such compounds have been isolated from sodium borohydride reductions containing sufficient borohydride to complete the reduction to the tetrahydro-state.7"10 Since 1,4-dihydropyridines having a 3-substituent which is electron-withdrawing have also been... 

Electrophilic substitution of pyridine is further hindered by the tendency of the nitrogen atom to attack electrophiles and take on a positive charge. The positively charged pyridinium ion is even more resistant than pyridine to electrophilic substitution. 

An equally serious problem is that the nitrogen lone pair is basic and a reasonably good nucleophile—this is the basis for its role as a nucleophilic catalyst in acylations. The normal reagents for electrophilic substitution reactions, such as nitration, are acidic. Treatment of pyridine with the usual mixture of HN03 and H2SO4 merely protonates the nitrogen atom. Pyridine itself is not very reactive towards electrophiles the pyridinium ion is totally unreactive. 

As we have seen, pyridine attacks electrophiles through its nitrogen atom. This produces the reactive species, the N-bro mo-pyridinium ion, which is attacked by the benzene. Pyridine is a better nucleophile than benzene and a better leaving group than bromide. This is another example of nucleophilic catalysis. 

Indolizine gives a stable pyridinium ion and does not polymerize in the presence of acid. Indole undergoes acid-catalyzed dimerization the 3//-indolium ion acts as an electrophile and attacks an unprotonated molecule to give the dimer 178. Protonation of the dimer in turn gives an electrophilic species from which a trimeric product can be derived. iV-Methylisoindole undergoes acid-catalyzed polymerization, indicating that protonation at C(l) gives a reactive electrophilic intermediate. 

A PET in intramolecular CPs between pyridinium ions and bromide, chloride or thiocyanate ions for polymerization initiation is described, too [137-139]. As expected, an equilibrium exists among free ions, ion pairs, and CT, which is shifted to the free ions in polar solvents and to the complex in a less polar solvent That complex serves as the photosensitive species for the polymerization (see Scheme 10). The photodecomposition of the CT yields radicals of the former anion and N-alkylpyridinyl radicals. Probably, the photopolymerization is initiated only by X- radicals, whereas latter radicals terminate the chain reaction. By addition of tetrachloromethane, the polymerization rate is increased owing to an electron transfer between the nucleophilic pyridinyl radical and CC14 (indirect PET). As a result, the terminating radicals are scavenged and electrophilic -CQ3 radicals are produced. 

Pyridinium, quinolinium, and isoquinolinium cations are the major species undergoing electrophile substitution reactions under acidic conditions [90AHC(47)1]. As expected from Table XXIII, the electrophilic reaction of pyridinium ion occurs at the 3-position, and an electrophile attacks at the 5- and 8-positions of quinolinium and isoquinolinium cations. Electrophile reactivity of 1 is rather low because of its electron accepting character. Molecular orbital calculations of its orientation did not give a consistent conclusion. Electron density and superdelocalizability (electrophile) predict that position 1 will be the most reactive towards an electrophile, while inspection of the localization energy (electrophile) predicts that electrophilic reaction takes place at position 4. 

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