Pyridinium salts, benzylic

Pyridinium salts, l-aryl-4-methoxy-2,6-dimethyl-synthesis, 3, 762 Pyridinium salts, N-aryloxy-rearrangements, 2, 354 Pyridinium salts, 1-benzyl-covalent amination, 2, 239 Pyridinium salts, N-benzyl-reactions... 

While examining possible approaches to the sempervirine ring system Stevens found that when the pyridinium salt 238 was treated with acid a Pomeranz-Fritzch type eychzation occurred leading to 239. Without the protective influence of the benzyl group, attack at the indole nitrogen atom took place yielding 240. 

An intriguing use of a quaternary ammonium salt in a two-phase reaction is to be found with the regeneration of 1 -benzyl-1,4-dihydronicotinamide by sodium dithionite in a biomimetic reduction of thiones to thiols [12], The use of sodium dithionite in the presence of sodium carbonate for the 1,4-reduction of the pyri-dinium salts to 1,4-dihydropyridines is well established but, as both the dithionite and the pyridinium salts are soluble in water and the dihydropyridine and the thione are insoluble in the aqueous phase and totally soluble in the organic phase, it is difficult to identify the role of the quaternary ammonium salt in the reduction cycle. It is clear, however, that in the presence of benzyltriethylammonium chloride, the pyridine system is involved in as many as ten reduction cycles during the complete conversion of the thione into the thiol. In the absence of the catalyst, the thione is recovered quantitatively from the reaction mixture. As yet, the procedure does not appear to have any synthetic utility. 

The first synthesis of the unstable pyrido[2,1 -ajisoindole system has been achieved by a cyclization of this type A-benzyl-2-bromo-pyridinium salts (336) on irradiation yield the pyrido[2,l-a]iso-... 

All these various pyrylium salts are converted by ammonia, or alkyl, arylkyl or aryl primary amines into the corresponding pyridinium salts.25 The probable reaction sequence is given in the illustrative example 1-benzyl-2,4,6-triphenyl-pyridinium tetrafluoroborate26 (Expt 8.31). 

Alkyl-substituted pyridinium salts also exchange benzylic hydrogens with deu-terated solvents with reversed regioselectivity in comparison with pyrylium salts because their y-anhydrobases are less stable than a-anhydrobases [57],... 

In studies of small molecules, it has been shown that only certain benzylic carbons are reactive enough to form pyridinium salts in refluxing pyridine and iodine (21). There are at least three modes of activating this carbon. 

In the first case [1] Y can be either a hydroxy, alkoxy or nitro group. The first two groups are important but variable constituents in coals and the last is probably minor or non-existent. The second active class of species are the alkyl-pyridines [2]. The final case [3] includes substituents on the benzyl carbon where X can be an ether or carbonyl functional group. The general mechanism of this reaction is most probably the base catalyzed iodination of the benzyl carbon with subsequent displacement of the iodide by the pyridine to form the pyridinium salt. In all three modes of activation, the single aromatic ring can be replaced with polycyclic rings. 

V-Benzyl and iV-Alkoxy Pyridinium Salts as Thermal and Photochemical Initiators for Cationic Polymerization... 

Cationic polymerizations induced by thermally and photochemically latent N-benzyl and IV-alkoxy pyridinium salts, respectively, are reviewed. IV-Benzyl pyridinium salts with a wide range of substituents of phenyl, benzylic carbon and pyridine moiety act as thermally latent catalysts to initiate the cationic polymerization of various monomers. Their initiation activities were evaluated with the emphasis on the structure-activity relationship. The mechanisms of photoinitiation by direct and indirect sensitization of IV-alkoxy pyridinium salts are presented. The indirect action can be based on electron transfer reactions between pyridinium salt and (a) photochemically generated free radicals, (b) photoexcited sensitizer, and (c) electron rich compounds in the photoexcited charge transfer complexes. IV-Alkoxy pyridinium salts also participate in ascorbate assisted redox reactions to generate reactive species capable of initiating cationic polymerization. The application of pyridinium salts to the synthesis of block copolymers of monomers polymerizable with different mechanisms are described. 

Grafting onto Polystyrene Possessing IV-Benzyl Pyridinium Salt... 

Table 2. H -NMR and IR spectral characteristics and melting points of various iV-benzyl pyridinium salts...

Benzyl pyridinum salts exhibit much better initiation activity than the aliphatic ammonium salts. In earlier investigations, on the basis of elemental analysis of the polymer obtained, the mechanism below was proposed for the initiation by pyridinium salts is shown in Scheme 3 [24]. 

The polymerization of BOE with N-benzyl pyridinium salts with various p-substituents in the pyridinum moiety was studied [26] in order to observe the... 

The position of the substituents on the pyridine moiety also affects the activity of the initiator. For example, p-MeO benzyl o-CN pyridinium salt 2k is 100 times more reactive than the corresponding p-CN derivative 2f. Thus, introduction of the o-cyano group into pyridine nucleo causes a large enhancement of the initiator activity. The reason for the activity enhancement by this modification can be accounted for by both electronic and steric effects of the o-cyano group of the pyridine group. In a typical comparison of the initiation activities of p-CN 2f and m-COOMe 21 substituted p-methoxy benzyl pyridinium salts in the polymerization of GPE, it was observed that p-substituted salt 2f is approximately 10 times more reactive than the corresponding o-substituted salt 21. In this case the steric effect of the substituents is negligible. A similar... 

Grafting onto Polystyrene Possessing N-Benzyl Pyridinium Salt Side Groups... 

In view of the fact that benzylic cations are responsible for the initiation, graft copolymerization is feasible when the polymer supported pyridinium salts are employed. Endo and co-workers [41] prepared vinyl monomers having pyridinium salt structure. In this case, p-chloromethyl styrene was used as the benzyl halide and the general synthetic strategy was followed. The polymerizable salt was then homo and copolymerized with styrene (Scheme 7). 

The homopolymer of the pyridinium salt monomer readily initiates the cationic polymerization of BOE upon heating to around 120 °C, to yield grafted copolymers. The copolymer with styrene similarly catalyzed the polymerization to form the corresponding grafted copolymers. The initiation activity of the copolymer in the polymerization was higher than that of the homopolymer but lower than that of the low-molar mass analogue, N-benzyl-p-cyanopyridinium hexafluoroantimonate. 

Another practical value of iV-benzyl pyridinium salts in synthetic applications was also demonstrated by Endo and co-workers [42] who utilized these salts as catalysts for acetalization of carbonyl compounds. 

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