Pyridinium salts, benzylic carbons

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. 

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. 

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. 

The catalysts of this ring opening polymerisation reactions are pyridinium salts (for example N-benzyl pyridinium p-toluene sulfonate [75]), p-toluene sulfonic acid [75], stannium or titanium compounds [68-74] etc. Other cyclic polymerisable cyclic carbonates are ethylene carbonate and propylene carbonate [68-74]. 

Preparation.—A quaternary ammonium anion-exchange resin, in its carbonate form, is the reagent in a new and convenient procedure for the hydrolysis of primary, allyl, and benzyl halides to the corresponding alcohols. A shortened (two-stage) sequence for the conversion of primary alkylamines to alcohols mediated by pyrilium salts (c/. 2,113 4,138) is outlined in Scheme l treatment of the intermediate pyridinium salt (1) with sodium 2-hydroxymethylbenzoate (or thiobenzoate) leads directly to isolation of the alcohol, rather than the esters obtained when sodium acetate or sodium trifluoroacetate are employed. 

Chapman et al. (1974) found that protonated heterocydes including benzthiazolium bromide (102), possessing an alkyl substituent (e.g. benzyl) on the carbon adjacent to the positive nitrogen reacted with methylvinylketone to form adduct (103) which can be converted into fused pyridinium salts (104,105) (Scheme-29) (Chapanan et al. 1974). 

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