Pyridinium salts, alkoxy

Perhaps more general are the transformations of pyridinium salts 349 (R = H, Me, or Br) into the 1-formylcarbazoles 350 in aqueous base. ° Several 1-hydroxy- or l-alkoxy-4-alkylcarbazoles (351) were prepared via... 

The highly nucleophilic 3,4-dimethoxyfuran, when treated with arene diazonium salts in aqueous pyridine, undergoes 1,4-addition to afford pyridinium salts (51). The pyridine is easily displaced by nucleophiles including alcohols and phenols, thereby affording 5-alkoxy or 5-phenoxy derivatives of 3,4-dimethoxyfuran-2(5H)-one (52 Scheme 19) (78HCA1033). 

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. 

Reaction of pyridine-N-oxides with triethyloxonium salt in methylene chloride or chloroform gives directly AT-alkoxy pyridinium salts with high yield. Quinolinium salts can also be prepared from the corresponding N-oxides [46], The synthetic procedure does not require the anion exchange reaction since triethyl oxonium salt possesses a non-nucleophilic counter anion, i.e. PFg ... 

Upon photolysis, pyridinium salts undetgo cleavage of the carbon-nitrogen bond to form pyridinium radical cation and alkoxy radical. The nitrogen centered radical cations were spectroscopically detected by laser flash photolysis... 

Fig. 2. Comparision of absorption maxima of IV-alkoxy pyridinium salts with the emission lines of mercury...

Crivello and Lam [69] have reported that the diaryliodonium salt-ascorbate redox system readily initiates the cationic polymerization of appropriate monomers. N-Alkoxy pyridinium salts were also shown [70] to participate in this redox process. The polymerization mechanism depicted below is quite similar to that described for the iodonium salts (Scheme 17). 

The use of iV-alkoxy pyridinium salts is not limited to cationic polymerization. Since, in addition to cationic species, ethoxy radicals are also formed upon direct and sensitized irradiation of pyridinium salts (see above), pyridinium salt based photoinitiating systems may be used to initiate the polymerization of vinyl monomers that are prone to free radical polymerization. Kayaman et al. [71] recently polymerized mono- and bi-functional acrylate monomers by photosensitization of pyridinium salts. It therefore appears that pyridinium salts can promote both cationic and free radical polymerization and are, thus, eminently suitable for use in hybrid systems. 

Synthesis of Block Copolymers by Using N-Alkoxy Pyridinium Salts... 

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