Pyrylium salts properties

Recent important developments consist in the synthesis of the unsubstituted pyrylium cation by Klagcs and Trager, the preparation of pyrylocyanines by Wizinger, the development of simple syntheses for alkyl-substituted pyrylium salts by Balaban and Nenitzescu, Praill, Schroth and Fischer, Schmidt, and Dorofeenko, the discovery of a variety of reactions by Dimroth and Hafner, and the study of physical properties by Balaban. 

Trimethylpyrylium perchlorate is a very versatile and useful starting material. Thus its reaction with cyclopentadienyl-sodium has made 4,6,8-trimethylazulene 12 easily available for general studies of the properties of azulenes 18 and for the synthesis of related compounds.14 In addition, pyrylium salts are readily converted to a variety of pyridine derivatives 9 15 as well as to derivatives of nitrobenzene16 and phenol.9 17,18 It is clear that its value as a starting material is such that it is receiving wide use. 

A.T. Balaban et al, eds., Pyrylium Salts. Syntheses, Reactions and Physical Properties. Academic Press, New York, 1982. 

A. T. Balaban, A. Dinculescu, G. N. Dorofeeko, G. W. Fischer, A. V. Koblik, V. V. Metzheritskii and W. Schroth, Pyrylium Salts. Synthesis. Reactions and Physical Properties, in Adv. Heterocycl. Chem.. Suppl 2, 1982. 

Many aryl-substituted pyrylium salts are intensely fluorescent. It is possible to predict these spectral properties by a consideration of the shape of the molecule, the nature of substituents and the length of the ir-electron system (75MI22203). Pyrylium salts have been used as Q-switches for neodymium and ruby lasers in acetonitrile (68MI22200). 

The properties of 1,3-oxazinium salts are similar to those of pyrylium salts.4194,219,255-259 1,3-Oxazinium salts are intermediates for the synthesis of derivatives of pyrazole, isoxazole, pyridine, pyrimidine, quinoline, isoquinoline, chromene, benzopyrylium salts, butadiene, etc.4... 

Benzo[c]Pyrylium Salts Syntheses, Reactions, and Physical Properties... 

Kuznetsov, E. V., Shcherbakova, I. V., Balaban, A. T., Benzo[c]pyrylium Salts Syntheses, Reactions, and Physical Properties, 50, 157. 

Bcnzo[/)]furan and derivatives, recent advances in chemistry of, Part 1, occurrence and synthesis, 18, 337 Benzo[c]furans, 26, 135 recent advances in the chemistry of, and related compounds, 73, 1 Benzofuroxans, 10, 1 29, 251 2//-l-Benzopyrans (chrom-3-enes), 18, 159 Bcnzofc] pyrylium salts syntheses, reactions, and physical properties, 50, 157 1,2-and 2,1-Benzothiazines and related compounds, 28, 73... 

All physical properties of pyrylium salts (unsubstituted or substituted with alkyl and/or aryl groups) prove the aromaticity of these cations vibrational spectra [66], mass-spectral fragmentations [67], magnetic properties [68-70], and electronic absorption spectra [71]. It should be mentioned that there is a close similarity between the electronic absorption bands of pyrylium salts and those of benzene, easily recognized by the marked bathochromic effect of substituents in y-position of pyrylium salts on one of these bands. Two-photon absorption spectra of 2,4,6-triarylpyrylium cations [72] may be used in optical data storage, lasing, and photodynamic therapy. 

Balaban AT, Dinculescu A, Dorofeenko GN, Fischer GW, Koblik AV, Mezheritskii W, Schroth W (1982) Pyrylium salts, Syntheses, reactions, and physical properties, Academic, New York. 

Thiopyrylium salts not only absorb in the visible region but also exhibit considerable fluorescence emission such that they have value as photosensitizers. The fluorescence properties of several thiopyrylium salts have been studied and are collated in Table 12. Although the fluorescence (Amax) is red-shifted by 30-60 nm compared with the corresponding pyrylium salts, the quantum yields ((pf) are appreciably lower, attributed to the higher efficiency of the forbidden singlet-triplet transition because intersystem crossing is efficient <1995BCJ2791>. 

Pyrrolopyridines, 9, 27 Pyrylium salts syntheses, 10, 241 syntheses, reactions, and physical properties, S2... 

A. V. Koblik, V. V. Mezheritskii, and W. Schroth, Pyrylium salts. Syntheses, reactions and physical properties, in "Advances in Heterocyclic Chemistry" (A. R. Katritzky, ed.), Suppl. Vol. 2, Academic Press, New York (1982). 

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