Triarylpyrylium salts

Triarylpyrylium salts react with hydrazine to give 4//-l,2-diazepines 472-1°s,ioo vja g unstable intermediates 3.110 The reaction fails with pyrylium salts containing alkyl groups in positions 2 and 6, with the exception of 2,4,6-tri-tert-butylpyrylium perchlorate. In some cases it is advantageous to use a thiapyrylium salt in place of the pyrylium salt. Selected examples are given. 

Compounds of this type (298) have been made from 4-aminothiophenol and 2,4,6-triarylpyrylium salts. "... 

This method is very convenient and is often the choice for the preparation of 2,4,6-triarylpyrylium salts. Of course, the substituents at C-2 and C-6 are identical. A wide range of substituted acetophenones and benzaldehydes has been used in this route and a choice of both catalyst and solvent is available. Once again, phenylacetylene may replace acetophenone. 

The fact that pyrylium salts easily form anhydrobases in water over a large range of pH values, and pseudobases at pH >5, makes such salts incompatible with biomedical applications, but the highly fluorescent 2,4,6-triarylpyrylium salts with phenyl or para-anisyl groups have various applications in reprographic industries. On the other hand, chalcogenopyrylium cations with other atoms (S, Se, Te) are less... 

Interest in the use of pyrylium salts in synthesis continues. 2-Methylpyridinium salts are converted into the 2-arylpyridinium compound on reaction with 2,4,6-triarylpyrylium salts in the presence of base. The methylpyridinium salt is deprotonated to the anhydrobase which behaves as an enamine, attacking the pyrylium salt at C-2, promoting a ring opening - ring closure sequence which culminates in the effective arylation of the pyridinium salt. 

T. Zimmermann and M. Pink, Ring transformations of heterocyclic compounds. XII. Novel spiroindolines via ring transformation of 2,4,6-triarylpyrylium salts with 2-methyleneindolines, J. Prakt. Chem. Chem. Zeit., 337, 368-374 (1995). 

Molecular oxygen inserts into substituted cyclopentadienes under acidic conditions producing pyrylium salts a hydroperoxide rearrangement is proposed <05JOC5768>. In a further development of the use of benzotriazole (Bt) in synthesis, P-lithiation of the vinyl ether 16 and quenching of the anion with chalcone affords an enol ether. Cyclisation with PCI5 produces 2,4,6-triarylpyrylium salts <05S245>. 

Pyrans. - Highly fluorinated 27/-pyrans have been synthesized by low-temperature condensation of the perfluoralkene (18) with an alkyl aryl ketone (19 R = CN, COCF3, or COCF2CF3). 2,4,6-Triarylpyrylium salts are converted, in good yield, into 2-alkoxy-2,4,6-triaryl-2/f-pyrans or 2-amino-2,4,5-triaryl-2//-pyrans on treatment with sodium alkoxides or... 

Systems Containing Two Different Heteroatoms 6.1 Oxazepines.- 2-Substituted triaryl-1,2-oxazepinium perchlorates, the first examples of seven-membered heterocyclic cations with eight n-electrons, have been prepared by the reaction of 2,4,6-triarylpyrylium salts with nitrones, or N-arylhydroxylamines,... 

Also, a European patent was applied for that describes cationic photoinitiators based on 2,4,6-triarylpyrylium salts with nonnucleophilic anions and electron donors. It is claimed that the photoinitiators rapidly photo crosslink mixtures of cycloaliphatic epoxides. Micro encapsulation of the salts in polystyrene was found to increase thermal and photo stability of these photoinitiators.The encapsulation material can be dissolved in monomers when polymerization is desired. 

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