Thiopyrylium, selenopyrylium, and

Doddi, G., Ercolani, G., Thiopyrylium, Selenopyrylium, and Telluropyrylium Salts, 60, 65. 

Thiopyrylium, selenopyrylium, and telluropyrylium salts, 60, 65 Thioureas in synthesis of heterocycles,... 

Thiopyrylium, selenopyrylium, and telluropyrylium salts are reviewed by G. Doddi (Rome, Italy) and G. Ercolani (Catania, Italy). Whereas the chemistry of the analogous pyrylium salts was the subject of a special supplementary volume in our series in 1982, no exhaustive previous review of the other chalcogenopyrylium salts has been available. 

Many further examples have been given of the preparation of mixtures of thiopyrylium salts and the corresponding tetrahydrothiopyrans, by the action of hydrogen sulphide and various acids on 1,5-diketones. Selenopyrylium salts have also been prepared by this method. 2-Amino-6-methylthio-thiopyrylium salts, obtained by methylation of the related 2H-thiopyran-2-thiones, react with aromatic amines to give 2-arylimines (42). 2-Amino-6-arylthiopyrylium salts have also been prepared, from 3-mercaptovinylacrylonitriles, by an extension of a reaction previously used for the synthesis of 2-aminopyrylium and pyridinium salts. Several theoretical treatments have appeared. Non-empirical calculations suggest that the 3d orbitals are used only to a trivial extent," but, in another study, the conclusion is reached that involvement of the d-orbitals may be important, and that contributions of forms of type (43) to the hybrid... 

As expected on the basis of the higher stability of sulfonium salts as compared with oxonium salts, thiopyrylium cations are more stable and less reactive than pyrylium cations hydride-accepting ability decreases in the order pyrylium> selenopyrylium> thiopyrylium [95], 2,4,6-Triphenyl-thiopyrylium salts react with ammonia and primary alkylamines forming the corresponding pyridine and pyridinium salts, respectively, but they do not react with aniline or its derivatives [96-99], As described below, the Se- or Te-analogs are less stable than the thiopyrylium salts. 

Pseudobase formation by the pyrylium, thiopyrylium, and selenopyrylium cations is complicated by ring-opening reactions (Section V,A) which preclude a simple direct measurement of pKR + for these cations. In a kinetic study, Williams80 has found the ring opening of 2,4,6-trisubstituted pyrylium cations 23 to be controlled by pKa values in the range of 5.0-6.7 for R2,... 

Pyrylium (1) and its chalcogen analogs, thiopyrylium (2), selenopyrylium (3), and telluropyrylium (4), are the parent structures of an important class of six-membered heteroaromatic cations. 

UV Spectral Data of Pyrylium, Thiopyrylium, and Selenopyrylium, Perchlorates in Acetonitrile" ... 

The chemical shift of the methyl group in methyl-substituted pyrylium, thiopyrylium, and selenopyrylium cations is reported in Table V. Whereas for pyrylium and thiopyrylium the order of chemical shift of methyl group is a > y > /3, in the case of selenopyrylium the order is a > 13 > y. The presence of additional methyl groups causes only small... 

In phenyl-substituted pyrylium ions, ortho protons of a- and y-phenyl groups resonate at lower fields than meta and para protons in thiopyrylium derivatives the separation between the ortho signals and the meta and para ones is lower and not always appreciable in selenopyrylium derivatives it is decidedly not significant (74UKZ287). 

C Chemical Shifts" (ppm). C, H Coupling Constants (Hz), One- and Three-Bond C, C Coupling Constants (Hz) of Pyrylium, Thiopyrylium, and Selenopyrylium Fluoroborates in CDiCN ... 

Disubstituted telluropyrylium cations 20 and 28 in pyridine with triphenylphosphine under aerobic conditions gave an oxidative dimerization to produce l,l-dioxo(telluropyranylidene)telluropyrans 272 and 273, respectively (87JOC2123). The reaction is also successful with the exclusion of oxygen if triphenylphosphine oxide is substituted for triphenylphosphine. The oxidative dimerization could not be extended to thiopyrylium cation 26 and selenopyrylium cation 27, which gave the corresponding bipyranylidenes 14 (Z = S, Se, R = Bu ). 

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