Thiopyrylium salts aromaticity

The aromatization of nonquaternized 2//-thiopyrans seems to be still exceptional as the conversions 59 — 126 or 86 to appropriate bis-thiopyrylium salt with Ph3CBF4 (85CL1119 81CC1143). On the other... 

In special cases aromatizations of thiopyrans to thiopyrylium salts were observed when P4S10 was used.105 A report6 of the isolation of 2-methyl-4//-thiopyran (3) after heating disodium 3-methylglutarate with at 180 to 250°C should be checked by making a comparison of the reaction product with an authentic sample of 3 prepared by another procedure.7... 

The agents used for oxidative aromatization of 2/f-thiopyrans were trityl perchlorate,42-155,156 tetrafluoroborate,267 and iodide.267 Thus 3,5-diphen-yl-2//-thiopyran (222) was found to aromatize either on its own to thiopyrylium salts 396 or by S-methylation to l-methyl-3,5-diphenylthiabenzene (398) via intermediate 397,267 as shown in Scheme 18. 

Agents Used for Aromatization of 4//-Thiopyrans to Thiopyrylium Salts... 

Di-te/Y-butyl-4//-thiopyran (363) as well as its 4-methyl derivative 243 were readily aromatized to thiopyrylium salts 403 using trityl tetra-fluoroborate.286 2,4,6-Triphenyl-4//-thiopyran (45) (R = Ph) was analogously converted to salts 404 by the action of methyl iodide, dimethyl sulfate, triethyloxonium tetrafluoroborate,39 perchloric acid,362 or oxygen in acetic acid.363... 

Charge-transfer complexes of thiopyrylium salts with aromatic hydrocarbons and olefins have been investigated. The n.m.r. spectra of thio- and seleno-pyrylium salts and their benzo- and thieno-derivatives have been examined. The results of further MO calculations have been advanced to support the earlier contention that J-orbitals are not significantly involved in bonding in thiopyrylium salts, despite the apparently contradictory evidence based on n.m.r. shifts. 

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... 

The photophysical properties of polyacene molecules depend markedly on the number of rings and the fluorescence behaviour of hexacene has now been compared with that of earlier members of the series. A similar comparison has been made of the photophysical properties of catacondensed aromatic poly-cycles. Fluorescence from an upper-excited singlet state has been described for benz[a]azulene derivatives while the fluorescence properties of some antiaromatic molecules have been described in detail. Several thiopyrylium and pyrylium salts have been studied and the effects of various substituents attached to the heterocycle have been examined in terms of the triplet yield. A full evaluation of the photophysical properties of 4-aminonaphthalimide, and its... 

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