Thiopyrylium salts, 4-

The synthesis of 2,4,6-trisubstituted thiopyrylium salts has been modified. Four new selenaphenanthrenium salts have been prepared. 3,5-Diphenylthio-pyrylium salts have been synthesized.  

Reduction of thiopyrylium salts to 2H- and 4/f-thiopyrans by trichlorosilane and by lithium aluminium hydride has been reported. Addition of organo-metallic reagents to thiopyrylium and selenopyrylium salts, which was (see Volume 2, p. 387) believed to yield thia- and selena-benzenes, has now been shown to give oligomers. Some authentic thia- and selena-benzenes have been prepared. Nucleophilic addition to, and nitration of, 9-phenyl-selenoxanthylium salts (28) has been studied. 

A series of interesting papers deals with the dimers of thiopyrylium 3-oxides (29) and their benzologues. Their structures and stereochemistry are highly dependent on the substitution pattern of the monomers (29). Dimers linked through the 2,6- and 2, 6 -carbons, and the 2,6- and 2, 4 -carbons, have been isolated, and their formation may be envisaged as the coupling of the appropriate resonance hybrids (30)—(32). 

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 

Methine salts, based on the thiopyran system, are included in an extensive theoretical and spectroscopic investigation of sulphur-containing dyestuffs.  

Ring-opening reactions occur when thiopyrylium fluoroborate is treated 

A further example has been reported of the formation of a benzene derivative (44) from the 2,4,6-triphenylthiopyrylium cation and a reactive carbanion, in this case dimethyloxosulphonium methylide. 

4fT-thiopyran-4-one by irradiation in acetonitrile (in the absence of oxygen). 2,6-Bisalkylthio-3,5-diphenyl-4 T-thiopyran-4-ones (37) are converted photochemically into cyclopentadienones (38) and sulphur. 2,6-Diphenyl-4 -thiopyrans undergo a disproportionation reaction in the presence of strong acids analogous to those previously reported for 2fi thiopyrans. The products are thiopyrylium salts and tetrahydro-thiopyrans.  

Adducts of type (39 X = CO-Ph or C02Me), containing the AH-thiopyran structure, are formed from l,2-dithiole-3-thiones and acetylenes  

Two groups of workers have observed that triphenylthiopyrylium salts (43) react with active-methylene compounds, in the presence of bases, to give tetrasubstituted benzenes, for example, triphenylbenzonitrile (46). 

These evidently complicated reactions must involve initial attack by a carbanion at the 2-position of the thiopyrylium ring, followed by ring-opening and elimination steps. 

Reduction of thiopyrylium iodide with zinc, and treatment of the product with triphenylmethyl fluoroborate, perchlorate, or iodide, yields the bisthiopyrylium salts (50 X = BF4, CIO4, or I), which are relatively stable in air but decompose rapidly in contact with water. 4-4-Coupling in the dication is indicated by the very simple n.m.r. spectrum. The dication is reduced with zinc in acetonitrile to the radical cation (51), the e.s.r. 

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Thiopyran and thiopyrylium salts synthesis and properties 98MI18. 

Thiepins synthesized via thiopyrylium salts (see Section 2.1.3.4.) normally cannot be isolated due to their rapid loss of sulfur, especially when 2,7-diisopropyl substituents are present, e.g. 3.79 In contrast, when substituted with tm-butyl groups, which stabilize thiepins, extrusion of sulfur only occurs in refluxing toluene.80 81... 

Addition reactions of thiopyrylium salts with nucleophiles (83AHC145, Section IV,B) have widely been used for the preparation of various thiopyrans in the last decade (92MI3). On the other hand, the application of catalytic processes still seems to be rare. 

Two principle approaches from thiopyrylium salts to thiopyrans accompanied by C-substitution have been found, e.g., the one-step additions of C-nucleophiles or the two-step procedures involving primary conversions of the salts to nucleophilic intermediates followed by attacks with appropriate electrophiles. 

A general preparation of 2-acetonyl-2,4,6-triaryl-2//-thiopyrans 58 was discovered in the reaction of corresponding 2,4,6-triarylthiopyrylium perchlorates with ethanolic acetone catalyzed with various dialkylammonium salts (86GEP235455, 86JPR573). This preparative procedure was successfully extended to cyclohexane- and cyclopentane- 1,2-diones as the carbonyl components (89JPR853 90GEP280324). The action of bases on thiopyrylium salts may caused their dimerization to thiopyranyl derivatives under suitable conditions (89KGS479). 

The equilibria between 27/-thiopyrans 63 (R = 4-MeOC6H4 R = Ph, 4-MeOC6H4) and their corresponding thiopyrylium salts were utilized to replace CI04, BF4-, Cl-, and CF3C02" ions by other counterions in the patent literature (88URP1447824). 

Any preparative utilization of the addition reactions of thiopyrylium salts with amines appears to be significantly dependent on the nucleophili-... 

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... 

Ultraviolet spectrophotommetry was used to follow the course of the photoreduction of thiopyrylium salts to their corresponding 4H-thiopyranyl radicals (85BCJ2600) as well as the kinetics of thiopyrylium salt transformations to thiopyrans with various nucleophiles (84JA7082, 84JOC1806 86JA3409). 

The synthetic utility of the ring expansion reaction was demonstrated by its application to the synthesis of thermolabile thiepins. When the diazo compound (66) obtained from benzo[c]thiopyrylium salt 65 was treated with palladium catalyst under the same conditions as in the case of 63, the product isolated was ethyl 2-naphthoate (68)48). The plausible reaction pathway is one comprising i) decomposition of 66 to the corresponding carbene intermediate, ii) ring expansion to the... 

Triphenylpyrylium tetrafluoroborate is a versatile and useful stable starting material. Its reaction with nitromethane under basic conditions has made 2,4,6-triphenylnitrobenzene easily available. In addition, pyrylium salts are readily converted to a variety of pyridine derivatives i i . 20 including alkyl- and arylpyridinium salts, to thiopyrylium salts," and to substituted azulenes. ... 

In contrast to the ease of reaction of ring nitrogen atoms in 77-deficient six-membered heterocycles with electrophiles, electrophilic heteroaromatic substitution at carbon of the unsubstituted compounds proceeds only under very drastic conditions and yields of products are usually very poor. This is also true with pyridinium, pyrylium and thiopyrylium salts,... 

The oxygen and sulfur heterocycles, including pyrylium and thiopyrylium salts, are readily reduced catalytically. Moreover, it is usually possible to control the degree of reduction both pyran-2- and -4-ones can readily be reduced to dihydro derivatives, in the former case the 5,6-dihydro compound. Examples of catalytic reduction of the oxo substituents to either the corresponding alcohol or to the methylene group are known, but these reactions normally occur only under pressure. 

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... 

Thiopyrylium salts are, in general, capable of accepting nucleophiles by addition at positions 2, 4, or 6 to give the corresponding 2H- and/or 4H-... 

The widely used reduction of thiopyrylium salts to thiopyrans has been accomplished with complex hydrides. Alternative reductions with ethanol-amine mixtures or with trichlorosilane are rarely used. 

Tetrasubstituted thiopyrylium salts also provide a variety of products. Thus symmetrically substituted 2,3,5,6-tetraphenylthiopyrylium ion with UA1H4 or the condensed tricyclic salt with NaBH4 gave exclusively 4H-thiopyrans 226a40 or 49 and 228.269,270 Dithieno-4//-thiopyran 16 was obtained similarly from the appropriate thiopyrylium pentachlorostannate.130 On the other hand, asymmetrically substituted substrates 229 with LiAlH4 afforded 2//-thiopyran 230 in yields of 60 to 73% accompanied by 13 to 14% of isomers 231 for the 4-phenyl derivatives.271... 

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