Thiopyrylium

Fig. 4. Dyes for WORM media (a) hydroxy squaryhum (9,10) (b) SQS = squaryhum core with thiopyrylium end group (11) (c) TPMP ...

Thiopyran and thiopyrylium salts synthesis and properties 98MI18. 

Dibenzo[/>,rf]thiopyrylium tetrafluoroborate (1) reacts with ethyl lithiodiazoacetate at — 120 C to form the diazo compound 2 which, with dimeric ( 3-allyl)chloropalIadium at 20 C, gives ethyl dibenzo[ ,d]thiepin-6-carboxylate (3), via a carbene intermediate.5 Compound 3 is quite stable the ethoxycarbonyl group can be hydrolyzed by alkali and decarboxylated to give the corresponding parent compound 4 in good yield. 

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... 

A number of examples of the typical thiopyran transformations to corresponding thiopyrylium ions and dihydro or tetrahydro derivatives have been described in the last decade. Similar behavior was found with a selenopyran. 

MO calculations were performed rarely for thiopyrans except for an MNDO study of 2 [84ZN(A)267], Charge distribution and orbital interaction concepts were explored in an interpretation of model reactions of thiopyrylium ions with azides giving 68 and the corresponding 3,5-unsub-stituted thiopyrans (84T3549) as well as for the equilibria between 1 and 2 or 167 and 168, respectively (92JOC4431). 

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... 

Thiopyrylium ions have been shown to behave as effective dienophiles. Shimizu and co-workers demonstrated that 2-benzothiopyrylium salts undergo [4+2+]-type cationic polar cycloadditions with various 1,3-dienes yield benzo-fused bicyclic sulfonium salts <1994J(P1)3129> (see Section 8.32.2.1.2 of CHEC-II(1996)). However, in this study the authors observed that cycloaddition of 2-benzothiopyrylium salt 40 with 2,3-dimethylbuta-l,3-diene 41a with prolonged reaction times caused a decrease in yield of the cycloadduct 22a (see entries 1 and 2 in Table 5 and Equation 10). In fact, stirring of the reaction mixture for 30 h did not afford the cycloadduct at all, see entry 3, but gave an undetermined complex mixture. 

N, P, S, or C, is thermodynamically and kinetically favored, yielding pyridine, pyridinium, phosphaben-zene, thiopyrylium, or benzenoid rings. Ring-opening occurs with alkali cyanides ring contraction to... 

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