2:4- „ „ „ selenophene

Dedeuteriation of selenophene by sulfuric acid in aqueous methanol at 20°C gave the approximate partial rate factors 2.1 x 108 and 4 x 104 for the 2- and 3-positions, respectively [72JPR(314)603]. Under these conditions, the Hammett p factor for the reaction was determined as approximately -8.4. Direct measurement of the exchange rates relative to that in benzene was not possible, so there may be some error in the corresponding cr+ values of -0.95 and -0.53. These cr+ values indicate that selenophene is more reactive than thiophene, as do other studies described below (Section 7.B). 

Further methylation, followed by the action of sodium ethoxide in ethanol, leads to substituted cyclopentanes. 

The reaction of 1,4-dibromo- or 1,4-diiodoalkanes with sodium or potassium sulfide provides thiolanes in good yield  

The ring transformation of tetrahydrofurans into thiolanes proceeds with hydrogen sulfide in the presence of aluminium oxide at 400°C  

I Thiolane is a colourless, water-insoluble liquid of bp 121°C. It has a distinctive odour similar to town gas. In combination with other compounds, it is responsible for the typical odour of 

Thiolane 1,1-dioxide, known by the trivial name sulfolane, is obtained industrially by catalytic hydrogenation of 3-sulfolene. Sulfolane, colourless crystals, mp 27.5°C, bp 285°C, is water soluble. Sulfolane is a polar aprotic solvent and is used for the extraction of sulfur compounds from industrial gases and for the extraction of aromatic substances from pyrolysis fractions. It also serves as a solvent for cellulose acetate, polyvinyl chloride, polystyrene, and polyacrylonitrile. 

The selenophene molecule is planar. The chemical shifts in the NMR spectra are found in the region typical for aromatic compounds. 

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In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, selenophene[233,234], and cyclobutadiene iron carbonyl complexpSS] react with alkenes to give vinyl heterocydes. The ease of the reaction of styrene with sub.stituted benzenes to give stilbene derivatives 260 increases in the order benzene < naphthalene < ferrocene < furan. The effect of substituents in this reaction is similar to that in the electrophilic aromatic substitution reactions[236]. 

Fig. 4. Simple organic ring compounds containing selenium selenophene [288-05-1] (1), selenolo[2,3- ]selenopliene [250-85-1] (2), seleno[3,2- ]selenopliene...

With its sextet of 7T electrons, thiophene possesses the typical aromatic character of benzene and other similarly related heterocycles. Decreasing orders of aromaticity have been suggested to reflect the strength of this aromatic character benzene > thiophene > pyrrole > furan (9) and benzene > thiophene > selenophene > teUurophene > fuian (10). 

The much less sophisticated PPP approximation has been shown to well reproduce the electronic spectral features not only of the monocyclic furan, pyrrole, thiophene, selenophene and tellurophene but also many of the benzo fused derivatives as well (79MI30101, 68JPC3975, 68MI30100). 

Pyrrole Furan Thiophene Selenophene Tellurophene Cyclopentadiene... 

The direct /c-3,h-3 coupling constants decrease regularly along the series O > NH > S > Se > Te, whereas the /c-z.h-z values show a peculiar behavior with the value for selenophene... 

In summary, most of the presently available criteria point to an order of decreasing aromaticity of benzene > thiophene > selenophene pyrrole > tellurophene > furan. 

Substituent Pyrrole Percent of syn form (51) Furan Thiophene Selenophene Tellurophene Phase... 

Mercapto derivatives of furan, thiophene, selenophene (77ACS(B)198) and pyrrole (72AJC985) all exist predominantly in the thiol form. 2-Mercaptobenzothiophene is also a thiol (70JCS(C)243i) whereas 2-mercaptoindole is mainly indoline-2-thione (89) (69CPB550). The finely balanced nature of this system is indicated by the fact that a 3-aryl, but not a 3-alkyl, substituent will stabilize the 2-thiol form, whereas for 3-aryl-fV-methyl derivatives the 2-thione tautomer is preferred (71CC836). 

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