Selenophenes—

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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