Thiophenes benzannelation

Dimerization of one-end-blocked benzo[c]thiophene (334a-c) caused a red shift of the absorption maxima of ca. 70 nm. Compared to the parent a,a -dihexylsexithiophene (ca. 444 nm), the benzannelated sexithienyl system 336c exhibited a Xmax value at a higher wavelength (520 nm) which confirms the enhancement of conjugation through anellation. 

To achieve a further benzannelation, benzo(b)thiophen-7-carbonitrile (196 b) has been converted by a four-step sequence — hydrolysis by KOH in cellosolve (150°, 1 h), reduction of the acid by sodium bis(methoxyethoxy)aluminiumdi-hydride, action of thionyl chloride on the carbinol, and reaction of the chloromethyl compound with sodium cyanide in DMSO — in a 75% overall yield to the oily benzo(b)thiophen-7-acetonitrile (197), which finally afforded the naphtho(l,2-b)-thiophen-9-carbonitriles (198a, b) ... 

For the synthesis of dibenzofuran (201), dibenzothiophenes (202), and carbazoles (203) by benzannelation, in the same manner, the 3-acetonitrile of benzo(b)furan, benzo(b)thiophen, and of indole are used ... 

Direct arylations of benzo[fo]furans, benzo[b]thiophenes, and indoles displayed different regioselectivity patterns. The chemical behavior of benzo[b]furans and benzo[fo]thiophenes are rather similar to those of non-benzannelated furans and thiophenes, respectively (Table 9.4). 

Benzannelation of 2-substituted thiophenes, benzothiophenes, pyrroles, benzofurans, and indoles has been reported with the IPr ligand (l,3-bis-(2,6-diisopropylphenyl)-imidazol-2-ylidene) on gold(I) (Scheme... 

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