Abstract
The arylation of 5-halogenothiophene-2-carbonitrile and methyl 5-halogenothiophene-2-carboxylate by a photochemical process was investigated. Whereas 5-bromothiophene-2-carbonitrile ( 3) is completely unreactive, the corresponding iodine derivative ( 4) furnishes the dehalogenation product. In contrast, methyl 5-iodothiophene-2-carboxylate ( 6) gives the corresponding aryl and heteroaryl derivatives in good yields on irradiation in the presence of various aromatic substrates (benzene, p-xylene, naphthalene, thiophene, 2-bromothiophene and 2-chlorothiophene). The different reactivities of compounds 3, 4 and 6 can be explained on the basis of the proposed mechanism which involves the formation of an exciplex: compounds 3 and 4 do not give the arylation products probably because of their low intersystem crossing efficiency; the different behaviour of compounds 3 and 4 can be explained by considering the triplet energy of the molecules. An application of this conversion to the synthesis of a naturally occurring bithiophene isolated from Arctium lappa, 5′-(1-propynyl)-2,2′-bithiophene-5-carboxylic acid ( 13), is reported.
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