Thiophenediazonium salts

Very little work has been done on fluoro derivatives of thiophenes. 2-Fluorothiophene was obtained in low yield from treatment of 2-iodothiophene with arsenic trifluoride. The action of fluoroboric acid on thiophenediazonium salts was unsuccessful. It may be useful for the preparation of 4-, 5-, 6- or 7-fluorobenzo[6]thiophenes from the appropriate amines. However, these are more conveniently prepared from fluorine-substituted benzenethiols by ring-closure reactions. For example 4,5,6,7-tetrafluorobenzo[6 Jthiophene was obtained by decarboxylation of the corresponding 2,3-dicarboxylic acid (equation 99) prepared by condensation of pentafluorobenzenethiol with diethyl acetylenedicarboxylate (Section 3.15.3.4.1). 2-Fluorothiophene has been prepared from 2-thienyllithium using perchloryl fluoride, and 2-fluorobenzo[ Jthiophene from the 2-lithio derivative in a similar manner (Section 3.14.3.9.1). 

Palladium-Catalyzed Cross-Coupling via Thiophenediazonium Salts... 

Thiophenediazonium salts 163 can be utiUzed as electrophilic coupling partners for differently functionaUzed alkenes 164 via a Matsuda-Heck (Scheme 64, left) as well as various potassium aryltrifluoroborates via Suzuki-Miyaura (Scheme 64, right) cross-coupling reactions giving rise to 3-substituted thiophenes 165 and 166... 

Scheme 64 Palladium-catalyzed cross-coupling via thiophenediazonium salts [377]...

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