2-Iodothiophene, reaction with terminal alkynes

A novel catalytic reaction of 3-iodothiophene-2-carboxylic acid with terminal alkynes leading to 4-alkynylthieno[2,3- ]-pyran-7-ones has been described <2006TL83>. This extension of the Sonogashira coupling occurs under Pd-Cu-catalysis (Equation 28). The reaction seems to proceed through an initial Sonogashira pathway to 3-( 1 -alkynyl)thiophene-2-carboxylic acid, which then reacts with a Pd(ll) species formed by insertion of a Pd(0) complex into the C-H bond of the second molecule of alkyne. Both Pd- and Cu-catalysts played cmcial roles, since the reaction failed if either was omitted. 

Palladium/copper catalyzed reactions of 3-iodothiophene-2-carboxylic acid 48 with two equivalents of a terminal alkyne gives selective access to 5-substituted 4-alkynylthieno[2,3-c]pyran-7-ones, for example the system 49 <06TL83, 06T9554>. 

Halofuran and halothiophene derivatives undergo photochemical reactions with arylalkenes and arylalkynes and with benzo[6]furan513,514. With the arylalkenes and aryl-alkynes, heteroarylation takes place at the terminal alkene or alkyne carbon atom, while benzo[6]furan is substituted at position 2. The experimental results are interpreted in terms of solvent-separated or contact radical ion pairs. Iodothiophene and iodofuran derivatives can also be used to synthesize derivatives of benzimidazole by means of photochemical coupling515. The reaction of 5-iodothiophene-2-carboxaldehyde (157) with benzimidazole (158) giving the coupling product 159 is illustrated in equation 131. 

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