Ullmann-type homocoupling

Hennings, D. D. Iwama, T. Rawal, V. H. Palladium-catalyzed (Ullmann-type) homocoupling of aryl halides a convenient and general synthesis of symmetrical biaryls via inter- and intramolecular coupling reactions. Org. Lett. 1999, 1, 1205-1208. 

The Ullmann-type reaction that is homocoupling of aryl or vinyl halides is conveniently mediated by copper at high temperature. The copper powder serves as a zerovalent metal. The classical Ullmann reaction reported in 1901 has long been employed by chemists to generate a carbon-carbon bond between two aromatic nuclei. 

Bromothiophene-5-carboxaldehyde (34) was converted to bisthiophene 35 via a Pd-catalyzed reductive homocoupling [40]. The reaction was also applicable to chloro- and iodothiophenes bearing many functional groups. In addition, an Ullmann-type reductive homocoupling of 2-iodothiophene utilizing catalytic Pd/C and three equivalents of Zn provided a practical entry to bisthiophene in 64% yield [41]. 

---