A coupling reaction in organic chemistry refers to a variety of reactions where two fragments are joined together. Often such reactions require the aid of a metal catalyst. In one important reaction type, a main group organometallic compound of the type R-M (R = organic fragment, M = main group center) reacts with an organic halide of the type R'-X with formation of a new carbon-carbon bond in the product R-R'. The most common type of coupling reaction is the cross coupling reaction. Richard F. Heck, Ei-ichi Negishi, and Akira Suzuki were awarded the 2010 Nobel Prize in Chemistry for developing palladium-catalyzed cross coupling reactions. Broadly speaking, two types of coupling reactions are recognized: Homocouplings joining two identical partners. The product is symmetrical Heterocouplings joining two different partners. These reactions are also called cross-coupling reactions. The product is unsymmetrical, . Homo-coupling types Coupling reactions are illustrated by the Ullmann reaction: Cross-coupling types Applications Coupling reactions are routinely employed in the preparation of pharmaceuticals. Conjugated polymers are prepared using this technology as well. References Organometallic chemistry Carbon-carbon bond forming reactions Catalysis