Coupling reaction

A coupling reaction or oxidative coupling in organic chemistry is a catch-all for a range of reactions in Organometallic chemistry where two hydrocarbon radicals are coupled with the aid of a metal containing catalyst.

Coupling reactions should be divided into two main classes, there are the cross couplings in which two different molecules react to form one new molecule. For example the nickel chloride catalyzed reaction of an aryl magnesium halide with an aryl halide to form a biaryl.

An example of the other type of coupling (homocoupling) would be a Ullmann reaction, this is the reaction of copper metal with two molecules of an aryl halide to form a biaryl. The Ullmann reaction often requires very high temperatures, and has partly been replaced in synthetic chemistry by palladium based reactions.

Many coupling reactions involve phenols. BINOL is the C-C coupling reaction product of 2-naphthol with copper(II) chloride and 2,6-xylenol dimerises as well with iodosobenzene diacetate.

A common metal in this type of chemistry is palladium often added in the form of tetrakis(triphenylphosphine)palladium(0). This is an air sensitive compound which is very good for coupling unsaturated halogen compounds with organometallics such as tributyltin hydride.

While many coupling reactions involve reagents that are extremely susceptible to presence of water or oxygen, it is unreasonable to assume that all coupling reactions need to be performed with strict exclusion of water. It is possible to perform palladium based coupling reactions in aqueous solutions using the water soluble sulfonated phosphines made by the reaction of triphenyl phosphine with sulfuric acid. In general the oxygen in the air is more able to disrupt coupling reactions, this is because many of these reactions occur via unsaturated metal complexes which do not have 18 valence electrons. For example in nickel and palladium cross couplings a zerovalent complex with two vacant sites (or labile ligands) reacts with the carbon halogen bond to form a metal halogen and a metal carbon bond. Such a zerovalent complex with labile ligands or empty coordination sites is normally very reactive towards oxygen.

Coupling types
Coupling reactions include (not exhaustive):

Miscellaneous reactions
In one study an unusual coupling reaction was described in which an organomolybdenum compound, [Mo3(CCH3)2(OAc)6(H2O)3](CF3SO3)2 not only sat on a shelf for 30 years without any sign of degradation but also decomposed in water to generate 2-butyne which is the coupling adduct of its two ethylidyne ligands. This according to the researchers opens another way for aqueous organometallic chemistry.

One method for palladium-catalyzed cross coupling reactions of aryl halides with fluorinated arenes, involves DMA. It is unusual in that it involves C-H functionalisation at an electron deficient arene.