Everything about Coupling Reaction totally explained
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's 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 don't 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, [Mo
3(CCH
3)
2(OAc)
6(H
2O)
3](CF
3SO
3)
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.
References
Further Information
Get more info on 'Coupling Reaction'.
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