What is the Ullmann Reaction?
The Ullmann reaction (also known as Ullmann coupling) is an organic named reaction that involves the coupling of two aryl halides in the presence of copper to yield a biaryl as the product. This coupling reaction is named after the German chemist Fritz Ullmann. The general format of the Ullmann reaction is illustrated below.
An illustration of the Ullmann reaction undergone by ortho-chloronitrobenzene in the presence of a copper-bronze alloy at a temperature of approximately 500 K to afford 2,2’-dinitrobiphenyl is provided below.
It can be noted that the Ullmann reaction is known to have relatively low yields, which is why palladium-catalyzed coupling reactions (such as the Heck reaction) are preferred over this reaction. Also, the reaction conditions for the Ullmann reaction are quite harsh.
Ullmann Reaction Mechanism
The mechanism of the Ullmann reaction involves the formation of an active copper(I) species upon the introduction of the aryl halide to an excess of metallic copper under relatively high temperatures (>200oC).
This copper(I) species undergoes further oxidative addition with another haloarene molecule, linking the two molecules (as illustrated below).
In the final step of the Ullmann reaction mechanism, the copper compound formed by the two aryl halide molecules undergoes reductive elimination, resulting in the formation of a new carbon-carbon bond between the two aryl compounds (as illustrated below).
Applications of the Ullmann Reaction
- Biphenylenes can be obtained from 2,2-diiodobiphenyl via the Ullmann reaction.
- This reaction can also be employed for the closure of five-membered rings.
- An unsymmetrical reaction can also be achieved given that one of the reactants is provided in excess.
- Chiral reactants can be coupled into a chiral product via this reaction.
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