Ullmann reaction

What is Ullmann reaction?

The Ullmann reaction, a copper-promoted reductive coupling between aryl halides, was first reported by Ullmann in 1901. This reaction is also referred to as the Ullmann cross-coupling or Ullmann reaction.

Ullmann reaction - general reaction scheme - Ullmann cross-coupling
Ullmann reaction

= I, Br, Cl
The reaction typically involves the use of aryl iodides due to their greater reactivity compared to aryl bromides and chlorides. However, aryl bromides and chlorides containing an electron-withdrawing group are also used to enhance the reaction rate. Copper powder quality affects the overall yield, with freshly prepared copper powder being more reactive than commercial copper powder. The Ullmann coupling can also be used to prepare unsymmetrical biaryls from two different aryl halides. In addition, aryl carbanions and neutral aryls can be coupled in the presence of a copper reagent. However, this reaction has some drawbacks, including the harsh reaction temperature, dehalogenation, and multiple couplings between polyhalogenated aryls.

References

Ullmann, F. and Bielecki, J., Ber., 1901, 34, 2174

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