The Ullmann reaction uses copper metal to convert aryl halides into biaryl compounds. It was first reported in 1901 by Fritz Ullmann and his student Bielecki, and is named after Fritz Ullmann.

Although the reaction mechanism is not yet fully understood, two major pathways have been proposed. One is a radical mechanism involving single-electron transfer (SET) to form aryl radicals, which couple to generate the biaryl product. The other involves two oxidative additions and a single SET event to produce a Cu(III) intermediate, which then undergoes reductive elimination to form the biaryl product.

• Reagents: Copper metal or Cu(I) salts (e.g., Cu₂O, Cu₂S)

• Reactants: Aryl halides or heteroaryl halides

• Products: Symmetrical biaryls (with Cu powder) or unsymmetrical biaryls (with Cu(I) salts)

• Reaction Type: Coupling

Experimental Tips:

• This reaction can also be catalyzed by palladium or nickel [1,2], which extends substrate scope and allows milder conditions.

• Reactivity order of halides: I > Br > Cl.

• Electron-withdrawing substituents on the aryl halides generally facilitate coupling.

• Bidentate ligands such as amino acids, quinolines, and Schiff bases, among others, can significantly enhance the efficiency of the reaction [3–5].

Reaction Mechanism

Original Literature

• Ullmann, F.; Bielecki, J. On Syntheses in the Biphenyl Series. Berichte der deutschen chemischen Gesellschaft, 1901, 34(2), 2174–2185.

Key References:

• Aryl−Aryl Bond Formation One Century after the Discovery of the Ullmann Reaction. Chem. Rev. 2002, 102(5), 1359–1470.

• Exceptional Coupling of Tetrachloroperylene Bisimide: Combination of Ullmann Reaction and C−H Transformation. J. Am. Chem. Soc. 2007, 129(35), 10664–10665.

• The Mechanism of the Modified Ullmann Reaction. Dalton Trans. 2010, 39(43), 10338–10351.