Ullmann reaction

Also known as: Ullmann coupling or Ullmann biaryl synthesis

Schematic of the Ullmann reaction. Reagents: aryl halide, Copper(0), heat. Product: biaryl.

The Ullmann reaction is an organic reaction used to couple two molecules of aryl halide to form a biaryl using copper metal and thermal conditions. The mechanism for the Ullmann reaction is not fully under-stood but there are two popular mechanisms. The radical mechanism begins with a single electron transfer (SET) from the copper metal to the alkyl halide to form an aryl radical. Two aryl radical then react to form the final biaryl product. The second proposed mechanism begins with oxidative addition of the copper to the aryl halide followed by a SET to form an organocuprate reagent. The organocuprate performs another oxidative addition on an aryl halide and after reductive elimination results in the final biaryl product.[1][2]


Mechanism of the Ullmann reaction. The Ullmann reduction mechanism is not fully understood. The two most popular mechanisms are shown below. 1. Radical Mechanism, reaction with a 2nd aryl radical. 2. Mechanism involving an aryl copper intermediate where the steps include: oxidative addition (aryl copper), single electron transfer (SET), another oxidative addition, and reductive elimination.


Ullmann, F.; Bielecki, J. Ber. Dtsch. Chem. Ges. 1901, 34, 2174–2185.
Ullmann, F. Justus Liebigs Ann. Chem. 1904, 332, 38–81.