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The Ullmann reaction is a coupling reaction in organic chemistry where aryl halides are converted into biaryls through a copper-catalyzed process. It was first reported by Fritz Ullmann and Jean Bielecki in 1901. The reaction has since been expanded to include the formation of carbon-nitrogen, carbon-oxygen, and carbon-sulfur bonds.

Here's a general overview of the original Ullmann coupling reaction:

Ar-X + Ar'-X + 2 Cu -> Ar-Ar' + 2 CuX

In this equation, Ar and Ar' represent aryl groups (e.g., phenyl), X represents a halide (e.g., bromide or iodide), and Cu represents copper.

The mechanism involves single electron transfer from copper(0) to the aryl halide to generate an aryl radical and a copper(I) halide. The aryl radical then reacts with another equivalent of copper(0) to form an Ar-Cu(I) species which undergoes oxidative addition with another molecule of aryl halide forming biaryl product along with regeneration of catalytic amount of Cu(I).

While the original Ullmann coupling has been largely superseded by other methods like Suzuki-Miyaura or Negishi couplings due to challenges related to harsh conditions required (like high temperatures), it still finds use especially in formation of C-O, C-N or C-S bonds where it often provides complementary reactivity compared to palladium-catalyzed processes. Variants like "Ullmann-Goldberg" method have also been developed which allow for milder conditions through use ligand-assisted bidentate coordination.