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Aryl-Aryl bond formation is a fundamental tool in modern organic synthesis, with its applications spanning from the production of natural products, pharmaceuticals, and agrochemicals, to the development of commercial dyes and polyaromatics. With over a century of history, it has been one of the first reactions to use a transition metal, primarily copper, in its higher oxidation states. Despite the significant advancements and the publication of over 700 articles in the past decade, the field continues to evolve, exploring new methodologies and diversifying the range of catalysts used. - Hassan, J., Sévignon, M., Gozzi, C., Schulz, E., & Lemaire, M. (2002). Aryl−Aryl bond formation one century after the discovery of the ullmann reaction. Chemical Reviews, 102(5), 1359-1470. https://doi.org/10.1021/cr000664r

The Ullmann reaction is a classic organic reaction used to synthesize biaryl compounds, which involves the coupling of two aryl halides in the presence of a copper catalyst. Here's a simplified explanation of the mechanism:
 * 1) Formation of the Copper Complex: The reaction begins with the copper catalyst (often copper(I) iodide) reacting with the aryl halide to form a copper complex. This is an oxidative addition step where the copper binds to the aryl group and the halide.
 * 2) Transmetallation: The aryl group is transferred from the copper complex to another equivalent of the aryl halide. This is the key step where the carbon-carbon bond is formed.
 * 3) Reductive Elimination: Finally, the copper is removed from the complex, regenerating the copper(I) catalyst and liberating the coupled biaryl product.

The reaction conditions typically include a strong base and a polar solvent, and the reaction is often carried out at high temperatures. It's worth noting that this reaction, while useful, is not always the most efficient or selective method for forming biaryl bonds. Modern variants of this reaction, such as the Buchwald-Hartwig amination and the Suzuki-Miyaura coupling, often provide better yields and selectivities.