Ullmann biaryl homocoupling

Hennings, D. D. Iwama, T. Rawal, V. H. Palladium-catalyzed (Ullmann-type) homocoupling of aryl halides a convenient and general synthesis of symmetrical biaryls via inter- and intramolecular coupling reactions. Org. Lett. 1999, 1, 1205-1208. 

Traditionally, the synthesis of symmetrical biaryls was routinely accomplished using the Ullmann reaction. Recently, palladium-catalyzed homocoupling of aryl halides has also been demonstrated to rival the utility of the Ullmann coupling. As illustrated in Scheme 21, using Pd(OAc)2 as the... 

Symmetrical biaryls are important intermediates for synthesising agrochemicals, pharmaceuticals and natural products (1). One of the simplest protocols to make them is the Ullmann reaction (2), the thermal homocoupling of aryl chlorides in the presence of copper iodide. This reaction, though over a century old, it still used today. It has two main disadvantages, however First, it uses stoichiometric amounts of copper and generates stoichiometric amounts of CuL waste (Figure 1, left). Second, it only works with aryl iodides. This is a problem because chemicals react by their molarity, but are quantified by their mass. One tonne of iodobenzene, for example, contains 620 kg of iodo and only 380 kg of benzene . 

A classic example is the homocoupling of aryl iodides in the presence of copper, known as the Ullmann reaction (Figure 1.17). This reaction gives symmetric biaryls,... 

The nickel-catalyzed reductive homocoupling of haloarenes is an excellent method for the synthesis of symmetrical biaryls (Eq. 22). The remarkable ability of [Ni(cod)2] to promote the stoichiometric formation of biaryls from aryl iodides and bromides was originally reported by Semmelhack, Helquist, and Jones [25a]. Unlike the Ullmann reaction [3], the Ni-promoted homocoupling of... 

Homocoupling reactions. Ni(0)-catalysts have been applied to the synthesis of symmetrical biaryls (72-75). Prior to the development of Ni(0) catalysts symmetric biaryls were obtained by the Ullmann reaction (equation 1). 

---