Biaryl synthesis homocoupling reactions

Apart from aryl halides, other very easily available substrates for nickel-catalysed biaryl (II) synthesis are aryl sulfonates (XII). Method D is very efficient in the homocoupling reactions of substituted aryl sulfonates in good to excellent yields [15], Table 6. Substituted aryl sulfonates are readily obtained from phenols and trifluoromethanesulfonic anhydride, benzenesulfonyl-, tosyl- or methanesulfonyl chloride in pyridine, or in a suitable inert solvent such as dichloromethane in the presence of triethylamine or Hiinig s base. Among other nickel complexes, Ni(dppe)Cl2 and Ni(dppf)Cl2 have been used (10 mo1%) as slightly less versatile catalysts for the homo-couplings of naphthyl sulfonates in refluxing THF, DMF or their mixtures [42]. 

Table 5. Synthesis of symmetrical biaryls by thallium(I) bromide-promoted homocoupling reaction of aryl Grignard reagents [14]...

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). 

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... 

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. 

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... 

Intermolecular dehydrogenative oxidative homocouplings of (hetero)arenes turned out to be among the most important methods for the synthesis of symmetrically substituted biaryls [122]. A recent illustrative example is oxidative coupling reactions of 2-naphthols, which were accomplished in an asymmetric fashion employing an inexpensive iron catalyst (Scheme 9.47) [123]. 

The reaction was exploited very recently in a solid phase synthesis of biaryls. Aryl zinc bromides undergo palladium catalyzed coupling reactions with aryl bromides bound to a polystyrene resin. The product can be released from the resin by transesterification [44]. Ni(0) catalysed homocoupling of arylzinc reagents could also be realised using aryl triflates [45], as well as aryl tosylates and mesylates [46]. 

In a handful of cases, two CCXIH groups have been activated for the synthesis of biaryls. Larrosa and coworkers reported for the first time the decarboxylative homocoupling of aromatic acids mediated by Pd and Ag [62a]. The reaction makes use of Pd(TFA)j as a catalyst and Ag CO as an additive to afford the desired biaryls in 76-95% yields. The only by-products observed were due to the proto-decarboxylation of the aryl carboxylic acid. Both metals are essential for the reaction, and the role of the Ag salt is not only as the terminal oxidant but also as a mediator of the decarboxylation process. The method is subject to some limitations on the substituents on the benzoic acids. Thus, m- and p-nitrobenzoic acids as well as benzoic acids ortho substituted with F, Br, or MeO failed to give decarboxylative homocoupling products. In all cases, protodecarboxylations to the corresponding arenes were the main products observed. The same problem was reported in the protocol developed by Deng and coworkers, where the best results were obtained with PdCl and PPhj in the presence of Ag COj [62b]. 

---