Aryl halides, from arene halogenations

Aryl halides, except fluorides, undergo metal-halogen exchange (1) with rates proportional to the polarizability of the C-X bond. The fact that p-cyanophenyllithium is obtained from treatment of the corresponding bromo-arene with butyllithium (2) attests to the preference for soft interaction. 

Few examples of this mechanism have been clearly demonstrated because of tfie difficulty in establishing that this path occurs from experimental data. The most well-established examples are reactions of nickel complexes with aryl halides Studied by Tsou and Kochi. The rate of the reaction of Ni(PEt3)jWith aryl halides was shown to be first order in nickel and in ArX and retarded by added PEtj, Ortho-methyl substituents had little effect on the rate. Because of the lack of steric effect, electron transfer was proposed to occur after formation of a TT-complex between Ni(PEt3)j and ArX, rather than by direct insertion of the metal into the carbon-halogen bond by a three-centered mechanism. Moreover, the products of the reaction included the Ni(I) species L3NiX and arene. Tliese products are likely to result from the pathway in Scheme 7.4, involving electron transfer from Ni(0) to the aryl halide and escape of the aryl radical from the solvent cage. Other studies of oxidative additions of aryl halides and sulfonates to Ni(0) complexes have been reported. " ... 

Whether the reaction is inter- or intramolecular, the Heck reaction generates vinyl(hetero)arenes or dienes from an alkene and a (hetero)aryl or alkenyl halide [130]. This reaction has great versatility and is applicable to a wide range of aryl and alkene species. Mechanistically, the Heck reaction varies from that depicted in Fig. 4.3. While the oxidative addition of the halogen species occurs, the transmetalation step is replaced by the coordination of the alkene. This is followed by a migratory insertion which essentially substitutes for the cross-coupling step. The product is released not by a reductive elimination, but by a 3-hydride elimination sequence (Fig. 4.5). 

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