Hetero arenes reduction

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). 

Scheme 11.5 The CMD mechanism and classification of different (hetero)arene substrates. The CMD mechanism is shown for one step in an unspecified catalytic cycle. Typically, the Pd(ii) centre contains another aryl group, which would see reductive elimination from intermediate B to give the organic product, leading to regeneration of the active Pd catalyst (not shown).

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