Nickel-catalysed reactions Heck reaction

Nickel-Catalysed Mizoroki-Heck-Type Reactions... 

Scheme 10.7 Nickel-catalysed MIzorokl-Heck-type reaction with salt 20.

Nickel-catalysed Mizoroki-Heck-type reaction with electrochemical... 

Scheme 10.9 Nickel-catalysed MIzoroki-Heck-type reaction with stoichiometric amounts of Zn (27).

Scheme 10.10 Nickel-catalysed MIzorokl-Heck-type reactions with aryl and alkyl bromides in the presence of pyridine (31).

Apart from palladium, nickel is also known to catalyse Heck vinylations [9-12]. A supported nickel catalyst was appUed for the Heck reaction of iodobenzene with butyl acrylate [13]. The supported aqueous-phase catalysis performed almost as well as the analogous homogeneous catalyst NiCl2(PPli3)2-Only the conversion was sHghtly lower. No leaching of metal into the product phase was observed (detection limit approx. 1 ppm). The immobihzed catalyst was successfully recycled three times without observing loss of activity and selectivity. 

Compared with palladium-catalysed Heck reactions, the studies of nickel-catalysed Heck reactions are far fewer. Theoretical studies show that oxidative addition and olefin insertion occur easier in the nickel system than in the palladium system. On the other hand, p-hydride elimination and catalyst regeneration through HX removal are more difficult in a nickel-based catalytic system." The former characters may make the nickel-catalysed Heck reaction of alkenyl or aryl chloride proceed relatively easily and the latter... 

Similar process takes place during palladium- and nickel-catalysed couplings of aryl halides via various diarylnickel(ll, III or IV)- or diarylpalladium(lV) intermediates. In the case of the related Heck reaction [19], palladium(IV) intermediate has been isolated and characterized [20]. 

Generally applicable nickel-catalysed arylation reactions of alkenes were accomplished through the use of relatively strong stoichiometric reducing reagents [29]. Hence, Perichon and coworkers [30] developed an electrochemical, highly selective nickel-catalysed Mizoroki-Heck-type reaction (Scheme 10.8). Importantly, this early report showed that not only aryl iodides and bromides, but also less reactive aryl chlorides could be used for luckel-catalysed Mizoroki-Heck-type reactions. 

The use of pyridine (31) as additive allowed for more selective and efficient nickel-catalysed arylations of styrenes (Scheme 10.10) [32, 33]. Aryl and alkyl bromides gave good yields of isolated products. With respect to the latter, secondary alkyl bromides proved superior to primary ones. However, use of methyl acrylate (1) as substrate yielded predominantly products originating from conjugate additions, rather than Mizoroki-Heck-type reactions. 

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