NHC-Ni-catalyzed Cross-coupling Reactions

The nickel-catalyzed reaction of a Grignard reagent with an aryl hahde, widely known as Corriu Kumada, constituted the first example of efficient crosscoupling reaction of aryl halides, long before the most popular Migjta Stille coupling. However, the classical protocol was not properly optimized and recent improvements were achieved using NHC Ni complexes. 

As classically observed in coupling reactions, electron-poor arenes were more reactive. Also, steric hindrance and ort/ro-substitution were not really deleterious, as generally observed in nickel catalysis. 

A year later, Hermann and co-workers showed that these Corriu-Kumada cross-coupling were efficiently obtained under NHC-Ni catalysis with aryl fluorides as starting materials. They showed that an in situ generated species worked similarly if not better than a pre-formed [(NHC)2Ni] complex. This observation suggested that the catalytic active species would be a zero-valent nickel coordinated with only one N-heterocyclic carbene ligand. The formation of a 12-electrons complex would be evidently favored in an in situ process. Both catalysts were active with electron-rich or electron-poor fluoroarenes as well as with congested organometallic species (Equation (10.12)). 

A polar pathway through oxidative addition was proposed in this highly selective transformation, in contrast with the classical radical pathways described for NiCl2-catalyzed reactions that produced higher amounts of ter-phenyl by-products. 

These novel mixed nickel carbene complexes were found to effect Kumada cross-couplings of -methoxyphenylmagnesium bromide with chloro- or bro-mobenzene, and 2-chloropyridine, but accompanied with significant amounts of 4,4 -dimethoxybiphenyl (15-25%). 

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