Nickel catalysts Kumada cross-coupling

An alternate approach to the asymmetric synthesis of axially chiral biaryls has been developed based on the nickel-catalysed Kumada cross coupling of dibenzothiophenes. Cleavage of the carbon—sulfur bond of achiral dibenzoth-iophene (10.25) with the nickel catalyst generated from ligand (10.26) leads to the... 

The use of a functionalized silica-supported salen-nickel complex has allowed Kumada cross-couplings to be performed in flow the corresponding polystyrene supported complex was shown to be inferior for a number of reasons. Catalyst 33 (Figure 4.7) with the longer tether was found to be more active than the benzyl ether tether used for catalyst 34. This was postulated to be due to the fact that catalyst 33 resided further away from the silica surface and hence was more available for reaction. Under the conditions used a maximum conversion of 65% was found for the 1 1 reaction of 4-bromoanisole and phenylmagnesium chloride, which was found to be comparable to that obtained in batch mode. However, during the reaction catalyst degradation was observed and the conversion reduced from 60% in the first hour to 30% in the fifth hour of the reaction [155,156]. 

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)). 

Other successful examples of catalysts containing NHC ligands are found in palladium- and nickel-catalyzed carbon-carbon bond formations. The catalyst development with these metals has focused in particular on Heck-type reactions, especially the Mizoroki-Heck reaction itself [Eq. (42)] and various cross coupling reactions [Eq. (43)], e.g., the Suzuki-Miyaura reaction ([M] = and the Kumada-Corriu reaction ([M] = MgBr). " Related reactions like the Sonogashira coupling [Eq. (44)]326-329 Buchwald-... 

Cross-coupling of arylmagnesium halides with fluorodiazines has been studied by Queguiner. Nickel catalysts were used for this Kumada-type process. The reactions work at room temperature in THE. Only one example on a 1,2-diazine core was studied, namely the coupling of 3-fluoro-6-phenylpyridazine with 4-methoxyphenylmagnesium bromide <2002JOC8991>. 

The asymmetric cross-coupling of 1-phenylethylmagnesium chloride 20a with vinyl bromide 21 a has been reported by Hayashi and Kumada to proceed in the presence of 0.5 or 1 mol% of nickel or palladium catalyst coordinated with a chiral ferrocenylphosphine ligand to give optically active 3-phenyl-1-butene 22 [28]. 

Kumada coupling. A report of biaryl synthesis from ArMgBr and Ar Cl highlights the use of a Ni carbenoid (1). Both bis(Ti -allyl)nickel and palladium complexes are also useful catalysts for the cross-coupling." ... 

Nickel-catalyzed Kumada-Gorriu cross-coupling reactions of a variety of monofluorinated aromatics have been aceomplished using nickel(O) complexes bearing the A-heterocyclic carbene ligand l,3-di(2 6 -diisopropylphenyl)imi-dazolin-2-ylidene (Scheme 42). The catalysts can also be prepared in situ from [Ni(acac>2] and an imidazolium salt. 

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