Ligands Kumada ligand

Kumada s group (252, 255, 256) has synthesized a range of ferrocenyl-phosphine ligands (28, 29) initially for use in asymmetric hydrosilylation. 

Kumada (44) showed that chiral ferrocenyl phosphine ligands on rhodium gave good optical yields of carbinols from prochiral ketones. 

One of the branches in ligand design was provided by Kumada and his introduction of the ferrocene backbone for BPPFA [99-101] (20a) and BPPOH [102] (20b). This development leads us to the next class of ligands - ferrocene-based. Other variations for development include changes in the backbone and incorporation of the phosphorus into a phospholane (see Section 23.6). 

Kumada s use of a ferrocene moved away from the C2-symmetrical motive, as planar chirality can result from the two ferrocene rings having different substituents. The development of this class of ligand is well documented [5, 125-127]. The best-known uses of these ligands are for reductions of carbon-heteroatom multiple bonds, as in the synthesis of the herbicide, Metolachlor [128, 129]. 

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-... 

For nickel(O) complexes prepared from Ni(r -cod)2 and an excess of the free NHC, it was shown that they exhibit outstanding catalytic activity in the Kumada-Corriu reaction at room temperature toward unreactive substrates like aryl chlorides and even aryl fluorides.Again, an essential element of these catalysts is the need for sterically demanding NHC ligands as observed for the palladium catalysts. 

Catalytic Properties. In recent years, NHC ligands have led to numerous breakthroughs in different highly useful reactions such as the Heck, ° Suzu-ki, Sonogashira, Kumada and StUle couplings, aryl amination, ° and amide a-arylation, and hydrosilylation. ... 

Kumada and co-workers have introduced the ferrocene-based ligand BPPFOH (62) for the rhodium-catalyzed asymmetric hydrogenation of carbonyl compounds. 

In a parallel study, it was found that chelating chiral diamines 208 or 209 are well suited as ligands to promote Kumada-type couplings of primary and secondary alkyl halides 202 with aryl Grignard reagents 203 (entry 4) [281]. This reaction was applicable to alkyl bromides and alkyl iodides, while alkyl chlorides gave only low yields. Acetal and ester functions are tolerated. A notable feature is the stereoretentive arylation of fra s-a-bromo acetals with excellent diastereo-selectivity. The involvement of radicals is supported by the stereoconvergent formation of cxo-phenvI norbornane from both endo- or exo-bromonorbomane (cf. Part 1, Fig. 9) and radical 5-exo cyclizations (see below). 

The cobalt-catalyzed radical Kumada coupling was extended to 1-silylvinyl- and silylethynylmagnesium compounds (entry 5) [282]. Co(acac)3 in the presence of TMEDA, which served as a ligand and solvent, proved to be the catalyst of choice however, a rather high loading of 20—40 mol% was required. The reaction... 

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