Asymmetric hydrogenations over chiral metal

Asymmetric Hydrogenations over Chiral Metal Complexes Immobilized in SILCAs I 257... 

The topic of this chapter is enantioselective hydrogenation over chiral or chirally modified solid catalysts. Diastereoselective hydrogenation of chiral compounds and asymmetric hydrogenation with heterogenized (supported, embedded) homogeneous transition metal complexes will not be discussed. 

The C02-philic perfluoroalkyl-substituted (R,S)-3-H F -BINAPHOS ligand [34] was successfully applied to enantioselective hydrogenation in the inverted SCCO2/H2O system. The complex [Rh(cod)2]BARF was chosen as metal source and the active catalyst was formed in situ. Using the same procedure as above, similar activities and more than 98% ee were obtained consistently over five subsequent cycles in the hydrogenation of methyl 2-acetamido acrylate. The results demonstrate the potential of the inverted SCCO2/H2O system for asymmetric synthesis of chiral biologically active products. 

The cationic rhodium catalysts are useful for asymmetric hydrogenation.152 In this variant, the presence of a chiral phosphine leads to differences in the rates of H2 addition to the two faces of a prochiral alkene. Where the alkene has groups such as C02Me suitably placed to bind to the metal, the selectivity can become very great enantiomeric excesses of the product over its enantiomer can reach 95-98% (equation 67). The mechanism has recently been elucidated by Halpern.153... 

Introduction. DIOP was the first example of a C2 chelating diphosphine for transition metal complexes to be used in asymmetric catalysis. It was also one of the first examples of a useful C2 chiral auxiliary. DIOP can be considered as an example of the first generation of chelating diphosphine ligands with a chiral carbon skeleton, which were followed over the next 20 years by many examples of chelating diphosphines, one of the most efficient of which is BINAP [2,2 -Bis(diphenylphosphino)-l,l -binaphthyl] The ready availability of DIOP has stimulated research in asymmetric catalysis beyond the area of asymmetric hydrogenation. 

Asymmetric hydrogenation of a,P-unsaturated ketones is rare. Most existing catalysts, either achiral or chiral, effect preferential saturatation of carbon-carbon double bonds over carbonyl groups under a hydrogen atmosphere [77]. This tendency is supposed to arise from the easier interaction of the metal center with an olefinic bond than with a carbonyl moiety. 

Torok, B., Felfoldi, K., Szakonyi, G., Molnar, A., Bartok, M. (1998) Ultrasonics in chiral metal catalysis. Effect of presonication on the asymmetric hydrogenation of ethyl pyruvate over Pt catalysts. Chemical Industries Series Catalysis of Organic Reactions, Herkes, F.E., ed.) (Dekker) 75, 129-140... 

In 9.13, asymmetric hydrogenation is possible if H2 prefers to add to one face over the other. Equation 9.7 shows how a prochiral alkene binds to an achiral metal to give two enantiomers that is, the complex is chiral even though neither the free ligand nor the metal were chiral. We can regard the chiral carbon indicated by the asterisk as having four different substituents, one of which is the metal. 

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