Used for Asymmetric Hydrogenation

Figure 1.16 lists other chiral ligands useful for asymmetric hydrogenation of ot-and/or p-keto esters.A Ru complex with BPE, a fully alkylated diphosphine. 

A few efficient bisphosphite ligands have been used for asymmetric hydrogenation of itaconates or a-dehydroamino acid derivatives. Reetz has developed a series of C2-symmetric bisphosphite ligands such as L14, which are based on the structure of 1,4 3,6-dianhydro-D-mannite [78]. The ligands exhibit excellent reactivity and enantioselectivity for the asymmetric hydrogenation of itaconates. 

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... 

Asymmetric hydrogenation. Simpler ligands of this type, but still based on a r-diphenylphosphinyl-2-(diphenylphosphino)methylpyrrolidine (PPPM), were first eported in 1976 (8, 57-59) to be useful for asymmetric hydrogenation of 2-ace-umidoacrylic acids. Since then a para-methoxy group on the phenyl groups has... 

The rate-determining step occurs during (3-hydride elimination. One model for enantioselection is shown in Figure 12-4, where the diamine ligand, (- sparteine, exhibits Q symmetry48 instead of C2. C2 symmetry is associated with dia-stereomeric structure 22b, (-)-a-isosparteine). C2 is also the inherent symmetry of chiral bisphosphine ligands used for asymmetric hydrogenation.49... 

Asymmetric hydrogenation. (R,/f)-Binaphane (1) is prepared from l,l -bi-2,2 -naphthol in five steps. Its Ru complex is useful for asymmetric hydrogenation of trisubstituted enamides. 

Lin and coworkers have reported zirconium phosphonate-derived Ru-BINAP systems (Scheme 16) [49]. Zirconium phosphonate-based chiral porous hybrid materials containing the Ru(BINAP)(diamine)Cl2 precatalysts showed excellent enantioselectivity (up to 99.2% ee) in the asymmetric hydrogenation of aromatic ketones. These catalysts were also readily recovered by centrifugation and reused for up to 10 times without significant loss of catalyst activity and enantioselectivity. Related zirconium phosphonates containing Ru(BINAP)(DMF)2Cl2 precatalysts were successfully used for asymmetric hydrogenation of p-keto esters with up to 95% ee [50]. 

A selection of monodentate phosphite and phosphoramidite ligands used for asymmetric hydrogenation. 

The basic compound of Brintzinger s ansa-titanocene complexes is ethylenebis-(tetrahydroindenyl)titanium dichloride, (EBTHI)TiCl2. Further analogues ((EBTHI)TiH, (EBTHI)Ti(Me)2, and (EBTHI)Ti(CO)2) have been wddely used for asymmetric hydrogenation, hydrosilylation, and Pauson-Khand reaction (121). Novel optically active titanium complexes containing a linked amido-cyclopentadienyl ligand have been developed and used for asymmetric hydrogenation (122). 

The immobilization of homogeneous catalysts is still a challenge in catalysis. This field is not discussed in detail in this article, but a promising result is worth to mention. A PYRPHOS-rhodium(I) complex was embedded at a specific site in a protein (biotin) and used for asymmetric hydrogenation of itaconic acid (243). 

Magnetically recoverable heterogenized nanoparticle supported chiral Ru complexes were obtained and used in highly enantioselective asymmetric hydrogenation of aromatic ketones (Hu et al., 2005). The catalysts can be recycled by magnetic decantation and used for asymmetric hydrogenation for up to 14 times without loss of activity and enantioselectivity. 

FIGURE 29.14. Iridium-based achiral Crabtree s catalyst 228, and its chiral analogs iridium P,N-and C,A -complexes used for asymmetric hydrogenation. 

We have already seen one example of a chelating bidentate phosphine, dppe, in Figure 2.1. As will be seen later, bidentate phosphines are industrially used for asymmetric hydrogenation and isomerization reactions. A parameter called bite angle is often used to explain the beneficial effects of some bidentate phosphines on the selectivity of hydroformylation reactions (see Section 5.2). 

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