Enantioselective catalysts homogeneous

Since the publication of the first edition of this book in 1996, the industrial application of enantioselective homogeneous catalysts has made significant progress. The list of processes suitable for the manufacture of enantiomerically enriched compounds is compiled in [6]. Few have actually been implemented as production processes and run on a regular basis but there is every reason to assume that this technology is here to stay. The number of commercial applications will increase in the near future because development chemists who realize technical processes will be more aware of the potential of enantioselective catalysis. More and more specialized technology companies such as Solvias, ChiRex, or ChiroTech are devel-... 

In the previous section, we explained the steric and energetic consequences of the zeo-hte micropore shape for the activation energy of an elementary zeohte-catalyzed reaction step. In this section we discuss transition-state selectivity whereby differences in selectivity are ascribed to a more or less optimum match of the reaction transition state with the micropore cavity. We will demonstrate that the difference in the selectivity for the reaction is determined by the probabihty that a preferred pre-transition state orientation can form rather than by differences in the activation barrier for the reaction step in which protonation occurs. This result is analogous to the finding that the preferred reaction channel for enantioselective homogeneous catalysts proceeds through the adsorption complex with the most favorable free energy (see Section 2.4.4). Similarly, we will discuss the importance of the pre-transition-state complex in enzyme catalysis in Chapter 7. 

In most cases homogeneous chiral catalysts afford higher enantioselectivities than heterogenous catalysts. Nevertheless, the development of heterogeneous chiral catalysts has attracted increasing interest because workup of the reaction, and recovery of often valuable chiral auxiliaries by simple filtration, is more convenient than in the case of homogeneous catalysts. 

The results obtained in reactions involving the two first examples showed a reduced catalytic activity compared to the homogeneous catalyst, a situation that may be due to diffusion problems. Enantioselectivity was similar or slightly lower than in solution, with 80% ee [21] and 58% ee [22] in the epox-idation of ds-/l-methylstyrene with NaOCl providing the best results. Only in the last example was an improvement in enantioselectivity reported from 51% to 91% ee in the epoxidation of a-methylstyrene. Recovery of the catalyst was only considered in one case [21] and a significant decrease in enantioselectivity was observed on reuse. 

Table 3.12 surveys current industrial applications of enantioselective homogeneous catalysis in fine chemicals production. Most chiral catalyst in Table 3.12 have chiral phosphine ligands (see Fig. 3.54). The DIP AMP ligand, which is used in the production of L-Dopa, one of the first chiral syntheses, possesses phosphorus chirality, (see also Section 4.5.8.1) A number of commercial processes use the BINAP ligand, which has axial chirality. The PNNP ligand, on the other hand, has its chirality centred on the a-phenethyl groups two atoms removed from the phosphorus atoms, which bind to the rhodium ion. Nevertheless, good enantio.selectivity is obtained with this catalyst in the synthesis of L-phenylalanine. 

Chiral heterogeneous catalysts, although have lower enantioselectivity and stability than homogeneous catalysts, are often preferable because of their handling and separation properties (5). Aim of this work was to shed light on the enantioselective hydrogenation of a,p-unsaturated acids or their... 

The current research areas with ruthenium chemistry include the effective asymmetric hydrogenation of other substrates such as imines and epoxides, the synthesis of more chemoselective and enantioselective catalysts, COz hydrogenation and utilization, new methods for recovering and recycling homogeneous catalysts, new solvent systems, catalysis in two or three phases, and the replace-... 

Early transition-metal complexes have been some of the first well-defined catalyst precursors used in the homogeneous hydrogenation of alkenes. Of the various systems developed, the biscyclopentadienyl Group IV metal complexes are probably the most effective, especially those based on Ti. The most recent development in this field has shown that enantiomerically pure ansa zirconene and titanocene derivatives are highly effective enantioselective hydrogenation catalysts for alkenes, imines, and enamines (up to 99% ee in all cases), whilst in some cases TON of up to 1000 have been achieved. 

In 1968, Knowles et al. [1] and Horner et al. [2] independently reported the use of a chiral, enantiomerically enriched, monodentate phosphine ligand in the rhodium-catalyzed homogeneous hydrogenation of a prochiral alkene (Scheme 28.1). Although enantioselectivities were low, this demonstrated the transformation of Wilkinson s catalyst, Rh(PPh3)3Cl [3] into an enantioselective homogeneous hydrogenation catalyst [4]. 

Blaser et al. carried out an extensive screening of homogeneous catalysts for the enantioselective hydrogenation of p-chlorophenylglyoxylic acid derivatives... 

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