Chiral heterogeneous catalyst

The method of Ito et at. 50) as applied by Bakos et at. 12) to the reduction of acctylacetone to either ( —)-(2R,4J )- or ( + H25,45)-2,4-pentanediol will serve to illustrate how a chiral heterogeneous catalyst has been used to prepare a chiral homogeneous ligand precursor. 

Abstract The immobilization of chiral catalysts through non-covalent methods, as opposed to covalent immobilization, allows an easier preparation of chiral heterogeneous catalysts with, in principle, less influence of the support on the conformational preferences of the catalytic complex. In this review the different possibilities for immobilization without forming a covalent bond between the chiral diazahgand and the support, which can be either solid or liquid, are presented. 

An alternative that has received a great deal of attention in recent years is the immobilisation of a chiral catalyst on a nonsoluble support (polystyrene resins, silica gel, zeolites, etc.), thereby creating a chiral heterogeneous catalyst. Unlike homogeneous catalysts, these supported complexes can be recovered from the... 

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... 

To summarize, chiral heterogeneous catalysts were prepared from rhodium-diphosphine complexes and aluminum-containing mesoporous materials. The bonding occurred via an ionic interaction of the cationic complex with the host. These catalysts were suitable for asymmetric hydrogenation of functionalized olefins. The catalysts can be recycled easily by filtration or centrifugation with no significant loss of activity or enantioselectivity. 

Several different immobilization methods are currently under investigation in order to immobilize various types of Jacobsen complexes in the mesopores of Al-MCM-48, Al-MCM-41 and Al-SBA-15 types of support materials. The novel chiral heterogeneous catalysts obtained will be characterized and their activity in different test reactions will be investigated. 

From a synthetic point of view, there are a few reaction types catalyzed by chiral heterogeneous catalysts which are useful for preparative chemists. But it is also evident that the scope of most catalytic systems is rather narrow and very high substrate specificity is observed. Compared to homogeneous or bio-catalysis, enantioselectivities are usually lower but there are exceptions. 

Heterogeneous catalysts modified by the addition of chiral substances have been used to hydrogenate olefins asymmetrically, but only a few effective chiral heterogeneous catalyst systems have been found. Palladium deposited on silk fibroin was used to hydrogenate 4-benzylidene-2-methyl-5-oxazolone asymmetrically to give, after hydrolysis, optically active phenylalanine (Fig. lc). The optical purity1 of the product was found to be dependent on the origin of the fibroin and its chemical pretreatment (4-6). 

In conclusion, chiral heterogeneous catalysts are prepared from chiral Rhodium diphosphine complexes and Al-MCM-41. The bonding supposedly occurs via an ionic interaction of the cationic complex with the host. Also a slight reduction of weak acidic sites of Al-MCM-41 has been observed. These catalysts are suitable for the hydrogenation of functionalised olefins. The organometallic complexes remain stable within the mesopores of the carrier at reaction conditions. The catalyst can be recycled by filtration or centrifugation. 

Blaser HJ (1991) Enantioselective synthesis using chiral heterogeneous catalysts. Tetrahedron Asymmetry 2 843... 

Raynor SA, Thomas JM, Raja R, Johnson BFG, Bell RG, Mantle MD (2000) A one-step enantioselective reduction of ethyl nicotinate to ethyl nipecotinate using a constrained chiral heterogeneous catalyst. Chem Commun 1925... 

For the enantioselective reduction of olefins, there are few alternatives to homogeneous hydrogenation because neither transfer hydrogenations with hydrogen donors such as HCOOH/NEt3 [28] nor chiral heterogeneous catalysts [12 c] are ready for larger-scale applications. 

Rh-MeBPE(33)-Al-MCM-48 catalyst exhibited 95% ee for the hydrogenation of methyl a-acetamidoacrylate, while its homogeneous counterpart showed 90% ee. These chiral heterogeneous catalysts can be reused at least four times without any activity loss, with a TON in excess of than 4000. 

Three types of chiral heterogeneous catalyst which we consider to be practically useful for synthetic purposes are listed and characterized in Figure 12.1. In the following sections, the scope and limitations with respect to the industrial apphcation of these three types of heterogeneous catalyst will be discussed, and the most important industrial applications described in some detail. It should be... 

Figure 12.1 A schematic representation and indication of the important properties of chiral heterogeneous catalysts.

Scheme 10.3S Hydrogenation of ethyl nicotinate using a chiral heterogeneous catalyst.

Most groups working in the field of chiral heterogeneous catalysts use an empirical approach in order to expand the scope of known catalytic systems. There are, however, some attempts to try new principles and ideas. Even though the ee s are still low, there are some interesting first results ... 

Smith et al. have found a new type of chiral heterogeneous catalyst based on Pt, Pd, Rh or Ru dispersed onto a >eto-cyclodextrine-epichlorohidrin copol5mier (CDP). The ability of these catalysts to catalyze the enantioselective hydrogenation of C=C bonds was tested with the hydrogenation of dimethylitaconate into dimethyl methylsuccinate and ( )-2-methyl-pent-2-enoic acid into 2-methylvaleric acid (Scheme 3.8.). 

Zeolites can be used as supports for the preparation of chiral heterogeneous catalysts in different ways... 

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