Enantioselective heterogeneous

Among the various strategies [34] used for designing enantioselective heterogeneous catalysts, the modification of metal surfaces by chiral auxiliaries (modifiers) is an attractive concept. However, only two efficient and technically relevant enantioselective processes based on this principle have been reported so far the hydrogenation of functionalized p-ketoesters and 2-alkanons with nickel catalysts modified by tartaric acid [35], and the hydrogenation of a-ketoesters on platinum using cinchona alk oids [36] as chiral modifiers (scheme 1). 

In this work a new approach is desribed, which can help to understand ED over heterogeneous catalysts We also hope that this approach can be used to find new modifiers for enantioselective heterogeneous catalytic reactions. The basis for this approach is the steric shielding known in organic chemistry [7,8]. A chiral template molecule can induce shielding effect (SE) in such a way that it preferentially interacts with one of the prochiral sites of the substrate. If a substrate is preferentially shielded its further reaction can take place only fi"om its unshielded site resulting in ED. 

An excellent enantioselective heterogeneous metal catalyst should be one in which the active metal sites are set in a chiral arrangement fulfilling the Ogston principles. Assuming no surface atom rearrangement, such a catalyst should furnish high enantioselectivities. 

A naturally occurring chiral metal structure is a screw dislocation (Fig. 3.4),11 which is a chiral arrangement observed in metal crystals but never resolved and tested for enantioselective heterogeneous catalysis. A possible method of making chiral arrangements like screw dislocations is by the glancing angle deposition technique, which can produce chiral sculptured thin films.12... 

Enantioselective heterogeneous catalytic reactions are of growing interest, as optically pure chiral compounds are of great importance in different areas like fine chemical, pharmaceutical, agrochemical, flavor, and fragrance industries. 

Liquid-phase oxidations catalyzed by polyoxometalates are covered by Mizuno, Kamata, and Yamaguchi (Chapter 13). Finally, the developing field of enantioselective heterogeneous catalysis is presented by Coman, Poncelet, and Parvulescu (Chapter 14). 

H. U. Blaser, B. Pugin, M. Studer, Enantioselective heterogeneous catalysis academic and industrial challenges, in D. E. de Vos, I. F. J. Vankelecom, P. A. Jacobs (Eds.), Chiral Catalyst Immobilization and Recycling, Wiley-VCFI, Weinheim, 2000, p. 1. 

Heterogeneous catalysis is obviously technically significant, and the development of enantioselective heterogeneous catalysis is highly desirable. Between 1922 and 1956, a tremendous number of trials were made, but without notable success (25). In 1956, the Akabori group... 

While the former three sections deal with investigations of model substrates and reactions, this part is dedicated to applications of enantioselective heterogeneous catalysts to solve real" synthetic problems both on a laboratory and on a commercial scale. With one exception, all the... 

From a technical or commercial point of view, enantioselective heterogeneous catalysts would be preferable to homogeneous catalysts because of their handling and separation properties, but only if their catalytic performance is satisfactory. It has been demonstrated that this is indeed possible. 

Enantioselective heterogeneous catalytic hydrogenation of acetophenone469 -471 to (R)-(-l-)-l-phenylethanol (ee 20%) in the presence of (S )-proline (the chiral auxiliary) was investigated. The effect of various catalytically active metals (Pt, Rh, Raney Ni, Pd), the reaction temperature and the amount of catalyst on the optical purity was studied. The correlation between the optical yield and the conversion, the concentration of the reactants, different pretreatment methods and additives was also investigated469 (equation 49). 

A. HECKELand D. Seebach, Preparation and characterization of TAD DO Ls immobilized on hydrophobic controlled-pore-glass silica gel and their use in enantioselective heterogeneous catalysis, Chem - Eur.J. 2002, 8, 559-572. 

Enantioselective heterogeneous catalytic hydrogenation using a chiral catalyst was pioneered by Aka-bori and Izumi, who prepared a palladium catalyst supported on silk fibroin. The oxime acetates of diethyl a-ketoglutarate or of ethyl phenylpyruvate were hydrogenated to form glutamic acid (7-15% ee) and phenylalanine (30% Similarly, a palladium-poly-L-leucine catalyst was used for the asym-... 

Systematic studies on the enantioselective heterogeneous catalytic hydrogenation of carbonyl compounds were carried out by Izumi using Raney nickel modified with various chiral reagents. Hydroxy acids or amino acids were used for the modification of the nickel catalyst, and (-i-)-tartaric acid (2R,3R)... 

Abstract Enantioselective heterogeneous catalysis requires surfaces with structures that are chiral at the atomic level. It is possible to obtain naturally chiral surfaces from crystalline inorganic materials with chiral bulk structures. It is also possible to create naturally chiral surfaces from achiral materials by exposing surfaces that have atomic stractures with no mirror symmetry planes oriented perpendicular to the surface. Over the past decade there have been a number of experimental and theoretical demonstrations of the enantiospecific physical phenomena and surface chemistry that arise from the adsorption of chiral organic compounds on the naturally chiral, high Miller index places of metals. 

It has been noted, especially since the thalidomide tragedy, that different chemical enantiomers can induce very different biochemical responses. The production of enantiomerically-pure compounds is critical to the pharmaceutical, agro-chemical and fine-chemical industries, and has fueled the need for enantioselective heterogeneous catalysis [1]. 

Ortega Lorenzo M, Haq S, Murray P, Raval R, Baddeley CJ (1999) Creating chiral surface for enantioselective heterogeneous catalysis (R, R)-tartaric acid on Cu(llO). J Phys Chem B 103 10661... 

The presence of the quinuclidine base functionality makes them effective ligands for a variety of metal-catalyzed processes (Chapters 2-4). The most representative example is the osmium-catalyzed asymmetric dihydroxylation of olefins [9]. The metal binding properties of the quinuclidine nitrogen also allow to use cinchona alkaloids as metal surface modifiers, for example, in the highly enantioselective heterogeneous asymmetric hydrogenation of a-keto esters (Chapter 2). Both... 

Figure 2.53 Overview of some of the possibilities for developing enantioselective heterogeneous catalysts, including a model of cinchonidine adsorbed on an Pt surface.

Blaser, H.U., Pugin, B. and Studer, M. (2000) Enantioselective heterogeneous catalysis academic and industrial... 

New Chiral Modifier for Enantioselective Heterogeneous Catalytic Hydrogenation... 

Improved enantioselectivity availability of cheaper chiral ligands and of enantioselective heterogeneous catalysts. 

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