Hydrogenation asymmetric heterogeneous

Harada, K. Asymmetric heterogeneous catalytic hydrogenation. In Morrison, J.D. (Ed.), Asymmetric Synthesis. Academic Press, Inc., Orlando, FL, 1985,... 

In broad terms there are three types of catalyst for transfer hydrogenation dehydrogenases heterogeneous and homogenous metal catalysts. Here, the first two are mentioned for completeness, and the main focus of this chapter will be asymmetric transfer hydrogenation with homogenous metal catalysts. 

In the present work, some promising results obtained with this kind of asymmetric heterogeneous catalyst, based on silica-supported Ni, Rh and Pt, chemically modified with chiral organotin compounds, are presented. The systems were tested in the enantioselective hydrogenation of ethyl pymvate, acetophenone and 3,4-dimethoxyacetophenone. The stabiUty of these catalysts was also studied to check if they could be reused. 

Review on stereoselective hydrogenation with heterogeneous catalysts K. Har-ada, Asymmetric Heterogeneous Catalytic Hydrogenation, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 5, Chap. 10, Academic Press, New York, 1985. 

Before launching into this subject, we briefly review the concept of an asymmetric synthesis, say a few words about some asymmetric heterogeneous hydrogenations, and outline basic information about soluble achiral rhodium-... 

The generally low-percent asymmetric synthesis in asymmetric heterogeneous hydrogenations may be due, in part, to a nonuniform distribution of chiral modifying agents over the catalytic surfaces. In the case of silk fibroin, metal clumping on the chiral support or dissociation of the metal from the fibroin may allow some reduction to occur in an achiral local environment. 

An asymmetric synthesis involving the hydrogenation of a prochiral olefin can be accomplished over an asymmetric heterogeneous catalyst such as Ni or Pd on d- or /-quartz. This asymmetric environment gives low optical yields. Better results are obtained over finely divided Ni in alkaline glucose solution or Pd deposited on silk fibroins which gives optical yields up to 70%. 

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 use of Sj-oia-diphenyl-Z-pyrrolidininethaiiol as a chiral modifier in asymmetric heterogeneous catalytic hydrogenation of ethyl pyruvate and isophorone is reported. Various solvents have been screened. The effect of modifier s concentration is described. The changes in optical yield of the saturated ketone as functions of the water content of the solvent has also been investigated. 

This chiral molecule is based on the (5)-proline, which in itself proved to be a good chiral auxiliary and synthon in asymmetric heterogeneous catalytic hydrogenation [11]. 

Asymmetric heterogeneous catalysis has been demonstrated by the enantioselec-tive and racemic hydrogenation of ethyl pyruvate over Pt-Al203 catalyst. Higher reaction rate and enantioselectivity (75%) were observed in nonpolar solvent (toluene) whereas in a polar solvent (ethanol) they decreased substantially (Scheme 13.5) [58]. 

Molvinger, K., Lopez, M., Court, J. (1999) Asymmetric reduction and hydrogenation over heterogeneous catalysts prepared by reacting nickel-boride with norephedrine, J. Mol. Catal. A. Chem. 150,261-213. 

Scheme 9.7 Asymmetric heterogeneous hydrogenation of a.y-keto esters in a microreactor [31].

DFT calculations are also used in asymmetric heterogeneous catalysis, for example, to address interaction between a reactant and a chiral modifier adsorbed on the metal catalyst surface to rationalize the mechanism of heterogeneous enantioselective [146] and diastereoselective [147] hydrogenations. 

Four general methods have been used for obtaining chiral ligands resolution of a racemic mixture, use of a chiral naturally occurring product 33), and asymmetric homogeneous or heterogeneous hydrogenation. 

The 96% ee is the highest so far achieved by the enantio-differentiating hydrogenation over an asymmetrically modified heterogeneous catalyst. Although it is difficult to separate factor i and E/ e+N), the present results indicated that both of them are well optimized and become almost xmity. 

Roche carries out asymmetric hydrogenation of a p-keto-ester for a pancreatic lipase inhibitor using their Ru (II) BIHEHP catalyst. For scaling up, Roche decided to use a heterogeneous catalyst, modified Ni /L-tartaric acid with NaBr, since this was economically more attractive. 

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