Chirally Modified Heterogeneous Hydrogenation Catalysts

Historically, the chiral modification of classical heterogeneous hydrogenation catalysts was the first successful approach to practically useful enantioselective catalysts [7]. (For a detailed update, see Chapter 11.) However, despite considerable efforts only two types of catalytic system are of practical importance for preparative purposes, namely Ni catalysts modified with tartaric acid, and cinchona-modified Pt (and to some extent Pd) catalysts. 

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Supported palladium and platinum modified by chiral compounds are largely used as pure heterogeneous hydrogenation catalysts. However, recent studies have been performed starting with catalysts of colloidal nature and particles with dimensions of only a few nanometers. Their development continues to attract substantial interest for three main reasons ... 

Chiral amines were always considered important targets for synthetic chemists, and attempts to prepare such compounds enantioselectively date back to quite early times. Selected milestones for the development of enantioselective catalysts for the reduction of C = N functions are listed in Table 34.1. At first, only heterogeneous hydrogenation catalysts such as Pt black, Pd/C or Raney nickel were applied. These were modified with chiral auxiliaries in the hope that some induction - that is, transfer of chirality from the auxiliary to the reactant -might occur. These efforts were undertaken on a purely empirical basis, without any understanding of what might influence the desired selectivity. Only very few substrate types were studied and, not surprisingly, enantioselectivities were... 

Heterogeneous asymmetric hydrogenation of 1,3-diketones is achieved by using a chirally modified Raney Ni catalyst (Scheme 1.51) [241]. Desired chiral diols are obtainable with about 90% ee. This method has been used for the synthesis of some naturally occurring compounds [242],... 

In syntheses of fine chemicals, heterogeneously catalyzed enantioselective hydrogenations have been gradually developing into a topic of great interest over the past few years, as enantiomerically pure substances are required in pharmaceuticals, biochemistry and food technology. For these purposes, chirally modified metal/carrier catalysts have been used, but the effect of the catalytic system is little known [1-8]. 

Klabunovskii, E.I., and Vedenyapin, A.A. (1977) Mechanism of as3Tnmetrizing action of chiral heterogeneous hydrogenation catalysts modified with optically active ligands, Zh. Fiz. Khim. (J. Phys. Chem., russ.), 51, 3005-3012, Chem. Abstr. 1978,88,61803c. 

The first attempts to use the asymmetric hydrogenation of prochiral compounds with C=C and C=N bonds over chiral modified heterogeneous Pt and Pd catalysts were not effective. Lipkin and Stewart found that the (+)-... 

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). 

New modifiers have traditionally been discovered by the trial-and-error method. Many naturally occurring chiral compounds (the chiral pool38) have been screened as possible modifiers. Thus, the hydrogenation product of the synthetic drug vinpocetine was discovered to be a moderately effective modifier of Pt and Pd for the enantioselective hydrogenation of ethyl pyruvate and isophorone.39 Likewise, ephedrine, emetine, strychnine, brucine, sparteine, various amino acids and hydroxy acids, have been identified as chiral modifiers of heterogeneous catalysts.38... 

The chapter Chiral Modification of Catalytic Surfaces [84] in Design of Heterogeneous Catalysts New Approaches based on Synthesis, Characterization and Modelling summarizes the fundamental research related to the chiral hydrogenation of a-ketoesters on cinchona-modified platinum catalysts and that of [3-ketoesters on tartaric acid-modified nickel catalysts. Emphasis is placed on the adsorption of chiral modifiers as well as on the interaction of the modifier and the organic reactant on catalytic surfaces. 

The heterogeneous catalytic hydrogenation of carbonyl compounds using chirally modified metal catalysts has been reviewed in recent years315,455-459. The conclusions can be summarized as follows ... 

Scheme 1 a The [2 + 2] cycloaddition product of prochiral trans 2-butene with Si dimers of the Si(100) surface leads to chiral adsorbate complexes, b Hydrogenation of prochiral a-keto esters over platinum is a heterogeneously catalyzed reaction leading to chiral alcohols. Using cinchonidin as chiral modifier makes this surface reaction enantioselective. In a similar fashion, TA-modified nickel is a highly enantioselective catalyst for /3-keto ester hydrogenation... 

A particularly successful approach to the catalytic hydrogenation of dialkyl ketones with hydrogen has been the use of the heterogeneous contact catalyst system - Raney nickel chirally modified with tartaric acid [18]. Here too, selectivity is enhanced by branching of the alkyl substituent in the alkyl methyl ketones (e.g., 85 % ee for the hydrogenation of isopropyl methyl ketone). With... 

Heterogeneous catalytic hydrogenations using chirally modified metal catalysts have been carried out successfully. This method is interesting chemically and could be a useful production method for chiral... 

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