Heterogeneous Reduction Catalysts

For the hydrogenation of a-functionalized ketones, the Pt on alumina system, modified with cinchonidine or its simple derivative 10,11-dihydro-O-methyl-cinchonidine, is the best catalyst [43, 44, 46]. This so-called Orito system [47] is 

Up to 72% ee has been achieved in the hydrogenation of a diphenyl-substituted reactant, (trans)-a-phenylcinnamic acid, with a Pd/Ti02 catalyst and cinchonidine at 1 bar in strongly polar solvent mixtures [49]. For aliphatic a,/ -unsaturated acids the enantioselectivities that can be attained are much lower. Therefore, for these type of substrates, homogeneous metal-catalysts are preferred. 

Another approach towards asymmetric heterogeneous catalysts is the immobilization of chiral homogeneous complexes via different methods. In this way the advantages of homogeneous catalysts (high activity and selectivity) and heterogeneous catalysts (easy recovery) can be combined. For a complete overview of this active research field the reader is referred to several reviews on this topic [50, 51]. The practical applicability of these catalysts is hampered by the fact that severe demands of recyclability and stability need to be obeyed. In certain cases promising results have been obtained as outlined here. 

The use of solid and soluble catalysts with covalently attached ligands  

The covalent anchoring of ligands to the surface or to a polymer is a conventional approach which often requires extensive modification of the already expensive ligands. The advantage is that most catalysts can be heterogenized by this approach and various supports can be used. An illustrative example is the use of the BINAP ligand that has been functionalized using several different 

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The SRC-II process, shown in Figure 2, was developed in order to minimise the production of soHds from the SRC-I coal processing scheme. The principal variation of the SRC-II process relative to SRC-I was incorporation of a recycle loop for the heavy ends of the primary Hquefaction process. It was quickly realized that minerals which were concentrated in this recycle stream served as heterogeneous hydrogenation catalysts which aided in the distillate production reactions. In particular, pyrrhotites, non stoichiometric iron sulfides, produced by reduction of iron pyrite were identified as being... 

Both heterogeneous and homogeneous CO reduction catalyst recipes often contain electrophilic components such as silica supports, metal oxides, and A1Cl3 [1,5,33,34,35,36]. 

The enantioselective hydrogenation of prochiral substances bearing an activated group, such as an ester, an acid or an amide, is often an important step in the industrial synthesis of fine and pharmaceutical products. In addition to the hydrogenation of /5-ketoesters into optically pure products with Raney nickel modified by tartaric acid [117], the asymmetric reduction of a-ketoesters on heterogeneous platinum catalysts modified by cinchona alkaloids (cinchonidine and cinchonine) was reported for the first time by Orito and coworkers [118-121]. Asymmetric catalysis on solid surfaces remains a very important research area for a better mechanistic understanding of the interaction between the substrate, the modifier and the catalyst [122-125], although excellent results in terms of enantiomeric excesses (up to 97%) have been obtained in the reduction of ethyl pyruvate under optimum reaction conditions with these Pt/cinchona systems [126-128],... 

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... 

Thus, the combination of increased acidity and increased solvent power can be exploited for cleaner acid-catalysed reactions of organic compounds, for example Friedel-Crafts alkylation, [31] or hydrolysis of esters [9,28]. The acidity of high-temperature D20 can be used to deuterate organic compounds but the process is more efficient with an additional heterogeneous acid catalyst [32]. A related application, but not a catalytic one, exploits the acidity of SC-H2O for the in situ generation of H2 from metallic Zn the H2 can then be used for the selective reduction of organic nitro-compounds [29,30], see Figure 9.1-2. 

Selective reduction of carbonyl groups.1 Heterogeneous reduction of carbonyl groups by triethoxysilane with CsF (or KF) as catalyst is highly selective, the reactivity order being aldehyde > ketone > ester. Double bonds, nitro, halo, and amido groups are inert to this reagent. 

This reaction was investigated recently by Flynn and Hulburt 19,20) with the view to utilize such information to elucidate the role of metal catalysts in the heterogeneous reduction of olefins. 

Phthalocyanine complexes are some of the few heterogeneous organic catalysts that have found their way into practical application. They are used to remove mercaptans from oil by selective oxidation into disulfides 98> and they also used in fuel cell cathodes, where they catalyze the reduction of oxygen 94 ... 

We recently reported that a heterogeneous copper catalyst prepared with a non-conventional chemisorption-hydrolysis technique is able to promote a hydrogen transfer reduction using a donor alcohol. In this case, the role of copper is cricial, both for activity and selectivity [20]. 

Scheme 13.1 4-tert-butylcyclohexanone reduction under hydrogen transfer conditions with heterogeneous copper catalysts. 

Here we summarize some of our results obtained by exploiting the hydrogen transfer ability of heterogeneous copper catalysts and therefore their activity in the reduction of polyunsaturated compounds, racemization and dehydrogenation of unactivated secondary alcohols, and isomerization of allylic alcohols. 

Heterogeneous copper catalysts prepared with the chemisorption-hydrolysis technique are effective systems for hydrogen transfer reactions, namely carbonyl reduction, alcohol dehydrogenation and racemization, and allylic alcohol isomerization. Practical concerns argue for the use of these catalysts for synthetic purposes because of their remarkable performance in terms of selectivity and productivity, which are basic features for the application of heterogeneous catalysts to fine chemicals synthesis. Moreover, in all these reactions the use of these materials allows a simple, safe, and clean protocol. 

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