Donor heterogeneous catalysts

Chemical catalysts for transfer hydrogenation have been known for many decades [2e]. The most commonly used are heterogeneous catalysts such as Pd/C, or Raney Ni, which are able to mediate for example the reduction of alkenes by dehydrogenation of an alkane present in high concentration. Cyclohexene, cyclo-hexadiene and dihydronaphthalene are commonly used as hydrogen donors since the byproducts are aromatic and therefore more difficult to reduce. The heterogeneous reaction is useful for simple non-chiral reductions, but attempts at the enantioselective reaction have failed because the mechanism seems to occur via a radical (two-proton and two-electron) mechanism that makes it unsuitable for enantioselective reactions [2 c]. 

Other salts of formic acid have been used with good results. For example, sodium and preferably potassium formate salts have been used in a water/organic biphasic system [36, 52], or with the water-soluble catalysts discussed above. The aqueous system makes the pH much easier to control minimal COz is generated during the reaction as it is trapped as bicarbonate, and often better reaction rates are observed. The use of hydrazinium monoformate salts as hydrogen donors with heterogeneous catalysts has also been reported [53]. 

Sulfur donors are well known as poisons for heterogeneous catalysts, and a consequence of this is that possible homogeneous systems, incor-... 

The catalytic reduction of nitro groups is usually achieved using heterogeneous catalysts, although the iridium complex 28 has been shown to be effective for the reduction of p-nitroanisole 29 to the corresponding aniline 30 using isopropanol as the hydrogen donor (Scheme 8) [30]. In the reduction of some nitroarenes, azo compounds (Ar-N=N-Ar) could be formed as by-products or as the major product by variation of the reaction conditions. 

Examples of electron tunneling reactions on the surface of heterogeneous catalysts have been discussed in Chap. 7. These reactions provide electron transfer between spatially separated donor and acceptor centres on the surface of heterogeneous catalysts as well as between the centres one of which is on the surface of the catalyst and the other is in the subsurface layer. Such processes are expected to be important for photocatalytic reactions, as well as for thermal catalytic reactions proceeding at low temperatures by heterolytic mechanisms. 

Fig. 17 a, b. Photosensitized H2-evolution systems a) basic configuration including a photosensitizer, S, an electron acceptor, A, and electron donor, D, and H2-evolution catalyst, b) H2-evolution system including MV2+ as electron acceptor and a noble metal colloid as heterogeneous catalyst... 

A one-pot quinoline synthesis starting with 2-aminobenzyl alcohol and a,/3-unsaturated ketones using ruthenium-grafted hydrotalcites as a heterogeneous catalyst has been reported (Scheme 64) <2004TL6029>. Molecular oxygen was used for the oxidation of ruthenium and the styryl quinoline 42 was produced in good yield. The use of other donors, for example, octanal and phenylacetonitrile, yielded 3-amylquinoline and 2-amino-3-phenylquinoline, respectively. 

To obtain the best lipase-catalyzed amidation resolution, the lipase operational conditions, i.e., additives, lipase preparations, acyl donors, and solvents, were further evaluated. When molecular sieve 4 A was added to control the water content in the enzymatic resolution, decompositions of aminonitrile intermediates were observed. Among a range of lipases, the resolution process by lipase PS-C I provided the highest conversion of amide products. Phenyl acetate 37 was chosen as acyl donor because its reaction led to marginal by-reactions. Thus, the lipase-catalyzed amidation resolution of the dynamic aminonitrile systems in the presence of zinc bromide as heterogeneous catalyst was performed by lipase PS-C I and phenyl acetate as acyl donor in dry toluene at 0 °C. 

Cycloheptatriene and derivatives thereof donate hydride readily to a variety of carbonium ion acceptors. The position of the end equilibrium depends on the thermodynamics of the exchange. " These reactions are prototypes of a broad area of carbonium ion chemistry wherein carbonium ions equilibrate via intra- and inter-molecular hydride shifts between a donor C—H bond, usually jp hybridized, and a carbonium ion acceptor. This chemistry is often achieved with heterogeneous catalysts and is of great industrial significance it lies outside the emphasis of this review, however. Excellent treatises are available, and a review has appeared on the use of carriers like adamantane to promote hydride transfer in hydrocarbons under strongly acidic conditions. ... 

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. 

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