Hydrogenation Using Heterogeneous Catalysts

The mechanistic description of catalytic hydrogenation of alkene is somewhat imprecise, partly because the reactive sites on the metal surface are not as well 

In most cases, both hydrogen atoms are added to the same face of the double bond (syn addition). If hydrogenation occurs by addition of hydrogen in two steps, as 

Addition of Hydrogen at Carbon-Carbon Multiple Bonds 

Reduction of Carbon-Carbon Multiple Bonds, Carbonyl Groups, and Other Functional Groups 

The preference for addition from the more hindered of the substituents in Entries 7 and 8 can be attributed to functional group interactions with the catalyst. Polar 

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Hydrogenations using heterogeneous catalysts usually require thermal condition above room temperature and H2 pressures higher than 10 Pa. Homogeneous catalysts are often more selective with individual reactions and make the reactions possible at lower temperatures and H2 pressures. 

A search for alternative energy supplies has triggered efforts to develop efficient homogeneous catalysts for Fischer-Tropsch-type syntheses via hydrogenation of carbon monoxide, a likely future key material available, for example, through oxidation of coal (33, 327, 328, 417, 418). Metal cluster systems have been used in attempts to emulate the presently used heterogeneous catalysts. The important reactions are methanation,... 

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. 

The synthesis of an N-alkylarylamine by the reductive alkylation of an aromatic primary amine with a ketone is used in the preparation of antioxidants for polymers and rubber. The alkylation of an amine with a ketone is typically carried out in the liquid phase using heterogeneous catalysts such as Pd, Pt, Rh, or Ru supported on carbon (1,2). The reaction of ADPA with MIBK yields an imine, which then is hydrogenated over a Pt or sulfur promoted-Pt catalyst to yield 6-PPD. 

Reasons for interest in the catalyzed reactions of NO, CO, and COz are many and varied. Nitric oxide, for example, is an odd electron, hetero-nuclear diatomic which is the parent member of the environmentally hazardous oxides of nitrogen. Its decomposition and reduction reactions, which occur only in the presence of catalysts, provide a stimulus to research in nitrosyl chemistry. From a different perspective, the catalyzed reactions of CO and COz have attracted attention because of the need to develop hydrocarbon sources that are alternatives to petroleum. Carbon dioxide is one of the most abundant sources of carbon available, but its utilization will require a cheap and plentiful source of hydrogen for reduction, and the development of catalysts that will permit reduction to take place under mild conditions. The use of carbon monoxide in the development of alternative hydrocarbon sources is better defined at this time, being directly linked to coal utilization. The conversion of coal to substitute natural gas (SNG), hydrocarbons, and organic chemicals is based on the hydrogen reduction of CO via methanation and the Fischer-Tropsch synthesis. Notable successes using heterogeneous catalysts have been achieved in this area, but most mechanistic proposals remain unproven, and overall efficiencies can still be improved. 

Photochemical H2-Evolution and Hydrogenation Processes Using Heterogeneous Catalysts.180... 

Rh and Ir complexes stabilized by tertiary (chiral) phosphorus ligands are the most active and the most versatile catalysts. Although standard hydrogenations of olefins, ketones and reductive aminations are best performed using heterogeneous catalysts (see above), homogeneous catalysis becomes the method of choice once selectivity is called for. An example is the chemoselective hydrogenation of a,/ -unsaturated aldehydes which is a severe test for the selectivity of catalysts. 

Examples of hydrogenation using homogeneous catalysts in moderate-scale industrial syntheses exist. Such catalysts are more selective and involve milder reaction conditions (i.e., 25°C and ca. 10 Pa Hj) than their heterogeneous counterparts . Asymmetric hydrogenation of a prochiral amino acid using chiral Rh catalysts yields L-dopa in high optical purity ... 

A number of steroidal dienes have been selectively hydrogenated at the sterically less-hindered position using various catalysts. Raney nickel and PtOa are the most commonly used heterogeneous catalysts for this purpose. Several examples of selectivity in heterogeneous catalytic hydrogenation of dienes are illustrated in Scheme 88. ... 

The stereoselective hydrogenation of folic acid to tetrahydrofolic acid (Scheme 12.18) might represent an attractive alternative to the present unselective process which uses heterogeneous catalysts (diastereomeric excess (de) = 0%). One major problem here is the insolubility of folic acid in most organic solvents. Initial results with up to 42% de but very low TONs and TOFs were obtained by Brunner et al. using a Rh/BPPM complex adsorbed onto silica gel in aqueous buffer solutions [62]. Unfortunately, however, the originally claimed 90% de [62a] turned out to be an analytical artifact [62c]. 

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