Hydrogenation Using Homogeneous Catalysts

In some cases an alternative sequence involving addition of hydrogen at rhodium prior to complexation of the alkene may operate.11 The phosphine ligands serve both to provide a stable soluble complex and to adjust the reactivity at the metal center. The a-bonded intermediates have been observed for Wilkinson s catalyst12 and for several other related catalysts.13 For example, a partially hydrogenated structure has been isolated from methyl a-acetamidocinnamate.14 

The stereochemistry of reduction by homogeneous catalysts is often controlled by functional groups in the reactant. Delivery of hydrogen occurs cis to a polar functional group. This behavior has been found to be particularly characteristic of an iridium-based catalyst that contains cyclooctadiene, pyridine, and tricyclohexylphosphine as ligands, known as the Crabtree catalyst 6 Homogeneous iridium catalysts have been found to be influenced not only by hydroxy groups, but also by amide, ester, and ether substituents.17 

Addition of Hydrogen at Carbon-Carbon Multiple Bonds 

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

Presumably, the stereoselectivity in these cases is the result of coordination of iridium by the functional group. The crucial property required for a catalyst to be stereodirective is that it be able to coordinate with both the directive group and the double bond and still accommodate the metal hydride bonds necessary for hydrogenation. In the iridium catalyst illustrated above, the cyclooctadiene ligand (COD) in the catalysts is released by hydrogenation, permitting coordination of the reactant and reaction with hydrogen. 

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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 ... 

The industrially used homogeneous catalysts for the hydroformylation of higher molecular olefins into aldehydes, which are hydrogenated to the corresponding surfactant alcohols, are cobalt carbonyl [47] or cobalt carbonylItert-phosphine complexes [48]. 

There is much current excitement and activity in the field of homogeneous hydrogenation using ruthenium catalysts. This is reflected in the recent, explosive increase in the number of research publications in this area, now rivaling those for rhodium catalysts (Fig. 3.1). Meanwhile, the price of rhodium metal has risen dramatically, becoming about ten times that of ruthenium, on a molar basis. The number of reports on the use of osmium catalysts has remained low, partly because of the higher price of osmium compounds - about ten times that of ruthenium - and partly because the activity of osmium catalysts is often lower. 

There were several attempts to use homogeneous catalysts in the hydrogenation of fatty acid esters. Pt, Pd and Mo complexes were investigated. The monoene selectivity and the activity of Pt catalysts were good, but their separation from the product remained an unsolved problem. They were active in a double-bond migration reaction which, however, was accompanied by the disadvantageous cis-trans isomerization9,31. 

Besides solid transition metals, certain soluble transition-metal complexes are active hydrogenation catalysts.4. The most commonly used example is tris(triphenylphosphine)-chlororhodium, which is known as Wilkinson s catalyst.5 This and related homogeneous catalysts usually minimize exchange and isomerization processes. Hydrogenation by homogeneous catalysts is believed to take place by initial formation of a rc-complex, followed by transfer of hydrogen from rhodium to carbon. 

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. 

Recall that in the majority of reactions using homogeneous catalysts in the liquid phase, a gas phase is also present, mainly hydrogen and/or carbon monoxide. This diffusion of gas in liquid can falsify the kinetics. In this section we consider the modeling of gas-liquid reactions in the absence of diffusional effects. 

Hydrogenation of Organic Compounds using Homogeneous Catalysts"... 

Several strategies have been developed to characterise the structural and functional role of particular molecular species of membrane lipid. The use of phospholipid exchange or transfer proteins have been described to exchange one molecular species of a lipid class for another or enrich membranes in specific molecular species, respectivly. Another method has been to hydrogenate the double bonds of polyunsaturated membrane lipids in situ using homogeneous catalysts and to study the consequences of lipid saturation on membrane functions [2]. With the isolation of mutant strains of Arabidopsis defective in desaturation of chloroplast fatty acids [3,4] the question can be addressed directly without the problems associated with the use of exchange or transfer proteins or introduction of transition metal catalysts. 

The hydrogenation of organic compounds using homogeneous catalysts has been reviewed. ... 

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