Hydrogenation catalyzed by transition metal

Aldehydes may sometimes pose a problem in transfer hydrogenations catalyzed by transition metals. They can poison the catalyst or decarbonylate, forming CO, which may coordinate to the metal complex and result in a change in activity (Scheme 20.26) [65, 66]. 

TJased on several kinetic investigations on hydrogenations catalyzed by transition metal complexes conducted over the last few years, certain general requirements must be fulfilled if a complex is to form an effective homogeneous catalyst in solution (see Ref. 1). One condition is that the catalytically active complex must be coordinatively unsaturated another that M-H or M-C bonds must be present in the complex. 

In recent years there has been much interest in homogeneous hydrogenations catalyzed by transition metal complexes (7). One facet of research in this area is the search for chiral catalysts (catalysts that are dissymmetric, i.e., optically active) that can be used to produce chiral compounds via asymmetric reactions. In this review, we survey asymmetric homogeneous hydrogenation reactions, that is reactions that create asymmetric carbon atoms by the addition of hydrogen across multiple bonds under the influence of soluble chiral catalysts. 

The design and study of the first chiral ligands for asymmetric hydrogenations catalyzed by transition metal complexes occurred in the early 1970s and the field of asymmetric catalysis is still expanding [Brunner and Zettlmeier 1993], fii the field of chiral rea-... 

The racemization mechanism of sec-alcohols has been widely studied [16,17]. Metal complexes of the main groups of the periodic table react through a direct transfer of hydrogen (concerted process), such as aluminum complexes in Meerwein-Ponn-dorf-Verley-Oppenauer reaction. However, racemization catalyzed by transition metal complexes occurs via hydrogen transfer processes through metal hydrides or metal dihydrides intermediates (Figure 4.5) [18]. 

Catecholborane and pinacolborane are especially useful in hydroborations catalyzed by transition metals.163 Wilkinson s catalyst Rh(PPh3)3Cl is among those used frequently.164 The general mechanism for catalysis is believed to be similar to that for homogeneous hydrogenation and involves oxidative addition of the borane to the metal, generating a metal hydride.165... 

Hydrogenation Reactions Catalyzed by Transition Metal Complexes... 

Hydrogenation Reactions Catalyzed by Transition Metal Complexes, 17, 319 Infrared Intensities of Metal Carbonyl Stretching Vibrations, 10, 199 Infrared and Raman Studies of ir-Complexes, 1, 239 Insertion Reactions of Compounds of Metals and Metalloids, 5, 225 Insertion Reactions of Transition Metal-Carbon Bonded Compounds 1. Carbon Monoxide Insertion, 11, 87... 

Ionic hydrogenations of C=C and C=0 bonds were reported prior to the development of ionic hydrogenations mediated or catalyzed by transition metals. Tri-fluoroacetic acid (CF3C02H) as the proton donor and triethylsilane (HSiEt3) as the hydride donor are most commonly used, though a variety of other acids and several other hydride donors have also been shown to be effective. A review [1] by Kursanov et al. of the applications of ionic hydrogenations in organic synthe-... 

Homogeneous Hydrogenation. The mild reaction conditions used in the homogeneous hydrogenation of C02 catalyzed by transition metal complexes allows the partial hydrogenation of C02 to yield formic acid and derivatives ... 

The Metathesis of Unsaturated Hydrocarbons Catalyzed by Transition Metal Compounds J. C. Mol and J. A. Moulijn One-Component Catalysts for Polymerization of Olefins Yu. Yermakov and V. Zakharov The Economics of Catalytic Processes J. Dewing and D. S. Davies Catalytic Reactivity of Hydrogen on Palladium and Nickel Hydride Phases... 

The use of chemiluminescence reactions for the detection of metal ions by liquid chromatography was recently reported [59,60]. The detectors made use of the chemiluminescence produced in the reaction between luminol and hydrogen peroxide which is catalyzed by transition metals. The column effluent was mixed with the reagents in order to yield the chemiluminescence. The reaction was fast and was carried out at room temperature. By varying the pH of the buffer, selectivity towards certain metals was also achieved. For example, at pH 10-11 nickel could be analyzed but lead and aluminium were inactive at pH 13-14, the converse was true [59]. Aminco-Bowman has marketed a liquid chromatographic system in which amino acids and amines are analyzed by means of the fluorescence produced on reaction with the reagent fluorescamine. Fluorescamine does not fluoresce, but it does react with primary amino groups to produce fluorescent derivatives. The reaction is instantaneous and may be carried out at room temperature, usually at pH 9. This detection system promises to be far more sensitive than the ninhydrin detection system and is much more easily adapted to HPLC. 

Abstract The applications of hybrid DFT/molecular mechanics (DFT/MM) methods to the study of reactions catalyzed by transition metal complexes are reviewed. Special attention is given to the processes that have been studied in more detail, such as olefin polymerization, rhodium hydrogenation of alkenes, osmium dihydroxylation of alkenes and hydroformylation by rhodium catalysts. DFT/MM methods are shown, by comparison with experiment and with full quantum mechanics calculations, to allow a reasonably accurate computational study of experimentally relevant problems which otherwise would be out of reach for theoretical chemistry. 

The highly efficient stereoselective transformations catalyzed by transition metals that contain BINAP have resulted in extensive efforts in the development of both new Ru-BINAP catalysts and chiral atropisomeric bisphosphines based on biaryl backbones and biheteroaryl backbones.49 Coupled with the various classes of Ru(BINAP) catalysts and chiral bisphosphines, the number of efficient industrial asymmetric hydrogenations are sure to increase because optimization for fine precision and easy optimization in catalyst activity and enantioselectivity made easier.57 66... 

With one exception (the Me/Cl exchange in Table I), all redistributions of siloxanes catalyzed by transition metals observed to date occur only when at least one silicon-hydrogen bond is present in the molecule. We have established that hexamethyldisiloxane (EE) and hexamethylcyclotri-siloxane (D3) are completely inert under the same conditions that cause extensive redistribution of hydridosiloxanes. 

Hydrogenation Reactions Catalyzed by Transition Metal Complexes, 17, 319 Infrared Intensities of Metal Carbonyl Stretching Vibrations, 10, 199... 

Of the three possible substrates, thiophene, benzo[ ]thiophene, and dibenzo[, thiophene, benzo[ ]thiophene is the most easily hydrogenated to the dihydro derivative this is ascribable to the more pronounced olefinic character of the C(2)-C(3) double bond in benzo[, ]thiophene as compared to that in thiophene. There is no example in the literature of the hydrogenation of dibenzothiophene either to the tetrahydro or the hexahydro stage. The hydrogenation of benzol ]thiophene is catalyzed by transition metals such as Ru, Os, Rh, and Ir. An excellent overview of homogeneous catalytic hydrogenation of thiophenic substrates has been presented recently <2004JOM (689)4277>. 

Kumada et al. have examined a number of chiral ferrocenylphosphines as ligands for asymmetric reactions catalyzed by transition metals. They are of interest because they contain a planar element of chirality as well as an asymmetric carbon atom. They were first used in combination with rhodium catalysts for asymmetric hydrosilylation of ketones with di- and trialkylsilanes in moderate optical yields (5-50%). High stereoselectivity was observed in the hydrogenation of a-acetamidoacrylic acids (equation 1) with rhodium catalysts and ferrocenylphosphines. ... 

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