Transition metal complexes hydrogenation catalyzed

A number of other soluble transition metal complexes may catalyze hydrogenation of styrene. These include HCo(CO)4, SnCl/H2PtCl2, KNi2(CN)2, RhCl(PPh3)3, RhH(CO)(PPh3)3, ... 

A substantial percentage of the transition metal complexes which catalyze the hydrogenation of olefins and acetylenes have a [Pg.84]

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]. 

Two significant communications indicate the considerable potential of transition metal complexes as multifunctional homogeneous catalysts in the silane field (5, 53). Here the same catalyst activates silanes toward different substrates and it is probable that all proceed via a common metal hydrido intermediate. Both Co2(CO)8 and (Ph3P)3CoHX [X = H2, N2, or (H)Si(OEt)j] catalyze 0-silylation and hydrosilylation the hydrogen on Si may be replaced by R O, R COO, R CONH, or R3SiO [e.g., Eqs. (117)-(120)], and excellent yields of silylated product result. Phenolic groups do... 

Hydrogenation Reactions Catalyzed by Transition Metal Complexes... 

Asymmetric hydrogenations catalyzed by supported transition metal complexes have included use of both chiral support materials (poly-imines, polysaccharides, and polyalcohols), and bonded chiral phosphines, although there have been only a few reports in this area. 

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... 

Table 6.21 Hydrogenation of benzene and naphthalene catalyzed by silica-supported Group IV-VI transition-metal complexes.

The use of ionic liquids has been successfully studied in many transition metal-catalyzed hydrogenation reactions, ranging from simple alkene hydrogenation to asymmetric examples. To date, almost all applications have included procedures of multiphase catalysis with the transition-metal complex being immobilized in the ionic liquid by its ionic nature or by means of an ionic (or highly polar) ligand. 

Transition Metal-Complex Catalyzed Hydrogenation of Carbonyl Compounds... 

In summary, many attempts have been made at achieving enantioselective reduction of ketones. Modified lithium aluminum hydride as well as the ox-azaborolidine approach have proved to be very successful. Asymmetric hydrogenation catalyzed by a chiral ligand-coordinated transition metal complex also gives good results. Figure 6-7 lists some of the most useful chiral compounds relevant to the enantioselective reduction of prochiral ketones, and interested readers may find the corresponding applications in a number of review articles.77,96,97... 

Thus far, we have discussed the transition metal complex-catalyzed hydrogenation of C=C, C=0, and C N bonds. In this section, another type of transition metal complex-mediated reaction, namely, the hydroformylation of olefins, is presented. 

Many transition metal complexes catalyze homogeneous activation of molecular hydrogen in solution, forming hydrido complexes. Such complexes include pentacyanocobaltate(II) anion, [Co(CN)5], many metal carbonyls, and several complexes of rhodium, iridium, and palladium. 

The catalyzed exchange of hydrogen in an organic molecule by deuterium from the solvent or from deuterium gas has been much used to study alkane activation by transition metal complexes it is discussed... 

The previous examples involve reduction (hydrogenation) of organic molecules, but transition metal complexes can also catalyze oxidation. For example, the Wacker process, which has been widely used to convert ethylene to acetaldehyde, depends on catalysis by palladium(II) in the presence of copper(II) in aqueous HC1. The role of the copper chloride is to provide a means of using air to reoxidize the palladium to palladium(II). Once again, Zeise-type coordination of the ethylene to the metal center is believed to be involved ... 

Asymmetric synthesis (i) has gained new momentum with the potential k use of homogeneous catalysts. The use of a transition metal complex with chiral ligands to catalyze a synthesis asymmetrically from a prochiral substrate is beneficial in that resolution of a normally obtained racemate product may be avoided. In certain catalytic hydrogenations of olefinic bonds, optical purities approaching 100% have been attained (2,3,4,5) hydrogenations of ketones (6,... 

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 ... 

Racemic diphosphines may be resolved by using transition metal complexes that contain optically active olefinic substrates (Scheme 11) (24). When racemic CHIRAPHOS is mixed with an enantiomerically pure Ir(I) complex that has two ( —)-menthyl (Z)-a-(acetam-ido)cinnamate ligands, (S,5)-CHIRAPHOS forms the Ir complex selectively and leaves the R,R enantiomer uncomplexed in solution. Addition of 0.8 equiv of [Rh(norbomadiene)2]BF4 forms a catalyst system for the enantioselective hydrogenation of methyl (Z)-a-(acetamido)cinnamate to produce the S amino ester with 87% ee. Use of the enantiomerically pure CHIRAPHOS-Rh complex produces the hydrogenation product in 90% ee. These data indicate that, in the solution containing both (S,S)-CHIRAPHOS-Ir and (/ ,/ )-CHIRAPHOS-Rh complexes, hydrogenation is catalyzed by the Rh complex only. 

This paper presents the results of an investigation of the oxidation of substituted olefins in the presence of hydrocarbon-soluble transition metal complexes. Results indicate that the initial interaction of oxygen with the olefin probably does not occur within the coordination sphere of the metal. The best interpretation appears to be autoxidation of the olefin, initiated either by the metal or by metal catalyzed decomposition of peroxidic impurities. The initial product of an olefin having allylic hydrogens is an allylic hydroperoxide species this is usually the case in radical initiated autoxidations. Nonetheless, with some metal complexes the product profile differs markedly from that observed when radical initiators are used. In the presence of several complexes, oxidation is... 

---