Hydrogenation, catalytic, alkene formation

One of the most famous catalysts, which operates through a mechanism involving formation of a mono-hydride (M—H), is [RuCl2(PPh3)3].38-40 In the catalytic hydrogenation of alkenes (Equations (1) and (2)) it shows very high selectivity for hydrogenation of terminal rather than internal C=C bonds. 

The same authors also reported the dispersion of palladium nanoparticles in a water/AOT/n-hexane microemulsion by hydrogen gas reduction of PdClJ and its efficiency for hydrogenation of alkenes in organic solvents [79]. UV-visible spectroscopy and TEM analysis revealed the formation of Pd nanoparticles with diameters in the range of 4 to 10 nm. Three olefins (1-phenyl-l-cyclohexene, methyl trans-cinnamate, and trans-stilbene) were used as substrates for the catalytic hydrogenation experiments under 1 atm of H2 (Table 9.12). All of the Start-... 

Reduction with metals in weakly acidic solvents is not restricted to arenes. A useful related reaction reduces alkynes to trans-alkenes, and provides a useful alternative to catalytic hydrogenation, which favors formation of cw-alkenes (Section 11-2A) ... 

A further step towards optimised conditions in the catalytic transfer hydrogenation of alkenes was achieved with the introduction of the ionic liquid N-butyl-N -methylimidazolium hexafluorophosphate (BMIMPFg) as a solvent. The reduction of alkenes with formates and Pd/C in BMIMPF6 leads to saturated hydrocarbons in high yields. With an alkyne, a mixture of cis/trans alkenes and the saturated alkane was obtained (Scheme 4.5). Sufficiently pure products were isolated by a simple liquid-liquid... 

As shown in Scheme 3.19, two competing pathways are possible with regard to allylic oxidation. The alkene 1 can either undergo abstraction of an allylic hydrogen and subsequent formation of the allylic alcohol 2 and the enone 3 (path A), respectively, or alternatively epoxidation of the C=C double bond occurs to give derivative 4 (path B). In order to develop a suitable catalytic system for path A, it is of utmost importance to achieve high chemoselectivity in addition to high catalytic... 

Mechanism 8-8 Formation of Halohydrins 352 8-10 Catalytic Hydrogenation of Alkenes 355 8-11 Addition of Carbenes to Alkenes 358 8-12 Epoxidation of Alkenes 360... 

All these ligands have extensive chemistry here we note only a few points that are of interest from the point of view of catalysis. The relatively easy formation of metal alkyls by two reactions—insertion of an alkene into a metal-hydrogen or an existing metal-carbon bond, and by addition of alkyl halides to unsaturated metal centers—are of special importance. The reactivity of metal alkyls, especially their kinetic instability towards conversion to metal hydrides and alkenes by the so-called /3-hydride elimination, plays a crucial role in catalytic alkene polymerization and isomerization reactions. These reactions are schematically shown in Fig. 2.5 and are discussed in greater detail in the next section. 

The partial reduction of substrates containing triple bonds is of considerable importance not only in research, but also commercially for stereoselectively introducing (Z)-double bonds into molecular frameworks of perfumes, carotenoids, and many natural products. As with catalytic hydrogenation of alkenes, the two hydrogen atoms add syn from the catalyst to the triple bond. The high selectivity for alkene formation is due to the strong absorption of the alkyne on the surface of the catalyst, which displaces the alkene and blocks its re-adsorption. The two principal metals used as catalysts to accomplish semireduction of alkynes are palladium and nickel. 

Ni, and Pd or Pt on carbon. For example, the hydrogenation of alkenes by Ni powder is enormously enhanced (>105-fold) by ultrasonic irradiation.10 This dramatic increase in catalytic activity is due to the formation of uncontaminated metal surfaces from interparticle collisions caused by cavitation-induced shockwaves. 

The reason for the formation of the trans products is thought to be because of migration of the double bond in a partially hydrogenated product on the catalyst surface. Although catalytic hydrogenation of alkenes may be accompanied by migration of the double bond, no evidence of migration normally remains on completion of the reduction. Sometimes, however, a tetrasubstituted double bond is formed which resists further reduction. 

Titanium. Catalyses of hydrogenation of alkenes, alkynes, carbonyl-, and nitro-compounds have been described. The effect of the nature of the ligand L and of the alkene to be reduced on reactivity in catalytic hydrogenation by Ti(7r-C5H5)2L2 has been quantitatively studied. The dependence of rate constants on solvent for reduction of decene in the presence of Ti(7r-C5H5)Me+ is interpreted in terms of electrostatic interaction between the active ionic species and the solvent. There is also a thermochemical report relevant here, and that is of a determination of the heats of mixing of cyclohexene and of hex-l-ene with titanium tetrachloride. The heats of mixing are close to zero, which implies very small heats of complex formation between these alkenes and titanium. ... 

Naphthaleneytterbium demonstrates a high activity not only in stoichiometric reactions but also in many catalytic processes. It catalyzes at room temperature the polymerization of styrene, methylmetacrylate, ethylacrylate, isoprene, ethylene oxide, the copolymerization of ethylene oxide with styrene, piperilene and carbon dioxide [65], the hydrogenation of alkenes, alkynes, ketones, aldehydes [78], the formation of alkylenecarbonates from epoxides and CO2 [65]. 

Fischer-Tropsch Process. The Hterature on the hydrogenation of carbon monoxide dates back to 1902 when the synthesis of methane from synthesis gas over a nickel catalyst was reported (17). In 1923, F. Fischer and H. Tropsch reported the formation of a mixture of organic compounds they called synthol by reaction of synthesis gas over alkalized iron turnings at 10—15 MPa (99—150 atm) and 400—450°C (18). This mixture contained mostly oxygenated compounds, but also contained a small amount of alkanes and alkenes. Further study of the reaction at 0.7 MPa (6.9 atm) revealed that low pressure favored olefinic and paraffinic hydrocarbons and minimized oxygenates, but at this pressure the reaction rate was very low. Because of their pioneering work on catalytic hydrocarbon synthesis, this class of reactions became known as the Fischer-Tropsch (FT) synthesis. 

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