Hydrogenation of phenylacetylene

The complex OsHCl(CO)(P Pr3)2 is also an effective catalyst precursor for the selective hydrogenation of benzylideneacetone to 4-phenylbutan-2-one.97 In contrast to the hydrogenation of phenylacetylene, kinetic studies on the hydrogenation of benzylideneacetone by OsHCl(CO)(P Pr3)2 show that the reaction is independent of the pressure of hydrogen and first order with respect to the concentration... 

High chemoselectivities (up to 96%) were reached in the hydrogenation of phe-nylacetylene to styrene (DMF, 30 °C, 1 atm H2 pressure, 1% catalyst loading, TOFs up to 4.2 h-1) catalyzed by chloride Pd(II) complexes [Pd(NN S)Cl] containing thiosemicarbazone or thiobenzoylhydrazone ligands (5 in Scheme 4.5) [45]. Instead, minor reactivities and selectivities were obtained with NN N" pyridyl-hydrazone-containing Pd(II) complexes in the hydrogenation of phenylacetylene [46]. 

A similar H2 activation mechanism was proposed for the [Pd(NN S)Cl] complexes (5 in Scheme 4.5) in the semi-hydrogenation of phenylacetylene [45] after formation of the hydride 14 (Scheme 4.9), coordination of the alkyne occurs by displacement of the chloride ligand from Pd (15). The observed chemos-electivity (up to 96% to styrene) was indeed ascribed to the chloride anion, which can be removed from the coordination sphere by phenylacetylene, but not by the poorer coordinating styrene. This was substantiated by the lower che-moselectivities observed in the presence of halogen scavengers, or in the hydrogenations catalyzed by acetate complexes of formula [Pd(NN S)(OAc)]. Here, the acetate anion can be easily removed by either phenylacetylene or styrene. 

Catalytic studies and kinetic investigations of rhodium nanoparticles embedded in PVP in the hydrogenation of phenylacetylene were performed by Choukroun and Chaudret [90]. Nanoparticles of rhodium were used as heterogeneous catalysts (solventless conditions) at 60 °C under a hydrogen pressure of 7 bar with a [catalyst]/[substrate] ratio of 3800. Total hydrogenation to ethylbenzene was observed after 6 h of reaction, giving rise to a TOF of 630 h 1. The kinetics of the hydrogenation was found to be zero-order with respect to the al-kyne compound, while the reduction of styrene to ethylbenzene depended on the concentration of phenylacetylene still present in solution. Additional experi-... 

Scheme 12.6 Hydrogenation of phenylacetylene using a colloidal Pd-catalyst system.

In contrast, 1,5-cyclo-octadiene remains coordinated during the catalytic cycle of hydrogenation of phenylacetylene to styrene, catalyzed by the related iridium complex [Ir(C0D)( Pr2PCH2CH20Me)]BF4. This complex, which contains an ether-phosphine-chelated ligand, catalyzes the selective hydrogenation reaction via a dihydrido-cyclo-octadiene intermediate. The reaction is first order in each of catalyst, phenylacetylene and hydrogen [11] the proposed catalytic cycle is shown in Scheme 2.3. 

Fig. 22. Photoinduced hydrogenation of phenylacetylene in an organized water-oil two phase system. The photosystem is localized in the aqueous phase and the heterogeneous catalyst is present in the oil phase. Electron transfer communication between the two phases is mediated by C8V +...

Another indication that electronic properties of Pd may be important in hydrogenation reactions originates from the work of Carturan et al. (167), who investigated palladium supported on vitreous materials in hydrogenation of phenylacetylene. A relatively better catalytic activity of the catalyst with smaller alkaline content (Na20) suggests that an electron transfer from Pd to the support is smaller in the case of less acidic (containing more alkaline) supports. Similar metal particle sizes (2.8-3.4 nm) exhibited by all the catalysts rule out an explanation that takes into account a surface sensitivity of this reaction. 

Lopez et al. [27] prepared Pd/SiC>2 catalysts under both acidic (pH = 3) and basic (pH = 9) conditions in the sol-gel step and reported that an acid medium promotes the formation of small metal crystallites. This finding is consistent with the formation of a micro-porous silica gel network at a low pH. By comparing samples prepared by the sol-gel method and impregnation, these authors found in the former a stronger metal-support interaction which they ascribed to the square planar palladium complex used as a precursor. Finally, their results showed that the method of preparation as well as the conditions used in each method impact on how these catalysts deactivate in the hydrogenation of phenylacetylene. 

Hydrogenation of phenylacetylene Pd alloy Pd alloy spiral tube 5 =92% Cis 10 times that of premixed system Gryaznov, 1992... 

Ti(Cp)2(CO)2l is a catalyst for the hydrogenation of phenylacetylene to ethylbenzene, while alkyl-substituted terminal alkynes are reduced to alkenes. Electron rich titanium(II) complexes, [Cp2Ti(PhC OPh)(PMe3)], [(MeCp)2Ti(PhC=CPh)(PMe3)] and [CpCp Ti(PhCsCPh)] are also catalyst precursors for the hydrogenation of alkynes to alkanes at 20 C under atmospheric pressure of hydrogen. "... 

Effects of crystalline structure and acidity differentiate the catalytic behavior of ZSM5, USY and mordenite zeolites. Compounded with the nature of the support, the location of nickel particles leads to very peculiar behaviors in the formation of low-temperature coke during the hydrogenation of phenylacetylene. The principal differences in the high temperature deactivation are determined by the size specificity of the zeolitic supports, and by the high acidity available to the reactant molecules, especially for the USY support. The contribution of nickel to coke formation at low temperatures occurs mainly at the internal surface of Ni/mordenite and Ni/USY and at the external surface of Ni/ZSM-5. This conclusion is supported by the TPR patterns as well as by the relative values of low, intermediate and high temperamre coke for each individual support. 

Catalytic homogeneous hydrogenation of phenylacetylene to styrene was achieved with Ru5(CO)i3(/t5-C)(PhC2H)(PtP(/-Bu)3) as catalyst precursor. Also formed was the hydride-containing cluster Ru5(CO)i2(/t5-C)(PtP(/-Bu)3)(PhC2H)(/r-H)2. The Pt atom promoted catalytic activity. ... 

Heterogeneous Ru catalysts were prepared by thermolysis of Ru3(/t-H)(/t3,77 -ampy)(CO)9 supported on silica and alumina. The face-bridging ampy ligand does not prevent cluster fragmentation on the support surface, and mononuclear Ru surface species are formed. The heterogeneous catalysts are poorer for hydrogenation of phenylacetylene than the cluster is in homogeneous solution. ... 

A real example following the H2-decoordination mechanism is provided by the selective hydrogenation of 1-alkynes to alkenes with the ruthenium(II) complex [(PP3)RuH(Ti2-H2)]BPh4 [PP3 = P(CH2CH2PPh2)3l [25]. A kinetic study of the hydrogenation of phenylacetylene at ambient or sub-ambient pressure showed that the reaction is first-order in catalyst concentration, second order in H2 pressure and independent of substrate concentration. At very low 1-alkyne concentration (<0.12 M), a first order dependence with respect to the substrate was observed. 

The planar clusters [W2Pd2Cp2(CO)6(PR3)2] (3e, 3f) catalyze the hydrogenation and isomerization of 1,5-cyclooctadiene and the hydrogenation of phenylacetylene to a mixture of styrene (major) and ethylbenzene. These clusters also catalyze the photoinitiated hydrosilation of 1-pentene and butadiene oligomerization. ... 

Figure 6.2. Hydrogenation of phenylacetylene catalyzed by a solution of [Rh2Cl(CO)3(dppm)2lRhCl2(CO)2] in methanol (2.00 x 10 M 22°C).

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