Alkene hydrogenation catalyst

A well-understood catalytic cycle is tliat of the Wilkinson alkene hydrogenation (figure C2.7.2) [2]. Like most catalytic cycles, tliat shown in figure C2.7.2 is complex, involving intennediate species in tire cycle (inside tire dashed line) and otlier species outside tire cycle and in dead-end patlis. Knowledge of all but a small number of catalytic cycles is only fragmentary because of tire complexity and because, if tire catalyst is active, tire cycle turns over rapidly and tire concentrations of tire intennediates are minute thus, tliese intennediates are often not even... 

In the mechanism for alkene hydrogenation shown m Figure 6 1 hydrogen atoms are transferred from the catalyst s surface to the alkene Although the two hydrogens are not transferred simultaneously they both add to the same face of the double bond... 

Both objectives have been met by designing special hydrogenation catalysts. The most frequently used one is the Lindlar catalyst, a palladium on calcium carbonate combination to which lead acetate and quinoline have been added. Lead acetate and quinoline partially deactivate ( poison ) the catalyst, making it a poor catalyst for alkene hydrogenation while retaining its ability to catalyze the addition of H2 to the triple bond. 

Alkene hydrogenation occurs on the surface of metal particles which act as a catalyst for the reaction. This usually means that both hydrogens are added to the same face of the alkene syn addition). 

Platinum and palladium are the most common catalysts for alkene hydrogenations. Palladium is normally used as a very fine powder supported" on an inert material such as charcoal (Pd/C) to maximize surface area. Platinum is normally used as PtC, a reagent called Adams catalyst after its discoverer, Roger Adams. 

Figure 7.7 MECHANISM Mechanism of alkene hydrogenation. The reaction takes place with syn stereochemistry on the surface of insoluble catalyst particles.

Complete reduction to the alkane occurs when palladium on carbon (Pd/C) is used as catalyst, but hydrogenation can be stopped at the alkene if the less active Lindlar catalyst is used. The Lindlar catalyst is a finely divided palladium metal that has been precipitated onto a calcium carbonate support and then deactivated by treatment with lead acetate and quinoline, an aromatic amine. The hydrogenation occurs with syn stereochemistry (Section 7.5), giving a cis alkene product. 

These pentahydrides have attracted attention as catalysts for hydrogenation of the double bond in alkenes. IrH5(PPr3)2 catalyses vinylic H-D exchange between terminal alkenes and benzene, the isomerization of a,f3-ynones, isomerization of unsaturated alcohols and dehydrogenation of molecules such as secondary alcohols [176],... 

The mechanism of homogeneous hydrogenation catalyzed by RhCl(Ph3P)3 ° involves reaction of the catalyst with hydrogen to form a metal hydride (PPh3)2RhH2Cl (43), which rapidly transfers two hydrogen atoms to the alkene. 

Scheme 4.17 Simplified alkene hydrogenation mechanism using Wilkinson s catalyst...

The choice of the metals is strictly related to the catalytic application. As we shall show later, the catal54ic reaction most commonly investigated with polymer supported M / CFP catalysts is hydrogenation (Table 3). The overwhelming majority of catalytic studies concerns the hydrogenation of alkenes and by far the most commonly employed metal is palladium, followed by platinum. Examples of rhodium and ruthenium hydrogenation catalysts supported on pol5uneric supports are very rare. 

Most low-valence metal complexes are generally deactivated by air and sometimes also by water. Carbon monoxide, hydrogen cyanide, and PH3 frequently act as poisons for these catalysts. Poisoning by strongly co-ordinating molecules occurs by formation of catalytically inert complexes. An example is the poisoning of Wilkinson s catalyst for alkene hydrogenation ... 

The mechanism of alkene hydrogenation catalyzed by the neutral rhodium complex RhCl(PPh3)3 (Wilkinson s catalyst) has been characterized in detail by Halpern [36-38]. The hydrogen oxidative addition step involves initial dissociation of PPI13, which enhances the rate of hydrogen activation by a factor... 

Since edges (and presumably ledges) are now associated with double bond migration,98 and since apparent trans addition is a function of the double bond migration ability of various catalysts, perhaps such locations can produce both processes. The fact that tetrasubstituted alkenes hydrogenate much more slowly than tri-, di-, or monosubsituted alkenes would allow greater... 

Jacinto, M.J., Landers, R. and Rossi, L.M. (2009) Preparation of supported Pt(0) nanopartides as efficient recyclable catalysts for hydrogenation of alkenes... 

Another example of selective C=C bond hydrogenation has arisen from mechanistic studies on an iron m-hydride dihydrogen complex, [Fe(PP3)(FI)(H2)](BF4) [PP3 = P(CH2CH2PPh2)3], a catalyst inactive with alkene substrates. Scheme 6 shows that no decoordination of dihydrogen is required in any step of the cycle and that the vacant site is created by unfastening of one of the P-donor atoms (species (16)).50 Extensive studies on catalytic alkene hydrogenation by analogous tripodal (polyphosphine) Rh, Os, and Ir complexes have been carried by Bianchini and co-workers.51,52... 

The trinuclear cluster [(/i-H)2Ru3(/i3-0)(C0)5(DPPM)2] is also an efficient catalyst for alkene hydrogenation reaction, for which Bergounhou proposed the catalytic Scheme 73.38... 

Hydrogenation catalysts from non-platinum group have been also reported. For example, some organolanthanides of formula [ (r/5-C5Me5)2MH)2] are active catalyst for alkene hydrogenation.384 It has been proposed on the basis of kinetic studies that first the dimer dissociates according to (Equation (18)),... 

The monosulfonated PPh derivative, Ph2P(m-C6H4S03K) (DPM) and its rhodium complex, HRh(CO)(DPM)3 have been synthesized and characterized by IR and NMR spectroscopic techniques. The data showed that the structure was similar to [HRh(CO)(PPh3)3]. The catalytic activity and selectivity of [HRh(CO)(DPM)3] in styrene hydroformylation were studied in biphasic catalytic systems.420 421 Rh1 complexes [Rh(acac)(CO)(PR3)] with tpa (131), cyep (132), (126), ompp (133), pmpp (134), tmpp (135), PPh2(pyl), PPh(pyl)2, and P(pyl)3 were characterized with NMR and IR spectra. Complexes with (131), (132), and (126) were catalysts for hydrogenation of C—C and C—O bonds, isomerization of alkenes, and hydroformylation of alkenes.422 Asymmetric hydroformylation of styrene was performed using as catalyst precursor [Rh(//-0 Me)(COD)]2 associated with sodium salts of m-sulfonated diarylphosphines.423... 

Other catalytic hydrocarbon reactions indude decomposition of olefins over a powdered nickel catalyst [84], hydrogenation of alkenes, hydrocracking of cycloalk-enes, and water-gas shift reactions [64]. 

Consequently, it is I ICo(CN)s3 that functions as a catalyst in hydrogenation processes. In the first step of the process shown in Figure 22.9, the alkene coordinates to HCo(CN)s3 as one hydrogen atom is added to the molecule so that only one double bond remains. The monoene is bonded to the cobalt in rf fashion. In the second step, another HCo(CN)53- transfers hydrogen to the alkene, which undergoes reductive elimination and leaves, having been converted to 1-butene. 

Alkene hydrogenation as a side reaction, which other than the direct efficiency loss it represents, is not particularly challenging from the separations viewpoint. Lighter al-kenes are usually separated in a flash stage prior to separation of the product from the catalyst. Higher molecular weight alkenes may be separated from the catalyst along with the product. 

The First Catalysts for Alkene Hydrogenation Mechanistic Considerations... 

Scheme 3.2 Preparation of the alkene hydrogen catalyst RuHCI(PPh3)3.

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