Hydrogenation, of olefins

A lot of work has been done with the simplest olefin, ethene. The following points can now be considered as well established. 

With all metals from the third to tenth column (Sc to Ni, etc.) of the Periodic Table, a clean metallic surface is too reactive and, at the temperature of freely running hydrogenation, most of the surface is always covered by fragments of ethene. The degree of the C-H and C-C bond dissociation depends on the temperature and pressure (see Chapter 4). The dissociative adsorbed forms can react to give ethane (or methane), but this is a rather slow reaction [64,65], compared with hydrogenation of the weakly adsorbed ethene. 

At the remaining surface, where reactive species are adsorbed, a competition takes place between adsorbed hydrogen and ethene. The formal kinetic equation in the form of a power law is then (approx.) rate = k pgr Ph2 where a is near to zero or slightly negative. Since adsorption of hydrogen is dissociative, one would expect p 2, if H atoms are added one by one. The reasons why a can be different are discussed elsewhere (see Section 5.2 on syngas reactions). The reactive form 

The order in activity of various metals (with well defined surfaces) in hydrogenation of ethene was first established by Beeck et al. [70] and the data (see 

It can be concluded that the most active metals are those that show the lowest heats of adsorption, but which extensively adsorb both reaction components (note metals like Cu, Ag, Zn, etc. do not do so and are much less active than those shown). Metals in the third to sixth group not only adsorb ethene dissociatively, but in contact 

The actual hydrogenation [7-3] to [7-4] and [7-4] to [7-1] illustrates a cis ligand orientation effect. Two ligands oriented cis to each other in either a six or four coordinated species are considered favorably situated for subsequent reaction. In some cases a new ligand is formed as in [7-3] to [7-4], and in other cases a new compound is produced [7-4] to [7-1]. 

As indicated earlier, the other coordination principles involved shall be discussed in subsequent examples. 

Halpern has utilized the principles of reactivity or coordination compounds to explain reaction mechanisms, some examples of which involve metal 7r-complexes. His concept of hydrogenation of olefins is based upon the formation and cleavage of a hydrido transition metal complex. 

Complexes of Cu(II), Cu(I), Ag(I), Hg(II), Hg(I), Co(I), Co(II), Pd(II), Pt(II), Rh(I), Rh(II), Ru(II), Ru(III), and Ir(I) have catalyzed homogeneous hydrogenation reactions in solution. In each case H2 is split by the catalyst with the formation of a reactive transition-metal hydride (or hydrido) complex as an intermediate. Three distinct mechanisms have been advanced, as given below. The first mechanism involves heterolytic splitting  

The second and third mechanisms involve either homolytic splitting or oxidative addition  

Zirconium oxide exhibits hydrogenation activity following heat treatment at relatively low temperatures the maximum activity is obtained at the pretreatment temperature of 873 K. It is noteworthy that the transfer hydrogenation of 1,3-butadiene using cyclohexadiene as the hydrogen source occurs at the maximum rate when Zr02 is pretreated at 1073K.  

Ail active rare earth oxides show maximum hydrogenation activity following pretreatment at about 923 This temperature also gives the maximum activity 

The characteristic features of olefin hydrogenation on solid base catalysts are as follows.  

Conjugated dienes undergo hydrogenation much faster than monoenes. For instance, 1,3-butadiene undergoes hydrogenation at 273 K over solid base catalysts, while butenes appreciably react above 473 K over alkaline earth oxides, and above 373 K over rare earth oxides. The products in diene hydrogenation consist exclusively of monoenes, no alkanes being formed at 273 K. 

3-butadiene hydrogenation, 2-butenes are preferentially yielded over solid base catalysts, while 1-butene is the main product over conventional hydrogenation catalysts. 

RSC Catalysis Series No. 2 Chiral Sulfur Ligands Asymmetric Catalysis By Helene Pellissier Helene Pellissier 2009 

In general, of the mixed phosphorus-thioether ligands that have been used in the asymmetric hydrogenation of prochiral olefins, the thioether-phosphinite ligands have provided some of the best results. As an example, a new class of thioether-phosphinite ligands developed by Evans et al. has recently proved to be very efficient for the rhodium-catalysed asymmetric hydrogenation of a 

Another class of chiral diphosphine ligands bearing two interconnected thiophene rings, has been successfully developed by Sannicolo et al. These bis(diphenylphosphino)[Z)]thiophene ligands, called tetraMe-BITIANP and 

Hydrogenation of olefin with thiophene-based ligands. 

Several S/N ligands have also been investigated for the asymmetric hydrogenation of prochiral olefins. Thus, asymmetric enamide hydrogenations have been performed in the presence of S/N ligands and rhodium or ruthenium catalysts by Lemaire et al., giving enantioselectivities of up to 70% ee. Two 

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Chiral ligands for homogeneous hydrogenation of olefins and ketones... 

Hydrogenation of olefins, enols, or enamines with chiral tVilkinson type catalysts, e.g., Noyort hydrogenation. Hydroboration of olefins with chiral boranes. Sharpless epoxi-dation of allylic alcohols. 

The catalysts with the simplest compositions are pure metals, and the metals that have the simplest and most uniform surface stmctures are single crystals. Researchers have done many experiments with metal single crystals in ultrahigh vacuum chambers so that unimpeded beams of particles and radiation can be used to probe them. These surface science experiments have led to fundamental understanding of the stmctures of simple adsorbed species, such as CO, H, and small hydrocarbons, and the mechanisms of their reactions (42) they indicate that catalytic activity is often sensitive to small changes in surface stmcture. For example, paraffin hydrogenolysis reactions take place rapidly on steps and kinks of platinum surfaces but only very slowly on flat planes however, hydrogenation of olefins takes place at approximately the same rate on each kind of surface site. 

As a result of rather extensive work on the hydrogenation of olefins 139,151 mechanism originally proposed by Horiuti and Polanyi is currently accepted ... 

The hydrogenation of -olefins requires isomerization of the double bond to the 14 position prior to hydrogenation. 

A very significant recent development in the field of catalytic hydrogenation has been the discovery that certain transition metal coordination complexes catalyze the hydrogenation of olefinic and acetylenic bonds in homogeneous solution.Of these catalysts tris-(triphenylphosphine)-chloror-hodium (131) has been studied most extensively.The mechanism of the deuteration of olefins with this catalyst is indicated by the following scheme (131 -> 135) ... 

Double-bond migrations during hydrogenation of olefins are common and have a number of consequences (93). The extent of migration may be the key to success or failure. It is influenced importantly by the catalyst, substrate, and reaction environment. A consideration of mechanisms of olefin hydrogenation will provide a rationale for the influence of these variables. 

Studies on the dimerization and hydrogenation of olefins with transition metal catalysts in acidic chloroaluminate(III) ionic liquids report the formation of higher molecular weight fractions consistent with cationic initiation [L7, 20, 27, 28]. These... 

Hydrogenation of olefinic unsaturation using ruthenium (Ru) catalyst is well known. It has been widely used for NBR hydrogenation. Various complexes of Ru has been developed as a practical alternative of Rh complexes since the cost of Ru is one-thirtieth of Rh. However, they are slightly inferior in activity and selectivity when compared with Rh catalyst. 

Rennard and Kokes (39) in their paper stated directly that their purpose was just to study the catalytic activity of palladium hydride in the hydrogenation of olefins, in this case ethylene and propylene. Kokes (39a) in his article recently published in Catalysis Reviews summarizes the results of studies on such catalytic systems. 

A MOF constructed from rhodium paddlewheel clusters linked to porphyrinic ligands already discussed in Section 4.3.1.1 shows an interesting synergetic behavior when the porphyrinic rings are loaded with metals like Cu , Ni , or Pd . In the hydrogenation of olefins, the hydride species at the rhodium center is transferred to the coordinated olefin adsorbed on a metal ion in the center of the porphyrin ring to form an alkyl species, and next this alkyl species reacts with a hydride species activated at the rhodium center to form the alkane [81]. 

Scheme 3 Plausible pathway for hydrogenation of olefin catalyzed by the Fe-H complex...

Scheme 6 Proposed mechanism for catalytic hydrogenation of olefin with 5...

Redox-type reactions show by far the worst performance in meeting the golden atom economical threshold. Three reductions meet this criterion with (AE)min values of 1 hydrogenation of olefins using the Lindlar catalyst (1952), Noyori stereoselective hydrogenation reaction (1985), and Zincke disulphide cleavage reaction (1911) whereas, oxidations... 

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