Isomerization alkene hydrogenation and

The clusters [AuOs3(/A-X)(CO)10(PPh3)] have been attached to phosphine-functionalized silica for X = H or Cl (175,176) or polymer (styrene-divinylbenzene) for X = H (176). On both supports, the immobilized hyd-rido cluster was found to be inactive for alkene hydrogenation and isomerization, whereas the supported Cl-containing species catalyzed alkene hydrogenation. The different behavior was initially incorrectly attributed to different metal framework structures for the two clusters, but, in fact, both species adopt similar butterfly skeletal geometries (12,54). 

The hydrogenation and isomerization of alkenes can usually be described by the classical Horiuti-Polanyi mechanism. According to that mechanism, in a deuterium atmosphere, double bond migration incorporates deuterium into the allylic position. 

The kinetics of secondary hydrogenation and isomerization of 1-alkenes as represented by the reaction scheme is characterized by a negative reaction order with respect to carbon monoxide.13 15... 

The use of modifiers in controlling the selectivity in liquid phase alkyne hydrogenation has been studied. Modifiers that are more strongly bound than the intermediate alkene inhibit hydrogenation and isomerization giving high stereo- and chemo-selectivity. One modifier increased the rate of alkyne hydrogenation... 

Selectivity. The insertion of the coordinated 1-alkene creating an alkyl intermediate may proceed either by Markownikoff or anti-Mark-ownikoff addition, but only Markownikoff addition can lead to isomerization. A mechanism which describes both hydrogenation and isomerization may thus be expressed ... 

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. 

Figure 2 Mechanism of hydrogenation and isomerization of alkenes by heterogeneous catalysts.

The move from a cobalt- to a rhodium-based process that was seen in methanol carbonylation (Section 4.2.4) is echoed in hydroformylation, thus the late 1960s saw the development of rhodium catalysts here too. Wilkinson and his colleagues found that RhH(CO)(PPh3)2 was an outstanding catalyst as it was very selective to aldehyde products (no alcohol formation, no alkene hydrogenation or isomerization occurred) and that very high n-ji- aldehyde selectivities of 20 1 for a... 

Some reactions such as alkene isomerization, alkene hydrogenation, and H2 -I- D2 exchange can be used as sensitive chemical probes of the coordination environment of metal atoms associated with surface-bound metal clusters. Other catalytic reactions such as CO -I- H2 and alkane hydroge-nolysis, which are sensitive to metal ensemble sizes, are applied as a further structural probe. Several attempts have been made to stabilize cluster frameworks in such a way that catalytic activity is maintained. One of the more promising approaches involves the introduction of a capping group into the... 

Because ruthenium catalysts are relatively unreactive for alkene hydrogenation and they are poor for double-bond isomerization, these catalysts are particularly effective for the selective hydrogenation of monosubstituted alkenes in the presence of di- and tri-substituted olefins at ambient temperature under 2-3 atmospheres of hydrogen (Eqn. 15.28). Water in the reaction medium... 

Fig. 24-B-l, Catalytic cycle of hydrogenation and isomerization of 1-alkenes by RhH(CO)(PPh3)3 at 25° and 1 atm pressure.

Wilkinson noted that HRh(CO)(PPh3)2 was very selective for the formation of aldehyde products (no alcohol formation, no alkene hydrogenation or isomerization) and that very high linear to branched aldehyde selectivities of 20 1 for a variety... 

A number of studies have examined alkene hydrogenation catalyzed by Ru3(CO)i2 and substituted derivatives, many reported prior to 1993. One study reported that Ru3(CO)i2 and Ru3(CO)9(PPh3)3 were catalyst precursors for hydrogenation and isomerization of 1-hexene at 303-363 K. After the hydrogenations, the IR spectra of the solutions showed H4Ru4(CO)i2 and H4Ru4(CO)i2 (PPh3) ( =1—4), respectively. In addition to hexane, the isomerization products were to r-2-hexene (up to 61%), m-2-hexene (up to 23%), and /ra r-3-hexene (up to 20%). ... 

Scheme 46 Hydrogenation and isomerization of alkenes catalyzed by Rh/Ta heterobimetallic...

Catalysts of this type are of prime theoretical interest since they allow modeling of the size effects on the catalytic activity. They may be used to exemplify basic theories of the catalysis multiple theory, theory of active ensembles, and theory of skeletal catalysis. Many attempts were made to assess the influence of catalyst particle size on activity. For example, the size of nickel particles was varied from that of an individual atom to that of the bulk metal. Quantitative estimates were made of the effect of the Ni /Si02 and Ni Cu ,/Si02 particle size on the course of hydrogenation. Similar studies were performed of the behavior of deposited polynuclear complexes in the hydrogenation and isomerization of alkenes. ... 

---