Ruthenium, homogeneous alkene hydrogenation

In Section 5.9, we saw that alkenes can be converted to alkanes by catalytic hydrogenation by a variety of catalysts, such as palladium and platinum. These are heterogeneous catalysts. We also noted that homogeneous catalytic hydrogenation can be carried out by Wilkinsons catalyst, Ru[(PPh3)3Cl. We now return to that subject to discuss the reaction mechanism. We will find that hydrogenation by Wilkinson s catalyst occurs in a catalytic cycle that is strikingly similar to the catalytic cycles of the reactions we have discussed thus far in this chapter. The transition metal in the Wilkinson catalyst, however, is ruthenium, not palladium. 

Catalytic hydrogenation is mostly used to convert C—C triple bonds into C C double bonds and alkenes into alkanes or to replace allylic or benzylic hetero atoms by hydrogen (H. Kropf, 1980). Simple theory postulates cis- or syn-addition of hydrogen to the C—C triple or double bond with heterogeneous (R. L. Augustine, 1965, 1968, 1976 P. N. Rylander, 1979) and homogeneous (A. J. Birch, 1976) catalysts. Sulfur functions can be removed with reducing metals, e. g. with Raney nickel (G. R. Pettit, 1962 A). Heteroaromatic systems may be reduced with the aid of ruthenium on carbon. 

The most widely used method for adding the elements of hydrogen to carbon-carbon double bonds is catalytic hydrogenation. Except for very sterically hindered alkenes, this reaction usually proceeds rapidly and cleanly. The most common catalysts are various forms of transition metals, particularly platinum, palladium, rhodium, ruthenium, and nickel. Both the metals as finely dispersed solids or adsorbed on inert supports such as carbon or alumina (heterogeneous catalysts) and certain soluble complexes of these metals (homogeneous catalysts) exhibit catalytic activity. Depending upon conditions and catalyst, other functional groups are also subject to reduction under these conditions. 

We have already alluded to the diversity of oxidation states, the dominance of oxo chemistry and the cluster carbonyls. Brief mention should be made too of the tendency of osmium (shared also by ruthenium and, to some extent, rhodium and iridium) to form polymeric species, often with oxo, nitrido or carboxylato bridges. Although it does have some activity in homogeneous catalysis (e.g. of m-hydroxylation, hydroxyamination or animation of alkenes, see p. 558, and occasionally for isomerization or hydrogenation of alkenes, see p. 571), osmium complexes are perhaps too substitution-inert for homogeneous catalysis to become a major feature of the chemistry of the element. The spectroscopic properties of some of the substituted heterocyclic nitrogen-donor complexes may yet make osmium an important element for photodissociation energy research. 

Chlorohydridotris(triphenylphosphine)ruthenium(ll) was the first complex in which homogeneous hydrogenation of alkenes was shown to follow the alkyl route." It can be prepared from dihydrogen and [RuCl2(PPh3)3] in the presence of base (equation 41). Most other alkyl route catalysts are also monohydrido complexes. They are usually specific for terminal alkenes. The behavior of several exo-nirfo-dicarbaborane complexes of rhodium has been reviewed." ... 

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