Hydrogenation, catalytic, alkene homogeneous catalysts

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. 

As recently recognized by the Nobel Chemistry award committee, the conceptualization, development, and commercial application of enantioselective, homogeneous hydrogenation of alkenes represents a landmark achievement in modem chemistry. Further elaboration of asymmetric hydrogenation catalysts by Noyori, Burk, and others has created a robust and technologically important set of catalytic asymmetric synthetic techniques. As frequently occurs in science, these new technologies have spawned new areas of fundamental research. Soon after the development of... 

The steric effects may be more pronounced in heterogeneous catalysts than in homogeneous reactions in solution. The rigid, solid surface restricts the approach of the reactants to the active centers and interaction between the reactants. The steric requirements are quite stringent when a two-point adsorption is necessary and when, in consequence, the internal motion of the adsorbed molecules is limited. In this way, the stereoselectivity of some heterogeneous catalytic reactions, for example, the hydrogenation of alkenes on metals (5) or the dehydration of alcohols on alumina and thoria (9), have been explained. 

Alkyne hydrogenation remains a largely forgotten area of homogeneous catalytic hydrogenation. There are clear commercial benefits to be obtained in the conversion of alkynes to alkenes however, homogeneous catalysts are not usually sufficiently selective towards the alkene intermediate. 

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