Metal Complexes as Hydrogenation Catalysts

Hydrogenation plays a key role in chemical synthesis. Homogeneous hydrogenation has become increasingly important in recent decades, mainly due to its application in the industrial production of specialty chemicals.1–6 Homogeneous hydrogenation is a chemical transformation during which one or more H atoms are incorporated into the product of the reaction, by the action of an active catalyst present in the same phase of the reactants. Hydrogen(H 2) is the simplest molecule and its properties are fully understood. Because this clean resource is available in abundance at a very low cost, catalytic hydrogenation is a core technology in both research and industry. The term ‘‘homogeneous hydrogenation’’ is generally referred to homogeneous catalyzed addition of H2 to unsaturated organic substrates; however, also the processes ‘‘transfer hydrogenation’’ and ‘‘hydrogenolysis’’ must be taken into account.5 The former comprises all catalyzed hydrogentran sfer reactions where hydrogen donors (DH2) other thansimple molecular hydrogenare involved as reactants in hydrogen addition, while the latter indicates those reactions, subsequent to hydrogenation by H2 or DH2, in which the original substrate undergoes fragmentation leading to new products. However, hydrogenation involving activation and addition of H2 constitutes, without doubt, the main field of homogeneous hydrogenation where the wider number of studies and examples of catalysts can be found. Molecular hydrogen is rather unreactive at ambient conditions, but many transition and lanthanide metal ions are able to bind and therefore activate H2, which results intran sformation into H (hydride) H (hydrogenrad ical) or Hþ (proton), and subsequent transfer of these forms of hydrogento the substrate.7,8 In this context, not only metal hydride but also dihydrogen complexes of transition metal ions, play a key role,9,10 especially since the first structural characterizationof one of these species in1984 by Kubas.11 At present, the economic impact of industrial homogeneous hydrogenation is rapidly growing. Some hydrogenation reactions are very useful in practical applications in organic synthesis on the laboratory scale and more recently some enantioselective hydrogenation reactions have become very important in the industrial manufacture of fine chemicals and pharmaceuticals. The expanding interest in this area and the resulting increased knowledge in the field of asymmetric catalysis resulted in the award of the 2001 Nobel Prize in Chemistry to three prominent scientists, among whom William S. Knowles12 and Ryoji Noyori13 were honored for their work on asymmetric catalytic hydrogenation. The last decades have witnessed a continually increasing effort into the study of homogeneous hydrogenation,1–6 mainly because activation and addition of H2 has provento be amenable for detailed investigations under gentle conditions, with respect to more unreactive small molecules such as, for example, CO or N2; and also because the characterization of the product distribution of alkene hydrogenation requires cheaper and simpler spectroscopic techniques compared to reactions carried out under high pressure and high temperature. In addition, knowledge of the mechanisms can be very important for the growing field of heterogeneous hydrogenation, and for biological studies on the structure–activity relationship of several naturally occurring hydrogenase enzymes. Here we focus onthe most representative aspects of the catalytic mechanisms so far elucidated and the catalytic systems developed, without covering all the literature; recent reviews are cited where appropriate.