Homogeneously Catalyzed Hydrogenation Reactions

The standard work of Evans [2] as well as a survey of the papers produced in the Journal of Labeled Compounds and Radiopharmaceuticals over the last 20 years shows that the main tritiation routes are as given in Tab. 13.1. One can immediately see that unlike most 14C-labeling routes they consist of one step and frequently involve a catalyst, which can be either homogeneous or heterogeneous. One should therefore be able to exploit the tremendous developments that have been made in catalysis in recent years to benefit tritiation procedures. Chirally catalyzed hydrogenation reactions (Knowles and Noyori were recently awarded the Nobel prize for chemistry for their work in this area, sharing it with Sharpless for his work on the equivalent oxidation reactions) immediately come to mind. Already optically active compounds such as tritiated 1-alanine, 1-tyrosine, 1-dopa, etc. have been prepared in this way. 

Today, many examples have been reported which demonstrate the potential of the PH IP technique as a powerful analytical tool to investigate the reaction mechanisms of homogeneously catalyzed hydrogenations [12]. 

There are many other examples of highly efficient catalytic asymmetric syntheses. These include the asymmetric dihydroxylation of alkenes and certain homogeneous catalyzed hydrogenations. The latter will be discussed in the context of redox reactions in Sections 17.3.2 and 17.4.7. Further examples for catalytic asymmetric syntheses also mentioned in this book are the proline-catalyzed cyclohexenone annulations in Figure 12.19. 

The addition of hydrogen to olefins is made possible only with noble metal catalysts. They allow for a multistep, low-energy reaction pathway. The noble metal catalyst may be soluble in the reaction mixture in that case we have a homogeneously catalyzed hydrogenation (cf. Section 14.4.7). But the catalyst may also be insoluble then we deal with a heterogeneously catalyzed hydrogenation. 

Note Other examples of very efficient catalytic asymmetric syntheses are certain homogeneously catalyzed hydrogenations. They are discussed in connection with the redox reactions in Section 14.4.7. 

Discussion Point DPI While there are many examples of industrial processes that depend on heterogeneously catalyzed hydrogenation reactions, there are very few (apart from the enantioselective processes) which are homogeneously catalyzed. Suggest some reasons for this. 

It is known that Rh2Cl2(PPh3) does not catalyze hydrogenation reactions in a homogeneous phase. However, a hydride species forms when it is supported on a polymer. This catalytic activity is apparently attributed to formation of a dimer by means of a halogen bridge ... 

Besides enzymatic reactions, many homogenously catalyzed hydrogenations follow Scheme 2.2. An example is the asymmetric hydrogenation dehydroamino acid derivatives with rhodium or ruthenium catalysts. On the basis of Scheme 2.2 the following rate equation can be derived ... 

It is clear from the limited number of homogeneous Pd-catalyzed hydrogenation reactions with dihydrogen presented above that this area of chemical research is still awaiting a more concerted developmental effort. Homogeneous Pd-catalyzed transfer hydrogenation protocols, on the other hand, have been shown to be more useful up to now. These reactions are discussed in detail in the aforementioned sections. 

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