Examples of catalytic hydrogenation

As a final example of catalytic hydrogenation activity with polymer-stabilized colloids, the studies of Cohen et al. should be mentioned [53]. Palladium nanoclusters were synthesized within microphase-separated diblock copolymer films. The organometallic repeat-units contained in the polymer were reduced by exposing the films to hydrogen at 100 °C, leading to the formation of nearly monodisperse Pd nanoclusters that were active in the gas phase hydrogenation of butadiene. 

The small number of examples of transition metal hydrides known to undergo addition across > C=0 is due to lack of study rather than lack of reactivity. There are numerous examples of catalytic hydrogenation and hydrosilation of ketones that involve insertions of >C=0 into M-H bonds The complex Cp2ZrH2 reacts readily with (CH3)2C = 0 to give Cp2Zr(OC3H7)2 . 

Few examples of catalytic hydrogenation of partially saturated thiophene derivatives are known, because sulfur normally poisons the catalyst. Nevertheless, hydrogenations are sometimes possible when a larger amount of catalyst is used. One example, with a high degree of diastereoselectivity, is hydrogenation of the biotin precursor l18. 

The addition of hydrogen is called hydrogenation. Because the preceding reactions require a catalyst, they are examples of catalytic hydrogenation. The metal catalysts are insoluble in the reaction mixture and therefore are classified as heterogeneous catalysts. A heterogeneous catalyst can easily be separated from the reaction mixture by filtration. It can then be recycled, which is an important property, since metal catalysts tend to be expensive. 

Metal clusters have also proved to have great possibilities as homogeneous catalysts for the reaction of the reduction of carbon monoxide. Selected examples of catalytic hydrogenation of carbon monoxide by metal clusters are shown in Table 2.32. 

Table 2.32. Examples of catalytic hydrogenation of carbon monoxide by metal clusters...

There have been only a few examples of reduction of the C=N+ function of catalytic hydrogenation since the reductions with complex hydrides are so easy to do in the laboratory. A possible reduction of an iminium salt 45 to 46 with platinum oxide was reported by McKay et al. (91). A report that platinum oxide reduces 2l tio).jgj yjj.Qqyjp Qjj2idijjjujn perchlorate (25) in quantitative yield to 47 indicates that such reduction should be facile (47). 

When an oxidation or a reduction could be considered in a previous chapter, this was done. For example, the catalytic hydrogenation of alkenes is a reduction, but it is also an addition to the C=C bond and was treated in Chapter 15. This chapter discusses only those reactions that do not fit into the nine categories of Chapters 10-18. An exception to this rule was made for reactions that involve elimination of hydrogen (19-1-19-6), which were not treated in Chapter 17 because the mechanisms generally differ from those in that chapter. 

This method ensures the deposition of very reactive metal nanoparticles that require no activation steps before use. We shall review here the following examples of catalytic reactions that are of interest in line chemical synthesis (a) the hydrogenation of substituted arenes, (b) the selective hydrogenation of a, 3-unsaturated carbonyl compounds, (c) the arylation of alkenes with aryl halides (Heck reaction). The efficiency and selectivity of commercial catalysts and of differently prepared nanosized metal systems will be compared. 

An even more impressive example of catalytic efficiency has recently been disclosed by Novartis (Bader and Bla.ser, 1997). The key step in a proce.ss for the synthesis of the optically active herbicide, (S)-metolachlor involves asymmetric hydrogenation of a prochiral imine catalysed by an iridium-ferrocenyldipho-sphine complex (see Fig. 2.36). 

In the foregoing it has been discus.sed how a metal can dissociate H2. Fig. 3.6 explains the principle of catalysis with an example of the hydrogenation of ethylene, for which dissociative chemisorption of hydrogen is an elementary step in the catalytic cycle. The adsorption of alkenes, on the other hand, is non-dissociative. 

To our knowledge, the first examples of catalytic non-hydrogen-mediated reductive C-C bond formation involve the reductive dimerization (telomeriza-tion) of butadiene mediated by silane, see Ref. 6-12. 

If a reaction that must be investigated follows a reaction sequence as in Scheme 10.1, and if the reaction order for the substrate equals unity, it means that (with reference to Eq. (4 b)), the observed rate constant (k0bs) is a complex term. Without further information, a conclusion about the single constants k2 and fCM is not possible. Conversely, from the limiting case of a zero-order reaction, the Michaelis constant cannot be determined for the substrate. For particular questions such as the reliable comparison of activity of various catalytic systems, however, both parameters are necessary. If they are not known, the comparison of catalyst activities for given experimental conditions can produce totally false results. This problem is described in more detail for an example of asymmetric hydrogenation (see below). 

This was the first example of catalytic chemoselective reduction of a,/ -unsatu-rated ketones to allylic alcohols by hydrogen transfer and, unusually, did not require the use of a basic co-catalyst. 

Other examples of arene hydrogenation by Ziegler-type catalysts have been reported [35]. However, none of them is discussed at this point as they are generally poorly defined. Likewise, some hydrogenation catalytic systems in either oxo or water-gas-shift conditions are only reported in the list of references for sake of information [24, 36]. 

This section focuses on the preparation of fluorinated compounds through asymmetric hydrogenation/reduction reactions and nucleophilic additions by listing some examples. The first successful example of catalytic asymmetric hydrogenation of a fluoro-compound was reported by Konig et al.81... 

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