Hydrogenation reactions 404 Subject

The subject of catalyst selection for hydrogenation reactions has been summarized in several books (29,30). 

It may be considered that in the hydrogenation reaction of coal, the coal is subjected to consecutive changes in components and reactivity which results in a consecutive molecular lowering. 

A metal cluster can be considered as a polynuclear compound which contains at least one metal-metal bond. A better definition of cluster catalysis is a reaction in which at least one site of the cluster molecule is mechanistically necessary. Theoretically, homogeneous clusters should be capable of multiple-site catalysis. Many heterogeneous catalytic reactions require multiple-site catalysis and for these reasons discrete molecular metal clusters are often proposed as models of metal surfaces in the processes of chemisorption and catalysis. The use of carbonyl clusters as catalysts for hydrogenation reactions has been the subject of a number of papers, an important question actually being whether the cluster itself is the species responsible for the hydrogenation. Often the cluster is recovered from the catalytic reaction, or is the only species spectroscopically observed under catalytic conditions. These data have been taken as evidence for cluster catalysis. 

As we have seen so far, libraries of hydrogenation catalysts are never composed of more than a few dozen members, up to 100 to 200 at the most. Consequently, modern analytical equipment such as gas chromatography (GC) or high-performance liquid chromatography (HPLC) equipped with an auto-sampler or even flow-through NMR systems are sufficient to handle the analysis of the entire library. Nevertheless, a few groups have initiated research towards the development of fast, sometimes parallel, analytical procedures. A few reviews have appeared on this subject [59]. Here, we will concentrate on the methods developed to analyze hydrogenation reactions, or methods that could likely be applied. 

One of the success stories of transition metal catalysis is the rhodium-complex-catalyzed hydrogenation reaction. Asymmetric hydrogenation with a rhodium catalyst has been commercialized for the production of L-Dopa, and in 2001 the inventor, Knowles, together with Noyori and Sharpless, was awarded the Nobel Prize in chemistry. After the initial invention, (enantioselective) hydrogenation has been subject to intensive investigations (27). In general, hydrogenation reactions proceed... 

The B] 2S block copolymers- were subjected to the hydrogenation reaction in which only the 1,2 poly(Bd) were hydrogenated to improve its physical properties. These are illustrated in Table XV. 

Before undertaking a detailed examination of individual catalysed hydrogenation reactions, we consider some of the more general aspects of the subject and highlight some of the problems associated with heterogeneous reactions in general, and catalytic hydrogenation in particular. 

A new stereoselective synthesis of 1,2,3-trisubstituted cyclopentanes based on the Wag-ner-Meerwein rearrangement of a 7-oxabicyclo[2.2.1]heptyl 2-cation starts with the Diels-Alder product of maleic anhydride and a furan (78TL2165, 79TL1691). The cycloadduct was hydrogenated and subjected to methanolysis. The half acid ester (47) was then electrolyzed at 0 °C to generate a cationic intermediate via the abnormal Kolbe reaction (Hofer-Moest reaction). Work-up under the usual conditions provided the 2-oxabicyclo[2.2.1]heptane (48) in 83% yield. Treatment of this compound in turn with perchloric acid effected hydrolysis of the ketal with formation of the trisubstituted cyclopentane (49) in nearly quantitative yield (Scheme 11). Cyclopentanes available from this route constitute useful... 

The mechanism of transfer hydrogenation is complicated because of the need to activate the reagents in the correct order. Since several of the more successfiil transfer-hydrogenation catalysts are monohydride complexes, deuterium-labeling experiments to ascertain the source and destination of the transferred hydrogen are subject to some uncertainty. This complication arises from the simultaneous isotope exchange reactions that occur. 

These are classical catalytic reactions and have been the subject of fundamental study for many years [70-80], Most transition metals catalyze hydrogenation reactions to a greater or lesser extent and the reaction kinetics for all of these are... 

The data presented in the following section concern only the hydrogen reaction at cathodic potentials. Those on hydrogen termination are presented in Chapter 2 and on silicon dissolution in Chapter 5. It is to be noted that as a reduction reaction, hydrogen evolution has not been well investigated at cathodic potentials although it has been the subject of numerous studies on the phenomena at anodic or open-circuit potentials. 

Attempts to hydrogenate benzene were unsuccessful both at atmospheric and superatmospheric pressures. While very little carbon deposition was found on the catalyst used in the above experiments, it was subjected to a usual regeneration procedure (oxidation with air and reactivation with hydrogen). The regenerated catalyst showed a typical green color of crystalline Cr20s it showed a decreased activity for hydrogenation reaction, but maintained a full activity in the ketone synthesis mentioned above. 

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