Reduction reactions, fine chemical synthesis

Heterogeneous copper catalysts prepared with the chemisorption-hydrolysis technique are effective systems for hydrogen transfer reactions, namely carbonyl reduction, alcohol dehydrogenation and racemization, and allylic alcohol isomerization. Practical concerns argue for the use of these catalysts for synthetic purposes because of their remarkable performance in terms of selectivity and productivity, which are basic features for the application of heterogeneous catalysts to fine chemicals synthesis. Moreover, in all these reactions the use of these materials allows a simple, safe, and clean protocol. 

Catalytic liquid phase semihydrogenation of acetylenes is an important industrial and laboratory reaction, especially in fine chemical synthesis [1]. The use of supported metal catalysts for this selective hydrogenation readily facilitates the separation of organic products from the catalyst. However, liquid phase reactions with supported catalysts tend towards mass transport limitation [2] and, therefore, the support particles should be between 1 and 10 pm in size this avoids transport limitations and separation problems. With support particles of this size high temperature reduction in a flow of H2 gas is very difficult and to avoid this step it is possible to prepare supported metal particles by decomposing organometallic compounds under mild conditions [3-5]. 

Catalytic behaviors of solid base catalysts for fine chemicals synthesis as well as the fundamental reactions are described. The reactions included are double bond isomerization of olefins, addition of hydrogen and amines to conjugated dienes, dehydration, dehydrogenation, reduction, alkylation, aldol addition and condensation, Wittig-Horner and Knoevenagel reactions, dehydrocyclodimerization, and ring transformation. The characteristic features of different types of solid base catalysts, zeolites, metal oxides, solid superbases and non metal-oxides, are summarized. 

This reaction looks attractive because triethylsilane is non toxic and its by-products are low boiling point compounds easily removed by distillation. However, the alkanethiols have a strong bad smell and the remaining traces could be difficult to eliminate from the solution in certain cases, which make them undesirable in the synthesis of fine chemicals. Therefore, it appeared of interest to test the use of thiols supported on polyHIPE in these radical chain reductions. 

The possibility of using C02 for the synthesis of fine chemicals that are now derived from petroleum has prompted efforts to obtain a broader understanding of the coordination chemistry of CO2 during the past 20 years.1-21 Carbon dioxide utilization will inevitably center on metal complexes and their ability to bind C02. In the past decade, many C02—metal complexes have been prepared and the ligand has demonstrated a remarkable variety of coordination modes in its complexes. The sections below outline the synthesis, characterization by X-ray crystallography and IR spectroscopy, and some characteristic reactions of these compounds. Also discussed are C02 insertion reactions into M—X bonds and oxidative coupling reactions between C02 and unsaturated substrates which occur at some metal centers. Finally, a profile of the research on catalytic reductions of C02 is provided. Where possible, references are made to reviews rather than to the primary literature. 

Phase transfer catalysts have been grafted onto the surface of porous capsules to facilitate product purification after reaction, and many types of immobilized cells, mycelia, enzymes, and catalysts have been encapsulated in polymers such as PDMS, PVA, or cellulose. In the specific case of PVA, they are named Lenti-kats, as commercialized by Genialab and used for nitrate and nitrite reduction and in the synthesis of fine chemicals. These beads show minimized diffusion limitations caused by the swelling of the polymeric environment under the reaction conditions. To avoid catalyst leaching, enlargement can be realized by linking them to, e.g., chitosan. 

Research to develop new specific catalysts for fine chemicals must be applicable to a range of products and cannot be limited to only one compound. Furthermore, since development time is limited, the chemical feasibility of the reaction must be demonstrated in advance. Since synthesis of fine chemicals has not been fully studied up to now, there is a unique opportunity to review classical organic chemistry and to find and develop new selective catalysts for key reactions. Since the reactions must be general careful choice of reactions to investigate is key to success. We have demonstrated that for important reactions such as Friedel-Crafts, Carbonylation, Reductions and oxidations, it is possible to develop new catalysts for the selective synthesis of fine chemicals. 

Radical reactions are widely used for carbon-carbon bond formations. This has led to highly efficient novel synthetic methods that can be used in natural product synthesis as well as preparation of fine chemicals. Many of these processes involve a reductive final step (see for instance Volume 1, Chapter 1.3). Alternative methods that allow functionalization of carbon-centered radicals are highly desired. In this chapter, we will focus on oxygenation and amination reactions. 

Clearly, most biocatalytic reactions for the production of fine chemicals are used to obtain enantiopure or enantioemiched compoimds, and only a minor nimiber of syntheses lead to products without chiral centers. More than 65 ap-pHcations of immobilized enzymes or whole cells for industrial research and production have been treated in this review, and it can be stated that approximately 80% utilize the class of hydrolytic enzymes. This number reflects the ease of handling and the broad utility of these enzymes. The reported hydrolytic enzyme applications mainly involve lipases, whereas other hydrolases can only be found in fewer but nevertheless just as attractive cases. The broad field of asymmetric synthesis (e.g., asymmetric reduction/oxidation) is defi-... 

Industrial interest in soluble polymer-bound catalysts has been closely linked to the development of ultrafiltration membranes with sufficient long-term stability in organic solvents. Membranes fulfilling these requirements were prepared first in the late 1980s. Today, solvent-stable flat sheet membranes and membrane modules are available from several suppliers. As for the viability of ultrafiltration in organic solvents, rhodium-catalyzed hydroformylation of dicydopentadiene with continuous catalyst recovery and recycling has been demonstrated successfully on a pilot plant scale over an extended period of time [5]. The synthesis of other fine chemicals by asymmetric reduction and other reactions has also been carried out in continuously operated membrane reactors (also cf Section 7.5) [6-9]. The extent of commercial interest in catalysts bound to soluble polymers appears to fluctuate at intervals. Amongst other factors, the price of precious metals can be a driver. 

Note at the outset that asymmetric catalysis in the synthesis of fine chemicals is rarely a single-step process that converts a reactant directly to the final product. It is usually one of the steps in a total synthesis but is often the key step. Hence the analysis of the overall yield will be based on the methods described in Chapter 5. There are many types of reactions where asymmetric catalysis can be applied. The most important of these are C-C bond-forming reactions such as alkylation or nucleophilic addition, oxidation, reduction, isomerization, Diels-Alder reaction, Michael addition, deracemization, and Sharpless expoxidation (of allyl alcohols). A few representative examples (homogeneous and heterogeneous) are given in Table 9.6. 

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