Efficient Catalyst Recycle

For the production of chemicals, food additives and pharmaceutical products, homogeneous catalysis offers some attractive features such as a high selectivity and activity, e.g. in asymmetric synthesis. However, since most homogeneous catalysts are relatively expensive, their current industrial application is limited [3]. On the other hand, heterogeneous catalysts can easily be separated from the products and can be recycled efficiently. Membrane separations with emphasis on nanofil-tration and ultrafiltration will allow for a similar recyclability of homogeneous catalysts, which is important both from an environmental as well as a commercial 

The incorporation or complexation of transition metal fragments by dendrimers has led to a broad spectrum of metallodendrimers. Dendrimers are well-defined branched structures (Fig. 13.2). The dimensions of a dendrimer can easily be adjusted by changing its generation, which can be very practical for their application 

Seen the list of demonstrated applications, numerous possibilities exist for the integration of homogeneous catalysis and a membrane separation. A complicating factor, however, is the relatively limited availability of solvent-resistant membranes. This will require a substantial development effort to obtain more solvent-stable membranes, including both polymeric and inorganic ones. 

Pervaporation has become one of the standard membrane technologies with a large number of realized industrial applications. Pervaporation is used for the dehydration of organic compounds, the separation of organic compounds from aqueous solu- 

2 Membrane Systems for Improved Chemical Synthesis 531 feed feed 

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A copper-free Sonogashira coupling reaction in ionic liquids and its application to a microflow system for efficient catalyst recycling, Org. Lett. 4, 10 (2002) 1691-1694. 

The water-soluble ligand (TPPTS) was discussed earlier with regard to aldehyde reduction [17]. Similarly, in ketone transfer hydrogenation, high yields are obtained for a variety of substrates with the ability for efficient catalyst recycling at no expense of activity or selectivity (Fig. 15.10). 

With an annual production of up to 9.3 million tons in 1998, hydroformylation is the most important homogeneously catalyzed reaction [20,21], The reaction is performed almost exclusively by the use of cobalt or rhodium catalysts. The advantages of rhodium catalysts are milder reaction conditions and better n/iso ratios in product distribution. The toxicity of rhodium compounds as well as the high rhodium price [22] (between 20 and 75 g during the last five years) demand an efficient catalyst recycling. 

In this way, the authors have proven several significant advantages of the reactions performed in a microreactor shorter reaction times, improved atom efficiency, excellent product yields and purities, efficient catalyst recycling and the increased safety of the reaction, thanks to the closed system which prevents the release of the cyanide. 

The supramolecular interaction between the transition metal catalyst and the binding site is sufficiently strong to enable efficient catalyst recycling. In addition, the support can be readily re-functionalized with different catalyst systems, by washing with methanol to remove the first catalyst system and then attaching the new catalyst system by just stirring in apolar solvent such as toluene (Fig. 7). 

Both reactions could be carried out in a liquid-liquid two-phase system with the advantages of easy product separation and efficient catalyst recycling. In the presence of a water-soluble ruthenium catalyst with the ligand triphenylphosphine trisulfo-nate, simultaneous extraction of the organic product phase by N,N-dibutylformamide can be achieved [87, 88]. 

Of all the catalyst systems studied, Rh-TPPTS is the most suitable and commercially proven catalyst system for biphasic hydroformylation. Several modifications of the water-soluble catalysts using co-solvents [15], surfactants and micelle-forming reagents [16], a supercritical C02-water biphasic system [17], supported aqueous-phase catalysis [18], and catalyst-binding ligands (interfadal catalysis) [19] have been proposed to overcome the lower rates observed in biphasic catalysis due to poor solubilities of reactants in water (see Sections 2.2.3.2 and 2.3.3.3). So far, endeavors have been centered on innovating novel catalyst systems from the viewpoint of efficient catalyst recycle and rate enhancement, but limited information is available on the kinetics of biphasic hydroformylation. 

The success of the SHOP has shown that the difficulties can be overcome, leading to very efficient catalyst recycling. 

It is always to be recalled that the main purpose of using a soluble catalyst in a biphasic process is the separation of the catalyst from the final reaction mixture. This is a prerequisite for efficient catalyst recycling and may be beneficial for (but not synonymous with) the isolation of products in sufficiently pure state. However, in some cases special chemical or engineering consequences arise from the limited solubilities of the reactants, products, and catalysts in one or both phases. 

Epoxidation of olefins was catalyzed by the ruthenium(II) complex of the above perfluorinated y3-diketone in the presence of 2-methylpropanal (Scheme 50). Unfunctionalized olefins were epoxidized with a cobalt-containing porphyrin complex, and epoxidation of styrene derivatives was catalyzed by chiral salen manganese complexes (248) (Scheme 50). In the latter case, chemical yields were generally high, however, the products showed low enantiomeric excess with the exception of indene (92% ee). [Pd(C7Fi5COCHCOC7Fi5)2] efficiently catalyzed the oxidation of terminal olefins to methyl ketones with f-butylhydroperoxide as oxidant in a benzene-bromoperfluoro-octane solvent system (Scheme 50). In all these reactions, the product isolation and efficient catalyst recycle was achieved by a simple phase separation. 

Following the results with neutral and ionic precatalysts, it became obvious that immobilisation of neutral catalysts was the solution for an efficient catalyst recycling. Several groups have worked in this direction by modifying the stable and recyclable Hoveyda catalysts 3 and 4. In 2003, Mauduit and Guillemin reported the synthesis of an IL supported Hoveyda type catalyst 7 [38]. The synthesis was performed in nine steps starting from the commercially available methyl-3(4-hydrophenyl)propionate. Complex 7 was obtained in 32% overall yield (Scheme 8). 

Fukuyama, T., Shinmen, M., Nishitani, S., Sato, M., Ryu, I. (2002). A copper-free Sonogashira coupling reaction in ionic liquids and its application to a microflow system for efficient catalyst recycling. Org. Lett, 2002,4,10, (April 2002) 1691-1694, ISSN 1523-... 

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