Asymmetric homogeneous catalysis selectivity

The symmetry of the ligand can influence the product stereoselectivity and enantio-selectivity. Enantiomerically pure chemicals are extremely important for the agrochemical, pharmaceutical, and food industries. Many of the bidentate ligands used in these processes feature C2-type symmetry, dividing the space around the metal center into two empty quadrants and two full quadrants (see Section 3.1.3 on asymmetric homogeneous catalysis) [61,62], Ligand symmetry is also important in polymerization catalysis [63], where it can influence the polymer s tacticity (Figure 3.22). 

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... 

Interest in the chemistry of phosphines and phosphonium salts continues at a high level, and, as in previous years, considerable selection has been necessary in the preparation of this Report. A noticeable feature has been the large number of papers concerned with the preparation of chiral phosphines and their use in the homogeneous catalysis of asymmetric synthesis. Of these, only those involving some new aspect of organophosphorus chemistry are included here. 

Jessop and co-workers have pointed out that homogeneous catalysis in supercritical fluids can offer high rates, improved selectivity, and elimination of mass-transfer problems.169 They have used a ruthenium phosphine catalyst to reduce supercritical carbon dioxide to formic acid using hydrogen.170 The reaction might be used to recycle waste carbon dioxide from combustion. It also avoids the use of poisonous carbon monoxide to make formic acid and its derivatives. There is no need for the usual solvent for such a reaction, because the excess carbon dioxide is the solvent. If the reaction is run in the presence of dimethy-lamine, dimethylformamide is obtained with 100% selectivity at 92-94% conversion.171 In this example, the ruthenium phosphine catalyst was supported on silica. Asymmetric catalytic hydrogenation of dehydroaminoacid derivatives (8.16) can be performed in carbon dioxide using ruthenium chiral phosphine catalysts.172... 

Enantioselective catalysis that rivals enzymes in selectivity is a major development in homogeneous catalysis. As a result, many earlier processes in the pharmaceutical and perfumery industries are being replaced by more elegant syntheses using soluble catalysts in which handedness is introduced in the critical step of the process, thus avoiding the costly separation of racemic mixtures. In view of its importance in organic synthesis, enantioselective (or asymmetric) catalysis was briefly introduced in Chapter 6 and is again considered as a powerful synthetic tool in Chapter 9. 

Direct air epoxidation of propylene to propylene oxide suffers from selectivity problems. Epoxidation by alkyl hydroperoxide, as practiced by Arco, is based on the use of Mo(CO)g as a homogeneous catalyst. The most impressive use of homogeneous catalysis in epoxidation, however, is in the Sharpless asymmetric oxidation of allylic alcohols. In view of its importance, this enantioselective reaction is included in Chapter 9 which is devoted mainly to asymmetric catalysis. 

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