Asymmetric metal complex catalysts

Optically active a-amino acids are prepared by a cyanide addition to imines, known as the Strecker reaction. Several organobase catalysts and metal complex catalysts have been successfully applied to the asymmetric catalytic Strecker amino... 

The first industrially interesting production of a chiral pharmaceutical with a chiral metal complex catalyst is the asymmetric hydrogenation of cinnamic acid derivatives to give L-DOPA, see Fig. 6.21. 

On the other hand, if aluminosilicates are used as supports for chiral metal complex catalysts, they might reveal a contribution to asymmetric action as was found in the case of the hydrogenation catalyst [CoSalen] complex supported on hectorite... 

The use of asymmetric catalysts in chiral syntheses is taking on increasing importance. Asymmetric ligands or asymmetric metal complexes used in these transformations are quite expensive and need to be efficiently separated from reaction mixtures and recycled. Scheme 16 shows the preparation of a polymer-anchored dibenzophosphole-DIOP platinum-tin catalysts system. The asymmetric ligand places the Pt-SnClj system in a chiral environment. This catalyst has given the highest enantiometric excesses ever observed in catalytic hydroformylation. The initially achieved 70-83% e.e. values were improved to >95% by the use of triethylorthoformate (TEOF) as the solvent. ... 

A wide diversity of catalysts have been supported on polymers [14, 17, 18, 19], but perhaps the four most important groups are asymmetric catalysts, phase transfer catalysts, sulphonic acid catalysts and transition metal complex catalysts. 

Many chiral metal complexes with Lewis acid properties have been developed and applied to the asymmetric Diels-Alder reaction. High enantioselectivity is, of course, one of the goals in the development of these catalysts. Enantioselectivity is not, however, the only factor important in their design. Other important considerations are ... 

From a historical perspective it is interesting to note that the Nozaki experiment was, in fact, a mechanistic probe to establish the intermediacy of a copper carbe-noid complex rather than an attempt to make enantiopure compounds for synthetic purposes. To achieve synthetically useful selectivities would require an extensive exploration of metals, ligands and reaction conditions along with a deeper understanding of the reaction mechanism. Modern methods for asymmetric cyclopropanation now encompass the use of countless metal complexes [2], but for the most part, the importance of diazoacetates as the carbenoid precursors still dominates the design of new catalytic systems. Highly effective catalysts developed in... 

The l ,J -DBFOX/Ph-transition metal aqua complex catalysts should be suitable for the further applications to conjugate addition reactions of carbon nucleophiles [90-92]. What we challenged is the double activation method as a new methodology of catalyzed asymmetric reactions. Therein donor and acceptor molecules are both activated by achiral Lewis amines and chiral Lewis acids, respectively the chiral Lewis acid catalysts used in this reaction are J ,J -DBFOX/Ph-transition metal aqua complexes. 

In a catalytic asymmetric reaction, a small amount of an enantio-merically pure catalyst, either an enzyme or a synthetic, soluble transition metal complex, is used to produce large quantities of an optically active compound from a precursor that may be chiral or achiral. In recent years, synthetic chemists have developed numerous catalytic asymmetric reaction processes that transform prochiral substrates into chiral products with impressive margins of enantio-selectivity, feats that were once the exclusive domain of enzymes.56 These developments have had an enormous impact on academic and industrial organic synthesis. In the pharmaceutical industry, where there is a great emphasis on the production of enantiomeri-cally pure compounds, effective catalytic asymmetric reactions are particularly valuable because one molecule of an enantiomerically pure catalyst can, in principle, direct the stereoselective formation of millions of chiral product molecules. Such reactions are thus highly productive and economical, and, when applicable, they make the wasteful practice of racemate resolution obsolete. 

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