Chiral metal complexes asymmetric synthesis

In the field of pharmaceuticals and pesticides, the desired big activity of the molecule is in a pure enantiomer (chiral molecule). This has been made possible by using either enzymes or chiral metal complexes (asymmetric catalysts). These chiral metal complexes, though expensive, are important for industrial application (i.e., have a high turnover frequency). A typical example is the synthesis of (S)-me olachlor, a herbicide. It is prepared by asymmetric... 

Enantiometrically pure alcohols are important and valuable intermediates in the synthesis of pharmaceuticals and other fine chemicals. A variety of synthetic methods have been developed to obtain optically pure alcohols. Among these methods, a straightforward approach is the reduction of prochiral ketones to chiral alcohols. In this context, varieties of chiral metal complexes have been developed as catalysts in asymmetric ketone reductions [ 1-3]. However, in many cases, difficulties remain in the process operation, and in obtaining sufficient enantiomeric purity and productivity [2,3]. In addition, residual metal in the products originating from the metal catalyst presents another challenge because of the ever more stringent regulatory restrictions on the level of metals allowed in pharmaceutical products [4]. An alternative to the chemical asymmetric reduction processes is biocatalytic transformation, which offers... 

Asymmetric induction in the ylide formation/[l,2]-shift has also been studied with chiral metal complexes. Katsuki and co-workers examined the reaction of ( )-2-phenyloxetane with 0.5 equiv. of /< //-butyl diazoacetate in the presence of Gu(i) catalyst. With chiral bipyridine ligand 53, trans- and m-tetrahydrofurans 54 and 55 are obtained with 75% and 81% ee, respectively (Equation (6)). This asymmetric ring expansion was applied by the same group to their enantioselective synthesis of translactone. 

Chiral sulfoxides have emerged as versatile building blocks and chiral auxiliaries in the asymmetric synthesis of pharmaceutical products. The asymmetric oxidation of prochiral sulfides with chiral metal complexes has become one of the most effective routes to obtain these chiral sulfoxides.We have recently developed a new heterogeneous catalytic system (WO3-30% H2O2) which efficiently catalyzes both the asymmetric oxidation of a variety of thioethers (1) and the kinetic resolution of racemic sulfoxides (3), when used in the presence of cinchona alkaloids such as hydroquinidine 2,5-diphenyl-4,6-pyrimidinediyl diether [(DHQD)2-PYR], Optically active sulfoxides (2) are produced in high yields and with good enantioselectivities (Figure 9.3). ... 

For a general review about methods for the synthesis of optically active fS-lactones, see H. W. Yang, D. Romo, Tetrahedron 1999, 55, 6403-6434 (b) For a selected recent chiral metal complex-catalyzed asymmetric approach, see S. G. Nelson, Z. Wan, Org. Lett. 2000, 2, 1883-1886. 

Chiral non-racemic cyclopropanes are a common motif in natural and synthetic biologically active compounds [85]. They represent an important target in asymmetric synthesis, and a range of catalytic methods have been developed for their synthesis [6, 47, 86-90]. Many of the existing methods make use of a chiral metal complex as catalyst [6], but organocatalytic methods have also been developed. In this section we will review methods using a substoichiometric amount of a chiral ylide as a catalyst for cyclopropanation [91]. 

Asymmetric pericyclic reaction represents one of the most straightforward protocols to access enantioenriched cyclic compounds and triggers continuing interest in organic synthesis. Fruitful results have been achieved by the catalysis of chiral metal complexes over the past decades [1], On the other hand, recently small organic molecules have also contributed a lot to this area owing to the rapid development of asymmetric organocatalysis [2]. 

The development of novel chiral metal complexes and chiral ligands is crucial for both progress and development of asymmetric catalytic synthesis [1-3]. Within this area, the appearance of planar-chiral ferrocenes as ligands in asymmetric catalysis has been an important advancement [4-7]. While most of these complexes bear side chains or atom groups with stereogenic centres, it is often the 1,2-disubstitution pattern of the n-complexed ring that creates an inherent planar chirality [8] and exercises efficient stereochemical control. 

Belokon and coworkers [218,219] reported a novel class of chiral metal complexes for the asymmetric synthesis of a-amino acids under PTC conditions. The use of (45 ,55)-... 

In an idealistic sense, a chemical approach which uses a small amount of a chiral catalyst to produce either enantiomer, cleanly and efficiently from a prochiral precursor, is the preferred method. For such asymmetric catalysis the efficiency of chiral multiplication can be infinite. The use of chiral metal complexes as homogeneous catalysts has become one of the most powerful economically and environmentally sound strategies for the preparation of enantiopure compounds. An excellent comprehensive review of asymmetric catalysis in organic synthesis has recently been published by Noyori [30]. 

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