Chiral ligand elements

The hydride-metal-nitrogen-proton motif and chiral ligand elements for enantioselectivity... 

These polymers, as helical chiral ligand lacking any other elements of chirality, were tested as chiral Hgands for the palladium-catalyzed allyhc substitution, and in the test reaction, enantiomeric excesses of up to 33% were obtained (higher than those reached with monomer 115 as unique Hgand). 

The mesoporous character of MCM-41 overcomes the size limitations imposed by the use of zeolites and it is possible to prepare the complex by refluxing the chiral ligand in the presence of Mn +-exchanged Al-MCM-41 [34-36]. However, this method only gives 10% of Mn in the form of the complex, as shown by elemental analysis, and good results are only possible due to the very low catalytic activity of the uncomplexed Mn sites. The immobihzed catalyst was used in the epoxidation of (Z)-stilbene with iodosylbenzene and this led to a mixture of cis (meso) and trans (chiral) epoxides. Enantioselectivity in the trans epoxides was up to 70%, which is close to the value obtained in solution (78% ee). However, this value was much lower when (E)-stilbene was used (25% ee). As occurred with other immobilized catalysts, reuse of the catalyst led to a significant loss in activity and, to a greater extent, in enantioselectivity. 

For the case of chiral ligands (with which this section is exclusively concerned), it is easy to convince oneself that the representation (n)u of <5n, whose diagram is shown in Fig. 16, is chiral for every skeleton. For, the representation is totally symmetric under all pure permutations, in particular under those belonging to , and antisymmetric under all group elements involving to, in particular under those in the coset of D in . The representations of [Pg.63]

In one step, the chirality elements are counted with reference to the skeleton, and in another, the local chirality elements of the separated chiral ligands are enumerated. The resulting sum is the total number of chirality elements. This enumeration corresponds to the number of independent asymmetric C-atoms as assumed by van t Hoff. Furthermore, it yields the number of chirality elements in systems with chiral units of a different nature, e.g. some belonging to Ruch s class B. This simply... 

Chiral Lewis acid catalysts are powerful tools for asymmetric synthesis, combining a metal or metaloid central atom with a chiral ligand [1, 2], Such chiral Lewis acids activate electrophiles 1 for a nucleophilic attack. Various metals can be used as the center element (Scheme 1). 

The chiral fullerene C75 was also asymmetrically osmylated using the chiral ligands and (Scheme 8.8) [63]. In this way an optically active allotrope of a pure element was prepared. C75 contains 15 different types of [6,6] bonds. The pronounced regioselectivity of C7Q towards osmylation [58] suggests that specific bonds in C75 may be favored for an attack by OSO4. An analysis of the ab initio calculated curvature of 75 shows that two of the five pyracylene-type bonds are particularly distorted, which could enhance their reactivity [64]. Indeed HPLC analysis of C75[Os04L ] shows that two regioisomers are predominantly formed upon osmylation of C75 [63]. 

From several pioneering works described in the original book [2], there is no doubt that carefully designed chiral Lewis acids have vast potential for the asymmetric synthesis of carbon skeletons and others. Choice of a proper metal and design of a suitable chiral ligand may be the most important elements in obtaining successful results. 

In addition, the reader may realize that axis of rotation can still be present in some chiral Cp-metal complexes (e.g., a C2 axis in the enantiomeric forms in 22 and 23, a C5 axis in 24). With rotation axes present the systems are not asymmetric, only dissymmetric (i.e., lacking mirror symmetry). This is, however, sufficient to induce the existence of enantiomeric forms (218). Moreover, it is known from numerous examples that chiral ligands with C2 symmetry can provide for a higher stereoselectivity in (transition metal-catalyzed) reactions than comparable chiral ligands with a total lack of symmetry. The effect is explained by means of a reduced number of possible competing diastereomeric transition states (218). Hence, rotational symmetry elements may be advantageous for developing useful Cp-metal-based catalytic systems. 

Addition of the elements of Si—H to a carbonyl group produces silyl ethers which are synthetically equivalent to chiral secondary alcohols since the silyl groups are easily hydrolyzed. Hydrosilylation can be catalyzed by acids or transition metal complexes. Enantioselective hydrosilylation of prochiral ketones has been extensively studied using platinum or rhodium complexes possessing chiral ligands such as BMPP (86), DIOP (87), NORPHOS (88), PYTHIA (89) and PYBOX (90)." ... 

Kozlowski, M. C., Panda, M. Computer-Aided Design of Chiral Ligands. Part 2. Functionality Mapping as a Method To Identify Stereocontrol Elements for Asymmetric Reactions. J. Org. Chem. 2003, 68, 2061-2076. 

Asymmetric induction from chiral ligands on the metal center can be used to produce enantiomerically enriched products from simple prochiral carbonyl compounds. More often, this aldol control element is employed to reinforce or overturn the inherent stereochemical bias of a chiral ketone or aldehyde. 

Asymmetric catalysis encompasses the use of both biocatalysts (e.g., enzymes) and chemical catalysts that possess an element of chirality (e.g., a transition metal complex bearing a chiral ligand). From a commercial perspective, the interest in asymmetric catalysis emanates from inherent economic and ecological benefits that are associated with the capacity to produce a large volume of valuable enantiomerically enriched material through the agency of a negligible quantity of a chiral catalyst. Asymmetric catalytic processes may involve kinetic resolution of a racemic substrate, or preferably, direct transformation of a prochiral substrate into the desired chiral molecule. 

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