Metal-center chirality transfer

CuBr/QUINAP System The CuBr/QUlNAP system was initially used in the enan-tioselective synthesis of proparyl amines via the reaction of alkynes and enamines (Scheme 5.5). It was rationalized that the enamines reacted with protons in terminal alkynes in the presence of copper catalyst to form zwitterionic intermediates in which both the generated iminiums and alkyne anions coordinate to the copper metal center. After an intermolecular transfer of the alkyne moiety to the iminium ion, the desired products were released and the catalyst was regenerated. The combination of CuBr as catalyst and the chiral ligand QUEMAP is crucial for the good reactivities and enantioselectivities seen in the reaction. Another potential... 

When activated by metallic catalysts, hydrogen may be transferred from the metallic center to unsaturated organic molecules. The nature and reactivity of transition metal hydrides depend on the central metals as well as on the electronic and steric properties of the ligands. Metal hydrides with optically active ligands are chiral and thus, are capable of asymmetric hydrogenation. 

This is mainly due to the fact that by means of chiral ligands it is comparatively facile to transfer absolute stereochemical information to a cat-alytically active metal center. However, the success of some of these reactions (e.g. the Sharpless asymmetric epoxidation or the Noyori hydrogenation) must not hide the fact that the number of powerful transition metal-catalyzed C-C coupling reactions, which proceed reliably with high enantioselectivity, is still rather small. 

If the metal enolate contains a center of chirality, diastereoselection may be exhibited in the C—C bond formation process. Evans has identified three classes of metal enolates in which chirality transfer may occur (i) endo- and exo-cyclic enolates such as (27) or (28), which contain a chiral center ( ) in a ring bonded to the enolate at two points (ii) acyclic enolates such as (29) or (30), in which the moiety containing the chiral center ( ) is bonded to the enolate at only one point and (iii) chelated enolates such as (31) or (32), in which the chiral center is a part of the chelate ring. (Z)-Endocyclic enolates are also possible for large ring cyclic ketones. 

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