Central chirality bidentate ligands

Coordination to the central P atom of two different types of symmetrical bidentate ligands leads to structures of type P(aa)2(bb), which are this time C2-symmetric as detailed in Fig. 16. The same chiral descriptors A and A apply to these compounds. Derivatives like 3,4,14-17 and 19-22 fit this description and have only been reported in racemic form so far. If the ligand bb is itself chiral, then diastereomers are generated. This will be described in the next section. 

Bidentate oxazoline-imidazolylidene ligands, in which both units are linked by a chiral paracyclophane, have been studied in Bolm s group [129]. In this case, the planar chirality of the pseudo-orfho-paracyclophane is combined with the central chirality of an oxazoline (Scheme 48). Compounds 70 were tested in the asymmetric hydrogenation of olefins displaying moderate selectivity (ee s of up to 46% for dimethylitaconate in the presence of 70b). 

Chiral separation using ligand-exchange chromatography involves the reversible complexation of metal ions and chiral complexing agents. The central ion, usually or forms a bis complex with bidentates ligands. 

The development of a vast number of chiral ligands has been the key aspect in the development of the field. The potential for variation of these ligands is almost endless by variation of the donor atoms and the ligand backbone. Bidentate chiral ligands which possess central chirality, planar chirality and axial chirality have aU been applied in a range of transformations. The most eflfective donor atoms in these ligands are P,P, N,N and P,N systems [28]. 

Fig. 6. Most widely used bidentate phosphorus ligands with planar and mixed planar-central chirality.

Another interesting finding regarding the chirality of trischelate complexes is that the heUcity of the metal coordination sphere can be transferred to the periphery of the coordinated ligands in two different ways. In complexes with bidentate ligands such as dithiolenes, catecholato, and dithiocarbamato, the chirality is attenuated as one moves away from the central metal, whereas for those with... 

All four chiral auxiliaries are available derivatives of ephedrine. Structurally, they are vicinal amino-alcohols, which are able to act as l,4-N,0-bidentate ligands for many metal cations. The stability of such complexes generally enhances with the hardness of the central metal ion [17]. 

FIGURE 29.6. Selected bidentate ligands with central chirality. 

FIGURE 29.9. Selection of ferrocene-based chiral ligands (A) ferrocene-based diphosphines with planar chirality only (B) bidentate ferrocene-based ligands with ntixed planar and central chirality and (C) ligands with planar and axial chirality as well as planar, central, and axial chirahty. 

---