Central chirality monodentate ligands

Whereas chirality in tetrahedral compounds of carbon requires four different groups to be bonded around the tetrahedral carbon atom, this is not necessarily the case for other central atoms with other stereochemistries. For example, octahedral complexes have more relaxed rules. Whereas a chiral tetrahedral organic compound is, as a consequence of the rule for chirality, asymmetric (or totally lacking in symmetry), chiral octahedral complexes need not be asymmetric, but may have axes of rotation (they are then dissymmetric). The common rule for chirality is simple - a compound must have non-superimposable mirror images. For an octahedral complex, this can occur even when three different pairs of monodentate ligands are coordinated, as discussed later. We shall look a little more closely at four- and six-coordinate complexes below. 

A classical method for the preparation of enantiopure compounds is the resolution of racemate. However, it is much more effective to use the selective synthesis of the desired enantiopure substance via enantioselective approach. Stereoselective methods of synthesis have been widely developed in organic chemistry. The method of asymmetric synthesis has been known since the nineteenth century and asymmetric catalysis has witnessed an enormous amount of development in recent decades as shown in Chapter 3. In contrast, the asymmetric synthesis of coordination compounds has only recently become a subject of systematic investigation. This is no doubt related to the fact that the chirality of coordination compounds is a much more complex phenomenon than that of organic compounds, because of higher coordination and the multitude of possible central atoms. Furthermore, while in organic chemistry the chiral tetrahedral carbon centres can be prepared without racemization, in contrast T-4 metal centres are very often labile. In fact it is even difficult to prepare compounds with a metal centre coordinated to four different monodentate ligands, and thus the possibility of obtaining one enantiomer is excluded in most cases. 

FIGURE 29.2. Several examples of monodentate phosphorous ligands with central chirality on... 

FIGURE 293. Examples of monodentate phosphine ligands with central chirality on carbon atoms of backbones. 

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