General Features of Chiral Ligands and Complexes

The design of a chiral catalyst often starts with an achiral metal complex that exhibits some activity in the desired catalytic reaction. Once such a complex is identified, the ligands are modified in such a way that a chiral environment is created. Further fine tuning of the ligand and other reaction conditions may then be undertaken to optimize the enantioselectivity of the reaction. 

The ligands that we are going to discuss in this chapter are in most cases no different from the general categories that we have already encountered in the previous chapters. They can be broadly divided into two types. The first 

The macrocyclic chemistry of tetradentate Schiff base complexes has been known for long time. However, the successful use of such a complex as an enantioselective catalyst in epoxidation reactions is a relatively recent finding. In these reactions complex 9.9 or an analogue is used. One of the possible routes for the synthesis of intermediate 9.2 of Table 9.1 involves the use of a similar catalyst. While complex 9.9 works well with unfunctionalized alkenes, for the epoxidation of allylic alcohols, dialkyl tartarates, 9.10, are the preferred ligands. As we shall see, the mechanisms of epoxidation in these two cases are different. Also for the tartarate-based system titanium is the metal of choice (see Section 9.3.3). 

The ligands in 9.7-9.9 are all based on N,0 or N,N donor-atom-based chelating systems. In contrast, the ligands in 9.10-9.12 are all bidentate alkoxides, with 0,0 donor atoms. The chirality in binapthol, 9.11, arises from the fact that due to steric interactions the molecule does not have a plane of symmetry. The two naphthyl rings lie in different planes. The complex 9.12 has binaphthol as the ligand. Complexes such as 9.12 are used as catalysts in nitroaldol condensation reaction (see Section 9.5.4). 

The chiral motifs of both 9.10 and 9.11 have been exploited to make chiral phosphines. Thus 9.13, abbreviated as DIOP, and the phosphine ligand in 9.16, 

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