Achiral anionic ligands

A way to confer enantiofacial selectivity in cyclizations of achiral olefinic organolithi-ums is by the use of chiral ligands. The ability to discriminate between the enantiotopic faces of an inactivated carbon—carbon jr-bond tethered to a formally carbanionic center considerably extends the synthetic utility of anionic cyclizations119. 

The terbium complex, in contrast, exhibits greatest sensitivity for chloride. Using the achiral ligand (R = H) exhibits similar selectivity for these anions, but with rather less sensitivity. 

The nucleophile is the enolate anion from ethyl 2-fluoroacetate 30 formed with NaH in the presence of n-Hex4N+Br. The complex 28 would be achiral except for the ligand which directs the nucleophile to one end of the allyl cation on the opposite face to the palladium. With only 1 mol% of the catalyst up to 96% ee was achieved though only one centre in 31 is controlled. 

As in the previous example, the chirality of the ligand is in close proximity to the palladium binding site, while the pendant group is achiral. Introduction of the crown ether was designed to enhance coordination to the counterion. A ternary complex including the crown ether, potassium cation, and the enolate anion was proposed in which the bulky crown ether blocks approach of the enolate to Cl and provides a chiral pocket around carbon C3 [40]. Enantioselectivities up to 75% were reported [40], which were later revised to 65% [38]. 

It may seem that the previous discussion leads to so many restrictions that one despairs at the use of d-d CD as a stereochemical probe at all. The source of this apparent dilemma is that the CD is sensitive not only to the absolute configuration of the chelate system, but also to any variations in the achiral chromophore. Such a negative conclusion is readily alleviated, however, if the restrictions are considered together with established empirical rules. In addition, the sensitivity to the achiral chromophore, though a partial hindrance to the establishment of a simple CD-stereochemical correlation to cover all complexes, is actually an appreciable advantage to the study of the effects of, for example, added anions on the metal-ligand bonding. We now illustrate these features with some examples. 

Fig. 2. Representative examples of anionic, cationic, non-ionic, achiral or chiral, monomeric or polymeric tertiary phosphines used as ligands in aqueous-organic biphasic catalysis...

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