Ligand structures chirality

Ahn et al. [20] reported the synthesis of homochiral bis(oxazolinyl)-biferrocene ligands (structure 11 in Scheme 7), which also have both planar and central chirality. With these complexes, 2-(phenyl)cyclopropane carboxy-lates were obtained in up to 99% ee and a trans/cis ratio of 88/12. 

Zhang et al. [49] prepared a chiral ruthenium complex coordinated by a pyridine-bis(imine) ligand (structure 43 in Scheme 21). 

In addition, Rowlands has involved chiral sulfoxide-containing ligands for the catalytic addition of McsSiCN to aldehydes. " The ligand structure was based on a phenolic oxazoline scaffold with introduction of the sulfur substituent via cysteine derivatives. The best enantioselectivities of up to 61% ee were obtained with the bulkiest tert-butyl substituted ligand (Scheme 10.42). The effect of the sulfoxide configuration was studied, showing that the use of... 

Figure 6.11 Typical tridentate ligand structure incorporating a chiral amino alcohol and modified diamine-based tridentate ligand structure attached to the solid support for parallel catalyst library strategy.

These, in turn, can be condensed with amines to give imines 4 or ketimines 5 and 6, or reduced to give amino alcohols 7-9, respectively. The ligand structure is therefore vastly variable. Steric factors, such as flexibility of backbone and side-chains, as well as electronic factors (for example sp versus sp conflguration of the N-donors) can be easily modulated. The introduction of central chirality via chiral amine side-chains is also possible. The interaction of planar and central chirality, usually referred to as chiral cooperativity [11-13], can thus be studied in a ligand system which has both planar and central chiral elements. 

The copper-catalyzed enantioselective Michael addition of organometallic reagents to enones was the first successful application of phosphoramidite chiral ligands in catalysis [4, 43]. Since this early report, substantial enhancement of the enantioselectivity and/or of the substrate scope has been achieved through an untiring effort to optimize the ligand structure [5a, 44]. 

When the /6-amino alcohol ligand is chiral and optically pure, there are four potentially low-energy TS structures that may lead to products. Several chiral ligands have been shown... 

If one of the ligands is chiral and its absolute configuration is known, one cao readily determine the absolute configuration at the metal atom by X-ray (or neutron) diffraction. Of the two enantiomeric structures consistent with the X-ray data, the one having the correct configuration about the known chiral center is chosen. For example, consider tris(R-propylenediaaune)cobalt(III). This was synthesized ... 

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