Ligands chiral geometry

The exclusive formation of the ( )-enamine in spite of the double bond geometry of the starting substrate is another noticeable feature of the isomerization. The present enantioselective isomerization requires prochiral allylamines free from geometrical isomers. In the isomerization, one specific feature is the stereochemical correlation between substrate geometries, product configurations, and the ligand chirality, as shown in Scheme 9. 

Complexation of copper salts with both achiral and chiral ligands offers additional potential for modulation of Lewis acidity, reactivity, and control of stereochemistry. Most notably, the application of chiral copper complexes in enantioselective transformations has steadily increased over the past 15 years. From the extensive investigations of Cu(II)-chiral bisoxazoline complexes to more recent combinations of Cu(I) and Cu(ll) salts with chiral ligands, chiral copper Lewis acids continue to attract considerable attention for several reasons, [3]. The first of which is their ready availability and/or accessibility. Second, chiral copper Lewis acids are moderately Lewis acidic, but more importantly, their Lewis acidity is easily modified by choice of oxidation state, counterion, and ligand. Finally, chiral Cu(l) and Cu(ll) complexes offer predictable and tunable coordination geometries about... 

The protocol of the allylic alkylation, which proceeds most likely via a c-allyl-Fe-intermediate, could be further improved by replacing the phosphine ligand with an M-heterocyclic carbene (NHC) (Scheme 21) [66]. The addition of a ferf-butyl-substituted NHC ligand 86 allowed for full conversion in the exact stoichiometric reaction between allyl carbonate and pronucleophile. Various C-nucleophiles were allylated in good to excellent regioselectivities conserving the 71 bond geometry of enantiomerically enriched ( )- and (Z)-carbonates 87. Even chirality and prochirality transfer was observed (Scheme 21) [67]. 

In 2005, Carretero et al. reported a second example of chiral catalysts based on S/P-coordination employed in the catalysis of the enantioselective Diels-Alder reaction, namely palladium complexes of chiral planar l-phosphino-2-sulfenylferrocenes (Fesulphos). This new family of chiral ligands afforded, in the presence of PdCl2, high enantioselectivities of up to 95% ee, in the asymmetric Diels-Alder reaction of cyclopentadiene with A-acryloyl-l,3-oxazolidin-2-one (Scheme 5.17). The S/P-bidentate character of the Fesulphos ligands has been proved by X-ray diffraction analysis of several metal complexes. When the reaction was performed in the presence of the corresponding copper-chelates, a lower and opposite enantioselectivity was obtained. This difference of results was explained by the geometry of the palladium (square-planar) and copper (tetrahedral) complexes. 

There can be significant differences in the detailed structure and mechanism of these catalysts. For example, the geometry of the phosphine ligands may affect the reactivity at the metal ion, but the basic elements of the mechanism of enantioselection are similar. The phosphine ligands establish a chiral environment and provide an appropriate balance of reactivity and stability for the metal center. The reactants bind to the metal through the double bond and at least one other functional group, and mutual interaction with the chiral environment is the basis for enantioselectivity. The new stereocenters are established under the influence of the chiral environment. 

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