Chiral hydrophobic ligands

Moreover, catalytic asymmetric ring-opening reactions of me o-epoxides with indoles, alcohols, and thiols proceeded smoothly in the presence of catalytic amounts of Sc(DS)3 and chiral bipyridine ligand 3 in water to afford (3-amino alcohols in high yield and enantioselectivity (Scheme 15.19 and Scheme 15.20)." Note that an excellent hydrophobic, asymmetric environment has been created in water. 

The LACS system has been successfully applied to asymmetric catalysis. Catalytic asymmetric ring-opening reactions of meso epoxides with aromatic amines proceeded smoothly in the presence of 1 mol% of Sc(DS)3 and 1.2 mol% of chiral bipyridine ligand (3) in water to give P-amino alcohols in high yields with excellent enantioselectivities (Scheme 12.75) [172]. It has been noted that excellent chiral hydrophobic environments have been created in water. 

In the same area, Moreau et al. have developed new hydrophobic ionic liquids containing chiral camphorsulfonamide units and showed that they could be used as ligands for the Ti-catalysed addition of ZnEt2 to benzaldehyde. The ionic... 

In the course of our investigations to develop new chiral catalysts and catalytic asymmetric reactions in water, we focused on several elements whose salts are stable and behave as Lewis acids in water. In addition to the findings of the stability and activity of Lewis adds in water related to hydration constants and exchange rate constants for substitution of inner-sphere water ligands of elements (cations) (see above), it was expected that undesired achiral side reactions would be suppressed in aqueous media and that desired enanti-oselective reactions would be accelerated in the presence of water. Moreover, besides metal chelations, other factors such as hydrogen bonds, specific solvation, and hydrophobic interactions are anticipated to increase enantioselectivities in such media. 

The fundamental behaviour of stationary phase materials is related to their solubility-interaction properties. A hydrophobic phase acts as a partner to a hydrophobic interaction. An ionic phase acts as a partner for ion-ion interactions, and surface metal ions as a partner for ligand complex formation. A chiral phase partners chiral recognition, and specific three-dimensional phases partner affinity interactions. 

CSPs has, overall, a hydrophobic character (very similar to RP phases with C4-C8 ligands) which stems from contributions of the chiral selectors itself and (capped) linker groups (only a portion of the linkers are utilized for selector attachment) which constitutes a kind of hydrophobic basic layer on the support surface. Hence under typical RP-conditions, hydrophobic interactions between lipophilic residues of the solute and hydrophobic patches of the sorbent may be active and thus a reversed-phase like partition mechanism may be superimposed upon the primary ion-exchange process k = A rp -I- A ix). This A Rp-retention contribution may be especially important for eluents with high aqueous content. 

A series of tetrahedral assemblies built from four metal ions (e.g., Ga3+, Al3+, In3+, Fe3+, Ti4+, Ge4+) and six bis-catechol ligands has been prepared by Raymond s group (Fig. 4a). These chiral hosts possess a small hydrophobic cavity... 

Figure 4.14 describes three systems where recognition is size- and shape-selective [ref. 9]. In one system, two [1,3] hexagons assembled with a receptor in the presence of [1,4] or [1,2,3] hexagons (Fig. 4.14). In a second system, two ligand-receptor pairs (2a and la 2b and lb) formed in the presence of each other. The pattern of the hydrophobic faces on the ligands and receptors was chiral, and the receptors and ligands assembled in a way that juxtaposed enantiomeric chiral faces. In a third system, one [1,4] hexagon selectively assembled with a receptor. Receptors that selectively assembled two or three [1,4] hexagons were also fabricated.

Figure 4.14. MESA can be used to model receptor/ligand interactions. In (a), two [1,3] hexagons - with dark edges and clear centers - assemble into the receptor. In (b), the ligands, 2a and 2b, assemble into the receptor, la and lb, based on the chirality of the pattern of the hydrophobic faces. In (c), the dark [1,4] hexagon assembles into the...

---