Chirality discrimination, hydrogen bonds

When a CSP is applied, the separation mechanism is based on the differences in the interaction between the chiral selector in the stationary phase and the enantiomers of the solute. Depending on the nature of the selector and the type of the solute, the stereoselective interaction can be based on interactions of one or more different types such as inclusion complexation, Tr-jr-interaction, dipole stacking, hydrogen bonding, electrostatic interaction, hydrophobic interaction, and steric interaction [35]. In order to obtain chiral discrimination between the enantiomers, a three-point interaction is required between at least one of the enantiomers and the CSP [36]. The interactions can be of attractive as well as repulsive nature (e.g., steric and electrostatic interactions). 

The cage-type peptide cyclophanes (7 and 8) exhibit discrimination toward steroid hormones, as effected by hydrophobic and n-n interactions. In addition, the chirality-based discrimination between a- and -estradiol as well as between D- and L-amino acids bearing an aromatic moiety is performed on the basis of their capacity of forming efficient hydrogen bonding with the host molecules in aqueous media [41, 43]. 

A significant ability to discriminate between chiral amines based on the quenching of S-di-2-naphthylprolinol fluorescence emission was reported by Diamond et al. [32], fl-Phenylethylamine (PEA) was seen to have a much greater efficiency as a quencher than the S-enantiomer. l- and D-norephedrine, which have structural conformation similarities to PEA, were also observed to have an enantiomeric selectivity. The mechanism of chiral recognition is proposed to be a combination of hydrogen bonding and 3D chirally restricted space. 

A chiral Ru hydride 23 is formed and it is assumed that the hydrogen transfer occurs via metal-ligand bifunctional catalysis. The N-H linkage may stabilize a transition state 24 by formation of a hydrogen bond to the nitrogen atom. Stereochemistry is determined by formal discrimination of the enantiofaces at the sp2 nitrogen atom of the cyclic imine. 

I. Alkorta, J. Elguero, Discrimination of hydrogen-bonded complexes with axial chirality. J. Chem. Phys. 117, 6463-6468 (2002)... 

E. Murguly, R. McDonald, N. R. Banda, Chiral Discrimination in Hydrogen-bonded [7]Helicenes, Org. Lett. 2000, 2, 3169-3172. 

As observed for the molecular clips reported above, the chiral scaffold is pivotal in promoting homo- or heterochiral self-discrimination. Amide hydrogen bonds were implemented on helicene chiral scaffolds as well, but in this case dimerization of the monomers was characterized by homochiral enantioselective self-recognition, that is self-association between molecules with the same helicity (Fig. 17B) [44], These species dimerized in solution with association constants of 207 M 1 by means of four non-covalent bonding interactions and, in combination with the peculiar helical shape of the monomers, forms only homochiral dimers. 

Chiral discrimination of guests by inclusion in hydrogen-bonded capsules of tetraureas is a highly interesting and challenging problem. The analysis of such systems by NMR spectroscopy is complicated by two facts ... 

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