Diastereoisomeric ion pairs

Lammerhofer and Lindner [90] explained the chiral resolution of N-derivatized amino acids by CEC. The authors explained the formation of the transient diastereomeric ion-pairs between negatively charged analyte enantiomers and a positively charged chiral selector by multiple intermolecular interactions which might be differentially adsorbed to the ODS stationary phase. Furthermore, they claimed that the enantioseparation was achieved because of different observed mobilities of the analyte enantiomers originating from different ion-pair formation rates of the enantiomers and/or differential adsorption of the diastereoisomeric ion-pairs to the ODS stationary phase [90]. 

Though precise geometries of the ion pair inclusion complexes are unknown, it is clear that the cation size plays an important role in modifying the central cavity of the ligand 54 which allows the regulation of the chiral discrimination. Larger anions can not be included in the central cavity which is evident as a lack of chiral discrimination, however, less structured diastereoisomeric ion pair complexes were observed. 

Clearly, the outcome of such processes is determined by a diastereoisomerism existing somewhere along the reaction pathway. This could, for example, be in the approach of two materials to each other, in a transition state or reaction intermediate, or in the properties of the final product. The nature of the approach, interaction or bonding, is immaterial. For example, it has been shown recently that there are real and significant differences in the energetics of aggregation of chiral ions in solution to make diastereoisomeric ion pairs [31]. As shown in Table 1, even very simple chiral ion pairs can differ by 200-500 cal/mol in their heats of formation from the free ions. Such a difference can account for the difference between a reaction yield of 50 50 and 60 40. 

Derivatives of L-proline and 10-camphorsulfonic acid have been used to resolve enantiomeric amines through the formation of diastereoisomeric ion pairs. The concept of reciprocity in these separations. has been shown to occur in such a way that if the R enantiomer of acid HA resolves base B into its R- and 5-enantio-mers, an enantiomer of B (for example, R-B) can be used to resolve R-HA and 5-HA. 

The addition of a homochiral counterion to the mobile phase for the LC resolution of enantiomers can yield enantioseparations with only slight differences in the properties of the resulting diastereoisomeric ion pairs. 

The product exhibits a remarkable atropisomerism [61] determined by the rotation barrier around the bonds of quaternary carbon center. After optimization of the reaction conditions with different phosphorous triftamides and by varying the temperature, solvent, catalyst loading, and concentration, the bis indole 17 was obtained with a remarkable 72% ee. In the supposed mechanism, a stabilized vinylic carbenium indolyl intermediate (Figure 26.4) undergoes a Snl-type reaction by formation of a diastereoisomeric ion pair. 

The idea that the instantaneous equilibrium between the different conformations of dissymmetric complex ions could be influenced by ion-pair formation finds some support in recent experiments of Mason and coworkers with diastereoisomeric cobalt(III) complexes. Thus, Mason and Norman (31) have shown that the circular dichroism spectra of D-Co(d-pn)3" and L-Co(d-pn)3" (pn = propylenediamine) are differently changed by oxo-anions such as P04, S04 and 8203 . According to these authors the oxo-anion and D-Co(d-pn)3+ (or D-Coen3" ), but not L-Co(d-pn)3 should have a preferred mutual orientation in the ion pairs. At low concentrations the effect of sulfate and thiosulfate ions on the circular dichroism of D-Co(d-pn)3" is similar to that of phosphate but with these anions at concentrations > 0.2M the previous changes are reversed— probably due to breaking down the specific orientation of the ion pair in the denser ionic atmosphere. It is also interesting that Mason and Norman (32) have found that Co(NH3)6 associated with d-tartaric acid produces a pronounced Cotton effect. These results show that some... 

In general the metal complexes are charged. It is thus possible to convert the racemic mixture of such a complex into a pair of diastereoisomeric species with different physico-chemical properties, in particular solubihty, by association with an enantiomerically pure chiral coimterion [19]. Examples of frequently used such ions are shown in Fig. 3. Then the separation can be achieved by ... 

The reaction of nitriles with aromatic aldehydes is carried out at heating the reactants to 50-70°C with a 1 -h 10 (v/v) mixture of concentrated sulfuric acid and glacial acetic acid. The cycloaddition reaction is regiospecific. The oxazines 21 (equation 10) are formed as diastereoisomeric pairs which are free from their regioisomeric products in the limit of the NMR analysis. Precursors used were benzonitrile and acetonitrile as well as acetaldehyde, benzaldehyde and its substituted derivatives, and a number of the olefins having various structures. Until now, the reaction of aldehydes with nitriles was interpreted as an extension of the Ritter reaction. The initial O-protonation of aldehyde 22 is postulated to form in the presence of acid the hydroxycarbenium ion 23 which then reacts as a cationoid electrophile with the nitrile (equation 11) giving the nitrilium ion 24. 

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