Ion pairs, diastereomeric

Repeated deprotonation of 278 removed due to a high H/D kinetic isotope effect the 1-proton, forming the dideuterio compound 279 with low diastereoselectivity . It is quite likely that a dynamic thermodynamic resolution is the origin. Intermediate 277 is configurationally labile, enabling an equilibration of the diastereomeric ion pairs 277 and epi-211. Similar studies were undertaken with 1-phenyl-l-pyrid-2-ylethane (280) and l-(4-chlorophenyl)-l-(pyrid-2-yl)-3-(dimethylamino)propane (281) (50% eef. An improvement of the achieved enantiomeric excesses resulted when external chiral proton sources, such as 282 or 283, were applied (84% ee for 280 with 283 and 75% ee for 281). 

The mechanism proposed to account for these observations involves two diastereomeric ion pairs [10] and [11] (1 ). 

Figure 16. Structure of diastereomeric ion pairs of poly(2-vinylpyridine) MgBz.

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]. 

W. Steuer, M. Schindler, G. Schill, and F. Erni, Supercritical fluid chromatography with ion-pairing modifiers separation of enantiomeric 1,2-aminoalcohols as diastereomeric ion pairs, J. Chromatogr., 447 287 (1988). 

Tetrahedron 6 represents one of the rare examples of supramolecular assemblies where complete enantiomeric resolution can be achieved. Treatment of racemic 6 with the chiral auxiliary (S) - N-melhylnicolinium (s-nic, 38) preferentially precipitates the diastereomeric ion-pair of (AAAA)-638. Once resolved, the chiral auxiliary can be replaced by achiral counter-ions such as tetramethyl- or tetraethylammonium ions, while the tetrahedra maintain their enantiopurity for at least 8 months, even upon extended heating in D2O (Fig. 24) [119]. 

Fig.24 Overview of the synthesis and resolution of solution stable, chiral tetrahedron 6 [119]. a Formation of racemic, homochiral (AAAA)-6 and (AAAA)-6 b Chiral resolution and separation upon addition of s-nic ions (38) by formation of diastereomeric ion-pair (AAAA)-6-38 c Ion-exchange of chiral auxiliary 38 by achiral counterions such as NMe4+ and NEt4+ maintains the chirality of the resolved tetrahedra (AA AA)-6...

Bartik, P. et al. Advanced statistical evaluation of the complex formation constants from electrophoretic data II diastereomeric ion-pairs of (R,S)-N-(3,5- dinitrobenzoyl)leucine and tert-butylcarbamoylquinine. Anal. Chine Acta 2004, 506, 105-113. 

Diastereomeric ion pairs relevant to different types of chirality. 

When an achiral substrate 241 is deprotonated in the presence of a chiral ligand, such as (-)-sparteine, two diastereomeric ion pairs 242 and epi-lAl result these differ in their energies and reactivity [Eq. (66)]. 

Aspartic acid has a very acid pi (3.0) and was present in the anionic form at the eluent pH. Under such experimental conditions diastereomeric ion pairs can be formed between L-aspartic acid and the enantiomers of alkaloids, which were largely in the cationic form. However, electrostatic interactions, even if necessary, did not seem to be sufficient to explain the formation of diastereomers leading to enantiomeric separation. In fact, the use of L-glutamic acid as chiral selector did not produce any separation. Therefore, it was deduced that for the resolution of alkaloids, further interactions such as hydrogen bonding and steric interactions are required. 

The diastereomeric ion pairs [(A(+)/CS) and (A(-)/CS)] were held together through ionic interactions, such as carboxylate group/ammonium ion interactions, hydrogen bonding, steric interactions, and van der Waals forces. The fundamental parameters determining the success of chiral resolution on the aforementioned CCSPs were the following ... 

This behavior was different from what was occurring between ibuprofen enantiomers and L-arginine, which gave rise to diastereomeric ion pairs (or salts). 

The influence of molar ratio racemate/chiral selector on the enantioseparation was scarcely investigated even though, in the case of silica gel achiral layers, it appeared crucial for enantiomeric resolution of ibuprofen [24]. In fact, only diastereomeric ion pairs formed with a molar ratio ibuprofen/(—)-brucine (2 1) gave rise to well-resolved spots. 

The effects of different temperatures on the separation of all three j8-blockers using L-aspartic acid as a chiral selector were also investigated. It has been found that 17°C was the most suitable temperature for the resolution of the examined jS-blockers, providing desired mobility to the diastereomeric ion pair formed anionic species of L-aspartic acid and protonated cations of amino moieties of the corresponding )3-blockers. The presence of chiral selector in situ was established by treating the developed chromatograms with ninhydrin that produced a characteristic color with aspartic acid in both spots of the resolved enantiomers [18]. This method was very sensitive, enabling detection of 0.26 fig atenolol and 0.23 tig of each metoprolol and propranolol. 

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