Abstract
Chiral molecules possess enantiomers that have non-superimposable chemical structures but exhibit identical nuclear magnetic resonance (NMR) spectra. This feature prevents the use of NMR spectroscopic methods for the determination of enantiomeric excesses (ee) of chiral molecules, using simple mixtures of their enantiomers. Recently, however, it was reported that the addition of a symmetrical prochiral molecule (a reporter or host) into a solution of chiral analyte can lead to estimation of ee through interactions involving rapid exchange of the chiral analyte (guest) in the formed host–guest complex. This is due to the ee-dependent splitting of NMR resonances of the prochiral host molecule based on averaging the chemical shift non-equivalency caused by the presence of a chiral guest. The mechanism is not dependent on diastereomer formation, and 1:1 host–guest complexes can also show ee-dependent NMR peak splitting. Prochiral molecules capable of ee sensing using the NMR technique are now referred to as so-called prochiral solvating agents (pro-CSAs). pro-CSAs represent a family of reagents distinct from the commonly used NMR chiral derivatizing reagents (where chiral auxiliaries are used to derivatize enantiomers to diastereomers) or chiral solvating agents (where chiral auxiliaries interact in an asymmetric manner with analyte enantiomers). pro-CSA methods are unique since neither pro-CSA nor NMR contains chiral factors, making the technique neutral with respect to chirality. Here, we review our recent work on this matter involving several different nominally achiral receptor molecules whose unique guest binding properties and solution characteristics (especially with regard to NMR spectroscopy) allow for the estimation of ee in the corresponding chiral guests.
Highlights
The benzylic CH2 of L is prochiral, and its nuclear magnetic resonance (NMR) resonance is capable of acting as an ee reporter group for chiral guests bound to the Zn (II) cation in L·2Zn·3C following the exchange of co-ligands C. (Figure 9a)
This review chronologically summarizes our synthetic and methodological advancements in detecting enantiomeric excess of various chiral analytes based on rapid analyte exchange in host–guest systems
We have shown that achiral porphyrin-based macrocyclic hosts, such as OxP, TPP, OxPBz2 or (TAPZn)2 Binaph, can be utilized as potent ee sensors for various guests in two modes of operation, i.e., forming host–guest complexes by acid–base or hydrogen-bonding interactions
Summary
In the case of NMR spectroscopy, until relatively recently, it was not possible to use it to analyze ee without using an intrinsically chiral analytical probe substance because NMR is essentially insensitive to the enantiomer identity Our work on this subject, which is the topic of this review, was initially based on non-chiral CSA, where information about ee is extracted based on the enantiotopicity of host proton NMR resonances in rapidly exchanging guests in guest–host complexes [42,43,44]. We have considered other corresponding small molecule systems [45] as well as receptors from the wellestablished porphyrin dimer family [46] and coordination complexes [47] The latter are highly promising as analytical reagents because of the large available range of ligand–.
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