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Distinction of Enantiomers by NMR REVIEWS Disinction of enantiomers by NMR spectroscopy using chiral orienting media Burkhard Luy Abstract | NMR spectroscopy is a very important analytical tool in modern organic and inorganic chemistry. Next to the identification of molecules and their structure determination, it is also used for the distinction of enantiomers and the measurement of enantiomeric purity. This article gives a brief review of the techniques being developed for enantiomeric differentiation by virtue of chiral alignment media and their induction of enantiomerically dependent anisotropic NMR parameters like residual dipolar couplings. An overview of existing chiral alignment media, a brief introduction into the basic theory and measurement of the various anisotropic parameters, and several example applications are given. 1. Introduction valuable compounds one doesn’t necessarily want to NMR spectroscopy is one of the most important irreversibly modify the substance. analytical tools in modern organic and inorganic Another possibility for the distinction of chemistry as it is the only tool that allows the enantiomers is the orientation of the molecule of determination of molecular structures at atomic interest in a so-called chiral alignment medium. resolution in solution. It is used to identify the In this case, the molecule is partially aligned constitution, conformation and configuration and anisotropic NMR parameters like residual of countless molecules every day. However, the quadrupolar couplings, residual dipolar couplings magnetic field used for the induction of the Zeeman and residual chemical shift anisotropy can be splitting is per se achiral so that enantiomers measured2–4. As the orientation in a chiral have identical properties and therefore identical alignment medium is different for the two NMR spectra. For the distinction of enantiomers enantiomers, resulting anisotropic parameters differ with NMR therefore special sample preparation is as well and the determination of enantiomeric purity needed. is possible. Most commonly used are chemical modifi- This article provides a brief overview of the cations with chiral substances of known chirality existing chiral alignment media and how they which leaves a diastereomeric pair in the end can be used to distinguish enantiomers. After that can be distinguished with conventional NMR an introduction of the various chiral alignment Department Chemie, LS spectroscopy. The most popular example is the media, individual anisotropic NMR parameters are OC II, Technische Mosher-ester and the many related compounds1. introduced with the corresponding pulse sequences Universitat¨ Munchen, ¨ But the method relies on the existence of an optimized for the differentiation of enantiomers. Lichtenbergstr. 4, 85747 appropriate functional group in the molecule of In addition, examples for enantiomeric distinction Garching, Germany. [email protected] interest and especially when working with highly are summarized which demonstrate the width of Journal of the Indian Institute of Science VOL 90:1 Jan–Mar 2010 journal.library.iisc.ernet.in 119 REVIEW Burkhard Luy potential applications. We also mention on the side cesium N-dodecanoyl-L-alaninate16, poly-γ-ethyl- the use of chiral lanthanide shift reagents. L-glutamate (PELG) and poly-ε-carbobenzyloxy-L- Although we try to give an overview of the lysin (PCBLL)17, a lyotropic liquid crystal based on state of the art in chiral discrimination using chiral glucopon/buffered water/n-hexanol18, amino acid- orienting media, the article is by no means complete. based anionic surfactants19, and currently developed The field is progressing rapidly these days and new chiral polyguanidines20. While the cholesteric developments can be expected every day. phases generally work with relatively narrow ranges with respect to temperature and solvent applicability, 2. Chiral orienting media the poly(amino acids) PBLG, PELG, PCBLL and The first partially aligned spectrum was published by the polyguanidines can be used with most apolar 5 Spence et al. in 1953 and stayed widely unnoticed. organic NMR solvents like CDCl3,CD2Cl2, or THF. Only the famous benzene spectrum in a liquid In addition, PBLG can also be used with the polar crystalline phase published by Saupe and Englert organic solvent DMF21 and in mixtures with up to in 1963 6 and the full theoretical explanation of the 50% DMSO22. observed signals one year leater7 started a whole The most exceptional role for the chiral new and very active field in NMR spectroscopy. distinction of enantiomers is certainly held by Only a few years later, Sackmann first proposed8 the lyotropic liquid crystalline phases of PBLG. and then experimentally verified9 the distinction The first distinction of enantiotopic deuterons of enantiomers within an optically active liquid with PBLG was reported by Samulski13 before the crystalline phase. Courtieu-group used the same polymer to develop Today three kinds of alignment media are known, the large variety of tools for the distinction of liquid crystalline phases, stretched polymer gels enantiomers21,23 (see section 3) which they applied and alignment via paramagnetic centers, which all to an even wider range of molecules (for some have their distinct behavior as is summarized in the examples see section 4). Usually PBLG is applied following with the focus on chiral media with the in its L-form, but also the D-form, called PBDG, ability to distinguish enantiomers. is available. With the two enantiomers in hand it was proven that a reversal of symmetry of both 2.1. Chiral liquid crystalline phases polymer and solute results in identical anisotropic Liquid crystalline phases were the first alignment NMR parameters24,25. It could also be shown that media used for chiral distinction and are still a 1:1 mixture of PBLG/PBDG does not result in a the most widely used for organic solvents. First split spectrum, but in the corresponding spectrum chiral liquid crystalline alignment media for the of an achiral substance that cannot distinguish distinction of enantiomers were a mixture of enantiomers anymore24,26. This is clear evidence cholesteryl chloride and cholesteryl myristate9, that the effect of the alignment medium is highly a membranuous phase with optically active averaged over time. The structure of PBLG is decyl-2-sulfate10,11 and poly-γ-benzyl-L-glutamate also quite well characterized to have an α-helical (PBLG)12,13. Lateron further chiral phases were secondary fold (see Figure 1A), which matches found with several cholesteric solvents14,15, the current opinion that a chiral secondary fold is Figure 1: Helical secondary structures of PBLG and gelatin. (A) Ordered α-helices of the liquid crystalline phase of PBLG21; (B) the triple helix of gelatin. 120 Journal of the Indian Institute of Science VOL 90:1 Jan–Mar 2010 journal.library.iisc.ernet.in Disinction of enantiomers by NMR spectroscopy using chiral orienting media REVIEW needed to induce sufficiently different alignment other polar organic solvents37–39. The same year properties to a solute molecule to be able to conventional gelatin was found to be the first chiral distinguish them. medium capable to distinguish enantiomers in pure Liquid crystalline alignment media, however, water40,41. Several studies on enantiomers followed also have profound disadvantages, the most with the underlying polymer collagen42 and gelatin important of them being the existence of a lower in combination with a specially designed stretching limit of alignment. As could be shown recently27, apparatus 43–45 that allows to rapidly and freely this limit correlates with the persistence length scale the alignment to a desired strength4,44,46. of the polymer chain used and the general rule Gelatin and collagen form triple helices as applies that the longer the individual chains of the a chiral secondary structure (Figure 1B) which polymer are, the lower is the achievable lower limit apparently results in a stereospecific alignment of of alignment. Only with special techniques like chiral molecules comparable to the α-helix of PBLG variable angle sample spinning (VASS)28–30, which and other poly(amino acids). As with all stretched imply the use of specialized hardware, the alignment polymer gels no lower limit for the alignment of solutes can be further reduced. Because of this strength applies which results in a higher flexibility limitation, most studies in liquid crystalline phases of applicable pulse sequences and the more general are limited to relatively small molecules and by use of dipolar couplings for the distinction of using insensitive detection methods like natural enantiomers. An extension to conventional gelatin abundance deuterium (NAD) detection (section was recently published with gelatin cross-linked by 3.2). accelerated electrons, so-called e−-gelatin, which Besides PBLG only PELG and PCBLL are allows enantiomeric distinction also at temperatures frequently used as chiral liquid crystalline alignment above the melting temperature of conventional media today. With the current development of chiral gelatin and with pure DMSO as the solvent 47. polyguanidines, however, there seems to be another, A second water-compatible gel-based alignment very cheap liquid crystal with similar properties medium was also introduced in 2009 with various when it comes to enantiomeric distinction. types of carrageenan48 as polysaccharide-based It should be noted that the distinction of polymers with similar properties to gelatin. enantiomers can also be achieved with achiral liquid The
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