Proton-Decoupled Carbon-13 NMR Spectroscopy in a Lyotropic Chiral Nematic Solvent as an Analytical Tool for the Measurement of the Enantiomeric Excess

Abstract

Organic solutions of poly-γ-(benzyl-l-glutamate) (PBLG) generate a sufficient differential ordering effect (DOE) to discriminate enantiomers using proton decoupled carbon-13 NMR in natural abundance. Discrimination between enantiomers is observed through the carbon-13 chemical shift anisotropy (CSA) differences. This method is successfully applied to a large number of chiral molecules including a case of axial chirality and offers the advantage that no labeling or chemical modification of molecules is needed. In most cases, the chemical shift differences are large enough to measure the enantiomeric excess with accuracy. We show that this new tool is an attractive and powerful alternative to the existing enantiomeric analytical techniques.

Application of a 1H δ‐resolved 2D NMR experiment to the visualization of enantiomers in chiral environment, using sample spatial encoding and selective echoes

“…[25] The resulting 5 mm NMR tube was then sealed in order to avoid solvent evaporation, and centrifugated head to tail until an optically homogeneous birefringent phase was obtained. [26] Figure 6 shows the difference between (a) the isotropic proton spectrum which was recorded on (±)3-butyn-2-ol dissolved in CDCl 3 , and (b) the proton spectrum of the same product dissolved in the chiral liquid-crystal. Due to residual 1 H -1 H dipolar couplings which are of the order of a few tenths of Hertz in this PBLG/CDCl 3 solvent, the observed linewidth and multiplicities do not allow for detecting subtle variations of resonance frequencies which could witness a chiral differentiation process.…”

“…Data were processed using zero-filling up to 4096 points in t 1 , an exponential filter and automatic baseline correction in both dimensions. This spectrum was recorded in about 6.5 h. Some regions, which are extracted from the positive projection of the columns of this spectrum, are shown in (b -bottom) for the proton sites Me 8 and Me 10 , and in (c -bottom) for CH 3 and CH 4 , as well as the corresponding region from the standard proton spectrum (top). media, where semiselective pulses cause coherence transfers which yield artifacts that cannot be removed by phase cycling, or any other coherence transfer pathway selection technique.…”

Samarium Iodobinaphtholate: An Efficient Catalyst for Enantioselective Aza‐Michael Additions of O‐Benzylhydroxylamine to N‐Alkenoyloxazolidinones

“…For products 3a-c, enantiomeric excesses were determined by HPLC, whereas the enantiomeric analyses of 3d and 3e were obtained using proton-decoupled 13 C NMR in a polybenzyl-l-glutamate (PBLG)/CD 2 Cl 2 liquid crystal. [23] [a] The absolute configuration of the major enantiomer (R) of 3a was assigned by comparison with literature.…”

Analysis of the 13C NMR Spectra of Molecules, Chiral by Isotopic Substitution, Dissolved in a Chiral Oriented Environment: Towards the Absolute Assignment of the pro‐R/pro‐S Character of Enantiotopic Ligands in Prochiral Molecules

“…As we will demonstrate below, this apparent complexity originates from three distinct NMR effects: the chiral discrimination in the PBLG phase, the enrichment of the mixture and the 1 H/ 2 H isotope effect observed on 13 C chemical shift of deuterium labelled compounds. To understand the analysis and the following discussion, we have to remember that the NMR resonance frequency, ñ i , of a 13 C nucleus i contains both an isotropic, s iso i , and an anisotropic, Ds i , contribution to the electronic shielding and may be written for a pair of enantiomers as: [1,9,10] …”