Chiral Discrimination in the 13C and 2H NMR of the Crown and Saddle Isomers of Nonamethoxy-Cyclotriveratrylene in Chiral Liquid-Crystalline Solutions

• Published in: The journal of physical chemistry. B 2007-08, Vol.111 (32), p.9453-9467
• Main Authors: Lafon, Olivier, Lesot, Philippe, Zimmermann, Herbert, Poupko, Raphy, Luz, Zeev
• Format: Article
• Language: English
• Subjects: Carbon Isotopes; Deuterium; Liquid Crystals - chemistry; Magnetic Resonance Spectroscopy; Molecular Structure; Polycyclic Compounds - chemistry; Solvents - chemistry; Stereoisomerism
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp070564q

We report 2H and 13C NMR spectra of the crown and saddle isomers of nonamethoxy-tribenzocyclononene (1), dissolved in lyotropic achiral and chiral liquid-crystalline solutions based on poly-γ-benzyl-glutamate and poly-γ-benzyl-l-glutamate (PBG and PBLG). The 2H−{H} measurements include spectra of compound 1 deuterated in the ring methylene and in the aromatic sites as well as of the methyl groups in natural abundance. Carbon-13 spectra were recorded in natural abundance as well as in two isotopomers enriched in the ring methylene and one of the methoxy groups. The crown isomer (c-1) is rigid with C 3 symmetry and can be separated into its enantiomers using a chiral high-performance liquid chromatography column. The NMR spectra of racemic c-1 in PBLG solutions exhibit two sets of lines due to the enantiomers. The peaks were identified by comparing the spectra with those of the neat enantiomers. Analysis of the 2H quadrupolar splittings and the 13C residual chemical shift anisotropies shows that the dominant factor determining the chiral discrimination is the difference in the ordering of the two enantiomers in the chiral liquid crystals. The saddle isomer (s-1) is highly flexible, undergoing fast pseudorotation between six conformers. The “frozen” conformers have C 1 symmetry and are therefore chiral. Three of these comprise one enantiomer, and the other three the second one. However, the rapidly interconverting species has, on the average, a C 3 h symmetry and is therefore achiral. The methylene groups in the latter are, however, prostereogenic, and their hydrogen/deuterium−carbon bonds constitute enantiotopic pairs. The 2H NMR spectra of the s-1 methylene-deuterated in PBLG solutions exhibit, in fact, enantio-discrimination with two quadrupolar doublets. This is in contrast to rigid prochiral molecules with a threefold symmetry axis, which normally do not show such discrimination. A detailed analysis of the effect is presented, and it is argued that the discrimination observed for s-1 reflects the different ordering of its enantiomers during the pseudorotation cycle.