Enantiomeric and Enantiotopic Analysis of Cone-Shaped Compounds with C3 and C3v Symmetry Using NMR Spectroscopy in Chiral Anisotropic S
详细信息 查看全文
- 作者：Philippe Lesot ; Denis Merlet ; Muriel Sarfati ; Jacques Courtieu ; Herbert Zimmermann ; and Zeev Luz
- 刊名：Journal of the American Chemical Society
- 出版年：2002
- 出版时间：August 28, 2002
- 年：2002
- 卷：124
- 期：34
- 页码：10071 - 10082
- 全文大小：197K
- 年卷期：v.124,no.34(August 28, 2002)
- ISSN：1520-5126
文摘
We describe the enantiomeric and enantiotopic analysis of the NMR spectra of compounds derivedfrom the functionalized cone-shaped core, cyclotriveratrylenes (CTV), dissolved in weakly oriented lyotropicchiral liquid crystals (CLCs) based on organic solutions of poly-
-benzyl-L-glutamate. The CTV core lacksprostereogenic as well as stereogenic tetrahedral centers. However, depending on the pattern of substitution,chiral and achiral compounds with different symmetries can be obtained. Thus, symmetrically nonasubstitutedCTVs (C3 symmetry) are optically active and exhibit enantiomeric isomers, while symmetrically hexasubstituted (C3v symmetry) derivatives are prochiral and possess enantiotopic elements. In the first part weuse 2H and 13C NMR to study two nonasubstituted (-OH or -OCH3) CTVs, where the ring methylenes arefully deuterated, and show for the first time that the observation of enantiomeric discrimination of chiralmolecules with a 3-fold symmetry axis is possible in a CLC. It is argued that this discrimination reflectsdifferent orientational ordering of the M and P isomers, rather than specific chiral short-range solvent-solute interactions that may affect differently the magnetic parameters of the enantiomers or even theirgeometry. In the second part we present similar measurements on hexasubstituted CTV with flexible sidegroups (-OC(O)CH3 and the, partially deuterated bidentate, -OCH2CH2O-), having on the average C3vsymmetry. No spectral discrimination of enantiotopic sites was detected for the -OC(O)CH3 derivative.This is consistent with a recent theoretical work (J. Chem. Phys. 1999, 111, 6890) that indicates that in C3vmolecules no chiral discrimination between enantiotopic elements, based on ordering, is possible. In contrast,a clear splitting was observed in the 2H spectra of the enantiotopic deuterons of the side groups in thetri(dioxyethylene)-CTV. It is argued that this discrimination reflects different ordering characteristics of thevarious, rapidly (on the NMR time scale) interconverting conformers of this compound. Assuming two twistedstructures for each of the dioxyethylene side groups, four different conformers are expected, comprisingtwo sets of enantiomeric pairs with, respectively, C3 and C1 symmetries. Differential ordering and/or fractionalpopulation imbalance of these enantiomeric pairs leads to the observed spectral discrimination of sites inthe side chains that on average form enantiotopic pairs.
-benzyl-L-glutamate. The CTV core lacksprostereogenic as well as stereogenic tetrahedral centers. However, depending on the pattern of substitution,chiral and achiral compounds with different symmetries can be obtained. Thus, symmetrically nonasubstitutedCTVs (C3 symmetry) are optically active and exhibit enantiomeric isomers, while symmetrically hexasubstituted (C3v symmetry) derivatives are prochiral and possess enantiotopic elements. In the first part weuse 2H and 13C NMR to study two nonasubstituted (-OH or -OCH3) CTVs, where the ring methylenes arefully deuterated, and show for the first time that the observation of enantiomeric discrimination of chiralmolecules with a 3-fold symmetry axis is possible in a CLC. It is argued that this discrimination reflectsdifferent orientational ordering of the M and P isomers, rather than specific chiral short-range solvent-solute interactions that may affect differently the magnetic parameters of the enantiomers or even theirgeometry. In the second part we present similar measurements on hexasubstituted CTV with flexible sidegroups (-OC(O)CH3 and the, partially deuterated bidentate, -OCH2CH2O-), having on the average C3vsymmetry. No spectral discrimination of enantiotopic sites was detected for the -OC(O)CH3 derivative.This is consistent with a recent theoretical work (J. Chem. Phys. 1999, 111, 6890) that indicates that in C3vmolecules no chiral discrimination between enantiotopic elements, based on ordering, is possible. In contrast,a clear splitting was observed in the 2H spectra of the enantiotopic deuterons of the side groups in thetri(dioxyethylene)-CTV. It is argued that this discrimination reflects different ordering characteristics of thevarious, rapidly (on the NMR time scale) interconverting conformers of this compound. Assuming two twistedstructures for each of the dioxyethylene side groups, four different conformers are expected, comprisingtwo sets of enantiomeric pairs with, respectively, C3 and C1 symmetries. Differential ordering and/or fractionalpopulation imbalance of these enantiomeric pairs leads to the observed spectral discrimination of sites inthe side chains that on average form enantiotopic pairs.