Use of a diamagnetic lanthanide complex for extending the scope of NMR determination of absolute configuration by the modified Mosher's method

Assignment of absolute configuration using chiral reagents and NMR spectroscopy

“…Monitoring the changes in several hydrogen resonances of the (R)-and (S)-MTPA derivatives in the presence of La(hfa) 3 can be used to confirm the configurational assignment. 22 Another reagent worth mentioning is (1R,2R)-2-(anthracene-2,3-dicarboximido)cyclohexane carboxylic acid, which is especially suited for distinguishing secondary alcohols with remotely disposed chiral centers. The anthryl ring is aligned in a geometry that causes shielding over a substantial length of the carbon chain of the secondary alcohol as seen in Figure 2.…”

Axially Chiral Trifluoromethylbenzimidazolylbenzoic Acid: A Chiral Derivatizing Agent for α-Chiral Primary Amines and Secondary Alcohols To Determine the Absolute Configuration

“…The methyl group in alanine derivatives 13 showed a difference of the same magnitude, but the Δδ PM value of the remote methyl ester group was reduced as the shielding effect decreased with distance. However, the Δδ PM is still higher compared to the reported alanine–MTPA amide (0.08 and 0.03 for the methyl and methylester group, respectively) . Phenylalanine derivative 14 showed a slightly lower difference than 13 .…”

“…The Δδ PM between the most relevant proton signals, the methyl group, and the aromatic hydrogens at the ortho positions were −0.18 and +0.27, respectively, and decreased to +0.09 at the meta positions, which was expected. Moreover, the Δδ PM of the methyl group with TBBA was substantially higher than the reported values (absolute values shown) for the derivatives with MTPA (0.04), MPA (0.07), or MPA in complex with barium perchlorate (0.09). ,, …”

Diamagnetic lanthanide tris β‐diketonates as organic‐soluble chiral NMR shift reagents

“…A similar shielding effect has been observed for hydrogen atoms remote from the ester group in menthol methoxyacetate in the presence of the tris lanthanum(III) chelate with 1,1,1,5,5,5-hexafluoro-2,3-pentanedionate. 8 The most striking comparison occurs for methyl p-tolyl sulfoxide and 1-phenylethylamine in which La(tfc) 3 and La(hfc) 3 deshield the hydrogen atoms, whereas La(dcm) 3 increases the shielding. Deshielding with these substrates is larger with La(hfc) 3 than La(tfc) 3 .…”

“…These changes can occur through electron-withdrawing effects of the electropositive lanthanide ion or diamagnetic anisotropy from p-systems on the ligands. 8 While small in magnitude, on a high field instrument, complexation shifts with these aqueous systems are often large enough to distinguish the spectra of two enantiomers, such as those of a-amino acids. 7 We wish to show that chiral tris b-diketonate complexes of diamagnetic lanthanum(III) and lutetium(III) can be used in organic solvents to distinguish the NMR spectra of enantiomers.…”