Role of barium(II) in the determination of the absolute configuration of chiral amines by 1H NMR spectroscopy.
The assignment of the absolute configuration of alpha-chiral primary amines by complexation of their MPA derivatives with Ba2+ and NMR analysis of the changes generated is presented. All that is required is (a) the derivatization of the amine of unknown configuration with one enantiomer of the auxiliary reagent (MPA), either (R) or (S)-alpha-methoxyphenylacetic acid, (b) the recording of the 1H NMR spectrum of the resulting amide in MeCN-d3, (c) the addition of Ba(ClO4)2 to the NMR tube, and (d) the recording of a second spectrum after a few minutes of shaking. The above steps take a few minutes and are followed by an analysis of the shifts (measured as Deltadelta(Ba)) produced on the L1 and L2 substituents of the amine by the addition of Ba2+ and their comparison with those expected from the conformational changes produced by the complexation. The conformational changes initiated by complexation have been subjected to NMR and CD studies, which showed that the formation of the complex shifts the equilibrium from an antiperiplanar (AP) to a synperiplanar (SP) form, leading to an increase of the shielding by the phenyl group of MPA of the substituent of the amine located on the same side. In addition, theoretical calculations [density functional theory (DFT)] provide further support for the formation, structure, and stability of the complexes. The general applicability of this method and the trustworthiness of the resulting configurational assignment were guaranteed with a series of amines of known absolute configuration and varied structures, used as test compounds. The method proposed is simple, fast, and inexpensive, and it requires a very small amount of sample, only one derivatization, and the recording of just two 1H NMR spectra at room temperature. A graphical guide to simplify the application of this method is included.
García
Rosa
R
Departamento de Química Orgánica, Facultad de Química, and Unidad de RMN de Biomoléculas Asociada al CSIC, Universidad de Santiago de Compostela, E-15782 Santiago de Compostela, Spain.
Seco
José M
JM
Vázquez
Saulo A
SA
Quiñoá
Emilio
E
Riguera
Ricardo
R