31P NMR spectroscopy as a powerful tool for the determination of enantiomeric excess and absolute configurations of α-amino acids

An easy method for the determination of the enantiomeric excess (ee) of mixtures of a-amino acids, and also for the elucidation of the absolute configuration of each component of the mixture, is reported. The method is based on the formation of diastereoisomers by reaction of the enantiomerically pure acetylacetonate derivative [Pd(acac-O, O')(P-2-dach)]ClO4 (4) [P-2-dach = (1R,2R)-C6H10(NHPPh2)(2)] with D,L-Mixtures of alpha-amino acids AaH (Pd:AaH = 1:1 molar ratio, refluxing MeOH). The reaction occurs with protonation of the acac ligand and N,O-coordination of the amino acidate group, giving the corresponding [Pd(Aa-N,O)(P-2-dach)]ClO4 complexes L-5 and D-6. The composition of these mixtures of amino acidate complexes was analyzed by integration of the corresponding peaks (four doublets, two for each diastereomer) in their P-31{H-1} NMR spectra. A series of 14 alpha-amino acids was studied (a, alanine; b, 2-aminobutyric acid; c, valine; d, phenylalanine; e, proline; f, leucine; g, isoleucine; h, norleucine; i, serine; j, threonine; k, methionine; 1, aspartic acid; m, glutamine; n, cysteine), and excellent agreement between the expected values of ee and those obtained from integration of the P-31{H-1} NMR spectra was obtained. Moreover, the position of the signals of each isomer is diagnostic, in such a way that the outer doublets are always due to the L-derivatives 5a-I, while the inner ones are due to the D-derivatives 6a-I, allowing the assignation of absolute configurations to each isomer in the mixture.