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 α-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‘)(P2-dach)]ClO4 (4) [P2-dach = (1R,2R)-C6H10(NHPPh2)2] with d,l-mixtures of α-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)(P2-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 31P{1H} NMR spectra. A series of 14 α-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; l, aspartic acid; m, glutamine; n, cysteine), and excellent agreement between the expected values of ee and those obtained from integration of the 31P{1H} 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−l, while the inner ones are due to the d-derivatives 6a−l, allowing the assignation of absolute configurations to each isomer in the mixture.本文报道了一种简便方法，可用于测定α-氨基酸（α-amino acids）混合物的对映体过量值（enantiomeric excess，ee），同时还可阐明该混合物中各组分的绝对构型。该方法基于对映体纯的乙酰丙酮衍生物[Pd(acac-O,O‘)(P2-dach)]ClO4（化合物4，其中P2-dach = (1R,2R)-C6H10(NHPPh2)2）与α-氨基酸AaH的外消旋混合物按1:1的摩尔比，在回流甲醇（MeOH）中发生反应，生成非对映异构体。反应过程中，acac配体质子化，氨基酸根基团以N,O配位模式结合，得到对应的[Pd(Aa-N,O)(P2-dach)]ClO4配合物l-5和d-6。通过对其31P{1H}核磁共振谱（31P{1H} NMR）中对应峰（共四组二重峰，每个非对映异构体对应两组二重峰）进行积分，可分析该氨基酸根配合物混合物的组成。本研究共考察了14种α-氨基酸：a为丙氨酸、b为2-氨基丁酸、c为缬氨酸、d为苯丙氨酸、e为脯氨酸、f为亮氨酸、g为异亮氨酸、h为正亮氨酸、i为丝氨酸、j为苏氨酸、k为甲硫氨酸、l为天冬氨酸、m为谷氨酰胺、n为半胱氨酸，且对映体过量值的实验测定结果与理论预期值具有极佳的一致性。此外，各异构体的信号位置具有诊断性特征：外侧的二重峰始终对应l型衍生物5a−5l，而内侧的二重峰则对应d型衍生物6a−6l，据此可对混合物中各异构体的绝对构型进行指认。