Determination of Enantiomeric Excess via 31P-NMR

Abstract

³¹P is a very attractive nucleus for nuclear magnetic resonance (NMR) analysis because of the large chemical dispersion and the simplicity of the spectra when compared with other nuclei (other than ¹⁹F). The ability to rapidly and quantitatively assay for enantiomeric excess (ee) measurements of alcohols, amines, thiols, and other chiral species using ³¹P-NMR is essential. Analysis of ee is particularly important in the pharmaceutical industry because a majority of medicinal compounds contain chiral centers, and enantiomers may possess different pharmacological activities. Although high-performance liquid chromatography (HPLC) with chiral stationary phase has become the standard method for ee determination, it can often be expensive and time consuming and requires purified samples. ³¹P-NMR offers advantages over many other nuclei as the interpretation is rapid and can have large chemical shift differences (Δδ) when comparing diastereomers. Proton decoupling avoids overlapping signals by preventing proton coupling, which could result in overlapping signals. With the rapid growth of asymmetric organocatalysis and asymmetric synthesis, the ability to quickly determine ee via phosphorus containing chiral derivatizing agents (CDAs) and chiral solvating agents (CSAs) is of utmost utility. It might be especially useful if used to determine ee directly in a reaction mixture, thereby alleviating any need for purification. This review focuses on ee determination by ³¹P-NMR.