Discrimination and Enantiomeric Excess Determination of Chiral Primary Amines Based on a Chiral-at-Metal Ir(III) Complex Using NMR Spectroscopy

Abstract

A three-component protocol involving enantiopure Δ-[Ir(ppy)₂(MeCN)₂](PF₆) (ppy is 2-phenylpyridine) and salicylaldehyde as chiral auxiliaries was successfully applied to discriminate the absolute configuration and determine the enantiopurity of primary amines and amine alcohols via ¹H NMR spectroscopy. The assembly reaction is rapid and quantitative, generating a pair of diastereomers that can be determined directly without physical separation. Single crystal structural analyses indicate that the shielding effects on the ligands imposed by a pair of diastereomers are different and generate sufficient resolution NMR signals for identification. The enhancement of stability via chelating coordination to Ir(III) ion and more than one pair of diastereotopic resonances in different detection regions of the three-component protocol ensure a high degree of accuracy in quantifying the ee value of chiral amines. The absolute errors in the ee determinations by ¹H NMR spectroscopy in different detection windows are within 2.0%. The linear relationship between the experimentally measured ee values and the gravimetrically prepared samples is found to be excellent. This finding would provide a complementary method for the discrimination and determination of the enantiopurity of chiral primary amines and amine alcohols in the screening of asymmetric reactions.