Effective enantiomeric identification of aromatic amines by tyrosine-modified pillar[5]arenes as chiral NMR solvating agents

Abstract

Endowed with electron-rich cavities and potential chiral platforms, chiral pillararenes are greatly useful; however, their chiral recognition remains a challenge due to chiral racemization caused by the rotation of phenolic units. Although pillar[n]arenes with bulky substituents and host–guest interaction have been reported to avoid this shortcoming, the chiral recognition of pillar[n]arenes as chiral NMR reagents has not been reported yet. Herein, we first reported two novel tyrosine-modified pillar[5]arenes L-PCSA and D-PCSA as chiral solvating agents (CSAs) for the enantioselective recognition of α-aromatic amines by ¹H NMR. The PCSAs showed excellent chiral discrimination and good complementarity for α-naphthylethylamine (G1) and α-phenylethylamine (G2) over a wide range of mole fractions (CSA%: 9%–95%). In addition, there is a linear relationship between the experimental and observed values of enantiomeric excess of G1 in the presence of L-PCSA, which indicated that L-PCSA can be used to rapidly and reliably analyze the enantiomeric purity of various naphthylethylamine samples. Interestingly, the NMR signals of the nonequivalent protons between the R and S configurations in the presence of stereoisomers of pillararenes are completely opposite, which can be used to establish a simple method to determine the absolute configuration of G1 and G2 successfully. Moreover, the chiral recognition of pillararenes on different substrates was expanded. At the same time, the enantioselective recognition behaviors are also discussed by means of the mole ratio plot, host–guest interaction and a proposed theoretical model of PCSAs with chiral substrates. This work provides a good example for designing effective pillararene-based chiral NMR reagents.