Integration of Wallach's Rule into Intermolecular Charge Transfer: A Visual Strategy for Chiral Purification

Abstract

Exploring the molecular packing and interaction between chiral molecules, no matter single enantiomer or racemates, is important for recognition and resolution of chiral drugs. However, sensitive and non‐destructive analysis methods are lacking. Herein, an intermolecular‐charge transfer (ICT) based spectroscopy is reported to reveal the differences in interaction between the achiral acceptor 1,2,4,5‐tetracyanobenzene (TCNB) and the chiral donors, including S, R, and racemic naproxen (S/R/rac‐NAP). In this process, S‐NAP+TCNB and R‐NAP+TCNB display a narrower band gap attributed to the newly formed ICT state. In contrast, the mixed rac‐NAP and TCNB exhibit almost no significant change due to the strong affinity between the stereoisomers according to the Wallach's rule. Thus, S/R‐NAP can be easily distinguished from rac‐NAP based on significantly different optical behavior. The single crystal analysis, infrared spectroscopy, fluorescence spectroscopy, and theoretical calculation of naproxen confirm the importance of carboxyl for this differentiation in molecular packing and interaction. In addition, the esterification derivatization of naproxen achieves the manipulation of the intermolecular interaction model of racemates from the absolute Wallach's rule to a coexisting form of Wallach's rule and ICT. Further, visualized chiral purification of naproxen by the simple cocrystallization method is achieved through the collaboration of ICT and Wallach's rule.