Helical Polymer-Based Polycarboxylic Acids as a Chiroptical Chemosensor for Detection and Quantification of Small Enantiomeric Imbalances of Chiral Amines in Water

Achiral dynamic helical polymers, poly(quinoxaline-2,3-diyl)s (P1 and P2) bearing achiral carboxylic acid side chains, i.e., carboxymethoxymethyl (in P1) and carboxyethoxymethyl (in P2), with different polymerization degrees were synthesized. They exhibited induced circular dichroism (ICD) in the presence of chiral amines such as 1-phenylalkylamines and nicotine, 1,2-amino alcohols such as valinol, leucenol, and prolinol, and the basic amino acid, arginine, in response to the induction of right- or left-handed helical conformation. The efficiency of helix induction depends on the compatibility of the structures of amines and polymers, with no clear structural correlation. The highly sensitive and formulated nature of ICD with the helical polymer-based polycarboxylic acids allowed their use as CD-based sensors to detect and quantify minute imbalances of enantiomeric excess of chiral molecules. We determined 0.2%–0.6% ee in the commercially available 1-phenylethylamine from three different suppliers, which have the label of “dl” or no indication of enantiopurity using P1 as a chemosensor.