Nuclear magnetic resonance signaling of molecular chiral information using an achiral reagent

Until now NMR spectroscopic detection of guest chirality using an achiral host has not been possible in the absence of a chiral medium or auxiliary since chiral discrimination is principally based on chiral discrimination by host and/or diastereomeric host-guest complex formation. In this paper, we demonstrate that an achiral oxoporphyrinogen works as a host capable of signaling chiral information of alpha-hydroxycarboxylic acids in (1)H NMR spectroscopy. In particular, enantiomeric excess (ee) can be determined by observing the splitting of (1)H NMR resonances of the achiral host. This differs from the case of chiral hosts (shift reagents) where % ee is generally determined from the ratio of peak areas due to diastereomeric host-guest complexes. UV/vis, CD, FT-IR, and NMR spectroscopic investigations suggest that the unusual phenomenon reported here is based on formation of a complex with 1:2 stoichiometry in concert with a protonation-driven tautomerization of the host.