Importance of molecular symmetry for enantiomeric excess recognition by NMR.

Recently prochiral solvating agents ( pro -CSA) came under the spotlight for the detection of enantiopurity by NMR. Chemical shift non-equivalency in achiral hosts introduced by the presence of chiral guests yields observable resonance signal splitting (Δ δ ) correlating to the enantiomeric excess (e.e.). In this work, symmetry is our lens to explain porphyrin-based supramolecular receptor activity in a chiral environment. Based on extensive NMR analyses of the atropisomeric receptors, the host symmetry is shown to be affected by porphyrin nonplanarity and further desymmetrized in the presence of a chiral guest. As such, the exposed porphyrin inner core (N-H), with its strong hydrogen bond abilities, for the first time, has been exploited in enantiomeric composition analysis. Our approach in e.e. detection by N-H signals appearing in a previously underutilized region of the spectrum (below 0 ppm) shows chemical shift splitting (Δ δ ) three times more sensitive to enantiomeric compositions than previously reported systems.