Elucidation of the Mechanism of Supramolecular Chirality Inversion in Bis(zinc porphyrin) by Dynamic Approach Using CD and 1H NMR Spectroscopy

Abstract

Dynamic CD and 1H NMR spectroscopies have been applied for studying the phenomenon of chirality inversion in the supramolecular system consisting of bis(Zn porphyrin) and (R)-amine upon stepwise titration with (S)-amine. The initially induced CD signals corresponding to the coupled B transitions of bis(Zn porphyrin) are gradually decreased during this process and transformed into its mirror images that are the Cotton effects with the same extrema positions, but with opposite signs. In the 1H NMR spectra of this system upon titration there is inversion of the proton signals which are in close proximity to the neighboring porphyrin plane, and are thus the most affected by its ring current effect. The observed spectral changes are caused by shifting of the equilibrium from the left- to right-handed screw in bis(Zn porphyrin) upon titration with the amine of the opposite absolute configuration. The CD and 1H NMR experiments used to determine the reaction kinetics explicitly corroborate each others results. This chirality inversion process has no total energy change (ΔG° = 0), as shown by the titration curves passing through their zero values at the racemic point of an enantiomeric mixture.