Chirality without Stereoisomers: Insight from the Helical Response of Bond Electrons

The association between molecular chirality and helical characteristics known as the chirality-helicity equivalence is determined for the first time by quantifying a chirality-helicity measure consistent with photoexcitation circular dichroism experiments. This is demonstrated using a formally achiral S(N)2 reaction and a chiral S(N)2 reaction. Both the achiral and chiral S(N)2 reactions possess significant values of the chirality-helicity measure and display a Walden inversion, i. e. an inversion of the chirality between the reactant and product. We also track the chirality-helicity measure along the reaction path and discover the presence of chirality at the transition state and two intermediate structures for both reactions. We demonstrate the need for the chirality-helicity measure to differentiate between steric effects due to eclipsed conformations and chiral behaviors in formally achiral species. We explain the significance of this work for asymmetric synthetic reactions including the intermediate structures where the Cahn-Ingold-Prelog (CIP) rules cannot be used.

Automated summary

This study demonstrates, for the first time, the equivalence between molecular chirality and helical characteristics through the quantification of a chirality-helicity measure consistent with photoexcitation circular dichroism experiments, showing the necessity of this measure to differentiate between chiral behaviors in formally achiral species. Additionally, phenomena such as Walden inversion, transition states, and intermediate structures reveal the significance of this relationship in asymmetric synthetic reactions.