Chirality without stereoisomers: insight from the helical response of bond electrons

Theassociation between molecular chirality and helical characteristics known asthe chirality-helicity equivalence is determined, for the first time, byquantifying a chirality-helicity measure consistent with photoexcitationcircular dichroism experiments. This is demonstrated using a formally achiral S_N2reaction and a chiral S_N2 reaction. Both the achiral and chiral S_N2reactions possess significant values of the chirality-helicity measure anddisplay a Walden inversion, i.e. an inversion of the chirality between thereactant and product. We also track the chirality-helicity measure along thereaction path and discover the presence of chirality at the transition stateand two intermediate structures for both reactions. We demonstrate the need forthe chirality-helicity measure to differentiate between steric effects due toeclipsed conformations and chiral behaviors in formally achiral species. Weexplain the significance of this work for asymmetricsynthetic reactions including the intermediate structures where the Cahn–Ingold–Prelog (CIP) rulescannot be used.