A generalized handedness labeling strategy: Addressing latent handedness in chiral structures

We propose a strategy for treating the problem of assignment of handedness in a generalized way. The need for this generalization arises from an inher ent pitfall of definitions of handedness: Given such a definition there exi sts at least one chiral structure for which handedness cam-tot be assigned. We demonstrate this situation for the helical and the CIP handedness conve ntions as well as for Ruch's potato, and provide an argument that the inabi lity to assign handedness is a property of any labeling procedure. We categ orize chiral structures for which handedness cannot be assigned as Latent H andedness structures. These structures-which define bounds for the range of applicability of labeling procedures-are analyzed in detail and serve here as a basis for the proposed generalized handedness assignment procedure. S ince latent handedness structures show up particularly along chiral enantio merization pathways, we concentrate on these processes with special attenti on to problems of helical transitions and helical handedness switching, whi ch are the focus of much recent experimental work. Two interesting categori es of enantiomerization pathways (J. Math. Chem., 23, 13 (1998)) are highli ghted in the context of latent handedness: chiral enantiomerization routes which are totally of latent handedness, namely chiral processes for which i t is not possible to assign handedness at any point; and enantiomerization pathways all points of which are of the same handedness except for the end points. Interestingly, quite often the chiral latent handedness structure a nd the chiral transition states are similar; an explanation is provided.