Stereoisomerism and Local Chirality

Abstract

The traditional linkage between stereoisomerism and local chirality that is expressed in terms such as “asymmetric carbon atom” or “element of chirality” represents a source of conceptual confusion in modern stereochemistry. Molecular segments must be viewed from two separate and distinct aspects: their character as stereogenic units and their local symmetry. The first is dependent on bonding connectivity (constitution) and is rooted in graph and permutation group theory, whereas the second is independent of constitution and is rooted in the theory of symmetry groups. Although these two aspects are in principle distinct and serve different purposes, they happen to overlap in the case of the regular tetrahedral permutation center. It is for this reason that the concepts of chirality and stereogenicity are most closely associated in organic stereochemistry where this center plays a dominant role. The present analysis clarifies stereochemical concepts, sheds new light on the meaning of stereochemical terminology, and ipso facto disposes of a number of notions introduced into stereochemistry since van't Hoff's day. To complete our analysis of stereochemical theory, a new treatment of prochirality is proposed. A theoretical framework is constructed that assigns membership in one of three classes of prochirality to any achiral molecular model according to symmetry. © 1984, American Chemical Society. All rights reserved.