Pseudoasymmetry paradox: A suggestion to introduce the term pseudostereogenicity and address reflection issues

Abstract

The confusion about the nomenclature of a tetrahedral carbon attached to a pair of enantiomorphic ligands (AR and AS) with the other two ligands different from each other as well as from A has been critically reexamined. Until now, this carbon has been known as a pseudoasymmetric center. However, according to Mislow, since asymmetry is the property of a molecule as a whole, it seems appropriate to replace ‘pseudoasymmetry’ with the term ‘pseudostereogenicity’. This is more relevant, particularly for molecules where different achiral ligands other than the enantiomorphic ones are attached to the pseudoasymmetric center. The molecule is not asymmetric as it has a S1 or σ plane. Another contentious issue is whether chiral molecules can be included in the domain of “pseudostereogenicity” and the related reflection invariance of the configuration (rectus remains rectus or sinister remains sinister). For chiral molecules containing such centers, the expectation is that the absolute configuration of the pseudoasymmetric center should invert upon reflection which is not the case as per the existing system. This article addresses all these issues like inclusion of chiral molecules bearing pseudoasymmetric center and making the center reflection variant by a simple introduction of a pilot atom based approach, which is already used in CIP system for planar chirality.