Axial chirality

Axial chirality is a special case of chirality in which a molecule does not possess a stereogenic center (the most common form of chirality in organic compounds) but an axis of chirality - an axis about which a set of substituents is held in a spatial arrangement which is not superposable on its mirror image. Axial chirality is most commonly observed in atropisomeric biaryl compounds where the rotation about the aryl-aryl bond is restricted, for example biphenyl, binaphthyls e.g., 1,1'-bi-2-naphthol, and certain dihydroanthracenone compounds ^[1] . Certain allene compounds also display axial chirality. The enantiomers of axially chiral compounds are usually given the stereochemical labels R_a and S_a, although the plus (P) or minus (M) notation is occasionally employed.^[2] P/M is used particularly for molecules which resemble a helix, such as hexahelicene, in which case a right-handed helix is denoted P and a left-handed helix is denoted M.^[3]

References

1. ^ Absolute stereochemistry of fungal metabolites: Icterinoidins A1 and B1, and atrovirins B1 and B2 Melvyn Gill and Peter M. Morgan Arkivoc (RI-1154C)2004 Online article
2. ^ Compendium of Chemical Terminology, axial chirality
3. ^ VLU: Additional Chirality Elements - Chemgapedia[1]