Aromaticity and Optical Activity

The relationship between aromaticity and optical activity is investigated in comparisons of heterocycles with 4n + 2 and 4n π-electrons, in cyclic ketones with and without aromatic resonance structure representations, in tautomers and pericyclic reaction partners in which only one compound of each pair is aromatic, and in partially hydrogenated cyclo-C18 derivatives with both radial and tangential π-orbitals. In all comparisons, aromaticity is correlated to diminished optical activity. A heuristic explanation of this observation is grounded in the electric dipole–magnetic dipole polarizability contribution to optical activity in which the sense of electric dipoles and magnetic dipoles become uncoupled when electrons can circulate around a ring with either sense. These observations form a basis for making broad structure–optical activity correlations from inspection of molecular structure.