Theoretical and Synthetic Chemistry of σ-Aromaticity

Since the discovery of benzene in 1825, aromaticity, which accounts for the exceptional stability and reactivity of benzene, has long been a topic of discussion in many fields of chemistry, and diverse classes of molecules possessing aromatic character have been developed. Throughout these studies, aromaticity has generally been limited to electronic delocalization through π-symmetric orbitals and, therefore, should more precisely be referred to as π-aromaticity. Against this background, a fundamental question is whether aromaticity can also be generated by electronic delocalization through orbitals possessing symmetries other than π symmetry. We herein review the theoretical and synthetic studies on σ-aromaticity, which arises from electronic delocalization through σ-symmetric orbitals.