Extended valence bond theory, aromaticity, and the Woodward-Hoffmann Rules

Excited State Aromaticity and Antiaromaticity: Opportunities for Photophysical and Photochemical Rationalizations

“…Although the discussion did not address aromaticity in electronically excited states Craig was the first to apply VB theory for the description of the lowest few singlet states of CBD. Later on, van der Hart and co-workers addressed aromaticity through an extended VB theory in which the Heitler-London bond functions are exchanged to general two-electron functions . Kuwajima addressed aromaticity and antiaromaticity in terms of ring-permutations and showed that the contribution from permutations which permute electrons circularly around the ring accounts for the difference between even and odd-parity annulenes in terms of destabilizing and stabilizing effects, respectively …”

Are Multicentre Bond Indices and Related Quantities Reliable Predictors of Excited-State Aromaticity?

“…Despite its somewhat vaguely defined qualitative nature, the concept of aromaticity has had huge impacts on organic chemistry, starting with the formulation of the Hückel aromaticity rules [ 1 , 2 ] and encompassing a broad research area including the elucidation of the link between cyclic delocalization and energetic stabilization of conjugated (poly)cyclic hydrocarbons [ 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 ], the role of cyclic conjugation in inducing the ring currents [ 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 ] responsible for the special magnetic properties of aromatic compounds, and revealing the links between electron counts, orbital topology and selection rules in pericyclic reactions [ 20 , 21 , 22 , 23 ]. The fact that the phenomenon of aromaticity can be associated with a very wide range of structural, energetic, and magnetic properties [ 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 16 , 17 , 18 , 24 , 25 ] has given impetus to numerous attempts to define measures or indices that are intended to characterize the “extent” of aromaticity in quantitative terms [ 16 , 17 , 18 , 19 , …”

Electron reorganization in allowed and forbidden pericyclic reactions: multicenter bond indices as a measure of aromaticity and/or anti‐aromaticity in transition states of pericyclic electrocyclizations

“…According to this concept, inspired by the earlier study by Evans and Warhurst,10 who noticed that the transition states of Diels–Alder reactions are topologically equivalent (isoconjugated) to those of benzene, the thermally allowed pericyclic reactions proceed via aromatic transition states. This simple idea has subsequently found independent theoretical support by Aihara11 and Van der Hart et al 12. and, more recently, also in our previous studies,13, 14 in which the assumed aromaticity of transition states for allowed reactions (as well as anti‐aromaticity for the forbidden ones) has been demonstrated using the so‐called molecular similarity indices 15–18.…”