Aromaticity and stability of furofurans and thienothiophenes

The topological resonance energy (TRE) calculated for a polycyclic π‐system cannot always be used to predict the thermodynamic or kinetic stability of the π‐system. For example, furo[3,4‐c]furan (1d) and thieno[3,4‐c]thiophene (2d) are aromatic with large positive TREs and large negative nucleus independent chemical shift (NICS) values at the ring centers. However, these heterobicycles are predicted to be very high in energy with very small HOMO–LUMO energy separations. These unusual properties of the heterobicycles are closely associated with the fact that classical resonance structures cannot be drawn for them. In general, the kinetic stability of a polycyclic π‐system reflects not only the aromaticity of the π‐system but also the kinetic stability of the polyene reference.