Rupturing aromaticity by periphery overcrowding

The balance between strain relief and aromatic stabilization dictates the form and function of non-planar pi-aromatics. Overcrowded systems are known to undergo geometric deformations, but the energetically favourable pi-electron delocalization of their aromatic ring(s) is typically preserved. In this study we incremented the strain energy of an aromatic system beyond its aromatic stabilization energy, causing it to rearrange and its aromaticity to be ruptured. We noted that increasing the steric bulk around the periphery of pi-extended tropylium rings leads them to deviate from planarity to form contorted conformations in which aromatic stabilization and strain are close in energy. Under increasing strain, the aromatic pi-electron delocalization of the system is broken, leading to the formation of a non-aromatic, bicyclic analogue referred to as 'Dewar tropylium'. The aromatic and non-aromatic isomers have been found to exist in rapid equilibrium with one another. This investigation demarcates the extent of steric deformation tolerated by an aromatic carbocycle and thus provides direct experimental insights into the fundamental nature of aromaticity.

Automated summary

The balance between strain relief and aromatic stabilization determines the structure and function of non-planar pi-aromatics. By increasing the strain energy beyond aromatic stabilization energy, we observed a rearrangement of the system and the rupture of its aromaticity. The introduction of steric bulk leads to contorted conformations where aromatic stabilization and strain are close in energy. Under high strain, the aromatic pi-electron delocalization breaks and a non-aromatic, bicyclic analogue called 'Dewar tropylium' is formed. This study provides experimental insights into the extent of steric deformation tolerated by an aromatic carbocycle and the nature of aromaticity.