Rupturing aromaticity by periphery overcrowding

The balance between strain relief and aromatic stabilization dictates the form and function of non-planar π-aromatics. Overcrowded systems are known to undergo geometric deformations, but the energetically favourable π-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. Increasing the steric bulk around the periphery of π-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 π-electron delocalization of the system is broken, leading to formation of a non-aromatic, bicyclic analogue referred to as ‘Dewar tropylium’. The aromatic and non-aromatic isomers exist in rapid equilibrium with one another. This investigation demarcates the extent of steric deformation tolerated by an aromatic carbocycle and provides direct experimental insights into the fundamental nature of aromaticity.

Promeet K. Saha, Abhijit Mallick, Andrew T. Turley, Aisha N. Bismillah, Andrew Danos, Andrew P. Monkman, Alyssa-Jennifer Avestro, Dmitry S. Yufit, Paul R. McGonigal