Rupturing Aromaticity by Periphery Overcrowding

Abstract The balance between strain relief and aromatic stabilization dictates the form and function of nonplanar π-aromatics. Overcrowded systems undergo geometric deformations, but the energetically favourable π-electron delocalization of their aromatic ring(s) is typically preserved. Here, we use peripheral overcrowding to increment the strain energy of an aromatic system beyond its aromatic stabilization energy, causing the ring system to rearrange and aromaticity to be ruptured. Firstly, increasing steric bulk around the periphery of π-extended tropylium ring systems leads them to deviate from planarity. We observe contorted conformations with end-to-end twists of up to 45.2°. Aromatic stabilization and strain energies are closely matched at this critical point. Upon further increasing the strain, the aromatic π-electron delocalization of the system is broken—the aromatic tropylium ring rearranges to a nonaromatic, bicyclic analogue, ‘Dewar tropylium’. Remarkably, by counterbalancing strain against aromatic stabilization energy, we observe that these aromatic and nonaromatic isomers can exist in equilibrium with one another. Dynamic NMR measurements reveal that the isomers interconvert rapidly. This investigation of the antagonistic relationship between aromaticity and strain provides direct experimental insights into the fundamental nature of aromaticity and demarcates the extent of steric deformation tolerated by an aromatic carbocycle.