Rupturing aromaticity by periphery overcrowding

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*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


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. We noted that 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 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. [Figure not available: see fulltext.].

Original languageEnglish
Number of pages11
JournalNature Chemistry
Early online date6 Mar 2023
Publication statusE-pub ahead of print - 6 Mar 2023

Bibliographical note

Funding Information:
P.K.S. and A.T.T. acknowledge the Engineering and Physical Sciences Research Council (EPSRC) for doctoral training grants. A.M., A.P.M., A.-J.A. and P.R.M. thank Universities UK International and the Department for Business, Energy and Industrial Strategy for a Rutherford Strategic Partnership Grant. A.N.B. and P.R.M. acknowledge the support of a Leverhulme Trust Research Project Grant (RPG-2020-218). P.R.M. and A.M. thank the Faraday Institution for funding (FIRG046). A.-J.A. thanks the Royal Society and Global Challenges Research Fund for the award of a Dorothy Hodgkin Fellowship (DHF\R1\180106) and an Enhancement Award (RGF\EA\181065). The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript. We thank D. Szczepanik for advice about EDDB calculations.

Publisher Copyright:
© 2023, The Author(s).

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