Anti-Aromaticity Relief as an Approach to Stabilize Free Radicals
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A new strategy to stabilize free radicals electronically is described by conjugating formally antiaromatic substituents to the free radical. With an antiaromatic substituent, the radical acts as an electron sink to allow configuration mixing of a low-energy zwitterionic state that provides antiaromaticity relief to the substituent. A combination of X-ray crystallography, VT-EPR and VT-UV/Vis spectroscopy, as well as computational analysis, was used to investigate this phenomenon. We find that this strategy of antiaromaticity relief is successful at stabilizing radicals, but only if the antiaromatic substituent is constrained to be planar by synthetically imposed conformational restraints that enable state mixing. This work leads to the counterintuitive finding that increasing the antiaromaticity of the radical substituent leads to greater radical stability, providing proof of concept for a new stereoelectronic approach for stabilizing free radicals.
This is the peer-reviewed version of the following article: Zhang, Rui, Arkady Ellern, and Arthur H. Winter. "Anti‐Aromaticity Relief as an Approach to Stabilize Free Radicals." Angewandte Chemie International Edition (2021)., which has been published in final form at DOI: 10.1002/anie.202110870. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving. Copyright 2021 Wiley-VCHGmbH. Posted with permission.