Substituent Effects on the Reactivity of the 2,4,6-Tridehydropyridinium Cation, an Aromatic σ,σ,σ-Triradical

Jinshan Gao, Bartłomiej J. Jankiewicz, Huaming Sheng, Lindsey Kirkpatrick, Xin Ma, John J. Nash, Hilkka I. Kenttämaa

Research output: Contribution to journalArticle

3 Scopus citations

Abstract

2,4,6-Tridehydropyridinium cation (7) undergoes three consecutive atom or atom group abstractions from reagent molecules in the gas phase. By placing a π-electron-donating hydroxyl group between two radical sites, their reactivity can be quenched by enhancing their through-space coupling via a favorable resonance structure. Indeed, 3-hydroxy-2,4,6-tridehydropyridinium cation (8) abstracts only one atom or group of atoms from reagents. On the other hand, an electron-withdrawing cyano group between two of the radical sites (9) destabilizes the analogous resonance structure and diminishes through-space coupling between the radical sites, resulting in abstraction of three atoms, just like 7. However, the cyano-substituent also increases acidity to the point that 9 reacts predominantly via proton transfer instead of undergoing radical reactions. Therefore, acidic triradicals may undergo nonradical, barrierless proton transfer reactions faster than radical reactions, which are usually accompanied by barriers. Examination of the analogous cyano-substituted mono- and biradicals revealed behavior similar to that of the corresponding unsubstituted species, with the exception of substantially greater reactivities due to their greater (calculated) vertical electron affinities. Finally, the 3-cyano-2,6-didehydropyridinium cation with a singlet ground state (S-T splitting: –11.9 kcal mol–1) was found to react exclusively from the lowest-energy triplet state by fast proton transfer reactions.

Original languageEnglish
Pages (from-to)6582-6589
Number of pages8
JournalEuropean Journal of Organic Chemistry
Volume2018
Issue number46
DOIs
StatePublished - 13 Dec 2018

Keywords

  • Acidity
  • Cations
  • Radical reactions
  • Reactivity
  • Substituent effects

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