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Hyperconjugative Antiaromaticity Activates 4H‑Pyrazoles as Inverse-Electron-Demand Diels–Alder Dienes

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journal contribution
posted on 07.10.2019, 13:36 by Brian J. Levandowski, Nile S. Abularrage, K. N. Houk, Ronald T. Raines
The Diels–Alder reactivity of 4,4-difluoro-3,5-diphenyl-4H-pyrazole was investigated experimentally and computationally with endo-bicyclo­[6.1.0]­non-4-yne. The computationally predicted rate enhancement from hyperconjugative antiaromaticity induced by fluorination of cyclopentadienes at the 5-position extends to five-membered heterocyclic dienes containing a saturated center. 4,4-Difluoro-4H-pyrazoles are new electron-deficient dienes with rapid reactivities toward strained alkynes.