Hyperconjugative π → σ*CF Interactions Stabilize the Enol Form of Perfluorinated Cyclic Keto–Enol Systems
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Author(s)
Levandowski, Brian
Raines, Ronald T
Houk, K. N.
Date Issued
April 2019
Journal
Journal of Organic Chemistry
Publisher
American Chemical Society (ACS)
Citation
Levandowski, Brian J. et al. "Hyperconjugative π → σ*CF Interactions Stabilize the Enol Form of Perfluorinated Cyclic Keto–Enol Systems." Journal of Organic Chemistry 84, 10 (April 2019): 6432–6436 © 2019 American Chemical Society
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Author's final manuscript
Abstract
Lindner and Lemal showed that perfluorination of keto-enol systems significantly shifts the equilibrium toward the enol tautomer. Quantum mechanical calculations now reveal that the shift in equilibrium is the result of the stabilization of the enol tautomer by hyperconjugative π→ σ∗CF interactions and the destabilization of the keto tautomer by the electron withdrawal induced by the neighboring fluorine atoms. The preference for the enol tautomer further increases in smaller perfluorinated cyclic keto-enol systems. This trend is in contrast to the nonfluorinated compounds, where the enol is strongly disfavored in the smaller rings. The fluoro effect overrides the effect of the ring size that controls the equilibria in nonfluorinated compounds. The increased overlap of the enol πbond with the σ∗CF orbitals of the allylic C-F bonds results in the increased preference for the enol tautomer in smaller perfluorinated keto-enol systems. We show here why the effect is much greater than in 3,3-difluorocyclooctyne.
MIT Department
Massachusetts Institute of Technology. Department of Chemistry
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DOI of Published Version
https://doi.org/10.1021/acs.joc.9b00825
