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Endocyclic Oxygen in 3‑Fluorodihydro‑2H‑pyran-4(3H)‑one That Does Not Induce the Gauche Effect
journal contribution
posted on 2014-08-14, 00:00 authored by Thayná F. B. Silva, Laize A. F. Andrade, Josué M. Silla, Claudimar J. Duarte, Roberto Rittner, Matheus P. Freitas2-Fluorocyclohexanone
undergoes chair inversion, giving rise to axial and equatorial conformers,
with the equatorial form being highly preferred in solution, for example,
87% in chloroform and 93% in methylene chloride. Modifications in
the conformational preferences can modify macroscopic properties of
2-fluoro ketones. The introduction of an endocyclic oxygen in 2-fluorocyclohexanone
to give 3-fluorodihydro-2H-pyran-4(3H)-one would be expected to create a gauche effect in the axial conformer
along with the O–C–C–F moiety, inducing an increase
of its population. However, small changes were verified in the conformational
populations both in the gas phase and solution because the carbonyl
group plays an important role for the hyperconjugation in the equatorial
conformer, despite experiencing strong dipolar repulsion with the
fluorine atom. These data were obtained theoretically and by NMR spectroscopy,
while the nature of the interactions governing these conformational
shifts were investigated on the basis of natural bond orbital analysis.