Conformational Preference of 2′-Fluoro-Substituted Acetophenone Derivatives Revealed by Through-Space ¹H–¹⁹F and ¹³C–¹⁹F Spin–Spin Couplings
2021
Otake, Chinatsu | Namba, Takuya | Tabata, Hidetsugu | Makino, Kosho | Hirano, Kiriko | Oshitari, Tetsuta | Natsugari, Hideaki | Kusumi, Takenori | Takahashi, Hideyo
The conformational properties of 2′-fluoro-substituted acetophenone derivatives were elucidated based on Hᵅ–F and Cᵅ–F through-space spin–spin couplings (TS-couplings), which occur between two atoms constrained at a distance smaller than the sum of their van der Waals radii. This study revealed that 2′-fluoro-substituted acetophenone derivatives in solutions form exclusively s-trans conformers by analyzing their NMR spectra focused on the TS-couplings. The magnitudes of the coupling constants ⁵J (Hᵅ, F) and ⁴J (Cᵅ, F) correlate linearly with the value of the dielectric constant of the solvents. Furthermore, s-trans conformations of the two derivatives were confirmed by X-ray crystallographic analysis. These conformational preferences were consistent with the DFT calculations. The s-cis conformer, in which fluorine and oxygen atoms lie in a syn-periplanar mode, may be subject to strong repulsion between the two polar atoms and become unstable. The s-trans preference of the 2′-fluoro-substituted acetophenone derivatives may be utilized in drug design.
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