Hindered rotations about single bonds are of immense scientific significance due to their applications in the investigation of intramolecular interactions, in asymmetric synthesis and catalysis as well as in many other fields. Although slow rotations of many bulky tetrahedral moieties are frequently reported, restricted F3C—C(sp~3) single-bond rotations have only been sporadically documented. As they are generated by strong noncovalent interactions between the CF3 group and its surrounding atoms (X), hindered CF3 rotations offer an ideal opportunity for investigating various C—F···X interactions. In particular, C—F···H— N/O hydrogen bondings have received much attention in the fields of physical organic and biological chemistry, whereas prevalence and energetic importance of C—F···H—C interactions are frequently debated. To address these problems, we have investigated CF3—C(sp~3) rotations in O-alkyl-9-dehydro-9-trifluoromethyl-9-epiquinidine scaffolds. Extremely short C—F···H—C distances have been observed both in solution and in the solid state, providing insights into the nature of C— H···F—C interactions.
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