Tautomerism and conformational analysis in 3-fluoropiperidin-2-one inform on F···HO intramolecular hydrogen bond

Abstract

Whether hydroxyl hydrogen is directed towards a Lewis base substituent in an alkyl chain due to intramolecular hydrogen bonds or to avoid repulsion between the hydroxyl oxygen and the substituent is a debatable topic and difficult to probe. This is still more challenging when fluorine is the Lewis base substituent. This work reports a theoretical approach capable of dissecting the contributions from hydrogen bond and repulsion, allowing to decide which interaction dominates the orientation of the hydroxyl group in a high-energy tautomer of 3-fluoropiperidin-2-one. A comparison between the potential energy curves for the hydroxyl rotation in the axial and equatorial conformers revealed that repulsion between fluorine and oxygen contributes by ca. 1.2 kcal mol−1 to destabilize the cis isomer, while intramolecular hydrogen bond between fluorine and the hydroxyl group amounts by ca. 0.9 kcal mol−1 favoring the trans isomer. These are not the major effects governing the conformational equilibrium of the studied compound, which is rather dictated by N⋯O steric repulsion, but the obtained outcomes can be valuable to understand the role of F⋯H(O) hydrogen bonds in conformational analysis and, ultimately, in the design of performance fluorinated aliphatic alcohols.