APPLICATIONS OF NMR AND THEORETICAL CALCULATIONS TO STUDY THE O-H∙∙∙N INTRAMOLECULAR HYDROGEN BOND EFFECT ON THE CONFORMATIONAL EQUILIBRIUM OF CIS-3-N-ETHYLAMINOCYCLOHEXANOL AND CIS-3-N,N-DIETHYLAMINOCYCLOHEXANOL

Textbooks show that in cis-1,3-disubstituted cyclohexane molecule, the conformer with the substituents in the diaxial positions is higher in energy and hence, it presents a low population in the conformational equilibrium. The diaxial conformer is destabilized by the repulsion between the substituent groups and the axial hydrogen atoms on the same side of the ring. This interaction is known as the 1,3-diaxial interaction. In this study, the solvent effect on the conformational equilibria of cis-3-N-ethylaminocyclohexanol (cis-3-EACOL) and cis-3-N,N-diethylaminocyclohexanol (cis-3-DEACOL) has been assessed through the spin-spin coupling constant (³J_HH). The results show that the diaxial conformation of cis-3-EACOL decreases from 92% in CCl₄ (∆G_ee−aa = 1.48 kcal mol⁻¹) to 10% in DMSO-d₆ (∆G_ee−aa = −1.31 kcal mol⁻¹). For cis-3-DEACOL the diaxial conformer decreased from 36% in CCl₄ (∆G_ee−aa = −0.33 kcal mol⁻¹) to 7% in Pyridine-d₅ (∆G_ee−aa = −1.55 kcal mol⁻¹). The stabilization of the diaxial conformer in nonpolar solvents takes place due to the O-H···N intramolecular hydrogen bond, which overcomes the 1,3-diaxial steric interactions.

Keywords:
conformational analysis; intramolecular hydrogen bond; aminocyclohexanols; QTAIM; NCI