Conformer dipole moment and syn-1,3-diaxial steric effect on the conformational equilibrium of the cis isomer of some 1,3-disubstituted cyclohexanes

The theoretical values of the dipole moment for the diaxial (ax-ax) conformer are larger than for the eq-eq conformer of some cis-3-halocyclohexanols (halo=Cl, Br and I) and cis−3-halo-1-methoxycyclohexanes (halo=F, Cl, Br and I). The experimental 3JHH coupling constants are rather large (>10 Hz) indicating that the diequatorial (eq-eq) conformer is predominant for all compounds in the solvents studied. Dipole moments and NMR data lead to the conclusion that the conformational equilibra of these 1,3-derivatives are not controlled by the solvent polarity, as is observed for the 1,2-disubstituted cyclohexanes, but are dictated by the syn-1,3-diaxial steric effect, which predominates over stabilization of the ax-ax conformer by an intramolecular hydrogen bond (IAHB).