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 (haloZCl, Br and I) and cisK3-halo-1-methoxycyclohexanes (haloZF, Cl, Br and I). The experimental 3JHH coupling constants are rather large (O10 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). q 2005 Elsevier B.V. All rights reserved