Axial/Equatorial Stability Reversal in all-trans-Polyalkylcyclohexanes

The sterically crowded hexaalkylcyclohexane all-trans-hexaisopropylcyclohexane (2) was prepared by catalytic hydrogenation (Rh/alumina and Pd/C, 950 psi H₂, 130 °C) of an isomer (3) of dodecamethyl[6]radialene. Solution data, X-ray diffraction, and molecular mechanics calculations indicate that 2 prefers a conformation in which the isopropyl groups are located at axial rather than equatorial positions. Molecular mechanics calculations were carried out on a large number of all-trans-polyalkylcyclohexanes in order to find out what are the minimal steric requirements necessary for an appreciable (or exclusive) population of the all-axial conformer. Although some exceptions were indicated by the calculations, in general increasing the number and bulk of the alkyl substituent (Me ⟶ Et ⟶ i-Pr) results in the relative stabilization of the axial conformer. The calculations also predict that the preferred conformation of all-trans-polyethylcyclohexanes involves either anliperiplanar or eclipsed arrangements of one or more ethyl groups.