Origin of the axial-alkyl preference of (R)-α-phellandrene and related compounds investigated by high-level ab initio MO calculations. Importance of the CH/π hydrogen bond

Ab initio MO calculations were carried out, at the MP2/6-311++G(d,p)//MP2/6-311G(d,p) level, to investigate the Gibbs energy of conformational isomers of (R)-α-phellandrene and related 5-alkyl-1,3-cyclohexadienes. It has been found that the conformer bearing the 5-alkyl group in axial orientation is more stable than the equatorial congener. The result is consistent with experimental evidence that the axial-isopropyl conformer prevails in the conformational equilibrium of α-phellandrene. The reason for the stability of the folded conformer has been sought in the context of the CH/π hydrogen bond. A number of short non-bond distances have been disclosed in the axial conformers, between CHs in the 5-alkyl group and sp2-carbons of the cyclohexadiene ring. We suggest that the stability of the folded conformation often observed in conjugated diene compounds of natural origin, such as α-phellandrene and levopimaric acid, is attributed to an attractive molecular force, the CH/π hydrogen bond.