Long-range substituent influence on the equatorial/axial conformational equilibrium of cyclohexanol and cyclohexanthiol esters

Abstract

The structures and relative energies of the conformers of the cyclohexyl esters of general formula Cy-XCOCR3 (X=O; R=H, F, Cl and X=S; R=H, Cl) and Cy-XCR3 (X=O, S) have been calculated at theoretical levels including HF/6-31G∗, HF/6-3l1G∗∗, MP2/6-311G∗∗, B3LYP/6-311G∗ and CBS-4M. The different stability of axial and equatorial conformers is analysed as a function of conformational enthalpy (ΔH0), entropy (ΔS0) and free energy (ΔG0) as well. The HF and DFT approaches reproduce the correct higher stability of the equatorial conformers of these molecules while the MP2 treatment inverts this order of stability in the case of X=O. The theoretical results reproduce as well the higher relative stability of the equatorial conformer in the case of X=S relative to those with X=O, yet less satisfactory is the agreement with the trend of experimental results as a function of the different R substituents. The origin of the different stability of the conformers as a function of X and R is discussed in detail; hyperconjugative interactions σC–C/σC–H→σC–X∗ prove to dominate the position of the conformational equilibria.