Conformational analysis of trans‐2‐halocyclohexanols and their methyl ethers: a 1H NMR, theoretical and solvation approach

Abstract

AbstractThe conformational equilibria of trans‐1‐methoxy‐2‐chloro‐ (1), trans‐1‐methoxy‐2‐bromo‐ (2) and trans‐1‐methoxy‐2‐iodocyclohexane (3), and their corresponding alcohols (4–6), were studied through a combined method of NMR, theoretical calculations and solvation theory. They can be described in terms of the axial–axial and equatorial–equatorial conformations, taking into account the main rotamers of each of these conformations. From the NMR experiments at 183 K in CD2Cl2–CS2, it was possible to observe proton H2 in the ax–ax and eq–eq conformers separately for 1 and 2, but not for 3, which gave directly their populations and conformer energies. In the alcohols the proportion of the ax–ax conformer was too low to be detected by NMR under these conditions. Those HH couplings together with the values at room temperature, in a variety of solvents allowed the determination of the solvent dependence of the conformer energies and hence the vapor state energy difference. The ΔE (Eax–Eeq) values in the vapor state for 1, 2 and 3 are −0.05, 0.20 and 0.55 kcal mol−1, respectively, increasing to 1.10, 1.22 and 1.41 kcal mol−1 in CD3CN solution (1 kcal = 4.184 kJ). For 4–6 the eq–eq conformation is always much more stable in both non‐polar and polar solvents, with energy differences ranging from 1.78, 1.94 and 1.86 kcal mol−1 (in CCl4) to 1.27, 1.49 and 1.54 kcal mol−1 (in DMSO), respectively. Comparison of the hydroxy and methoxy compounds gives the intramolecular hydrogen bonding energy for the alcohols as 1.40, 1.36 and 1.00 kcal mol−1 (in CCl4) for 4, 5 and 6, respectively. Copyright © 2002 John Wiley & Sons, Ltd.