Theoretical investigation of the substituent effects in the conformational isomerism of bromoalkoxycyclohexanes

Isodesmic reactions and the energy of intramolecular interactions in bromoalkoxycyclohexanes (alkoxy = OMe, OEt, OiPr and OtBu) were computed to evaluate the effect of bromine and alkoxy groups when bonded together in a cyclohexane ring to give cis and trans-1,2, 1,3 and 1,4 isomers. According to the enthalpy energies obtained from the isodesmic reactions, the bromine atom is preferred to be introduced axially in methoxycyclohexane to give trans-1-bromo-2-methoxycyclohexane; otherwise, equatorial introduction of the bromine atom is favoured. Either direct or indirect intramolecular interactions involving OR and Br, obtained from comparison with the conformational energies of the monosubstituted cyclohexanes, indicate a preference for bromine axially oriented, except for the cis-1,3 isomer, in which the OR and Br groups experience high steric hindrance to each other. The effect of R is generally invariant.