VERY SLOW ROTATION ABOUT A CARBON-CARBON SINGLE BOND IN o-TOLYL-1,1'-DIADAMANTYLCARBINOL AND RELATED COMPOUNDS

Article abstract of DOI:10.1016/S0040-4020(01)97984-0

Previous work on the ap and sp rotamers of o-tolyldi-t-butylcarbinol (1a and 2a) has been extended to 1-adamantyl derivatives which are found to have abnormally high barriers to the ap->sp rotation.A semi-quantitative explanation of the activation enthalpies for this conformational change is proposed.Rotameric ap and sp o-tolyl-1-adamantyl-t-butylcarbinols (1b and 2b) and o-tolyl-1,1'-diadamantylcarbinols (1c and 2c) are synthesised by the addition of o-tolyl-lithium to 1-adamantyl-t-butylketone and 1,1'-diadamantylketone, respectively.In both cases the major product is the less stable ap isomer.The rotation rates of 1b and 1c in dodecane correspond to activation energies of 33.9 and 39.1 kcal/mol, respectively, even higher than that for 1a, 29.8 kcal/mol.Steric energies are calculated by molecular mechanics for model structures corresponding to the ap and sp isomers and to two points on the rotation pathway, giving extrapolated theoretical activation enthalpies in qualitative agreement with the experimental data.The enhanced steric effect of the 1-Ad group can be attributed to three factors : restricted rotation of the methylene groups as opposed to the methyl groups in t-Bu : increase in bond angle strain due to the terminal cyclohexane ring; increase in non-bonding interactions by buttressing effects of the terminal cyclohexane methine and methylene hydrogens.