Importance of Entropy in the Diastereoselectivity of 5-Substituted 2-Methyladamant-2-yl Cation

The diastereofacial selectivity of 2-methyl-5-X-adamant-2-yl cations IX (X = CN, Cl, Br, CH3O, COOCH3, C6H5, CH3, and (CH3)3Sn) toward methanol was studied in the gas phase at 750 torr and in the 40-120 C temp. range and compared with that of IF (X = F) and ISi (X = (CH3)3Si) measured previously under similar conditions. Detailed anal. of the energy surface of the IMe (X = CH3) ion reveals that the activation barrier of its syn addn. to methanol is significantly lower than that of the anti attack. In the 40-100 C range, such a difference is strongly reduced by adverse entropic factors which are large enough to invert the IMe diastereoselectivity from syn to anti at T > 69 C. The behavior of IMe diverges markedly from that of IF and ISi. Large adverse entropic factors account for the predominant syn diastereoselectivity obsd. in the reaction with IF (X = F), notwithstanding the anti enthalpy barrier is lower than the syn one. Adverse entropy plays a minor role in the reaction with ISi (X = (CH3)3Si) which instead exhibits a preferred anti diastereoselectivity governed by the activation enthalpies. Depending on the electronic properties of X, the kinetic behavior of the other IX ions obeys one of the above models. The gas-phase diastereoselectivity of IX ions responds to a subtle interplay between the .sigma.-hyperconjugative/electrostatic effects of the X substituent and the activation entropy terms. .sigma.-Hyperconjugation/field effects det. the pyramidal structure and the relative stability of the syn and anti conformers of IX as well as the relative stability of their addn. transition structures and their position along the reaction coordinate. The diastereoselectivity of IX in the gas phase is compared with that measured in soln. and with theor. predictions.