| Abstract
| - Secondary 5-X-adamant-2-yl cations IX (X = F, Si(CH3)3) have been generated in the gas phase (totalpressure = 760 Torr) from protonation-induced defluorination of epimeric 2-F-5-X-adamantanes 1X andtheir kinetic diastereoselectivity toward CH318OH investigated in the 40−160 °C range. The experimentalresults indicate that the facial selectivity of IX is insensitive to the composition of the starting 1X epimersas well as to the presence and the concentration of a powerful base (N(C2H5)3). This kinetic picture,supported by B3LYP/6-31G* calculations, is consistent with a single stable pyramidalized structure forIX, that is, (Z)-5-F-adamant-2-yl (IZF) and (E)-5-Si(CH3)3-adamant-2-yl cations (IESi). The temperaturedependence of the IX diastereoselectivity lends support to the intermediacy of noncovalent adducts[IX•CH318OH], characterized by a specific C2−H+···O18(H)CH3 hydrogen bonding interaction. Theirconversion to the covalently bonded O-methylated (Z)- (IIZX) and (E)-5-X-adamantan-2-ols (IIEX; X =F, Si(CH3)3) is governed by activation parameters, whose magnitude depends on the specific IX faceaccommodating CH318OH. The gas-phase diastereoselectivity of IX toward CH318OH is compared to thatexhibited in related gas-phase and solution processes. The emerging picture indicates that the factorsdetermining the diastereoselectivity of IX toward simple nucleophiles in the gaseous and condensed mediaare completely different.
|
