Gas-phase diastereoselectivity of secondary 5-substituted (X)-adamant-2-yl (X=F, Si(CH3)3) cations.

Secondary 5-X-adamant-2-yl cations IX (X = F, Si(CH3)3) were generated in the gas phase (total pressure = 760 torr) from protonation-induced defluorination of epimeric 2-F-5-X-adamantanes 1X and their kinetic diastereoselectivity toward CH318OH studied in the 40-160 C range. The facial selectivity of IX is insensitive to the compn. of the starting 1X epimers as well as to the presence and the concn. of a powerful base (N(C2H5)3). This kinetic picture, supported by B3LYP/6-31G* calcns., is consistent with a single stable pyramidalized structure for IX, i.e., (Z)-5-F-adamant-2-yl (IZF) and (E)-5-Si(CH3)3-adamant-2-yl cations (IESi). The temp. dependence of the IX diastereoselectivity lends support to the intermediacy of noncovalent adducts [IX.bul.CH318OH], characterized by a specific C2-H+O18(H)CH3 hydrogen bonding interaction. Their conversion 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 face accommodating CH318OH. The gas-phase diastereoselectivity of IX toward CH318OH is compared to that exhibited in related gas-phase and soln. processes. The emerging picture indicates that the factors detg. the diastereoselectivity of IX toward simple nucleophiles in the gaseous and condensed media are completely different.