Preparation, ¹³C NMR and IGLO/DFT Studies of Trifluoromethyl Substituted Allyl Cations

2-(Adamantylidenemethyl)-2-adamantyl cation (5) was prepared by ionization of 2-(adamantylidenemethyl)-2-adlamantanol (9) in FSO ₃ H/SO ₂ ClF or neat FSO ₃ H at -78°C. In spite of its high inherent steric strain, carbocation 5 is stable up to 80°C. Its rotational barrier was estimated to be 16.8 kcal/mol on the basis of the coalescence temperature of the ¹³ C NMR signals of its diastereomeric ß-methylene carbons, which is in agreement with a ΔH ^‡ value of 16.5 kcal/mol calculated by MNDO. Similarly, 2-adamantylidene-1,1-dicyclopropylethyl cation (6) was prepared by the ionization of 1,1-dicyclopropyl-2-(2-hydroxy-2-adamantyl)ethanol (12) in SbF ₅ -FSO/SO ₂ ClF or FSO ₃ H/SO ₂ ClF. Carbocation 6 shows a rotational barrier of less than 5 kcal/mol. The small rotational barrier in 6 is interpreted as due to the positive charge stabilizing influence of the adjacent cyclopropyl groups resulting in an unsymmetrically charge-delocalized allylic cation.