摘要:
2-(Adamantylidenemethyl)-2-adamantyl cation (5) was prepared by ionization of 2-(adamantylidenemethyl)-2-adamantanol (9) in FSO3H/SO2ClF or neat FSO3H at -78-degrees-C. In spite of its high inherent steric strain, carbocation 5 is stable up to 80-degrees-C. Its rotational barrier was estimated to be 16.8 kcal/mol on the basis of the coalescence temperature of the C-13 NMR signals of its diastereomeric beta-methylene carbons, which is in agreement with a DELTAH(double dagger) value of 16.5 kcal/mol calculated by MNDO. Similarly, 2-adamantylidene-l,l-dicyclopropylethyl cation (6) was prepared by the ionization of 1,1-dicyclopropyl-2-(2-hydroxy-2-adamantyl)ethanol (12) in SbF5-FSO/SO2ClF or FSO3H/SO2ClF. 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.