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 FSO3H/SO2ClF or neat FSO3H 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 13C 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 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.
G. A. Olah et al., "Preparation, 13C NMR and IGLO/DFT Studies of Trifluoromethyl Substituted Allyl Cations," Journal of Organic Chemistry, vol. 57, no. 24, pp. 6431-6434, American Chemical Society (ACS), Nov 1992.
The definitive version is available at http://dx.doi.org/10.1021/jo00050a014
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