2- Cyclopropyl-2-norbornyl cation

The 3-spirocyclopropyl-2-cyclopropyl-2-norbornyl cation 97, R = C-C3H5 is stable even up to -20 °C, whereas the phenyl and methyl analogues rearranged to the allylic cations 98 at -70 °C and -90 °C, respectively (equation 58). 

The 2-cyclopropyl-2-norbornyl cation (83) was evidently present as two conformers at — 90°C due to slow rotation about the bond to the cyclopropyl, but the individual structures could not be assigned. The CH2 groups of the cyclopropyl group were both deshielded but by different amounts (43.0 and 56.0 ppm), and it was suggested that this indicated more effective donation from one of the C-C bonds of the cyclopropyl which... 

Using methods such as those discussed for the norbornyl cation, nonclassical structures have now been established for a number of carbocations. " Representative examples are shown below. The 7-phenyl-7-norbornenyl cation 19 exists as a bridged strucmre 20, in which the formally empty p orbital at C7 overlaps with the C2—C3 double bond. This example is of a homoallylic cation. The cyclopropyl-carbinyl cation 21, historically one of the first systems where nonclassical ions were proposed, has been shown to exist in superacids mainly as the nonclassical bicyclo-butonium ion 22, although it appears as if there is a small amount of the classical 21 present in a rapid equilibrium. Cations 23 and 24 are examples of p-hydridobridged... 

An ESCA study by Olah and coworkers " succeeded in observing the ESCA spectrum of the 2-norbornyl cation (126) and compared it with those of the related 2-methyl-2-norbomyl cation (142) and other trivalent carbenium ions such as the cyclopentyl (75), 1-methyl-1-cyclopentyl (141), and tert-butyl (50) cations. The Is electron spectrum of the 2-norbomyl cation shows no high binding energy carbenium center and a maximum separation of <1.5 eV is observed between the two equivalent cyclopropyl -type carbons... 

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