Observation in Different Superacids

The first cyclopropylmethyl cation directly observed was the tricyclopropyl-methyl cation 16 by Deno54. Its 1H NMR spectrum in H2S04 consists of a single sharp line at S 2.26. In the 300 MHz JH NMR spectrum in S02ClF/SbF5 solution, 

A wide range of studies55,60-62) have indicated that a cyclopropyl group is equal or more effective than a phenyl group in stabilizing an adjacent carbocation center. 

In contrast to classical tertiary and secondary cyclopropylmethyl cations (showing substantial charge delocalization into the cyclopropane ring but maintaining its identity) primary cyclopropylmethyl cations in contrast show compeltely cr-delocalized non-classical carbonium ion character (see subsequent discussion). 

A recent X-ray characterization of the cyclopentyl cation 33 generated with methyl carboranes has shown a perfect planar structure around the carbocation center [bond angles CH3-C+-CH2= 124.9 and 125.2°, CH2-C+-CH2= 109.9° bond distances CH3-C+ = 1.46 A, CH2-C+ = 1.45 A).123 

Sorensen and co-workers150 have prepared tertiary cycloalkyl cations of different ring sizes, n = 4 (small ring), n = 5-7 (common rings), n = 8-11 (medium rings), 

A series of aryl-substituted cycloalkyl cations 38 and 39 and other aryl-substituted cyclic systems (40,41,42) have been studied in connection with the application of the tool of increasing electron demand.67,153-156 

The direct observation of the cyclopropyl cation 44 has evaded all attempts, owing to its facile ring opening to the energetically more favorable allyl cation 45 [Eq. (3.29)].158 

Many cyclopropyl cation precursors indeed readily rearrange to allyl cations under stable ion conditions.159-162 However, a distinct cyclopropyl cation 46 showing a significant 2n aromatic nature has been prepared by the ionization of 11-methyl-ll-bromotricyclo[4.4.1.01,6]undecane in SbF5-S02ClF solution at — 120°C163 [Eq. (3.30)]. 

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