Tricyclopropylmethyl cation

The tricyclopropylmethyl cation is half-formed (Cqh = C, ) in 22% H2SO4, which shows it to be far more stable than the triphenylcarbonium ion which is half-formed at 50% H2SO4 (Deno et al., 1962). Its U.V. spectrum contains an intense absorption maximum at 270 mp, e = 22,000. 

The relationship between charge density and the NMR chemical shifts is, however, only qualitative and should be used with caution. Other factors, such as neighboring anisotropic effects of the substituents, should also be considered. The cationic center of triphenylmethyl cation (<5I3C 211.2 ppm), for example, is much shielded from that of tricyclopropylmethyl cation (270.9 ppm), which may erroneously lead to the conclusion that a phenyl is more stabilizing than the cyclopropyl group. 

Figure 3.9. 1H NMR spectrum (300 MHz) of the tricyclopropylmethyl cation in SbF5-S02ClF at —60°C.

Several very stable carbocations are included in the Other Carbocations part of Table 4.15. The tricyclopropylmethyl cation, for example, is more stable than the... 

There are some amazingly stable cations. 1,2-Dipropylcyclopropenyl cation (IV) and aryl-substituted cyclopropenyl cations (Breslow, 1957 Breslow and Yuan, 1958 Breslow and Hover, 1960 Breslow and Chang 1961), the cycloheptatrienyl cation (V) (Doering and Knox, 1954 Turner et al., 1960), and the heptamethylcyclohexadienyl cation (VI) (Doering et al., 1958) are half-formed from their alcohols and/or olefins in the very dilute acids of pH 1-4. Tricyclopropylmethyl cation (VII) (Deno et al., 1963c) is half-formed from its alcohol in 22% H2SO4. 

Let us now examine two rather extreme cases the hydride exchange between tricyclopropylmethyl and cumyl cations and the proton exchange between guaiazulene and cr-methylstyrene, respectively (reactions 31 and 32) ... 

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