2- Phenylnorbornyl cation

Reduction of either the exo or endo isomer of 2-phenyl-2-norbornanol with trifluoroacetic acid and triethylsilane, triphenylsilane, or phenylsilane in dichloro-methane gives endo-2-phenylnorbomane quantitatively (Eq. 24).164 The stereospecific formation of only the endo-hydrocarbon can be understood on the basis that only exo approach by organosilicon hydride toward the 2-phenylnorbornyl cation intermediate is kinetically competitive for product formation.164... 

The spectra of other norbornyl cations have also been investigated at low temperatures. Spectra of the tertiary 2-methyl- and 2-ethylnorborny cations show less delocalization,153 and the 2-phenylnorbornyl cation (54) is essentially classical,154 as are the 2-methoxy-155 and 2-chloronorbornyl cations.156 We may recall (p. 170) that methoxy and halo groups also... 

The question of the high exolendo rate ratio was examined with exo- and endo-2-phenyl-2-norbornanol. The tertiary and benzylic 2-phenylnorbornyl cation is classical, as evidenced by its NMR spectrum. Since the relative rates of hydrolysis of exo- and endo-2-phenylnorbornyl p-nitrobenzoate in 60% aqueous dioxane at 50°C are 140, and the reactions proceed by rate-determining formation of classical carbonium ions. Brown concluded that the exo-endo rate ratio of 340 for norbornyl brosylates does not require assisted ionization of exo-norbornyl brosylate. ... 

Another characteristic property associated with the nonclassical norbornyl cation, the high exofendo rate ratio, was also scrutinized. This was done by examining the rate ratio for related tertiary cations that have classical structures, such as the 2-phenylnorbornyl cation. It was found that the exo and endo 2-phenyl-norbornyl p-nitrobenzoates exhibited an exo j endo rate ratio of 140 1 in aqueous... 

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