The Norbornyl Cation and Other Nonclassical Carbocations

The acetolyses of both exo-2-norbornyl brosylate and ent/o-2-norbomyl brosylate produce exclusively exo-2-norbomyl acetate. The exo-brosylate is more reactive than the endo isomer by a factor of 350. Furthermore, enantiomerically enriched cxo-brosylate gave completely racemic exo-acetate, and the enJo-brosylate gave acetate that was at least 

SECTION 5.12. THE NORBORNYL CATION AND OTHER NONCLASSICAL CARBOCATIONS 

Both acetolyses were considered to proceed by way of a rate-determining formation of a carbocation. The rate of ionization of the enrfo-brosylate was considered normal, because its reactivity was comparable to that of cyclohexyl brosylate. Elaborating on a suggestion made earlier concerning rearrangement of camphene hydrochloride, Winstein proposed that ionization of the exo-brosylate was assisted by the C(l)—C(6) bonding electrons and led directly to the formation of a nonclassical ion as an intermediate. 

This intermediate serves to explain the formation of racemic product, since it is achiral. The cation has a plane of symmetry passing through C-4, C-5, C-6, and the midpoint of 

Carbon-6, which bears two hydrogens, is pentacoordinate and serves as the bridging atom in the cation. 

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