Endo systems, 2-norbomyl cation

The problem of norbomyl cation stabilities vs. solvolysis rate discrepancies in the norbomyl system has been addressed in an important paper.159 The classical and non-classical norbomyl cations do not resemble the 2-endo- and 2-exo -norbomyl solvolysis transition states very closely. The authors conclude that Brown was wrong, but that Winstein was not entirely right either.159 A substituent in the benzene ring has little effect upon the kinetics of the acid-catalysed hydrolysis of 2-exo-norbomyl phenyl ether.160 The FTIR spectra of matrix-isolated 2-methylbenzonorbomen-2-yl cations have been examined at —196 °C the structure can best be represented as (108), rather like a phenonium cation, but at higher temperatures a transition takes place to a structure that is more nearly represented as (109), with some 71-bridging.161 The stereoselectivities of some 7-methyl-7-norbom(en)yl cations have been investigated (110) has a classical structure and reacts in a stereo-random manner, whereas (111) is... 

To summarize the situation at present, the nonclassical norbomyl cation seems to be the species observed in superacid media, but the reservation has been expressed that the species observed may not be the same as the intermediate in solvolytic reactions, in which much of the electrophilicity of the cationic center may be attenuated by solvation. The cationic intermediate generated in solvolysis reactions may also be the nonclassical ion, but the observable properties of rapid solvolysis, high exo/endo capture ratio, and high exo/endo rate ratio are not sufficient to require this conclusion. Similar properties can be shown to obtain in model systems that surely react through intermediates in which bridging is absent. 

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