Norbomyl cations substituent effects

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... 

Brown, H. C., Heydkamp, W. R., Breuer, E., Murphy, W. S. 1964. Comparison of effect of substituents at 2-position of norbomyl system with their effect in representative secondary aliphatic and alicyclic derivative. Evidence for absence of non-classical stabilization of norbomyl cation. J. Am. Chem. Soc. 86 3565-3566. 

The results (4) do not support the conclusion that the rates for the norbomyl derivatives exhibit major enhancements. For example, the rate of solvolysis of camphene hydrochloride (/) is faster than that of the tetraniethylcyclopentyl chloride (7) by a factor of only 5.7. Such a factor is far too small to argue for a major new stabilizing phenomenon in the ion from 1. Removal of the gem-dimethyl substituents decreases the rate from 13,600 for 1 to 355 for 8. A similar effect is observed in the cyclopentyl derivatives, 7 and 9. These changes in reactivity are far more compatible with relief of steric strain13 than with resonance stabilization of the cations 12 ... 

---