The selectivity principle and the question of the bridged structure of the 2-norborynyl cation.
Low exo/endo rate ratios in the solvolysis of certain norbornyl derivatives have been attributed to the involvement of classical (unbridged) 2-norbornyl cations. Consequently, the high exo/endo rate ratio in the parent system, 2-norbornyl arenesulfonates, has been interpreted as involving rate enhancement in the exo isomer arising from sigma-participation. It is pointed out that this treatment involves neglect of the Selectivity Principle. Taking the relative reactivity of cyclopentyl to cyclohexyl as a representative test system, it is pointed out that those reactions which reveal low exo/endo rate ratios in norbornyI also reveal low cyclopentyl/cyclohexyl rate ratios. It is proposed that the low exo/endo rate ratios in norbornyl, observed in the above reactions, arise from the low selectivities of the test reactions. Consequently, the results cannot be utilized to support the argument that low exo/endo rate ratios reflect the absence of sigma-participation and high exo/endo rate ratios reflect the presence of sigma-participation.
Richard B. Wetherill Laboratory, Purdue University, West Lafayette, Indiana 47907.
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Brown
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Peters