Solvolysis of 8-Vinyl-Exo-8-Bicyclo[3.2.1]octyl 3,5-Dinitrobenzoate. Evidence for Stabilization by Carbon-Carbon Hyperconjugation in the Transition State for Formation of a Tertiary Allylic Cation

The high exo/endo reactivity ratio (8000) for acetolysis of the epimeric 8-bicyclo[3.2.1]octyl tosylates may derive from preferential “vertical” stabilization of the transition state for ionization of the exo tosylate by carbon-carbon hyperconjugation. Support for this suggestion is found in the observation that a high reactivity ratio is maintained and that high product stereospecificity is manifest in the solvolysis of the epimeric 8-vinyl-8- bicyclo[3.2.1]octyl 3,5-dinitrobenzoates (k_exo/k_endo = 515). Since the “leveling capacity” (10⁸) of the vinyl group exceeds the exo/endo rate ratio exhibited by the secondary tosylates by a factor of 10⁴, incorporation of a vinyl substituent at the site of ionization should engender complete cessation of neighboring group participation if such participation were nonvertical in nature.