Studies on the Molecular Geometry of the Norbornyl Cation. I. Synthesis and Acetolysis of the exo- and endo-4,5-exo-Trimethylene-2-Norbornyl P-Toluenesulfonates

4,5-exo-Trimethylene-2-norbornene (10) has been synthesized from a monosubstituted cyclopentadiene 9 using an intramolecular Diels-Alder reaction, and from this intermediate exo- and endo-4,5-exo-trimethylene-2-norbornyl p-toluenesulfonates (11, R = Ts, and 16, R = Ts) have been prepared. These sulfonates undergo acetolysis (25°) at relative rates of 8.6:1. The ratio of rate constants (25°) for acetolysis of the exo-sulfonate 11, R = Ts, and 2-exo-norbornyl p-toluenesulfonate is 1:85, where as the corresponding ratio for the endo-sulfonate 16, R = Ts, and 2-endo-norbornyl p-toluenesulfonate is 1:2.5. The depressed rate for the tricyclic exo-sulfonate 11, R = Ts, relative to 2-exo-norbornyl p-toluenesulfonate is readily explained in terms of bridging of carbon in the transition state for ionization, but seems to be contrary to expectations based on ionization to a localized (classical) carbonium ion. Thus, the present results favor the bridged-ion mechanism for acetolysis of 2-exo-norbornyl arenesulfonates.