Why the Classical and Nonclassical Norbornyl Cations Do Not Resemble the 2-endo- and 2-exo-Norbornyl Solvolysis Transition States1,†

In order to analyze the solvolysis behavior of epimeric norbornyl derivatives, the dissociative mechanisms of protonated 2-exo- (1, X = OH2+) and 2-endo-norbornanol (2, X = OH2+), 1-methyl-2-exo- (7) and 2-endo-norbornanol (8), and 1-phenyl-2-exo- (9) and 2-endo-norbornanol (10) were studied ab initio at the B3LYP/6-311+G*//B3LYP/6-31G* level. In agreement with the experimental solvolysis data, the activation energy (including the 1.2 kcal mol-1 ground state energy difference) for dissociation of exo-1 (X = OH2+) is 3.7 kcal mol-1 lower than that of endo-2 (X = OH2+). This value is much smaller than the 14 kcal mol-1 energy difference favoring the isolated nonclassical (3) over the classical (5) 2-norbornyl cation. That the rate acceleration reflects only a small part of the driving force available poses a general interpretative problem in neighboring group participation. Winstein's hypothesis, that “bridging lags behind ionization” is not the full explanation for this discrepancy. Brown's hypothesis, tha...