Why the Classical and Nonclassical Norbornyl Cations Do Not Resemble the 2-endo- and 2-exo-Norbornyl Solvolysis Transition States^1,†

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, that there is “steric hindrance to ionization from the (norbornyl) endo face”, is not correct as the interaction of the (partially positively charged) endo-hydrogen (C6) and the leaving group is attractive in the transition state. Although the structure of the C₇H₁₁⁺ moiety in the exo-transition state is unsymmetrical, its energy is only 1.3 kcal mol^-1 higher than that of the fully relaxed nonclassical norbornyl cation (3). The norbornyl cation moiety in the 2-endo transition structure (also computed by removing the water molecule and retaining the C₇H₁₁⁺ geometry) is 4.3 kcal mol^-1 more stable than the classical 2-norbornyl cation but 8.8 kcal mol^-1 less stable than the fully bridged ion. Hence, the changes in geometry and charge distribution in the solvolysis transition structures reduce the energy difference of the classical and nonclassical cation moieties in the endo and exo transition structures to 7.5 kcal mol^-1. This is reduced further by the stronger leaving group interaction in the 2-endo over the 2-exo transition structure. The leaving group interaction with the developing carbocation in the 2-endo-norbornyl transition structure is stronger than in the 2-exo-transition structure. This difference (which exemplifies the general behavior of participating systems) arises since the stabilizing interactions of the neighboring group and of the leaving group must compete. Consequently, the effectiveness of both the neighboring group and the leaving group interactions is reduced relative to anchimerically unassisted solvolysis, and only a fraction of the potential driving force is reflected in the stabilization of the transition structure of participating systems. This is shown even more dramatically by the very modest effect (which was confirmed computationally) of a 1-methyl or a 1-phenyl substituent on the rate of 2-exo-norbornyl solvolyses (less than 100-fold acceleration), despite the huge increase in driving force.