Nucleophilic substitution reactions of 2-norbornyl arenesulfonates with anilines

Abstract

The kinetics and mechanism of the nucleophilic substitution reactions of exo- and endo-2-norbornyl arenesulfonates with anilines are investigated in methanol and acetonitrile at 60.0 °C. Rate constants for three distinct competing processes, solvolysis k_s, unimolecular k₁ and bimolecular k₂, are separately determined by plotting k_obsvs. aniline concentration [Nu], k_obs = k₁ + k₂[Nu], where k₁ = k_s + k_n with k_n as the nonsolvolytic S_N1 rate constant. The k_n/k_s value ranges from 6 to 7. The extent of leaving group departure in the transition state expressed by ρ_Z (where Z is a substituent in the leaving group) is always (for k_s, k₁ and k₂) greater for exo than for endo suggesting a greater degree of bond cleavage in the exo system. The cross-interaction constants, ρ_XZ, are zero for k_s and k_n, but are the smallest ever obtained with distinctly non-zero value (ρ_XZ ⩽ 0.01) for k₂. The transition state structures of the S_N2 pathway are of a very loose, open or ‘exploded’ type as judged by the very small magnitudes of ρ_X (where X is a substituent in the nucleophile) and ρ_XZ coupled with the large values of ρ_Z. The reactions of exo-2-norbornyl arenesulfonates in the aprotic solvent, CH₃CN, are characterized by a much smaller ρ_Z for k₁ but a larger value of ρ_Z for k₂ than those in CH₃OH. All the experimental results support a preassociation mechanism for the bimolecular substitution process (k₂).