Nucleophilic substitution reactions of 2-norbornyl arenesulfonates with anilines

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 degrees C, Rate constants for three distinct competing processes, solvolysis k(s), unimolecular k(1) and bimolecular k(2), are separately determined by plotting k(obs) vs. aniline concentration [Nu], k(obs) = k(1) + k(2)[Nu], where k(1) = k(s) + k(n) with k(n) as the nonsolvolytic S(N)1 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 rho(z) (where Z is a substituent in the leaving group) is always (for k(s), k(1) and k(2)) greater for exo than for endo suggesting a greater degree of bond cleavage in the exo system. The cross-interaction constants, rho(xz), are zero for k(s) and k(n), but are the smallest ever obtained with distinctly non-zero value (rho(xz) less than or equal to 0.01) for k(2). The transition state structures of the S(N)2 pathway are of a very loose, open or 'exploded' type as judged by the very small magnitudes of rho(x) (where X is a substituent in the nucleophile) and rho(xz) coupled with the large values of rho(z). The reactions of exo-2-norbornyl arenesulfonates in the aprotic solvent, CH3CN, are characterized by a much smaller rho(z) for k(1) but a larger value of rho(z) for k(2) than those in CH3OH, All the experimental results support a preassociation mechanism for the bimolecular substitution process (k(2)).