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 ks, unimolecular k1 and bimolecular k2, are separately determined by plotting kobsvs. aniline concentration [Nu], kobs = k1 + k2[Nu], where k1 = ks + kn with kn as the nonsolvolytic SN1 rate constant. The kn/ks value ranges from 6 to 7. The extent of leaving group departure in the transition state expressed by ρZ (where Z is a substituent in the leaving group) is always (for ks, k1 and k2) greater for exo than for endo suggesting a greater degree of bond cleavage in the exo system. The cross-interaction constants, ρXZ, are zero for ks and kn, but are the smallest ever obtained with distinctly non-zero value (ρXZ ⩽ 0.01) for k2. The transition state structures of the SN2 pathway are of a very loose, open or ‘exploded’ type as judged by the very small magnitudes of ρX (where X is a substituent in the nucleophile) and ρXZ coupled with the large values of ρZ. The reactions of exo-2-norbornyl arenesulfonates in the aprotic solvent, CH3CN, are characterized by a much smaller ρZ for k1 but a larger value of ρZ for k2 than those in CH3OH. All the experimental results support a preassociation mechanism for the bimolecular substitution process (k2).