Collect. Czech. Chem. Commun. 1999, 64, 1833-1848

Elimination of Substituted Fluoren-9-ylmethyl Benzenesulfonates: Hammett Substituent Effects at a Mechanistic Borderline

Finbar G. Larkin, Rory A. More O'Ferrall* and Donal G. Murphy

Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland


Rates of elimination of fourteen substituted fluoren-9-ylmethyl benzenesulfonates have been measured in methanolic sodium methoxide and 90% aqueous ethanolic solutions of triethylamine, trimethylamine and 4-methyl morpholine. For the sodium methoxide, a linear Hammett plot with ρ = 0.74, consistent with reaction by an E2 mechanism, is observed. For the amine bases the Hammett plots are curved, suggesting a transition from an E2 mechanism for electron-withdrawing substituents to an irreversible E1cB mechanism with a smaller value of ρ for electron-donating substituents. The evidence for a change of mechanism is weakened by systematic and random deviations of substituents from correlations which span small changes in reactivity (less than ten-fold), by a surprisingly large value of ρ = 2 implied for the concerted (E2) reaction and by the possible influence of negative hyperconjugation. Nevertheless, it is consistent with independent evidence that the borderline between concerted and stepwise mechanisms is associated with chemically distinguishable reaction paths, even though pronounced carbanion character (and probably a small extent of bond-breaking to the leaving group) ensures a degree of similarity of structure and sensitivity to substituents of their transition states.

Keywords: β-Eliminations; Mechanistic borderline; Hammett relationship; Reaction kinetics; Base catalysis; E2 and E1cB mechanisms.