Eliminations from (E)-2,4-Dinitrobenzaldehyde O-Aryloximes Promoted by R3N in MeCN. Effects of β-Aryl Group and Base-Solvent on the Nitrile-Forming Transition-State

The transition state was shifted toward theE1cb corner by an electron-withdrawing β-aryl substituentand toward the E1 intermediate and the product by astronger base in the reaction coordinate diagram. On theother hand, when the leaving group was changed from 2,4-dinitrophenoxide to picrate, the ρ value decreased but the βvalue increased. The results are in conflict with the predic-tion of the reaction coordinate diagram because both ρ and βvalues should be decreased with a better leaving group. Theunusual changes in the transition-state structure noted withthe leaving group variation have been interpreted as result-ing from a large difference in the anion-stabilizing ability ofthe leaving groups. However, the validity of this interpreta-tion remains to be verified. To determine whether a change in mechanism could berealized by introducing an even more electron-withdrawingβ-aryl substituent, we have now studied the reactions of ( E)-2,4-dinitrobenzaldehyde O-aryloxime with R