Eliminations from (E)-O-Arylbenzaldoximes Promoted by Tertiary Amines in Acetonitrile. Effects of Aryl Substituents, Base Strength, and Leaving Group upon the Nitrile-Forming Transition State

Elimination reactions of (E)-O-(2,4-dinitrophenyl)benzaldoximes (1) and (E)-O-picrylbenzaldoximes (2) with R₃N in MeCN have been investigated. The reactions are quantitative, producing only benzonitriles and aryloxides. The observation of second-order kinetics, β= 0.43-0.81, and |β_lg| = 0.39-0.73 is consistent with an E2 mechanism. For reactions of 1 with R₃N in MeCN, the β and ρ values increased significantly with electron-withdrawing aryl substitutents and stronger bases respectively. The corresponding changes in the β and ρ values for 2 were in the same direction but much less. When the leaving group was changed from 2,4-dinitrophenoxide to picrate, however, β increased and ρ decreased. Changes in transition-state parameters with alteration in the reactant structure are interpreted as resulting from variation in a nitrile-forming transition state.