Eliminations from (E)-2,4-dinitrobenzaldehyde O-aryloximes promoted by R₃N/R₃NH⁺ in 70 mol% MeCN(aq). Effects of leaving group and base-solvent on the nitrile-forming transition-state

Elimination reactions of (E)-2,4-(NO₂)₂C ₆H₂CH=NOC₆H₃-2-X-4-NO₂ (1a-e) promoted by R₃N/R₃NH⁺ in 70 mol % MeCN(aq) have been studied kinetically. The reactions are second-order and exhibit Brönsted β = 0.80- 0.84 and |βlg| = 0.39-0.42, respectively. For all leaving groups and bases employed in this study, the β and |βlg| values remained almost the same. The results can be described by a negligible pxy interaction coefficient, p_xy = δβ/pKlg = δβlg/pK-azip ̃ 0, which describes the interaction between the base catalyst and the leaving group. The negligible p_xy interaction coefficient is consistent with the (E1cb) _irr mechanism. Change of the base-solvent system from R ₃N/MeCN to R₃N/R₃NH⁺-70 mol % MeCN(aq) changed the reaction mechanism from E2 to (E1cb)_irr. Noteworthy was the relative insensitivity of the transition state structure to the reaction mechanism change.