Elimination reactions of (E)-2,4,6-trinitrobenzaldehyde O-benzoyloximes promoted by R₂NH/R₂NH₂⁺ in 70 mol% MeCN (aq). Effects of the β-aryl group and leaving group on nitrile-forming transition states

Elimination reactions of (E)-2,4,6-(NO₂)₃C₆H₂CH=NOC(O)C₆H₄X (3) promoted by R₂NH/R₂NH₂⁺ in 70 mol% MeCN (aq) have been studied. The reactions produced elimination products and exhibited second-order kinetics. The β and |β_1g| values remained nearly the same for all leaving groups and bases. The results can be described by the negligible p_xy interaction coefficient, p_xy = ∂β/∂pK_lg = ∂β_lg/∂pK_BH ≈ 0, which provides a strong support for the (E1cb)_irr mechanism. For eliminations from (E)-ArCH NOC=(O)C₆H₄X (1, 3) and (E)-2,4,6-(NO₂)₃C₆H₂CH=NOAr′ (2, 3), the change of the β-aryl group (Ar) from 2,4-dinitrophenyl (1) to 2,4,6-trinitrophenyl (3) increased the rate by 270-fold without appreciable change in the transition state structure. On the other hand, the leaving group (OAr′) variation from benzoate (3) to 4-nitrophenoxy (2) induced a change in reaction mechanism from (E1cb)_irr to E2. These results have been attributed to the cyclic transition state for the nitrile-forming eliminations involving the benzoate leaving group.