Elimination Reactions of (E)-2,4,6-Trinitrobenzaldehyde O-Benzoyloximes Promoted by R2NH/R2NH2+ in 70 mol% MeCN (aq). Effects of the beta-Aryl Group and Leaving Group on Nitrile-Forming Transition States

Abstract

Elimination reactions of (E)-2,4,6-(NO2)(3)C6H2CH=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 beta and vertical bar beta(lg)vertical bar 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) = partial derivative beta/partial derivative pK(1g) = partial derivative beta(1g)/partial derivative pK(BH) approximate to 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)(3)C6H2CH=NOAr' (2, 3), the change of the beta-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.