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

Abstract

Elimination reactions of (E)-2,4-(NO2)(2)C6H2CH=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 Bronsted beta = 0.80-0.84 and vertical bar beta(lg)vertical bar = 0.39-0.42, respectively. For all leaving groups and bases employed in this study, the beta and vertical bar beta(lg)vertical bar values remained almost the same. The results can be described by a negligible p(xy) interaction coefficient, p(xy) = partial derivative beta/pK(lg) = partial derivative beta(lg)/pK(BH) perpendicular to 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 R3N/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.