Base-Promoted, Ketene-Forming Elimination Reactions. Mechanistic Borderline between E2 and E1cb Mechanisms

Elimination reactions of have been studied under various conditions. When X was moderately electron-withdrawing, Y = H, and base-solvent was NH-MeCN, the reaction proceeded by the E2 mechanism via an E1cb-like transition state. Concave downward curve was noted in the Hammett plots. When X = 4-, Y = Cl, , and the base-solvent was in 70 mol % MeCN(aq), the reaction proceeded by the E2 mechanism. The mechanism changed to a competing E2 and E1cb when X = 4- and Y = H, MeO, and to the E1cb when X = 2,4-(, and Y = . From these results, a plausible pathway of the change of the mechanism from E2 to the E1cb extreme is proposed.