Base-promoted, ketene-forming elimination reactions. Mechanistic borderline between E2 and E1cb mechanisms

Elimination reactions of XC₆H₄CH₂CO ₂C₆H₃-2-Y-4-NO₂ have been studied under various conditions. When X was moderately electron-withdrawing, Y = H, and base-solvent was R₂NH-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-NO₂, Y = Cl, CF₃, NO ₂, and the base-solvent was R₂NH/R₂NH ₂⁺ in 70 mol % MeCN(aq), the reaction proceeded by the E2 mechanism. The mechanism changed to a competing E2 and Elcb when X = 4-NO ₂ and Y = H, MeO, and to the E1cb when X = 2,4-(NO₂) ₂, and Y = NO₂. From these results, a plausible pathway of the change of the mechanism from E2 to the E1cb extreme is proposed.