Ketene-forming eliminations from aryl phenylacetates promoted by R₂NH/R₂NH₂⁺ in aqueous MeCN. Mechanistic borderline between E2 and E1cb

Elimination reactions of 2-X-4-NO₂C₆H₃CH₂C(O) OC₆H₃-2-Y-4-NO₂ [X = H (1), NO₂ (2)] promoted by R₂NH/R₂NH₂⁺ in 70 mol % MeCN(aq) have been studied kinetically. The base-promoted eliminations from 1 proceeded by the E2 mechanism when Y = Cl, CF₃, and NO₂. The mechanism changed to the competing E2 and E1cb mechanisms by the poorer leaving groups (Y = H, OMe) and to the E1cb extreme by the strongly electron-withdrawing β-aryl group (2, X = NO₂). The values of β = 0.14 and |β_1g| = 0.10-0.21 calculated for elimination from 1 (Y = NO₂) indicate a reactant-like transition state with small extents of proton transfer and C_α-OAr bond cleavage. The extent of proton transfer increased with a poorer leaving group, and the degree of leaving group bond cleavage increased with a weaker base. Also, the changes in the k₁ and k_-1/k₂ values with the reactant structure variation are consistent with the E1cb mechanism. From these results, a plausible pathway of the change of the mechanism from E2 to the E1cb extreme is proposed.