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 beta-aryl group (2, X = NO2). The values of beta = 0.14 and \beta(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-alpha-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 k(1) and k(-1)/k(2) 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.