Ketene-Forming Eliminations from Aryl Phenylacetates Promoted by R<sub>2</sub>NH/R<sub>2</sub>NH<sub>2</sub><sup>+</sup> in Aqueous MeCN. Mechanistic Borderline between E2 and E1cb

Elimination reactions of 2-X-4-NO<sub>2</sub>C<sub>6</sub>H<sub>3</sub>CH<sub>2</sub>C(O)OC<sub>6</sub>H<sub>3</sub>-2-Y-4-NO<sub>2</sub> [X = H (<b>1</b>), NO<sub>2</sub> (<b>2</b>)] promoted by R<sub>2</sub>NH/R<sub>2</sub>NH<sub>2</sub><sup>+</sup> in 70 mol % MeCN(aq) have been studied kinetically. The base-promoted eliminations from <b>1</b> proceeded by the E2 mechanism when Y = Cl, CF<sub>3</sub>, and NO<sub>2</sub>. 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 (<b>2</b>, X = NO<sub>2</sub>). The values of β = 0.14 and |β<sub>lg</sub>| = 0.10−0.21 calculated for elimination from <b>1</b> (Y = NO<sub>2</sub>) indicate a reactant-like transition state with small extents of proton transfer and C<sub>α</sub>−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 <i>k</i><sub>1</sub> and <i>k</i><sub>-</sub><sub>1</sub>/<i>k</i><sub>2</sub> 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.