Ketene-forming elimination reactions from aryl thienylacetates promoted by R₂NH/R₂NH₂⁺ in 70 mol % MeCN(aq). Effect of the β-aryl group

(Chemical Equation Presented) Ketene-forming eliminations from ArCH ₂CO₂C₆H₃-2-X-4-NO₂ (Ar = thienyl, 1) promoted by R₂NH/ R₂NH₂⁺ in 70 mol % MeCN(aq) have been studied kinetically. When X = CF₃ and NO₂, the reactions exhibit second-order kinetics as well as β = 0.30-0.64 and |β_1g| = 0.31-0.52 that decrease with a better leaving group. Hence, an E2 mechanism is evident. As the leaving group is made poorer (X = H, OCH₃, and CI), E2 transition state becomes more skewed toward the proton transfer, as revealed by the increase in Brönsted β to 0.5-0.64, and the Elcb mechanism competes. The changes in the k₁ and k_-1/k₂ values with the reactant structure variation provide additional support for the competing Elcb mechanism. By comparing with existing data for 4-YC₆H₄CH₂CO ₂C₆H₃-2-X-4-NO₂, the effect of β-aryl group on ketene-forming elimination is assessed.