Ketene-Forming Elimination Reactions from Aryl Thienylacetates Promoted by R2NH/R2NH2 + in 70 mol % MeCN(aq). Effect of the β-Aryl Group

Ketene-forming eliminations from ArCH2CO2C6H3-2-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 |βlg| = 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 Cl), 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 E1cb mechanism competes. The changes in the k 1 and k -1/k 2 values with the reactant structure variation provide additional support for the competing E1cb mechanism. By comparing with existing data for 4-YC6H4CH2CO2C6H3-2-X-4-NO2, the effect of β-aryl group on ketene-forming elimination is assessed.