Ketene-forming elimination reactions from aryl thienylacetates promoted by R2NH/R2NH2+ in 70 mol % MeCN(aq). Effect of the beta-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 beta = 0.30-0.64 and |betalg| = 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 beta to 0.5-0.64, and the E1cb mechanism competes. The changes in the k1 and k-1/k2 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 beta-aryl group on ketene-forming elimination is assessed.
Cho
Bong Rae
BR
Department of Chemistry, Korea University, 1-Anamdong, Seoul, 136-701, Korea.
Pyun
Sang Yong
SY