Elimination Reactions of Aryl Furylacetates Promoted by R2NH-R2NH2+ in 70 mol % MeCN(aq). Effects of beta-Aryl on the Ketene-Forming Transition-State

Abstract

Ketene-forming elimination from 2-X-4-nitrophenyl furylwacetates (1a-d) promoted by R2NH-R2NH2+ in 70 mol % MeCN(aq) has been studied kinetically. When X = Cl and NO2, the reactions exhibited second-order kinetics as well as Bronsted beta = 0.37-0.54 and I vertical bar beta(lg)vertical bar = 0.31-0.45. The Bronsted beta decreased with a poorer leaving group and vertical bar beta(lg)vertical bar increased with a weaker base. The results are consistent with an E2 mechanism. When the leaving group was changed to a poorer one [X=H (1a) and OCH3 (1b)], the reaction mechanism changed to the competing E2 and E1cb mechanisms. A further change to the Elcb mechanism was realized for the reaction of la with i-Pr2NH/i-Pr2NH2+ in 70 mol % MeCN-30 mol % D2O. By comparing the kinetic results in this study with the existing data for ArCH2C(O)OC6H3-2-X-4-NO2, the effect of the beta-aryl group on the ketene-forming elimination was assessed.