Elimination reactions of aryl furylacetates promoted by r ₂NH-r₂NH₂ ⁺ in 70 mol% MeCN(aq). effects of β-aryl on the ketene-forming transition-state

Ketene-forming elimination from 2-X-4-nitrophenyl furylacetates (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 Brönsted β = 0.37-0.54 and |βlg| = 0.31-0.45. The Brönsted β decreased with a poorer leaving group and |βlg| 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 E1cb mechanism was realized for the reaction of 1a 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-NO ₂, the effect of the β-aryl group on the ketene-forming elimination was assessed.