Reactions of Aryl Phenylacetates with Secondary Amines in MeCN. Structure−Reactivity Relationship in the Ketene-Forming Eliminations and Concurrent E2 and E1cb Mechanisms

Abstract

Elimination reactions of aryl esters of arylacetic acids 1 and 2 promoted by R2NH in MeCN have been investigated kinetically. The reactions are second-order and exhibit β = 0.44−0.84, |βlg| = 0.41−0.50, and ρH = 2.0−3.6. Brønsted β and |βlg| decrease with the electron-withdrawing ability of the β-aryl substituent. Hammett ρH values remain nearly the same, but the |βlg| value increases as the base strength becomes weaker. Both ρH and β decrease with the change of the leaving group from 4-nitrophenoxide to 2,4-dinitrophenoxide. The results are consistent with an E2 mechanism and a reaction coordinate with a large horizontal component corresponding to proton transfer. When the base−solvent system is changed from R2NH−MeCN to R2NH/R2NH2+−70 mol % MeCN(aq), the Brønsted β, ρH, and |βlg| decrease. Finally, the ketene-forming elimination reactions from p-nitrophenyl p-nitrophenylacetate promoted by R2NH/R2NH2+ buffers in 70 mol % MeCN(aq) have been shown to proceed by concurrent E2 and E1cb mechanisms.