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.
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