Published on Web 12/06/2002
Kinetic and Thermodynamic Acidities of Substituted
1-Benzyl-1-methoxy-2-nitroethylenes. Strong Reduction of the
Transition State Imbalance Compared to Other Nitroalkanes
Claude F. Bernasconi,* Mahammad Ali,† and Janette C. Gunter
Contribution from the Department of Chemistry and Biochemistry of the UniVersity of
California, Santa Cruz, California 95064
Received August 30, 2002
Abstract: Acidity constants of six substituted 1-benzyl-1-methoxy-2-nitroethylenes (2-Z with Z ) m-NO2,
m-CF3, m-Cl, H, p-Me, p-MeO) and their respective nitronic acids were determined in 50% DMSO-50%
water (v/v) at 20 °C. Kinetic data were obtained on the reversible deprotonation of all six 2-Z by OH- and
piperidine and on the reversible deprotonation of 2-NO2 by piperazine, 1-(2-hydroxyethyl)piperazine, and
morpholine in the same solvent. These data allowed a determination of the Brønsted coefficients R
(dependence on acidity of 2-Z) and â (dependence on amine basicity). The fact that R > â indicates the
presence of a transition state imbalance which, however, is much smaller than that for the deprotonation
of arylnitromethanes. The reasons for this reduction in the imbalance and their relevance to a recent study
of the deprotonation of Fischer carbene complexes are discussed.
Introduction
lags behind the progress of proton transfer at the transition state.
This is shown, in exaggerated form, in eq 1 for a generalized
carbon acid with the π acceptor Y. The lag in the charge
Proton transfers involving nitroalkanes have generated an
unusual amount of interest over many years.1-16 A major reason
is that these reactions show some extreme features regarding
reactivity and transition state structure. One such feature is that
they have the highest intrinsic barriers17 or lowest intrinsic rate
constants17 for proton transfers in solution.8a,18 Another feature,
directly related to the first, is that the transition state imbalance
which is characteristic of proton transfers from carbon acids
activated by π acceptors18 manifests itself more strongly than
for any other type of carbon acids.2,8d,18
delocalization is responsible for the high intrinsic barriers of
these reactions.18 The increased barriers arise because the
transition state cannot take advantage of the resonance stabiliza-
The term “imbalance” refers to the observation that the de-
localization of the incipient negative charge into the π acceptor
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* Address correspondence to this author. E-mail: bernasconi@
chemistry.ucsc.edu.
† Current address: Department of Chemistry, Jadavpur University,
Kolkata 700 032, India.
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10.1021/ja0211398 CCC: $25.00 © 2003 American Chemical Society
J. AM. CHEM. SOC. 2003, 125, 151-157
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