a larger degree of proton transfer, more negative charge
development at the â-carbon, and a greater extent of the
leaving group departure. It occurred to us that the effect
of the â-aryl group would be more pronounced in the
imine-forming eliminations for the following reasons.
First, the energy difference between CsN and CdN
bonds is significantly larger than that between CdN and
CtN bonds. Second, both the CâsH bond and developing
partial multiple bonds are parallel to the π-orbital of the
â-aryl group in the imine-forming transition state, whereas
they are orthogonal to it in the nitrile-forming transition
state. Hence, the â-aryl group is expected to stabilize the
imine-forming more than the nitrile-forming transition
state.
To explore the relative importance of these factors, we
have investigated the reactions of N-alkyl-N-chloro-
thenylamines 1-4 with MeONa-MeOH and Et2NH-
MeCN. We have determined kH/kD, Hammett F, ∆Hq, and
∆Sq values for the elimination from 1-4. The mechanism
of the elimination reactions, the effect of base-solvent,
and the steric effect have been studied. From these
results, the effect of changing the â-aryl group from Ph
to thienyl on the imine-forming transition states is
assessed.
Elimination Reactions of
N-Alkyl-N-chlorothenylamines Promoted
by MeONa-MeOH and Et2NH-MeCN.
Effect of the â-Aryl Group on the
Imine-Forming Transition State
Sang Yong Pyun,† Dong Choon Lee,†
Yoon Je Seung,† and Bong Rae Cho*,‡
Department of Chemistry, Pukyong National University,
Pusan 608-737, Korea, and Department of Chemistry,
Korea University, 1-Anamdong, Seoul 136-701, Korea
chobr@korea.ac.kr; sypyun@pknu.ac.kr
Received February 28, 2005
Elimination reactions of N-alkyl-N-chlorothenylamines 1-4
with MeONa-MeOH and Et2NH-MeCN have been studied
kinetically. The elimination reactions are regiospecific,
producing only the conjugated imines. The reactions are
second order and exhibit substantial values of Hammett F
and kH/kD, and an E2 mechanism is evident. The relative
rates of elimination for Me/Et/i-Pr/t-Bu substituents are
1/0.5/0.2/0.02 with MeONa-MeOH and 1/0.4/0.2/0.06 with
Et2NH-MeCN. The transition state structure changes
toward more product-like as the base is changed from
MeONa-MeOH to Et2NH-MeCN. Comparison with exist-
ing data reveals that the structure of the transition state is
relatively insensitive to the â-aryl group variation.
Reactions of 1-4 with MeONa-MeOH and Et2NH-
MeCN produced only N-thenylidenealkylamines. Rates
of the elimination reactions were measured by monitoring
the appearance of the absorption at the λmax for the
products. Excellent pseudo-first-order kinetic plots that
covered at least 3 half-lives were obtained. Pseudo-first-
order rate constants (kobs) are summarized in Tables S1
and S2 in Supporting Information. The plots of kobs versus
base concentration for 1-4 are straight lines passing
through the origin, indicating that the reactions are
second-order, first order to the substrate and first order
to the base (Figure S1 and S2). The second-order rate
constants k2 were obtained either from the slopes of
straight lines or by dividing the kobs by base concentra-
tion. Values of k2 for eliminations from 1-4 are sum-
marized in Tables 1 and 2. Relative rates of elimination
for 1a/2a/3a/4a at 25 °C are 1/0.5/0.2/0.02 with MeONa-
MeOH and 1/0.4/0.2/0.06 with Et2NH-MeCN.
Extensive studies of the structure-reactivity relation-
ships in the imine-forming elimination reactions from
XC6H4CH2N(Y)R have led to the qualitative understand-
ing of the relationship between the reactant structure
and the E2 transition state.1-5 In contrast, virtually
nothing is known about the corresponding eliminations
involving heterocyclic aromatic compounds.
Earlier, we reported that the nitrile-forming elimina-
tion from (Z)-ArCHdNOC(O)Ar′ proceeds by the E2
mechanism via a symmetrical transition state.6,7 When
the â-aryl group was changed from Ph to thienyl to furyl,
the rates remained nearly the same and the transition
state structure changed slightly toward product-like with
* Address correspondence to this author. Phone: 82-2-3290-3129
† Pukyong National University.
The influence of the N-alkyl group upon the rates for
the MeONa-promoted eliminations from 1a-4a cor-
related reasonably well with Charton’s equation using
ν' values.12,13 On the other hand, the rate data for the
‡ Korea University.
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(9) Thies, M.; Schoeneberger, H.; Borah, K. J. Arch. Pharm. (Wein-
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(5) Cho, B. R.; Pyun, S. Y. J. Am. Chem. Soc. 1991, 113, 3920-
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(6) Cho, B. R.; Cho, N. S.; Lee, S. K. J. Org. Chem. 1998, 63, 4685-
4690.
(11) Wilson, C. V.; Stenberg, J. F. Organic Syntheses; Wiley: New
York, 1963; Collect. Vol. IV, pp 564-565.
(12) Charton, M. J. Am. Chem. Soc. 1975, 97, 1552-1556.
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10.1021/jo050368k CCC: $30.25 © 2005 American Chemical Society
Published on Web 06/01/2005
J. Org. Chem. 2005, 70, 5327-5330
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