Rein ter p r eta tion of Cu r ved Ha m m ett P lots in Rea ction of
Nu cleop h iles w ith Ar yl Ben zoa tes: Ch a n ge in Ra te-Deter m in in g
Step or Mech a n ism ver su s Gr ou n d -Sta te Sta biliza tion
Ik-Hwan Um,* Hyun-J oo Han, J ung-Ae Ahn, Swan Kang,1 and Erwin Buncel*,†
Department of Chemistry, Ewha Womans University, Seoul 120-750, Korea,
and Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
ihum@mm.ewha.ac.kr
Received August 21, 2002
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A kinetic study is reported for the reaction of the anionic nucleophiles OH-, CN-, and N3 with
aryl benzoates containing substituents on the benzoyl as well as the aryloxy moiety, in 80 mol %
H2O-20 mol % dimethyl sulfoxide at 25.0 °C. Hammett log k vs σ plots for these systems are
consistently nonlinear. However, a possible traditional explanation in terms of a mechanism
involving a tetrahedral intermediate with curvature resulting from a change in rate-determining
step is considered but rejected. The proposed explanation involves ground-state stabilization through
resonance interaction between the benzoyl substituent and the electrophilic carbonyl center in the
two-stage mechanism. Accordingly, the data are nicely accommodated on the basis of the Yukawa-
Tsuno equation, which gives linear plots for all three nuceophiles. Literature reports of the
mechanism of acyl transfer processes are reconsidered in this light.
In tr od u ction
philic attack at the carbonyl center occurs concertedly
with leaving group departure (eq 1), or whether reaction
occurs via a discrete tetrahedral intermediate (eq 2).
Due to their importance in biological processes as well
as in synthetic applications, numerous studies have been
performed to investigate the mechanism of acyl transfer
processes.2-12 One aspect of interest is whether nucleo-
† Queen’s University.
(1) Present address: Department of Biochemistry, Oxford Univer-
sity.
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J . Am. Chem. Soc. 1999, 121, 2508-2515. (c) Adalsteinsson, H.; Bruice,
T. C. J . Am. Chem. Soc. 1998, 120, 3440-3447.
In ester aminolysis, it is now firmly established that
the two-step mechanism obtains, in which the rate-
determining step (RDS) is dependent on the relative
basicity of the amine and the leaving group.3-5 Thus the
RDS changes from breakdown of the intermediate to its
formation as the attacking amine becomes more basic
than the leaving group by 4-5 pKa units.3-5
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In a series of important studies by Williams and co-
workers it was concluded that acyl transfer to aryloxide
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10.1021/jo026339g CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/26/2002
J . Org. Chem. 2002, 67, 8475-8480
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