BULLETIN OF THE
Article
Quinuclidinolysis of O-Aryl Thionobenzoates
KOREAN CHEMICAL SOCIETY
benzylamine although the former is ~2.1 pKa units more basic
than the latter. Steric hindrance exerted by the tertiary amine is
responsible for the decreased reactivity. (2) The Hammett plot
for the reactions of 2a–2f with quinuclidine consists of two
intersecting straight lines, while the Yukawa–Tsuno plot for
the same reaction exhibits an excellent linear correlation with
ρX = 1.10 and r = 0.71, indicating that the nonlinear Hammett
plot is not due to a change in the RDS but is caused by GS sta-
bilization through resonance interactions. (3) Brønsted-type
plot for the reactions of 1a with quinuclidines is linear with
βnuc = 0.89, which is typical for reactions reported to proceed
through a stepwise mechanism. However, the reactions have
been concluded to proceed through a concerted mechanism.
(4) Instability of a plausible intermediate T forces the reac-
tions to proceed through a concerted mechanism in order to
decrease steric hindrance. (5) The current reactions proceed
through a forced concerted mechanism with a tight TS (e.g.,
large bond formation with only a little leaving-group
expulsion).
constants (kquin) from the slope of linear plots of kobsd vs. qui-
nuclidine concentrations.
Products Analysis.
4-Nitrophenoxide or 3,4-
dinitrophenoxide ion was liberated quantitatively and identi-
fied as one of the products by comparison of the UV–vis spec-
tra after completion of the reaction with the authentic sample
under the same reaction conditions.
Acknowledgment. The publication cost of this paper is met
by the Korean Chemical Society.
References
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Experimental
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Materials. O-Aryl thionobenzoates (1a and 2a–2f) were pre-
pared as reported previously.10b The crude product was puri-
fied by column chromatography (silica gel, methylene
chloride/n-hexane 50/50). Their purity was checked by their
melting point and 1H NMR spectra. Quinuclidines and other
chemicals were of the highest quality available. Double dis-
tilled water was further boiled and cooled under nitrogen just
before use. Due to the low solubility of the substrates in pure
water, aqueous DMSO (80 mol% H2O/20 mol% DMSO) was
used as the reaction medium.
Kinetics. The kinetic study was performed with a UV–vis
spectrophotometer equipped with a constant-temperature
circulating bath. The reactions were followed by monitoring
the appearance of the leaving 4-nitrophenoxide or 3,4-
dinitrophenoxide ion. All the reactions were carried out under
pseudo-first-order conditions in which quinuclidine concen-
trations were at least 20 times greater than the substrate con-
centration. The quinuclidine stock solution of ~0.2 M was
prepared by dissolving 2 equiv of quinuclidine hydrochloride
and 1 equiv of standardized NaOH solution to make a self-
buffered solution in a 25.0-mL volumetric flask.
Typically, the reaction was initiated by adding 5 μL of a
0.02 M solution of the substrate in acetonitrile to a 10-mm
quartz UV cell containing 2.50 mL of the thermostated reac-
tion mixture made up of solvent and an aliquot of the quinu-
clidine stock solution. All the solutions were transferred by
gas-tight syringes. Generally, the quinuclidine concentration
was varied over the range 20–200 × 10−3 M, while the sub-
strate concentration was 2 × 10−5 M. Pseudo-first-order
rate constants (kobsd) were calculated from the equation: ln
(A∞ – At) = −kobsdt + C. The plots of ln (A∞ – At) vs. time were
linear over ~90% of the total reaction. Usually, five different
quinuclidine concentrations were employed, and replicate
values of kobsd were determined to obtain the second-order rate
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Bull. Korean Chem. Soc. 2015, Vol. 36, 1551–1556
© 2015 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim