SUBSTITUENT EFFECTS ON THE DEHYDRATION OF ARENE HYDRATES IN AQUEOUS SOLUTION
at ionic strength, I = 0.5 (NaClO4). A plot of 10-pH(obs) against the concentra-
tion of hydronium ion yield the activity coefficient as slope.
under the supervision of RMOF and the authors would like
to thank the University College Dublin for support of this work.
For the more recent synthesis/study of all other arene hydrates
reported herein, the authors acknowledge EPSRC (UK) for
financial support.
1H NMR analysis of the dehydration of arene hydrates
A typical experiment involved accurately transferring buffer or dilute acid
solution (890 μL) by microlitre syringe to a reaction vial. To this solution,
internal standard stock solution (10 μL) was added. Substrate stock
solution (100 μL) was added to the buffered solution and mixed vigor-
ously. An aliquot of the reaction mixture (750 μL) was transferred to an
NMR tube, and the reaction was followed in situ by 1H NMR spectroscopy.
1H NMR spectra of the solution dehydration reactions of hydrates were
recorded on an Oxford Varian Inova 400 or 500 spectrometer with a
relaxation delay of 18 s, sweep width of 7996.8 Hz, and acquisition time
of 6 s, and a 90° pulse angle. Spectra were run with 32 transients with a
total acquisition time of 12 min and 32 s.
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UV–visible spectrophotometry
For UV–visible spectrophotometric analyses, Cary 100 and Cary 50
spectrophotometers were used. Quartz cuvettes (1 cm, 3 mL) fitted with
Teflon caps were employed, and the temperature in the cell compart-
ment was maintained at 25.0 0.1 °C.
A typical kinetics run was carried out by accurately pipetting 3 mL of
buffer or HClO4 solution into a cuvette and allowing it to equilibrate at
25 °C. The reaction was initiated by addition of the substrate stock
solution (30 μL) in acetonitrile by microlitre syringe. Mixing was achieved
by inverting the cuvette three times. The cuvette was then inserted into
the spectrophotometer, and the collection of the absorbance versus time
data was started at this point. The kinetic reactions were generally
followed at the λmax of the reactants, and where possible the kinetic
reactions were followed for both the appearance of products and the
disappearance of the reactants.
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Data analysis
The results obtained were analysed using the Sigma-plot (Version 8.02)
statistics software. The absorbance versus time data were fitted to a
single exponential equation (Eqn 3) and/or the double exponential
equation (Eqn 4).
y ¼ y0 þ Aeꢀbx
(3)
(4)
y ¼ y0 þ Aeꢀbx þ Ceꢀdx
The first-order rate constants are obtained as the parameters b and d
in the equations above.
For data that fitted well to Eqn 3, the first-order rate constant was also
obtained from the slope of a semilogarithmic plot of the change in absor-
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followed for approximately 10 half-lives so as to reliably estimate A∞.
For reactions performed in acetic acid buffers, the observed pseudo-
first-order rate constants at a given pH are quoted both as the mean of
the values at different buffer concentrations and also as the intercept,
kint, of the plot of kobs against buffer concentration at a fixed pH.
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[10] The mono-epoxides used in the final ring-opening step were a
mixture of diastereomers.
SUPPORTING INFORMATION
[11] A ratio of kHClO4/kDClO4 ∼ 1 was observed for the dehydration re-
action of arene hydrate 8e. A rate constant of kDClO4 = 2.0 ( 0.2) ꢂ
10–4 s-1 was determined by 1H NMR spectroscopy for the reaction
of 8a in 2.5 mM DClO4 in D2O solution. A rate constant of kHClO4 =
2.25 ( 0.08) ꢂ 10–4 s-1 can be calculated for reaction of 8a in 2.5
mM HClO 4 in H2O solution from the second order rate constant
for acid-catalyzed dehydration kH = 0.0899 M-1s-1, thus giving a
kHClO4/ kDClO4 of ~1.
The details of synthetic procedures and the kinetic data are in-
cluded in the Supporting Information.
Acknowledgements
This report originated with the previously unpublished study
[12] Preliminary calculations using Gaussian 2003 B3Lyp6-31g** level of
of arene hydrates 8b and 8c during the PhD studies of ACO
theory have shown that the presence of the hydroxyl group in the
J. Phys. Org. Chem. 2013, 26 989–996
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