Decarboxylation of 6-Nitrobenzisoxazole-3-carboxylate with Ultrasonic Irradiation. The Possibility of the Formation of Supercritical Water

Full text of DOI:10.1021/jo980370u

6
048
J . Org. Chem. 1998, 63, 6048-6049
Ta ble 1. Ra te Con sta n ts for th e Deca r boxyla tion of
Deca r boxyla tion of
-Nitr oben zisoxa zole-3-ca r boxyla te w ith
Ultr a son ic Ir r a d ia tion . Th e P ossibility of
th e F or m a tion of Su p er cr itica l Wa ter
6
-Nitr oben zisoxa zole-3-ca r boxyla te
temp (°C)a k (s-1
Method A
6
)
-
-
6
6
ultrasound
30.4
8.32 × 10
8.29 × 10
3.10 × 10
8.23 × 10
1.26 × 10
1.35 × 10
6.17 × 10
3.58 × 10
1.89 × 10
3
4
4
4
4
1.4
0.0
0.4
8.4
9.7
Takashi Ando,*,† Mitsue Fujita, Takahide Kimura, and
†
†
-5
-5
Yasuhiko Kondo‡
-
-
-
-
-
4
4
6
5
4
Department of Chemistry, Shiga University of Medical
Science, Seta, Otsu, Shiga 520-2192, J apan and Department
of Natural Science, Osaka Women’s University,
Sakai, Osaka 590-0035, J apan
stirring
30.4
4
5
0.5
0.7
Method B
Received February 26, 1998
-
-
-
-
-
6
6
6
5
5
ultrasound
30.0
30.2
6.56 × 10
6.35 × 10
6.95 × 10
2.26 × 10
2.04 × 10
2.33 × 10
In 1995, Hoffmann and co-workers proposed the pos-
sible formation of transient supercritical water in the
hydrolysis of p-nitrophenyl acetate with ultrasonic ir-
radiation.1 Their proposal depends on the observations
of rate acceleration of 2 orders of magnitude, the rate
independence of pH and ionic strength, and characteristic
activation parameters. As it is an established idea that
acoustic cavitation produces high temperatures and
pressures exceeding the critical values of water (647 K
and 221 bar), there is a distinct possibility of the
formation of supercritical water, although its lifetime and
volume might be questionable. Considering the rapidly
increasing importance of supercritical water as a reaction
medium of chemical transformations, we tried to detect
the possible formation of supercritical water with a
different approach.
3
3
3
3
0.7
7.9
8.5
8.9
-5
0.90 × 10-4
48.5
48.8
-
-
4
4
1.14 × 10
0.97 × 10
4
9.3
stirringb
30.4
(6.80 ( 0.13) × 10
(3.71 ( 0.12) × 10
(1.65 ( 0.05) × 10
-6
-
5
4
5
0.5
0.7
-4
a
For stirring, (0.1 °C; for ultrasound, see the text. b Average
of two runs.
decarboxyation rate of benzisoxazole-3-carboxylates must
vary considerably with ultrasonic irradiation if a super-
critical state is formed.
Decarboxylation was carried out for 6-nitrobenzisox-
azole-3-carboxylic acid (1) as a substrate in phosphate-
buffered solutions at pH 7.0 under argon. Reaction
solutions were prepared with two methods: (A) dilution
of a concentrated ethanol solution of 1 with a buffered
solution or (B) direct dissolution into a buffered solution
and filtration. The reaction was followed spectrophoto-
metrically at 395 nm. Isosbestic points were observed
in all the kinetic experiments with both ultrasonic
irradiation and magnetic stirring. A horn type sonicator,
Astrason Sonicator XL 2020 (Heat System-Ultrasonics,
The rates of decarboxylation of benzisoxazole-3-car-
boxylates (eq 1) are strongly dependent on the reaction
medium. In the detailed solvent-effect studies, Kemp and
co-workers showed that the first-order rate varies up to
8
orders of magnitude on going from reaction in water
2
0 kHz), was used for ultrasonic irradiation at an acoustic
to reaction in dipolar aprotic solvents such as N,N-
4
dimethylformamide.2 This has been explained by (1)
intensity of 12 ( 1 W. Excellent linear plots (r > 0.999)
were obtained for more than two half-lives except for the
kinetic runs at low temperatures with ultrasonic irradia-
tion, in which precipitation of metal powder hindered the
accuracy (r > 0.997 for one half-life).
stabilization of the starting carboxylates by hydrogen
bonding in protic media and (2) stabilization of the
charge-delocalized transition state through dispersion
interactions with polarizable media. The reaction has
The results are summarized in Table 1. It is evident
that the effect of ultrasonic irradiation is not large. The
ratios of the calculated rate constants at 40 °C with
ultrasonic irradiation versus magnetic stirring are 1.01
for method A and 0.87 for method B. Two methods of
preparation of reaction solutions, A and B, were adopted
to avoid the possible disturbance of the formation of
been widely used to investigate the microenvironment
of the medium.3 As supercritical water has solvent
properties completely different from those of water under
normal conditions, it is reasonable to expect that the
*
Corresponding author. Phone: +81-77-548-2108. Fax: +81-77-
5
48-2405. E-mail: ando@sums.shiga-med.ac.jp.
†
1
Shiga University of Medical Science.
Osaka Women’s University.
supercritical water with an alcohol cosolvent, but this
‡
choice does not have much effect. Activation parameters
are calculated as shown in Table 2. All the kinetic data
(
1) Hua, I.; H o¨ chemer, R. H.; Hoffmann, M. R. J . Phys. Chem. 1995,
9, 2335-2342.
2) Kemp, D. S.; Paul, K. G. J . Am. Chem. Soc. 1975, 97, 7305-
312.
3) (a) Bunton, C. A.; Minch, M. J .; Hidalgo, J .; Sepulveda, L. J .
9
(
2
are similar to those reported by Kemp et al.
7
Somewhat slower rates with ultrasonic irradiation,
especially apparent for the method B series, can be
attributed to the temperature inhomogeneity inside the
(
Am. Chem. Soc. 1973, 95, 3262-3272. (b) Shirai, M.; Smid, J . J . Am.
Chem. Soc. 1980, 102, 2863-2865. (c) Kunitake, T.; Okahata, Y.; Ando,
R.; Shinkai, S.; Hirakawa, S. J . Am. Chem. Soc. 1980, 102, 7877-
7
1
5
881. (d) Grate, J . W.; McGill, R. A.; Hilvert, D. J . Am. Chem. Soc.
993, 115, 8577-8584. (e) Lee, J .-J .; Ford, W. T. J . Org. Chem. 1993,
8, 4070-4077.
(4) Kimura, T.; Sakamoto, T.; Leveque, J .-M.; Sohmiya, H.; Fujita,
M.; Ikeda, S.; Ando, T. Ultrason. Sonochem. 1996, 3, S157-S162.
S0022-3263(98)00370-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 07/23/1998

Notes
J . Org. Chem., Vol. 63, No. 17, 1998 6049
Ta ble 2. Activa tion P a r a m eter s for th e
Deca r boxyla tion of 6-Nitr oben zisoxa zole-3-ca r boxyla tea
in the decarboxylation of 1 does not necessarily mean that
supercritical water is not formed in the water with
ultrasonic irradiation, as the present reaction may not
be an adequate probe for the formation of supercritical
water.
Aqueous sonochemistry is generally attributed to the
initial decomposition of water for producing hydroxyl
radical and hydrogen or pyrolysis of substrates in the
process of cavitation. Another possibility of the mecha-
nism, the formation of supercritical water, must be
pursued further from a different point of view considering
the importance of aqueous sonochemistry in the indus-
trial application.
q
H (kJ mol-1)
∆S (J K mol-1
q
-1
∆
)
Method A
120 (4)
135 (1)
ultrasound
stirring
53 (14)
99 (1)
Method B
116 (3)
125 (3)
ultrasound
stirring
38 (10)
70 (9)
a
Estimated errors are shown in parentheses.
reaction vessel. Evolution of heat inside the vessel near
the horn and cooling outside the vessel cause the tem-
perature deviation of 1.3 °C at the maximum at the
reaction temperature of 50 °C. Throughout the series of
kinetic experiments, the temperature was measured at
the same position showing a rather high temperature.
It is also worth noting that differences in the activation
Exp er im en ta l Section
6-Nitr oben zisoxa zole-3-ca r boxylic a cid (1) was prepared
according to the literature,3
a,e,6
mp 188.0-189.5 °C (lit. mp 189-
6
1
90 °C).
q
q
parameters, smaller ∆H and ∆S with ultrasound, are
apparently greater than estimated errors (Table 2).
However, the temperature inhomogeneity described above
makes the detailed discussion difficult.
Deca r boxyla tion of 1 was carried out in phosphate-buffered
solutions at pH 7.0 under argon. Reaction solutions were
prepared with two methods. (A) 1 (18.3 mg) was dissolved in
ethanol (0.5 mL), and 0.25 mL of the ethanol solution was added
into 9.2 mL of the buffered solution. (B) 1 (10-12 mg) was
directly dissolved in a buffered solution (12 mL), and 9.4 mL of
the solution was used after filtration. The reaction was followed
spectrophotometrically at 395 nm by using a Shimadzu UV-
Vis recording spectrophotometer (UV-260).
Ultr a son ic ir r a d ia tion was administered by using an As-
trason Sonicator XL2020 (Heat System-Ultrasonics, 20 kHz).
The reactor was a cylindrical and jacketed glass vessel fitted
with an immersible 0.5 in. horn. The temperature inside the
reaction vessel was monitored with a thermocouple. In all the
sonochemical experiments, the electric power was 165 W and
the acoustic intensity was 12 ( 1 W.4 The same glass vessel
was used for reactions with magnetic stirring.
It is not easy to estimate the properties of such
transient supercritical water if it is formed. It is gener-
ally considered that supercritical water behaves like
organic solvents with low densities and low dielectric
constants such as acetone.5 The rate constant of decar-
6
boxylation of 1 in acetone was reported to be 3 × 10
times faster than that in water.2 Although there is a
possibility of a rate decrease caused by the higher ion
product of supercritical water, it must not be so large as
the decarboxylation rate is almost constant for the range
of pH above 3.2 Thus, the formation of supercritical
water with ultrasonic irradiation could not be proved in
this study. However, the absence of a sonochemical effect
J O980370U
(
5) Shaw, R. W.; Brill, T. B.; Clifford, A. A.; Ecker, C. A.; Franck, E.
(6) Lindemann, H.; Ciss e´ e, H. J ustus Liebigs Ann. Chem. 1929, 469,
44-57.
U. Chem. Eng. News 1991, 69 (51), 26-39.