Mechanistic study of oxidative aromatization using activated carbon-molecular oxygen system in the synthesis of 2-arylbenzazoles:focus on the role of activated carbon

Article abstract of DOI:10.1246/bcsj.82.482

Nineteen activated carbons, including sixteen tailor-made and three commercially available ones, were employed to investigate their role in the one-pot synthesis of 2-arylbenzazoles starting from 2-aminophenol, 1,2-diaminobenzene, or 2-aminobenzenethiol, and aldehyde via oxidative cyclization of intermediate phenolic Schiff base promoted by the activated carbon-molecular oxygen system. Tailor-made activated carbons were prepared from wood, coconut shell, and coal by the steam activation method or the chemical activation method. All activated carbons were characterized to clarify their properties, such as metal contaminants, specific surface area, porosity, and surface functionality. After examining their effects on reactivity in the oxidative aromatization, we found that the essential role of activated carbon in this oxidation system is concerned not with metal contaminants, specific surface area, pore volume, mean pore diameter, and surface oxygen groups evolving as CO₂ at 900 °C but with surface oxygen groups evolving as CO during heating at 900 °C, such as carbonyl groups on the surface of activated carbon. Additionally, in the synthesis of 2-arylbenzoxazole, it was found that activated carbon would also promote the cyclization of the intermediate Schiff base.

Full text of DOI:10.1246/bcsj.82.482

4
82
Bull. Chem. Soc. Jpn. Vol. 82, No. 4, 482–488 (2009)
© 2009 The Chemical Society of Japan
Mechanistic Study of Oxidative Aromatization Using Activated Carbon­
Molecular Oxygen System in the Synthesis of 2-Arylbenzazoles:
Focus on the Role of Activated Carbon
1
2
2
1
Yuka Kawashita, Juichi Yanagi, Tatsuo Fujii, and Masahiko Hayashi*
1Department of Chemistry, Graduate School of Science, Kobe University, Nada, Kobe 657-8501
2Japan EnviroChemicals, Ltd., Konohana, Osaka 554-0051
Received October 21, 2008; E-mail: mhayashi@kobe-u.ac.jp
Nineteen activated carbons, including sixteen tailor-made and three commercially available ones, were employed to
investigate their role in the one-pot synthesis of 2-arylbenzazoles starting from 2-aminophenol, 1,2-diaminobenzene, or 2-
aminobenzenethiol, and aldehyde via oxidative cyclization of intermediate phenolic Schiff base promoted by the activated
carbon­molecular oxygen system. Tailor-made activated carbons were prepared from wood, coconut shell, and coal by the
steam activation method or the chemical activation method. All activated carbons were characterized to clarify their
properties, such as metal contaminants, specific surface area, porosity, and surface functionality. After examining their
effects on reactivity in the oxidative aromatization, we found that the essential role of activated carbon in this oxidation
system is concerned not with metal contaminants, specific surface area, pore volume, mean pore diameter, and surface
oxygen groups evolving as CO2 at 900 °C but with surface oxygen groups evolving as CO during heating at 900 °C, such
as carbonyl groups on the surface of activated carbon. Additionally, in the synthesis of 2-arylbenzoxazole, it was found
that activated carbon would also promote the cyclization of the intermediate Schiff base.
Benzazole ring moieties are often found in compounds that
oxidation of secondary benzylic alcohols18 to corresponding
ketones were also performed. In this article, we will report the
mechanism of this activated carbon­molecular oxygen system
in the synthesis of 2-arylbenzoxazoles, with focus on the role
of activated carbon.
exhibit a variety of biological activities, including antitumor,
antimicrobial, and antiviral properties.1 In the case of 2-
substituted benzoxazoles, three general methods are available
for their synthesis. One is the coupling of 2-aminophenols with
carboxylic acid derivatives catalyzed by strong acids or
requiring microwave conditions.2 Recently, the palladium-
Results and Discussion
catalyzed coupling reactions of bromo- or iodobenzene with
benzimidazole or benzoxazole were reported. Another method
is the oxidative cyclization of phenolic Schiff bases derived
from the condensation reaction of 2-aminophenols with alde-
A variety of functionalized 2-arylbenzoxazoles, 2-arylbenz-
imidazoles, and 2-arylbenzothiazoles were synthesized in high
yields in a one-pot reaction using the activated carbon­
3
11,19
molecular oxygen system (Scheme 1 and Table 1).
The use
4
hydes. In the last reaction, various oxidants, such as DDQ,
of activated carbon available from Tokyo Chemical Industry
Co., Ltd., Shirasagi KL (Japan EnviroChemicals, Ltd.), and
5
6
+
¹ 7
8
9
Mn(OAc) , PhI(OAc) , Th ClO , BaMnO , NiO , and
3
2
4
4
2
1
0
µ
Pb(OAc)4, have been used. However, stoichiometric or excess
amounts of all these oxidants were required. Therefore, a more
efficient process is strongly desired.
Darco KB (Aldrich Inc.) are recommended. During the course
of detailed study of this reaction, we found that
the activated carbons had different reactivities. Therefore, we
investigated the reaction mechanism, particularly the role of
activated carbon.
In response to those requirements, we developed practical
and environmentally friendly one-pot syntheses of 2-arylbenz-
11
oxazoles. The reactions proceeded smoothly, starting from 2-
aminophenols and aldehydes under oxygen atmosphere in the
presence of activated carbon using xylene as solvent. Although
several dehydrogenation reactions using activated carbon as
catalyst have been employed in the vapor-phase reaction of
Rate-Determining Step. To investigate which reaction
step in Scheme 1 was promoted by activated carbon, several
experiments were performed. First, time courses of the
2
starting material benzaldehyde (R = H in 2 in Scheme 1),
1
2
the intermediate 2-(benzylideneamino)phenol (R = R = H
1
2
13
1
2
ethanol and ethylbenzene, ours is the first approach to the
synthesis of functional molecules using activated carbon as
catalyst in the liquid phase. The present activated carbon­
molecular oxygen system is also applicable to the syntheses
in 3a), and the product 2-phenylbenzoxazole (R = R = H in
1
5a) were measured by H NMR analysis (Figure 1). Reactions
1
were carried out using 0.5 mmol of 2-aminophenol (R = H in
2
1a), 0.5 mmol of benzaldehyde (R = H in 2), and 50 mg of
1
4
15
15
16
µ
of anthracenes, pyridines, pyrazoles, and indoles by
oxidative aromatization of the corresponding dihydro com-
activated carbon (Darco KB) in p-xylene-d (2 mL) at 120 °C
1
0
under oxygen atmosphere. Anthracene (0.1 mmol) was used as
an internal standard to estimate the yield of the product by
1
7
pounds. Benzylic oxygenation reaction of alkylarenes and
Published on the web April 11, 2009; doi:10.1246/bcsj.82.482

Y. Kawashita et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 4 (2009)
483
O2
XH
X
activated carbon
OHC
R2
R2
+
R1
xylenea
R1
N
NH2
1
1
1
a: X = O
b: X = NH
c: X = S
2
5a: X = O
5b: X = NH
5c: X = S
XH
N
X
R2
R1
R1
N
H
R2
3
3
3
a: X = O
b: X = NH
c: X = S
4a: X = O
4b: X = NH
4c: X = S
a
o-, m-, p-xylene mixture.
Scheme 1. One-pot synthesis of 2-arylbenzazoles.
Table 1. One-Pot Synthesis of Various 2-Arylbenzazolesa)
O2
OH
O
activated carbon
p-xylene-d10
+
OHC
O2
XH
NH2
R1 = H in 1a
N
activated carbon
120 °C
R2 = H in 2
R1 = R2 = H in 5a
R2
+
OHC
R1
NH2
xyleneb)
100
X
R2
R1
N
80
Entry
X
R1
R2
Temp/°C Time/h Yield/%c)
d)
6
4
0
0
1
2
3
4
5
6
7
8
9
0
O
O
O
O
O
O
O
O
H
H
H
H
H
H
H
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
50
4
4
4
6
4
4
4
24
29
21
2
3
2
2
2
3
3
3
4
78
79
76
88
87
86
82
82
67
83
79
60
72
67
72
79
82
81
72
82
86
d)
e)
e)
CH3
OCH3
Cl
CN
NO2
H
20
d)
CH3
NO2
NO2 OCH3
H
O
O
0
100
200
300
time/min
1
NO2 Cl
Darco KB :
product,
product,
Schiff base,
Schiff base,
aldehyde
aldehyde
1
1
NH
NH
NH
NH
NH
S
S
S
S
S
H
H
H
H
H
H
H
H
H
H
H
H
Darco G-60 :
12
13
14
15
16
17
18
19
20
21
CH3
OCH3
Cl
NO2
H
CH3
OCH3
Cl
CN
NO2
Figure 1. Time course of 2-phenylbenzoxazole, Schiff
base, and aldehyde using activated carbon as catalyst.
µ
Darco KB and G-60, which have different properties
as shown in Table 2, were employed. Reactions were
performed using 2-aminophenol (0.5 mmol), benzaldehyde
(0.5 mmol), and activated carbon (50 mg) in p-xylene-d10
(2 mL) at 120 °C under O2. Yields were determined by
d)
d)
50
50
50
50
1
4
4
H NMR analyses. Anthracene (0.1 mmol) was used as
S
50
internal standard.
a) The ratio of 2-aminophenol, 1,2-diaminobenzene, and 2-
aminobenzenethiol to aldehyde was 1:1, 1.1:1, and 1:1,
respectively. As for activated carbon, one gram of Darco KB
1
1
2
H NMR analyses. We found that Schiff base (R = R = H in
µ
3a) was generated at an early stage in both reactions employing
high- and low-reactivity activated carbons (Darco KB and G-
6
to prospective cyclization product (R = R = H in 4a) of
Schiff base (R = R = H in 3a) did not appear in the H NMR
spectra measured at room temperature when the reaction was
µ
(
Aldrich Inc.) was used per 8 mmol of aldehyde for Entries 1­15
0, respectively). In addition, we found that the peak assignable
and 0.63 g of Shirasagi KL (Japan EnviroChemicals, Ltd.)
was used per 5 mmol of aldehyde for Entries 16­21. b) o-,
m-, p-xylene mixture. c) Isolated yields by recrystallization
unless otherwise noted. d) Isolated yields by silica gel column
1
2
1
2
1
1
1
2
chromatography. e) H NMR analysis.
incomplete. To confirm that cyclization product (R = R = H

4
84
Bull. Chem. Soc. Jpn. Vol. 82, No. 4 (2009)
Oxidation Using Activated Carbon­O2 Oxygen System
OH
N
such as Zn, Co, Mn, Fe, Cr, Mg, V, Cu, Pd, Ca, Na, and K,
were measured by ICP-AES in washed and unwashed samples.
We confirmed that the amounts of those metal contaminants
decreased after washing (concerning the contents and amounts
of metal contaminants, see Supporting Information). The yields
O
OCH3
N
H
OCH3
R1 = H, R2 = OCH3 in 3a
R1 = H, R2 = OCH3 in 4a
1
2
of 2-(4-methoxyphenyl)benzoxazole (R = H and R = OCH
3
-
N=CH
in 5a) using washed activated carbons were higher than those
using unwashed activated carbons (Figure 3). Therefore, we
concluded that the driving factor of this oxidation system was
not the metal contaminants in activated carbon but other
factors. The reason why washed activated carbons exhibited
higher yields than unwashed activated carbons will be
discussed in the next section.
-
OH
1
10 ˚C
8
0 ˚C
0 ˚C
5
Role of Activated Carbon: Investigation of the Effects of
Specific Surface Area, Pore Volume, Mean Pore Diameter,
and Surface Functionality. Then, we investigated the role of
2
0 ˚C
1
activated carbons in the reaction of 2-aminophenol (R = H in
8.4
8.2
8.0
7.8
7.6
7.4
7.2
7.0
6.8
6.6
2
(
ppm)
1a in Scheme 1) with 4-methoxybenzaldehyde (R = OCH in
3
2
) via oxidative cyclization of intermediate 2-[(4-methoxy-
1
Figure 2. Temperature effect on H NMR spectrum of inter-
1
2
1
benzylidene)amino]phenol (R = H and R = OCH in 3a). To
3
mediate 2-[(4-methoxybenzylidene)amino]phenol (R = H
and R = OCH3 in 3a in Scheme 1).
2
examine the effects of specific surface area, pore volume, mean
pore diameter, and surface functionality, we employed nineteen
activated carbons, including sixteen tailor-made activated
carbons (Entries 1­16 in Table 2) and three commercially
available activated carbons (Entries 17­19). These activated
carbons were obtained by using different raw materials and
activation methods, and therefore, they have different specific
surface areas, pore volumes, mean pore diameters, and surface
oxygen group contents and amounts, as summarized in Table 2.
in 4a) was produced, a >C­H peak should appear at around
ppm instead of the 8.23 ppm peak that was assignable to
6
­
N=C­H.
Next, to investigate the effect of temperature we measured
H NMR spectra of the intermediate 2-[(4-methoxybenzyl-
idene)amino]phenol (R = H and R = OCH in 3a in
1
1
2
3
Scheme 1) in p-xylene-d10 at 20, 50, 80, and 110 °C, respec-
Specific surface area was measured using the Brunauer­
1
20,21a,21c
tively. As shown in Figure 2, cyclization product (R = H and
Emmett­Teller (BET) method
with nitrogen gas as
2
R = OCH in 4a) was not detected in all cases. On the other
adsorbate. Pore volume and mean pore diameter were calcu-
3
1
20,21a,21b
hand, cyclization products 4c was detected in the H NMR
lated according to the Cranston­Inkley (CI) method
spectra even at room temperature (see Supporting Information).
Thus, we concluded that in the reaction of 1a with 2, the
equilibrium preferred Schiff base 3a to cyclization product 4a
even at 110 °C without activated carbon because the nucleo-
philicity of O is less than that of S. Based on these results,
activated carbon would promote not only the dehydrogenation
step (4a ¼ 5a) but also the cyclization step (3a ¼ 4a).
Role of Activated Carbon: Investigation of the Effect of
Metal Contaminants. We investigated the role of activated
from the volume of adsorbed N . Oxygen content was
determined by GC measurement of evolved gases, such as
2
CO and CO , at 900 °C, which originated in decomposed
2
surface oxygen groups of 1 g of activated carbon.
Figure 4 shows the relationship between the properties of
activated carbons and the yields in oxidative aromatization. It is
clear that chemically activated carbons (filled symbols and
in Figure 4) had higher reactivity than steam-activated carbons
(unfilled symbols
,
, and in Figure 4). Then, we turned
1
carbon in the reaction of 2-aminophenol (R = H in 1a in
our attention to surface oxygen groups because the chemical
activation method was usually conducted at a lower temper-
ature than the steam activation method and thus more surface
oxygen groups remained on the surface of activated carbons
obtained by the former method. As shown in Figure 4f, surface
oxygen groups that evolved as CO have the best tendency to
increase reaction yields among all the properties examined. The
2
Scheme 1) with 4-methoxybenzaldehyde (R = OCH in 2)
3
via oxidative cyclization of intermediate 2-[(4-methoxybenzyl-
1
2
idene)amino]phenol (R = H and R = OCH in 3a). First, we
3
examined the effect of metal contaminants in activated carbon
on reactivity in the oxidative aromatization, employing six
tailor-made activated carbons (samples A to F). These activated
carbons were made from wood (sample A), coal (sample B),
and coconut shell (samples C, D, E, and F). As for the
activation method, we used two methods: one was chemical
total oxygen amounts evolved as CO and CO and the oxygen
2
amounts evolved as CO did not have a strong relationship with
2
the reaction yields (Figures 4d and 4e). Here, CO would be
derived from phenol, carbonyl, and quinone groups, and CO2
would be derived from carboxyl and lactone groups that existed
activation using ZnCl (samples A, E, and F) and the other was
2
steam activation (samples B, C, and D).
2
0,21a
Samples A­F were treated with 3% HCl aq for 10 min and
then with water ten times with boiling. The reaction yields
using unwashed activated carbons (samples A to F) were
compared with those using washed activated carbons (samples
A¤ to F¤), respectively. Metal contaminants in activated carbon,
on original activated carbon surfaces.
Specific surface
area, pore volume, and mean pore diameter appeared to be little
related to the reaction yield (Figures 4a, 4b, and 4c).
Furthermore, to confirm the effect of oxygen groups evolved
as CO, the reaction was performed using activated carbon

Y. Kawashita et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 4 (2009)
485
O2
OH
O
0
.5 g of activated carbon
+
OHC
OCH3
OCH3
xylene
20 °C, 8 h
NH2
N
R1 = H, R2 = OCH3 in
5a
1
8
mmol
R1 = H in 1a
8 mmol
R2 = OCH3 in 2
1
00
Before wash
After wash
90
80
70
60
50
40
30
20
10
0
A
A’
B
B’
C
C’
D
D’
E
E’
F
F’
Activated carbon
Figure 3. Reaction yields using washed and unwashed activated carbons.
Table 2. Properties of Activated Carbons
b)
Vc)
/mL g¹1 /nm
Dd)
O-COe)
/mg g
e)
e)
Activation
method
S
O-CO2
/mg g
O-Total
/mg g
Entry Samplea)
Material
2
¹1
¹1
¹1
¹1
/m g
1
2
3
4
5
6
7
8
9
0
A
A¤
B
B¤
C
C¤
D
D¤
E
E¤
F
F¤
G
H
I
J
ZnCl2
ZnCl2
Steam
Steam
Steam
Steam
Steam
Steam
ZnCl2
ZnCl2
ZnCl2
ZnCl2
Steam
Steam
ZnCl2
ZnCl2
H3PO4
Wood
Wood
Coal
Coal
Coconut shell
Coconut shell
Coconut shell
Coconut shell
Coconut shell
Coconut shell
Coconut shell
Coconut shell
Wood
Wood
Wood
Wood
Wood
Wood
Coal
1455
1446
1042
1038
1230
1251
1640
1640
1413
1400
1279
1300
1033
995
1.310
1.294
0.541
0.536
0.553
0.567
0.776
0.777
0.742
0.730
0.569
0.589
0.603
0.575
1.007
0.803
1.260
1.313
0.625
3.60
3.58
2.08
2.06
1.80
1.81
1.89
1.90
2.10
2.09
1.78
1.81
2.34
2.31
3.63
3.24
3.42
3.49
2.60
31.3
32.4
15.0
17.7
12.2
16.6
15.0
20.0
29.3
39.6
31.1
34.7
22.6
19.9
37.1
43.4
29.9
30.8
13.5
17.0
14.0
7.4
6.6
5.7
6.1
6.5
6.0
15.7
18.7
11.8
12.4
26.2
27.7
35.3
42.6
11.9
14.3
7.3
48.3
46.4
22.4
24.3
17.9
22.8
21.5
26.0
45.0
58.3
42.9
47.1
48.8
47.6
72.5
86.1
41.8
45.2
20.8
1
1
1
12
13
14
15
16
17
18
19
1111
991
1474
1505
964
Darco KB
Darco KB-B H3PO4
Darco G60 Steam
a) X¤ indicates sample X that was treated with 3% HCl aq for 10 min and then with water ten times with boiling.
b) Specific surface area. c) Pore volume. d) Mean pore diameter. e) Amount of oxygen calculated from the amount of
CO and/or CO2 gas that evolved from 1 g of corresponding activated carbon at 900 °C.
whose surface oxygen groups were removed at 850 °C under
nitrogen atmosphere. As shown in Table 3, the yield was low
compared with that of the original activated carbon. This result
is evidence that surface oxygen groups evolved as CO during
the heating to 850 °C play an important role in the present
oxidative aromatization. One of the reasons why the reaction
gradually proceeded would be that molecular oxygen regen-
erated active surface oxygen groups and/or some effective
surface oxygen groups remained in spite of the heat treatment
at 850 °C. Similarly, washed activated carbons (in the previous
section) might promote this reaction more efficiently than
unwashed activated carbons because of the increase in the
number of surface oxygen groups evolved as CO at 900 °C
(Table 2 and Figures 3 and 4).

4
86
Bull. Chem. Soc. Jpn. Vol. 82, No. 4 (2009)
Oxidation Using Activated Carbon­O2 Oxygen System
O2
OH
O
0
.5 g of activated carbon
+
OHC
OCH3
OCH3
xylene
20 °C, 8 h
NH2
mmol
R1 = H in 1a
N
1
8
8 mmol
R1 = H, R2 = OCH3 in 5a
R2 = OCH3 in 2
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
90
(a)
A’
A
KB
(b)
A’
A
80
70
60
50
40
30
20
10
J
I
J
I
KB
E’
KB-B
E’
F
F
F’
KB-B
F’
E
E
D’
D
D’
B’
D
B’
C’
C’
H
H
G
B
G
C
C
B
G60
G60
8
00
1000
1200
1400
1600
KB
1800
0.4
0.6
0.8
1.0
1.2
1.4
−1
specific surface area/m2
g−1
pore volume/mL g
9
0
9
0
(c)
(d)
A’
A’
A
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
80
A
I
J
I
E’
E
J
KB
F’
E’
KB-B
70
F
F’
KB-B
F
E
60
50
40
30
20
10
D’
D
C’
D’
D
B’
B’
C’
H
G
H
G
C
C
B
B
G60
G60
1
.0
1.5
2.0
2.5
3.0
3.5
4.0
0
20
40
60
80
100
mean pore diameter/nm
amount of oxygen evolved as [CO + CO2]/mg g−1
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
90
80
70
60
50
40
30
20
10
(
e)
A’
(f)
A’
A
A
J
I
J
I
KB
F
E’
KB
E
E’
KB-B
KB-B
E
F
F’
F’
D’
B’
D’
G
D
C’
D
B’
C’
C
C
H
B
B
H G
G60
G60
0
10
20
30
40
50
10
20
30
40
50
−
1
amount of oxygen evolved as CO /mg g−1
amount of oxygen evolved as CO2/mg g
Figure 4. Effect of properties of activated carbons listed in Table 2 on reaction yields. (a) Specific surface area, (b) pore volume,
c) mean pore diameter, (d) amount of oxygen evolved as [CO + CO2], (e) amount of oxygen evolved as CO2, and (f) amount
of oxygen evolved as CO. : Chemical activation (wood), : chemical activation (coconut shell), : steam activation (wood),
(
:
steam activation (coconut shell), and : steam activation (coal).
Experimental
Conclusion
Syntheses of 2-Arylbenzoxazoles (Table 1, Entry 1).
A
Functionalized 2-arylbenzoxazoles, 2-arylbenzimidazoles,
mixture of 2-aminophenol (873 mg, 8 mmol), benzaldehyde
and 2-arylbenzothiazoles were synthesized via oxidative
aromatization of their dihydro compounds using the activated
carbon­molecular oxygen system. Detailed mechanistic studies
revealed that surface oxygen groups evolving as CO, such as
carbonyl groups on the surface of activated carbon, promoted
the dehydrogenation step in this activated carbon­molecular
oxygen system. Additionally, in the synthesis of 2-arylbenz-
oxazoles, activated carbon would also promote the cyclization
of intermediate Schiff base. It should be mentioned that
recovery and reuse of activated carbon were available.
µ
(
849 mg, 8 mmol), and Darco KB (1 g) in xylene (15 mL) was
placed in a 100-mL three-necked flask under oxygen atmosphere
and stirred at 120 °C for 4 h. Then, the reaction mixture was filtered
using Celite. After concentration of the filtrate, the product was
isolated by silica gel column chromatography to give a pale yellow
crystalline solid (1.2 g, 78%).
Syntheses of 2-Arylbenzimidazoles (Table 1, Entry 11).
A
µ
mixture of 1,2-diaminobenzene (952 mg, 8.8 mmol) and Darco
KB (1 g) in xylene (10 mL) was placed in a 100-mL three-necked
flask under oxygen atmosphere and stirred at 120 °C. After 0.5 h,

Y. Kawashita et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 4 (2009)
487
Table 3. Effect of Surface Oxygen Groups
This was placed in a washing vessel with filter cloth and 2% HCl
aq was added thereto. Stirring and washing were continued for 2 h
and then the water was drained off. Subsequently, the residue was
washed with water at 50 °C at a rate of 0.25 L h for 4 h. The
washed activated carbon was dried in an electric dryer held at
115 « 5 °C. The dried activated carbon was pulverized with a ball
mill. Thus, sample C was obtained.
O2
OH
0.5 g of activated carbon
+
OHC
OCH3
¹1
xylene
20 °C, 8 h
NH2
1
8
mmol
8 mmol
R = OCH3 in 2
R1 = H in1a
2
O
N
OCH3
Preparation of Sample D (Table 2, Entry 7). Coconut shell
was pulverized and regulated to a particle diameter within the
range of from 2.36 to 1.18 mm. It was heated to 350­550 °C over
1 h and then was subjected to stream activation at 850 °C for 4 h.
This was placed in a washing vessel with filter cloth and 8% HCl
aq was added thereto. Stirring and washing were continued for 2 h
and then the water was drained off. Subsequently, the residue was
1
2
R = H, R = OCH3 in 5a
Yield/%a)
Entry
Activated carbon
1
2
Sample Kb)
Pre-treated sample K
91
41
c)
1
¹1
a) H NMR analyses after silica gel column chromatography.
washed with water at 50 °C at a rate of 0.25 L h for 4 h. The
washed activated carbon was dried in an electric dryer held at
115 « 5 °C. The dried activated carbon was pulverized with a ball
mill. Thus, sample D was obtained.
¹
1
b) Oxygen amount evolved as CO and CO2 is 32.1 mg g
¹1
and 15.6 mg g , respectively. c) Surface oxygen group was
removed at 850 °C under nitrogen atmosphere. Oxygen amount
¹
1
¹1
evolved as CO and CO2 is 15.3 mg g and 13.0 mg g
respectively.
,
Preparation of Sample E (Table 2, Entry 9). To 50 g of dry
wood flour, 83 g of 60% ZnCl2 aq was added, mixed well, and
placed in a crucible, which was then covered with a lid. This was
placed in an electric furnace and was heated from 100 to 250 °C
over 2 h and from 250 to 550 °C over 1 h, held at that temperature
for 30 min, and then cooled. This was placed in a washing vessel
with filter cloth and 8% HCl aq was added thereto. Stirring and
washing were continued for 2 h and then the water was drained off.
Subsequently, the residue was washed with water at 50 °C at a rate
benzaldehyde (849 mg, 8 mmol) in xylene (5 mL) was added
¹1
slowly at the rate of 0.1 mL s with stirring for 2 h. Then, the
reaction mixture was filtered using Celite. After concentration of
the filtrate, the desired product was isolated by recrystallization to
give a yellow crystalline solid (1.2 g, 79%).
Syntheses of 2-Arylbenzothiazoles (Table 1, Entry 16).
mixture of 2-aminobenzenethiol (505 mg, 5 mmol), benzaldehyde
531 mg, 5 mmol), and Shirasagi KL (625 mg) in xylene (8 mL)
A
¹1
of 0.25 L h for 4 h. The washed activated carbon was dried in an
electric dryer held at 115 « 5 °C. The dried activated carbon was
pulverized with a ball mill. Thus, sample E was obtained.
(
was placed in a 100-mL three-necked flask under oxygen
atmosphere and stirred at 50 °C for 3 h. Then, the reaction mixture
was filtered using Celite. After concentration of the filtrate, the
product was isolated by silica gel column chromatography to give
a pale yellow crystalline solid (0.84 g, 79%).
Preparation of Sample F (Table 2, Entry 11). To 50 g of dry
wood flour, 50 g of 60% ZnCl2 aq was added, mixed well, and
placed in a crucible, which was then covered with a lid. This was
placed in an electric furnace and was heated from 100 to 250 °C
over 2 h and from 250 to 550 °C over 1 h, held at that temperature
for 30 min, and then cooled. This was placed in a washing vessel
with filter cloth and 8% HCl aq was added thereto. Stirring and
washing were continued for 2 h and then the water was drained off.
Subsequently, the residue was washed with water at 50 °C at a rate
Preparation of Sample A (Table 2, Entry 1). To 50 g of dry
wood flour, 140 g of 60% ZnCl2 aq was added, mixed well, and
placed in a crucible, which was then covered with a lid. This was
placed in an electric furnace and was heated from 100 to 250 °C
over 2 h and from 250 to 550 °C over 1 h, held at that temperature
for 30 min, and then cooled. This was placed in a washing vessel
with filter cloth and 8% HCl aq was added thereto. Stirring and
washing were continued for 2 h and then the water was drained off.
Subsequently, the residue was washed with water at 50 °C at a rate
¹1
of 0.25 L h for 4 h. The washed activated carbon was dried in an
electric dryer held at 115 « 5 °C. The dried activated carbon was
pulverized with a ball mill. Thus, sample F was obtained.
Preparation of Samples A¤, B¤, C¤, D¤, E¤, and F¤ (Table 2,
Entries 2, 4, 6, 8, and 12). Samples A, B, C, D, E, and F were
boiled with 3% HCl aq for 10 min, respectively. Each sample was
cooled, filtrated, and then the residue was boiled with water ten
times. Subsequently they were filtrated and dried in an electric
dryer held at 115 « 5 °C. Then samples A¤, B¤, C¤, D¤, E¤, and F¤
were obtained, respectively.
¹1
of 0.25 L h for 4 h. The washed activated carbon was dried in an
electric dryer held at 115 « 5 °C. The dried activated carbon was
pulverized with a ball mill. Thus, sample A was obtained.
Preparation of Sample B (Table 2, Entry 3).
Daxi coal
produced in Shanxi Province, China was used as a raw material.
This charcoal was pulverized and regulated to a particle diameter
within the range of from 2.36 to 1.18 mm. It was heated to 350­
Preparation of Sample G (Table 2, Entry 13). Commer-
cially available steam-activated carbon flour (made in Japan),
steam-activated carbon flour (made in Southeast Asia), and ZnCl2
activated carbon flour were mixed in the ratio of 4:5:1. 600 g of the
mixture was stirred with 6 L of 2% HNO3 aq for 2 h at 50 °C. This
was placed in a washing vessel with filter cloth. Subsequently, the
5
8
50 °C over 1 h and then was subjected to stream activation at
50 °C for 5 h. This was placed in a washing vessel with filter cloth
and 8% HCl aq was added thereto. Stirring and washing were
continued for 2 h and then the water was drained off. Subsequently,
the residue was washed with water at 50 °C at a rate of 0.25 L h
for 4 h. The washed activated carbon was dried in an electric dryer
held at 115 « 5 °C. The dried activated carbon was pulverized with
a ball mill. Thus, sample B was obtained.
¹
1
¹1
residue was washed with water at 50 °C at a rate of 2.5 L h for
24 h. The washed activated carbon was dried in an electric dryer
held at 115 « 5 °C. Thus, sample G was obtained.
Preparation of Sample C (Table 2, Entry 5). Coconut shell
was pulverized and regulated to a particle diameter within the
range of from 2.36 to 1.18 mm. It was heated to 350­550 °C over
Preparation of Sample H (Table 2, Entry 14). Commer-
cially available steam-activated carbon flour (made in Southeast
Asia) and ZnCl2-activated carbon flour were mixed in the ratio of
9:1. 600 g of the mixture was stirred with 6 L of 2% HNO3 aq for
1
h and then was subjected to stream activation at 850 °C for 3 h.

4
88
Bull. Chem. Soc. Jpn. Vol. 82, No. 4 (2009)
Oxidation Using Activated Carbon­O2 Oxygen System
2
h at 50 °C. This was placed in a washing vessel with filter cloth.
Science and Technology, Japan. The authors thank Professor
Jun-ichiro Setsune of Kobe University for help in temperature-
controlled H NMR analysis.
Subsequently, the residue was washed with water at 50 °C at a rate
¹1
1
of 2.5 L h for 24 h. The washed activated carbon was dried in an
electric dryer held at 115 « 5 °C. Thus, sample H was obtained.
Preparation of Sample I (Table 2, Entry 15). Commercially
available steam activated carbon derived from wood (made in
Southeast Asia) 600 g was stirred with 6 L of 2% HNO3 aq for 4 h
at 75 °C. This was placed in a washing vessel with filter cloth.
Subsequently, the residue was washed with water at 50 °C at a rate
of 2.5 L h for 24 h. The washed activated carbon was dried in an
electric dryer held at 115 « 5 °C. The dried activated carbon was
pulverized with a ball mill. Thus, sample I was obtained.
Supporting Information
Experimental procedure and compound characterization data.
This material is available free of charge on the web at http://
www.csj.jp/journals/bcsj/.
¹1
References
1
a) T. L. Gilchrist, Heterocyclic Chemistry, 3rd ed., Addison-
Preparation of Sample J (Table 2, Entry 16). Commercially
available steam activated carbon derived from wood (made in
Southeast Asia) 600 g was stirred with 6 L of 2.5% HNO3 aq for
Wesley Longman, Ltd., England, 1998. b) D. Lednicer, Strategies
for Organic Drugs Synthesis and Design, Wiley & Sons, New
York, 1998, Chaps. 8 and 9. c) T. Eicher, S. Hauptmann, The
Chemistry of Heterocycles, Wiley-VCH GmbH & Co. KGaA,
Weinheim, 2003.
4
h at 75 °C. This was placed in a washing vessel with filter cloth.
Subsequently, the residue was washed with water at 50 °C at a rate
¹1
of 2.5 L h for 24 h. The washed activated carbon was dried in an
electric dryer held at 115 « 5 °C. The dried activated carbon was
pulverized with a ball mill. Thus, sample I was obtained.
2
a) K. Bougrin, A. Loupy, M. Soufiaoui, Tetrahedron 1998,
54, 8055. b) R. S. Pottorf, N. K. Chadha, M. Katkevics, V. Ozola,
E. Suna, H. Ghane, T. Regberg, M. R. Player, Tetrahedron Lett.
2003, 44, 175.
Preparation of Sample K (Table 3, Entry 1). To 500 g of dry
wood flour, 2.3 kg of 60% ZnCl2 aq was added, mixed well, and
placed in a crucible. This was placed in an electric furnace and was
heated from 100 to 250 °C over 2 h and from 250 to 550 °C over
3
a) F. Bellina, S. Cauteruccio, R. Rossi, Eur. J. Org. Chem.
2006, 1379. b) F. Derridj, S. Djebbar, O. Benali-Baitich, H.
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1
h, held at that temperature for 1 h, and then cooled. This was
4
5
J. Chang, K. Zhao, S. Pan, Tetrahedron Lett. 2002, 43, 951.
R. S. Varma, D. Kumar, J. Heterocycl. Chem. 1998, 35,
placed in a washing vessel with filter cloth and 8% HCl aq was
added thereto. Stirring and washing were continued for 2 h and
then the water was drained off. Subsequently, the residue was
washed with water at 50 °C at a rate of 2.5 L h for 4 h. The
washed activated carbon was dried in an electric dryer held at
1539.
6
1997, 38, 2621.
R. S. Varma, R. K. Saini, O. Prakash, Tetrahedron Lett.
¹1
7
K. H. Park, K. Jun, S. R. Shin, S. W. Oh, Tetrahedron Lett.
115 « 5 °C. The dried activated carbon was pulverized with a ball
1996, 37, 8869.
mill. Thus, sample K was obtained.
8
R. G. Srivastava, P. S. Venkataramani, Synth. Commun.
Preparation of Pre-Treated Sample K (Table 3, Entry 2).
1988, 18, 1537.
2
00 g of sample K (before ball milling) was placed in 6-L rotary
9
K. Nakagawa, H. Onoue, J. Sugita, Chem. Pharm. Bull.
type electronic funnel held at 850 °C.
1964, 12, 1135.
The cooled activated carbon was pulverized with a ball mill.
Thus, sample K¤ was obtained.
Experiments to Investigate the Role of Activated Carbon.
The yields of the synthesis of 2-(4-methoxyphenyl)benzoxazole
were used to compare the properties of activated carbons. The
reaction was carried out as follows. A mixture of 2-aminophenol
10 F. F. Stephens, J. D. Bower, J. Chem. Soc. 1949, 2971.
11 Y. Kawashita, N. Nakamichi, H. Kawabata, M. Hayashi,
Org. Lett. 2003, 5, 3713.
12 F. Carrasco-Marín, A. Mueden, C. Moreno-Castilla, J.
Phys. Chem. B 1998, 102, 9239.
13 M. F. R. Pereira, J. J. M. Órfão, J. L. Figueiredo, Appl.
Catal., A 1999, 184, 153.
(
8 mmol), 4-methoxybenzaldehyde (8 mmol), and the correspond-
ing activated carbon (500 mg) in xylene (15 mL) was placed in a
14 N. Nakamichi, H. Kawabata, M. Hayashi, J. Org. Chem.
2003, 68, 8272.
15 N. Nakamichi, Y. Kawashita, M. Hayashi, Synthesis 2004,
1015.
16 Y. Nomura, Y. Kawashita, M. Hayashi, Heterocycles 2007,
74, 629.
17 H. Kawabata, M. Hayashi, Tetrahedron Lett. 2004, 45,
5457.
1
1
00-mL three-necked flask under oxygen atmosphere and stirred at
20 °C for 8 h. Then, the reaction mixture was filtered using Celite.
After concentration of the filtrate, the mixture of the desired
product, the intermediate Schiff base, and the aldehyde was
obtained by silica gel column chromatography and the yield was
1
determined by H NMR analysis.
The amounts of metal contaminants, such as Zn, Co, Mn, Fe, Cr,
Mg, V, Cu, Pd, Ca, Na, and K were measured by ICP-AES (Varian
Liberty Series II). Specific surface area was measured using the
18 Y. Sano, T. Tanaka, M. Hayashi, Chem. Lett. 2007, 36,
1414.
2
0,21a,21c
Brunauer­Emmett­Teller (BET) method
with nitrogen gas
19 Y. Kawashita, C. Ueba, M. Hayashi, Tetrahedron Lett.
2006, 47, 4231; see also: M. Hayashi, Chem. Rec. 2008, 8, 252.
20 H. Marsh, F. Rodríguez-Reinoso, Activated Carbon,
Elsevier, Amsterdam, Boston, Heidelberg, London, New York,
Oxford, Paris, San Diego, San Francisco, Singapore, Sydny and
Tokyo, 2006, Chap. 4.
21 a) Y. Sanada, M. Suzuki, K. Fujimoto, Kasseitan, Kodansha
Ltd., Tokyo, 1992. b) R. W. Cranston, F. A. Inkley, Adv. Catal.
1957, 9, 143. c) S. Brunauer, P. H. Emmett, E. Teller, J. Am. Chem.
Soc. 1938, 60, 309.
as adsorbate by using ASAP2400 (Micromeritics). Pore volume
and mean pore diameter were calculated according to the
2
0,21a,21b
Cranston­Inkley (CI) method
from the volume of adsorbed
N2. Measurement of the amount of surface oxygen groups was
carried out by GC after wet gas meter analysis of 25 L desorbed
gas at 900 °C (See Supporting Information).
This research was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,