An efficient synthesis of 2-substituted benzothiazoles in the presence of FeCl3/Montmorillonite K-10 under ultrasound irradiation

Article abstract of DOI:10.1016/j.ultsonch.2012.09.010

2-Substituted benzothiazoles have been synthesized via one-pot reaction from aromatic aldehydes and o-aminothiophenol in the presence of FeCl ₃/Montmorillonite K-10 in absolute methanol at 25-30°C under ultrasound irradiation. The remarkable ad

Full text of DOI:10.1016/j.ultsonch.2012.09.010

Ultrasonics Sonochemistry 20 (2013) 627–632
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journal homepage: www.elsevier.com/locate/ultson
Short Communication
An efficient synthesis of 2-substituted benzothiazoles in the presence of
FeCl /Montmorillonite K-10 under ultrasound irradiation
3
a,
⇑
a
a
b
a
Guo-Feng Chen , Hui-Ming Jia , Li-Yan Zhang , Bao-Hua Chen , Ji-Tai Li
a
College of Chemistry and Environmental Science, Hebei University, Key Laboratory of Chemical Biology of Hebei Province, Baoding 071002, PR China
College of Pharmaceutical Sciences, Hebei University, Baoding 071002, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 June 2012
Received in revised form 31 August 2012
Accepted 8 September 2012
Available online 16 October 2012
2-Substituted benzothiazoles have been synthesized via one-pot reaction from aromatic aldehydes and
o-aminothiophenol in the presence of FeCl /Montmorillonite K-10 in absolute methanol at 25–30 °C
3
under ultrasound irradiation. The remarkable advantages are an inexpensive and easily available reagent,
a simple procedure, mild conditions, short reaction times and moderate to good yields.
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Benzothiazoles
o-Aminothiophenol
Aromatic aldehydes
FeCl
3
/Montmorillonite K-10
Ultrasound
1
. Introduction
2 5 4 3
[32], P O [33], Dowex 50W [34], Fe(HSO ) [35], etc. have all been
used in the reaction. Although these methods are suitable for spe-
cific synthetic conditions, sometimes, a number of these methods
suffer from one or more disadvantages such as long reaction times,
expensive reagents, drastic reaction conditions, low yields, tedious
work up procedures and co-occurrence of several side reactions. As
a consequence, the development of simple synthetic routes for
widely used organic compounds from readily available reagents is
one of the major tasks in organic synthesis.
In the last few years, the application of ultrasound in synthetic
organic chemistry has aroused more and more people’s interest
[36]. Sonochemistry offers a more versatile and facile pathway
for a large variety of syntheses in comparison to classical methods.
Thus, a large number of organic reactions can be carried out in high
yield, short reaction time or mild conditions under ultrasound irra-
diation [37].
Benzothiazole and their derivatives are often found in heterocy-
clic compounds, which exhibit a variety of biological activities, such
as antitumor, antimicrobial and antiviral properties [1–4]. They
also can be used as antioxidants, vulcanization accelerators in
industry and as a dopant in light-emitting organic electrolumines-
cent devices [5–7]. Because of the above importance, the synthesis
of substituted benzothiazoles has become a focus of intense re-
search in recent years. Several synthetic methodologies have been
developed for the synthesis of 2-substituted benzothiazoles. The
most synthetic approaches to the libraries are based on the
condensation of o-aminothiophenol with substituted nitriles [8],
carboxylic acids [9–12], acyl chlorides [13,14] and esters [15], or
with aldehydes followed by oxidation. Subsequently, with the avail-
ability of a vast number of aldehydes, the second approach has
recently become very popular. Various oxidative reagents and cata-
lysts, such as 4-methoxy-TEMPO [4a], cetyltrimethyl ammonium
Recently, there has been an ongoing effort to use supported
catalysts or reagents. This is mainly due to their advantages, such
as non-corrosivity, high selectivity, mild reaction conditions and
bromide [16], KAl(SO
4
)
2
ꢀ12H
19], NaOAc [20], poly[4-diacetoxyiodo] styrene [21], Sc(OTf)
22], trichloroisocyanuric acid [23], I [24], HClO /PANI [25], silica
2 2 2 2 2
O [17], H O /CAN [18], H O /HCl
[
[
3
easy of work-up [38]. Among of them, FeCl
Montmorillonite K-10 (FeCl /Montmorillonite K-10) has attracted
considerable attention because of its desirable characteristics. As
reported in previous papers, FeCl /Montmorillonite K-10 has been
3
supported on
2
4
3
gel [26], PTSA [27], activated carbon-molecular oxygen [28], Ionic
Liquid [29], bakers’ yeast [30], SDS [31], glucose oxidase–peroxidase
3
used as the catalyst in the preparation of 1,2-diaryl-1,2-ethanedi-
one [39] to afford the desired products in higher yields. To the best
of our knowledge, however, there are no reports on the synthesis of
⇑
Corresponding author. Address: College of Chemistry and Environmental
Science, Hebei University, Wusi East Road No. 180, Baoding 071002, PR China.
Fax: +86 312 5079628.
3
the title compound in the presence of FeCl /Montmorillonite K-10
under ultrasound irradiation. Considering the above points and in
E-mail address: chenguofeng@hbu.cn (G.-F. Chen).
1
350-4177/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ultsonch.2012.09.010

6
28
G.-F. Chen et al. / Ultrasonics Sonochemistry 20 (2013) 627–632
S
Table 1
SH
FeCl3/Montmorillonite K-10, air
CH OH, u.s.
R
The effect of solvent and temperature on the synthesis of 2-phenylbenzothiazole
+
RCHO
2(a-q)
a
N
(3a).
3
NH2
Entry
Solvent
OH
COOC
Cl
T/°C
Time/h
Isolated yield [%]
1
3(a-q)
1
2
3
4
5
6
7
8
9
10
C
CH
CH
2
H
5
25–30
25–30
25–30
25–30
25–30
25–30
25–30
25–30
25–30
30–35
35–40
25–30
1
1
1
1
1
1
1
1
1
1
1
1
83.8
81.0
84.3
76.8
79.2
77.9
83.5
14.9
85.0
85.0
83.2
72.5
Scheme 1. Synthesis of 2-substituted benzothiazoles.
3
2 5
H
2
2
DMF
CH CN
THF
CHCl
1,4-Dioxane
continuation of our work on ultrasound-promoted organic reac-
tions [40], herein, we report a new and simple synthesis of 2-
substituted benzothiazoles promoted by FeCl /Montmorillonite
3
3
3
K-10 via condensation of o-aminothiophenol and aldehydes under
CH
3
CH
3
CH
3
CH
3
OH
OH
OH
OH
ultrasound irradiation (Scheme 1).
1
1
1
2
b
2
. Experimental
a
o-Aminothiophenol (1.0 mmol) and benzaldehyde (1.0 mmol), FeCl
3
/Montmo-
rillonite K-10 was 0.1 mmol (based on FeCl
presence of air.
3
), the reactions were carried out in the
2.1. Apparatus, materials and measurements
b
Operated in nitrogen atmosphere.
All chemicals were obtained from commercial suppliers and
were used without further purification. Melting points were uncor-
rected and determined using an X-4 apparatus. IR spectra were ob-
C
NMR: d 168.1, 154.2, 135.1, 133.6, 131.0, 129.0, 127.6, 126.3,
125.2, 123.3, 121.6; m/z (ESI): 212 [M+H] .
1
+
tained using a NICOLET 380 FT-IR spectrometer instrument.
H
1
3
NMR (600 MHz) and C NMR (150 MHz) spectra were recorded
on a Bruker AVANCE III 600 spectrometer using CDCl as the sol-
2.3.2. Compound 3b
3
vent with tetramethylsilane (TMS) as internal standard at room
temperature. Mass spectrometric data were determined on Agilent
Technologies 6310 Lon Trap LC/MS. Sonication was performed
using a Shanghai Branson BUG 40–06 ultrasonic cleaner (with a
frequency of 40 kHz and a nominal power 250 W).
2-(4-Chlorophenyl)benzothiazole: white crystals, IR (KBr,
ꢁ1
cm ): 3054, 1589, 1508, 1474, 1434, 1399, 1315, 1287, 1251,
1
1090, 1012, 965, 828, 756; H NMR: d
H
8.09 (d, J = 8.1 Hz, 1H,
ArH), 8.05 (d, J = 8.5 Hz, 2H, ArH), 7.92 (d, J = 8.0 Hz, 1H, ArH),
7.51–7.54 (m, 1H, ArH), 7.49 (d, 1H, J = 8.6 Hz, ArH), 7.41–7.44
1
3
(
1
m, 1H, ArH); C NMR: d
C
166.6, 154.1, 137.1, 135.1, 132.1,
+
29.3, 128.7, 126.5, 125.4, 123.3, 121.7; m/z (ESI): 246 [M+H] .
3
2.2. Preparation of FeCl /Montmorillonite K-10
2
.3.3. Compound 3c
FeCl
3
/Montmorillonite K-10 (10% w/w) was prepared by follow-
O) 8 g was mixed
2
-(4-Methylphenyl)benzothiazole: white crystals, IR (KBr,
ing process. Hydrated ferric chloride (FeCl
3
ꢀ6H
2
ꢁ
1
cm ): 3058, 3023, 1608, 1484, 1457, 1434, 1312, 1286, 1253,
1
1
ArH), 7.50–7.53 (m, 1H, ArH), 7.38–7.41 (m, 1H, ArH), 7.32 (d,
J = 7.9 Hz, 2H, ArH), 2.45 (s, 3H, CH
with methanol (72 mL) and Montmorillonite K-10 (43.2 g). The
mixture was stirred at room temperature for 1 h and methanol
was removed under reduced pressure. The resulting yellow-green
powder was dried at 120 °C for 4 h. The content of FeCl was deter-
3
mined to be about 10%.
1
227, 1122, 959, 834, 817, 761; H NMR: d
H, ArH), 8.02 (d, J = 8.2 Hz, 2H, ArH), 7.91 (d, J = 8.0 Hz, 1H,
H
8.10 (d, J = 7.9 Hz,
1
3
3 C
); C NMR: d 168.3, 154.2,
1
41.5, 134.9, 131.0, 129.7, 127.5, 125.0, 126.3, 123.1, 121.6, 21.5;
+
m/z (ESI): 226 [M+H] .
2
.3. General procedure for synthesis of 2-substituted benzothiazoles
2
.3.4. Compound 3d
Aromatic aldehydes (2, 1.0 mmol), o-aminothiophenol (1,
.0 mmol), FeCl /Montmorillonite K-10 (160 mg, 0.1 mmol, based
) and absolute methanol (5 mL) were added into a 25 mL
conical flask. The reaction flask was placed in the ultrasonic cleaner
bath, where the surface of reactants was slightly lower than the
water level and irradiated at 25–30 °C for the period of time (son-
ication was continued until aromatic aldehydes disappeared as
indicated by TLC) as indicated in Tables 1–4. After completion of
the reaction, the reaction mixture was dissolved in ethyl acetate
2
-(4-Hydroxyphenyl)benzothiazole: brown crystals, IR (KBr,
1
3
ꢁ1
cm ): 3063, 2995, 1605, 1540, 1523, 1482, 1465, 1430, 1380,
on FeCl
3
1
1
317, 1284, 1249, 1224, 1167, 1108, 1073, 977, 827, 798, 755; H
NMR (DMSO-d ): d 10.25 (s, 1H, OH), 8.07 (d, J = 7.9 Hz, 1H,
ArH), 7.98 (d, J = 8.1 Hz, 1H, ArH), 7.94 (d, J = 8.5 Hz, 2H, ArH),
6
H
7
2
1
.49–7.51 (m, 1H, ArH), 7.39–7.41 (m, 1H, ArH), 6.94–6.96 (m,
13
H, ArH); C NMR (DMSO-d
6 C
): d 167.9, 161.0, 160.8, 154.2,
34.6, 129.5, 126.9, 125.4, 124.6, 122.8, 122.6, 116.6, 116.5; m/z
+
(
ESI): 228 [M+H] .
and FeCl
concentrated and purified by silica-gel column chromatography
200–300 mesh) using petroleum ether or the mixture of petro-
3
/Montmorillonite K-10 was filtered off. The filtrate was
2.3.5. Compound 3e
(
2
-(4-Methoxyphenyl)benzothiazole: light yellow crystals, IR
leum ether and ethyl acetate as eluent to give a light yellow crys-
talline solid. All of the products described herein were previously
reported in the literatures [16–35]. The authenticity of the
products was established by spectroscopic data and by comparing
their melting points with literature values.
ꢁ1
(
KBr, cm ): 3064, 2995, 2937, 2835, 1605, 1591, 1555, 1522,
1
1
484, 1466, 1455, 1434, 1412, 1310, 1303, 1286, 1257, 1225,
182, 1171, 1074, 1027, 968, 832, 791; H NMR: d
1
H
8.06–8.07
(
m, 3H, ArH), 7.90 (d, J = 8.0 Hz, 1H, ArH), 7.48–7.51 (m, 1H,
ArH), 7.37–7.39 (m, 1H, ArH), 7.02–7.03 (m, 2H, ArH), 3.91 (s, 3H,
OCH ); C NMR: d 167.9, 161.9, 154.2, 134.9, 129.1, 126.5,
3 C
126.2, 124.80, 122.8, 121.5, 114.4, 55.5; m/z (ESI): 242 [M+H] .
1
3
+
2
.3.1. Compound 3a
-Phenylbenzothiazole: light yellow crystals, IR (KBr, cmꢁ1):
062, 1589, 1510, 1477, 1456, 1433, 1283, 1252, 1224, 1158,
2
3
9
1
2.3.6. Compound 3f
1
62, 765; H NMR: d
H
8.11–8.14 (m, 3H, ArH), 7.93 (d, J = 8.0 Hz,
2-(4-N,N-Dimethylphenyl)benzothiazole: light yellow crystals,
1
3
ꢁ1
H, ArH), 7.51–7.54 (m, 4H, ArH), 7.42 (t, J = 8.0 Hz, 1H, ArH);
C
IR (KBr, cm ): 3052, 2895, 2806, 1609, 1576, 1540, 1507, 1485,

G.-F. Chen et al. / Ultrasonics Sonochemistry 20 (2013) 627–632
629
Table 2
The synthesis of 2-substituted benzothiazoles derivatives in the presence of FeCl
3
/Montmorillonite K-10 under ultrasound.
Entry
R
Product
Time [h]
Isolated yield [%]
m.p., [°C] (lit)
1
2
3
4
5
6
7
8
9
C
C
C
C
6
6
6
6
H
H
H
H
5
5
5
5
3a
3a
3a
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
1.0
1.0
1.0
1.0
0.7
1.0
1.2
1.4
1.4
4.9
1.7
3.9
1.3
5.0
2.4
1.5
1.4
1.7
2.9
5.0
85.0
67.5
58.7
32.8
86.0
85.7
95.3
81.4
72.0
75.4
77.8
73.8
82.6
33.4
83.1
51.4
89.2
51.1
73.3
72.3
–
112–114(112–114) [17]
a
b
c
4-ClC
4-CH
4-OHC
4-CH OC
4-(Me) NC
2-ClC
2-CH
2-HOC
3-ClC
3-NO
3,4-Cl
2,4-Cl
4-HO-3-MeOC
2-HO-3-MeOC
3,4-OCH
2-Furyl
2-NO
6
H
C H
3 6 4
4
114–116(114–115) [17]
80–84(83) [17]
6
H
4
230–234(231) [31]
123–124(121) [31]
180–182(173) [31]
80–82(80–82) [20]
110–112(120–122) [16]
132–134(129–131) [24]
98–100(94–95) [25]
200–204(183–185) [16]
123–124(118–120) [4c]
150–152(144–145) [4b]
180–182(162–164) [34]
175–177(162–163) [35]
129–130(125–127) [16]
110–113(106) [31]
–
3
6 4
H
2
6 4
H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
6
H
4
3
6 4
OC H
6 4
H
6
H
4
2
C
6
H
4
2
C
C
6
H
H
3
2
6
3
3m
3n
3o
3p
3q
6
6
H
H
3
3
2 6 3
OC H
d
2
C
6
H
)
4
3.8
d
2,4-(NO
CH
CH
2
2
C
6
H
3
5.0
4.8
–
–
–
–
–
–
d
3
CH
(CH
2
CH
2
d
3
2
)
5
4.8
a
b
c
In the presence of FeCl
In the presence of Montmorillonite K-10.
3
.
Without FeCl
No reaction.
3
/Montmorillonite K-10.
d
2
.3.8. Compound 3h
Table 3
2
-(2-Methoxyphenyl)benzothiazole: light yellow crystals, IR
Contrast experiments of 3(a–d) in the presence of FeCl
ultrasound or without ultrasound.
3
/Montmorillonite K-10 with
ꢁ1
(KBr, cm ): 3035, 3015, 2965, 2936, 2831, 1617, 1598, 1581,
1
1
558, 1540, 1521, 1499, 1460, 1429, 1309, 1284, 1248, 1216,
Entry
R
Product
Time [h]
Isolated yield (%)
1
174, 1160, 1115, 1017, 962, 945, 757, 728; H NMR: d
H
8.58
A
B
(
dd, 1H, J
1
= 7.8 Hz, J = 1.6 Hz, ArH), 8.14 (d, J = 8.2 Hz, 1H, ArH),
2
1
2
3
4
C
6
H
5
3a
3b
3c
3d
1.0
0.7
1.0
1.2
85.0
86.0
85.7
95.3
82.6
58.8
70.3
68.9
7.96 (d, J = 7.9 Hz, 1H, ArH), 7.47–7.54 (m, 2H, ArH), 7.40 (t,
4-ClC
4-CH
4-OHC
6
H
4
J = 7.3 Hz, 1H, ArH), 7.17 (t, J = 7.4 Hz, 1H, ArH), 7.08 (d,
3
C
6
H
4
13
3 C
J = 8.3 Hz, 1H, ArH), 4.07 (s, 3H, CH ); C NMR: d 163.1, 157.3,
6 4
H
1
1
52.2, 136.2, 131.8, 129.6, 125.9, 124.6, 122.8, 122.4, 121.2,
11.7, 55.7; m/z (ESI): 242 [M+H] .
Condition: A, ultrasound; B, stirring alone without ultrasound.
+
2.3.9. Compound 3i
2
-(2-Hydroxyphenyl)benzothiazole: white crystals, IR (KBr,
ꢁ1
Table 4
cm ): 3057, 2923, 1621, 1589, 1543, 1522, 1483, 1457, 1437,
3
Synthesis of 2-phenylbenzothiazole (3a) with recovered FeCl /Montmorillonite K-10.
1
7
419, 1315, 1272, 1251, 1219, 1165, 1150, 1033, 973, 871, 860,
1
57, 742; H NMR: d
H
12.55 (s, 1H, OH), 8.00 (d, 1H, J = 8.0 Hz
= 7.9 Hz,
= 1.1 Hz, ArH), 7.51–7.54 (m, 1H, ArH), 7.40–7.44 (m, 2H, ArH),
.14 (dd, 1H, J = 8.3 Hz, J = 0.8 Hz, ArH), 6.97–6.99 (m, 1H, ArH);
169.4, 158.0, 151.9, 132.7, 132.6, 128.4, 126.7, 125.5,
Number of cycles
Isolated yield (%)
Fresh
85.0
Recycle I
78.6
Recycle II
73.4
Recycle III
69.0
ArH), 7.91 (d, 1H, J = 7.9 Hz ArH), 7.71 (dd, 1H, J
1
J
2
7
1
2
1
3
C NMR: d
22.2, 121.5, 119.5, 117.9, 116.8; m/z (ESI): 228 [M+H] .
C
+
1
1
8
2
7
456, 1431, 1369, 1314, 1287, 1255, 1227, 1188, 1062, 963, 843,
1
H
00, 752; H NMR: d 8.02 (d, 1H, J = 8.0 Hz, ArH), 7.98–8.00 (m,
2.3.10. Compound 3j
H, ArH), 7.86 (d, 1H, J = 8.4 Hz, ArH), 7.45–7.48 (m, 1H, ArH),
2
-(3-Chlorophenyl)benzothiazole: white crystals, IR (KBr,
1
3
.32–7.34 (m, 1H, ArH), 6.75–6.78 (m, 2H), 3.07 (s, 6H, CH
3
);
C
ꢁ1
cm ): 3053, 1622, 1588, 1569, 1541, 1501, 1473, 1458, 1434,
1
7
NMR: d
21.4, 111.7, 40.2; m/z (ESI): 255 [M+H] .
C
168.8, 154.4, 152.2, 134.6, 128.9, 126.0, 124.2, 122.3,
423, 1294, 1268, 1245, 1232, 1161, 1076, 943, 886, 861, 783,
+
1
1
59, 732; H NMR: d
H
8.14 (t, 1H, J = 1.8 Hz ArH), 8.11 (d, 1H,
J = 8.2 Hz, ArH), 7.96–7.97 (m, 1H, ArH), 7.93 (d, 1H, J = 8.4 Hz,
2
.3.7. Compound 3g
ArH), 7.52–7.55 (m, 1H ArH), 7.47–7.49 (m, 1H, ArH), 7.42–7.45
(m, 2H, ArH); C NMR: d 166.3, 154.0, 135.3, 135.2, 135.1,
C
1
3
2
-(2-Chlorophenyl)benzothiazole: white crystals, IR (KBr,
ꢁ
1
cm ): 3055, 1590, 1564, 1492, 1456, 1431, 1317, 1299, 1271,
250, 1061, 1016, 966, 757, 748; 1H NMR: d
8.23–8.26 (m, 1H,
ArH), 8.17 (d, J = 8.1 Hz, 1H, ArH), 7.98 (d, J = 8.0 Hz, 1H, ArH),
130.9, 130.2, 127.4, 126.5, 125.7, 125.6, 123.5, 121.7; m/z (ESI):
+
1
H
246 [M+H] .
7
1
1
.55–7.58 (m, 2H, ArH), 7.43–7.48 (m, 3H, ArH); 13C NMR: d
64.2, 152.5, 136.1, 132.8, 132.3, 131.8, 131.2, 130.8, 127.1,
26.3, 125.5, 123.5, 121.4; m/z (ESI): 246 [M+H] .
C
2.3.11. Compound 3k
2-(3-Nitrophenyl)benzothiazole: light yellow crystals, IR (KBr,
+
ꢁ1
cm ): 3085, 1623, 1593, 1579, 1570, 1558, 1530, 1472, 1458,

6
30
G.-F. Chen et al. / Ultrasonics Sonochemistry 20 (2013) 627–632
1
8
432, 1363, 1345, 1311, 1288, 1244, 1128, 1101, 987, 888, 843,
2.3.17. Compound 3q
2-(2-Furanyl)benzothiazole: brown crystals, IR (KBr, cm ):
3143, 3121, 3048, 1598, 1541, 1522, 1502, 1473, 1455, 1433,
1
ꢁ1
10, 761; H NMR: d
= 8.1 Hz, J
H, ArH), 7.96 (d, J = 8.0 Hz, 1H, ArH), 7.70 (t, J = 7.9 Hz, 1H, ArH),
.56 (t, J = 7.5 Hz, 1H, ArH), 7.47 (t, J = 7.6 Hz, 1H, ArH); 13C NMR:
H
8.93 (s, 1H, ArH), 8.42 (d, J = 7.7 Hz, 1H,
ArH), 8.34 (dd, 1H, J
1
7
1
2
= 1.3 Hz, ArH), 8.13 (d, J = 8.2 Hz,
1420, 1385, 1313, 1253, 1244, 1218, 1155, 1012, 897, 882, 769,
1
761, 747; H NMR: d
H
8.08 (d, J = 8.2 Hz, 1H, ArH), 7.90 (d,
d
1
C
164.9, 153.9, 148.8, 135.3, 135.2, 133.0, 130.1, 126.8, 126.0,
25.1, 123.7, 122.3, 121.8; m/z (ESI): 257 [M+H] .
J = 7.9 Hz, 1H, ArH), 7.62 (s, 1H, FuranH), 7.51 (t, J = 7.3 Hz, 1H,
ArH), 7.40 (t, J = 7.3 Hz, 1H, ArH), 7.22 (d, J = 2.8 Hz, 1H, FuranH),
+
1
3
6
1
2
C
.61 (d, J = 1.5 Hz, 1H, FuranH); C NMR: d 157.6, 153.7, 148.8,
44.7, 134.3, 126.5, 125.2, 123.1, 121.6, 112.5, 111.5; m/z (ESI):
2
.3.12. Compound 3l
+
02 [M+H] .
2
-(3,4-Dichlorophenyl)benzothiazole: white crystals, IR (KBr,
ꢁ
1
cm ): 3061, 1587, 1473, 1464, 1456, 1419, 1388, 1374, 1310,
1
1
279, 1257, 1245, 1230, 1136, 1027, 985, 824, 786, 757, 725;
NMR: d 8.23 (d, J = 2.1 Hz, 1H, ArH), 8.09 (d, J = 8.2 Hz, 1H, ArH),
.93 (d, J = 7.9 Hz, 1H, ArH), 7.90 (dd, 1H, J = 8.3 Hz, J = 2.0 Hz,
ArH), 7.57 (d, J = 8.3 Hz, 1H, ArH), 7.52–7.55 (m, 1H, ArH), 7.43–
H
H
3
. Results and discussion
7
1
2
In the initial experiment, we investigated various conditions in
.45 (m, 1H, ArH); 1 C NMR: d
3
7
1
C
165.0, 154.0, 135.1, 133.5, 131.0,
the model reaction of o-aminothiophenol (1) with benzaldehyde
2a) in the presence of FeCl /Montmorillonite K-10 under ultra-
sound irradiation and the results were summarized in Table 1.
The effect of various solvents (such as EtOH, CH COOC , CH Cl
DMF, CH CN, THF, CHCl , 1,4-dioxane and MeOH) on the model
+
29.0, 126.7, 126.5, 125.7, 123.5, 121.7; m/z (ESI): 280 [M+H] .
(
3
3
H
2 5
2
2
,
2
.3.13. Compound 3m
-(2,4-Dichlorophenyl)benzothiazole: white crystals, IR (KBr,
2
3
3
ꢁ
1
reaction was conducted in the presence of FeCl /Montmorillonite
cm ): 3067, 1624, 1583, 1549, 1508, 1481, 1457, 1449, 1377,
1
7
3
K-10. The results indicated that the solvents had a significant effect
on the product yield. The use of 1,4-dioxane as solvent gave poor
316, 1258, 1220, 1162, 1106, 1078, 1060, 964, 859, 826, 794,
1
54, 725; H NMR: d
H
8.26 (d, J = 8.5 Hz, 1H, ArH), 8.15 (d,
yields (Table 1, Entry 8). Solvents like DMF, CH
moderate yields (Table 1, Entries 4, 5 and 6). The best conversion
was observed when the reaction was performed in CH OH (Table 1,
3
CN and THF gave
J = 8.2 Hz, 1H, ArH), 7.97 (d, J = 8.0 Hz, 1H, ArH), 7.55–7.58 (m,
H, ArH), 7.45–7.48 (m, 1H, ArH), 7.42 (dd, 1H, J = 8.5 Hz,
163.0, 152.4, 136.6, 136.1, 133.3,
2
1
1
3
J
2
= 2.0 Hz, ArH); C NMR: d
C
3
Entry 9). Based on these results, CH
solvent for further investigations.
3
OH was then selected as the
1
2
32.5, 130.8, 130.6, 127.6, 126.5, 125.7, 123.5, 121.4; m/z (ESI):
+
80 [M+H] .
The effect of reaction temperature on the model reaction in the
presence of FeCl /Montmorillonite K-10 under ultrasound irradia-
3
2
.3.14. Compound 3n
tion was also observed. As shown in Table 1, when the temperature
was 25–30, 30–35 and 35–40 °C, the yield of 3a was 85.0% (Entry
2
-(3-Methoxy-4-hydroxyphenyl)benzothiazole: brown crystals,
ꢁ1
IR (KBr, cm ): 3099, 3004, 2934, 1616, 1604, 1584, 1559, 1529,
1
1
9
), 85.0% (Entry 10) and 83.2% (Entry 11) respectively. Conse-
478, 1462, 1427, 1387, 1364, 1315, 1279, 1256, 1194, 1124,
quently, the reaction temperature had little effect on the reaction.
In addition, the reaction was slow and gave unsatisfactory yield
033, 1011, 894, 872, 776, 756, 727; 1H NMR: d
9.85 (s, 1H,
H
OH), 8.06 (d, J = 8.2 Hz, 1H, ArH), 7.89 (d, J = 7.9 Hz, 1H, ArH),
.75 (d, J = 1.8 Hz, 1H, ArH), 7.56 (dd, 1H, J = 8.2 Hz, J = 1.9 Hz,
ArH), 7.48–7.51 (m, 1H, ArH), 7.37–7.39 (m, 1H, ArH), 7.03 (d,
(
(
72.5%) of 2-phenylbenzothiazole (3a) under nitrogen atmosphere
Table 1, Entry 12), it showed that aerial oxygen played an oxidant
7
1
2
role in this reaction.
13
J = 8.2 Hz, 1H, ArH), 4.00 (s, 3H, CH
3 C
); C NMR: d 168.2, 154.0,
To test the generality of this reaction, a series of aromatic
aldehydes with both electron-donating and electron-withdrawing
substituents were reacted with o-aminothiophenol under the opti-
mized reaction conditions, and the corresponding 2-substituted
benzothiazoles (3a–q) were obtained under ultrasound irradiation
or silent conditions. The results are summarized in Tables 2
and 3.
As shown in Tables 2 and 3, a series of aromatic aldehydes were
successfully employed to prepare the corresponding benzothiazole
derivatives in excellent yields. In general, the yields of benzothiazole
derivatives are similar or higher than those described in literatures.
The outstanding character of the present procedure is to save the
reaction time and improve the yield.
1
1
48.6, 147.0, 134.8, 126.3, 126.2, 124.9, 122.7, 122.0, 121.5,
14.8, 109.3, 56.2; m/z (ESI): 258 [M+H] .
+
2.3.15. Compound 3o
2
-(2-Hydroxy-4-methoxyphenyl)benzothiazole: brown crys-
ꢁ
1
tals, IR (KBr, cm ): 3055, 2937, 2837, 1583, 1540, 1470, 1460,
1
7
7
1
438, 1293, 1279, 1247, 1177, 1117, 1084, 1003, 936, 777, 756,
1
25; H NMR: d
.89 (d, J = 7.9 Hz, 1H, ArH), 7.75 (d, J = 1.4 Hz, 1H, ArH), 7.56 (dd,
H, J = 8.2 Hz, J = 1.7 Hz, ArH), 7.49 (t, J = 7.6 Hz, 1H, ArH), 7.38
H
9.85 (s, 1H, OH), 8.06 (d, J = 8.1 Hz, 1H, ArH),
1
2
(
t, J = 7.6 Hz, 1H, ArH), 7.03 (d, J = 8.2 Hz, 1H, ArH), 3.99 (s, 3H,
1
3
3 C
CH ); C NMR: d 168.2, 154.0, 148.6, 147.0, 134.8, 126.3, 126.2,
According to the method catalyzed by 4-methoxy-TEMPO in
presence of oxygen [4a], the time and yield of 3a were 9 h and
8
1
24.9, 122.7, 122.0, 121.5, 114.8, 109.3, 56.2; m/z (ESI): 258
+
[
M+H] .
0% respectively heating at 120 °C in xylene for the condensation
of o-aminothiophenol and benzaldehyde. The present procedure
gave 85% yield within 1 h (Table 2, Entry 1) at 25–30 °C. In the
reaction catalyzed by Dowex 50 W in water under aerial oxygen
[34], 3d was obtained in 92% yield at 70 °C for 10 h, whereas the
present procedure afforded 3d in 95.3% yield within 1.2 h at 25–
30 °C (Table 2, Entry 7).
2
.3.16. Compound 3p
0
0
2
-(3 ,4 -Methylenedioxyphenyl)benzothiazole: light yellow
ꢁ1
crystals, IR (KBr, cm ): 3058, 2987, 2908, 1602, 1497, 1473,
1
8
440, 1350, 1254, 1132, 1108, 1071, 1031, 988, 940, 881, 834,
1
H
04, 756, 727; H NMR: d 8.05 (d, J = 8.1 Hz, 1H, ArH), 7.88 (d,
J = 7.9 Hz, 1H, ArH), 7.61–7.64 (m, 2H, ArH), 7.48–7.51 (m, 1H,
ArH), 7.37–7.40 (m, 1H, ArH), 6.92 (d, J = 8.0 Hz, 1H, ArH), 6.07 (s,
In the present procedure, furfuraldehyde (2q) also afforded the
desired products in moderate yields (72.3%) within 5.0 h (Table 2,
Entry 20). 2-Nitrobenzaldehyde and 2,4-dinitrobenzaldehyde
(Table 2, Entries 21 and 22) were failed. Aliphatic aldehyde was
less reactive than arylaldehyde, no product was obtained when
1
3
2
1
2
2 C
H, CH ); C NMR: d 167.6, 154.0, 150.1, 148.4, 134.8, 128.0,
26.3, 125.0, 122.9, 122.5, 121.5, 108.7, 107.5, 101.7; m/z (ESI):
+
56 [M+H] .

G.-F. Chen et al. / Ultrasonics Sonochemistry 20 (2013) 627–632
631
aliphatic aldehyde was used (Entries 23 and 24). It seems that this
protocol has its limitations.
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We also did the same scale reaction in the presence of FeCl
3
[
6] C.P. Ye, N.G. Wang, H.Q. Jia, X.J. Xu, Synthesis of 2-(4-substituted
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alone. The yield of 3a was 67.5% after 1.0 h under ultrasound (Ta-
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of 3a was found when only using the Montmorillonite K-10 after
(
2002) 1008–1010.
[
7] A. Heynderickx, R. Guglielmetti, R. Dubest, J. Aubard, A. Samat, Sulfinyl-and
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.0 h under ultrasound (Table 2, Entry 3), however, the reactivity
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).
[
[
3
1
9] S. Mourtas, D. Gatos, K. Barlos, Solid phase synthesis of benzothiazolyl
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We also did the experiment for the reaction of o-aminothiophenol
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.0 h. It seems that FeCl /Montmorillonite K-10 plays an important
[
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(
3
1
3
role in the reaction (Table 2, Entry 4).
[
In order to verify the effect of ultrasound irradiation, we also did
the contrast experiments under stirring without ultrasound irradi-
ation. As shown in Table 3, compound 3a, 3b, 3c and 3d were ob-
tained in 82.6%, 58.8%, 70.3% and 68.9% yields within 1.0, 0.7, 1.0
and 1.2 h respectively. Whereas under ultrasound irradiation, 3a,
[
3
b, 3c and 3d were obtained in 85.0%, 86.0%, 85.7% and 95.3%
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condesation of o-aminothiophenol with aromatic aldehydes and
improve the result. The phenomenon of cavitation produced by
ultrasound may be responsible for competitive advantages of pres-
ent procedure [36].
The recycling performance of the FeCl
the model reaction was also inverstigated. After completion of the
reaction, the isolated FeCl /Montmorillonite K-10 was washed with
ethyl acetate and reactivated at 120 °C for 4 h, and can be reused
three times, the yield of 3a decreased from 85.0 to 69.0% (Table 4).
This may due to the leaching of the active component (i.e. iron),
which can be seen from the color change of the solvent when
washed with ethyl acetate in the post-processing [41]. Further work
[
16] X.L. Yang, C.M. Xu, S.M. Lin, J.X. Chen, J.C. Ding, H.Y. Wu, W.K. Su, Eco-friendly
synthesis of 2-substituted benzothiazoles catalyzed by cetyltrimethyl
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3
/Montmorillonite K-10 in
[
17] S.S. Pawar, D.V. Dekhane, M.S. Shingare, S.N. Thore, Alum (KAl(SO
4
)
2
ꢀ12H
2
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3
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[
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2 2
O
3
is necessary to strongly bind the FeCl on Montmorillonite K-10.
[
[
[
[
20] M. Okimoto, T. Yoshida, M. Hoshi, K. Hattori, M. Komata, K. Tomozawa, T.
Chiba, Electrooxidative cyclization of benzylideneaminothiophenols to the
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4
. Conclusion
In summary, we have found an efficient and practical procedure
for the preparation of 2-substituted benzothiazoles in the presence
of FeCl /Montmorillonite K-10 under ultrasound irradiation. This
methodology offers the competitive advantages of mild reaction
conditions, short reaction times, high yields and an inexpensive
and easily available reagent.
3
23] H.L. Xiao, J.X. Chen, M.C. Liu, D.J. Zhu, J.C. Ding, H.Y. Wu, Trichloroisocyanuric
acid (TCCA) as
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[24] Y. Li, Y.L. Wang, J.Y. Wang, simple iodine-promoted synthesis of 2-
a mild and efficient catalyst for the synthesis of 2-
A
substituted benzothiazoles by condensation of aldehydes with 2-
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Acknowledgements
[25] M. Abdollahi-Alibeik, S. Poorirani, Perchloric acid–doped polyaniline as an
efficient and reusable catalyst for the synthesis of 2-substituted
benzothiazoles, Phosphorus Sulfur Silicon Relat. Elem. 184 (2009) 3182–3190.
The project was supported by Natural Science Foundation of
Hebei Province (B2011201072), the Science Project of the Hebei
Education Department (ZD200910) and Foundation of Hebei Uni-
versity (2009-163).
[
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