Alumina as a support and catalyst for the synthesis of benzothiazoles in solvent-free condition

Article abstract of DOI:10.1002/jccs.201200413

Arylbenzothiazole (4) and benzothiazolylcoumarins (5) were prepared by the condensation of 2-aminothiophenol (1) and hydroxybenzaldehydes (2) and 3-ethoxycarbonylcoumarinnin (3), respectively, in solvent- free condition. Of the solid support used,γ-Al2O3 demonstrated the best activity. Alinear regression line obtained from correlation between relative formation rate and Hammett constants with slope of 0.653 suggested that this condensation process is less sensitive to the substituents on the phenyl ring due to low efficacy of polarization in the solid state. In general, good yields (>80% yield) were obtained for the formation of 4. Conversely, yields of 5 were poor, because of γ-Al2O3 catalytic decomposition of 3 to form 2 (without P2O5) and coumarin (with P2O5).

Full text of DOI:10.1002/jccs.201200413

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Alumina as a Support and Catalyst for the Synthesis of Benzothiazoles in Solvent-free
Condition
Mei-Fang Ding, Chia-Pei Chen and Shaw-Tao Lin*
Department of Applied Chemistry, Providence University, 200, Chung-Chi Road, Sha-Lu Distract, Taichung 43301, Taiwan
(Received: Jul. 27, 2012; Accepted: Jan. 23, 2013; Published Online: Mar. 11, 2013; DOI: 10.1002/jccs.201200413)
Arylbenzothiazole (4) and benzothiazolylcoumarins (5) were prepared by the condensation of 2-amino-
thiophenol (1) and hydroxybenzaldehydes (2) and 3-ethoxycarbonylcoumarinnin (3), respectively, in sol-
vent-free condition. Of the solid support used, g-Al2O3 demonstrated the best activity. A linear regression
line obtained from correlation between relative formation rate and Hammett constants with slope of 0.653
suggested that this condensation process is less sensitive to the substituents on the phenyl ring due to low
efficacy of polarization in the solid state. In general, good yields (>80% yield) were obtained for the for-
mation of 4. Conversely, yields of 5 were poor, because of g-Al2O3 catalytic decomposition of 3 to form 2
(without P2O5) and coumarin (with P2O5).
Keywords: Arylbenzothiazoles; Benzothiazolylcoumarins; Solvent-free; Alumina; Hammett’s plot.
INTRODUCTION
Benzothiazoles are an extremely important class of
and benzaldehydes was by acidic condensation. Similarly,
this study uses a-Al2O3, g-Al2O3, SiO2 and nano-TiO2 as
the supports and catalyst, to examine their efficiency in the
condensation reaction. 2-Hydroxybenzaldehye (2n) was
chosen to optimize the reaction, because it is less volatile.
Prior to use, all of the supports were heated up to 150 °C,
for 30 min, except for the g-Al2O3 which was activated at
450 °C. Ten-fold mole of solid support was used. To per-
form the reaction, 1 and 2, in dry MeOH solution, were in-
troduced to the activated solid support. After removing
MeOH under reduced pressure, followed by heating at a
specific temperature, for 10 min, the resultant was passed
through silica gel, with EtOH as an eluent, to give a reason-
ably pure product.
compounds which occur widely as biologically active natu-
ral products. They are also present in marketed drugs, or
drug candidates.1 Aminothiophenols (1) are commonly re-
acted with benzaldehydes (2) and coumarin derivatives (3)
to yield arylbenzothiazoles (4) and benzothiazolycumarins
(5), respectively. To form compounds 4, the reactions are
performed using various solvents and long reaction peri-
ods,2 while formation of compounds 5 occurs in PPA solu-
tion at 180 °C.3
Although the Suzuki reaction can be used for the
preparation of arylcoumarins via the coupling of 3-bromo-
cumarin and aryl-boronic acid,4 the preparation of benzo-
thizolylboronic acid is a challenging task. Organic synthe-
sis in the absence of solvent has been the subject of much
research because the preparative procedures have several
advantages, such as environmental compatibility, easy
work-up, enhanced selectivity, a reduction of by-products
and much improved reaction rates.5 Microwave irradiation
is also known to accelerate the reaction and the selectivity,
because of the penetration of radiation and focused heat-
ing.6 This work studies the preparation of a series of com-
pounds 4 and 5 in solvent-free conditions either normal
heating or by microwave irradiation. The substituent effect
on the formation of compounds 4 is also studied.
Table 1 shows that the condensation processes take
place readily at room temperature, but with very low yield
(entry 1). Presumably the condensation can be occurred
during the mixing process. An increase in the reaction tem-
peratures, increases the yields of 4n regardless of the heat-
ing methods, but the yields are relatively lower at 160 °C.
This can be attributed to the vaporization of the reactant(s)
(entries 8, 9). Low molar numbers of Al2O3 (five-fold, en-
tries 4, 5) produce low yields of product. Of the four solid
supports, g-Al2O3 gave the best results followed by SiO2
and TiO2. Therefore, ten-fold g-Al2O3 and a temperature of
120 °C were used to examine the reactions of 1 and substi-
tuted benzaldehydes. (Scheme I)
RESULTS AND DISCUSSION
During the reactions of substituted benzaldehydes 2
and 1, the progress of reaction was monitored by TLC anal-
In general the condensation of 2-aminothiophenol 1
* Corresponding author. E-mail: sdlin@pu.edu.tw
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Ding et al.
Table 1. Yields for condensations of 1 and 2n catalyzed by
various solids for different reaction conditionsa
Scheme I
Support
Entry
Temp (°C)
Method
Yield (%)
(mmol)
1
g-Al2O3 (10)
g-Al2O3 (10)
g-Al2O3 (10)
g-Al2O3 (5)
25
80
no heat
microwave
heat
< 5%
42%
40%
65%
60%
80%
84%
78%
80%
68
2
3
80
4
120
120
120
120
160
160
120
120
120
120
120
120
microwave
heat
R: H (a); 3-OCH3 (b); 4-OCH3 (c); 3-CH3 (d); 4-CH3 (e); 3-Br (f);
3-Cl (g); 4-Cl (h); 2-NO2 (i); 3-NO2 (j); 4-NO2 (k); 3-CN (l);
4-CN (m); 2-OH (n); 4-OH (o)
5
g-Al2O3 (5)
6
g-Al2O3 (10)
g-Al2O3 (10)
g-Al2O3 (10)
g-Al2O3 (10)
a-Al2O3 (10)
a-Al2O3 (10)
Nano-TiO2(10)
Nano-TiO2(10)
SiO2(10)
microwave
Heat
7
8
microwave
heat
9
ysis. Once the compound 2 is absence from the TLC plate,
the product is harvested, by passing through a short silica
gel column. Table 2 shows the time for completion of the
reaction depends on the substituent of aldehydes. The iso-
lated yields for all of aldehydes are higher than 80%. The
required reaction times for irradiation are substantially
shorter than those required for the normal heating method,
10
11
12
13
14
15
microwave
heat
70
23
microwave
heat
25
72
microwave
heat
SiO2(10)
74
a. One mmole of both 1 and 2 were used. Reaction period is 10
min.
a
Table 2. Yields for condensation of 1 and 2 in the presence of g-Al2O3
Time
(min)
Reported mp
Observed mp
Entry
R
H
Yield (%)(1)
(°C)
(°C)
1
30
30b
30c
30d
10e
60
114 ~ 115a
110 ~ 112
85%
75%
49%
45%
83%
85%
89%
90%
84%
82%
86%
92%
90%
88%
90%
92%
83%
80%
84%
81%
83%
85%
84%
80%
88%
2
3
4
5
6
3-OCH3
4-OCH3
81.5 ~ 827b
121 ~ 1222a
83 ~ 85
7
30
125~126.5
8
15e
180
15e
60
9
3-CH3
63 ~ 647c
62~63
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
4-CH3
3-Br
85 ~ 862a
83 ~ 842a
86~87
85~86
90
15e
120
120
15e
15
3-Cl
4-Cl
957d
116~1172a
95
115-117
2-NO2
3-NO2
4-NO2
3-CN
138 ~ 1402a
183 ~ 1852a
228 ~ 2302a
1587d
134~136
184~186
230~232
160
60
30
180
20e
60
4-CN
2-OH
170 ~ 1707d
127 ~ 1287d
170~172
130
10
10e
4-OH
30
227 ~ 2287d
226~228
a. 1 mmole of aminothiophenol and benzaldehyde were heated at 120 °C in the
presence of 10 mmole g-Al2O3; the progress of the reaction was monitored, using
TLC. All of the products have been described previously in literatures and were
identified by their NMR spectra and melting points. The yields are isolated yields. b.
First recycle. c. Second recycle. d. Third recycle. e. Heating by microwave irradiation.
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CHEMICAL SOCIETY
Alumina for Synthesis of Benzothiazole in Solvent-free
Table 3. The correlation between relative reaction rates for the
condensation of aminothiophenol and substituted
varying from 1/9, for 2l to 1/3, for 2a. The g–Al2O3 it was
activated in the previously process before it was reused.
The recycling runs show that the activity of g–Al2O3 decays
very quickly.(entries 2, 3, and 4). This indicates that the
strong interaction between the alumina and the impurities
deteriorates the activity of alumina.
hydroxylbenzaldehydes and Hammatt’s constantsa
Entry
R
Relative reaction rate
s9
1
4'-OH
4'-OCH3
4'-CH3
3'-CH3
H
0.24
0.52
0.63
0.75
1
-0.92
-0.78
-0.31
-0.07
0
2
3
To more closely examine the influence of the sub-
stituents on the reaction rate, reactions were performed us-
ing normal heating method. Equal molarities of substituted
benzaldehyde, benzaldehyde 2a and 2-aminothiophenol
were used. After 10 min, the product was treated in the nor-
mal fashion and analyzed using HPLC. The relative reac-
tion rate was determined, based on the ratio of the peak ar-
eas of products. The plot for the relative rates versus s val-
ues is shown in Table 3 and Figure 1. There appear to be the
inflections in the correction lines, corresponding to a slope
of 0.653 (R2 = 0.947) for the groups from p-methoxy to
p-nitro group. The positive slopes suggest that the reaction
rate is affected by the substituents on the aryl ring, which
can be directly attributed to the resonance effect. The small
slope suggests the less polarization of the carbonyl group,
in the solid state. This r value is smaller than that for con-
densation of aldehydes with Medleu’m acid (+1.226)7a and
amino-acetonitrile (+1.86),8b in the solution state and com-
parable with that for condensation with ethyl cyanoacetate
(+0.48).8c
4
5
6
3'-Cl
0.86
1.05
1.04
1.08
1.21
1.35
1.41
1.50
0.11
0.12
0.23
0.39
0.62
0.66
0.71
0.81
7
3'-OCH3
4'-Cl
8
9
3'-Br
10
11
12
13
3'-CN
4'-CN
3'-NO2
3'-NO2
a. the ratios were estimated according to the area ratio from
HPLC analyses using RP-18 as stationary phase, eluting with
MeOH : H2O (80 : 20 volume ratio) and using a UV detector at a
wavelength of 254 nm.
Scheme II
R1: H (1a); Cl (1b); CH3 (1c); NO2 (1d)
The condensations of ethoxycarbonylcoumarin 3a
and 1a were also carried out in the solid support with P2O5,
instead of using PPA at 180 °C.
R2: H (3a); 6-OCH3 (3b); 8-OCH3 (3c); 6-NO2 (3d)
R1, R2: H, H (5aa); H, 6-OCH3 (5ab); H, 8-OCH3 (5ac); H, NO2
(5ad); Cl, H (5ba); CH3, H (5ca); 6-NO2, H (5da)
Under this conditions, low yields of benzothiazolyl-
coumarin 5, along with coumarin and compound 1 were ob-
tained regardless of the heating method. Apparently, com-
pound 3a is decarboxylated by P2O5. When this reaction
was performed with microwave irradiation, the mixture
tended to be tarnished. From Table 4 shows that a pro-
longed reaction time does not improve the yield, which in-
dicates that the decarboxylation of compound 3a, using
heating methods, is a higher efficient process.
CONCLUSIONS
g-Alumina is able to catalyze the condensation of 2-
aminothiophenol and hydroxylbenzaldehydes and 3-eth-
oxycarbonylcoumarin to form arylbenzothiazole and ben-
zothiazolylcoumarins, respectively, in solvent-free condi-
tions. However, the yields for the formation of coumarins
were relative poor due to the competition of decomposition
along the condensation.
EXPERIMENTAL
General. All melting points are uncorrected and were de-
termined in open capillaries. 1H and 13C NMR were recorded at
400 and 100 MHz, respectively, on a Bruker Advance 400 spec-
Fig. 1. Correlation between the relative reaction rate
and Hammett’s constants (s).
J. Chin. Chem. Soc. 2013, 60, 645-649
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Table 4. Yields of 5aa from condensation of 1a and 3a, under
different used solid supporting materials and heating
methodsa
peared from the TLC plate, the mixture was allowed to cool to
ambient temperature, filtered through a silica gel (5.0 g) and
washed by ethanol (20 mL). The pure compound 4a was obtained
(83% yield) upon evaporation of the solvent. The yields and mps
are summarized in Table 2.
Solid Supporting
Material
Yield
Entry
Method
Time (min)
(%)
1
g-Al2O3
g-Al2O3
g-Al2O3
g-Al2O3
a-Al2O3
a-Al2O3
a-Al2O3
SiO2
microwave
microwave
heat
10
60
35%
37%
34%
34%
28%
30%
25%
20%
24%
22%
10%
12%
8%
Competition reaction for the formation of arylbenzo-
thiazoles. A mixture of substituted benzaldehyde (1.0 mmole),
benzaldehyde 2a (1.0 mmole) and 2-aminothiophenol 1 (0.125 g,
1.0 mmole) in dried methanol (5.0 mL) was added to activated
g-Al2O3 (1.10 g, 10 mmole) in a round bottom flask. After stirring
for 30 min and removing the methanol under reduced pressure,
the resultant solution was heated in an oil-bath at 120 2 °C for
10 min. The mixture was filtred through silica gel and washed us-
ing methanol (20 mL), prior to HPLC analysis. The relative reac-
tion rates were estimated according to the ratio of the product ar-
eas in the chromatographic analysis.
2
3
30
4
heat
180
10
5
microwave
microwave
heat
6
60
7
180
10
8
microwave
microwave
heat
9
SiO2
60
10
11
12
13
SiO2
180
10
Nano-TiO2
Nano-TiO2
Nano-TiO2
microwave
microwave
heat
60
180
Typical procedure for the preparation of benzothiazolyl-
coumarins (5). Without P2O5: A mixture of ethoxycarbonyl-
coumarin 3a (0.218 g, 1.0 mmole) and 2-amino-5-methylthio-
phenol 1c (0.139 g, 1.0 mmole) in dried methanol (5.0 mL) was
added to activated g-Al2O3 (1.10 g, 10 mmole) in a round bottom
flask. After stirring for 30 min and removing the methanol under
reduced pressure, the resultant solution was heated in an oil-bath
at 120 °C. The progress of reaction was monitored by using TLC
analysis. Once the ethoxycarbonylcoumarin had disappeared
from the TLC plate, the mixture was allowed to cool to ambient
temperature, filtered through a silica gel (5.0 g) and washed by
ethanol (20 mL). The pure compound 5a was obtained (95%
yield) upon evaporization of the solvent. With P2O5: A mixture of
ethoxycarbonylcoumarin 3a (0.218 g, 1.0 mmole) and 2-amino-
5-methylthiophenol 1c (0.139 g, 1.0 mmole) in dried methanol
trometer at ambient temperature with TMS as an internal stan-
dard. HPLC analyses were performed on a Perkin-Elmer series
2001C pump with a Perkin-Elmer diode array detector LC295 de-
tection at 254 nm. a-Al2O3, SiO2 and nano-TiO2 were activated at
150 °C for 30 min prior to use while g–Al2O3 was activated at 450
°C.
Typical procedure for the preparation of arylbenzo-
thiazoles (4). Amixture of benzaldehyde 2a (0.106 g, 1.0 mmole)
and 2-aminothiophenol 1 (0.125 g, 1.0 mmole) in dried methanol
(5.0 mL) was added to activated g-Al2O3 (1.10 g, 10 mmole) in a
round bottom flask. After stirring for 30 min and removing the
methanol under reduced pressure, the resultant solution was
heated in an oil-bath at 120 °C. The progress of reaction was mon-
itored by using TLC analysis. Once the benzaldehyde had disap-
Table 5. Yields of 5, from condensations of 1 and 3 catalyzed by g-Al2O3, using an irradiation
power of 300Wa
Reported mp
Observed mp
R1
R2
Time (min)
Yield (%)
(°C)
(°C)
5aa
5ab
5ac
5ad
5ad
5ba
5ba
5ca
5da
H
H
H
10
20
20
30
60
10
30
15
30
211~2123b
222~2243b
239.5~2403b
211~212
222~223
238~240
35%
25%
22%
0%
6-OCH3
H
8-OCH3
6-NO2
6-NO2
Cl
H
H
0%
H
205~207
205~208
32%
34%
35%
0%
Cl
H
CH3
H
H
NO2
a. 1 mmole of aminothiophenol and 3-ethoxycarbonylcoumarins were heated at 120 °C in the
presence of 10 mmole g-Al2O3 and P2O5 (0.5 g); the progress of the reaction was monitored
using TLC. All of the known products were identified by their NMR spectra and melting
points. The yields are isolated yields.
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Alumina for Synthesis of Benzothiazole in Solvent-free
D.; Westwell, A. D.; Stevens, M. F. G. J. Med. Chem. 1999,
42, 4172; (c) Hutchinson, I.; Chua, M. S.; Browne, H. L.;
Trapani, V.; Bradshaw, T. D.; Westwell, A. D.; Stevens, M. F.
G. J. Med. Chem. 2001, 44, 1446; (d) Palmer, P. J.; Trigg, R.
B. Pure Appl. Chem. 2008, 80, 707–715; (e) Palmer, P. J.;
Trigg, R. B.; Warrington, J. V. J. Med. Chem. 1971, 14, 248;
(e) Lau, C. K.; Dufresne, C.; Gareau, Y.; Zamboni, R.;
Labelle, M.; Young, R. N.; Metters, K. M.; Rochette, C.;
Sawyer, N.; Slipetz, D. M.; Charette, L.; Jones, T.;
McAuliffe, M.; McFarlane, C.; Ford-Hutchinson, A. W.
Bioorg. Med. Chem. 1995, 5, 1615.
(5.0 mL) was added to activated g-Al2O3 (1.10 g, 10 mmole) in a
round bottom flask. After stirring for 30 min, removing the meth-
anol under reduced pressure and adding P2O5 (about 0.5 g), the re-
sultant solution was heated in an oil-bath at 120 °C. The progress
of the reaction was monitored using TLC analysis. Once 3a had
disappeared from the TLC plate, the mixture was allowed to cool
to ambient temperature, filtered through a silica gel (5.0 g) and
washed by ethanol (20 mL). It was then separated using a silica
gel column with hexane-EtOAc (9:1) as an eluent. 3-(6’-Methyl-
benzothizaolyl)coumarin (5ca), coumarin and 2-aminothiophe-
nol were obtained in sequence from the column. Compound 5ca:
a yellowish-green solid, 1H NMR d 2.53 (3H, s), 7.34-7.41 (2H,
m), 7.45 (1H, d, J = 8.4 Hz), 7.64 (1H, td, J = 8.0, 1.6 Hz), 7.73
(1H, dd, J = 8.4, 1.6 Hz), 7.74 (1H, s), 7.97 (1H, d, J = 8.4 Hz),
9.18 (1H, s); MS m/z (%) 293 (M+., 100), 265 (38), 236 (7); Calcd
for C17H11NO2S: C, 69.61; H, 3.78; N, 4.77. Found: C, 69.58; H,
3.77; N. 4.81.
2. (a) Chen, L.; Yang, C.; Li, S.; Olin, J. Spectrochim. Acta
Part. A 2007, 68, 317; (b) Abdelkrim, B. A.; Bakkas, S.;
Soufiaoui, M. Tetrahedron Lett. 1977, 38, 6395; (c)
Mukhopadhyay, C.; Datta, A. J. Heterocycl. Chem. 2009, 46,
91.
3. (a) Ayyangar, N. R.; Srinivasan, K. V.; Daniel, T. Dyes Pigm.
1990, 13, 301; (b) Chao, R. Y.; Ding, M. F.; Chen, J. Y.; Lee,
C. C.; Lin, S. T. J. Chin. Chem. Soc. 2010, 57, 213-221.
4. (a) Schiedel, M. S.; Chrstoph, A. B.; Petter, B. J. Organomet.
Chem. 2002, 653, 200; (b) Suzuki, A. Chem. Commun. 2005,
4759; (c) Adam, F. L.; Dai, C.; Fu, G. C. J. Am. Chem. Soc.
2000, 122, 4020.
3-(6’-Chlorobenzothiazolyl)coumarin (5ba): a green solid,
1H NMR d 7.41 (1H, t, J = 8.0 Hz), 7.46 (1H, d, J = 8.4 Hz), 7.50
(1H, dd, J = 8.8, 2.0 Hz), 7.67 (1H, td, J = 8.0, 1.6 Hz), 7.75 (1H,
dd, J = 8.0, 1.2 Hz), 7.96 (1H, d, J = 2.0 Hz), 8.01 (1H, d, J = 8.8
Hz), 9.09 (1H, s); 13C NMR d 160.4, 159.8, 153.8, 150.8, 141.8,
137.9, 133.5, 131.4, 129.5, 127.4, 125.3, 123.6, 121.3, 119.9,
118.9, 116.8; MS m/z (%) 315/313 (M+., 35/100), 287/285
(20/57), 222 (14); Calcd for C16H8ClNO2S: C, 61.25; H, 2.57; N,
4.46. Found: C, 61.18; H, 2.65; N, 4.42.
5. (a) Tanaka, K.; Toda, F. Chem. Rev. 2000, 100, 1025; (b)
Loupy, A. Top. Curr. Chem. 2000, 206, 153; (c) Lonpy, A.;
Song, S. J.; Sohn, S. M.; Lee, Y. M.; Kwon, T. W. J. Chem.
Soc., Perkin Trans. 1 2001, 1220.
6. (a) Gedye, R.; Smith, F.; Westaway, K.; Ali, H.; Baldisera,
L.; Laberge, L.; Rousell, J. Tetrahedron Lett. 1986, 27, 279;
(b) Caddick, S. Tetrahedron 1995, 51, 10403; (c) Struss, S.;
Trainor, R. W. Aust. J. Chem. 1995, 48, 1665.
ACKNOWLEDGEMENTS
7. (a) Medien, H. A. A. Z. Naturforsch 2002, 57b, 1320; (b)
Perosa, A.; Selva, M.; Tundo, P. J. Chem. Soc. Perkin 2,
1999, 2485; (c) Cabello, J. A.; Campelo, J. M.; Garcia, A.;
Luna, D.; Marinas, J. M. J. Org. Chem. 1984, 49, 5195.
8. (a) Tale, R. H. Org. Lett. 2002, 4, 1641; (b) Wattenberg, L.
W.; Page, M. A.; Leong, J. L. Cancer Res. 1968, 28, 2539; (c)
Hachiya, H.; Kirano, K.; Satoh, T.; Miura, M. Org. Lett.
2009, 8, 1737; (d) Sharghi, H.; Asemani, O. Synth. Commun.
2009, 39, 860.
The authors are grateful to the National Science
Council of ROC (NSC-95-2113-M-126-005) for financial
support and the National Center for High-Performance
Computing for technique information support.
REFERENCES
1. (a) Bradshaw, T. D.; Wrigley, S.; Shi, D. F.; Schulz, R. J.;
Paull, K. D.; Stevens, M. F. G. Br. J. Cancer 1998, 77, 745;
(b) Kashiyama, E.; Hutchinson, I.; Chua, M. S.; Stinson, S.
F.; Phillips, L. R.; Kaur, G.; Sausville, E. A.; Bradshaw, T.
9. Hansh, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.
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