Solvent-free synthesis of indole-based thiosemicarbazones under microwave irradiation

Article abstract of DOI:10.3184/030823410X12628833270368

A rapid, effi cient and environmentally friendly methodology has been developed for the synthesis of indole-3-carboxaldehyde thiosemicarbazones by using aluminum oxide as the solid support under microwave assisted solvent-free conditions. Compared with the conventional heating method, this method gave the target products in good yield.

Full text of DOI:10.3184/030823410X12628833270368

JOURNAL OF CHEMICAL RESEARCH 2010
RESEARCH PAPER 57
JANUARY, 57–60
Solvent-free synthesis of indole-based thiosemicarbazones under
microwave irradiation
Lingling Liu, Jie Yang, Zhigang Zhao*, Peiyu Shi and Xingli Liu
College of Chemistry and Environmental Protection Engineering, Southwest University for Nationalities, Chengdu 610041, P.R. China
A rapid, efficient and environmentally friendly methodology has been developed for the synthesis of indole-
3-carboxaldehyde thiosemicarbazones by using aluminum oxide as the solid support under microwave assisted
solvent-free conditions. Compared with the conventional heating method, this method gave the target products in
good yield.
Keywords: solvent-free conditions, microwave irradiation, indole-based thiosemicarbazones
Table 1 Effect of microwave powers on yields
Thiosemicarbazones and their metal complexes have attracted
considerable interest due to their antiviral, antibacterial,
antimalarial, antifungal, and tumour inhibitory biological
activities.1–6 Moreover, thiosemicarbazones bearing an aro-
matic heterocyclic moiety seem to possess enhanced biologi-
cal activities.7–8Among them, indole-based thiosemicarbazones
have aroused particular interest.
Power/W
Yield/%
240
52
280
64
360
79
425
90
460
81
500
70
As shown in Table 1, we irradiated the reaction using
different powers under the same reaction time (3 min). As a
result, 425 W was the optimum power. The yield improved up
to this power but it decreased under the highest power, owing
to carbonisation of this mixture.
Microwave irradiation has been applied to a large and
expanding range of chemical transformations in recent years,
and this method of energy transfer to a reaction mixture can
provide impressive enhancements in product yield, selectivity,
and reaction rate.9 This method has some obvious advantages:
the process is fast and simple; the reaction can be performed
under atmospheric pressure in a domestic microwave oven;
no high-pressure and high temperature apparatus is needed.10
Microwave irradiation provides a rapid and homogeneous
heating of the entire sample, facilitates formation of uniform
nucleation centres, and it is also energy efficient and
environmentally friendly. Other advantages of microwave
technique include its capability to screen a wide range of
experimental conditions (such as time and temperature).11–15
Moreover, with the development of “green chemistry”, the
focus has now shifted to less cumbersome solvent-free
methods, giving facile reactions to provide high yields of pure
products, thus eliminating or minimising the use of organic
solvents.16,17 We report here the synthesis of indole-based
thiosemicarbazones under solvent-free irradiation using neutral
aluminum oxide as mineral support and microwave organic
reaction enhancement in good yields. The synthetic route is
shown in Scheme 1.
Table 2 Effect of microwave irradiation time on yields
Time/min
Yield/%
1
79
2
82
3
90
4
71
5
65
As shown in Table 2, we irradiated the reaction using
different time under the same power (425 W). It was found
that 3 min was the optimum time, and more byproducts were
identified when the reaction time was increased.
Table 3 Effect of supports on yields
Supporter Zeolite
K2CO3
Neutral
Al2O3
Silica
gel H
Alkaline
Al2O3
Yield/% 71
75
90
54
69
As shown in Table 3, we irradiated the reaction using
different supports under the same time (3 min) and power
(425 W). This supports significantly affected the yield of
products. A low yield was obtained when using silica gel H,
zeolite, alkaline Al2O and K2CO as supports and irradiating
for 3 min. However, 3the product3 was obtained in a yield of
90% when using neutral Al2O3 as support.
Results and discussion
As shown in Scheme 1, intermediate 3a–j were obtained
using a prior method.18 Condensation of 3a–j with indole-
3-carboxaldehyde (4) afforded the products 5a–j under
solvent-free conditions using microwave irradiation.
As shown in Table 4, we carried out a comparison of
the synthesis of 5a–j between microwave irradiation in the
Structure elucidation of 5a–j were accomplished on the
basis of their elemental analysis and spectral data. The IR
spectra of compound 5a–j exhibited a characteristic strong
absorption at 3149–3412 cm−1 due to N–H stretching vibration;
The strong bands between 1500 and 1614 cm−1 indicated the
absorption of C=N; The strong absorption bands falling within
the range of 1193–1291 cm−1 were assigned to the C=S. In the
1H NMR spectra, the singlet peaks between δ 11.59–11.79
ppm were assigned to the protons in the =NH, the CH=N
proton appeared as a singlet peaks at δ 8.16–9.00 ppm. Their
mass spectra showed the expected molecular ions with a high
intensity.
Table 4 Synthetic comparison of thiosemicarbazones 5a–j
between solvent-free conditions under microwave irradiation
and conventional heating
a
b
Comp. Conventional method
Microwave method tC /tMW
t/min
Yield/%
t/min
Yield/%
5a
5b
5c
5d
5e
5f
360
240
360
360
240
300
240
300
300
360
71
85
69
60
73
75
80
79
69
68
3.0
2.0
3.0
4.0
3.0
3.0
3.0
3.0
2.0
4.0
90
96
90
85
91
92
94
88
91
88
120
120
120
90
80
100
120
100
150
90
In searching for the best reaction conditions for the reaction,
we examined the synthesis of 5a and varied the microwave
irradiation power, time and different supports. The typical
results are shown in the tables as follows.
5g
5h
5i
5j
TCa was the time of conventional heating method; TMWb was the
time of microwave irradiation method
* Correspondent. E-mail: zzg63129@yahoo.com.cn

58 JOURNAL OF CHEMICAL RESEARCH 2010
Scheme 1
INOVA 400 MHz spectrometer using DMSO-d6 as solvent and TMS
as internal standard. Mass spectra were determined on FinniganL-
CQDECA instrument. Elemental analysis was performed on a Carlo-
Erba-1106 autoanalyzer. Microwave irradiation was carried out with
a MCL-3 microwave oven which was modified from domestic
microwave oven and tested at full power (700 W) to conform to the
performance index before use. All the solvents were purified before
use.
solvent-free conditions and conventional heating. Compared to
conventional thermal heating, microwave irradiation greatly
decreased the reaction time from 240–360 min to 2–4 min.
It was obvious that yields were increased from 60–85% to
85–96%. From these data, we conclude that microwave
irradiation method is a rapid, efficient, and environmentally
friendly methodology for this synthesis.
In conclusion, we have developed a very rapid, efficient and
eco-friendly method for the preparation of thiosemicarbazones.
The reaction was conducted in the presence of aluminum
oxide, without using solvent, and assisted by microwave
irradiation. The salient features of this method include the
simple reaction set-ups, high yields of products, and short
reaction time.
General procedure for the preparation of substituted thiosemicarba-
zides 3a–j
A typical reaction was carried out as follows: A solution of substituted
amine (0.01 mol) in ethanol (10 mL) and concentrated aqueous
ammonia (2 mL) and was added carbon disulfide dropwise (0.8 mL),
and the mixture was stirred at 15–20 °C for 1–2 h to form a solid.
Sodium chloroacetate (1.2 g) was added to the stirred mixture and
then 85% hydrazine hydrate(1.2 mL) was added. Stirring continued
at 60 °C for 4 h to obtain a solid. The crude product was then
recrystallised from ethanol to obtain each thiosemicarbazide in
62–80% yields. The melting point of the thiosemicarbazide 3a–j is
shown in Table 5.
Experimental
Melting points were determined on a micro-melting point apparatus
and were uncorrected. IR spectra were obtained on 1700 Perkin-Elmer
1
FTIR using KBr disks. H NMR spectra were recorded on a Varian

JOURNAL OF CHEMICAL RESEARCH 2010 59
Table 5 The melting point of thiosemicarbazide 3a–j
4-(p-Chlorophenyl)-3-thiosemicarbazone of indole-3-carboxalde-
hyde (5e): Yellow crystal; yield 91%, m.p. 200–202 °C; IR (KBr)
(cm−1): 3405, 3307, 1613, 1543, 1492, 1268, 741; 1H NMR (DMSO-
d6, 400 MHz) δ: 7.13–7.23 (m, 2H, ArH), 7.41–7.45 (m, 3H, ArH),
7.67–7.71 (m, 2H, ArH), 7.92 (d, J = 2.8 Hz, 1H, indole-CH in
2-moiety), 8.25 (d, J = 7.6 Hz, 1H, ArH), 8.41 (s, 1H, NCH), 9.67 (s,
1H, NH), 11.67 (s, 1H, NH), 11.70 (s, 1H, indole-NH); ESI-MS m/z
(%): 367 ( [M+K]+, 100). Ana1.Calcd for C16H13ClN4S: C, 58.44; H,
3.98; N, 17.07. Found: C, 58.42; H, 3.97; N, 17.14%.
Product
Formula
M.p./°C
Lit M.p./°C
3a
3b
3c
3d
3e
3f
C H11N OS
C8H N3OS
C8H11N3OS
C8H11N 3OS
C7H9N3ClS
C7H8N3BrS
C7H8N3S
157–158
155–157
147–149
160–162
178–180
165–168
133–134
113–114
135–137
137–139
16119
15920
15221
16022
17823
163–16524
136–13725
114–11526
138–13927
136–13828
3g
3h
3i
C23H79N33OS
C H11N S
C171H10N33OS
4-(p-Bromophenyl)-3-thiosemicarbazone of indole-3-carboxalde-
hyde (5f): Pale yellow crystal; yield 92%, m.p. 191–192 °C; IR (KBr)
(cm−1): 3398, 3294, 3182, 1614, 1547, 1199, 741; 1H NMR (DMSO-
d6, 400 MHz) δ: 7.16 (t, J = 7.2 Hz, 1H, ArH), 7.23 (t, J = 7.2 Hz, 1H,
ArH), 7.43–7.48 (m, 1H, ArH), 7.57 (dd, J = 2.8 Hz, J = 1.6 Hz, 2H,
ArH), 7.65 (d, J = 8.8 Hz, 2H, ArH), 7.92 (d, J = 3.2 Hz, 1H, indole-
CH in 2-moiety), 8.24 (d, J = 7.6 Hz, 1H, ArH), 8.41 (s, 1H, NCH),
9.66 (s, 1H, NH), 11.67 (s, 1H, NH), 11.70 (s, 1H, indole-NH);
ESI-MS m/z (%): 371 ( [M-1]−, 100). Ana1.Calcd for C16H13BrN4S:
C, 51.48; H, 3.51; N, 15.01. Found: C, 51.40; H, 3.50; N, 14.97%.
4-Methyl-3-thiosemicarbazone of indole-3-carboxaldehyde (5g):
Pale yellow crystal; yield 94%, m.p. 209–210 °C; IR (KBr) (cm−1):
3j
General procedure for the preparation of thiosemicarbazones 5a–j
Conventional method: Five drops of acetic acid were added to a
mixture of the substituted thiosemicarbazide (3) (1 mmol) and indole-
3-carboxaldehyde (4) (1 mmol) in refluxing ethanol (10 mL). The
solution was refluxed for 4–6 h at 85 °C, and the mixture was allowed
to cool to room temperature and filtered. The crude product was then
recrystallised from ethyl alcohol in 60–85% yields.
1
3365, 3236, 1606, 1544, 1441, 1237, 735; H NMR (DMSO-d6, 400
MHz) δ: 3.08 (d, J = 4.8 Hz, 3H, CH3), 7.12 (t, J = 7.2 Hz, 1H, ArH),
7.18 (t, J = 8 Hz, 1H, ArH), 7.43 (d, J = 8.4 Hz, 1H, ArH), 7.80
(d, J = 2.4 Hz, 1H, indole-CH in 2-moiety), 7.91 (m, 1H, NH), 8.28
(d, J = 8 Hz, 1H, ArH), 8.29 (s, 1H, NCH), 11.17 (s, 1H, NH), 11.59
(s, 1H, indole-NH); ESI-MS m/z (%): 233 ( [M+1]+, 100). Ana1.Calcd
for C11H12N4S: C, 56.87; H, 5.21; N, 24.12. Found: C, 56.83; H, 5.20;
N, 24.10%.
4-Hydroxyethyl-3-thiosemicarbazone of indole-3-carboxaldehyde
(5h): Pale yellow crystal; yield 88%, m.p. 204–206 °C; IR (KBr)
(cm−1): 3380, 3170, 1614, 1549, 1236, 749; 1H NMR (DMSO-d6, 400
MHz) δ: 3.59–3.70 (m, 4H, CH2), 4.91 (t, J = 4.8 Hz, 1H, OH), 7.15
(t, J = 7.2 Hz, 1H, ArH), 7.22 (t, J = 7.2 Hz, 1H, ArH), 7.44 (d, J =
8 Hz, 1H, ArH), 7.82 (d, J = 2.8 Hz, 1H, indole-CH in 2-moiety), 7.96
(t, J = 4.4 Hz, 1H, NH), 8.16 (d, J = 7.6 Hz, 1H, ArH), 8.30 (s, 1H,
NCH), 11.28 (s, 1H, NH), 11.62 (s, 1H, indole-NH); ESI-MS m/z (%):
263 ( [M+1]+, 100). Ana1.Calcd for C12H14N4OS: C, 54.94; H, 5.34;
N, 21.36. Found: C, 54.97; H, 5.33; N, 21.42%.
4-(α-Naphthyl)-3-thiosemicarbazone of indole-3-carboxaldehyde
(5i): Pale yellow crystal; yield 91%, m.p. 212–213 °C; IR (KBr)
(cm−1): 3261, 3149, 3054, 1602, 1544, 1490, 1221; 1H NMR (DMSO-
d6, 400 MHz) δ: 7.09 (t, J = 7.6 Hz, 1H, ArH), 7.20 (t, J = 7.6 Hz, 1H,
ArH), 7.45 (d, J = 8 Hz, 1H, ArH), 7.53–7.59 (m, 3H, ArH), 7.68
(d, J = 7.2 Hz, 1H, ArH), 7.88 (d, J = 8.4 Hz, 1H, ArH), 7.92 (d,
J = 2.8 Hz, 1H, indole-CH in 2-moiety), 7.94–7.96 (m, 1H, ArH),
7.97–8.01 (m, 1H, ArH), 8.36 (d, J = 8 Hz, 1H, ArH), 8.48 (s, 1H,
NCH), 9.85 (s, 1H, NH), 11.68 (s, 1H, NH), 11.68 (s, 1H, indole-NH);
ESI-MS m/z (%): 367 ( [M+Na]+, 100). Ana1.Calcd for C20H16N4S:
C, 69.74; H, 4.68; N, 16.27. Found: C, 69.73; H, 4.66; N, 16.26%.
4-Phenyl-3-thiosemicarbazone of indole-3-carboxaldehyde (5j):
Yellow crystal; yield 88%, m.p. 197–199 °C; IR (KBr) (cm−1): 3412,
3320, 3119, 2965, 1601, 1545, 1278; 1H NMR (DMSO-d6, 400 MHz)
δ: 7.13–7.22 (m, 3H, ArH), 7.40 (t, J = 7.6 Hz, 2H, ArH), 7.45 (d,
J = 8 Hz, 1H, ArH), 7.66 (d, J = 7.6 Hz, 2H, ArH), 7.92 (d, J = 2.8 Hz,
1H, indole-CH in 2-moiety), 8.23 (d, J = 7.6 Hz, 1H, ArH), 8.41
(s, 1H, NCH), 9.61 (s, 1H, NH), 11.60 (s, 1H, NH), 11.69 (s, 1H,
indole-NH); ESI-MS m/z (%): 295 ( [M+1]+, 100). Ana1.Calcd for
C16H14N4S: C, 65.28; H, 4.79; N, 19.03. Found: C, 65.27; H, 4.78;
N, 19.00%.
Microwave irradiation method: The substituted thiosemicarbazide
(3) (1 mmol), indole-3-carboxaldehyde(4) (1 mmol) and neutral
aluminium oxide (0.8 g) were put in a porcelain mortar, then acetic
acid (5 drops) was added. After grinding, the mixture was put in a
round-bottom flask (25 mL) and was placed in the microwave oven.
Then it was irradiated for 2–4 min at 240–510 W. The reaction
mixture was cooled to room temperature and was dissolved in DMSO
and filtered. The filtrate was added to water and the product was
precipitated. The product was recrystallised from DMSO and H2O in
85–96% yields. The physical and spectra data of the compounds 5a–j
are as follows.
4-(m-Methoxyphenyl)-3-thiosemicarbazone of indole-3-carboxal-
dehyde (5a): Pale yellow crystal; yield 90%, m.p. 192–194 °C; IR
1
(KBr) (cm−1): 3411, 3315, 1598, 1557, 1291, 1196, 735; H NMR
(DMSO-d6, 400 MHz) δ: 3.76 (s, 3H, OCH3), 6.78 (dd, J = 2 Hz, J = 2
Hz, 1H, ArH), 7.14 (t, J = 7.2 Hz, 1H, ArH), 7.19 (t, J = 6.8 Hz, 2H,
ArH), 7.26 (t, J = 8.4 Hz, 1H, ArH), 7.44 (d, J = 8.4 Hz, 2H, ArH),
7.93 (d, J = 2.8 Hz, 1H, indole-CH in 2-moiety), 8.22 (d, J = 7.6 Hz,
1H, ArH), 8.41 (s, 1H, NCH), 9.59 (s, 1H, NH), 11.63 (s, 1H, NH),
11.70 (s, 1H, indole-NH); ESI-MS m/z (%): 671 ( [2M+Na]+, 100).
Ana1.Calcd for C17H16N4OS: C, 62.94; H, 4.97; N, 17.27. Found:
C, 62.93; H, 4.97; N 17.25%.
4-(o-Methoxyphenyl)-3-thiosemicarbazone of indole-3-carboxal-
dehyde (5b): Pale yellow crystal; yield 96%, m.p. 178–180 °C; IR
1
(KBr) (cm−1): 3391, 3280, 1600, 1549, 1241, 1193, 744; H NMR
(DMSO-d6, 400 MHz) δ: 4.03 (s, 3H, OCH3), 6.96–7.00 (m, 1H, ArH),
7.10–7.17 (m, 2H, ArH), 7.24–7.29 (m, 2H, ArH), 7.48–7.52 (m, 1H,
ArH), 7.93(d, J = 2.4 Hz, 1H, indole-CH in 2-moiety), 8.25–8.28
(m, 1H, ArH), 8.43 (s, 1H, NCH), 9.00 (d, J = 7.6 Hz, 1H, ArH), 10.03
(s, 1H, NH), 11.76 (s, 1H, indole-NH), 11.79 (s, 1H, NH); ESI-MS
m/z (%): 325 ( [M+1]+, 100). Ana1.Calcd for C17H16N4OS: C, 62.94;
H, 4.97; N, 17.27. Found: C, 62.81; H, 4.98; N, 17.21%.
4-(p-Methoxyphenyl)-3-thiosemicarbazone of indole-3-carboxal-
dehyde (5c): Pale yellow crystal; yield 90%, m.p. 207–209 °C; IR
1
(KBr) (cm−1): 3307, 3149, 2983,1607, 1553, 1513, 1242, 744; H
NMR (DMSO-d6, 400 MHz) δ: 3.77 (s, 3H, OCH3), 6.93 (d, J = 8.8
Hz, 2H, ArH), 7.12 (t, J = 6.8 Hz, 1H, ArH), 7.18 (t, J = 6.8 Hz, 1H,
ArH), 7.42 (dd, J = 8.8 Hz, J = 10.4 Hz, 3H, ArH), 7.90 (d, J = 2.4 Hz,
1H, indole-CH in 2-moiety), 8.25 (d, J = 8 Hz, 1H, ArH), 8.39 (s, 1H,
NCH), 9.47 (s, 1H, NH), 11.50 (s, 1H, NH), 11.67 (s, 1H, indole-NH);
ESI-MS m/z (%): 671 ( [2M+Na]+, 100). Ana1.Calcd for C17H16N4OS:
C, 62.94; H, 4.97; N, 17.27. Found: C, 62.83; H, 4.95; N, 17.25%.
4-(m-Hydroxyphenyl)-3-thiosemicarbazone of indole-3-carboxal-
dehyde (5d): Pale green crystal; yield 85%, m.p. 211–212 °C; IR
We thank the Natural Science Foundation of the State Ethnic
Affairs Commission of P.R.China (Project No.09XN08) for
the financial support.
Received 4 November 2009; accepted 4 January 2010
Paper 090863 doi: 10.3184/030823410X12628833270368
Published online: 22 January 2010
1
(KBr) (cm−1): 3386, 3303, 3236, 3079, 1611, 1553, 1233, 740; H
NMR (DMSO-d6, 400 MHz) δ: 6.61 (dd, J = 1.6 Hz, J = 2 Hz, 1H,
ArH), 7.06 (d, J = 8.4 Hz, 1H, ArH), 7.13–7.23 (m, 4H, ArH), 7.46 (d,
J = 8 Hz, 1H, ArH), 7.91 (d, J = 2.8 Hz, 1H, indole-CH in 2-moiety),
8.20 (d, J = 7.6 Hz, 1H, ArH), 8.40 (s, 1H, NCH), 9.48 (s, 1H, OH),
9.49 (s, 1H, NH), 11.57 (s, 1H, NH), 11.69 (s, 1H, indole-NH);
ESI-MS m/z (%): 643 ( [2M+Na]+, 100). Ana1.Calcd for C16H14N4OS:
C, 61.92; H, 4.55; N, 18.05. Found: C, 61.78; H, 4.56; N, 18.07%.
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