Xanthan sulfuric acid: An efficient and biodegradable solid acid catalyst for the synthesis of bis(indolyl)methanes under solvent-free conditions

Article abstract of DOI:10.1016/j.crci.2013.04.013

Xanthan sulfuric acid (XSA) is found to be an efficient catalyst for the electrophilic substitution reaction of indole with aromatic aldehydes to afford the corresponding bis(indolyl)methanes at room temperature under solvent-free conditions. The catalyst

Full text of DOI:10.1016/j.crci.2013.04.013

C. R. Chimie 16 (2013) 829–837
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Full paper/Memoire
Xanthan sulfuric acid: An efficient and biodegradable solid acid catalyst
for the synthesis of bis(indolyl)methanes under solvent-free conditions
*
Zeba N. Siddiqui , Saima Tarannum
Department of Chemistry, Aligarh Muslim University, 202002 Aligarh, India
A R T I C L E I N F O
A B S T R A C T
Article history:
Xanthan sulfuric acid (XSA) is found to be an efficient catalyst for the electrophilic
substitution reaction of indole with aromatic aldehydes to afford the corresponding
bis(indolyl)methanes at room temperature under solvent-free conditions. The catalyst
was characterized for the first time with the help of powder XRD, SEM–EDX and DSC–TGA.
The attractive features of this green, new methodology are excellent yield of products,
clean reaction profile, reusability of the catalyst, energy sustainable protocol, simple
experimental and easier work-up procedures.
Received 1 January 2013
Accepted after revision 24 April 2013
Available online 10 July 2013
Keywords:
Xanthan sulfuric acid
Indole
´
ß 2013 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Bis(indolyl)methanes
Solvent-free conditions
Heterogeneous catalyst
1. Introduction
have proven to be useful to chemists for lab-scale as well as
for large-scale conversions [5,6].
Replacement of conventional toxic and pollutant
Brønsted and Lewis acid catalysts with environmentally
benign and reusable solid heterogeneous catalysts is an
active area of current research. Solid acid catalysts have
advantages such as ease of product separation, recycling of
the catalyst and environmental acceptability as compared
to liquid acid catalysts [1]. Carbon-based solid acid
catalysts have many advantages. They are insoluble in
common organic solvents, cause low corrosion, and are
environmentally benign. Also the products could be easily
separated from the reaction mixture and the catalyst is
recoverable without decreasing its activity [2]. Because of
their strong protonic acid sites, they generally exhibit
higher catalytic activity than conventional catalysts [3,4].
In recent years, much effort has been directed towards the
development of new organic transformations under
environmentally friendly conditions. In this context,
organic reactions under solvent-free grinding conditions
Among natural biopolymers, xanthan is the most
abundant bacterial exopolysaccharide, being produced
through fermentation [7]. It has been widely studied
during the past several decades because it is a biodegrad-
able material and
a renewable resource. Its unique
properties make it an attractive alternative to conventional
organic or inorganic supports in catalytic applications [8].
It is very stable under a wide range of temperatures and pH
values [9]. Recently, xanthan sulfuric acid (XSA) has
emerged as a promising biopolymeric solid-support acid
catalyst for acid catalysed reactions, such as the synthesis
of 3,4-dihydropyrimidin-2(1H)-ones [10], thiadiazolo ben-
zimidazoles [11], 4,40-(arylmethylene)bis(1H-pyrazol-5-
ols) [12], 14-aryl-14H-dibenzo[a,i]xanthene-8,13-diones
[13], N-substituted pyrroles [14],
[15] and coumarins [16].
a-amino phosphonates
Indoles are convenient starting materials for the
synthesis of series of compounds with promising
a
practical uses. Bis(indolyl)methanes and bis(indoly-
l)ethanes are important derivatives of indole. Bis(indo-
lyl)methanes are the most active cruciferous substances
for promoting beneficial oestrogen metabolism in women
and men [17]. They are also effective in the prevention of
*
Corresponding author.
E-mail addresses: siddiqui_zeba@yahoo.co.in, zns.siddiqui@gmail.-
com (Z.N. Siddiqui), saimatarannum.amu@gmail.com (S. Tarannum).
1631-0748/$ – see front matter ß 2013 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.crci.2013.04.013

830
Z.N. Siddiqui, S. Tarannum / C. R. Chimie 16 (2013) 829–837
cancer due to their ability to modulate certain cancer
causing oestrogen metabolites [18]. Moreover, these
compounds may normalize abnormal cell growth asso-
ciated with cervical dysplasia. Several methods have been
reported in the literature for the synthesis of bis(indo-
lyl)methanes using heterogeneous catalysts including
zeolite [19], ion exchange resins [20], PPh3–HClO4 [21],
silica-supported sodium hydrogen sulphate and amber-
lyst-15 [22], H3PO4–SiO2 [23], metal hydrogen sulphates
[24], P2O5–SiO2 [25], KHSO4–SiO2 [26], SiO2–POCl2 [27],
LiHSO4–SiO2 [28], SiO2–ClCH2COOH [29], Ce(OTf)4 [30],
Zeokarb-225 [31], Nano-TiO2 [32], PEG–SO3H [33], silica
sulfuric acid [34] and cellulose sulfuric acid [35]. Many of
the methods used have disadvantages such as long
reaction periods, use of hazardous solvents, use of
expensive reagents or preformed reagents, poor yields of
products, and the fact that they are not environmentally
friendly. For these reasons, there is a great effort to replace
the conventional catalysts by eco-friendly and green-
process catalysts.
It is, therefore, of interest to examine the behaviour of
XSA as a catalyst for the synthesis of bis(indolyl)methanes.
To the best of our knowledge, there is no report on the
synthesis of bis(indolyl)methanes using XSA as a catalyst.
Herein, we describe the use of XSA as a highly efficient,
biodegradable and recyclable solid acid catalyst for the
synthesis of bis(indolyl)methanes by the reaction of indole
with different heterocyclic/aromatic aldehydes under
solvent-free conditions in excellent yields. The compounds
were identified on the basis of spectral data. The structure
and morphology of the catalyst was established for the first
time with the help of powder X-ray diffraction (XRD),
scanning electron microscopy (SEM), energy dispersive X-
ray analysis (EDX), differential scanning calorimetry (DSC),
and thermo-gravimetric analysis (TGA).
asymmetric stretching vibrations of the SO2 group,
respectively. Another stretching absorption band at 599
to 659 cmꢀ1 was attributed to the S–O functional group.
The spectrum also showed a strong broad band for OH
stretching absorption in the range from 3261 to 3467 cmꢀ1
[36].
2.1.2. Powder X-ray diffraction (XRD) analysis of free xanthan
and XSA
The structure of the prepared catalyst was identified by
powder XRD. X-ray patterns of the free xanthan and
catalyst xanthan–OSO3H were recorded in the 2u = 20–798
range. The XRD of free xanthan showed no considerable
peaks of crystallinity (Fig. 1a) [37]. However, the XRD of
XSA (Fig. 1b) showed small amount of crystallinity with
the characteristic peaks of SO3 groups.
2.1.3. SEM-EDX analysis of the catalyst
To study the surface morphology of the catalyst, SEM
micrographs of the catalyst were employed. The SEM
images of the catalyst (Fig. 2) clearly show the strand-like
nature of the sample.
Further, EDX analysis (Fig. 3) of the catalyst showed the
presence of S, O and C elements, suggesting formation of
the expected catalytic system.
2.1.4. DSC-TGA of the catalyst (XSA)
DSC analysis (Fig. 4) of the catalyst (XSA) was
performed in the temperature range between 20 and
300 8C at a constant heating rate of 10 8C/min under
nitrogen atmosphere. The DSC curve shows an irreversible
endothermic transition in the region between 100 and
136 8C, which may be due to the loss of a water molecule
from the polymer matrix. This analysis also shows that the
catalyst is stable up to 270 8C, after which it shows some
exothermic transition.
TG analysis (Fig. 5) of the catalyst (XSA) was performed
in the temperature range between 20 and 500 8C at a
constant heating rate of 20 8C/min under nitrogen atmo-
sphere. The TG curve shows the weight loss at 61.5 8C,
which is due to the loss of water molecules trapped in the
support matrix. The weight loss of 42% at a temperature of
283.9 8C may be due to the decomposition of xanthan.
2. Results and discussion
2.1. Catalyst characterization
2.1.1. FT-IR spectrum of xanthan sulfuric acid (XSA)
The FT-IR spectrum of the catalyst showed absorption
bands at 1263 and 1374 cmꢀ1 for the symmetric and
Fig. 1. Powder XRD pattern of (a) free xanthan and (b) fresh catalyst.

Z.N. Siddiqui, S. Tarannum / C. R. Chimie 16 (2013) 829–837
831
Fig. 2. The SEM images of freshly synthesized catalyst (XSA) at different magnifications.
Since 3-position of indole is the preferred site for
electrophilic reactions, substitution occurred exclusively
at this position, and N-substituted products were not
detected in the reaction mixture.
The structural assignment of all the compounds (3a–h)
was done by collection of elemental and spectroscopic data
(IR, NMR and MS). The IR spectrum of the newly
synthesized compound (3a) exhibited strong absorption
bands at 3467 and 3400 cmꢀ1 for two NH groups of the
indole moiety. The proton nuclear magnetic resonance
spectroscopy exhibited sharp singlets at
d = 1.98 and 5.87
for three methyl and one methine protons (Ha), respec-
tively. Fifteen aromatic protons (five protons of a phenyl
group of pyrazole moiety and ten protons of indole unit)
Fig. 3. EDX analysis of the catalyst (XSA).
Further, weight loss of 23.7% at a temperature of 373.7 8C
may be due to the decomposition of the SO3 group attached
to the support [38].
were discernible as multiplet at
protons of the indole moiety were present as singlet at
= 10.47. The 13C NMR spectrum showed signals at
= 13.4
d 6.80–7.57. Two NH
d
d
and 29.6 for methyl and methine carbon, respectively.
Other carbon signals appeared at their appropriate
positions and will be discussed in the experimental
section. Further, evidence of the formation of 3a was
obtained by mass spectrometry, which showed a mole-
cular ion peak at m/z = 436.
2.2. Synthesis of bis(indolyl)methanes
In the present study, a series of bis(indolyl)methanes
were prepared by the condensation of indole (1) and
different aldehydes (2a-h) in the presence of XSA (Scheme
1). The reactions proceeded smoothly to afford bis(indo-
lyl)methanes (3a–h) in good to excellent yields. The results
are summarized in Table 1.
A plausible mechanism for the formation of 3a in the
presence of XSA is displayed in Scheme 2.
Fig. 5. TGA of catalyst (XSA).
Fig. 4. DSC of the catalyst (XSA).

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Z.N. Siddiqui, S. Tarannum / C. R. Chimie 16 (2013) 829–837
Scheme 1. Synthesis of bis(indolyl)methanes using XSA under grinding condition.
Table 1
XSA catalyzed synthesis of bis(indolyl)methanes.
Entry
1
Aldehyde
Product
Timea (min)
25
Yieldb (%)
92
2
20
86
c
3
30
88

Z.N. Siddiqui, S. Tarannum / C. R. Chimie 16 (2013) 829–837
833
Table 1 (Continued )
Entry
4
Aldehyde
Product
Timea (min)
25
Yieldb (%)
90
5
35
86
6
30
84
7
32
86
8
35
90
a
Reaction progress monitored by TLC.
Isolated yield.
b
c
New compounds.
2.3. Catalytic reaction
carried out in the presence of xanthan gum, a highly sticky
orange reaction mixture was obtained. Though the
reaction mixture indicated the formation of a product,
no pure product could be isolated. When XSA was used as a
solid acid catalyst, the product was obtained in excellent
yields (84–92%) in a shorter time period (20–35 min). The
reaction of 5-chloro-3-methyl-1-phenylpyrazole-4-car-
boxaldehyde (1 mmol) with indole (2 mmol) in the
presence of XSA was used as a model reaction.
To study the appropriate reaction conditions for the
synthesis of bis(indolyl)methanes, the condensation of
aldehyde (2a) and indole (1) was initially examined in the
presence of chlorosulfonic acid. It was observed that
chlorosulfonic acid promoted the reaction with impure
products (28%) with generation of harmful wastes, which
pose environmental problems. When the reaction was

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Z.N. Siddiqui, S. Tarannum / C. R. Chimie 16 (2013) 829–837
Scheme 2. Proposed mechanism for the formation of 3a.
completed in a relatively shorter time period, but with a
2.3.1. Effect of different catalysts
moderate yield of the product (Table 2, entry 6). With NiCl2
and AlCl3, only trace amounts of the products were
obtained (Table 2, entries 7, 8). When XSA was used as a
catalyst, the reaction was completed in a shorter reaction
time, with an excellent yield of the product (Table 2, entry
9).
In order to emphasize the efficiency of XSA in
comparison with other catalysts, the model reaction was
carried out with various catalysts such as L-proline, Zn (L-
proline)2, zinc acetate, sulfamic acid, sulfuric acid,
benzenesulfonic acid, NiCl2, and AlCl3 (Table 2). It was
observed that L-proline and Zn (L-proline)2 could not
catalyse the reaction (Table 2, entries 1, 2). When the
reaction was performed with sulfamic acid, it was
completed after a long time period with impure products
(Table 2, entry 3). In a comparative study, the model
reaction was also carried out in sulfuric acid and
benzenesulfonic acid. The result showed that the reaction
took a longer time for completion (more than 4 h) and that
the products were obtained in trace amounts (Table 2,
entries 4, 5). Using zinc acetate, the reaction was
2.3.2. Effect of solvents
In order to study the solvent’s effect, the model reaction
was carried out in different protic and aprotic solvents
such as CH3COOH, MeOH, EtOH, (CH3)2CHOH, CH2Cl2, and
CH3CN. When the reaction was performed in EtOH, MeOH,
and (CH3)2CHOH, lower yields of the product were
obtained after a longer time period (Table 3, entries 4, 5,
6). Using CH3COOH, the reaction was completed in a
relatively shorter time period, with impure products (Table
Table 3
Table 2
Comparative study for the synthesis of bis(indolyl)methanes using
solution conditions versus the solvent-free method.
The screening of different catalysts on the model reaction under solvent-
free condition.
Entrya
Solvent
Temperature
Timeb
Yieldc (%)
Entrya
Catalyst
Timeb
Yieldc (%)
1
2
3
4
5
6
7
8
Grinding
No solventd
CH3COOH
EtOH
RT
25 min
35 min
3 h
92
1
2
3
4
5
6
7
8
9
L
-proline
–
–
70 8C
RT
48
Zn ( -proline)2
L
–
–
Mixture
72
Sulfamic acid
Sulfuric acid
Benzenesulfonic acid
Zinc acetate
NiCl2
2 h
60
RT
7 h
5 h
Trace
Trace
72
MeOH
RT
8 h
64
4.5 h
40 min
1 h
(CH3)2CHOH
CH2Cl2
RT
23 h
43 h
46 h
54
RT
36
Trace
Trace
92
CH3CN
RT
28
AlCl3
50 min
25 min
XSA
a
Reaction of 5-chloro-3-methyl-1-phenylpyrazole-4-carboxaldehyde
(1 mmol) with indole (2 mmol) in the presence of 200 mg of catalyst.
a
Reaction of 5-chloro-3-methyl-1-phenylpyrazole-4-carboxaldehyde
(1 mmol) with indole (2 mmol) in the presence of 200 mg of catalyst.
b
Reaction progress monitored by TLC.
c
b
Isolated yield.
Reaction progress monitored by TLC.
d
c
Mp of indole (1) = 52 8C [39], Mp of (2a) = 148 8C [40].
Isolated yield.

Z.N. Siddiqui, S. Tarannum / C. R. Chimie 16 (2013) 829–837
835
Table 4
Effect of catalyst loading on the synthesis of bis(indolyl)methane.
Entry
Catalyst (mg)
Timea
Yieldb (%)
1
2
3
4
5
6
7
60
4 h
35
42
65
84
92
90
20
100
140
180
200
220
None
2.5 h
2 h
1 h
25 min
20 min
10 h
a
Reaction progress monitored by TLC.
Isolated yield.
b
3, entry 3). In CH2Cl2 and CH3CN, again only trace amounts
of the product were obtained (Table 3, entries 7, 8). When
the reaction was carried out under grinding conditions,
both the yield and the reaction time were significantly
improved (Table 3, entry 1). In comparison with the
solvent-free grinding condition, the model reaction was
also performed under solvent-free heating. It was observed
that the reaction was completed in a relatively shorter time
period, but the product was obtained in lower yields (Table
3, entry 2).
Fig. 7. Powder XRD pattern of a recovered catalyst after four runs.
was applied for all recycling studies. The results (Fig. 6)
revealed that the catalyst exhibited a good catalytic
activity up to four consecutive cycles.
2.3.3. Loading of the catalyst
The recovered catalyst was identified by powder XRD,
SEM, and EDX analysis. It was observed that the peaks
remained the same (Fig. 7) and also that no change in the
morphology of the catalyst (Fig. 8) was observed as
compared to the fresh catalyst, suggesting that the catalyst
was active up to four cycles. EDX analysis (Fig. 9) also
showed that no leaching due to hydrolysis of the supported
xanthan hydrogen sulphate occurred up to four runs.
Furthermore, acid–base titration was done to confirm the
number of acidic groups and it was found to be
0.58 mequiv/g for the recovered catalyst (after four runs)
against 0.62 mequiv/g for the fresh catalyst [36].
The model reaction was carried out using different
amounts (60, 100, 140, 180, 200 and 220 mg) of XSA for the
optimization of the catalyst. It was found that 200 mg of
catalyst is sufficient for the fruitful completion of the
reaction. It was found that when the reaction was carried
out without the use of a catalyst, it required longer reaction
time for completion with the formation of by-products
(Table 4, entry 7). As the amount of the catalyst increased, a
reduction in time period and an enhancement in the
product yield was observed. A maximum yield of the
product (92%) and a shorter time period (25 min) for the
completion of reaction were observed when the amount of
the catalyst was 200 mg. Further, an increase in the
amount of the catalyst reduced the time period, but the
yield of the product was lowered (Table 4, entry 6).
3. Experimental
3.1. General
2.4. Recycling study of catalyst
Melting points of all synthesized compounds were
taken in
a Riechert Thermover instrument and are
Using the model reaction, indole, aldehyde and XSA
were ground together in a mortar with a pestle at room
temperature for a specified time period. On completion of
reaction, methanol was added and the reaction mixture
filtered. The recovered catalyst was washed with methanol
thoroughly (4 ꢁ 10 mL), dried in an oven at 80 8C for 2 h
and used for the subsequent cycles. The same procedure
Fig. 6. Recyclability of the catalytic system.
Fig. 8. SEM image of the recovered catalyst after four runs.

836
Z.N. Siddiqui, S. Tarannum / C. R. Chimie 16 (2013) 829–837
pestle at room temperature for a specified period. On
completion of reaction (as monitored by TLC), methanol
(20 mL) was added and the reaction mixture filtered. The
catalyst was washed with methanol several times. The
solvent was evaporated under reduced pressure to obtain
the product in almost pure form, which was further
purified by crystallization from suitable solvents.
3.4. Spectroscopic data
3.4.1. 4-[Bis(indol-3-yl)methyl]-5-chloro-3-methyl-1-
phenylpyrazole (3a)
Fig. 9. EDX analysis of the recovered catalyst after four runs.
Mp: 210–212 8C. IR (KBr)
(NH), 1596 and 1541(C5C). 1H NMR (DMSO-d6, 300 MHz):
1.98 (s, 3H, CH3), 5.87 (s, 1H, Ha), 6.80–7.57 (m, 13Ar–
H + 2Hb), 10.47 (s, 2H, NH). 13C NMR (300 MHz):
148.58,
n
max/cmꢀ1: 3467 and 3400
uncorrected. The IR spectra (KBr) were recorded on
PerkinElmer RXI spectrometer. 1H NMR and 13C NMR
spectra were recorded with a Bruker DRX-300 and Bruker
Avance II 400 spectrometer using tetramethylsilane (TMS)
as an internal standard and DMSO-d6/CDCl3 as a solvent.
ESI–MS were recorded with a Quattro II (ESI) spectrometer.
Elemental analyses (C, H and N) were conducted using the
Elemental vario EL III elemental analyser and their results
were found to be in agreement with the calculated values.
5-Chloro-3-methyl-1-phenylpyrazole-4-carboxaldehyde,
5-azido-3-methyl-1-phenylpyrazole-4-carboxaldehyde
and 3-formylchromone were synthesized by the reported
procedures [40–42]. Xanthan was purchased from Otto
Chemie Pvt Ltd. Other chemicals were of commercial grade
and used without further purification. The homogeneity of
the compounds was checked by thin layer chromatography
(TLC) on glass plates coated with silica gel G254 (E. Merck)
a using chloroform–methanol (3:1) mixture as the mobile
phase and visualized using iodine vapours. X-ray diffrac-
d
d
138.35, 136.75, 128.78, 127.48, 126.64, 124.37, 123.64,
121.00, 118.92, 118.34, 115.40, 111.41, 29.66, 13.45. ESI–
MS: M+436 (m/z). Anal. calcd for C27H21N4Cl: C, 74.38; H,
4.82; N, 12.84. Found: C, 73.37; H, 4.85, N, 12.82.
3.4.2. 4-[Bis(indol-3-yl)methyl]-5-azido-3-methyl-1-
phenylpyrazole (3b)
Mp: 220–222 8C. IR (KBr)
(NH), 1542 and 1521 (C5C). 1H NMR (DMSO-d6, 300 MHz):
1.98 (s, 3H, CH3), 5.87 (s, 1H, Ha), 6.80–7.57 (13 Ar–
H + 2Hb), 10.47 (s, 2H, NH). 13C NMR (300 MHz):
146.58,
n
max/cmꢀ1: 3421 and 3374
d
d
134.75, 133.95, 126.36, 125.24, 124.28, 123.80, 122.32,
120.86, 118.35, 117.66, 114.82, 110.35, 30.72, 14.26. ESI–
MS: M+443 (m/z) Anal. calcd. For C27H21N7: C, 73.20; H,
4.74; N, 22.12. Found: C, 73.24; H, 4.77; N, 22.09.
3.4.3. 3-[Bis(indol-3-yl)methyl]chromone (3c)
tograms (XRD) of the catalyst were recorded in the 2
range between 20 and 808 with a scan rate of 48 minꢀ1 on a
Rigaku Minifax X-ray diffractometer with Ni-filtered Cu K
u
Mp: 236–240 8C. IR (KBr)
(NH), 1635 (CO), 1571 (C5C). 1H NMR (DMSO-d6,
300 MHz): 6.08 (s, 1H, Ha), 6.88–8.13 (m, 9Ar–
H + 2Hb + Hc+Hd + He), 10.69 (s, 2H, NH). 13C NMR
(300 MHz): 176.25, 156.08, 154.53, 136.94, 133.34,
n
max/cmꢀ1: 3389 and 3226
a
d
˚
radiation at a wavelength of 1.54060 A. The SEM-EDX
characterization of the catalyst was performed on a JEOL
JSM-6510 scanning electron microscope equipped with an
energy dispersive X-ray spectrometer operating at 20 kV.
DSC data were obtained with DSC-60 and TGA with DTG-
60H (simultaneous DTA-TG apparatus) Shimadzu instru-
ments.
d
126.75, 125.52, 123.77, 123.69, 121.13, 118.96, 118.11,
115.94, 111.47, 29.39. ESI–MS: M+390 (m/z). Anal. calcd.
for C26H18N2O: C, 80.07; H, 4.62; N, 7.18. Found: C, 80.11;
H, 4.65; N, 7.21.
3.4.4. 3-[Bis(indol-3-yl)methyl]-6-methylchromone (3d)
3.2. Preparation of the catalyst
Mp: 244–246 8C. IR (KBr)
(NH), 1646 (CO), 1619 (C5C). 1H NMR (DMSO-d6,
300 MHz): 2.35 (s, 3H, CH3), 6.42(s, 1H, Ha), 6.89–8.74
(m, 9Ar–H + 2Hb+Hc + Hd+He), 10.27 (s, 1H, NH), 11.90 (s,
1H, NH). 13C NMR (300 MHz):
175.64, 162.23, 156.34,
151.26, 135.71, 133.82, 128.62, 127.52, 124.46, 122.85,
121.24, 117.88, 116.74, 115.56, 113.60, 20.82. ESI–MS:
M+404 (m/z). Anal. calcd. for C27H20N2O2: C, 80.27; H, 4.95;
N, 6.93. Found: C, 80.29; H, 4.92; N, 6.89.
n
max/cmꢀ1: 3289 and 3146
The catalyst (XSA) was synthesized by the reported
procedure [36]. To a magnetically stirred mixture of
xanthan (5.0 g) in CHCl3 (15 mL), chlorosulfonic acid
(1.00 g) was added dropwise at 0 8C during 2 h. HCl gas
was removed from the reaction vessel immediately. After
completion of the addition, the mixture was stirred for 3 h.
Then, the mixture was filtered and washed with methanol
(25 mL) and dried at room temperature to obtain XSA as a
white powder (5.30 g).
d
d
3.4.5. 3-[Bis(indol-3-yl)methyl]-6-chlorochromone (3e)
Mp: 240–242 8C. IR (KBr)
1640 (CO), 1604 (C5C). 1H NMR (DMSO-d6, 300 MHz):
6.47 (s, 1H, Ha), 7.03–8.23 (m, 9Ar-H + 2Hb + Hc + Hd + He),
12.04 (s, 2H, NH). 13C NMR (300 MHz):
178.46, 165.32,
n
max/cmꢀ1: 3304 (NH), m
3.3. General procedure for the synthesis of
bis(indolyl)methanes
d
d
A mixture of indole (2 mmol), aldehyde (1 mmol) and
XSA (200 mg) were ground together in a mortar with a
158.21, 150.60, 134.17, 128.26, 126.81, 124.24, 123.87,
121.66, 118.23, 117.65, 115.82, 112.28, 25.62. ESI–MS: M+

Z.N. Siddiqui, S. Tarannum / C. R. Chimie 16 (2013) 829–837
837
References
424 (m/z). Anal. calcd. for C26H17N2O2Cl: C, 73.59; H, 4.01;
N, 6.60. Found: C, 73.62; H, 4.03; N, 6.63.
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3.4.6. 3, 30-(Thiophen-2-ylmethylene)bis(indole) (3f)
Mp: 148–150 8C. IR (KBr)
(NH), 1545 (C5C). 1H NMR (DMSO-d6, 300 MHz):
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NMR (300 MHz): 149.35, 136.61, 126.37, 126.13, 124.55,
n
max/cmꢀ1: 3404 and 3384
d
6.09 (s,
C
d
123.26, 120.95, 118.30, 118.25, 111.37, 35.14. ESI–MS: M+
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8.53. Found: C, 76.80; H, 4.83; N, 8.61.
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3.4.7. Tris(1H-indol-3-yl)methane (3 g)
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1553 (C5C). 1H NMR (DMSO-d6, 300 MHz):
6.81–8.04 (m, 12Ar-H + 3Hb), 10.46 (s, 3H, NH). 13C NMR
n
max/cmꢀ1: 3396 and 3054 (NH),
d6.03 (s, 1H, Ha),
(300 MHz): d136.75, 126.91, 123.36, 120.70, 118.49, 111.23,
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(NH), 1603 (C5C). 1H NMR (DMSO-d6, 300 MHz):
1H, Ha), 6.65–7.39 (m, 13Ar–H + 2Hb), 7.89 (s, 2H, NH). 13
NMR (300 MHz): 144.68, 136.75, 128.41, 127.89, 126.74,
n
max/cmꢀ1: 3404 and 3194
5.88 (s,
d
C
d
125.72, 123.70, 120.92, 119.21, 118.31, 118.21, 111.30,
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In summary, we have developed a simple, novel and
efficient protocol for the synthesis of bis(indolyl)methanes
using XSA as biodegradable, reusable, heterogeneous
catalyst under solvent-free conditions at room tempera-
ture. The advantages of the present methodology are
excellent yields of products, inexpensive catalyst, energy-
sustainable protocol, shorter time period, simple opera-
tional procedure and purification of products merely by
recrystallization.
Acknowledgements
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UGC, NewDelhi, isgratefullyacknowledgedforawarding
research fellowship to ST and Special Assistance Programme
Scheme (Departmental Research Support Phase 1). The
authors are thankful to the Centre of Nanotechnology,
Department ofApplied Physics and University Sophisticated
Instrument Facility (USIF), AMU, Aligarh for providing
powder X-Ray diffractometer and SEM-EDX facilities. The
authors would also like to thank SAIF, Punjab University,
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