Synthesis and characterization of Polyindole and its catalytic performance study as a heterogeneous catalyst

Article abstract of DOI:10.1007/s12039-016-1040-1

The catalytic performance study of polyindole as a heterogeneous catalyst is reported for the synthesis of 3,3'-arylmethylene-bis-1H-Indole derivatives using various substituted aldehydes and indole under reflux reaction condition with good to excellent y

Full text of DOI:10.1007/s12039-016-1040-1

c
J. Chem. Sci. Vol. 128, No. 3, March 2016, pp. 467–475. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-016-1040-1
Synthesis and characterization of Polyindole and its catalytic
performance study as a heterogeneous catalyst
PRAKASH CHHATTISEa,b, KALPANA HANDOREa, AMIT HORNEa,
KAKASAHEB MOHITEb, ATUL CHASKARc, SABRINA DALLAVALLEd
and VASANT CHABUKSWARa,∗
aDepartment of Chemistry, Nowrosjee Wadia College, University of Pune, 19, Joag Path, Pune 411 001, India
bDepartment of Chemistry, Haribhai V Desai College, Pune 411 002, India
cNational Centre for Nanosciences and Nanotechnology, Mumbai University, Mumbai, India
dDepartment of Food, Environmental and Nutritional Sciences, Università degli Studi di Milano, Milan, Italy
e-mail: vvchabukswar@gmail.com
MS received 19 October 2015; revised 18 December 2015; accepted 6 January 2016
Abstract. The catalytic performance study of polyindole as a heterogeneous catalyst is reported for the syn-
thesis of 3,3’-arylmethylene-bis-1H-Indole derivatives using various substituted aldehydes and indole under
reflux reaction condition with good to excellent yield. Polyindole was synthesized by chemical oxidative poly-
merization using citric acid as a dopant. The synthesized polymer was well characterized by various spectro-
scopic techniques like FT-IR, XRD, FESEM, etc. The XRD pattern confirms the partially crystalline nature of
polyindole. The FESEM images of polyindole revealed the formation of irregularly shaped particulate nature
with size in the range of 0.2 to 6 micron. In FT-IR spectrum, the major peak at ∼ 3400 cm−1 indicates N-
H stretching and at 1564−1624 cm−1 indicates C-C stretching of benzenoid ring of indole. The presence of
peak at ∼ 3400 cm−1 indicates that the polymerization does not occur at nitrogen. The present protocol has
certain advantages like recyclability, low loading of the catalyst, low-cost and efficient use of polyindole as a
heterogeneous catalyst.
Keywords. Polymerization; polyindole; arylmethylene bis-1H-Indole; heterogeneous catalyst; recyclability.
1. Introduction
methods have been studied to effect the condensation
reaction of indole with a variety of aldehydes in the
literature. Most of the reported methods have employed
conventional Lewis acids as well as protic acids as
catalysts. Perumal et al., synthesized bisindolyl alkanes
using InCl3 as Lewis acid catalyst.7 An excess amount
of Lewis acid is needed for the synthesis of heterocyclic
compounds as it can be destroyed by the nitrogen atom
of heterocyclic compounds. Mishra et al., reported synthesis
of 3, 3’-arylmethylene bis-1H Indole with ZrOCl2·8H2O
which required prolonged reaction time, ∼ 10 to 16 h.8
Kurosh and Masoumeh Sharifi-Kiasaraaie synthesised
bisindolyl alkanes using [BMIM]BF4 as ionic liquid
within 3-4 h with low yield of the corresponding product.9
Various other catalysts used are ZrOCl2·8H2O/ silica gel,10
silica supported NaHSO4/amberlyst-15,11 FeCl3/ionic
liquid,12 lithium perchlorate,13 Cu(BF4).SiO2,14 Ionic
As a part of recent investigations in the field of poly-
mers, conducting polymers like polyaniline and its ana-
logues have received significant attention as catalysts
for diverse organic reactions and transformations. 3,3’-
arylmethylene bis-1H-Indole and its derivatives are
well known and an important class of heterocyclic com-
pounds which are used in pharmaceuticals as bioactive
intermediates, agrochemicals and in material science.1
They exhibit a broad spectrum of biological activities
such as antimicrobial and antifungal,2 antibacterial,3
antitumor,4 and antioxidantetc agents.5 Recently, bis
(5-methoxy-3-indolyl) methanes have been used as
DNA-based electrochemical biosensors that consid-
erably reduce the growth of cancer cell lines such as
HOP-92 (lung), A498 (renal), and MDAMB-231/ITCC
(breast).6 The condensation of aldehydes with indole
is an important reaction in organic chemistry to
afford 3,3’-arylmethylene bis-1H Indole. Various
17
18
19
liquid,15 KHSO4,16 In(OTf)3, Dy(OTf)3, I2, NBS,19
H-Y zeolite,20 Ion exchange resin,21 H3PMo12O40.xH2O,22
10
Montmorillonite K10,23 Sb2(SO4)3,24 Ln(OTf)3, and clay.24
Most of the reported methods suffer from various
disadvantages such as prolonged reaction time, harsh
reaction conditions, use of expensive Lewis acids,
∗For correspondence
467

468
Prakash Chhattise et al.
toxic metal ions and tedious work-up procedures. In 2.3 Characterization
search of better catalytic systems, herein we have stud-
X-ray diffraction (XRD) analysis of the synthesized
ied polymer-supported reagents for the synthesis of 3,
3’-arylmethylene bis-1H Indole. Polyindole plays an
important role in organic synthesis enabling chemists to
prepare various organic compounds efficiently, remov-
ing excess reagents and byproducts by simple filtration
and simplifying work-up procedure and purification.
The present study demonstrates the synthesis of polyin-
dole by oxidative chemical polymerization and its
effective use as a heterogeneous catalyst for the synthe-
sis of 3, 3’-arylmethylene bis-1H Indole avoiding use
of expensive and toxic reagents or solvents (scheme 1).
polyindole was carried out using BrukerAXS model D-
8, (10 to 70◦ range, scan rate =1◦ min−1) equipped with
a monochromator and Ni-filtered Cu Kα radiation (λ =
1.542 Å). For this purpose, the polymer sample was dis-
persed in ethanol and a small droplet was applied on the
aluminium sheet. The prepared sample was sputtered
with Au and used for FESEM analysis. FTIR analysis
was carried out using Shimadzu A 800 equipped with
diffuse reflectance mode.
2.4 General procedure for the synthesis of 3,
3’-arylmethylene bis-1H Indole
2. Experimental
In a 25 mL round bottom flask, mixture of indole (2.0
mmol), aldehyde (1.0 mmol) and polyindole (5 wt%)
with respect to aldehydes in acetonitrile (5 mL) was
refluxed with constant stirring for 1 to 2 h. The progress
of the reaction was monitored by thin layer chromatog-
raphy TLC (n-hexane/ethyl acetate 30%), on Merck’s
silica gel plates (60 F254). After completion of the reac-
tion, the catalyst was filtered and the resulting crude
product was extracted with ethyl acetate. All the prod-
ucts were purified by using neutral silica gel (60-120
mesh) and column chromatography using hexane and
ethyl acetate as eluents. 1H and 13C-NMR spectra were
recorded in CDCl3 and DMSO-d6 as the solvent on
Varian 400 MHz NMR spectrometer. Proton chemical
shifts (δ) were recorded relative to TMS (δ = 0) as
internal standard and expressed in ppm. Coupling con-
stants (J) are given in Hertz. IR spectra were recorded
on a Shimadzu A 800R in the range of 400–4000 cm−1.
Melting points were determined on Buchi M-560.
2.1 Materials and methods
All the chemicals and solvents (Aldrich, Fluka, and
Merck) were of analytical grade and were used as
received without further purification.
2.2 Synthesis of Polyindole
For the synthesis of polyindole, 10 mmol of indole
monomer and 10 mL of methanol were mixed in a
round bottom flask under nitrogen atmosphere with
constant stirring. The nitrogen purging was continued
for 1 h followed by addition of 10 mmol citric acid.
To the above reaction mixture, Copper chloride solu-
tion (10 mmol) was added dropwise under constant stir-
ring at low temperature (0-5◦C). The reaction mixture
was sonicated for 1 h. the greenish black precipitate
of the polymer was isolated by filtration and washed
with distilled water until the supernatant liquid became
colourless. Copper chloride and citric acid are water
soluble and hence easily washed away. Finally, the
2.5 Characterization data
polymer material was dried under vacuum at 50-60◦C 2.5a 3, 3’- (phenylylmethylene)bis- 1H–Indole (table 3
in an oven. The obtained polyindole was characterised entry 1): 1H NMR (DMSO-d6, 400 MHz, Me4Si): δ 5.8
by various spectral techniques such as FT-IR, XRD (1H, CH), 6.7 (2H, JHH = 2 Hz, CH2 ), 6.86 (2H, JHH
=
and SEM.
7.6 and 14.6 Hz, CH2), 7.1(2H, JHH = 8 and 14.80 Hz,
R
1 R2
O
Polyindole(PIn)
CH3CN/Reflux
2
N
R1
R2
H
HN
N
75-82°C
1-3 h
H
Alkyl, aryl
R1-
82-95%
R2- H, Alkyl
Scheme 1. Synthesis of 3, 3’-arylmethylene bis-1H Indole.

Synthesis and Characterization of Polyindole
469
JHH =8.4Hz, Ph), 7.2 (2H, JHH =8.0Hz, Ph), 7.3 (2H,
JHH =8.4 Hz, Ph), 10.8 (1H, bs, NH); M.p. 222–224◦C.
19
26.7
2.5d 3, 3’- ((3,4dimethoxyphenyl) methylene) bis- 1H
–Indole (table 3, entry 7): 1H NMR (CDCl3,400 MHz,
Me4Si): δ 3.7 (3H, OMe), 3.8 (3H, OMe), 5.8 (1H, CH),
6.6 (2H, Ph), 6.77 (1H, JHH = 8.4 Hz, Ph), 6.83 (1H,
Ph), 6.9 (1H, Ph), 7.0 (2H, JHH =7.6 and 16 Hz, Ph),
7.1 (2H, JHH = 8.0 and 14.8 Hz, Ph), 7.3(4H, dd, JHH
=
8.0 Hz, Ph), 7.9 (2H, bs, Ph);13C NMR (CDCl3,200
MHz,) δ 40.0, 50.6, 110.9, 111.0, 112.2, 119.1, 119.8,
119.9, 120.5,121.8, 123.5, 127.0, 136.7, 147.2, 148.6;
M.p.196–198◦C; m/z379.54.
10
20
30
40
50
60
70
80
2
deg.
2.5e 3, 3’- ((4 N,Ndimethylphenyl) methylene) bis-1H
–Indole (table 3, entry 8): 1H NMR (CDCl3,400 MHz,
Me4Si): δ 2.9 (6H, Nme2), 5.8 (1H,CH), 6.67 (4H, Ph),
Figure 1. XRD of polyindole.
CH2), 7.02 (1H, JHH = 7.2 and 14.40 MHz, Ph), 7.4 (4H,
JHH = 8 and 16 Hz, Ph), 7.2(4H), 10.8 (2H, br, NH);
13C NMR (CDCl3,200 MHz,): δ 144.8,136.4, 128.19,
127.9, 126.54, 125.64, 123.41, 120.74, 118.98,118.03,
117.97, 111.32; M.p. 124–126◦C; m/z 321.38
6.9 (2H, m, Ph), 7.2–1.12 (4H, m, Ph), 7.3 (2H, JHH
8.4 Hz, Ph), 7.4 (2H, JHH = 8.0, Ph),7.8 (2H, bs, NH);
=
M.p. 206–208◦C; m/z 366.
2.5f 3, 3’- ((4-Fluorophenyl) methylene) bis-1H–
Indole (table 3, entry 9): 1H NMR (CDCl3,400
MHz, Me4Si): δ 5.8(1H, CH), 6.8(2H, Ph), 6.93–7.02
(4H,Ph), 7.1–7.2 (2H, m, Ph), 7.27–7.30 (3H,Ph), 7.4
(4H, dd, JHH = 2 and 7.60 Hz, Ph), 7.8(2H, br, NH);
M.p. 82–84◦C; m/z 338.8
2.5b 3, 3’- ((4-methoxyphenyl) methylene) bis-1H –
Indole (table 3, entry 4): 1H NMR (CDCl3,400 MHz,
Me4Si): δ 3.7 (3H, 3OMe), 5.8 (1H, CH), 6.6 (2H, Ph),
6.8 (2H, dd, JHH = 4.80 Hz, Ph), 7.0 (2H, Ph), 7.14
(2H, JHH = 7.2 and 14.4 Hz, Ph), 7.25 (2H, m,Ph),
7.37 (3H, dd, JHH = 12.0 Hz, Ph), 7.8 (2H, bs, NH);
M.p.186–188◦C; m/z 351.25.
2.5g 3, 3’- ((4-chlorophenyl) methylene) bis-1H –
Indole (table 3, entry 10): 1H NMR (DMSO-d6400
MHz, Me4Si): δ 5.9 (1H), 6.8 (2H, br), 7.4-7.80 (12H),
10. 2 (2H, bs, NH); M.p. 74-76◦C.
2.5c 3, 3’- ((4-nitrophenyl) methylene) bis-1H –
Indole (table 3, entry 6): 1H NMR (DMSO-d6,400
MHz, Me4Si): δ 6 (1H, CH), 6.8 (2H, JHH = 8.4,
CH), 6.7 (2H, d), 6.84 (2H, JHH = 8.0 and 15.2 Hz, 2.5h 3, 3’- ((3hydroxy,4-methoxyphenyl) methylene)
Ph), 7.01 (2H, JHH = 7.60 and 14.8 Hz, Ph), 7.1 (2H, bis-1H –Indole (table 3, entry 13): 1H NMR (CDCl3,
Figure 2. FE-SEM image of polyindole.

470
Prakash Chhattise et al.
It shows two reflection peaks, first one at 19◦ is a broad
peak and another sharp peak is at 26.7◦. The broad
peak confirms the polymerization of indole. Due to poor
crystalline structure, polymeric materials usually show
broad reflection peaks. Additionally, the sharp peak at
26.7◦ validates partial crystalline nature of polyindole.
3400
1624
1564
3.2 FE-SEM Analysis
1451
1385
1100
1040
The morphology and dimensions of polyindole were
examined by FESEM and high and low resolution
FESEM images are reproduced in figure 2. The syn-
thesised polyindole reveals the formation of irregu-
larly shaped particles with size ranging from 0.2 to 6
micron. The high resolution image depicts the presence
of spherical particles with smooth surface which are
fused together to form chunk like morphology.
735
1000
500
1500
2000
2500
3000
3500
4000
Wavenumber (cm-1)
Figure 3. FTIR spectra of polyindole.
400 MHz, Me4Si): δ3.8 (3H, OMe), 5.5 (1H, OH), 5.8
(1H, CH), 6.6 (2H, Ph), 6.7 (1H, JHH = 8.4 Hz,Ph), 6.83
3.3 FT-IR Analysis
(1H, dd, JHH = 8.0 Hz, Ph), 6.9 (1H, s), 7.0 (2H, JHH
=
7.2 and 14.4 Hz, Ph), 7.1 (2H, JHH = 7.6 and 15.2 Hz,
Ph), 7.3 (2H, JHH = 8.0 Hz, Ph), 7.4 (2H, JHH = 8.0
Hz, Ph),7.8 (2H, bs, NH); M.p. 110–111◦C; m/z 365.46
(M−).
The FT-IR spectrum of polyindole is shown in figure 3.
The major peak at ∼3400 cm−1 indicates N-H stretching,
peak at 1564-1624 cm−1 indicates C-C stretching of
benzenoid ring of indole, peaks at 1451 cm−1 and 1385
cm−1 indicate C-N and C=N stretchings, respectively,
and the peak at 735 cm−1 indicates out-of-plane defor-
mation of benzene. The presence of peak at ∼3400
cm−1 indicates that the polymerization does not occur
at nitrogen.
2.5i Tri (1-H-indol-3-yl) methane (table 3, entry 16):
1H NMR (DMSO-d6400 MHz, , Me4Si) δ 6.4 (1H, CH),
6.6 (2H, JHH = 8.4, CH), 6.7 (2H) 6.84 (2H, JHH = 8.0
and 15.2 Hz, Ph), 7.01 (2H, JHH = 7.60 and 14.8 Hz,
Ph), 7.1 (2H, JHH = 8.4Hz, Ph), 7.2 (2H, JHH = 8.0Hz,
Ph), 7.3 (2H, JHH = 8.4 Hz, Ph), 9.1 (1H, bs, NH), 10.7
(2H, bs, OH); M.p. 162–164◦C.
3.4 Synthesis of 3, 3’-arylmethylene bis-1H Indole
using polyindole
3. Results and Discussion
Initially, the optimization of the general reaction con-
dition was carried out by the model reaction between
p-nitrobenzaldehyde and indole using polyindole
(PInd) as a catalyst. The aldehyde and indole in 1:2
3.1 XRD Analysis
The synthesized polyindole was analysed by powder
X-ray diffractometry to identify the crystalline phases.
XRD pattern of polyindole is depicted in figure 1.
Table 2. Effect of Solvent.
Time
(min)
Yield
%
Entry
Solvent
Table 1. Optimization of the amount of catalyst.
1
2
3
4
5
6
7
8
Ethanol
Acetonitrile
DCM
Chloroform
Benzene
THF
60
90
55
45
60
55
75
60
92
95
84
80
70
82
80
78
Entry
PInd (x wt %)
Yield
1
2
3
4
5
6
0.5
1.0
2.5
5.0
7.5
72%
83%
90%
95%
96%
96%
DMF
1,4 dioxane
10.0
Reaction Conditions: Aldehyde (1 mmol), Indole (0.117 g,
2 mmol), catalyst (x wt%); stirred in 5 mL acetonitrile at
reflux temperature, time 1.5 h. Isolated yields.
Reaction Conditions: Aldehyde (1 mmol), Indole (0.117 g,
2 mmol), catalyst (5 wt%); stirred in 5 mL acetonitrile at
reflux temperature, time 1.5 h. Isolated Yields.

Synthesis and Characterization of Polyindole
471
Table 3. Polyindole catalyzed synthesis of 3, 3’arylmethylene bis-1H indole.
Melting Point(◦C)
Entry
Aldehyde
Product
Time (h) Yieldsa (%)
Found
Reported
H
O
H
N
N
H
H
1
2
3
2.5
2.5
2.5
82
86
84
124-126
114-119
–
125-26
119-121
–
OH
H
O
H
N
N
H
HO
H
OH
O
H
H
N
H
N
H
OH
OCH3
H
O
H
N
H
N
H
H3CO
4
5
3
88
85
186-188
125-127
186-87
HO
H
O
H
N
H
N
H
OH
2.5
–
NO2
H
O
H
N
H
N
H
O2N
6
7
1.5
95
88
222-224
196-198
217-219
198-200
OCH3
OCH3
O
H
H
H3CO
N
H
N
H
OCH3
2
NMe2
H
O
H
N
H
N
Me2N
H
8
2.5
85
206-208
210-212

472
Prakash Chhattise et al.
Table 3. (contd.)
Melting Point(◦C)
Entry
Aldehyde
Product
Time (h) Yieldsa (%)
Found
Reported
F
O
H
H
N
H
N
F
H
9
1
87
82-84
80-82
Cl
H
O
H
N
H
N
Cl
H
10
11
1.5
1.5
90
89
74-76
76-78
CN
O
H
H
N
H
N
NC
H
213-215
209-212
OCH3
OCH3
H3CO
H
O
H
H3CO
OCH3
OCH3
N
H
N
H
12
13
3
85
89
220-222
110-111
–
OCH3
OH
H
O
H
H3CO
N
H
N
H
OH
2.5
110-12
F
O
F
F
F
H
O
H
F
F
F
F
O
N
H
N
H
14
15
1
1
80
90
–
–
H
O
H
N
H
N
H
100-102
100-102

Synthesis and Characterization of Polyindole
473
Table 3. (contd.)
Melting Point(◦C)
Entry
Aldehyde
Product
Time (h)
Yieldsa (%)
Found
Reported
HN
O
H
H
N
N
H
N
H
H
16
2.5
–
85
–
162-164
160
2
85
Reaction Conditions: Aldehyde (1 mmol), Indole (0.117 g, 2 mmol), catalyst (5 wt%); stirred in 5 mL
acetonitrile at reflux temperature, time 1.5 h. Isolated Yields.
H
R
1R2
N
H
N
O
O
H
N
H
R1
R2
n
N
N
n
H
O
H
N
N
H
H
H
N
R
R2
Polyindole
Catalyst
-H2O
R2
R1
R
H
1 R2
N
H
NH
N
H
N
H
Scheme 2. Plausible mechanism for the synthesis of 3, 3’-arylmethylene bis-1H indole.
Table 4. Reusability of polyindole.
mole ratio was refluxed in acetonitrile (5 mL) with 5
wt% of catalyst for 1.5 h affording 95% yield of 3, 3’-
arylmethylene bis-1H Indole. In order to optimize the
appropriate concentration of polyindole for the synthe-
sis of 3, 3’-arylmethylene bis-1H Indole derivatives, the
catalyst amount was varied from 0.5 wt% to 10 wt%, the
results of which are depicted in table 1. With increase
in catalyst amount, the yield of the reaction increased
from 72% to 96%, respectively. There was not much
increase in the yield of product when 7.5 to 10 wt%
of the catalyst was used. The reaction was also carried
out with CuCl2 but the yield of the desired product was
very low, less than 10% after prolonged reaction time.
Based on these results, 5 wt% catalyst was used in fur-
ther studies. The effect of various solvents on the yield
of 3, 3’-arylmethylene bis-1H Indole was investigated
and results are shown in table 2. The reaction was com-
plete in acetonitrile within 1.5 h with 5 wt% polyin-
dole affording 95% yield of the desired product. How-
ever the reaction, was sluggish in benzene yielding 70%
yield of 3,3’-arylmethylene bis-1H Indole. The extent
of yield of 3, 3’-arylmethylene bis-1H Indole was in the
order, CH3CN>EtOH>THF>DCM>CHCl3∼DMF>1,4-
dioxane>benzene. This observation suggests that among
the studied solvents, acetonitrile, being highly polar
Entry
No. of cycle
Yield (%)
1
2
3
4
5
1
2
3
4
5
95
90
85
79
75
Reaction Conditions: Aldehyde (1 mmol), Indole (0.117 g,
2 mmol), catalyst (5 wt%); stirred in 5 mL acetonitrile at
reflux temperature, time 1.5 h. Isolated Yields.
aprotic solvent enhances the activity of catalyst com-
pared to other polar aprotic and non-polar solvents and
gave the excellent yield in 1.5 h under reflux reaction
condition. For further investigation we used 10 wt%
catalyst and acetonitrile as a solvent.
To explore the further applicability and performance
of this reaction, we extended the methodology for
the synthesis of variety of 3, 3’-arylmethylene bis-1H
Indole derivatives using a variety of aromatic aldehy-
des with indoles and substituted indoles. The results
are summarized in table 3. Benzaldehyde and hydrox-
ybenzaldehydes gave almost more than 85% yield

474
Prakash Chhattise et al.
Table 5. Comparison with other catalysts. Entry 9 is this work.
Sr. No.
Catalysta
Solvent
Temp(◦C)b
Time (h)c
Yield(%)d
1
2
3
4
5
6
InF3.H2O
Ln(OTf)3
LiClO4
Aqueous
EtOH/H2O
CH3CN
-
Room-Temp
Room-Temp
100
10-15
12
5
3-12
4
9416
988b
9011
ZrOCl2.8H2O
[BMIM]BF4
Montmorillonite
K10
50
100
948a
Solvent free
Trace9
Solvent free
MeOH
CH2Cl2
Room -Temp
Room -Temp
Room -Temp
75
1-6
1-3
1
9723a
9024
8525
95
7
8
9
Sb2(SO4)3
Zeolite
Polyindole
CH3CN
1-2
(table 3, entries 1, 2, 3, and 5). Di and trisubstituted under optimized reaction conditions and was found
methoxybenzaldehydes under similar reaction condi- that yield of 3, 3’-arylmethylene bis-1H Indole was
tion led to ∼82% yield of 3, 3’-arylmethylene bis- almost comparable up to two runs without any signifi-
1H Indole (table 3, entry 7, 12 and 13) compared cant loss of activity. The reusability of the catalyst was
to mono substituted methoxybenzaldehyde (table 3, checked for four cycles and the corresponding yields
entry 4) because of the electron donating ability of obtained in each cycle are summarized in table 4. The
the methoxy group. Aldehydes possessing electron- lower activity of the polyindole catalyst after third cycle
withdrawing groups on the aromatic ring (table 3, entry was due to loss of some catalyst during washing and
6, 11) afforded 95 and 90% yield of the desired product. drying. The detailed comparison of the present pro-
Halogenated and amino benzaldehydes gave around 80 tocol and various other methodologies is depicted in
to 90% yield (table 3, entry 8, 9, 10 and 14). It was table 5.
observed that the reaction proceeds at a faster rate
with all the aldehydes possessing electron-withdrawing
substituent on the aromatic ring than aldehydes pos-
sessing electron-releasing substituent. Additionally, the
4. Conclusions
methodology was further extended to unsaturated alde- A simple methodology for an efficient synthesis of
biologically active 3, 3’-arylmethylene bis-1H indoles
using polyindole as a heterogeneous catalyst has been
reported. The present protocol offers several advantages
like short reaction time, high yield of the desired prod-
ucts, reusability of the catalyst, simple workup proce-
dure and minimal use of toxic and hazardous chemicals
compared to the traditional methods of synthesis.
hydes and substituted indoles, affording almost 85%
yields of 3, 3’-arylmethylene bis-1H Indole (table 3,
Entry 15 and 16) without affecting the functionality
in the indole ring which confirms the mild nature of
polyindole.
The plausible mechanism for the synthesis of 3, 3’-
arylmethylene bis-1H indole is given in scheme 2. The
reaction proceeds through the activation of the carbonyl
group by the polyindole catalyst through –NH bond,
which makes the carbonyl carbon more electrophilic
and induces the condensation with a second molecule of
indole to give 3, 3’-arylmethylene bis-1H Indole with
the recovery of polyindole catalyst.
Supplementary Information (SI)
Additional information related to characterization of the
catalyst and synthesized compounds (XRD, SEM, FT-
1
IR, HNMR and 13CNMR) are given in the supporting
information and are available at http://www.ias.ac.in/
Journals/Journal_of_Chemical_Sciences.
3.5 Reusability of catalyst
In order to study the reusability of polyindole cata-
lyst and thus make the process economical, the reac-
tion of 4-nitrobenzaldehyde with indole was studied for
a desired time under reflux reaction condition. Polyin-
dole catalyst was recovered, filtered and washed with
Acknowledgements
Authors sincerely acknowledge ISRO, University of
Pune for financial assistance. We also thank Centre
for Materials for Electronic technology (C-MET) and
water repeatedly followed by ethanol and then dried Haribhai V. Desai College for characterization and
in an oven at 65-70◦C for half an hour. It was reused Nowrosjee Wadia College, Pune for providing lab facilities.

Synthesis and Characterization of Polyindole
475
13. Yadav J S, Reddy B V S, Murthy C V S R, Kumar G M
and Madan C 2001 Synthesis 5 783
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