Green protocol for the synthesis of bisindolylmethanes and evaluation of their antimicrobial activities

Article abstract of DOI:10.1080/10426500802590178

A simple, efficient, and ecofriendly procedure for the preparation of bis(indolyl)methanes via electrophilic substitution reactions of 2-phenylindoles with aldehydes is described. The reaction takes place in the presence of glacial acetic acid and a catalytic amount of anhydrous ferric chloride. All the synthesized compounds have been characterized by elemental analysis and spectral data (IR and ¹H NMR). They have also been evaluated for their antibacterial and antifungal activities, and some of them have shown promising results against S. aureus, E. coli, A. flavus, and A. niger.

Full text of DOI:10.1080/10426500802590178

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Phosphorus, Sulfur, and Silicon
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Green Protocol for
the Synthesis of
Bisindolylmethanes
and Evaluation of Their
Antimicrobial Activities
Anjali Tiwari a & Meenakshi Jain a
a Centre of Advanced Studies, Department of
Chemistry , University of Rajasthan , Jaipur, India
Published online: 03 Nov 2009.
To cite this article: Anjali Tiwari & Meenakshi Jain (2009) Green Protocol for the
Synthesis of Bisindolylmethanes and Evaluation of Their Antimicrobial Activities,
Phosphorus, Sulfur, and Silicon and the Related Elements, 184:11, 2835-2845, DOI:
10.1080/10426500802590178
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Phosphorus, Sulfur, and Silicon, 184:2835–2845, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500802590178
Green Protocol for the Synthesis of Bisindolylmethanes
and Evaluation of Their Antimicrobial Activities
Anjali Tiwari and Meenakshi Jain
Centre of Advanced Studies, Department of Chemistry, University
of Rajasthan, Jaipur, India
A
simple, efficient, and ecofriendly procedure for the preparation of
bis(indolyl)methanes via electrophilic substitution reactions of 2-phenylindoles
with aldehydes is described. The reaction takes place in the presence of glacial
acetic acid and a catalytic amount of anhydrous ferric chloride. All the synthesized
compounds have been characterized by elemental analysis and spectral data (IR
and 1H NMR). They have also been evaluated for their antibacterial and antifungal
activities, and some of them have shown promising results against S. aureus, E.
coli, A. flavus, and A. niger.
Keywords Anhydrous ferric chloride; bis(indolyl)methanes; ecofriendly procedure; elec-
trophilic substitution reaction; green chemistry techniques; 2-phenylindole
INTRODUCTION
The indole nucleus is associated with a wide spectrum of biological
activities. The introduction of a phenyl group at the 2-position of the
indole ring enhances biological activities such as monoamine oxidase
inhibition,1,2 electrochemical behavior,3 mammary tumor inhibiting ac-
tivity, and estrogenic and anti-estrogenic properties.4 As such, bisin-
dolylalkanes and their derivatives constitute an important group of
bioactive metabolites of terrestrial and marine origin5 and are useful
for drug design.6 Bisindolylmethanes are the most active cruciferous
substances for promoting beneficial estrogen metabolism and inducing
Received 23 August 2008; accepted 2 October 2008.
Dedicated to the late Prof. Vijai N. Pathak.
The authors are thankful to the Head, Department of Chemistry, University of Ra-
jasthan, Jaipur, India for elemental analysis and spectral (IR and 1H NMR) data. We
are also thankful to the Head, Department of Biotechnology, JECRC, Jaipur, India for
biological screening of these compounds. One of the authors (A.T.) is also thankful to
the Department of Science and Technology (SDST), Jaipur, India for the sanction of the
student project.
Address correspondence to Anjali Tiwari, Centre of Advanced Studies, Department of
Chemistry, University of Rajasthan, Jaipur 302 004, India. E-mail: at.chem@gmail.com
2835

2836
A. Tiwari and M. Jain
apoptosis in human cancer cells.7 They have also shown potent CNS de-
pressant activities, tranquilizing effects, and neuroplegic activities.8,9
Therefore, there is a great deal of interest in the synthesis of these
compounds. Various methods are available in the literature10–21 for the
synthesis of bisindolylmethanes, but there are only scant reports on
the ecofriendly synthesis of bisindoles of 2-phenylindoles. In contin-
uation to our earlier work on various 2-phenylindoles,22 we prepared
some bisindolylmethane derivatives by improved green chemistry tech-
niques. As a part of this program, we wish to report various strategies
for the synthesis of 2,2ꢀ-diaryl-3,3ꢀ-alkylidene/arylidene bisindoles and
to assess the efficiency of these synthetic routes and make a compara-
tive study. We have used various green chemistry techniques, viz., mi-
crowave, grindstone chemistry, and ultrasonication for this synthesis.
Glacial acetic acid and inexpensive catalysts such as anhydrous ferric
chloride (anhyd. FeCl3) are used to obtain these target molecules.
RESULTS AND DISCUSSION
A series of 2,2ꢀ-diaryl-3,3ꢀ-alkylidene/arylidene bisindoles have been
synthesized using new, improved green chemistry techniques. These
compounds are stable and contain the indole and appropriate aldehyde
in a 2:1 ratio (Table I).
2-Arylindoles are prepared by the method of Joshi et al.23 2-
Arylindole (20 mmol) reacts with the appropriate aldehyde (10 mmol)
in the presence of (a) glacial acetic acid or (b) a catalytic amount of
anhydrous ferric chloride (anhy. FeCl3) by employing various green
chemistry techniques to give the desirable title compounds. The results
obtained are presented in Table II.
The results reveal that microwave-assisted reactions take place in
the shortest time period (using anhy. FeCl3) and give highest yield
(using glacial acetic acid) of the title compounds. The synthetic steps
are illustrated in Scheme 1.
In the IR spectra of title compounds (2a–e), the >NH absorption
band appears at 3464–3342 cm−1 and the aromatic C−H stretching
absorption peak appears at 3050 cm−1. The aliphatic C−H stretching
vibration is observed at 2850–2836 cm−1 and the aromatic >C=C<
absorption band is observed at 1679 cm−1. An absorption band due to
C-Br appears at 550 cm−1
.
The 1H NMR spectra of 2-arylindole revealed the presence of a broad
resonance signal in the region of δ 8.0–8.6 ppm, which is attributed to
>N-H proton, and the methine proton signal (=CH, at C-3) appears

2837

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A. Tiwari and M. Jain
TABLE II Results and Conditions of the Synthesis of 2,2ꢀ-Diaryl-3,3ꢀ-
alkylidene/arylidene Bisindoles (2a–e)
Yield (%)
(b)∗
Time required (min.)
(b)
(a)
(i)
(a)
(i)
Compd.
No.
(ii)
(iii)
(i)
(ii)
(iii)
(ii)
(iii)
(i)
(ii)
(iii)
2a
2b
2c
2d
2e
84
82
86
85
83
83
78
83
79
77
75
73
76
73
72
77
74
78
81
77
75
70
73
76
75
69
67
71
70
66
7
7
5
6
6
10
20
15
10
20
80
70
50
80
60
4
5
3.5
4.5
4
10
20
10
10
10
60
60
50
70
40
∗Isolated yield.
(a) = using glacial CH3COOH.
(b) = using anhy. FeCl3.
(i) = by microwave irradiation.
(ii) = by grinding.
(iii) = by ultrasound irradiation.
1
as a sharp singlet at δ 6.7–6.9 ppm. In the H NMR spectra of title
compounds (2a–e), the disappearance of the resonance signal due to a
methine proton at C-3 position shows the formation of bisindoles. Com-
pound (2a) shows additional resonance signals at δ 4.36–4.56 ppm as
a double doublet, which is due to >CH2 protons. In compounds (2b–e),
the appearance of a sharp singlet at δ 5.5–6.0 ppm due to the arylidene
–CH proton also supports the bisindole formation. The aromatic and
1
>N-H resonance signals remain unaltered. The IR and H NMR data
of all synthesized compounds are presented in Table III.
EXPERIMENTAL
All the melting points were determined in open glass capillary tubes
and are uncorrected. The IR spectra (νmax in cm−1) were recorded on a
Perkin Elmer 557 grating infrared spectrophotometer as KBr pellets.
1H NMR spectra were recorded on a Bruker spectrophotometer (300
MHz) using CDCl3 as a solvent. TMS was used as internal standard
(chemical shift in δ, ppm). The microwave-assisted reactions were car-
ried out in a domestic MW oven (LG MS–194A) operating at 800 W,
and ultrasound irradiation reactions were carried out in an ultrasonic
bath (Toshniwal–SW-4) operating at 150 W generating 37 KHz output
frequency. The purity of the compounds was checked by TLC using sil-
ica gel-G as adsorbent, UV light or iodine accomplished visualization.

Green Protocol for Bisindolylmethanes
2839
SCHEME 1
2-Arylindoles were prepared by the method of Joshi et al.23 Compounds
2a12 and 2d24 have been synthesized earlier by conventional methods,
but we report some new routes for the synthesis of these compounds.
Some 2,2ꢀ-diaryl-3,3ꢀ-alkylidene/arylidene bisindoles (2a-e) were syn-
thesized by the following routes:
Method (i): Microwave Activation Method
(a) Using Glacial Acetic Acid
2-Aryl-1H-indole (1 mmol), the appropriate aldehyde (0.5 mmol),
and glacial acetic acid (2 mL) were taken in a conical flask (100 cm3)
and irradiated with microwaves for 5–7 min at 800 W. The color of

2840
A. Tiwari and M. Jain
TABLE III Spectral Data of 2,2ꢀ-Diaryl-3,3ꢀ-alkylidene/arylidene
Bisindoles (2a–e)
Compd.
no.
IR (KBr) ν
cm−1
1H NMR (CDCl3) δ ppm
max
2a
3464 (NH str.), 3050 (aromatic
C−H str.) 2836 (aliphatic C−H
str.), 1679 (aromatic C = C str.)
3400 (NH str.), 3050 (aromatic
C−H str.), 2840 (aliphatic C−H
str.), 1679 (aromatic C = C str.),
550 (C-Br), 1236 (C-F)
3386 (NH str.), 3050 (aromatic
C−H str.), 2836 (aliphatic C−H
str.), 1679 (aromatic C = C str.),
550 (C-Br), 1158 (C-O-C)
3400 (NH str.), 3050 (aromatic
C−H str.), 2850 (aliphatic C−H
str.), 1679 (aromatic C = C str.),
1229 (C-F)
3342 (NH str.), 3050 (aromatic
C−H str.), 2836 (aliphatic C−H
str.), 1679 (aromatic C = C str.),
1329 (C-F), 1136 (C-O-C)
4.36—4.56 (dd, CH2,2H), 6.7–7.9
(m, ArH, 18H), 8.0 (s, NH, 2H)
2b
2c
2d
2e
5.5 (s, CH, 1H), 6.8–7.7 (m, ArH,
21H), 8.0 (s, NH, 2H)
6.0 (s, CH, 1H), 3.8 (s, OCH3,
3H), 6.7–7.3 (m, ArH, 21H), 8.0
(s, NH, 2H).
5.5 (s, CH, 1H), 6.8–7.9 (m, ArH,
21H), 8.0 (s, NH, 2H).
5.5 (s, CH, 1H), 3.8 (s, OCH3,
3H), 6.5–8.0 (m, ArH, 21H), 8.1
(s, NH, 2H).
reaction mixture gradually turned from light brown to dark brown.
After completion of the reaction (checked by TLC), the reaction mixture
was allowed to cool to room temperature and then poured into ice cold
water. It was filtered and recrystallized from benzene to afford the pure
product.
(b) Using Anhydrous FeCl3
In a conical flask (100 cm3), a mixture of well-ground anhydrous
FeCl3 (0.2 mmol, 0.03 g) and silica gel (3g) was combined with a so-
lution of 2-aryl-1H-indole (1 mmol) and the appropriate aldehyde (0.5
mmol) in dichloromethane (15 mL). Under reduced pressure, the mix-
ture was dried to a powder and then irradiated with microwaves for
3.5–5 min at 235 W. The color of the mixture gradually turned from
pink to maroon. After completion of the reaction (checked by TLC), the
reaction mixture was allowed to cool to room temperature and extracted
with diethyl ether (3 × 15 mL) followed by washing with water (3 × 15
mL). The ether layer was dried over anhydrous magnesium sulfate and
then evaporated to afford the crude compound. The crude compound

Green Protocol for Bisindolylmethanes
2841
was purified by silica gel column chromatography using benzene:ethyl
acetate (80:20) as eluent to afford the pure title compound.
Method (ii): Grindstone Method
(a) Using Glacial Acetic Acid
2-Aryl-1H-indole (1 mmol), the appropriate aldehyde (0.5 mmol),
and glacial acetic acid (2 mL) were taken in a mortar and ground using
a pestle for 10–20 min to give a dark brown colored mixture. After
completion of the reaction (checked by TLC), the reaction mixture was
poured into ice cold water, filtered, washed well, and recrystallized from
benzene to afford the pure compound.
(b) Using Anhydrous FeCl3
The mixture of 2-aryl-1H-indole (1 mmol) and the appropriate alde-
hyde (0.5 mmol) were taken in a mortar, then anhy. FeCl3 (0.2 mmol,
0.03g) was added to it and ground well using a pestle for 10–20 min
to give a dark brown colored semi-viscous substance. After completion
of the reaction (checked by TLC), the reaction mixture was extracted
with diethyl ether (3 × 15 mL) followed by washing with water (3 × 15
mL). The ether layer was dried over anhy. magnesium sulfate and then
evaporated to afford the crude compound, which was purified by silica
gel column chromatography to afford the pure title compound.
Method (iii): Ultrasonic Irradiation Method
(a) Using Glacial Acetic Acid
A solution of 2-aryl-1H-indole (1 mmol) in benzene (10 mL), the ap-
propriate aldehyde (0.5 mmol) in benzene (5 mL), and glacial acetic acid
were taken in a conical flask (100 cm3) and irradiated with ultrasonic
waves for 50–80 min at 40 ◦C. After completion of the reaction (checked
by TLC), the reaction solution was extracted with benzene (3 × 15 mL)
followed by washing with water (3 × 15 mL). The benzene layer was
dried over anhy. magnesium sulfate. After removal of the benzene layer,
we obtained the pure title compound.
(b) Using Anhydrous FeCl3
A solution of 2-aryl-1H-indole (1 mmol) in benzene (10 mL), the
appropriate aldehyde (0.5 mmol) in benzene (5 mL), and anhy. FeCl3
(0.2 mmol, 0.03 g) were taken in a conical flask (100 cm3) and irradi-
ated with ultrasonic waves for 40–70 min at 40 ◦C. After completion of
the reaction, as indicated by TLC, the reaction solution was extracted
with benzene (3 × 15 mL) followed by washing with water (3 × 15

2842

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2844
A. Tiwari and M. Jain
mL). The benzene layer was dried over anhy. magnesium sulfate. The
benzene layer was removed in vacuo to afford the crude compound,
which was purified by silica gel column chromatography using ben-
zene/ethylacetate as the eluent to afford the desired compound. The
characteristic and analytical data of these compounds are given in Ta-
ble I. The comparative data of various procedures are tabulated in Table
II.
Antibacterial and Antifungal Activities of
2,2ꢀ-Diaryl-3,3ꢀ-alkylidene/arylidene Bisindoles (2a–e)
All the synthesized title compounds were screened for their antibac-
terial activity against Gram-positive bacteria S. aureus and Gram-
negative bacteria Escherichia coli at different concentrations using the
inhibition zone technique25 (Table IV). All the synthesized title com-
pounds were also screened for their antifungal activity against the
fungi Aspergillus flavus and Aspergillus niger at different concentra-
tions, and results obtained were recorded in Table V.
Streptomycin and mycostatin were used as reference compounds
for evaluating the antibacterial and antifungal activities, respectively.
Compounds 2c and 2e showed greater activity due to the presence of
the methoxy group in the bisindole moiety. The possible use of these
compounds as drugs is being explored. All other compounds showed
significant antibacterial and antifungal activities at 1000, 800, 400,
and 200 ppm concentrations, respectively.
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