An Efficient Approach for the Synthesis of 1,1-bis(2-phenyl-3-indolyl)ethylene Using ZnO Nanocatalyst

Article abstract of DOI:10.1002/jhet.3053

A combination of nanocatalyst and green chemical route (mechanochemistry) was used to generate a series of substituted 1,1-bis(2-phenyl-3-indolyl)ethylene derivatives (4a–e) that were synthesized by reacting various 2-arylindoles (1a–e) and acetyl chlorid

Full text of DOI:10.1002/jhet.3053

Month 2017
An Efficient Approach for the Synthesis of 1,1-bis(2-phenyl-3-indolyl)
ethylene Using ZnO Nanocatalyst
b
Yogita Madana
and Ragini Gupta
*
aDepartment of Chemistry, Amity School of Applied Sciences, Amity University Rajasthan, Jaipur 303 007, India
bDepartment of Chemistry, Malaviya National Institute of Technology, Jaipur 302 017, India
*
E-mail: guptaragini@yahoo.com; rgupta.chy@mnit.ac.in
Received July 4, 2017
DOI 10.1002/jhet.3053
Published online 00 Month 2017 in Wiley Online Library (wileyonlinelibrary.com).
A combination of nanocatalyst and green chemical route (mechanochemistry) was used to generate a
series of substituted 1,1-bis(2-phenyl-3-indolyl)ethylene derivatives (4a–e) that were synthesized by reacting
various 2-arylindoles (1a–e) and acetyl chloride in absolute alcohol with or without ZnO nano as catalyst via
two approaches, that is, classical and green solvent-free route. The use of ZnO nano particles is preferred to
absence of catalyst in terms of short reaction time and mild reaction conditions with reusability of the
catalyst. All the synthesized compounds were characterized on the basis of their elemental and spectral data
(IR, proton magnetic resonance, 13C NMR, and Mass).
J. Heterocyclic Chem., 00, 00 (2017).
INTRODUCTION
synthesis, pollutant removal, energy production,
medicine, health, and in many other discipline.
Nanomaterials have led to a dramatic expansion of their
wide utilization as a solid support material for the
design of environmentally benign heterogeneous
catalysts to address various economic and environmental
issues [16,17]. In the context of green chemistry, use of
nanosized catalysts is one of the key areas of research
for the welfare of mankind. Nanocatalyst strikes in mind
for their use in the production of biological active
building blocks due to its higher activity, selectivity, and
easy separation. In past decades, nanocrystalline metal
oxides have been used as an effective catalyst for the
synthesis of various heterocyclic moieties. The recent
literature survey revealed that ZnO nanoparticles as
heterogenous catalyst [18] as received much attention
due to the advantages: that is, minimum execution time,
easy workup procedure, low corrosion, economically
cheap chemical, commercially available, easily
synthesized [19,20], and recycling of it. In recent years,
ZnO nanoparticles have excellent potential in expressing
abundant biological properties such as antimicrobial
[21,22], antioxidant, anticancer, and other activities [23].
Conventional methods of organic synthesis are too slow
to meet the demand for generation of heterocyclic
compounds. These methods also have certain other
disadvantages such as low yield, tedious work up, and
high reaction time. Therefore, the arena of chemistry is
now shifted toward the innovative techniques that mainly
focus on environmental aspects [1,2]. Thus, the fields of
green chemistry [3] such as solvent free synthesis, use of
green solvents, microwave irradiation, grind stone
technology and ultrasound irradiation [4–9], catalysis,
and nanocatalysis [10–12] have emerged to meet the
increasing requirement of new heterocyclic derivatives
for drug discovery, where speed is of essence.
The role of nanomaterials in modern technologies is
becoming increasingly and interestingly significant
because of their unique physicochemical properties and
broad-scale applicability in numerous fields of
catalysis, imaging, photonics, nanoelectronics, sensors,
biomaterials, and biomedicine [13–15]. Recently, catalysis
by nanoparticles has brought a revolution in various
fields, that is, chemical transformations in organic
© 2017 Wiley Periodicals, Inc.

Y. Madan and R. Gupta
Vol 000
ZnO nanoparticles were also suggested as good
RESULTS AND DISCUSSION
photocatalytic agents [24]. ZnO nanoparticles have been
used as catalyst in organic transformations such as O-
acylation of alcohol and phenol [25], synthesis of β-
acetamido ketones/ester [26], benzimidazole derivatives
[27], and bis(pyrazolyl)methanes [28] in aqueous media
and in Biginelli [12] and Hantzsch reactions [29]. In
continuation to our work to develop new green effective
chemical route to synthesize bioactive heterocycles, we
wanted to explore the feasibility of ZnO nanocatalyst for
the synthesis of bisindolylethylene derivatives (4a–e).
Bisindolylethylene and their derivatives exhibit a broad
spectrum of biological activities such as antibacterial,
cytotoxic, antioxidative, and insecticidal activities
[30,31]. These compounds also form a class of bioactive
metabolites of terrestrial and marine origin [32,33].
Compounds having conjugated polyolefin bond or central
ethylene bond in which either of the ends is associated
with heterocyclic moiety also constitute the major class
of fluorescent cyanine dyes. Many of the compounds of
this larger dye group consist of indole moiety as
heterocyclic end [34]. A broad application of dye group
as fluorescent probes in nucleic acid imaging [35] have
also been observed. Three new series of bis-indole
derivatives were synthesized based on p-phenylenediamine
and 4,40-ethylenedianiline moieties using facile and
efficient condensation of three positional isomeric indole-
carboxaldehyde derivatives with bifunctional amines upon
microwave irradiation [36].
1,1-Bis(2-phenyl-3-indolyl)ethylene (4a) was synthesized
conventionally by refluxing 2-phenylindoles and acetyl
chloride (RANKEM, Gurugram, Haryana, India) in
alcohol for 4–5 h [37]. To explore the versatility of the
procedure, we have reacted of various substituted-2-
phenylindoles (1a–e) with acetyl chloride in the presence
of ZnO nanocatalyst via two approaches, that is,
classical and green grindstone mechanochemical route.
This protocol is preferred over the conventional
procedure in terms of short reaction time and mild
reaction conditions with reusability of the catalyst. All
the synthesized compounds have been characterized by
their elemental and spectral data analyses (IR,
phosphorus magnetic resonance, 13C NMR, and Mass).
In the conventional approach, compounds (4a–e) were
synthesized by refluxing 2-arylindoles (1a–e) with acetyl
chloride in absolute alcohol (Merck Limited, Vikroli East,
Mumbai, India) for 4–7 h (Table 1) with or without ZnO
nano. Alternately, this reaction was performed by adopting
grindstone mechanochemistry that is preferred to
conventional approach as it includes the benefit of safe
environmental conditions and better yield with short
reaction time. Based on the illustrated results in Table 2,
the yields of the desired products were obtained maximum
under grinding for 35 min at 2 mol% of ZnO nanoparticles
for all the other derivatives synthesized. Therefore, the
amount of ZnO nanoparticles was chosen to be 2 mol% for
all the other derivatives synthesized. In this methodology,
when ZnO nano was used as catalyst, product formation
started in a few minutes that confirmed by the generation of
a new spot other than the reactants on TLC plate (Scheme 1).
In the IR-spectrum of compound 4a, broad absorbance
from 3414 to 3418 cmꢀ1 is assigned to –NH stretching
vibration. Ethylenic =CH stretching is observed at
2983 cmꢀ1 that is slightly at lower wave numbers due to
conjugation. In the PMR spectrum of compound 4a,
ꢀNH proton of indole moiety appeared as a singlet
centered at δ 8.19 ppm (D2O exchangeable). A multiplet
from δ 7.48–7.88 ppm is observed for aromatic protons.
Disappearance of the singlet from
δ
6.4 ppm
(corresponding to methine proton of indole ring) and
appearance of a new characteristic singlet of ethylenic
protons at δ 5.37 ppm confirmed the formation of product
4a. 13C NMR spectrum of the compound 4a showed
characteristic peak at δ 129.18 ppm indicating the
presence of ethylenic carbon atom that was previously
not present (in reactant). Further confirmation was
obtained by mass spectra of 4a that displayed M+1 peak
at 411.1866 corresponding to molecular formula
C30H22N2. Finally, ESI-MS spectra of compounds (4a–e)
gave an accurate molecular ion peak at m/z 479 (4b), 569
(4c), 438 (4d), and 447 (4e) that agreed well with their
corresponding molecular formulae.
In X-ray diffraction patterns, all diffraction peaks of the
samples were indexed to the hexagonal phase of ZnO
Table 1
Synthesis of 2-aryl-3-(1-(2-aryl-1H-indol-3-yl) vinyl)-1H-indoles (4a–e).
Conventional reflux without catalyst
Reflux with ZnO nano
Grinding with ZnO nano
Compound
X
Yield (%)
Time (h)
Yield (%)
Time (h)
Yield (%)
Time (min)
4a
4b
4c
4d
4e
H
Cl
Br
CH3
F
50
41
47
44
42
6
7
6.5
7.5
5
74
73
71
72
70
1.5
5.2
4.5
4.0
1
88
86
89
85
85
35
45
40
45
30
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Synthesis of 1,1-bis(2-phenyl-3-indolyl)ethylene Derivatives
Table 2
recoverability and reusability of the catalyst, the zinc
oxide nanoparticles were separated by centrifugation and
then were washed three to four times with acetone
(Merck Limited, Vikroli East, Mumbai, India) and
methanol (RANKEM, Gurugram, Haryana, India) and
then dried at 450°C for 5 h. The separated catalyst was
used several times with a slightly decreased activity.
A possible mechanism (Scheme 2) indicates that ZnO
acts as a mild Lewis acid catalyst and enhances the
electrophilicity of the C atom of the carbonyl group of
acetyl chloride with its Lewis acid site Zn+2 by the
binding of ZnO to the O atom in acetyl chloride to give
intermediate I. Intermediate I attacks the nucleophilic
center C-3 of 2-arylindoles (1a–e) to give intermediate II,
which then loses HCl gas, and water sequentially to give
the desired products (4a–e) is evolved.
Optimization of the concentration of ZnO nanoparticles for the synthesis
of 2-aryl-3-(1-(2-aryl-1H-indol-3-yl) vinyl)-1H-indoles (4a).
Serial
number
ZnO nanoparticle
(mmol%)
Time
(min)
Yield
(%)
1.
2.
3.
4.
5.
6.
0
1
48
45
50
35
49
38
66
73
79
88
77
77
1.5
2
2.5
3.0
Scheme 1. Synthesis of 2-aryl-3-(1-(2-aryl-1H-indol-3-yl)vinyl)-1H-
indole derivatives (4a–e).
MATERIALS AND METHODS
Melting points were determined in open glass capillaries
and are reported uncorrected. The IR absorption spectra
ѵ
max. in cmꢀ1) were recorded on Perkin-Elmer FTIR
Spectrophotometer (PerkinElmer, Inc., Thane, Maharashtra,
India) at Indian Institute of Technology, Bombay. H NMR
1
(hexagonal phase, with lattice constants a = 3.249 Å,
c = 5.206 Å, Joint Committee on Power Diffraction
Standards [JCPDS] No. 361451). A most intense peak at
2θ = 36.4689° is obtained along (1 0 1) orientation
indicating that the growth direction of the ZnO was along
c-axis orientation. Because the patterns do not show the
presence of Zn or any other phase, SEM image reveals
that the ZnO is in nano scale.
were recorded on JEOL-AL 300 Spectrophotometer (JEOL
INDIA Pvt. Ltd., New Delhi, India) (300 MHz), and 13C
NMR spectra were recorded (75 MHz) at University of
Rajasthan, Jaipur using CDCl3 as solvent, and TMS was
taken as internal standard. Chemical shifts are reported in
parts per million (ppm, δ) downfield from TMS. ESI-MS
spectra were recorded on Q-TOF MS ES+ micro (YA-105)
mass spectrophotometer (Bruker India Pvt. Ltd., Goregaon,
Mumbai, India). The ESI mass spectra and CHN analyses
Reusability is one of the important properties of this
catalyst. After completion of the process, to study the
Scheme 2. Plausible reaction mechanism of the reaction.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Y. Madan and R. Gupta
Vol 000
1200
were recorded at IIT, Bombay, India. Scanning electron mi-
101
croscope (SEM) image of ZnO nanoparticles was recorded
using JEOL-JSM 6610LV (Jeol INDIA Pvt. Ltd., New Delhi,
India) at Malaviya National Institute of Technology, Jaipur.
The nano particles were characterized for their structure
using X-ray diffraction spectroscopy (XPert Pro by
PANalytical [PANalytical India, Gurugram, Haryana, India])
at Department of Physics, University of Rajasthan, Jaipur.
The type of X-ray tube is copper and chromium and detector
used in scintillation counter. The purity of the compounds
was assured by thin-layer chromatography (precoated silica
gel 60 mesh, MERCK (MERCK Ltd., Mumbai, India), as ad-
sorbent, UV light, or iodine accomplished visualization). All
common reagents and solvents were used as obtained from
commercial suppliers without further purification. Various
2-arylindoles (1a–e) were synthesized according to literature
methods [38,39].
1000
800
600
400
200
0
100
002
110
103
102
112
201
200
20
30
40
50
60
70
2
Figure 2. X-ray diffraction of ZnO nanoparticles.
angle and intensity of the characteristic peaks obtained in
XRD pattern clearly indicate that it is consistent with that
of the standard JCPDS card No. 36-1451. The average
size of particles has been calculated from the values of
full width at half maximum (FWHM) of peaks using
Debye Scherrer formula [41]
EXPERIMENTAL
Preparation of ZnO nanoparticles. ZnO nanoparticles
were synthesized by using previously reported method
D ¼ Kλ=βCosθ;
[40].
In
a
typical
procedure,
sodium
hydroxide (RANKEM, Gurugram, Haryana, India)
solution was added dropwise to an aqueous solution of
ZnSO4.6H2O in the molar ratio of 1:2 with continuous
stirring. The slurry so obtained was further stirred for the
next 12 h to give a precipitate that was filtered and
washed with distilled water (Millipore, Bengaluru,
Karnataka, India) (3 × 50 ml). It was then dried at 100°C
in an oven for 12 h, further grinded to a fine powder in
pestle and mortar, and subsequently calcinated for 2 h at
550°C in a muffle furnace. The sample was then
characterized using SEM (Fig. 1), and X-ray powder
diffraction (XRD) (Fig. 2) techniques. The diffraction
where K is constant, λ is the wavelength of X-rays employed
radiation, β is corrected FWHM, and θ is Bragg angle. The
size of ZnO nanoparticles was found to be 35 nm.
General experimental procedure for the synthesis of
2-aryl-3-(1-(2-aryl-1H-indol-3-yl) vinyl)-1H-indole derivatives
(4a–e). Method A: Conventional methodology without
catalyst. 2-arylindoles 1a–e (1 mmol) and acetyl chloride
2 (1 mmol) were taken in a round bottom flask and refluxed
for the time as mentioned in Table 1 according to the
procedure of Borthe and Groth [27]. The progress of the
reaction was monitored via thin-layer chromatography. At
the end of the reaction, a blue color developed indicating
the formation of the desired products (4a–e). The reaction
mixture was cooled overnight, and the solid so obtained
was filtered and recrystallized with pet ether (bp 60–80°C).
Further purification by column chromatography using silica
gel (60–120 mesh) as stationary phase and solvents of
increasing polarity as mobile phase gave off white crystals
of pure substituted 2-aryl-3-(1-(2-aryl-1H-indol-3-yl)
vinyl)-1H-indoles (4a–e) in 96% pet ether (bp 60–80°C)
and 4% ethyl acetate.
Method B: (i) Using ZnO nanocatalyst by classical
approach.
A mixture of 2-arylindoles 1a–e (1 mmol)
and ZnO nano 3 (2 mol%) in absolute alcohol (10 ml.)
were taken in a round bottom flask and acetyl chloride 2
(1 mmol) was added dropwise. A blue color developed
instantly indicating the start of product formation (as
monitored by TLC). To complete the reaction, the
reaction mixture was refluxed for additional 1.5 h. Same
work up procedure was followed (as mentioned in
Figure 1. Scanning Electron Microscope (SEM) image of ZnO
nanoparticle.
Journal of Heterocyclic Chemistry
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Month 2017
Synthesis of 1,1-bis(2-phenyl-3-indolyl)ethylene Derivatives
classical method) to afford off white crystals of pure 2-aryl-
2-p-tolyl-3-(1-(2-p-tolyl-1H-indol-3-yl)
vinyl)-1H-indole
(4d). Mp 182°C, IR (cmꢀ1, KBr): 3436 (br, ꢀNH str.),
3051 (ꢀCH str., Aromatic), 3024 (=CH2 str., methylene),
1H NMR (300 MHz., CDCl3, 30°C in δ ppm) 8.19 (s,
1H, NH indole), 7.48–7.88 (m, 18H, ArH), 5.26–5.34 (br
s, 2H, vinylidene H), 13C NMR (75 MHz, CDCl3, 30°C
in δ ppm) 29.66, 30.18, 75.96, 76.45, 76.59, 76.78,
110.14, 121.95, 121.76, 122.12, 122.87, 126.77, 128.04,
128.12, 129.04, 129.07, 129.13, 132.09, 134.49, 192.81,
MS (ESI) m/z = 439.2194 [M+1], molecular formula
C32H26N2 Anal. Calcd. C 87.64 (83.63) H 5.98 (5.96) N
3-(1-(2-aryl-1H-indol-3-yl) vinyl)-1H-indoles (4a–e).
(ii) Using ZnO nanocatalyst in mechanochemical
approach. 2-arylindoles 1 (1 mmol) and preheated ZnO
nano 3 (2 mol%) were mixed together in mortar and
grinded with pestle for 0.5 h with addition of CH3COCl
2 (1 mmol) and leave it for overnight. If it becomes
sticky, inert medium such as sand and silica gel (260
mesh) was also be added to provide friction among
molecules for proper grinding. Completion of the reaction
was monitored via TLC and contents were transferred
into a beaker. The products were filtered and washed well
with cold pet ether. Same work up procedure was
followed (as mentioned in classical method) to afford off
white crystals of pure 2-aryl-3-(1-(2-aryl-1H-indol-3-yl)
6.39 (6.43).
2-(4-fluorophenyl)-3-(1-(2-(4-fluorophenyl)-1H-indol-3-yl)
vinyl)-1H-indole (4e). Mp 180°C, IR (cmꢀ1, KBr): 3418
(ꢀNH str.), 3028 (ꢀCH str., Aromatic), 2978, 2883
(=CH2 str., methylene), 1HNMR (300 MHz., CDCl3,
30°C in δ ppm) 8.16 (br s, 1H, NH indole), 7.28–7.78
(m, 18H, ArH), 5.16–5.28 (s, 2H, vinylidene H), 13C
NMR (75 MHz, CDCl3, 30°C in δ ppm) 29.68, 30.31,
75.85, 76.38, 76.51, 76.76, 110.19, 121.65, 121.53,
122.42, 122.82, 126.64, 128.03, 128.12, 129.04, 129.08,
vinyl)-1H-indoles (4a–e).
Spectral data of 2-phenyl-3-(1-(2-phenyl-1H-indol-3-yl)
vinyl)-1H-indole derivatives (4a–e) 2-phenyl-3-(1-(2-phenyl-
1H-indol-3-yl) vinyl)-1H-indole (4a).
m.p. 245°C, IR
(cmꢀ1, KBr): 3421 (m, ꢀNH str.), 3058 (ꢀCH str.,
1
Aromatic), 2982, 2853 (=CH2 str., methylene), H NMR
(300 MHz., CDCl3, 30°C in δ ppm) 8.36 (br s,1H, NH
indole), 7.31–7.58 (m, 18H, ArH), 5.36–5.38 (s, 2H,
vinylidene H), 13C NMR (75 MHz, CDCl3, 30°C in δ
ppm) 29.68, 30.28, 75.94, 76.35, 76.55, 76.78, 110.16,
121.85, 121.73, 122.02, 122.97, 126.67, 128.01, 128.05,
129.01, 129.06, 129.18, 132.07, 134.40, 194.82, MS (ESI)
m/z = 411.1866 [M+1], molecular formula C30H22N2 Anal.
129.28,
132.31,
134.42,
189.62,
MS
(ESI)
m/z = 446.1566 [M + 1], molecular formula C30H20F2N2
Anal. Calcd. C 80.63 (80.70) H 4.59 (4.51) F 8.48 (8.51)
N 6.29 (6.27).
CONCLUSION
Calcd. C 87.74 (87.77) H 5.40 (5.44) N 6.82 (6.81).
2-(4-chlorophenyl)-3-(1-(2-(4-chlorophenyl)-1H-indol-3-yl)
vinyl)-1H-indole (4b). Mp 188°C, IR (cmꢀ1, KBr): 3414
This protocol offers several advantages in terms of shorter
reaction time, mild reaction conditions, and better yield with
reusability of the catalyst as well as greener way in term of
solvent free conditions. Mechanochemistry in combination
with nanocatalyst enhances the efficiency from economic as
well as a green point of view. These compounds show
fluorescence and can also be utilized as probe. They also
have skeleton similar to cyanine dyes; therefore, future
possibilities lie in these kinds of products.
(ꢀNH str.), 3050 (ꢀCH str., Aromatic), 2980, 2878
(=CH2 str., methylene), 663 (>C-Cl str.), 1H NMR
(300 MHz., CDCl3, 30°C in δ ppm) 8.19 (br s, 1H, NH
indole), 7.48–7.88 (m, 18H, ArH), 5.26–5.34 (s, 2H,
vinylidene H), 13C NMR (75 MHz, CDCl3, 30°C in δ
ppm) 28.68, 31.28, 76.04, 76.55, 76.95, 77.38, 111.26,
121.15, 121.23, 122.82, 122.97, 126.79, 128.16, 128.35,
129.19, 129.26, 129.28, 132.17, 134.50, 194.62. MS
(ESI) m/z
= 479.1566 [M+1], molecular formula
C30H20Cl2N2 Anal. Calcd. C 75.12 (75.16) H 4.23 (4.21)
Cl 14.48 (14.79) N 5.84 (5.89).
Acknowledgments. The author Yogita Madan is grateful to
Council of Scientific and Industrial Research, Delhi for their
financial support and IIT, Bombay, University of Rajasthan,
Jaipur and Malaviya National Institute of Technology, Jaipur for
spectral facility, SEM & lab facility.
2-(4-bromophenyl)-3-(1-(2-(4-bromophenyl)-1H-indol-3-yl)
vinyl)-1H-indole (4c). Mp 178°C, IR (cmꢀ1, KBr): 3419
(ꢀNH str.), 3052 (ꢀCH str., Aromatic), 2982 (=CH2 str.,
methylene), 585 (>C-Br str.), 1H NMR (300 MHz.,
CDCl3, 30°C in δ ppm) 8.29 (br s, 1H, NH indole), 7.45–
7.88 (m, 18H, ArH), 5.31–5.36 (s, 2H, vinylidene H),
13C NMR (75 MHz, CDCl3, 30°C in δ ppm) 29.38,
30.38, 75.84, 76.25, 76.35, 76.68, 110.26, 121.75,
121.83, 122.22, 122.77, 126.68, 128.11, 128.15, 129.11,
129.16, 129.28, 132.17, 134.42, 194.70, MS (ESI)
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet