Cu doped CdS nanoparticles: A versatile and recoverable catalyst for chemoselective synthesis of indolo[2,3-b]quinoxaline derivatives under microwave irradiation

Article abstract of DOI:10.1016/j.molcata.2014.07.022

CdS and Cu doped CdS NPs has been obtained successfully using safe agents through a simple aqueous chemical method and characterized by XRD, TEM, SEM, EDAX, UV/VIS and ICP-AES. These investigations revealed that the particle size of the synthesized materi

Full text of DOI:10.1016/j.molcata.2014.07.022

Journal of Molecular Catalysis A: Chemical 394 (2014) 244–252
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Cu doped CdS nanoparticles: A versatile and recoverable catalyst for
chemoselective synthesis of indolo[2,3-b]quinoxaline derivatives
under microwave irradiation
a,∗
a
a
b
Anshu Dandia , Vijay Parewa , Shuchi Maheshwari , Kuldeep S. Rathore
a
Centre of Advanced Studies, Department of Chemistry, University of Rajasthan, Jaipur 302004, India
Semiconductor and Polymer Science Laboratory, Department of Physics, University of Rajasthan, Jaipur, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
CdS and Cu doped CdS NPs has been obtained successfully using safe agents through a simple aqueous
chemical method and characterized by XRD, TEM, SEM, EDAX, UV/VIS and ICP-AES. These investigations
revealed that the particle size of the synthesized materials were uniformly distributed in the range of
Received 2 January 2014
Received in revised form 1 July 2014
Accepted 2 July 2014
2
–4 nm and confirm the Cu doping in the lattice of CdS NPs. These nanoparticles were exploited to
Available online 22 July 2014
study their catalytic activities toward the chemoselective synthesis of indolo[2,3-b]quinoxalines by the
reaction of isatins with 1,2-diamines in ethylene glycol under microwave irradiation. The method showed
remarkable selectivity for indolo[2,3-b]quinoxalines over 3-imino-isatin, spirobenzimidazole, and ring-
opened quinoxalinone derivatives. The catalytic activity of Cu doped CdS NPs was found to be about
Keywords:
Chemoselectivity
Doping
CdS and copper doped CdS NPs
Heterogeneous catalyst
Indolo[2 ;3-b]quinoxalines
1
8-fold higher under microwave irradiation (MW) as compared to the conventional method. Nanocatalyst
plays a dual role of catalyst as well as susceptor, and enhances the overall capacity to absorb MW in the
reaction mixture. Doping of Cu promotes the activity and selectivity of CdS nanoparticles indicated by
high TOF value with good chemoselectivity. The surface acidity of NPs was measured by FTIR spectra of
chemisorbed pyridine. Simple workup, mild reaction conditions, low cost, easy separation, and reusability
of the catalyst are some advantages of this method.
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
of high atom efficiency, simplified isolation of product, and easy
recovery and recyclability of the catalyst [3].
Catalysis is becoming a strategic field of science because it repre-
The quinoxaline ring system is probably the most ubiqui-
tous privileged heterocyclic scaffold to build pharmacologically
and biologically active molecules [4]. In addition to medicinal
uses, quinoxaline derivatives have found technical applications
as semiconductors [5], dyes [6], and as suitable ligands in coor-
dination chemistry [7]. Furthermore, indole substituted with
various heterocyclic rings has been found in a fascinating array
of bioactive natural products and pharmaceutical compounds [8].
An indolo[2,3-b]quinoxaline nucleus which accumulates quinox-
aline as well as indole moieties integrates properties of both,
and the synergism of both the heterocyclic moieties in a
single nucleus results in the formation of some worthwhile
molecules from the biological point of view [9] (Fig. 1). Deriva-
tives of indolo[2,3-b]quinoxaline exhibit promising biological
activities such as anticancer, anticonvulsant, antibacterial and
antiviral activity [10]. Further, 6H-indolo[2,3-b]quinoxaline com-
pounds, NCA0424 and NCA0465 represent an important series
of DNA intercalating agents endowed with antiviral and cyto-
toxic activities [11a] and Allosteric dual Akt1 and Akt2 inhibitor
sents a new way to meet the challenges of energy and sustainability.
These challenges are becoming the main concerns of the global
vision of societal challenges and world economy. The societal pres-
sure has been at the origin of the concept of nanoscience, which
is an exponentially growing research field in modern science that
involves the synthesis and application of nanoparticles of differ-
ent sizes and shapes [1]. The exciting prospect of nanoscience is
its potential use in almost any conceivable domain. Every field
from medicine and electronics to manufacturing and fashion stand
to be benefitted from advances in nanotechnology [2]. Though
nano-scale technology is multifaceted in its application, the use
of nano-material as catalyst is perhaps the most intriguing. Fur-
ther, the nano-material catalysed reactions provide the advantages
∗ Corresponding author. Tel.: +91 1412525845/+91 9414013436 (Mobile).
E-mail addresses: dranshudandia@yahoo.co.in, dandiaanshu1957@gmail.com
A. Dandia), kuldeep ssr@yahoo.com (K.S. Rathore).
(
http://dx.doi.org/10.1016/j.molcata.2014.07.022
381-1169/© 2014 Elsevier B.V. All rights reserved.
1

A. Dandia et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 244–252
245
CH3
N
N
N
N
O
CH3
N
N
N
N
N
NH
N
N
N
H
CH3
CH OH
2
N
H C
CH3
2
H C
CH3
3
Allosteric dual Akt1 and Akt2 inhibitor
CH
2
OH
B 220
NCA0424
CH OH
2
H C
CH2
3
N
H
N
CH
2
OH
N
N
N
N
O
NPS-2390
N
N
CH
Tricyclic benzo[g]quinoxaline leads
3
NCA0465
Fig. 1. Quinoxalines and indoloquinoxalines as clinically used drugs.
afforded modest activity in cell-based IPKA Akt assays [11b].
Compound B-220 (2,3-dimethyl-6-(2-dimethylaminoethyl)-6H-
indolo[2,3-b]quinoxaline) has shown remarkable activity against
herpes virus [12a], tricyclic benzo[g]quinoxaline acts as platelet
derived growth factor receptor kinase inhibitor [12b] and NPS-2390
has been identified as an active mGluR1 fragment [12c].
morphology could facilitate the controlled tuning of the catalytic
properties [19]. Cu has been extensively investigated as promoter
elements to develop new or improved catalytic system in the past
[20]. Doping with copper affected measurable changes in the sur-
face and catalytic properties of CdS nanoparticles.
During our continuous efforts on the development of novel
green protocols for heterocyclic frameworks and nanoparticles
synthesis [21], herein, we describe the synthesis, characterization
and catalytic application of CdS and Cu doped CdS nanoparticles for
chemoselective synthesis of indolo[2,3-b]quinoxalines by the reac-
tion of isatins with 1,2-diamine in ethylene glycol under microwave
irradiation (Scheme 1).
A
general procedure for the synthesis of indolo[2,3-
b]quinoxalines involves the condensation reaction of isatin
analogs with 1,2-diamines [13]. The product distribution of
the reaction of isatins with 1,2-diamines has been reported to
strongly depend on the reaction conditions. The reaction gave
a mixture of compounds [13b,e], indolo[2,3-b]quinoxalines (A),
3
-imino-isatin (B), spirobenzimidazole (C), and ring-opened
quinoxalinone (D) and sometimes B–D, have been isolated as the
main product instead of indolo[2,3-b]quinoxalines [13c,d,14]. This
fact resulted in our enthusiasm for either exclusive formation of
the indolo[2,3-b]quinoxaline derivatives.
2. Experimental
2.1. General
A literature survey suggested that there are very few reports
where quinoxaline moiety is assimilated with indole moiety
All the chemicals used were of research grade and were used
without further purification. IR spectra were recorded on a Shi-
1
[
13,14]. However, the reported methods suffer from many draw-
madzu FT IR- 8400S spectrophotometer using KBr pellets. H and
13
backs like utilization of hazardous chemicals, longer reaction time,
unsatisfactory yields, burdensome product isolation procedure and
co-occurrence of several side reactions with less selectivity of
the process. Recently Jain et al. described [15] the synthesis of
indolo[2,3-b]quinoxalines in aqueous medium with surfactant. The
main disadvantage lies with the use of these surfactants in water
is that removal of these materials from water is much expensive.
Another disadvantage of using surfactants is that they are not eas-
ily bio-degradable. This has many environmental implications [16].
Therefore, further development of the catalytic system will require
advanced materials that can selectively catalyze chemical reactions
with high reactivity and can be recycled through simple separation
and regeneration processes. In this context, we have used various
doped nanoparticles as a heterogeneous recyclable catalyst in het-
erocyclic synthesis under environmentally benign conditions [17].
Inspired by this work, we are interested in studying the catalytic
activity of CdS and Cu doped CdS NPs.
C NMR spectra were recorded in DMSO-d6 and CDCl3 using
TMS as an internal standard on a Bruker spectrophotometer at
300, 75 MHz and 400, 100 MHz. Mass spectrum of representa-
tive compound was recorded on JEOL SX-102 spectrometer at
70 eV and Agilent 1100 LC–MS spectrometer. The reactions were
carried out using an ultrasonic processor probe (Processor SONO-
PROS PR-1000 MP, OSCAR ULTRASONICS with power input 230 V,
50 Hz, 4 A and power variac 0–230 V and 3 A) operating at 20 kHz,
750 W with 6 mm/12 mm tip diameter probes. The microwave-
assisted reactions were carried out in a MAS-II microwave oven
(2450 MHz, Sineo Microwave Chemistry Technology Company,
Shanghai, China) with a maximum power output of 1000 W. This
system is equipped with a power and temperature feedback control
switch.
2.2. Catalyst preparation
CdS and Cu doped CdS NPs is an important semiconductor owing
to its unique electronic and optical properties and its potential
applications in solar energy conversion, nonlinear optical photo-
electrochemical cells and heterogeneous photocatalysis [18]. Since
the properties of the catalyst surfaces are closely correlated with
the catalytic activities, the precise modification of the catalyst sur-
face by introducing another component (dopant) or changing the
Nanoparticles of CdS were prepared at room temperature by
dropping simultaneously 50 ml of 1 M solution of CdSO and 50 ml
4
of 1 M solution of Na S into 200 ml of distilled water contain-
2
ing 50 ml of 0.1 M solution of EDTA, which was vigorously stirred
using a magnetic stirrer. The high insolubility of CdS formed out
of the chemical reaction caused the formation of a number of new
nuclei while preventing the growth of already existing ones, thus

2
46
A. Dandia et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 244–252
Z
NH2
O
Z
NH
O
N
HN
X
X
N
N
Y
Y
B
C
O
X
1
N
Y
O
H N
2
2
H N
2
Z
N
X
N
X
N
Y
N
Z
NH
Y
O
N
H
Z
A
D
Scheme 1. Chemoselective synthesis of indolo[2,3-b]quinoxaline derivatives.
limiting the particle size. The role of EDTA was to stabilize the par-
ticles against aggregation which may lead to an increase in the
particle size. The doping of copper has been done by adding 2 wt% of
metal sulphate (CuSO ) to CdSO (at the beginning of the reaction)
acetate; the organic layer was removed under reduced pressure
to afford the crude product. The pure products were obtained by
crystallization from ethanol.
4
4
for the formation of CdS:Cu NPs. The precipitate was separated from
the reaction mixture and was dried at room temperature. After suf-
ficient drying, the precipitate was crushed to fine powder with the
help of mortar and pestle.
2
.5. General procedure for the recyclability of the catalyst
The catalyst could be recycled easily by solvent extraction of the
product from the reaction mixture. After completion of the reaction,
0 ml of ethyl acetate was added to the reaction mixture. The reac-
1
2
.3. Catalyst characterization
tion mixture was sonicated until complete dissolution of products
in ethyl acetate was observed. The two layers were then separated.
The ethylene glycol layer was sonicated for 5–10 min and reused
for the same experiment for over five cycles.
The wide angle X-ray diffraction pattern of the sample was
obtained using Bragg–Brentanno geometry on Panalytical X’pert
Pro diffractometer in 2ꢀ range of 15–85 with Cu K␣ radiation
source (ꢁ = 1.5406 A˚ ). The X-ray tube was operated at 45 kV and
◦
3. Results and discussion
4
0 mA. TEM measurements of the sample were carried out using a
JEOL—transmission electron microscope. Sample for the TEM was
prepared by making a clear dispersion of nanoparticles in dimethyl
formaldehyde and putting a drop of it on a carbon coated cop-
per grid. The EDAX measurements of the synthesized nanoparticles
were performed to confirm the copper doping using a FEI Quanta
Simple aqueous chemical method [22] has been used to prepare
these nanomaterials. The advantages of this method lies in its sim-
plicity, cost effectiveness, environment friendliness, easier scaling
up for large scale synthesis while avoiding the use of high pressure,
temperature and toxic chemicals. The morphology and structure of
the prepared nanoparticles were characterized by TEM, XRD, SEM
and EDAX analyses, which confirmed the successful preparation of
the nanoparticles.
2
00F SEM fitted with an EDAX. The optical absorption spectra of
the nanoparticles were recorded using ocean optics USB 2000 spec-
trophotometer in the solution form.
2.4. Catalyst activity measurement
3.1. XRD
Indole-2,3-dione 1 (1 mmol) and 1,2-diamine 2 (1 mmol) and
mol% of Cu doped CdS NPs in ethylene glycol (10 ml) was
Fig. 2 shows the X-ray diffraction patterns of all samples of CdS
and Cu doped CdS catalyst prepared by wet chemical method. The
samples show intense and broad diffraction peaks at 2ꢀ values of
about 26.55, 44.03 and 52.08 correspond to the (1 1 1), (2 2 0) and
(3 1 1) planes of cubic phase of CdS. Addition of Cu into the CdS
matrix does not cause any significant change in the XRD profile of
5
introduced in a 50 ml round-bottomed flask. The flask was placed in
the microwave cavity and subjected to irradiation for appropriate
◦
time at 80 C using a maximum power of 300 W. When the reac-
tion was complete, the reaction mixture was extracted with ethyl

A. Dandia et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 244–252
247
Fig. 5. SEM image of Cu doped CdS NPs.
Fig. 2. XRD patterns of nanoparticles.
It can be seen that all the nanoparticles have an average size in
the 2–4 nm range, which is in consistent with the results obtained
through the calculation by Scherrer’s equation. Spherical morphol-
ogy of Cu-doped CdS NPs was further confirmed by the SEM image
CdS [22b–23]. The average particle size for pure CdS and Cu doped
CdS has been found to be 3 nm and 2 nm, respectively, according to
Scherrer’s equation [24].
(Fig. 5).
3.2. TEM
3.3. EDAX
Size of the nanoparticles was characterized by Transmission
EDAX of both the samples were shown in Fig. 6. The EDAX data
substantiate the doping of Cu in CdS matrix. Dopant concentration
of the catalyst was also confirmed by ICP-AES analysis.
Electron Microscopy (Fig. 3). TEM images confirmed that the par-
ticles are in nano range and they are approximately spherical in
shape, whose size distribution is given by the histogram (Fig. 4).
Fig. 3. TEM image of nanoparticles.
Fig. 4. Histogram of nanoparticles.

2
48
A. Dandia et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 244–252
Fig. 6. EDAX spectrum of the nanoparticles.
undoped sample is attributed to the absorption of CdS nanoparti-
cles. With the Cu doping concentration the absorption edge showed
a blue shift. This slight shifting in absorption edge is the outcome of
the quantum confinement effect produced due to increased nuclea-
tion rate with doping. The trend of band gap variation after doping
observed in present study is similar to that reported earlier [24].
3.5. Surface acidity measurements
The surface Lewis acidity of this material was confirmed through
the adsorption of pyridine vapors [25] on the surface of the NPs.
FT-IR spectra of pyridine adsorbed on the catalyst are included
Fig. 7. UV spectrum of the nanoparticles.
−
1
in spectral information and the band at around 1400 cm
is
attributed to the adsorbed pyridine at the surface Lewis acid
site, the intensity of this band increases with Cu doping (Fig. 8).
From the results of the acidity measurements, it can be concluded
that the incorporation of Cu enhances the surface acidity, which
increases the number of exposed metal sites [26,27] and therefore
3
.4. UV-spectra
Fig. 7 shows UV–vis spectra of the Cu-doped CdS and undoped
CdS nanoparticles. The absorption peak at 315 nm in the curve of
Fig. 8. FT-IR spectra of the pyridine-adsorbed samples. (a) CdS NPs (b) Cu doped CdS NPs.

A. Dandia et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 244–252
249
Table 1
Table 2
a
Comparison of catalytic activity of catalyst for the synthesis of indolo[2,3-
b]quinoxaline derivatives .
Effect of solvent on the synthesis of indolo[2,3-b]quinoxaline derivatives (3a) .
a
Entry
Solvent
Time (min)
Yield (%)*
Yield (%)*
−1
TOF (h )
Entry
Catalyst
Time (min)
1
2
3
4
5
Ethanol
10
10
10
10
10
69
54
46
74
95
1.
2.
3.
3.
4.
5.
6.
7.
–
60
38
36
34
30
25
10
10
28
42
44
47
56
71
95
95
–
298
330
373
504
767
2565
5415
Methanol
Acetonitrile
Water
CdSO4 (10 mol%)
CdCl2 (10 mol%)
CdNO3 (10 mol%),
Powder CdS (10 mol%)
CdS NPs (10 mol%)
Cu doped CdS NPs (10 mol%)
Cu doped CdS NPs (5 mol%)
Ethylene glycol
a
Reaction conditions: isatin (1 mmol), ortho-phenylenediamine (1 mmol) and
5
mol% of catalyst under microwave irradiation.
Isolated yield.
*
a
Reaction conditions: isatin (1 mmol) and ortho-phenylenediamine (1 mmol) in
ethylene glycol (5 ml) under microwave irradiation.
*
Isolated yield.
method (Table 3). The enhancement of catalytic activity in MW
might be due to the fact that nanocatalyst act as a susceptor and
absorb microwave irradiation [28], thus they can serve as an inter-
nal heat source for the reactions which enhance the overall capacity
to absorb MW in the reaction mixture and prevented the deac-
tivation of nanocatalyst during the reaction. The model reaction
was also studied by varying microwave power and temperature. It
improve the metal-time yields and also increase the reaction rate
per exposed metal site i.e., the turn over frequency.
3.6. Catalytic performance for the synthesis of
indolo[2,3-b]quinoxaline derivatives
◦
was concluded that 300 W power output at 80 C was required to
To examine the catalytic activity of nanoparticles in the chemos-
elective synthesis of indolo[2,3-b]quinoxalines, isatin (1 mmol)
and ortho-phenylenediamine (1 mmol) were chosen as model sub-
strates for the reaction in ethylene glycol (5 ml) under microwave
irradiation. A control experiment was conducted in the absence of
catalyst. The reaction was incomplete even after 60 min, though for-
mation of a small amount of 3a (28%) was observed. Thus the initial
efforts were focused on the systematic evaluation of various cata-
lyst systems. As shown in Table 1, when the reaction was carried out
in the presence of catalytic amount of CdS NPs, the desired product
accomplish maximum conversion to product. Furthermore, to take
advantage of the highly efficient green protocol, the reaction was
scaled up to 10 mmol.
In order to study the generality of this procedure, a variety of
substituted isatins were subjected to this reaction (Fig. 9). Isatins
bearing electron withdrawing groups reacted faster with slightly
improved yields as compared to the isatins having electron donat-
ing counter parts. The application of the reaction to heteroatomic
diamines has also been explored. The results suggest that, the reac-
tion proceeds well in the optimized conditions. Further, the analysis
of the final product by ICP-AES showed that there was no nanocat-
alyst present in the final product.
3
a was isolated with 71% yield. Further, the study of catalytic abil-
ity of Cu doped CdS nanoparticles showed that doping increases the
product yield up to 95% and reduces the reaction time (10 min) with
the decreased catalyst loading (5 mol%). It was found that the best
result in terms of turnover frequency (TOF) could be achieved by
using Cu doped CdS NPs catalyst (5 mol% loading). It shows that the
CdS NPs were active in reaction with a turn-over frequency (TOF)
In the reaction of isatins with 1,2-diamines, the formation
of the required indolo[2,3-b]quinoxalines is accompanied by the
occurrence of 3-imino-isatin, spirobenzimidazole, and ring-opened
quinoxalinone as side products and sometimes these products have
been isolated as the main product. The present protocol gives
indolo[2,3-b]quinoxalines selectively and exclusively. The struc-
−
1
of 767 h is observed. When the Cu doped CdS NPs were used,
−1
ture of the products 3a-k was established by IR, 1H, C NMR
13
the activity was septupled to (TOF) 5415 h . This finding indicates
that doping of Cu promotes the activity and selectivity of CdS NPs
and higher concentration of acidic sites gives more products in the
reaction. Therefore, lower catalyst loading is required for this trans-
formation compared to many other catalytic systems, thus showing
that this method is superior to the other methods in terms of yield
and reaction time.
and mass spectrometric studies. For instance, the IR spectrum of
−
1
the product 3a showed characteristic bands at 1625 cm corre-
sponding to the C N group in the cyclic ring system. In the 1
H
NMR of 3a, only one exchangeable hydrogen with chemical shift
between 11.94 and 12.21 ppm was observable. This hydrogen can
be assigned as the NH of indole moiety of the final structure along
with characteristic signals with appropriate chemical shift and cou-
pling constant for the eight protons of the two aromatic moieties.
In order to ascertain the effect of solvent, the reaction was car-
ried out using different solvents. The superiority of ethylene glycol
as a solvent as compared to commonly employed solvents is quite
evident from the results summarized in Table 2.
The model reaction was also investigated under different non-
conventional and conventional conditions and the overall findings
are given in Table 3. Under MW, the catalytic activity of Cu doped
CdS NPs was found to be 18-fold higher than the conventional
13
The C NMR spectrum of 3a demonstrated signals at ı 143.9 and
145.7 ppm due to two C N group in the tetra cyclic ring system. The
+
mass spectrum of 3a showed a molecular ion peak at 219.0 (M) . In
the IR spectrum, the disappearance of two C O of the oxindole ring
along with the appearance of a signal due to C N further proved the
formation of quinoxaline derivatives instead of other products. This
Table 3
a
Dependency of catalytic activity of catalysts under different nonconventional and conventional conditions .
◦
Yield (%)*
−1
TOF (h )
Entry
Condition
Catalyst
Temp. ( C)
Time (min.)
1.
2.
3.
4.
5.
6.
Conventional
Ultrasound
Microwave
Microwave
Microwave
Microwave
Cu doped CdS NPs (5 mol%)
Cu doped CdS NPs (5 mol%)
Cu doped CdS NPs (5 mol%)
Cu doped CdS NPs (5 mol%)
Cu doped CdS NPs (5 mol%)
Cu doped CdS NPs (5 mol%)
80
80
60
70
80
90
90
60
10
10
10
10
48
54
42
68
95
95
304
513
2394
3876
5415
5415
a
*
Reaction conditions: isatin (1 mmol), ortho-phenylenediamine (1 mmol) and 5 mol% of catalyst in ethylene glycol (5 ml).
Isolated yield.

2
50
A. Dandia et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 244–252
____________________________________________________________________________
N
N
H C
N
N
Cl
N
N
3
N
H
N
H
N
H
3
a, 10 min., 95 %
3b, 12 min., 93 %
3c, 10 min., 96 %
_
_
_
___________________________________________________________________________________________
H C
3
N
N
N
N
Br
N
N
N
H
N
N
H
CH3
3
CH
3
d, 14 min., 91 %
3e, 10 min., 96 %
3
f, 11 min., 94 %
___________________________________________________________________________________________
N
N
H C
N
3
N
N
N
H
N
N
N
H
N
N
CH Ph
2
3
g, 11 min., 94 %
3h, 10 min., 96 %
3i, 12 min., 95 %
___________________________________________________________________________________________
H C
N
N
3
Cl
N
N
N
H
N
N
H
N
CH
3
3
j, 12 min., 97 %
3k, 14 min., 90 %
Fig. 9. Products of chemoselective synthesis of indolo[2,3-b]quinoxaline derivatives.
was also confirmed by the 13C NMR spectrum in which no signals
due to C O of oxindole ring and spiro carbon was observed.
A possible mechanism for the formation of products 3a-k is
shown in Scheme 2. The Lewis acid sites of NPs were coordinated
to the oxygen of carbonyl groups. The coordination of NPs with
the carbonyl oxygen of 1 induces electrophilic activation of isatin,
which benefits the initial addition of 2 with 1 to give an amino-1,
2-diol. The resultant amino-1,2-diol undergoes dehydration to give
indolo[2,3-b]quinoxaline 3 as the end product.
3.7. Recycling of the catalyst
Reusability of the catalyst was evaluated by carrying out
repeated runs of the reaction on the same batch of the catalyst in
H N
2
H
N
HO
H
HO
O
N
H N
2
2
N
H
1
O
N
H
O
N
H
N
H
NH2
HO
=
Cu doped CdS
-
2H O
2
N
N
H
N
3
Scheme 2. Plausible mechanism.

A. Dandia et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 244–252
251
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