Site-specific role of bifunctional graphitic carbon nitride catalyst for the sustainable synthesis of 3,3-spirocyclic oxindoles in aqueous media

Article abstract of DOI:10.1039/d1ra03881h

Functionalized graphitic carbon nitride (Sg-C₃N₄) has been manufactured and used as a reusable catalyst for the one-pot production of various spiro-pyrano chromenes and spiro indole-3,1′-naphthalene tetracyclic systems in aqueous med

Full text of DOI:10.1039/d1ra03881h

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Site-specific role of bifunctional graphitic carbon
nitride catalyst for the sustainable synthesis of 3,3-
spirocyclic oxindoles in aqueous media†
Cite this: RSC Adv., 2021, 11, 28452
a
a
a
a
Anshu Dandia,* Dinesh Kumar Mahawar, Pratibha Saini, Surendra Saini,
ab
c
a
Shyam L. Gupta, Kuldeep S. Rathore and Vijay Parewa
*
3 4
Functionalized graphitic carbon nitride (Sg-C N ) has been manufactured and used as a reusable catalyst
0
for the one-pot production of various spiro-pyrano chromenes and spiro indole-3,1 -naphthalene
tetracyclic systems in aqueous media. An ultrasound-assisted method has been used for the
functionalization of g-C
characterized via different analytical tools. The catalytic site-specific role of Sg-C
various control experiments in one-pot reaction sequences. We recognized that Sg-C
3
N
4
. The catalytic functionalities and the structural integrity of the catalyst were
was confirmed via
acts as
3 4
N
3 4
N
a bifunctional acid–base catalyst for the first reaction sequence whereas it is an acidic catalyst for the
second reaction sequence during the one-pot production of various spiro-pyrano chromenes. In
3 4
addition, the bifunctional acid–base catalytic role of Sg-C N has been confirmed for the first reaction
sequence whereas it has a basic catalytic role for the second reaction sequence during the one-pot
0
production of spiro indole-3,1 -naphthalene tetracyclic systems. Diverse C–C, C–O, and C–N bonds,
Received 18th May 2021
Accepted 8th August 2021
six-membered cycles, stereogenic centers, and spiro frameworks were formed in a single reaction,
enhancing the biocidal profile and possibly resulting in the discovery of new medicinal properties. The
mild reaction environment, simple workup, easy separation, low cost, heterogeneity, and recyclability of
DOI: 10.1039/d1ra03881h
rsc.li/rsc-advances
3 4
Sg-C N are some rewards of this approach.
4
biotechnology, advanced electrodes and polymer composites.
It is the chemical genius of carbon that it can create different
1
Introduction
5
Carbon is one of the most multifaceted elements in the periodic nanostructures with entirely different properties. Amongst
table. Carbon-based substances have achieved insightful carbon-based substances, graphitic carbon nitride (g-C ), has
3 4
N
consequences in many areas of science and technology because elicited a huge focus in organic transformation because of its
1
of their noteworthy properties. In previous decades, many robust nature. g-C
N
4
as an abundant and nontoxic substance
3
scientists worked on developing inventive carbon-based can be effortlessly synthesized from carbon constituents, by the
2
6
compounds for wide-ranging applications. Carbon materials one-step polymerization of N-rich chemical compounds. Due
have a splendid ability to provide at the nano-scale because they to their distinctive properties they are potential candidates for
7
have fabulous thermal and electrical conductivity as well as a variety of applications in numerous elds. The signicance of
lightness and elevated mechanical strength that traditional g-C
bulk substances cannot possess. With the multiplicity of their sively recognized for numerous chemical reactions. However,
3
N
4
as a catalyst or catalytic support has also been exten-
3
8
nanostructures, these distinctive values can be attained over an the preparation of bifunctional g-C
enormously broad range of environments. Therefore, they are is still a difficult task for the chemist. Enormous efforts have
exhaustively investigated for innumerable applications in been made to introduce acidic groups into basic g-C . But in
and these methods the inherent reasonable basic groups of g-C
3 4
N with acid and base sites
9
N
3 4
optoelectronics
and
photonics,
nanomedicine
N
3 4
have been destroyed and the resulting material has very low or
a
even no basicity. Therefore, there is a still a need for a mild and
Centre of Advanced Studies, Department of Chemistry, University of Rajasthan,
Jaipur, India. E-mail: dranshudandia@yahoo.co.in; parewavijay@uniraj.ac.in; efficient protocol for the concurrent introduction of acid and
parewavijay.parewa@gmail.com
3 4
base sites in g-C N to make it a bifunctional catalyst.
b
Government Polytechnic College, Near Itarana Fly Over, Kalimori, Alwar, Rajasthan,
Organic compounds with a spiro heterocyclic moiety are
receiving wide scientic interest because of their distinctive
conformational and chemical characteristics as well as the
biological properties that are oen coupled with the asymmetric
3
01001, India
c
Department of Physics, Arya College of Engineering and IT, Jaipur, India. E-mail:
kuldeep_ssr@yahoo.com
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d1ra03881h
10
spiro carbon atom. More specically, the spiro-oxindole
1
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Fig. 1 Examples of spirocyclic oxindoles in natural products and pharmaceutical applications.
11
nucleus is found in a variety of biologically active heterocyclic motifs. For example, a novel class of marine toxins isolated
compounds and natural products and can be of interest in the from dinoagellates and shellsh, like pteriatoxin and pinna-
12
development of innovative clinically signicant heterocyclic toxins, shows that a spiro aza system is responsible for the
Scheme 1 The synthesis of various spiro-oxindole derivatives.
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these structural frameworks form the central units of numerous
naturally occurring compounds with important biological
13
activities.
Several spiro-oxindole derivatives show interesting biological
activities (Fig. 1): for example, as progesterone receptor modu-
14
lators, potent nonpeptide inhibitors of the p53–MDM2 inter-
1
5
16
17
action, and antitubercular, anticancer, anti-HIV, and
18
antimalarial agents. Notably, the spiro-oxindole subunit is
also found in spirotryprostatin A and B, which have been
recognized as innovative inhibitors of microtubule assembly.
19
In addition such spiro-oxindole heterocycles have been reported
to be potent therapeutic agents against malaria. Isopteropodine
and pteropodine have been found to modulate the utility of
20
muscarinic serotonin receptors.
As a result of the extensive variety of the biological relevance
of a 3,3-spirocyclic oxindole ring system with a quaternary
center, enormous endeavors have been made to construct these
bioactive spiro-oxindole derivatives by synthetic chemists.
Numerous solvents and catalysts have been used to achieve this
3 4 3 4
Fig. 2 XRD patterns of g-C N and Sg-C N .
biological activity. Synthetic derivatives of spiro-oxindole ring
systems, for example spirotryprostatin B, elacomine, alstonisine
and horsline, have become imperative synthetic targets as
21
transformation. Recently, Shirini et al. described the synthesis
3 4 3 4
Fig. 3 SEM images of Sg-C N and g-C N .
3 4
Fig. 4 EDAX analysis of g-C N .
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21k
of spiro-oxindoles using an Fe
3
O
4
/g-C
3
N
4
nanocomposite.
oxindole derivatives. By comparable experiments, we have
Albeit with a subtle boost in reaction conditions, somewhere identied the responsible catalytic functionality in this Sg-C N4
3
down the line they still fall short of the required exibility. for the construction of various spiro-oxindole derivatives
During our investigations on the synthesis of spiro derivatives (Scheme 1).
and a detailed literature survey, it was observed that traditional
methods suffer from many disadvantages: a multi-step process
2
Results and discussion
with a tedious workup procedure, prolonged reuxing in
elevated harsh conditions using volatile or corrosive solvents
and reactants with strong acidic/basic catalysts which may lead
to the formation of the target product in lower yields. Further-
more, indigenous overheating can result in substrate, product
or reagent decomposition, with a decrease in the yield of the
required product, the formation of a mixture of products and
a complicated isolation process with further purication using
chromatographic techniques. Furthermore, the major short-
coming of nearly all current methods is that the catalysts are
damaged in the workup process and cannot be recovered or
recycled, reducing the turn over number (TON) or turn over
frequency (TOF). In these environmentally conscious days, for
reasons of economy and pollution, environ-economic green
chemical procedures have to be developed to eliminate pollu-
tion problems, and replacing or reducing corrosive acids or
volatile organic solvents in the reaction medium are among the
most promising ways to reach these goals. Therefore, in an
extension of our attempts towards the enlargement of novel
synthetic methodologies for heterocyclic frameworks and
To achieve the goal of the convenient synthesis of bifunctional
g-C , we have rstly prepared pristine g-C from the easily
available starting material melamine. Bifunctionalization of g-
has been accomplished by a simple ultrasound-assisted
method through the reaction of pristine g-C and aqueous
H SO (30%). The as-prepared nanomaterials were character-
3
N
4
3 4
N
3 4
C N
3 4
N
2
4
ized by XRD, SEM, EDAX, XPS and FT-IR analyses.
The crystal or amorphous structures and phase purity levels
of the prepared g-C
3 4 3 4
N and Sg-C N catalysts were investigated
with X-ray diffraction (XRD) patterns (Fig. 2). There are two
ꢀ
ꢀ
sharp diffraction peaks at 12.4 and 27.4 , corresponding to the
100) and (002) reection of graphitic carbon of synthesized g-
. The diffraction peaks of the XRD pattern of the synthe-
sized Sg-C N catalyst (H SO -treated sample) are similar to
(
3 4
C N
3
4
2
4
those of the prepared catalyst g-C N . Despite the intensity of
3
4
the (002) peak of Sg-C N being higher with a minor change in
3
4
3 4
its 2q value compared to g-C N , this result indicates that the
graphitic-like structures of the prepared catalyst still remained
aer the acid treatment. The extra exposed graphitic-like
22,23
nanoparticle synthesis,
we have developed a bifunctional g-
structural units of Sg-C
the intensity.
3 4
N are responsible for the increase in
C N (by the simultaneous incorporation of –SO H and –NH
3
4
3
2
groups) by the reaction of melamine-derived g-C
3
N
4
and
The micro-structures of g-C N , as revealed in the SEM
3
4
aqueous H under ultrasound irradiation. The bifunctional
2
SO
4
images (Fig. 3) indicate the large aggregation of g-C N units.
3
4
g-C (Sg-C N ) has been characterized by diverse analytical
3
N
4
3 4
The SEM image of the acid-treated sample (Sg-C N ) reveals that
3
4
techniques and used as a metal-free catalyst for the synthesis of
various spiro-oxindole derivatives. Due to the admirable assis-
tance between acid and base sites, the catalyst demonstrates
a very high reactivity and selectivity for the synthesis of spiro-
there are no aggregations present in this sample because they
are shattered into tiny particles with a more exposed and
discrete surface, which is favorable for the strong interaction
between the substance and the active acidic sites. The ordered
3 4
Fig. 5 EDAX analysis of Sg-C N .
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3 4 3 4
Fig. 8 XPS survey scans of g-C N and Sg-C N .
3 4 3 4
Fig. 6 FT-IR spectra of g-C N and Sg-C N .
3 4
C N (Fig. 4 & 5). Aer acid treatment, the value of the carbon–
nitrogen ratio reduces. Additionally, elemental analysis of the
stacking of the CN layers was enhanced aer H
because H SO treatment could exfoliate the g-C
generate more exposed g-C
Elemental analysis of simple graphitic carbon nitride ach-
ieved by EDAX analysis veries the existence of C and N in g-
2
SO
4
treatment
acid-treated sample (Sg-C N ) veries the existence of S N, and C
3
4
2
4
N layers and
3 4
in the acid-treated sample. This proves that S was incorporated
aer the acid treatment. Similar results were also found in the
elemental analysis (CNOS) of the samples (Table S1†).
24
3 4
N units.
3 4 3 4 3 4
Fig. 7 XPS spectra: (a) the N 1s spectrum of Sg-C N , (b) the C 1s spectrum of Sg-C N , (c) the O 1s spectrum of Sg-C N , and the (d) S 2p
spectrum of Sg-C
3 4
N .
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a
Table 1 Optimization of the reaction conditions for the production of spiro-pyrano chromen (4a)
b
c
S. No.
Catalyst
Solvent
Time (min)
Yield (%)
TOF
1
2
3
4
5
6
7
8
9
—
H
H
H
H
H
H
H
H
H
H
H
H
2
2
2
2
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
O
O
O
O
5 h
60
60
60
60
60
60
60
10
10
20
10
30
30
30
30
42
52
59
28
—
14
43
20
34
96
57
96
28
17
39
36
n.c.
I
2
(20 wt%)
0.092
0.105
0.050
—
PTSA (20 wt%)
MWCNTs (20 wt%)
Graphite (20 wt%)
Activated carbon (20 wt%)
GO (20 wt%)
d
0.025
0.076
0.035
0.363
1.024
0.686
0.598
0.099
0.060
0.139
0.128
rGO (20 wt%)
g-C
3
N
4
(20 wt%)
10
11
12
13
14
15
16
Sg-C
Sg-C
Sg-C
Sg-C
Sg-C
Sg-C
Sg-C
3
3
3
3
3
3
3
N
4
(20 wt%)
(10 wt%)
(30 wt%)
(20 wt%)
(20 wt%)
(20 wt%)
(20 wt%)
N
4
N
4
N
4
N
4
N
4
N
4
3
CH CN
EtOH
THF
2 2
CH Cl
a
c
b
All reactions were performed with isatin (2.0 mmol), malononitrile (2.0 mmol), and 4-hydroxycoumarin (2.0 mmol) under reux. Isolated yield.
TOF (ꢁ10 mol g min ). Multi-walled carbon nanotubes.
ꢂ3
ꢂ1
ꢂ1
d
Scheme 2 Various control experiments for the synthesis of spiro-pyrano chromen.
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FT-IR spectra of the prepared carbon nitride materials g-
The occurrence of different surface atoms (chemical envi-
C N and Sg-C N are presented in Fig. 6. The sharp band at ronment) and oxidation states in the prepared g-C N and Sg-
3
4
3
4
3 4
ꢂ1
8
08 cm is assigned to the tris-s-triazine layers of the synthe- C N catalysts were examined using the X-ray photoelectron
sized samples. Broad peaks at around 3040–3380 cm are spectroscopic (XPS) method (Fig. 7, and S3†). The XPS decon-
3 4
ꢂ1
instigated by the N–H vibration modes for –NH
2
groups. An voluted C 1s spectra of the samples show two characteristic
ꢂ1
absorption band at 1635 cm
corresponds to the typical peaks with binding energies of 288.2 and 284.4 eV owing to the
2
stretching modes of CN heterocycles. A range of bands in the occurrence of sp -hybridized C elements in the prepared
ꢂ1
region of 1462–1570 cm can be related to the tris-s-triazine of samples. The three peaks at 397.7, 399.2 and 400.3 eV in the N
ꢂ1
2
the melamine constituent. A signal at 1407 cm is equivalent 1s spectrum of the samples belong to N element (sp -bonded),
to the stretching vibrations of C–N bonds in tertiary N-atoms in tertiary N element and primary N element, respectively. The two
the units of the prepared catalyst. Various bands in the range of peaks in the O 1s spectra of the samples with binding energies
ꢂ1
2
1
209–1314 cm correspond to sp hybridized C–(NH) entities. of 530.6 eV and 528.7 eV correspond to –OH functional groups.
ꢂ1
Aer the acid treatment, a sharp band at 649 cm is attributed On the other hand, a study of the XPS survey spectrum of g-C
3
N
4
to the presence of –SO
The appearance of two strong peaks at 1352 and 1148 cm
3
H functionality in the Sg-C
3
N
4
catalyst. with Sg-C
3
N
4
reveals that intensity of the C 1s spectrum is
ꢂ1
reduced while the intensity of the N 1s and O 1s spectra is
(
because of asymmetric and symmetric stretching vibrations) enhanced. A peak at around 232 eV in both survey spectra may
25
also conrms the occurrence of an –SO
peaks at around 3000–3400 cm are instigated by free amino
3
H group. The broad arise due to the Auger spectra of C KLL.
The XPS survey spectrum in Fig. 8 demonstrates the occur-
ꢂ1
and hydroxyl functionalities. These results conrm that acid- rence of C, N, O atoms in a simple carbon nitride sample,
treated samples initiate several new functional groups (NH & whereas the XPS spectrum of the Sg-C N sample has C, N, S
2
3 4
SO
3
H) in the carbon nitride nanosheet.
and O species without other impurities. XPS analysis of the
elemental S 2p spectrum of Sg-C allocates three peaks with
3 4
N
Scheme 3 The probable mechanistic pathway for the production of spiro-pyrano chromens.
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a
Table 2 Synthesis of spiro-pyrano chromens
a
All reactions were performed with substituted isatins (2.0 mmol), malononitrile/ethyl cyanoacetate (2.0 mmol), and 4-hydroxycoumarin (2.0
in water and were completed in 30 to 45 min.
3 4
mmol) using 20 wt% Sg-C N
binding energies of 164.0 eV (C–S bond), 165.8 eV (–N–S bond) acid treatment generated functionality plays a vital role in the
and 169.1 (–N–S bond). Furthermore, the characteristic peak at reaction.
1
69.1 eV is ascribed to the –S]O group in Sg-C
further veries the existence of –SO H functionalities in the in the reaction, Sg-C
prepared Sg-C N sample. FT-IR and X-ray photoelectron spec- (reuxed with NaOH and NaCl) to acquire base-treated Sg-C N4
3
N
4
, which
To conrm which functionality in Sg-C
3 4
N plays a vital role
3
3 4
N
was employed in basic conditions
3
4
3
troscopy examinations verify the healthy incorporation of SO H (BSg-C N ). The FT-IR intensity of the –SO H group in BSg-C N
3 4
3
3
4
3
and NH functionalities into g-C N aer acid treatment.
was relatively weak because of the deprotonation of the –SO H
3
2
3
4
Aer the identication of the fundamental nature of the group (Fig. S1; see ESI†). The intensity of the FT-IR band due to
2
samples, we assessed the catalytic activity of theses sample the sp -C mode was improved because of the additional room in
towards the synthesis of spiro-oxindoles. Our preliminary the p–p* network caused by the dehydration reaction forced by
attempts concentrated on optimization of the production of the base. The FT-IR band of –NH
spiro-pyrano chromen derivatives. The reaction of isatin 1 (2.0 conditions so this intensity remains unchanged. This BSg-C
mmol), malononitrile 2 (2.0 mmol) and 4-hydroxycoumarin 3 was further converted into BaSg-C N by acidic treatment with
2
groups is stable in basic
3 4
N
3
4
(
2.0 mmol) was selected as a model reaction. Without any 0.1 M HCl. The FT-IR intensity of the –SO H group can be
3
catalyst a lower yield of the product was formed. To enhance the restored because of reprotonation of the –SO H group (Fig. S2;
3
efficacy of the protocol, different metal-free materials were see ESI†). But the intensity of the FT-IR band due to –NH
screened as catalysts. As mentioned in Table 1, the most groups could not be restored due to the neutralization of basic
appealing outcome was obtained with Sg-C as the catalyst in groups by the acid treatment.
terms of turnover frequency (TOF) compared to other catalysts. To investigate the mechanism for the catalytic action of the
It was found that 20 wt% Sg-C in H O is enough to drive the catalyst, some control trials were executed using BSg-C and
reaction forward. Any excess of Sg-C N beyond this amount did BaSg-C N catalysts (Scheme 2). When the model reaction was
2
3 4
N
3
N
4
2
3 4
N
3
4
3 4
not show any further increase in yield. g-C N did not show any tried with BSg-C N , isatylidene malononitrile (X) was isolated
3
3 4
signicant impact on the reaction, which demonstrates that as a major product instead of spiro-pyrano chromen. While
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0
a
Table 3 Synthesis of spiro indole-3,1 -naphthalene tetracyclic systems
a
3 4
All reactions were performed with substituted isatins (2.0 mmol), malononitrile (4.0 mmol), and cyclic ketones (2.0 mmol) using 20 wt% Sg-C N
in water and were completed in 30 to 45 min.
BaSg-C
for the selective synthesis of spiro-pyrano chromen. However an sible for the Knoevenagel reaction are the –SO
excellent yield of spiro-pyrano chromen was observed with Sg- –NH groups and a further attack on 4-hydroxycoumarin was
C N . No side product was formed under these conditions facilitated by the –SO H groups of the Sg-C N catalyst in a one-
3
N
4
conrmed substantial recovery in catalytic activity
Thus, these reaction prole indicates that the sites respon-
3
H group and
2
3
4
3
3 4
(
Scheme 2a).
pot process.
Various solvents (like ethanol, CH CN, THF, DCM and H O)
To further conrm the site-specic role of this bifunctional
3
2
catalyst, we carried out the stepwise synthesis of the desired were also tested on the model reaction. The outcomes suggested
product under control conditions. When the reaction of isatins that the solvents had a remarkable impact on the product yield.
with malononitrile was run with Sg-C
Knoevenagel adducts were produced in excellent yield. While carried out in water. g-C
moderate yields were observed with BSg-C and BaSg-C demonstrates the highest yield of the product among the
catalysts (Scheme 2b). This outcome conrms the vital role of different examined concentrations (10%, 20%, 30% and 40%).
the –SO H group and –NH groups for the formation of an Based on these outcomes, a probable mechanistic pathway
Knoevenagel adduct. Moreover, we prepared the Knoevenagel for the synthesis of spiro-pyrano chromen is presented in
adduct and subjected this adduct with 4-hydroxycoumarin to Scheme 3. Firstly, Sg-C acts as a bifunctional catalyst and
the control conditions. Sg-C was found to be the best catalyst activates isatins and malononitrile for the Knoevenagel reaction
for the synthesis of the desired product. BSg-C gave an and produces isatylidene malononitrile (X). Aerwards, the
inferior yield of the product. Signicant improvement in cata- –SO H group of Sg-C facilitates Michael addition and forms
lytic activity was detected with BaSg-C (Scheme 2c). These intermediate Y. Subsequent cyclization of intermediate Y fol-
3
N
4
, corresponding The best transformation was noticed when the conversion was
3
N
4
treated with 30% aqueous H SO
2
4
3
N
4
3 4
N
3
2
3 4
N
3 4
N
3 4
N
3
3 4
N
3 4
N
experiments illustrate that –SO H groups are the sites in Sg- lowed by tautomerization produces desired product 3a.
3
C N responsible for the above transformation.
3
4
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Scheme 4 Various control experiments for the synthesis of a spiro indole-3,1 -naphthalene tetracyclic system (a–c).
To demonstrate the scope and versatility of this methodology reaction conditions and formed the corresponding functional-
under identical reaction conditions, a library of substituted ized spiro-oxindoles in excellent yield. The reaction also pro-
medicinally relevant spiro-pyrano chromens was produced ceeded prociently under similar experimental conditions with
(
Table 2). Isatins with different electron donating and with- other types of cyclic ketones (Table 3).
drawing groups on the aromatic ring do not inuence the We have also done some control experiments to conrm
reaction prole that much and give the desire product in which catalytic site in Sg-C was responsible for the above
3 4
N
excellent yield. N-substituted isatins also tolerate the reaction conversion (Scheme 4). To accomplish the appropriate condi-
0
conditions and produce the desire product in good to excellent tions to afford the spiro indole-3,1 -naphthalene tetracyclic
yield. The same one-pot method also proceeded efficiently with system, a modal reaction was performed between isatin, malo-
ethyl cyanoacetate in place of malononitrile. No noteworthy nonitrile and cyclohexanone under control conditions using,
deviation in yields of the products was detected in the case of Sg-C N , BSg-C N and BaSg-C N catalysts (Scheme 4a). An
3
4
3
4
3 4
ethyl cyanoacetate. The conversion completes well and the admirable yield of the product was viewed with Sg-C N . No side
3
4
desired compounds are isolated in good yields.
product was produced under these conditions. Selective
0
Encouraged by these results, we broadened the scope of the synthesis of a spiro indole-3,1 -naphthalene tetracyclic system
above approach for the production of other types of pharma- was also observed with BSg-C
3
N
4
, but the yield of the product
0
ceutically imperative spiro indole-3,1 -naphthalene tetracyclic was lower compared to Sg-C
3
N
4. While with BaSg-C
3
N
4
, Knoe-
26
0
systems. According to the literature, synthesis of these venagel adducts were formed instead of a spiro indole-3,1 -
compounds requires the previous production of Knoevenagel naphthalene tetracyclic system. We have already established the
adducts, which involves two extra steps in the synthesis of target bifunctional role of Sg-C N for the synthesis of isatylidene
3
4
27
compounds. We have effectively applied the above optimized malononitrile (Scheme 2b). Analogous results were found for
conditions for the production of these systems by a one-pot the formation of cyclohexanone malononitrile (Scheme 4b).
process using isatins, malononitrile and cyclohexanone in Moreover, we also carried out the reaction of isatylidene malo-
3 4
a 1 : 2 : 1 ratio. Various substituted isatins tolerated the nonitrile and cyclohexanone malononitrile with Sg-C N , BSg-
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0
Scheme 5 A probable mechanistic pathway for the synthesis of a spiro indole-3,1 -naphthalene tetracyclic system.
C
3
N
4
and BaSg-C
the desired product in excellent yield. BSg-C
active catalyst for the above transformation, but the yield of the the CN group forms an imine D. Isomerization of imine D yields
product was lower compared to Sg-C . A substantial loss in the corresponding product 6.
3
N
4
catalysts (Scheme 4c). Sg-C
3 4
N produced the activated double bond of X with further cyclization into
3 4
N
also acts as an anion C. Subsequent intramolecular nucleophilic addition on
3 4
N
catalytic activity was observed when the reaction was tried with
3 4
BaSg-C N . The desired product was not formed and the reac- Table 4 Recyclability experiments with Sg-C N for various spiro-
3
4
oxindole derivatives
tants remained unchanged under these conditions.
Hence, these results illustrate that the sites responsible for
the Knoevenagel reaction are the –SO H group and –NH
3 2
groups, and furthermore the reaction of isatylidene malononi-
trile and cyclohexanone malononitrile was encouraged by the
–
2 3 4
NH groups of the Sg-C N catalyst in a one-pot process.
Based on these outcomes, a probable mechanistic pathway
0
for the production of a spiro indole-3,1 -naphthalene tetracyclic
system is presented in Scheme 5. The –SO H group and –NH
3
2
groups of Sg-C
produce isatylidene malononitrile (X) and cyclohexanone
malononitrile (A). The –NH group of Sg-C activates the
cyclohexanone malononitrile and forms anion B, which attacks
3 4
N facilitate the Knoevenagel reaction and
2
3 4
N
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The high recyclability and the heterogeneous nature of crude product was subjected to purication by recrystallization
the used Sg-C N catalyst were also established (see ESI†). using ethanol.
3
4
The results show that the Sg-C N sheets are heterogenous
3
4
in nature and can be successfully reused for six cycles (Table
4
Conclusions
4
).
3 4
XRD, SEM and FT-IR of the reused catalyst conrmed the In summary, a functionalized graphitic carbon nitride (Sg-C N )
analogous characteristic nature of the catalyst compared to catalyst was prepared using an ultrasound-assisted technique
fresh catalyst. These outcomes suggest that the morphology and and it was employed for the one-pot production of various spiro-
0
structure of Sg-C
reaction.
3
N
4
sheets remain unchanged during the pyrano chromenes and a spiro indole-3,1 -naphthalene tetra-
cyclic system in aqueous media. Characterizations using
different analytical tools showed that bifunctional acidic
(
–SO H) and basic (–NH ) sites were created aer functionali-
3 2
3
Experimental section
zation. Additionally, the bifunctional nature (acidity/basicity) of
A general part including instruments used is provided in the the used catalyst Sg-C was conrmed via numerous control
3 4
N
ESI.†
.1. Synthesis of g-C N
tests in one-pot reaction sequences. The uniqueness of this
study is the simultaneous activation of the rst reaction
sequence by the synergistic participation of the bifunctional
3
3
4
acidic and basic sites present in Sg-C
3 4
N . The acidic sites
ꢀ
ꢀ
ꢂ1
Melamine (5 g) was heated at 550 C (at a rate of 4 C min
)
present in Sg-C N activated the second reaction sequence for
3
4
using a closed crucible with a lid in a muffle furnace for 3 h.
Aer that, the obtained material was ground into a ne powder
using a mortar and pestle and referred to as g-C N .
the one-pot production of various spiro-pyrano chromenes,
whereas the basic sites present in Sg-C N activated the second
3
4
3
4
reaction sequence for the one-pot production of a spiro indole-
0
3
,1 -naphthalene tetracyclic system. Sg-C
3
N
4
showed no loss in
3
.2. Synthesis of Sg-C
The as prepared g-C
ask. And then it was xed to a 12 mm tip diameter probe and and spiro frameworks were designed in a single reaction.
the solution was sonicated at 50% power of the processor and
30 W output in a 4 s pulse mode for 4 h. The resulting mixture
3 4
N
activity or catalytic functionality over several runs. Diverse C–C,
3 4 2 4
N and 30% aqueous H SO
were placed in C–O, and C–N bonds, six-membered cycles, stereogenic centers,

2
Conflicts of interest
was poured into ice-cold water and centrifuged (12 000 rpm for
0 min twice). The obtained solid material was dried under There are no conicts to declare.
vacuum for 24 h and referred to as Sg-C
1
3 4
N .
Acknowledgements
3.3. Synthesis of spiro-pyrano chromene derivatives
Financial assistance from the CSIR, UGC and DST-SERB, New
Delhi is gratefully acknowledged. We are thankful to the
Malaviya National Institute of Technology, Jaipur, SICART,
GUJARAT and Cochin University of Science & Technology, Kochi
for spectral analyses.
A
mixture of isatins (2.0 mmol), ethyl cyanoacetate/
malononitrile (2.0 mmol), and 4-hydroxycoumarin (2.0 mmol)
with 20 wt% Sg-C N in 20 ml of water were mixed. The solution
was heated under reux conditions for an appropriate time. The
progress of the conversion was monitored with TLC. Aer
3
4
3 4
completion of the conversion the Sg-C N and solid product
were ltered from the solution. The precipitate was dissolved in
ethyl acetate and the catalyst was recovered by centrifugation
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