Ce(IV) immobilized on halloysite nanotube–functionalized dendrimer (Ce(IV)–G2): A novel and efficient dendritic catalyst for the synthesis of pyrido[3,2-c]coumarin derivatives

Article abstract of DOI:10.1002/aoc.5948

In this study, a new and stable Ce(IV) immobilized on halloysite nanotube–functionalized dendrimer was designed, synthesized, and characterized using Fourier-transform infrared, elemental analysis, thermogravimetric analysis, field emission scanning electron microscopy, scanning electron microscopy–energy dispersive X-ray spectroscopy, transmission electron microscopy, dynamic light scattering, Brunauer–Emmett–Teller, and inductively coupled plasma optical emission spectroscopy techniques. This catalyst was efficiently used for the one-pot, single-step multicomponent synthesis of pyrido[3,2-c]coumarins from 4-aminocoumarin, aldehydes, and aryl ketones. The efficiency and selectivity of this catalytic system were also evaluated for the synthesis of pyrido[3,2-c]coumarins from terminal/internal alkynes instead of aryl ketones. In this respect, the regioselectivity of the products was successfully assigned by X-ray crystallographic analysis. All these reactions were best performed under solvent-free conditions in the presence of only 0.28 mol% of the catalyst, and such a one-pot multicomponent synthesis of pyrido[3,2-c]coumarins is reported for the first time. It is also worth noting that single-step and short reaction path for the synthesis of a variety of pyrido[3,2-c]coumarins along with excellent reusability of this dendritic catalyst makes this method economically and environmentally attractive.

Full text of DOI:10.1002/aoc.5948

Received: 29 April 2020
DOI: 10.1002/aoc.5948
Revised: 9 July 2020
Accepted: 13 July 2020
F U L L P A P E R
Ce(IV) immobilized on halloysite nanotube–functionalized
dendrimer (Ce(IV)–G2): A novel and efficient dendritic
catalyst for the synthesis of pyrido[3,2-c]
coumarin derivatives
1
1
Tahereh Ataee-Kachouei | Mahboobeh Nasr-Esfahani
|
1
1
1
Iraj Mohammadpoor-Baltork
| Valiollah Mirkhani | Majid Moghadam
|
1
2
Shahram Tangestaninejad
| Behrouz Notash
1
Department of Chemistry, Catalysis
In this study, a new and stable Ce(IV) immobilized on halloysite nanotube–
functionalized dendrimer was designed, synthesized, and characterized using
Fourier-transform infrared, elemental analysis, thermogravimetric analysis,
field emission scanning electron microscopy, scanning electron microscopy–
energy dispersive X-ray spectroscopy, transmission electron microscopy,
dynamic light scattering, Brunauer–Emmett–Teller, and inductively coupled
plasma optical emission spectroscopy techniques. This catalyst was efficiently
used for the one-pot, single-step multicomponent synthesis of pyrido[3,2-c]
coumarins from 4-aminocoumarin, aldehydes, and aryl ketones. The efficiency
and selectivity of this catalytic system were also evaluated for the synthesis of
pyrido[3,2-c]coumarins from terminal/internal alkynes instead of aryl ketones.
In this respect, the regioselectivity of the products was successfully assigned by
X-ray crystallographic analysis. All these reactions were best performed under
solvent-free conditions in the presence of only 0.28 mol% of the catalyst, and
such a one-pot multicomponent synthesis of pyrido[3,2-c]coumarins is
reported for the first time. It is also worth noting that single-step and short
reaction path for the synthesis of a variety of pyrido[3,2-c]coumarins along
with excellent reusability of this dendritic catalyst makes this method
economically and environmentally attractive.
Division, University of Isfahan, Isfahan,
81746-73441, Iran
2
Department of Inorganic Chemistry and
Catalysis, Shahid Beheshti University,
General Campus, Evin, Tehran,
1983963113, Iran
Correspondence
Iraj Mohammadpoor-Baltork, Department
of Chemistry, Catalysis Division,
University of Isfahan, Isfahan
81746-73441, Iran.
Email: imbaltork@sci.ui.ac.ir
Funding information
Research Council of the University
of Isfahan
K E Y W O R D S
Ce(IV)-supported catalyst, dendrimer, HNT-functionalized dendrimer, pyrido[3,2-c]coumarin,
reusable catalyst
1
| INTRODUCTION
been of great interest due to their ability to catalyze a
[
1,2]
wide range of chemical reactions.
Among them,
In recent years, application of transition metal com-
plexes, mainly lanthanides, in the field of catalysis has
Ce(IV) compounds, which are commercially available,
eco-friendly, strong Lewis acids, and one-electron
Appl Organomet Chem. 2020;e5948.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
1 of 17
https://doi.org/10.1002/aoc.5948

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ATAEE-KACHOUEI ET AL.
oxidizing agents, have been used as effective catalysts in
4-aminocoumarin, aldehydes, and aryl ketones or ter-
minal/internal alkynes. Therefore, the development of
an efficient one-pot, multicomponent synthesis of
pyrido[3,2-c]coumarin derivatives is an active ongoing
research area and is still highly desirable.
[3–10]
a variety of organic reactions.
However, because of
the homogeneity of these transition metal catalysts, their
recyclability and reusability are restricted. One way to
overcome this problem, which is important both environ-
mentally and economically, is the use of proper supports
In continuation of our study, relating to the expan-
sion of effective and valuable catalytic systems for use in
[11–15]
or stabilizers to fixate metallic particles.
In the
[
58–61]
past decades, dendrimers, which are well-defined,
hyperbranched, and three-dimensional synthetic macro-
molecules with internal cavities and chemical functional-
important synthetic protocols,
for the first time,
we described the synthesis of pyrido[3,2-c]coumarin
derivatives via a one-pot, single-step, multicomponent
reaction of 4-aminocoumarin, aldehydes, and aryl
ketones or terminal/internal alkynes in the presence of
only catalytic amount of a thermally, air- and moisture-
stable Ce(IV) immobilized on HNT-functionalized den-
drimer under solvent-free conditions (Scheme 1).
[16–18]
ities, have been used in various fields.
Recently,
the application of dendrimers for stabilizing metal
[19]
ions by encapsulation has been established.
In addi-
tion, dendrimer-encapsulated metallic particles (DEMs)
improve the catalytic activities compared to the other
ligand-stabilized particles by increasing the active
[20,21]
sites.
Due to the improvement of the catalytic activ-
ity as well as the simplified recyclability of DEMs,
heterogenization of DEMs by immobilizing on insoluble
2 | EXPERIMENTAL
2.1 | General
[22–25]
supports has gained much interest.
During the past
years, using organic or inorganic nanotubular materials as
support has attracted attention because of their exception-
ally large surface area and specific tube-shaped structure
The chemicals were purchased from Fluka (Switzerland)
and
Merck
(Germany)
chemical
companies.
[26]
with an empty cavity.
Halloysite nanotube (HNT), a
4-Aminocoumarin and cerium(IV) triflate were synthe-
[
62,63]
kind of naturally occurring aluminosilicate nanoclay
sized as mentioned in the literature.
HNTs were
[
27]
(
Al Si O (OH) ꢀ2H O with 1:1 ratio of Al/Si),
has
purchased from Sigma-Aldrich (Germany) chemical com-
pany. According to the supplier, the properties of HNTs
are as follows: diameter = 30–70 nm, length = 1–3 μm,
cation exchange capacity = 8.0 meq/g, and den-
2
2
5
4
2
gained great attention because of its widespread
[28–35]
applications
and advantages such as high mechani-
cal and thermal stability, low price, environmental friend-
liness, biocompatibility, large surface area, resistibility
against organic solvents, and ease of reusability. Owing to
these outstanding properties, using HNTs as support is of
practical importance and highly attractive in chemistry as
3
sity = 2.53 g/cm . Melting points were determined using
a Stuart Scientific SMP2 apparatus (UK). Fourier-
transform infrared (FT-IR) spectra were recorded on a
Jasco 6300 (Japan) instrument in the range of
[36–43]
−1
1
13
well as in industry.
400–4000 cm . H and C NMR (400 and 100 MHz)
Coumarins (2H-chromen-2-ones) and their fused
derivatives are important cores in many natural and
synthetic biologically active molecules with different
spectra were recorded on a Bruker-Avance 400 spectrome-
ter (Germany) using dimethyl sulfoxide (DMSO)-d as a
6
solvent. Elemental analysis was done on a LECO,
CHNS-932 analyzer (USA). Thermogravimetric analysis
(TGA) was carried out on a Mettler TG50 (USA) instru-
ment under argon flow at a uniform heating rate of
[44–47]
applications.
Among coumarin-fused heterocycles,
pyridocoumarins are fundamental parts of natural
compounds with an extensive range of pharmacological
properties such as antibacterial, anticoagulant,
anti-inflammatory, antimicrobial, antitumor, and anti-
ꢁ
−1
ꢁ
20 C min in the range of 30–700 C. The cerium content
of the catalyst was measured using an inductively
coupled plasma optical emission spectrometry, using a
Jarrell-Ash 1100 ICP analyzer (USA). Field emission
scanning electron microscopy (FE-SEM) measurements
and energy dispersive X-ray spectroscopy (EDS) were per-
formed on a Zeiss Sigma VP (Germany). Transmission
electron microscopy (TEM) measurements were carried
out on a Zeiss-EM10C-100kV (Germany). X-ray crystallo-
graphic analysis was performed using an STOE-IPDS 2T
single crystal diffractometer (Germany). Dynamic light
scattering (DLS) measurements were performed using a
Nano Particle Analyzer SZ-100 (Japan) in a distilled
[48–52]
diabetic activities.
Because of the significance
of pyridocoumarins, several synthetic methods of
pyrido[3,2-c]coumarin derivatives have been developed
[49,50,53–57]
over the past years.
Nevertheless, multistep
sequences, low diversity of pyrido[3,2-c]coumarins, long
reaction times, and the use of detrimental organic sol-
vent as well as non-reusable catalyst are limitations of
some of the previously reported methods. Furthermore,
to the best of our knowledge, there have been no
reports on the synthesis of pyrido[3,2-c]coumarin deriv-
atives via the one-pot, multicomponent reaction of

ATAEE-KACHOUEI ET AL.
3 of 17
SCHEME 1 Synthesis of pyrido[3,2-c]coumarins catalyzed by Ce(IV)–G2
water medium. The Brunauer–Emmett–Teller (BET)
measurements were taken by a BELSORP-mini 2 (Japan).
filtration, washed with distilled water and EtOH, and
dried in a vacuum oven at 70 C for 1 h. Then, a mixture of
ꢁ
BMP2@HNT (2 g), diethanolamine (DEA, 20 ml), and
DIPEA (20.8 ml) in 50 ml DMF was stirred under reflux
2
.2 | Preparation of HNT-functionalized
conditions for 24 h under N atmosphere. The synthesized
2
dendrimer (G2)
G2 with periphery OH groups was filtrated and washed
ꢁ
with EtOH and dried in a vacuum oven at 70 C for 1 h.
For the preparation of CP@HNT, HNT was first activated
ꢁ
by heating at 100 C for 2 h. Then, a mixture of acti-
vated HNT (4 g) and 3-chloropropyltrimethoxysilane
2.3 | Preparation of Ce(IV) immobilized
on HNT-functionalized dendrimer
(Ce(IV)–G2)
(
(
CPTS, 8 ml) was exposed to microwave irradiation
MW) (100 C, 25 W) under constant stirring for 1 h.
ꢁ
The mixture was filtered, washed with EtOH, and dried
ꢁ
at 80 C under vacuum to yield CP@HNT. Then, a mix-
A solution of Ce(OTf) (2 mmol) in dry tetrahydrofuran
4
ture of CP@HNT (3 g), K CO3 (4.14 g), and
(THF) (15 ml) was added slowly to a mixture of
HNT-functionalized dendrimer (G2, 1 g) in dry THF
(10 ml), and the mixture was stirred at room temperature
for 24 h. The solid material was filtered, washed with
2
3
,5-bis(bromomethyl)phenol
(BMP,
1
g)
in
ꢁ
dimethylformamide (DMF) (20 ml) was stirred at 80 C
under N2 atmosphere for 24 h. The solid material
ꢁ
(BMP1@HNT) was separated by simple filtration, washed
THF, and then dried in a vacuum oven at 80 C for 1 h to
with distilled water and EtOH, and dried in a vacuum
oven at 70 C for 1 h. Subsequently, a mixture of
synthesize Ce(IV)–G2 catalyst.
ꢁ
BMP1@HNT (3 g), bis(2-bromoethyl)amine hydro-
bromide (BEA, 4 g), diisopropylethylamine (DIPEA,
2.4 | Preparation of Ce(OTf) @HNT
4
5
.2 ml), and dry toluene (40 ml) was stirred under reflux
conditions for 24 h. After cooling, the solid material was
separated by filtration, washed with toluene and EtOH,
and then dried in a vacuum oven at 70 C for 1 h to yield
A solution of Ce (OTf) (1 g, 0.7 mmol) in 10 ml of dry THF
4
was added slowly to a mixture of HNT (0.5 g) in dry THF
(10 ml), and the mixture was stirred at room temperature
for 24 h. The solid material was filtered, washed with THF,
ꢁ
G1. For the synthesis of second generation, G2, a mixture
of G1 (2.6 g), K CO (8.28 g), and BMP (2.9 g) in 40 ml of
ꢁ
and thendried in a vacuum oven at 80 C for 1 h. The cerium
2
3
ꢁ
DMF was stirred at 80 C under N atmosphere for 24 h.
content of Ce(OTf) @HNT, measured by ICP, exhibited a
2
4
The solid material (BMP2@HNT) was separated by
valueof 0.014 mmolpergram of Ce(OTf) @HNT.
4

4
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ATAEE-KACHOUEI ET AL.
2
.5 | General procedure for the synthesis
and the crude product was purified by chromatography
on silica gel (n-hexane/ethyl acetate, 10:1) to afford the
pure product in 70–80% yields.
of pyrido[3,2-c]coumarins in the presence
of Ce(IV)–G2 catalyst
In a screw-cap glass tube with a magnetic stirrer, a mix-
ture of 4-aminocoumarin (1 mmol), aryl ketone or alkyne
3 | RESULTS AND DISCUSSION
(
1 mmol), aldehyde (1 mmol), and Ce(IV)–G2 (80 mg,
ꢁ
containing 0.0028 mmol Ce(IV)) was stirred at 130 C
under solvent-free conditions. The reaction progress was
checked by thin layer chromatography(eluent: n-
hexane/EtOAc, 10:1). At the end of the reaction, the mix-
ture was cooled to room temperature, and EtOAc (15 ml)
was added. The catalyst was separated by filtration and
washed with EtOAc (10 ml). The filtrate was evaporated,
3.1 | Preparation of Ce(IV) immobilized
on HNT-functionalized dendrimer
(Ce(IV)–G2)
So far, by combining different dendrons and functionalized
surfaces, several types of dendrimers such as
[
64,65]
poly(amidoamine) dendrimer,
poly(propyleneimine)
SCHEME 2 Preparation of halloysite nanotube–functionalized dendrimer (G2)

ATAEE-KACHOUEI ET AL.
5 of 17
[
66]
[67,68]
dendrimer,
triazine dendrimer,
poly(propylethe-
moieties. It is noteworthy that the frequency of stretching
bands of inner-surface OH groups of HNT does not alter
upon functionalization, indicating that grafting has
[69,70]
[71,72]
rimine) dendrimer
and carbosilane dendrimer
havebeensynthesized.Inthisstudy,wetriedtodesignanew
second generation of dendrimer using BMP and BEA. The
synthetic pathway of this HNT-functionalized dendrimer
G2)isillustratedinScheme2.
At first, the activated HNT was treated with CPTS
according to the reported procedure,
[
74,75]
occurred on the outer surface of HNT.
The amount of organic moieties of different steps of
functionalization of HNT surface was determined by ele-
mental analysis. First, the CP@HNT was investigated
and revealed only hydrogen and carbon on the surface.
The amount of organic moieties was found to be
0.42 mmol g− of HNT, based on the carbon result of the
elemental analysis. In the next step, the amount of
organic moieties in BMP1@HNT was evaluated to be
8.74 mmol g− based on the carbon result of the elemen-
tal analysis. The higher carbon content in BMP1@HNT
(
[73]
to yield
CP@HNT which is essential for growing dendritic mole-
cules. The reaction between CP@HNT and BMP pro-
duced BMP1@HNT, which upon reaction with BEA
provided the first generation of dendrimer, G1. In the fol-
lowing, G1 was transformed to BMP2@HNT by the reac-
tion with BMP. Finally, BMP2@HNT reacted with DEA
to form HNT-functionalized hydroxyl groups on the
molecular surface (G2) (Scheme 2). The synthesized G2
with OH groups on its periphery was applied for the
immobilization of Ce(IV) via the reaction of G2
with cerium(IV) trifluoromethanesulfonate to produce
Ce(IV)–G2 (Scheme 3).
The stepwise preparation of Ce(IV)–G2 was deter-
mined by FT-IR, TGA, and elemental analysis.
The functionalization of HNT surface was recognized
by FT-IR spectroscopy (Figure 1). In the FT-IR spectrum
of HNT, the characteristic bands were observed at 3694,
1
1
−
1
−1
(8.74 mmol g ) compared to CP@HNT (0.42 mmol g )
is due to the presence of BMP fragment on the surface of
HNT. The nitrogen content of G1, BMP2@HNT, G2, and
Ce(IV)–G2 was also measured by the elemental analysis.
The results were found to be 0.67, 0.49, 1.01, and 0.74%,
respectively. According to these results, the amount of
organic layers coated on HNT was about 0.239, 0.175,
0.072, and 0.053 mmol g− in G1, BMP2@HNT, G2,
and Ce(IV)–G2, respectively. Consequently, the conver-
sion of G1 ! BMP2@HNT, BMP2@HNT ! G2, and
G2 ! Ce(IV)–G2 was achieved with 73.18, 41.17, and
73.33% yields. The elemental analysis results are
presented in Table 1.
1
−
1
3
623, and 910 cm (inner surface –OH groups), 1029
and 466 cm (Si–O–Si), and 531 cm (Al–O–Si).
−1
−1
[37]
In
the spectra of CP@HNT, BMP2@HNT, and G2, the spe-
Figures 2 and 3 show the thermogravimetric and
derivative thermogravimetric (TG/DTG) analysis curves
for HNT, CP@HNT, BMP1@HNT, G1, and Ce(IV)–G2.
The weight loss in each step was studied as a function of
−
1
cific bands at 3050, 2921, 2857, and 1453 cm are related
to the C–H stretching and bending vibrations of aryl and
alkyl groups. Moreover, the bands that appeared at 1595
−1
ꢁ
(
C=C) and 1313 cm (C–O) are good evidence for the
temperature between 30 and 700 C. As shown in
functionalization of HNT with the desired organic
Figure 2, HNT experienced two weight losses, at 103.1
SCHEME 3 Preparation of Ce(IV) immobilized on halloysite nanotube–functionalized dendrimer (Ce(IV)–G2)

6
of 17
ATAEE-KACHOUEI ET AL.
FIGURE 1 Fourier-transform
infrared spectra of halloysite nanotube
(
HNT), CP@HNT, BMP2@HNT,
and G2
ꢁ
and 553.6 C, corresponding to the loss of adsorbed
conditions, the amount of OH groups found in HNT,
which correspond to inner ones, was also obtained (about
2.8 mmol g ). These results were completely comparable
[
76]
water and structural dehydroxylation, respectively.
According to the weight loss at 553.6 C, the quantity of
total hydroxyl groups of HNT was calculated to be
ꢁ
−1
with those obtained by the aforementioned direct deter-
−
1
[78]
6
.3 mmol g . The amount of inner hydroxyl groups in
mination of the amount of hydroxyl groups in HNT.
HNT was determined by their substitution with fluoride
of ammonium fluoride and subsequent titration with
H SO . Based on the obtained results, the amount of
As shown in Figure 3a–d, CP@HNT experienced two
weight losses, one at 162.67 C and one at 502.17 C.
ꢁ
ꢁ
BMP1@HNT experienced two weight losses at 75.25 and
2
4
ꢁ
these hydroxyl groups was found to be about
502.15 C. G1 experienced three weight losses at 66.95,
.8 mmol g− .
1 [77,78]
Consequently, the amount of outer
330.90, and 508.76 C, and Ce(IV)–G2 also experienced
ꢁ
2
ꢁ
hydroxyl groups in HNT was estimated to be about
three weight losses at 49.50, 352.83, and 520.83 C. Weight
−
1
ꢁ
3
.5 mmol g . To further approve this aim, the pristine
losses up to 200 C correspond to the removal of physically
HNT was first functionalized by CPTS (which occurred
adsorbed water molecules, whereas, the weight losses at
300 C and higher are due to the removal of organic moie-
[79]
ꢁ
on the outer surface of HNT)
and then treated with
ammonium fluoride and finally titrated. Under these
ties from the surface. This shows that the catalyst has high
TABLE 1 CHNS results for the preparation of Ce(IV)–G2
Elemental analysis (%)
−
1
Entry
Product
CP@HNT
BMP1@HNT
G1
C
H
N
S
HNT (mmol g
0.42
)
Yield (%)
–
1
2
3
4
5
6
1.53
2.32
2.22
2.32
2.43
2.50
3.19
–
–
11.55
12.21
15.30
15.28
15.48
–
–
8.74
–
0.67
0.49
1.01
0.74
–
0.239
2.73
BMP2@HNT
G2
–
0.175
73.18
41.17
73.33
–
0.072
Ce(IV)–G2
1.43
0.053

ATAEE-KACHOUEI ET AL.
7 of 17
FIGURE 2 Thermogravimetric and
derivative thermogravimetric curves of
halloysite nanotube
thermal stability. Consequently, the total weight losses of
CP@HNT, BMP1@HNT, G1, and Ce(IV)–G2 are 9.43,
HNT and Ce(IV)–G2 were also characterized using
TEM (Figure 5a,b). The tubular morphology of HNT is
also displayed in the TEM image of Ce(IV)–G2, indicat-
ing that the surface morphology of HNT does not alter
extremely in the presence of organic unit (Figure 5b).
The histogram of size distribution verified that the matrix
has an average diameter of about 30–60 nm (Figure 5c).
The cerium content of the catalyst, measured by ICP,
exhibited a value of 0.035 mmol Ce(IV) per gram of
Ce(IV)–G2 catalyst.
2
0.26, 21.17, and 23.30%, respectively.
The morphology of the surface of HNT and Ce(IV)–
G2 was determined by FE-SEM (Figure 4a,b). Both HNT
and Ce(IV)–G2 particles are tubular. The SEM–EDS anal-
ysis for HNT shows only the presence of O, Al, and Si,
whereas the SEM–EDS of Ce(IV)–G2 clearly demon-
strates the presence of C, N, F, O, Al, Si, S, and Ce ele-
ments in the construction of the prepared catalyst
(
Figure 4c,d). These results indicate the successful func-
The average hydrodynamic diameter of HNT and
Ce(IV)–G2 was evaluated using DLS (Figure 6). As
shown, the average hydrodynamic diameter of Ce(IV)–
tionalization of HNT by covalent attachment of designed
dendrimer along with immobilization of cerium(IV)
trifluoromethanesulfonate.
[80]
G2 (426.9 nm) is larger than that of HNT (399.7 nm)
FIGURE 3 Thermogravimetric and derivative thermogravimetric curves of (a) CP@HNT, (b) BMP1@HNT, (c) G1, and (d) Ce(IV)–G2

8
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ATAEE-KACHOUEI ET AL.
FIGURE 4 Field emission scanning electron microscopy images of (a) halloysite nanotube (HNT) and (b) Ce(IV)–G2. Scanning
electron microscopy–energy dispersive X-ray spectroscopy spectra of (c) HNT and (d) Ce(IV)–G2
FIGURE 5 Transmission electron microscopy image of (a) halloysite nanotube and (b) Ce(IV)–G2. (c) Histogram of size distribution
for Ce(IV)–G2

ATAEE-KACHOUEI ET AL.
9 of 17
FIGURE 6 Dynamic light scattering plots of halloysite nanotube and Ce(IV)–G2
2
FIGURE 7 (a) Barrett–Joyner–Halenda plots and (b) N adsorption–desorption isotherms of halloysite nanotube and Ce(IV)–G2
2
−1
3 −1
which indicates that dendrimer had been attached to the
surface of HNT.
(as,BET = 27.788 m g , total pore volume = 0.189 cm g )
are considerably lower than those of pristine HNT (a
s,-
−1
2
−1
3
The surface properties of HNT and Ce(IV)–G2 were
studied by nitrogen adsorption–desorption measure-
= 39.156 m g , total pore volume = 0.214 cm g ).
BET
These results show that dendrimer due to its huge struc-
ture covers the surface of HNT.
ꢁ
[37,82]
ments at −196 C (Figure 7). According to the Interna-
In addition, the
tional Union of Pure and Applied Chemistry
higher amount of pore diameter of Ce(IV)–G2) com-
classification, the isotherms of HNT and Ce(IV)–G2 are
pared to that of HNT can be related to the porous struc-
ture of dendrimer.
[81]
[36]
of type IV with H3 hysteresis loops.
Emmett–Teller (BET) and Barrett–Joyner–Halenda
BJH) methods were used for the calculation of
The Brunauer–
To summarize, the data obtained indicate that the
Ce(IV)–G2 catalyst has been successfully prepared. As far
as we know, the synthesis of Ce(IV) immobilized on
HNT-functionalized dendrimer is reported for the
first time.
(
specific surface areas and pore size distributions, respec-
tively. As shown in Table 2, the specific surface area,
as,BET, and the total pore volume of Ce(IV)–G2
TABLE 2 BET analysis results of HNT and Ce(IV)–G2
s,BET (m g−1)
2
Total pore volume (p/p
= 0.990) (cm g−1)
3
Mean pore diameter (nm)
Sample
HNT
a
0
39.156
27.788
0.214
0.189
21.904
27.143
Ce(IV)–G2

10 of 17
ATAEE-KACHOUEI ET AL.
3
.2 | Synthesis of pyrido[3,2-c]coumarin
Due to the novelty of this method, several reactions were
performed to determine the optimal conditions for the
synthesis of pyrido[3,2-c]coumarins in the presence of
various potential catalysts (entries 2–15). The results sur-
prisingly showed that the prepared dendritic catalyst
(Ce(IV)–G2) was the most effective catalyst for this
reaction, and the respective pyrido[3,2-c]coumarin 4be
was obtained in higher yield than those of pure Ce
derivatives catalyzed by Ce(IV)–G2
Because of the aforesaid disadvantages of the reported
synthetic methodologies of pyrido[3,2-c]coumarin, the
introduction of a one-pot, single-step, multicomponent
reaction for the synthesis of these heterocyclic com-
pounds is of practical importance. Accordingly, we ini-
tially tested the reaction between 4-aminocoumarin (1,
(OTf) , HNT, or even Ce(OTf)4 coated on HNT
4
1
3
mmol), 4-methylacetophenone (2b, 1 mmol), and
-methoxybenzaldehyde (3e, 1 mmol) in the absence of
(Ce(OTf) @HNT). The three variables, solvent, tempera-
4
ture, and the amount of catalyst were investigated
(entries 16–27). The best yield of the product was
obtained using 80 mg Ce(IV)–G2 catalyst (containing
the catalyst (Table 3). No respective product was
observed even at a high temperature (150 C) (entry 1).
ꢁ
[
a]
TABLE 3 Optimization of reaction conditions for the synthesis of 4
ꢁ
c
Entry
Catalyst (mol%)
Solvent (3 ml)
T ( C)
Yield (%)
1
2
3
4
5
6
7
8
9
–
–
150
140
–
5
BiCl
3
(2)
(2)
(2)
–
ZnCl
2
–
140
10
5
AlCl
3
–
140
CeCl
3
ꢀ7H
2
O (2)
–
140
5
Bi(OTf)
3
(2)
–
140
20
25
–
Triflic acid (3 ml)
–
140
Diethanolamine (2)
–
140
c
CAN (2)
–
140
5
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Ce(OTf)
Ce(OTf)
4
4
(0.28)
(2)
–
140
28
40
–
–
140
HNT(80 mg)
Ce(OTf) @HNT (2)
–
140
4
–
140
45
–
G2 (80 mg)
–
140
Ce(IV)–G2 (0.28)
Ce(IV)–G2 (0.28)
Ce(IV)–G2 (0.28)
Ce(IV)–G2 (0.28)
Ce(IV)–G2 (0.28)
Ce(IV)–G2 (0.28)
Ce(IV)–G2 (0.28)
Ce(IV)–G2 (0.28)
Ce(IV)–G2 (0.14)
Ce(IV)–G2 (0.50)
Ce(IV)–G2 (0.28)
Ce(IV)–G2 (0.28)
Ce(IV)–G2 (0.28)
–
140
70
5
DMF
140
Ethylene glycol
140
20
–
H
2
O
Reflux
Reflux
Reflux
Reflux
140
EtOH
CH CN
–
3
–
Toluene
5
DMSO
5
–
–
–
–
–
140
45
70
70
30
5
140
130
120
80
Abbreviations: DMF, dimethylformamide; DMSO, dimethyl sulfoxide; HNT, halloysite nanotube.
a
Reaction of 4-aminocoumarin 1 (1.0 mmol) with 4-methylacetophenone 2b (1.0 mmol) and 3-methoxybenzaldehyde 3e (1.0 mmol) was
investigated under solvent-free conditions for 10 h at different conditions.
b
Isolated yield.
c
Ceric ammonium nitrate.

ATAEE-KACHOUEI ET AL.
11 of 17
ꢁ
0
.0028 mmol Ce(IV), 0.28 mol%) at 130 C under solvent-
conditions. As shown in Scheme 4, acetophenone 2a
reacted effectively with 4-aminocoumarin 1 and different
aromatic aldehydes such as benzaldehyde 3a,
4-chlorobenzaldehyde 3b, 4-isopropylbenzaldehyde 3c,
and even 2-phenylpropanal 3d as an aliphatic aldehyde
free conditions (entry 25).
Then, the scope and generality of this catalytic system
were investigated in the synthesis of a series of
pyrido[3,2-c]coumarins under the optimized reaction
SCHEME 4 Scope of different aldehydes and aryl ketones under optimized reaction conditions
SCHEME 5 Synthesis of
compound 6 using diphenylacetylene
under optimized reaction conditions

12 of 17
ATAEE-KACHOUEI ET AL.
SCHEME 6 Synthesis of compounds
8
and 9 using phenylacetylene and methyl
propiolate under optimized reaction
conditions
FIGURE 8 Molecular structures of 8ja and 9bb
to produce the desired products 4aa–ad in 70–80% yields.
Moreover, the aryl ketones with electron-donating groups
such as 4-methyl and 4-cyclohexylacetophenones (2b and
electron-withdrawing or electron-donating groups to pro-
duce the corresponding pyrido[3,2-c]coumarins in
70–78% yields.
2
c) and electron-withdrawing groups such as haloge-
To further examine the efficiency and selectivity of
this catalytic system, this one-pot, multicomponent reac-
tion was also checked with diphenylacetylene 5a instead
of aryl ketones in the presence of Ce(IV)–G2. Under these
conditions, the desired polysubstituted pyrido[3,2-c]
nated acetophenones (2d–f) and 4-nitroacetophenone 2g,
as well as 2- and 3-acetylpyridines (2h and 2i) as the het-
erocyclic substrates, underwent smooth reaction with
4
-aminocoumarin and aromatic aldehydes bearing either

ATAEE-KACHOUEI ET AL.
13 of 17
SCHEME 7 Proposed mechanism for the synthesis of
8
from phenylacetylene
SCHEME 8 Proposed mechanism for the
synthesis of 9 from methyl propiolate
coumarins (6fa, 6ha, and 6ia) were obtained in 70–75%
yields (Scheme 5).
pyrido[3,2-c]coumarins (8ea, 8ja, and 8ka) were pro-
duced with excellent selectivity and good yields. How-
ever, treatment of methyl propiolate 7b with
4-aminocoumarin and aldehydes produced the reg-
ioisomeric 3,4-disubstituted pyrido[3,2-c]coumarins (9ab,
9bb, and 9fb) as sole product in 70–75% yields. The
observed regioselectivity of these products was further
confirmed using the X-ray crystallographic analysis
Finally, the applicability of this new one-pot, mul-
ticomponent reaction was examined using pheny-
lacetylene 7a and methyl propiolate 7b as terminal
alkynes (Scheme 6). It is worth mentioning that in the
reaction of phenylacetylene 7a with 4-aminocoumarin
and different aldehydes, the respective 2,4-disubstituted

14 of 17
ATAEE-KACHOUEI ET AL.
FIGURE 9 Reusability of Ce(IV)–
G2 and Ce(OTf) @HNT
4
(
1
Figure 8, CCDC of 8ja: 1981325 and CCDC of 9bb:
intermediate E which upon the elimination of water
forms intermediate F. Finally, aerial oxidation of
F forms the desired 2,4-disubstituted pyrido[3,2-c]cou-
marin 8 (Scheme 7).
981326). Disordered acetonitrile solvent molecule has
[83]
been removed by Squeeze.
As reported previously, Ce(IV) triflate can act as an
[6]
efficient Lewis acidic catalyst. Based on this observa-
tion and according to the aforementioned results, reason-
able mechanisms for the selective synthesis of 2,4- and
In the case of 3,4-disubstituted pyrido[3,2-c]coumarin,
the amino group of 4-aminocoumarin 1 initially attacks
the terminal carbon of activated methyl propiolate G by
Ce(IV)–G2 catalyst to yield intermediate H, wherein the
negative charge is stabilized by the adjacent carbonyl
group. Next, intermediate H attacks the activated alde-
hyde I to give J, which upon intramolecular cyclization
along with elimination of water produces K. The Inter-
mediate K is converted to L by hydrogen shift.
Finally, aerial oxidation of L gives the corresponding
3
,4-disubstituted pyrido[3,2-c]coumarins catalyzed by
Ce(IV)–G2 are suggested in Schemes 7 and 8.
First, the reaction between activated aldehyde A and
phenylacetylene 7a synthesizes intermediates B and
then C, which via Meyer–Schuster rearrangement is
converted to intermediate D.
4
[84]
Then, 1,4-addition of
-aminocoumarin 1 to the intermediate D results in
FIGURE 10 Fourier-transform
infrared spectra of fresh Ce(IV)–G2 and
Ce(IV)–G2 after the sixth run

ATAEE-KACHOUEI ET AL.
15 of 17
3
,4-disubstituted pyrido[3,2-c]coumarin 9 (Scheme 8). To
alkynes has also been achieved using this catalytic sys-
tem. As far as we know, this is a new synthetic method
for the preparation of pyrido[3,2-c]coumarins via such a
one-pot, single-step, three-component reaction directly
from 4-aminocoumarin. Furthermore, good yields, avoid-
ance of toxic organic solvents, and simple work-up proce-
dure, as well as the stability and reusability of the
catalyst, are noteworthy features of the present protocol
and make it useful and attractive for the synthesis of
pyrido[3,2-c]coumarins.
further confirm the suggested mechanism, the reaction
was also performed under argon atmosphere, and the
product yield was reduced considerably under these con-
ditions. This result ruled out the anomeric-based oxida-
[85,86]
tion of the intermediate L to the product 9.
As we know, this is a new synthetic method for the
preparation of pyrido[3,2-c]coumarin derivatives directly
from 4-aminocoumarin via such a multicomponent reac-
tion. Therefore, such a synthesis can be anticipated as an
appropriate practical method for the preparation of
pyrido[3,2-c]coumarin derivatives.
ACKNOWLEDGMENTS
We are grateful to the Research Council of the University
of Isfahan for financial support of this work.
1
All products were identified by their FT-IR, H NMR,
13
and C NMR spectra and CHNS elemental analysis
(Supporting Information).
The recycling and reusability of the Ce(IV)–G2
AUTHOR CONTRIBUTIONS
catalyst, which is economically and environmentally
Tahereh Ataee-Kachouei: Investigation; software.
important, was explored in the reaction between
Mahboobeh
Nasr-Esfahani:
Investigation.
Iraj
4
3
-aminocoumarin,
-methoxybenzaldehyde at 130 C under solvent-free con-
4-methylacetophenone,
and
Mohammadpoor-Baltork:
Project
administration;
ꢁ
supervision. Valiollah Mirkhani: Funding acquisition;
methodology. Majid Moghadam: Software. Shahram
Tangestaninejad: Conceptualization; formal analysis.
Behrooz Notash: Formal analysis; methodology.
ditions. At the end of the reaction, the mixture was
cooled to ambient temperature and then ethyl acetate
was added. The catalyst was separated by filtration,
washed with ethyl acetate, dried, and reused in the next
cycle. As shown in Figure 9, the catalyst could be reused
five consecutive times without a significant loss in its
catalytic activity. In addition, the leaching of Ce(IV) ion
from Ce(IV)–G2 catalyst was measured by ICP tech-
nique, and it was found that the leaching grade was
insignificant. Also, no considerable difference was
observed between FT-IR spectra of the fresh and
recycled catalyst after six runs (Figure 10). All these
results signify that the Ce(IV)–G2 catalyst is stable in
the reaction mixture. To show the fundamental role of
dendrimer in stabilizing Ce(IV) ions, the reusability of
FUNDING INFORMATION
We are grateful to the Research Council of the University
of Isfahan for financial support of this work.
ORCID
Iraj Mohammadpoor-Baltork https://orcid.org/0000-
0001-7998-5401
Majid Moghadam https://orcid.org/0000-0001-8984-
1225
Shahram Tangestaninejad https://orcid.org/0000-0001-
5263-9795
Ce(OTf) @HNT was compared with that of Ce(IV)–G2
4
under the same conditions. As shown, the corresponding
product was achieved in 45, 22, and 5% yields from the
first to third runs, demonstrating that substantial
leaching of Ce(IV) ions from the HNT support has
occurred during the reaction.
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[
Abbreviation: HNT, halloysite nanotube.
Abbreviations: BET, Brunauer–Emmett–Teller; HNT,
halloysite nanotube.
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
[
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