Use of an efficient polystyrene-supported cerium catalyst for one-pot multicomponent synthesis of spiro-piperidine derivatives and click reactions in green solvent

Article abstract of DOI:10.1002/aoc.4227

One-pot multicomponent reactions are very demanding in synthetic organic chemistry. Here we report a new polystyrene-supported cerium catalyst (PS-Ce-amtp) obtained via an easy two-step procedure, which was thoroughly characterized using various techniques. PS-Ce-amtp catalyses the environmentally benign one-pot multicomponent synthesis of spiro-piperidine derivatives through the reaction of substituted aniline, cyclic active methylene compound and formaldehyde at room temperature. The catalyst also exhibits excellent catalytic activity in one-pot synthesis of 1,4-disubstituted 1,2,3-triazoles via click reaction between in situ generated azides (derived from anilines and amines) and terminal alkynes. The catalyst can be recovered easily after reaction and reused five times without significant loss in its catalytic activity. The advantageous features of this catalyst are atom economy, operational simplicity, short reaction times, easy handling and high recycling efficiency.

Full text of DOI:10.1002/aoc.4227

Received: 26 September 2017
DOI: 10.1002/aoc.4227
Revised: 15 November 2017
Accepted: 16 November 2017
F U L L P A P E R
Use of an efficient polystyrene‐supported cerium catalyst for
one‐pot multicomponent synthesis of spiro‐piperidine
derivatives and click reactions in green solvent
Paramita Mondal1,2 | Swarbhanu Ghosh1 | Sabuj kanti Das3 | Asim Bhaumik3 |
Debashis Das2 | Sk. Manirul Islam1
1 Department of Chemistry, University of
One‐pot multicomponent reactions are very demanding in synthetic organic
Kalyani, Kalyani, Nadia 741235, India
2 Department of Chemistry, University of
Burdwan, Burdwan 713104WB, India
3 Department of Material Science, Indian
Association for the Cultivation of Science,
Kolkata 700032, India
chemistry. Here we report a new polystyrene‐supported cerium catalyst
(PS‐Ce‐amtp) obtained via an easy two‐step procedure, which was thoroughly
characterized using various techniques. PS‐Ce‐amtp catalyses the environmen-
tally benign one‐pot multicomponent synthesis of spiro‐piperidine derivatives
through the reaction of substituted aniline, cyclic active methylene compound
and formaldehyde at room temperature. The catalyst also exhibits excellent
catalytic activity in one‐pot synthesis of 1,4‐disubstituted 1,2,3‐triazoles via
click reaction between in situ generated azides (derived from anilines and
amines) and terminal alkynes. The catalyst can be recovered easily after
reaction and reused five times without significant loss in its catalytic activity.
The advantageous features of this catalyst are atom economy, operational
simplicity, short reaction times, easy handling and high recycling efficiency.
Correspondence
Asim Bhaumik, Department of Material
Science, Indian Association for the
Cultivation of Science, Kolkata, 700032,
India.
Email: msab@iacs.res.in
Debashis Das, Department of Chemistry,
University of Burdwan, Burdwan, 713104,
WB, India.
Email: ddas100in@yahoo.com
KEYWORDS
Sk. Manirul Islam, Department of
Chemistry, University of Kalyani, Kalyani,
Nadia, 741235, India.
click reaction, eco‐friendly catalysis, heterogeneous catalysis, multicomponent reaction, spiro‐
piperidine
Email: manir65@rediffmail.com
1 | INTRODUCTION
properties, CeCl3⋅7H2O has been intensively studied as
catalyst for a plethora of organic transformations such as
Lanthanide salt‐mediated Lewis acid‐catalysed reactions
have attracted tremendous interest over the years due to
their low toxicity, ease of handling, low cost, stability and
recoverability of reagents from aqueous work‐up.[1–4] In
particular, cerium chloride heptahydrate (CeCl3⋅7H2O)
has emerged as a potentially useful Lewis acid, which
imparts high regio‐ and chemoselectivity in various
organic transformations.[5,6] It is also a cheap, non‐toxic
and water‐tolerant catalyst. Due to its unique catalytic
hydrooxacyclization of unsaturated 3‐hydroxy esters,
Michael addition, dihydroxylation of unreactive olefins
and Julia olefination of cyclopropyl carbinols.[7–10] How-
ever, the main limitation from economic and environmen-
tal points of view is the use in stoichiometric amounts and
involvement in secondary reactions. The use of a core
metal salt as a catalyst is associated with a major problem
of recovery of the catalyst from the reaction medium
making reaction work‐up and product isolation tedious.
To overcome this problem, researchers have made
great efforts to find eco‐friendly reusable heterogeneous
Paramita Mondal and Swarbhanu Ghosh contributed equally.
Appl Organometal Chem. 2018;e4227.
wileyonlinelibrary.com/journal/aoc
Copyright © 2018 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4227

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MONDAL ET AL.
catalysts based on organic, inorganic and organic–inor-
ganic hybrid solid supports for various organic transfor-
mations.[11–16] Polystyrene is one of the most widely
studied heterogeneous and polymeric supports due to its
low cost, ready availability, environmental stability and
hydrophobic nature which protects water‐sensitive Lewis
acids from hydrolysis by atmospheric moisture until
suspended in an appropriate solvent where they can be
used in a chemical reaction.[17–19] Heterogeneous polysty-
rene‐supported acid catalysts have received much atten-
tion because of their non‐toxicity, non‐corrosiveness,
ease of handling, recovery and reuse, thermal stability,
mildness of reaction conditions, high selectivity and
simple work‐up procedure.[20]
and often these reactions need to be performed in the
presence of a relatively large amount of catalyst.
The development of environmentally benign and
efficient catalytic reactions has been attracting growing
interest over the years. Toxic organic solvents remain a
scientific challenge to the chemical industry due to
increasing environmental awareness. It is now widely
accepted that their use is considered to be one of the
major concerns in waste management in fine chemical
industries and they pose serious health as well as environ-
mental hazards. Among various solvent systems, water is
frequently employed in organic transformations because
it is a readily available, safe and environmentally
friendly.[52–57] The use of water and water‐miscible
solvent systems has become one of the most exciting
research endeavours in organic synthesis. Polyethylene
glycol is also used as a green solvent with widespread
industrial and medical applications.[58,59] They are
inexpensive and significantly less hazardous than other
organic solvents. They have good stability in both acidic
and basic media and are suitable reaction media for
various organic reactions.
Azide–alkyne cycloaddition reaction leads to the
production of five‐membered heterocycles containing
nitrogen (1,2,3‐triazoles).[60] Azide–alkyne cycloaddition
reaction products have found much utility as
photostabilizers, pharmaceutical agents, anti‐HIV
agents[61] and anti‐allergic products,[62] in biological
screening[63] and as antibacterial agents,[64] corrosion
inhibitors, photographic materials and dyes. Cu(I/II)‐
catalysed 1,3‐dipolar azide–alkyne cycloaddition is well
known, which exhibits strong regioselectivity under mild
reaction conditions in water as solvent and using little
amount of catalyst and in the absence of any reducing
agents or base.[65] The idea of ‘click’ chemistry was
proposed by Sharpless and co‐workers in 2001.[66] After
the discovery of click chemistry, azide chemistry has
been explored because reactions between alkynes and
azides result in the regioselective production of
1,2,3‐triazoles.[67] The Huisgen 1,3‐dipolar cycloaddition
reaction is considered as the most demanding click reac-
tion. Additionally, azides are very harmful to use freely
due to their toxic nature. These azides can be obtained
from their related anilines or amines by diazotization
followed by the displacement reaction between sodium
azide and alkyl halide or by sodium azide addition. Hence
a single‐step azide–alkyne cycloaddition reaction between
an alkyne and in situ produced azide from its related
precursor is very attractive.
Multicomponent reactions have received substantial
consideration in organic synthesis due to their many
advantages over conventional multistep synthesis.[21–29]
Multicomponent reactions allow the creation of several
bonds in a single operation and offer remarkable advan-
tages like convergence, operational simplicity, facile
automation, reduction in the number of work‐up steps,
and easy extraction and purification processes, and hence
minimize waste generation, rendering such transforma-
tions green. One of the important reactions in the field of
multicomponent synthesis is the formation of spiro‐
substituted piperidines.[30–34] Piperidine is a very common
organic compound, being best known as a representative
structural element within many pharmaceuticals, alka-
loids, eudistomidines H and I,[35] tetraponerines[36]
verbametrine[37] and verbamethine.[38] The piperidine
structural motif is also present in numerous natural
products such as piperine (present in black pepper), fire
ant toxin solenopsin, the nicotine analogue anabasine of
tree tobacco (Nicotiana glauca), lobeline of Indian tobacco
and the toxic alkaloid coniine from poison hemlock.
Various N‐substituted piperidines are synthetic intermedi-
ates for spermidine‐nitroimidazole drugs for the treatment
of A549 lung carcinoma.[39] Due to significant biological
activity and miscellaneous interesting physiological
activities, spiro‐substituted piperidines have received a
great deal of attention in recent years.[40–44] The spiro‐
piperidine nucleus is frequently found in several plant
alkaloids and animal toxins and widely used in the treat-
ment of cocaine abuse, epileptic disorder and depression,
in the prevention of various inflammatory diseases,
immune diseases such as autoimmune diseases or allergic
diseases, and HIV infection, and in 5‐HT2B receptor
antagonists.[40,41,45] Despite their great importance, to date
there are only a few reports in the literature documenting
the synthesis of these spiro‐substituted compounds.[46–51]
Although all these methods provide good yields, some of
the reactions are conducted with a homogeneous catalyst,
requiring high temperature and long time for completion,
In continuation of our interest in developing greener
compatible processes,[68–72] in the work reported
herein, we explored a novel and highly efficient synthetic
one‐pot reaction methodology for the synthesis of

MONDAL ET AL.
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spiro‐substituted piperidine compounds in three different
green reaction media and at room temperature in the pres-
ence of a highly active polystyrene‐supported cerium cata-
lyst (PS‐Ce‐amtp). We also investigated a single‐step
method for the generation of five‐membered 1,4‐disubsti-
tuted heterocyclic 1,2,3‐triazoles by the reaction between
alkynes and in situ produced organic azides from related
aromatic amines or anilines at room temperature catalysed
by PS‐Ce‐amtp. The catalyst is not only readily prepared
from cheap reagents, but also stable in air and can be easily
recovered from a reaction mixture by simple filtration.
products were obtained by further evaporation of this
organic layer under reduced pressure. The catalyst in the
remaining aqueous phase was filtered, washed with
isopropanol–ethyl acetate (1:10) mixture, dried and
reused for further catalytic cycles. Pure compounds were
obtained by further recrystallization of crude products.
2.3 | General Procedure for Synthesis of
1,4‐Disubstituted 1,2,3‐Triazoles
In a 25 ml round‐bottom flask, aromatic amine (1 mmol)
was poured and kept in an ice bath at 0–5 °C. A
solution of conc. H2O–HCl (1:1, v/v) was poured in
slowly and stirred for 5 min. Then sodium nitrite
(1 mmol) was added to 1 ml of chilled water and poured
dropwise into the solution. After mixing, NaN3
(1.2 mmol) was poured into the solution and kept under
magnetic stirring for 10 min. Then alkyne (1.2 mmol)
and PS‐Ce‐amtp (20 mg) were added to the reaction
mixture with continual stirring for 3 h at room temper-
ature. The PS‐Ce‐amtp catalyst was separated by filtra-
tion and cleaned with water, ethanol and acetone.
Extraction of other parts of the reaction mixture was
performed with ethyl acetate. The aqueous part present
in the mixture of the reaction was separated using
Na2SO4 and the reaction mixture was added to a small
amount of ethanol. After that, the separation of pure
product was performed by crystallization to afford the
resultant 1,4‐disubstituted 1,2,3‐triazoles.
2 | EXPERIMENTAL
2.1 | Materials and Methods
Chloromethylated polystyrene (PS‐Cl; 5.5 mmol Cl per
gram of resin) was purchased from Sigma‐Aldrich.
Starting materials were commercially available or were
synthesized by the reported procedures. All solvents and
reagents were obtained from commercial sources and
were used without further purification unless otherwise
stated.
The Fourier transform infrared (FT‐IR) spectra of
samples were recorded from 400 to 4000 cm−1 with a
PerkinElmer FT‐IR 783 spectrophotometer. A Mettler
Toledo TGA/SDTA 851 instrument was used for thermo-
gravimetric analysis (TGA). The morphology of the func-
tionalized polystyrene and complex was analysed using a
scanning electron microscopy (SEM) instrument (Zeiss
EVO40, UK) equipped with an energy‐dispersive X‐ray
(EDX) analysis facility. Diffuse reflectance UV–visible
spectra were obtained using a Shimadzu UV‐2401PC
double‐beam spectrophotometer having an integrating
sphere attachment for solid samples. The Ce content in
the catalyst was estimated using atomic absorption
spectrophotometry (AAS; Varian AA240). NMR spectra
were obtained with a Bruker AMX‐400 NMR spectropho-
2.4 | Synthesis of Catalyst
2.4.1 | Synthesis of polymer‐supported
ligand
PS‐Cl beads (1) were functionalized with aldehyde group
according to a literature procedure.[73] To a suspension
of the aldehyde‐substituted polystyrene beads (2; 5.0 g)
in methanol (20 ml), 2‐aminothiophenol (2.0 g) was
added, followed by stirring and refluxing for 20 h. After
filtration and washing with absolute methanol, the poly-
meric ligand (3; PS‐amtp) was obtained.
1
tometer (400 MHz for H NMR) using tetramethylsilane
as internal standard.
2.2 | General Procedure for Synthesis of
Spiro‐substituted Piperidines
2.4.2 | Synthesis of PS‐Ce‐amtp catalyst
A mixture of aromatic amine (2 mmol), active cyclic
methylene compound (4 mmol), formaldehyde (6 mmol,
37–41% aqueous solution) and catalyst (30 mg) in the sol-
vent (15 ml) was stirred at room temperature for the
required period of time. The appearance of a solid com-
pound denoted the formation of products. After comple-
tion of the reaction (monitored by TLC), the reaction
mixture was allowed to cool and extracted with a mixture
of isopropyl alcohol and ethyl acetate (1:10). Solid
An amount of 1 g of CeCl3⋅7H2O was added to a
suspension of PS‐amtp (2 g) in 25 ml of water with
continuous stirring. The pH of the solution was adjusted
to ca 9.0 using 25% ammonia solution. The mixture was
then continuously stirred overnight at room temperature
and the resulting catalyst (4) was separated by filtration,
washed with water and dried under vacuum at 60 °C
(Scheme 1).

4 of 15
MONDAL ET AL.
SCHEME 1 Synthesis of PS‐Ce‐amtp
catalyst
expected, the pure polystyrene bead had a smooth and flat
surface, while the anchored complex showed roughening
of the top layer.
3 | RESULTS AND DISCUSSION
3.1 | Characterization of Catalyst
High‐resolution TEM images of the PS‐Ce‐amtp mate-
rial are shown in Figure 2 to understand the morphology
and nanostructure of the polymer‐supported Ce catalyst.
As seen from these images, the Ce Schiff base complex
The outline for the preparation of the polymer‐supported
cerium Schiff base complex, PS‐Ce‐amtp, is given in
Scheme 1. Due to the insolubility of the complex in all
common organic solvents, the characterization was
limited to physicochemical properties, chemical analysis,
SEM, transmission electron microscopy (TEM), TGA,
and FT‐IR and UV–visible spectral data.
SEM images of PS‐Cl and the PS‐Ce‐amtp complex
were recorded to understand the morphological changes
occurring on the surface of the polymer matrix. In
Figure 1 the SEM images of PS‐Cl and the metal complex
immobilized on modified polystyrene are shown. As
FIGURE 1 Field emission SEM images of (A) PS‐Cl and (B) PS‐
Ce‐amtp catalyst
FIGURE 2 High‐resolution TEM images of PS‐Ce‐amtp catalyst
at (a) 100 nm and (b) 20 nm length scales

MONDAL ET AL.
5 of 15
particles of size 8–10 nm are dispersed onto the polymer
matrix throughout the specimens. Further, EDX analysis
also confirmed the presence of Ce metal on the polymer
matrix (Figure 3). From the EDX analysis it is seen that,
in the metal complex, there is no chlorine atom. Since
the immobilization of Ce(III) was performed in basic
medium, the active metal species was Ce(III) in the form
of Ce(OH)3. AAS analysis suggested a loading of 4.65 wt%
Ce in the PS‐Ce‐amtp material.
The mode of attachment of 2‐aminothiophenol and
metal onto the support was confirmed from FT‐IR
spectral bands (Figure 4). The sharp C―Cl peak (due to
CH2Cl groups) at 1264 cm−1[76] in the spectrum of starting
polymer material practically disappeared after introduc-
ing ―CHO group in the polymer and a new peak was
observed at 1702 cm−1 which indicated the C―O bond
stretching vibration of the ―CHO group. After ligand
formation, there was a new peak at 1634 cm−1, which
indicated the formation of C―N bond in Schiff base
moiety.[74] This band showed a decrease in intensity and
shifted to lower wavenumber after complexation with
metal. The thiophenolic (C―S) stretching frequency was
FIGURE 4 FT‐IR spectra of (A) (P)‐CHO, (B) PS‐amtp and (C)
PS‐Ce‐amtp catalyst
observed in the region of 1287 cm−1 (of the ligand), which
shifted to the lower frequency at 1268 cm−1 for the
complex, indicating coordination of metal through the
phenolic sulfur.[74,75] In the spectrum of the complex,
bands at 281 and 554 cm−1 were assigned to νCe―S and
νCe―N stretching frequencies, respectively.
The thermal stability of the PS‐amtp ligand and PS‐
Ce‐amtp material was investigated using TGA at a heating
rate of 10 °C min−1 in nitrogen atmosphere over a temper-
ature range of 30–600 °C. It is seen that the thermal
stability of the metal complex was slightly greater than
that of the Schiff base ligand (Figure 5). The slight weight
loss of the complex on heating below 200 °C might be
attributed to the release of physically adsorbed water.
The complex was stable up to 410 °C and further weight
loss at a higher temperature (above 410 °C) was attributed
to the decomposition of the complex. TGA suggested that
the PS‐Ce‐amtp material degraded at considerably high
temperature. The electronic spectrum of the PS‐Ce‐amtp
material was recorded in diffuse reflectance spectrum
mode using BaSO4 disc as reference, as shown in
Figure 6. The PS‐Ce‐amtp material showed a strong
absorption at 350 nm, which could be attributed due to
the presence of Ce(III).
3.2 | Catalytic Activity
3.2.1 | Activity of PS‐Ce‐amtp catalyst in
synthesis of spiro‐piperidine derivative
To explore appropriate reaction conditions is essential for
FIGURE 3 EDX spectra of (A) PS‐amtp ligand and (B) PS‐Ce‐
amtp catalyst
the targeted synthesis. Initially, the three‐component

6 of 15
MONDAL ET AL.
FIGURE 5 TGA weight loss plots for PS‐amtp ligand and PS‐Ce‐amtp catalyst
product was formed, which could not be isolated. In
search of a more efficient catalyst, a series of experi-
ments was performed with the model reaction and the
results are presented in Table 1. In the presence of
strong acid like HCl, only a trace amount of the product
was detected. This could be due to decomposition of the
product in the presence of strong acid. Next, we carried
out the reaction with various Lewis acids including
different metal salts to compare the catalytic activity.
Among the various homogeneous Lewis acids, it was
found that CeCl3⋅7H2O was the most effective, and
71% conversion was obtained within 3 h at room
temperature. Other homogeneous Lewis acids such as
FeCl3, CuCl2⋅2H2O and AlCl3 gave low to moderate
conversions. Compared to these homogeneous catalysts,
heterogeneous acid catalysts afforded better isolated
yields. Among them, PS‐Ce‐amtp showed excellent
FIGURE 6 UV–visible spectrum of PS‐Ce‐amtp catalyst
TABLE 1 Effect of various catalysts on model reactiona
condensation reaction of aniline (1), dimedone (2) and
formaldehyde (3) was taken as a model reaction to estab-
lish the feasibility of the strategy and for optimizing the
reaction conditions (Scheme 2). Here the maximum yield
of the spiro‐piperidine derivative was obtained when
dimedone, aromatic amine and formaldehyde were taken
in a ratio of 2:1:3.
Entry
Catalyst
Time
Yield (%)b
1
HCl
3 h
8
2
FeCl3
6 h
52
3
CuCl2⋅2H2O
AlCl3
6 h
38
4
6 h
67
The reaction did not proceed smoothly in the
absence of the catalyst and only a trace amount of the
5
CeCl3⋅7H2O
PS‐Cl
3 h
71
6
24 h
3 h
No product
7
PS‐Al‐amtp
PS‐Ce‐amtp
Silica‐Ce‐amtp
No catalyst
72
8
25 min
25 min
24 h
88
79
9
10
No product
aReaction conditions: aniline (2 mmol), dimedone (4 mmol), formaldehyde
(6 mmol), in water (15 ml) at room temperature. Reaction progress was mon-
itored by TLC.
SCHEME 2 Synthesis of spiro‐piperidine derivative
bIsolated yield.

MONDAL ET AL.
7 of 15
TABLE 2 Effect of various solvents on model reactiona
catalytic activity under the present reaction conditions.
The reaction did not proceed at all in the presence of
the parent polymer, crosslinked PS‐Cl, revealing that
the support had almost no activity in the absence of
Ce and Ce play the crucial role in this catalytic reaction.
Also, PS‐Ce‐amtp was found to be a more effective cat-
alyst than silica‐Ce‐amtp in terms of reaction time and
yield of product under identical conditions. To deter-
mine the minimum optimum concentration of the PS‐
Ce‐amtp catalyst, the model reaction was carried out
with various amounts of catalyst and the results are pre-
sented in Figure 7. It is observed that the use of just
30 mg of PS‐Ce‐amtp is sufficient for the completion of
the reaction in 25 min with 88% yield of the correspond-
ing product in water medium at room temperature.
With less than 30 mg of the catalyst, the reaction was
incomplete and resulted in a low yield of product. On
the other hand, an excessive amount of catalyst did
not increase the yield significantly. Further studies
showed that the reaction temperature has no such influ-
ence on the model reaction. On increasing the reaction
temperature, no improvement in the yield was observed.
The choice of an appropriate reaction medium is of
crucial importance for successful synthesis, so our next
experiment focused on the selection of solvent. The
results are summarized in Table 2. We screened various
organic solvents such as dichloromethane (DCM), meth-
anol, ethanol, acetonitrile and tetrahydrofuran (THF).
All solvents gave low to moderate yield of product and
the reaction required a long time for completion. Our
literature investigation at this stage revealed that there
are only very few reports on the synthesis of spiro‐piper-
idine using a green reaction medium at room tempera-
ture. In most cases, the reaction time was long and
moderate yield was obtained. Our main objective was
to explore green synthesis conditions that can give
Entry
Solvent
Time
Yield (%)b
1
DCM
2 h
65
2
Methanol
Ethanol
2 h
60
63
34
28
97
80
38
92
88
3
2 h
4
Acetonitrile
THF
5 h
5
5 h
6
PEG‐200
PEG‐400
PEG‐600
Water–ethanol (1:1)
Water
20 min
20 min
20 min
20 min
25 min
7
8
9
10
aReaction conditions: aniline (2 mmol), dimedone (4 mmol), formaldehyde
(6 mmol), PS‐Ce‐amtp catalyst (30 mg) at room temperature. Reaction prog-
ress was monitored by TLC.
bIsolated yield.
higher yield of products in shorter time than the con-
ventional reaction. To reduce the employment of ecolog-
ically suspect solvents we chose three different green
solvents: polyethylene glycol (PEG), water–ethanol (1:1)
and water. PEG is recognized as an attractive medium
for many organic reactions. First, we conducted our
model reaction in PEG. Because of the lower viscosity
of PEG‐200 than other PEGs (PEG‐400 and PEG‐600),
it gave excellent product yields within 20 min. Next,
we performed our model reaction in water–ethanol
medium. The piperidine derivative was formed in a
fairly high yield of 92% in 20 min. In our last set of
experiments we used pure water as the solvent. To our
delight, the results obtained were very satisfactory as
the product in good yield (88%) was obtained in a short
time period (25 min).
To further probe the potential of our methodology, the
scope of the reaction was extended to a wider range of
substituted aromatic amines using the PS‐Ce‐amtp
catalyst under optimized reaction conditions (Table 3).
The product yields were not significantly affected by the
electronic nature of the substituent on the aromatic ring
residue in the amine. Amines with both electron‐donating
and electron‐withdrawing groups react efficiently to give
spiro‐piperidine adducts in good yield. The reaction of
sterically hindered arylamine also proceeded smoothly
affording the desired product in good yield. In addition,
we also explored the reactivity of indane‐1,3‐dione (2a)
for this transformation. From the results, it is clear that
2a also showed excellent reactivity under the present
reaction conditions.
A possible reaction mechanism for the one‐pot forma-
tion of the spiro‐piperidine derivatives is shown in
Scheme 3 on the basis of reported literature. The spiro‐
FIGURE 7 Effect of amount of PS‐Ce‐amtp catalyst on cyclo‐
condensation reaction

8 of 15
MONDAL ET AL.
TABLE 3 Synthesis of spiro‐piperidine derivatives with various active methylene compounds and aromatic aminesa
Entry
Aniline
Active methylene
Product
Time (min)
Yield (%)b
1
20
20
25
PEG: 97
WE: 93
W: 86
2
3
30
30
30
PEG: 92
WE: 84
W: 82
35
35
35
PEG: 91
WE: 80
W: 78
4
5
35
35
40
PEG: 88
WE: 86
W: 80
20
25
25
PEG: 96
WE: 91
W: 86
6
7
40
40
40
PEG: 84
WE: 77
W: 70
20
20
25
PEG: 97
WE: 92
W: 88
(Continues)

MONDAL ET AL.
9 of 15
TABLE 3 (Continued)
Entry
Aniline
Active methylene
Product
Time (min)
Yield (%)b
8
35
35
35
PEG: 95
WE: 89
W: 85
9
25
25
25
PEG: 96
WE: 92
W: 87
aReaction conditions: aniline (2 mmol), dimedone (4 mmol), formaldehyde (6 mmol), in water (15 ml) at room temperature. Reaction progress was monitored by
TLC. Products were characterized by NMR spectral data. (PEG, polyethylene glycol; WE, water–ethanol (1:1); W, water).
bIsolated yield.
3.2.2 | Activity of PS‐Ce‐amtp catalyst for
azide–alkyne one‐pot click reaction
To explore the azide–alkyne one‐pot click chemistry, we
studied catalytic, high‐yielding, environmentally benign
and cheap reagents for click reaction and considered PS‐
Ce‐amtp as a preference. When organic azide (produced
from aromatic amine by the method of diazotization) and
alkyne were reacted in the presence of PS‐Ce‐amtp in
aqueous medium at room temperature, a five‐membered
heterocyclic 1,4‐disubstituted 1,2,3‐triazole was produced
(Scheme 4).
As water is abundant in nature and is an environ-
ment‐friendly and economical solvent, we initially tried
to carry out the click reaction in water medium for
the reaction between phenyl azide generated in situ
from aniline and phenylacetylene using 20 mg of PS‐
Ce‐amtp catalyst at room temperature. The reaction
proceeded well resulting in the production of the
SCHEME 3 Proposed reaction mechanism of spiro‐substituted
piperidine ring formation
corresponding five‐membered heterocycle with 98%
yield. We also performed this click reaction using
CeCl3⋅7H2O and PS‐Cl as catalyst. PS‐Ce‐amtp exhibited
better activity than CeCl3⋅7H2O (78% yield obtained with
the latter). PS‐Cl did not exhibit any activity for this click
reaction. Syntheses of spiro‐substituted piperidines and
1,4‐disubstituted 1,2,3‐triazoles are metal‐catalysed
reactions. When PS‐Cl, which is used as heterogeneous
support, was used as catalyst, we could not observe any
catalytic activity (Table 1, entry 6). Further, when homoge-
neous Ce salt was used as catalyst (Table 1, entry 5), con-
version was much lower after longer reaction time. The
immobilized polystyrene‐supported cerium complex
cyclization is proposed to proceed as a domino sequence
of Knoevenagel, Michael and double Mannich reactions.
First, the immobilized Ce catalyst activates both methy-
lene compound and formaldehyde which leads to the for-
mation of the standard active methylene–formaldehyde
adduct (2) through Knoevenagel condensation followed
by Michael addition. This undergoes two consecutive
Mannich reactions with aniline to form the final
spiro‐piperidine product (4), freeing the catalyst for the
subsequent cycle.

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MONDAL ET AL.
SCHEME 4 Production of 1,4‐
disubstituted 1,2,3‐triazoles from aromatic
amines by two‐step one‐pot click reaction
at room temperature in green solvent
catalysed by PS‐Ce‐amtp
catalyst was found to be more active than the homoge-
neous metal salt. This could be attributed to the
confinement effect where the immobilization of active
sites over the support played crucial role in facilitating
the reactions.
entries 1–6), although ortho‐substituted anilines required
longer reaction time than the others. Substituted p‐
methoxyphenylacetylene was also employed to react with
diversely substituted azides formed in situ from amines to
form the corresponding 1,2,3‐triazoles by this procedure.
A good yield was also obtained with anilines having
electron‐donating and electron‐withdrawing substituents
(Table 5, entries 7–10).
The proposed mechanism for the PS‐Ce‐amtp‐
catalysed click reaction is shown in Scheme 5. PS‐Ce‐
amtp generates μ‐coordinated aggregate with the
terminal alkyne, and subsequently by substitution of
the terminal proton of the corresponding alkyne
provides cerium acetylides (II). After that, the aromatic
organic azide generates linkage with the terminal Ce
composite, producing component III. Because of the
nucleophilic nature of terminal nitrogen of the organic
azide, a rearrangement generates metallacycles (IV),
which first supports the C‐4 position to attack. Subse-
quently, at carbon of C‐5, the attack of unshared pairs
of electrons on the nitrogen of N‐1 generates in a novel
metallacycle to provide component IV. Subsequently, in
the last step, PS‐Ce‐amtp can depart from species V
producing the 1,2,3‐triazole.
To optimize the reaction conditions, a series of exper-
iments altering catalyst amount, temperature and time for
the click reaction were performed in aqueous medium.
The experimental data are listed in Table 4. The best con-
version was found utilizing 20 mg of PS‐Ce‐amtp at room
temperature for 3 h (Table 4, entry 2). No yield was found
in the absence of PS‐Ce‐amtp catalyst (Table 4, entry 7).
To examine the scope and limitation of the click reac-
tion, a series of organic anilines and terminal alkynes
with structurally diverse functional groups were studied
and the results are presented in Table 5. In general, the
method is extremely high yielding and clean. A series of
various aromatic amines was investigated in the reaction
with phenylacetylene. The results showed that aryl and
benzyl azides formed in situ from substituted anilines
and benzylamines reacted rapidly with phenylacetylene
and the resultant five‐membered heterocycles were
obtained in elevated yield at room temperature within
3–4 h. The catalytic system was found to be very efficient
regardless of the presence of electron‐withdrawing (Cl, I,
2‐NO2) or electron‐donating (4‐OCH3, 3‐OH) substituents
in the aromatic ring. All the para‐, meta‐ and ortho‐
substituted aromatic amines reacted almost equally with
the alkyne and gave comparable product yields (Table 5,
To understand the efficiency of our catalytic system
we compared the PS‐Ce‐amtp catalyst with some reported
heterogeneous catalysts from the literature for the
cyclo‐condensation reaction[48,76] and one‐pot click
reaction.[77–80] In Table 6 is presented a comparison of
TABLE 4 Factors affecting the ‘click’ chemistry catalysed by PS‐Ce‐amtpa
Entry
Catalyst amount (mg)
Time (h)
Temperature (°C)
Yield (%)b
1
20
4
70
98
2
3
4
5
6
7
20
20
15
25
25
0
3
2
3
3
2
3
RT
RT
RT
RT
RT
RT
98
96
95
98
96
No Yield
aReaction conditions: phenylacetylene (1.2 mmol), aniline (1 mmol), water (10 ml).
bIsolated yield.

MONDAL ET AL.
11 of 15
TABLE 5 Click reaction between various kinds of aromatic amine derivatives and phenylacetylene catalysed by PS‐Ce‐amtpa
Entry
R
R1
Product
Time
Yield (%)b
1
2‐NO2
H
4
94
2
3
4
5
3‐OH
3‐Cl
2‐I
H
H
H
H
3
3
4
3
94
93
90
98
H
6
7
4‐OMe
2‐I
H
3
4
98
89
4‐OMe
8
3‐OH
3‐Cl
4‐OMe
4‐OMe
4‐OMe
3
3
4
92
92
92
9
10
2‐NO2
aReaction conditions: phenylacetylene (1.2 mmol), aromatic amine (1 mmol), water (10 ml), 20 mg PS‐Ce‐amtp at room temperature. Products were character-
ized by NMR spectral data.
bIsolated yield.
catalytic data for our PS‐Ce‐amtp catalytic system with
those published in the literature. As is evident from
Table 6, the PS‐Ce‐amtp catalyst showed higher catalytic
activity compared to previously published data. Reactions
were carried out at room temperature, reaction times of
minutes were necessary and, most significantly, these
reactions were carried out in aqueous medium with the
PS‐Ce‐amtp catalyst.

12 of 15
MONDAL ET AL.
dimedone, formaldehyde and aniline and the one‐pot
click reaction of aniline with phenylacetylene under the
optimized reaction conditions. After the first run, the cat-
alyst was recovered by extracting the mixture with (1:10)
isopropyl alcohol–ethyl acetate followed by filtration.
The catalyst then washed with ethyl acetate under
reduced pressure and dried. The catalyst was recovered
almost quantitatively and tested in up to five more reac-
tion cycles (Figure 8). It was found that, after five cycles,
the catalyst still had a high activity and gave the desired
product in good yield.
3.4 | Leaching Study
Catalyst lifetime and leaching of active metal species into
solution are important issues to be considered when a
SCHEME 5 Proposed pathway for click reaction catalysed by
PS‐Ce‐amtp
3.3 | Catalyst Recycling
One of the major thrusts of our present work was to find
high recycling efficiency of the catalyst and this is
obviously imperative in the context of financial viability
and sustainable development. The catalyst recycling
experiment was carried out using the model reaction of
FIGURE 8 Recycling efficiency of PS‐Ce‐amtp catalyst for
multicomponent and click reactions
TABLE 6 Comparison of catalytic activity of present catalyst in synthesis of spiro‐piperidine derivatives and click reactions with that of
other reported catalyst systems
Yield
Catalyst
Reaction conditions
(%)
Ref.
Synthesis of spiro‐piperidine derivatives
STA
Aniline (1 mmol), dimedone (2 mmol), formaldehyde (3 mmol),
water, RT, 6 h
74
81
88
[50]
[78]
Cu/SiO2
PS‐Ce‐amtp
4‐Chloroaniline (1 mmol), dimedone (2 mmol), formaldehyde
(3 mmol), DCM, RT, 10 h
Aniline (2 mmol), dimedone (4 mmol), formaldehyde (6 mmol),
water, RT, 25 min
Present study
Click reaction
Silver(I) acetate complex
Ag2CO3
Phenylacetylene, benzyl azide, caprylic acid, PhMe, 48 h, RT
Phenylacetylene, NMP, ethyl 2‐isocyanoacetate, 80 °C
Pyridine, azide, 24 h, RT
95
89
75
79
98
[79]
[80]
Cu(MeCN)4PF4 and silver acetylide
Silver acetate complex
PS‐Ce‐amtp
[81]
Phenyl acetylene, benzyl azide, caprylic acid, PhMe, 90 °C
Aniline (1 mmol), phenylacetylene (1.2 mmol), RT, 3 h
[82]
Present study

MONDAL ET AL.
13 of 15
ACKNOWLEDGEMENTS
S.M.I. gratefully acknowledges the Department of
Science and Technology, DST‐SERB, (project sanction
no. EMR/2016/004956), New Delhi, India, the Council
of Scientific and Industrial Research, CSIR (project
sanction no. 02(0284)/16/EMR‐II; date: 06‐12‐2016) and
West Bengal State Council of Science and Technology
(WB‐DST (Sanc.)/ST/P/S&T/4G‐8/2014, dated 04/01/
2016) for financial support. P.M. is grateful to the
University Grant Commission (UGC) for funding in
the form of a Dr D. S. Kothari post‐doctoral research fel-
lowship (award no. F4‐2/2006 (BSR)/CH/15‐16/023). S.
K.D. thanks UGC, New Delhi for a senior research
fellowship. We sincerely thank the Department of
Chemistry of the University of Burdwan for infrastruc-
tural facilities. We acknowledge the Department of
Science and Technology (DST) and UGC New Delhi,
India for providing support to the Department of
Chemistry, University of Kalyani under PURSE, FIST
and SAP programme.
FIGURE 9 SEM image of the recycled PS‐Ce‐amtp catalyst after
five cycles
heterogeneous catalyst is employed for large‐scale indus-
trial operation. Our present PS‐Ce‐amtp catalyst is very
stable in air and moisture. In order to determine the abso-
lute amount of metal species dissolved in solution caused
by leaching, AAS analysis was performed. The AAS anal-
ysis of the reaction mixture revealed that at the end of the
first run only 0.06 wt% of Ce metal was lost into the solu-
tion, indicative that Ce is tightly bound to the support. It
is seen that after five recycling experiments less than
0.2 wt% of Ce metal was leached into the solution. This
leaching level was negligible which was also confirmed
by the excellent recoverability and reusability of this het-
erogeneous catalyst. The morphology of the catalyst
obtained after five recycles observed using SEM
(Figure 9) also does not show any significant changes,
indicating that the structural integrity of the catalyst is
maintained throughout the recycling process.
ORCID
Sk. Manirul Islam http://orcid.org/0000-0002-3970-5468
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