Microwave-assisted facile synthesis of propargylamine library by robust nitro functionalized cross-linked polystyrene resin supported Cu NPs

Article abstract of DOI:10.1002/poc.3749

In the present investigation, we study catalytic activity of copper nanoparticles stabilized onto a nitro functionalized polystyrene resin (Cu NPs@ Nitro-Resin). The size of stabilized copper nanoparticles was found between 3 and 9?nm. This work reports a

Full text of DOI:10.1002/poc.3749

Received: 16 February 2017
DOI: 10.1002/poc.3749
Revised: 25 April 2017
Accepted: 30 June 2017
R E S E A R C H A R T I C L E
Microwave‐assisted facile synthesis of propargylamine
library by robust nitro functionalized cross‐linked
polystyrene resin supported Cu NPs
Anuj S. Sharma | Harjinder Kaur
| Nirav Barot
Department of Chemistry, School of
Abstract
Sciences, Gujarat University, Ahmedabad,
India
In the present investigation, we study catalytic activity of copper nanoparticles
stabilized onto a nitro functionalized polystyrene resin (Cu NPs@ Nitro‐Resin).
The size of stabilized copper nanoparticles was found between 3 and 9 nm. This
work reports a cost‐effective and sustainable protocol for the synthesis of
propargylamines. Herein, we have developed microwave‐assisted synthesis of
propargylamine from 3 components coupling of aldehyde, alkynes, and amines
(A3 coupling) in truly heterogeneous catalytic system. Reaction parameters,
such as solvent, catalyst concentration reaction time, and recyclability, were
investigated, and reaction conditions were optimized. The present method
has advantages such as environmentally benign, ease to handle, short reaction
time (≈25 min), excellent yields (98%), low E‐factor (0.15), and high atom
economy (94%).
Correspondence
Harjinder Kaur, Department of Chemistry,
School of Sciences, Gujarat University,
Ahmedabad, India.
Email: hk_ss_in@yahoo.com
Funding information
GUJCOST, Grant/Award Number: /MRP/
2014‐15/403
KEYWORDS
A3 coupling, microwave heating, nanocatalysis, propargylamines, resin supported nanoparticles
1 | INTRODUCTION
carefully. During the reaction process, there is not only
usual agglomeration and Ostwald ripening of nanoparti-
In the 21st century, there has been an increased awareness
towards green chemistry approach to synthesize of
important organic moieties. The need for developing sus-
tainable reaction protocols with state‐of‐the‐art use of
technology, alternate reaction media, energy saving,
waste minimization, etc has become vital. One such tech-
nique that has gained attention is synergistic use of
microwave reactors and nanoparticles as catalyst. In the
last few decades, microwave radiation in synthetic
organic chemistry became popular and attracted consid-
erable practical and theoretical attention along with the
development of resource friendly and environmentally
benign processes in terms of sustainable chemistry.[1,2]
Although use of homogeneous and heterogeneous cata-
lysts is common under microwave heating, there are
some important parameters that need to be looked
cles but also increased chances of sintering and hot spot
development even at low temperature as metal nanoparti-
cles are efficient absorbers of microwave. So careful choice
of solvents, supports, reaction temperature, etc needs to be
evaluated.[3–6]
Transition metal catalyzed multicomponent reactions
are a powerful synthetic strategy to generate multiple
molecular scaffolds and to increase structural as well as
skeletal, diversity from simple and readily available mole-
cules.[7] Three components coupling of aldehyde, alkynes,
and amines (A3 coupling) is one of the best examples of
such a process, and this transformation has received much
more attention in recent year. Propargylamine scaffolds
show remarkable biological activity and are used in drugs
such as rasagiline, a potent cardiovascular drug.[8] Besides,
they also act as enzyme inhibitors,[9] antitumor,[10]
J Phys Org Chem. 2017;e3749.
wileyonlinelibrary.com/journal/poc
Copyright © 2017 John Wiley & Sons, Ltd.
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SHARMA ET AL.
antibiotics,[11] herbicides,[12] and pharmaceutical agents.[13]
They are also useful as intermediate such as peptide
isosters, oxotremorine analogues, β‐lactams natural prod-
ucts, therapeutic drug molecules, and polyfunctional
amino derivatives in organic synthesis as well as some drug
molecules.[14] One of the general and traditional ways of
synthesizing propargylamine is through the amination
of propargylic halides, propargylic phosphonates, or
propargylic triflates and the nucleophilic addition of in
situ generated metal acetylides to imines and enamines.
However, all these methods require stoichiometric amounts
of reagents that are highly moisture sensitive, poor atom
economy, corrosive, generate large amounts of waste and
require tedious workup during the reaction.[15–20]
2 | RESULT AND DISCUSSION
The use of nanocatalysts has been rapidly increasing
owing to their extraordinary efficiency as well as selecti-
vity. In the past several years, supported metal nanopar-
ticles have appeared as sustainable replacements to
conventional homogeneous catalyst as they combine
the ease of catalyst separation with high catalytic activ-
ity. The availability of, supported transition metal nano-
particles, has attracted excessive consideration because
of their remarkable catalytic activity. The ideal sup-
ported nanocatalyst should be accessible, leach proof,
stable, and highly recyclable.[49] It is well known that
the insoluble resins have been probably the most fre-
quently used polymeric supports in heterogenous cataly-
sis. Organic polymers, especially polystyrene‐type resins,
represent one of the most popular commercialized sup-
ports. As compared to some other supports used for the
stabilization of active metal nanoparticles, polystyrene
resin is a good candidate because of its commercially
availability with low price. Its chemical and thermal
stability, high surface area, inert nature, reusability,
mechanical robustness, and plain filterability under
microwave as well as conventional heating.[50–53] In
addition, the metal nanoparticles are highly stabilized
in the polystyrene resin because of the electrostatic sta-
bilization and steric stabilization. Supporting copper
nanoparticles on polystyrene resin beads was performed
in a 2‐step process reported earlier by our group.[48]
High‐resolution transmission electron microscopy
(TEM) images indicated that the copper nanoparticles
present in the interior of polymeric matrix of the polysty-
rene resin were of 3 to 9 nm dimension (Figure 1A). Cop-
per loading on the resin as measured by atomic
absorption spectroscopy was 0.21 mmol per gram of the
resin. An X‐ray photoelectron spectroscopy study was
also performed to know the oxidation state of active sites
in the catalyst. The high‐resolution X‐ray photoelectron
spectroscopy of Cu 2p indicates the presence of a peak
at 940 to 945 eV that is a characteristic of Cu+2, mainly
due to the formation of CuO. The presence of lower Cu
2P3/2 binding energy peaks (933 eV) indicated the pres-
ence of Cu(0) (Figure 1C). The stabilized copper nano-
particles present into the resin beads were of Cu(0) and
Cu+2 state.
Recently, different homogenous catalysts such as
CuCl,[21] CuBr,[22] CuI,[23] Cu (I) PF6‐ligand,[24] CuCl2,[25]
Fe‐I2‐CuBr,[26] and Cu (I) pybox complex[27] have been
successfully used for the synthesis of propargylamine.
However, to overcome disadvantages associated with
homogenous catalysts such as poor recycling, stability,
and handling in high scale industrial synthesis of
propargylamine, heterogeneous catalysts have also been
designed.[28–44] They too consistently suffer from several
problems such as sintering and Ostwald ripening. Tu et al
reported an efficient tool to synthesize propargylamine
under microwave irradiation by using CuI catalyst.[45]
Recently, Ermolatev et al have reported a microwave‐
assisted decarboxylative coupling of a 2‐oxaacetic acid, an
amine, and an alkyne to afford polysubstituted
propargylamine by using CuBr as catalyst. However, the
main disadvantage associated with both these methods is
that it requires high amount of catalyst (15 mol%) and the
recyclability issues of homogeneous catalyst could make
this protocol less sustainable and accessible.[46] Recently,
Xiong et al have developed oyster shell waste supported
CuCl2 catalyst and investigated its activity towards A3 cou-
pling under the microwave irradiations. Nevertheless, the
issues of recyclability again reduced the adaptability of this
catalyst.[47] In our recent report, we reported the synthesis
and characterization of copper nanoparticles stabilized
onto a novel nitro functionalized polystyrene resin (Cu
NPs@ Nitro functionalized Resin) and investigated its cat-
alytic activity towards the C–N coupling of various aro-
matic amines with aryl halides and the oxidative C–C
homocoupling of phenyacetylene.[48] In the present inves-
tigation, we studied the feasibility and sustainability of this
catalyst for the synthesis of propargylamine under micro-
wave heating. The effect of various reaction parameters
such as solvent, temperature, reaction time, catalyst quan-
tity, etc was studied on the yield of products. To assess the
generality of the protocol, we tested many substituted
derivatives of aldehydes and phenyl acetylenes and ana-
lyzed the products by 1H‐NMR.
2.1 | Catalytic activity
Our initial investigation on the evaluation of the catalytic
activity started with the coupling reaction of benzalde-
hyde, piperidine, and phenyl acetylene under microwave
irradiation as model reaction (Scheme 1).

SHARMA ET AL.
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FIGURE 1 A, Transmission electron microscopy images of Cu NPs@ Resin. B, Histogram of Cu NPs@ Resin. C, High‐resolution X‐ray
photoelectron spectroscopy of Cu 2p
SCHEME 1 A3 coupling of
benzaldehyde, piperidine, and phenyl
acetylene catalyzed by Cu NPs@ Nitro‐
Resin. MW, microwave
The performance of metal supported heterogeneous
catalysts in organic reactions mainly depends upon the
accessibility of catalytic sites, which in turn depends on
the choice of support material, the polarity of substrate,
and solvents. To investigate the effect of solvent on the
performance of reaction, the model reaction was per-
formed in the presence of Cu NPs@ Nitro‐Resin catalyst
in different solvents such as toluene, acetonitrile, ethanol,
N,N‐dimethylformamide, dimethyl sulfoxide, 1,4 dioxane,
and water. The results are shown in Table 1. The best
yield of propargylamine (98%) was achieved in case of tol-
uene. This can be attributed to the better swelling of resin
in toluene and its nonreactive nature that decreases side
reactions. We also tried solvent‐free synthesis of
propargylamine, but the resins beads were stuck on the
surface of reaction vessel, and poor yield of product was
obtained.
It was found that temperature has a prominent effect
on synthesis of propargylamine. In our model reaction,
the synthesis of propargylamine was conducted at differ-
ent temperatures ranging from 60°C to 100°C shown in
Figure 2. From the results, we have found that the yield
of propargylamine increases significantly from 27% to
98% as temperature was raised from 60°C to 100°C. Thus,
TABLE 1 Effect of solvents on synthesis of propargylamine using
Cu NPs@ Nitro‐Resin
Entry
Solvents
Yield, %
1a
Toluene
Trace
2
3
4
5
6
7
Water
Trace
24
Ethanol
DMF
50
1,4 Dioxane
Acetonitrile
Toluene
25
85
98
FIGURE 2 Screening of temperature for the synthesis of
propargylamine conditions: benzaldehyde (1 mmol), piperidine
(1 mmol), phenyl acetylene (1.15 mmol), 180 mg catalyst
Reaction condition: benzaldehyde (1 mmol), piperidine (1 mmol), phenyl
acetylene (1.15 mmol), 180 mg catalyst (3.78 mmol% of Cu), 100°C, micro-
wave (100 W), 25 min, solvent (1 mL).
aAbsence of catalyst.
(3.78 mmol% of Cu), 1 mL toluene, microwave (100 W), 25 min

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SHARMA ET AL.
100°C was selected as the optimum temperature to
afforded higher yield of propargylamine (98%), and the
effect of other important parameters such as quantity of
catalyst, reaction time, and effect of solvents was studied
at this temperature.
To ascertain the yield of propargylamine, reaction
mixture was analyzed at intervals of 5 minutes. The reac-
tion was quenched at different intervals of time and prod-
uct isolated. The report indicates that after 5 minutes, the
yield of propargylamine was 15%, which increased to 98%
in 25 minutes. A time‐verses‐yield graph is shown in
Figure 3.
The presence of catalyst was necessary for the progress
of the reaction as in its absence trace amount of
propargylamine formed (Table 1). To study the effect of
catalyst concentration on reaction rate, we changed the
amount of catalyst from 80 to 180 mg while keeping other
parameters constant (Figure 4). As the quantity of catalyst
increased from 80 to 180 mg, the yield of propargylamine
enhanced up to 98%. Therefore, 180 mg of catalyst was
sufficient to afforded high yield (98%) of propargylamine.
FIGURE 4 Screening of quantity of catalyst on the synthesis of
propargylamine. Reaction conditions: benzaldehyde (1 mmol),
piperidine (1 mmol), phenyl acetylene (1.15 mmol), 180 mg catalyst
(3.78 mmol% of Cu), 1 mL toluene, 373 K, microwave (100 W),
25 min
the reaction was uniform irrespective of the nature of
the substituents (electron withdrawing or electron donat-
ing) on the aromatic ring. A wide range of substituted
aldehydes that included –X (Br, Cl, I), –OH, –CH3, –
OCH3, and –NO2 groups were compatible with this pro-
cedure. Almost all screened substrates underwent A3
coupling smoothly to afford the corresponding products
in 98 to 82% yields as summarized in Table 2. In case
of furfuraldehyde, excellent product was obtained in
short period (entry 13, Table 2). We also tried vanillin
that also afforded excellent yield of product in short time
(entry 7, Table 2). Different acetylene derivatives were
also tried for the synthesis of propargylamine. All the
derivatives gave corresponding products in 92 to 98%
yields, and results are summarized in Table 3. Both aro-
matic and aliphatic acetylenes gave high yield of prod-
ucts. Different substituents (electron withdrawing and
electron donation) of phenyl acetylene afford excellent
yield. 1‐Hexyne (entry 6, Table 3), ethynyl cyclopropane
(entry 5, Table 3), and trimethyl silyl (entry 8, Table 3)
also gave good yields. In addition, reaction was also tried
with aliphatic aldehydes such as paraformaldehyde and
acetaldehyde, and they afforded good yields of
propargylamine (entries 17 and 18, Table 3). All the
2.2 | Scope of different aldehydes and
acetylenes for the synthesis of
propargylamine
The generality of the protocol or substrate scope for the
synthesis of propargylamine was also investigated.
Under the optimized reaction conditions, different deriv-
atives of propargylamine could be synthesized. The
results are shown in Tables 2 and 3. To our delight,
1
products were characterized by matching their H‐NMR
spectra, summarized in Supporting Information. The
proposed reaction mechanism of A3‐coupling reaction
is well known in literature. It has been reported that
the reaction takes place by the activation of the Csp–H
bond of the terminal alkyne by the Cu NPs@ Resin.
After that alkynyl‐Cu intermediate goes to react with
FIGURE 3 Screening of time on the synthesis of propargylamine.
Reaction conditions: benzaldehyde (1 mmol), piperidine (1 mmol),
phenyl acetylene (1.15 mmol), 180 mg catalyst (3.78 mmol% of Cu),
1 mL toluene, 373 K, microwave (100 W)

SHARMA ET AL.
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TABLE 2 Scope of various aldehydes for the synthesis of
propargylamine
TABLE 2 (Continued)
Time, Yield,
Time, Yield,
Entry Reactant
Product
min
%
Entry Reactant
Product
min
%
9
18
95
1
25
98
2
20
96
10
11
20
18
95
92
3
4
20
25
94
89
12
13
20
15
99
98
5
6
7
25
25
20
85
84
98
14
25
72
89
15
16
20
15
8
25
82
98
(Continues)
(Continues)

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SHARMA ET AL.
TABLE 2 (Continued)
TABLE 3 Scope of various acetylenes for the synthesis of
propargylamine
Time, Yield,
Time, Yield,
Entry Reactant
Product
min
%
Entry Substrate
Product
min
%
17
20
95
1
2
25
98
18
18
65
15
18
25
97
95
92
Reaction condition: aldehyde (1 mmol), secondary amine (1 mmol), acetylene
(1.15 mmol), 180 mg catalyst (3.78 mmol% of Cu), 100°C, microwave (100 W),
25 min, toluene (1 mL).
3
4
the immonium ion generated from the reaction of aro-
matic aldehyde and secondary amine to afford the
propargylamine and regenerate the catalytic active sites
for another reaction.[32,33] In addition, Cu NPs@ Nitro‐
Resin exhibited high activity, environmentally begins
protocol, with negligible waste generation from the reac-
tion mixture after the workup. The green chemistry met-
rics calculation for the model reaction provided very
small E‐factor (0.15) and high atom economy (94%)
(see Supporting Information for calculations).[54,55]
5
6
20
20
92
95
2.3 | Recyclability
Recyclability is one of the biggest advantages of a hetero-
geneous catalyst and an important parameter that is
required for evaluation of catalytic performance. The
recovery and reusability of the Cu NPs@ Nitro‐Resin cat-
alyst was studied for coupling reaction of benzaldehyde,
secondary amine, and acetylene under optimized reaction
conditions. To attempt this, the Cu NPs@ Nitro‐Resin was
easily isolated from the reaction mixture by filtration and
washed with methanol and hot deionized water 3 times to
removed adsorbed organic products from its surface. After
cleaning and drying of the catalyst, it was ready to use for
the 6 catalytic batches. As shown in Figure 5, it was found
that the Cu NPs@ Nitro‐Resin catalyst could be reused for
5 runs with some loss of its activity after several runs.
Nitro functionalization is mainly responsible for enhanc-
ing the stability by restricting the metal leaching from
the resin beads. Also, it enhances the polarity of resin that
facilitates the movement of reactant towards the resin and
hence the catalyst. Additionally, the filtrate was analyzed
for the presence of copper by atomic absorption
7
8
20
20
98
94
Reaction condition: aldehyde (1 mmol), secondary amine (1 mmol), acetylene
(1.15 mmol), 180 mg catalyst (3.78 mmol% of Cu), 100°C, microwave (100 W),
25 min, toluene (1 mL).

SHARMA ET AL.
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ACKNOWLEDGEMENTS
Authors are grateful to GUJCOST, Gandhinagar, for the
financial support to carry out this work and to IIT
Bombay for TEM and high‐resolution TEM analysis,
SAIF‐Punjab University for 1H‐NMR analysis, and
MNIT‐Jaipur for atomic absorption spectrometer results.
ORCID
Harjinder Kaur http://orcid.org/0000-0002-7442-0097
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