Copper modified spherical MCM-41 nano particles: An efficient catalyst for the three-component coupling of aldehydes, amines and alkynes in solvent-free conditions

Article abstract of DOI:10.1039/c4ra04253k

Cu-MCM-41 nano particles have been shown to effectively catalyze the multi-component synthesis of propargylamines by the reaction of aldehydes, amines, and alkynes (A³coupling) effectively at 100 °C under solvent-free conditions. Spherical Cu-M

Full text of DOI:10.1039/c4ra04253k

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Copper modified spherical MCM-41 nano particles:
an efficient catalyst for the three-component
coupling of aldehydes, amines and alkynes in
solvent-free conditions
Cite this: RSC Adv., 2014, 4, 39759
*
Mohammad Abdollahi-Alibeik and Ali Moaddeli
Cu-MCM-41 nano particles have been shown to effectively catalyze the multi-component synthesis of
propargylamines by the reaction of aldehydes, amines, and alkynes (A3 coupling) effectively at 100 ꢀC
under solvent-free conditions. Spherical Cu-MCM-41 nanoparticles were prepared by a simple method
and characterized by XRD, TEM, SEM and FT-IR techniques. Cu-MCM-41 with a Si : Cu molar ratio of
20 : 1 shows the best catalytic activity. The catalyst was recovered by simple filtration and reused for
several cycles without considerable loss of activity.
Received 7th May 2014
Accepted 12th August 2014
DOI: 10.1039/c4ra04253k
www.rsc.org/advances
the use of toxic solvent, and the use of expensive and non-
reusable catalyst.
1. Introduction
Recently heterogenous catalysts have been considered by
chemists due to the unique advantages, such as simple
handling and storage, easy separation and regenerability.21
MCM-41 as one of the members of mesoporous silicas, has been
applied as heterogeneous catalyst and support in many organic
processes.22 MCM-41 possesses a high surface area and uniform
tubular channels (2D-hexagonal structure) with tunable pore
diameters in the range of 2–10 nm.23–25 These properties provide
good transportation channels for reactants to access active
centers and for products to move out. Due to the low catalytic
activities of MCM-41, modied MCM-41 has attracted consid-
erable attention.26–30 Transition metal (such as V,31 Fe,32 Cu,33
Mn,34 Co,35 Ni 36 and Mo 37) modied MCM-41 has emerged as
useful heterogeneous catalysts due to their versatile applica-
tions in synthesis and catalysis.
C–C bond formation reaction is one of the most important
processes in chemistry because they provide key steps in
building more complex molecules from simple precursors.
Transition metal catalyzed C–H bond activations and subse-
quent C–C bond formations have attracted great interest in
recent years.38
Copper is an efficient, cheap and non-toxic ingredient in
many heterogeneous and homogeneous catalysts particularly
for the synthesis of many organic compounds especially
through C–H bond activation for C–C bond formation.39
To the best of our knowledge, there are no examples of C–H
bond activation using copper-based mesoporous silica cata-
lysts. We expect that copper-modied mesoporous silica show
favorable activity in this type of reactions (Scheme 1). It is
considerable that in the presence study a novel method was
Propargylamines represent an important class of compounds,
which are found in various biologically active naturally existing
materials. These compounds have also been used as precursors
in the synthesis of heterocyclic compounds such as quinolines,1
phenanthrolines,2 pyrroles,3 pyrrolidines,4 indolizines5 and
oxazolidinones.6 Propargylamines can be utilized as versatile
and key synthetic intermediates for the preparation of several
natural products and bioactive compounds.7,8
In general, propargylamines have been constructed through
various methods like nucleophilic addition of in situ generated
metal acetylides to imines and enamines,9,10 amination of
propargylic electrophiles (halides, propargylic phosphates or
propargylic triates)11 and highly efficient three-component
coupling reaction through C–H activation (Mannich conden-
sation).12 Among these synthetic strategies, three-component
Mannich condensation of terminal alkynes, aldehydes, and
amines is more efficient and certainly the most popular method
due to building complex structures from simple starting mate-
rials which are available commercially. In addition, this reac-
tion is performed in an environmentally compatible fashion
through one-pot multi-component reaction (MCRs) in a cata-
lytic process and also atom-economical approach.
Various catalyst have been developed for this type of reac-
tion, such as CuSBA-15, iridium,13 gold,14 nickel,15 iron16 and
indium,17 and also various solvents such as toluene,18 acetoni-
trile19 and chloroforme20 have been used. However, many of
these methods suffer from at least one of disadvantages, such as
Department of Chemistry, Yazd University, Yazd 89195-741, Iran. E-mail: abdollahi@
yazd.ac.ir; moabdollahi@gmail.com; Fax: +98-351-8210644; Tel: +98-351-8122659
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Scheme 1
developed for the preparation of Cu-MCM-41 nanoparticles
under mild condition and without using hydrothermal method.
2. Experimental
2.1 Material and methods
All chemicals were commercial products. All reactions were
monitored by TLC (thin layer chromatography) and all yields
refer to isolated products. 1H and 13C NMR spectra were recorded
in CDCl3 on a Bruker DRX-400 AVANCE (400 MHz for 1H and 100
MHz for13C) spectrometer. Infrared spectra of the catalysts and
reaction products were recorded on a Bruker FT-IR Equinox 55
instrument spectrophotometer in KBr disks. XRD patterns were
recorded on a Bruker D8 ADVANCE X-ray diffractometer using
Fig. 1 SEM image of 20-CM.
were added into a 5 mL round bottom ask and were stirred at
100 ꢀC. The progress of the reaction was monitored by TLC.
Aer completion of the reaction, ethanol (2 mL) was added to
the reaction mixture and the solution was centrifuged at 3000
rpm for 2 min. Finally, the excess of solvent was removed under
reduced pressure to give the corresponding product (4a–n).
Further purication was achieved by thin layer chromatography
on silica gel using n-hexane/ethylacetate.
˚
nickel ltered Cu Ka radiation (l ¼ 1.5406 A). Scanning electron
micrograms (SEM) was obtained using KYKY-EM3200 Instru-
ment. Transmittance electron microscopy was performed with
Zeiss-EM10C at 80 KV. Potentiometric data was collected using
pH/mV meter, AZ model 86502-pH/ORP.
2.4 Physical and spectroscopic data for selected compounds
2.2 Preparation of Cu-MCM-41 nanoparticles
4-(1-(4-Chlorophenyl)-3-phenylprop-2-yn-1-yl)morpholine (4d).
IR (neat) nmax: 3080, 2964, 2856, 2822, 1597, 1488, 1452, 1402,
1320, 1286, 1115, 1096, 1004, 971, 851, 782, 755, 691 cmꢂ1;1H
NMR (400 MHz, CDCl3, ppm): d 7.51 (d, 2H), 7.45–7.42 (m, 2H),
7.27–7.25 (m, 5H), 4.69 (s, 1H), 3.70–3.61 (m, 4H), 2.55–2.52
The synthesis of nano-sized Cu-MCM-41 was carried out by
method of direct insertion of copper ion in sol–gel preparation
step at room temperature (RT) using tetraethylorthosilicate
(TEOS) as the Si source, cetyltrimethylammonium bromide
(CTAB) as the template, ammonia as the pH control agent and
Cu(OAc)2$H2O as the copper source with the gel composition
(molar ratio) of SiO2 : Cu(OAc)2$H2O : CTAB : NH4OH : H2O ¼
1.00 : 0.050 : 0.127 : 0.623 : 508.
Cetyltrimethylammonium bromide (1.04 g) was dissolved in
deionized water (200 mL) under stirring, and then temperature
was adjusted to 60 ꢀC for 15 min. To this solution, tetraethy-
lorthosilicate (5 mL) was added dropwise and then a solution of
Cu(OAc)2.H2O (appropriate amount of copper precursor in 5 mL
of deionized water) was added dropwise under ꢀvigorous stirring
while, the temperature was maintained at 60 C. The reaction
mixture was allowed to cool to RT and then pH of the solution
was adjusted to 10.5 by adding 25 wt% ammonia solution. The
mixture was stirred for 12 h at RT. The gel was recovered by
centrifuging and washed with ethanol (3 ꢁ 5 mL) and deionized
water (3 ꢁ 10 mL). The obtained solid was dried at 120 ꢀC for 2 h
and calcined in air at 550 ꢀC for 4 h. The obtained samples with
Si : Cu molar ratio of 10, 20 and 30 were denoted as 10-CM, 20-
CM and 30-CM respectively.
2.3 General procedure for the synthesis of propargylamine
derivatives
Aromatic/aliphatic aldehydes 1a–n (1 mmol), secondary amine
Fig. 2 FT-IR spectra of (a) MCM-41, (b) 10-CM, (c) 20-CM and
(1.1 mmol), phenylacetylene (1.2 mmol) and catalyst (40 mg) (d) 30-CM.
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Fig. 6 Potentiometric titration of (-) 10-CM, (ꢃ) 20-CM and (;) 30-CM.
Fig. 3 Low angle XRD patterns of (a) 10-CM, (b) 20-CM and (c) 30-CM.
Fig. 4 High angle XRD patterns of (a) 10-CM, (b) 20-CM and (c) 30-
CM.
Fig. 7 FT-IR spectra of (a) 20-CM, (b) pyridine adsorbed 20-CM
at ambient temperature and pyridine adsorbed 20-CM heated at
(c) 100 ꢀC, (d) 200 ꢀC, (e) 300 ꢀC, (f) 400 ꢀC and (g) 500 ꢀC.
(t, 4H);13C NMR (100 MHz, CDCl3, ppm) 136.5, 133.6, 131.9,
129.9, 128.44, 128.39, 128.38, 122.7, 88.9, 84.3, 67.1, 61.4, 49.8.
1-(1-(3,5-Dimethoxyphenyl)-3-phenylprop-2-yn-1-yl)pyrrolidine
(4m). IR (neat) nmax: 3080, 2997, 2958, 2930, 2835, 1597, 1489,
1460, 1427, 1345, 1293, 1204, 1155, 1065, 1029, 917, 757,
Fig. 5 TEM image of 20-CM.
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691 cmꢂ1;1H NMR (400 MHz, CDCl3, ppm): d 7.41–7.38 (m, 2H), decreased with increasing in copper content of the catalysts.
7.24–7.21 (m, 3H), 7.74 (d, J ¼ 2 Hz, 2H), 6.32 (t, J ¼ 2 Hz, 1H), These changes are due to the decrease in the long-range order of
4.78 (s, 1H), 3.71 (s, 6H), 2.64–2.75 (m, 4H), 1.74 (t, 4H);13C NMR the hexagonal meso-structure of MCM-41 by the incorporation
(100 MHz, CDCl3, ppm) 160.7, 141.5, 131.8, 128.3, 128.2, 123.1, of copper into the hexagonal channels of MCM-41. The main
106.4, 99.8, 87.0, 86.3, 59.3, 55.4, 50.4, 23.6.
peaks of samples is observed in 2q ¼ 1.98, 1.55, 1.45 for 10-CM,
20-CM and 30-CM, respectively. An increase in the d-values (a
decrease in 2q based on the Bragg's equation) and unitcell
parameters are also observed on incorporation of copper and
they increase with increasing copper content. These results are
probably due to the partial substitution of the structural Si4+ by
the Cu2+ ion, resulting in collapse of the hexagonal structure of
MCM-41 and the increase in unit-cell parameters on Cu incor-
poration is probably due to the larger size of Cu2+ compared to
Si4+.
As shown in Fig. 4a, high angle XRD pattern of 10-CM show
slightly CuO in tenorite phase while 20-CM and 30-CM with
lower Cu content (Fig 4b) lack this phase. This fact show high
dispersion of copper ions in the MCM-41 framework and
conrms absence of distinct phase for CuO especially for 20-
CM.
Mesostructure of the 20-CM sample was further studied by
TEM, as shown in Fig. 5. Porosity of the sample is clear and
pores size is observed in the range of 2.5–3 nm. No copper oxide
nanoparticles are observed in TEM image, suggesting that
copper is completely dispersed in MCM-41 framework. Partial
disordered mesoporous system is related to the incorporation
of Cu into the MCM-41 framework that conrms by XRD data.
The catalyst acidity characters, including the acidic strength
and the total number of acid sites were determined by poten-
tiometric titration. According to this method, the initial
3. Results and discussions
3.1 The catalyst characterization
Fig. 1 represents the results of scanning electron micrograms
(SEM) in order to investigate the particle size and morphology of
the catalysts. The SEM of Cu-MCM-41 shows spherical nano-
particles with sizes of <100 nm.
The FT-IR absorption spectra of pure silica MCM-41 and Cu-
incorporated MCM-41 samples with different loading amounts
of copper are shown in Fig. 2. In the range of 400–1600 cmꢂ1
,
three peaks at ꢄ465 cmꢂ1, ꢄ800 cmꢂ1 and ꢄ1090 cmꢂ1 are
corresponding to the rocking, bending (or symmetric stretch-
ing), and asymmetric stretching of the inter-tetrahedral oxygen
atoms in SiO2 of Cu-MCM-41, respectively and also the peak at
966 cmꢂ1 is assigned to the silanol group. The vibration
absorption band at 1089 cmꢂ1 is assigned to n(Si–O–Si) and a
slight red-shi to the lower frequencies is observed in Cu-MCM-
41 samples. In general, this shi of the absorption peaks toward
the lower wavenumbers is considered an indication of Cu
incorporation into the framework of silica tetrahedra.
The low angle XRD patterns of Cu-MCM-41 samples with
Si : Cu molar ratio of 10, 20 and 30 are shown in Fig. 3. The
intensity of the main peak of Cu-MCM-41 samples was
Scheme 2
Scheme 3
Table 1 Three-component coupling of benzaldehyde, morpholine and phenylacetylene catalyzed by Cu-MCM-41a
Entry
Catalyst
Temp. (ꢀC)
Catalyst amount (mg)
Cu contentb (mol%)
Solvent
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
20-CM
20-CM
20-CM
20-CM
20-CM
20-CM
20-CM
20-CM
10-CM
30-CM
20-CM
20-CM
20-CM
80
90
40
40
40
40
40
20
30
50
40
2.9
2.9
2.9
2.9
2.9
1.5
2
3.6
5.8
1.9
7.3
7.3
7.3
—
—
—
—
—
—
—
—
—
—
PhCH3
CH3CN
THF
180
180
180
150
150
180
180
180
180
180
360
360
24 h
60
80
85
79
74
65
72
80
80
68
85
50
60
100
110
120
100
100
100
100
100
100
Reux
Reux
9
10
11
12
13
40
100
100
100
a
All reactions were carried out with benzaldehyde (1 mmol), morpholine (1.1 mmol), phenylacetylene (1.2 mmol) and Cu-MCM-41 as catalyst.
Copper content of the catalyst was measured by atomic absorption instrumentation.
b
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Table 2 Cu-MCM-41 catalyzed synthesis of propargylaminesa
Entry
Substrate (1)
Substrate (2)
Product (4)
Timeb (min)
Yieldc (%)
a
180
85
b
210
240
210
80
92
80
c
d
e
120
180
90
80
85
93
f
g
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Table 2 (Contd.)
Entry
Substrate (1)
Substrate (2)
Product (4)
Timeb (min)
Yieldc (%)
h
120
85
i
90
45
30
90
88
85
j
k
l
35
90
m
45
92
N
20
75
a
Reaction conditions: aldehyde (1 mmol), phenylacetylene (1.2 mmol), amine (1.1 mmol), Cu-MCM-41 (40 mg) and temperature (90–100 ꢀC).
Reactions time is based on the consumption of aldehyde monitored by TLC. Isolated yield.
b
c
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Table 4 Reusability of the catalyst in the model reaction
electrode potential (Ei) indicates the maximum acid strength of
the surface sites.40 Therefore, a suspension of the catalyst in
acetonitrile was potentiometrically titrated with a solution of
0.02 M n-butylamine in acetonitrile. As shown in Fig. 6, 20-CM
displays higher acid strength than the other samples.
Entry
Fresh
Cycle 1
Cycle 2
Cycle 3a
Yield (%)
85
82
77
80
180
Time (min)
180
180
180
In order to obtain a clear distinction between Lewis and
Brønsted acid sites, FT-IR analysis of pyridine adsorbed on the
catalyst surface were carried out and results are displayed in
Fig. 7. The FT-IR spectrum of pyridine adsorbed 20-CM before
heat treatment (Fig. 7b) shows the contribution of pyridine
adducts in the region of 1400–1650 cmꢂ1. In this spectrum, the
peaks at 1448 and 1598 cmꢂ1 are attributed to pyridine bonded
Lewis acid sites of the 20-CM. The weak peak at 1543 cmꢂ1
assigned to Brønsted acid sites (corresponds to protonation of
pyridine on Brønsted acid sites) is so hard to nd in current
zoom. The weak peak at 1491 cmꢂ1 is attributed to the combi-
nation of pyridine bonded to Lewis and Brønsted acid sites. As
shown in Fig. 7c–g, with increasing in temperature, character-
istic peaks of Lewis acid sites are still remained at 1448 and
1599 cmꢂ1. These results show that Lewis acidity of the catalyst
is stronger than its Brønsted acidity.23
a
The catalyst was calcined at 550 ꢀC for 2 h before using in cycle 3.
according to the obtained results (Table 1, entries 6–10) 40 mg of
the catalyst was chosen as the best catalyst amount. The effect of
solvent was also investigated by performing the model reaction in
the presence of 40 mg catalyst in various solvents (Table 1, entries
11–13). The model reaction in the presence of toluene as solvent
showed convenient yield but it suffered from high loading of
catalyst, long reaction time and toxic solvent (Table 1, entry 11).
Thereaer, the above optimized reaction conditions
were explored for the synthesis of propargylamine derivatives
(Scheme 3) and the results are summarized in Table 2. As
exemplied in Table 2, this protocol is rather general for a wide
variety of electron-rich as well as electron-decient aromatic
aldehydes and also various secondary amines. Although a regular
trend is not observed base on the activity of benzaldehyde
derivatives but results in Table 2 show that pyrolidine have the
most activity among the applied amines. This procedure is also
3.2 Catalytic activity of Cu-MCM-41
To testify the catalytic activity, synthesis of propargylamines convenient for aliphatic aldehydes (Table 2, entry j).
through A3 coupling of aldehyde, amines and phenylacetylene is
selected to represent activity of the prepared catalysts.
We perform a comparative study of the reactivity of Cu-
MCM-41 with other reported heterogeneous Cu-based catalyst
In order to optimize the reaction conditions, initially, the for the synthesis of propargylamines through A3 coupling
reaction of benzaldehyde, phenylacetylene and morpholine was reaction of benzaldehyde, piperidine, and phenylacetylene
selected as model reaction (Scheme 2).
using Cu-based catalysts (Table 3) (Scheme 4). With an overall
The reaction was optimized for the various parameters such as look at Table 3, we can say that our method is comparable with
temperature, solvent and catalyst loading. To investigate the other catalytic systems in term of yield and reaction time. In
effect of reaction temperature, the reaction was performed addition to this, moderate reaction temperature, lack of
initially in solvent free condition at various temperature (Table 1 requirement for an inert atmosphere, and absence of solvent
entries 1–5). The best result was obtained at 100 ꢀC. Increasing in are some obvious benets with respect to the other methods.
ꢀ
the reaction temperature up to 120 C led to a decrease in the
Reusability of the catalyst was investigated in the model
yield of desired product due to the increasing in by-products and reaction under the optimized reaction conditions. The catalyst
emission of the volatile precursors from the reaction media.
was separated from the model reaction and reused two times with
To optimize the catalyst amount, the model reaction was moderate loss of the catalytic activity (Table 4). This result may be
performed in the presence of various amount of the catalyst and due to blockage of active sites of the catalyst and/or partial
leaching of copper from the catalyst. Enhancement in the activity
aer calcination of recovered catalyst from cycle 2 conrms
partial blockage of active sites of the catalyst (Table 4, cycle 3).
4. Conclusion
We introduced a new method for the synthesis of copper modi-
Scheme 4
ed MCM-41 at room temperature without using hydrothermal
Table 3 Comparison of Cu-MCM-41 with other Cu-based catalysts for the synthesis of propargylamines
Entry
Catalyst
Solvent
Temp. (ꢀC)
Time (h)
Yield%
1
2
3
4
5
Cu-MCM-41
CuNPs/TiO2
Cu(OH)x–Fe3O4
CuHAP
—
—
—
90
70
120
Reux
90
1.5
7
3
6
6
93 [this work]
98 (ref. 41)
>99 (ref. 42)
85 (ref. 19)
82 (ref. 43)
Acetonitrile
Toluene
Nano CuO
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