Cu(0) onto sulfonic acid functionalized silica/carbon composites as bifunctional heterogeneous catalysts for the synthesis of polysubstituted pyridines and nitriles under benign reaction media

Article abstract of DOI:10.1039/c6ra19840f

A series of novel and highly efficient bifunctional heterogeneous catalysts based on Cu(0) onto sulfonic acid functionalized silica/carbon composites derived from silica and inexpensive natural organic materials (starch, glucose, maltose, cellulose and chitosan) were successfully synthesized. The utility of the developed catalysts was explored for the one-pot synthesis of polysubstituted pyridines under solvent-free conditions and for the oxidative transformation of aldehydes to nitriles using aqueous ammonia and H₂O₂. Among the different catalysts screened, Cu(0)-SilC_cell-SO₃H was found to be the most active and selective. The prepared bifunctional composites were characterized by FTIR, TGA and CHNS, while the most active catalyst, Cu(0)-SilC_cell-SO₃H was further characterized by SEM, TEM, XRD, EDX, ICP-AES and XPS analysis. Further, the catalyst can be recovered and reused for atleast six runs without any significant impact on catalytic activity and selectivity. The high catalytic activity, thermal stability, simple recovery, reusability and eco-friendly nature of the catalyst makes the present protocol particularly attractive from the view point of green chemistry.

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Cu(0) onto sulfonic acid functionalized silica/carbon composites as
bifunctional heterogeneous catalysts for the synthesis of
polysubstituted pyridines and nitriles under benign reaction
media
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Madhvi Bhardwaj, Manmeet Kour and Satya Paul*
A series of novel and highly efficient bifunctional heterogeneous catalysts based on Cu(0) onto sulfonic acid
functionalized silica/carbon composites derived from silica and inexpensive natural organic materials (starch,
glucose, maltose, cellulose and chitosan) were successfully synthesized. The utility of the developed catalysts
was explored for the oneꢀpot synthesis of polysubstituted pyridines under solventꢀfree conditions and for the
oxidative transformation of aldehydes to nitriles using aqueous ammonia and H2O2. Among the different
catalysts screened, Cu(0)ꢀSilCcellꢀSO3H was found to be the most active and selective. The prepared
bifunctional composites were characterized by FTIR, TGA and CHNS, while the most active catalyst, Cu(0)ꢀ
SilCcellꢀSO3H was further characterized by SEM, TEM, XRD, EDX, ICPꢀAES and XPS analysis. Further, the
catalyst can be recovered and reused for atleast six runs without any significant impact on catalytic activity
and selectivity. The high catalytic activity, thermal stability, simple recovery, reusability and ecoꢀfriendly
nature of the catalyst makes the present protocol particularly attractive from the view point of green chemistry.
active sites in the multifunctional catalysts can be organic,
metallic or combination of both, and thus can work
Introduction
The challenges of the 21st century require scientific and
technological accomplishments that must be developed under
sustainable and environmentally benign practices. In this
vein, catalysis and green chemistry walk hand in hand in
promoting faster and cleaner transformations.1,2 The
designing of oneꢀpot multiple transformations also known as
tandem or domino reactions, completely demonstrates the
concepts of efficiency, atom economy and waste reduction.3
Thus, the key to attain multicatalytic events in the same
independently or synergistically to perform multicatalytic
events in the desired reactions.4,5 The heterogeneous catalysts,
for instance PdꢀSO3H/SiO2, Ir/Hꢀbeta zeolite, Pt/Hꢀbeta
zeolite, PdꢀH3PW12O40/SiO2 and many others have recently
been explored for their multifunctional applications.5,6
Despite the spectacular success in the field of multifunctional
catalysts, there is always much scope available for the
improvement at several levels. Among various types of
multifunctional catalysts, heterogeneous metal/sulphonated
bronsted acid catalysts have attracted considerable interest.7
With increasing demands upon our planet’s resources, more
and more research efforts are being focussed on the
preparation of new materials from renewable resources. Thus,
sulfonated carbon based composites prepared from
biomaterials and silica have been proved to be promising
heterogeneous catalysts in the conversion of biomass due to
reaction vessel
involve the use of highly selective
heterogeneous catalysts. Following this concept, chemists
have designed various heterogeneous multifunctional catalysts
with wellꢀdefined multisites to avoid unwanted pathways. The
Department of Chemistry, University of Jammu, Jammu-180 006, India.
Fax: +91-191-2431365; Tel: +91-191-2453969;
*E-mail: paul7@rediffmail.com
Electronic Supplementary Information (ESI) available: Spectral details
of all the products listed in Table 6 and 8 and copies of spectras of 1H, its excellent hydrothermal stability and catalytic activity.8ꢀ10
D2O and 13C of selected products.
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Composites containing metal NPs have attracted a great deal further transformation into other functional compounds.23
of attention, due to their unique optical, electrical, and Representative strategies for the synthesis of nitriles generally
catalytic properties.6,11,12 Recently, Cu(0) nanoparticles have involve the use of toxic reagents, high temperature, pressure,
drawn attention of chemists owing to their inexpensive and generate large amount of wastes.24ꢀ26 Hence, there is a
nature, easy preparation and the ability to replace other noble growing demand for a highly practical nitrile synthesis by
metals, such as Ag, Au and Pt.13 Conversely, the use of Cu(0) employing heterogeneous metal catalysts and using cheap and
nanoparticles is often complicated by the issues related with commercially available starting materials.
their oxidation and agglomeration in the absence of
Herein, we have synthesized novel and sustainable
stabilizers, thereby diminishing their activity.14 Recent
bifunctional heterogeneous catalysts based on Cu(0) NPs onto
advances in the design and preparation of Cu(0) nanoparticles
sulfonic acid functionalized silica/carbon composites [Cu(0)ꢀ
confirmed that they can be synthesized through different
SilCꢀSO3H]. The developed catalyst has been efficiently
preparation routes and using acid functionalized supports to
utilized for the oneꢀpot synthesis of polysubstituted pyridines
give tailored sizes, shapes and distributions, overcoming the
via nucleophilic addition/intermolecular cyclization under
main drawbacks of traditional synthetic methodologies.15
solventꢀfree conditions. Further, to explore the wider
Furthermore,
acid
functionalized
organic/inorganic
applicability of the developed catalyst, we have further used
Cu(0)ꢀSilCꢀSO3H for the oxidative transformation of
aldehydes to nitriles using aqueous ammonia and H2O2 at 100
oC. In both these reactions, excellent results were obtained.
To the best of our knowledge, the use of copper nanoparticles
onto sulfonic acid functionalized silica/carbon composites as
catalysts for the oneꢀpot synthesis of polysubstituted
pyridines and for the oxidative synthesis of nitriles from
aldehydes has not been previously reported in the literature.
composites generally exhibit important advantages over
traditional catalysts such as: (i) the control of the particle
shape by the acidic functional groups located in the polymer
chain; (ii) the improvement of catalytic selectivity; (iii) the
stabilization of nanoparticles by suppression of their
aggregation16, 17 and thus can act as efficient supports for the
immobilization of metal nanoparticles. Hence, with the
growing interest in the heterogeneous catalysis, it is certain
that metal nanoparticles based acid functionalized
organic/inorganic composites will continue to be a fast
moving topic for next several years.
Results and Discussion
Characterization of
Cu(0) onto sulfonic
acid
Pyridines and its derivatives are an important class of Nꢀ
heterocyclic compounds which have been found to be an
integral part of many natural products18ꢀ20 and have shown
functionalized silica/carbon composites [Cu(0)-SilC-
SO3H]
The preparation procedure of Cu(0)ꢀSilCꢀSO3H is represented
in Scheme 1. Initially, silica/carbon composites were
prepared by the partial carbonization of biomaterials (starch,
glucose, maltose, cellulose and chitosan) in the presence of
silica followed by sulfonation using sulfonic acid.27 This step
was followed by the immobilization of Cu(0) nanoparticles
onto the surface of sulfonated silica/carbon composites by in
situ reduction of copper chloride with NaBH4. The prepared
Cu(0)ꢀSilCꢀSO3H composites were characterized by FTIR,
TGA and CHNS, while the most active catalyst,
potent
pharmacological
properties.21
Recently,
multicomponent reactions (MCRs) have gained tremendous
attention from medicinal and organic chemists. These
methods offer rapid and convergent construction of complex
molecules without isolation and purification of intermediates,
minimizing waste, effort, time, and cost. Thus, several elegant
and effective methods based on MCRs have been reported for
the synthesis of pyridine derivatives.22
Further, nitriles are an important class of compounds in
organic synthesis, because of their versatility and potential for
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Scheme 1 General procedure for the synthesis of Cu(0)ꢀSilCꢀSO3H.
Table 1. Major absorption frequencies in FTIR (υmax in cmꢀ1) of
Cu(0)ꢀSilCcellꢀSO3H was further characterized by SEM, TEM,
XRD, EDX, ICPꢀAES and XPS analysis.
Cu(0)ꢀSilCꢀSO3H.a
SO2 str.
The surface chemical structure of Cu(0)ꢀSilCꢀSO3H was
characterized by FTIR spectroscopy. The FTIR spectrum
showed bands in the region of 1639ꢀ1648 cmꢀ1 which were
assigned to aromatic C=C stretching modes in polycyclic
aromatic rings. The absorption bands in the range of 1402ꢀ
1406 cmꢀ1 and 1100ꢀ1106 cmꢀ1 were observed due to the
asymmetric and symmetric stretching modes of SO2 group,
indicating the successful incorporation of SO3H groups into
the carbon framework. Further, the broad bands ranging from
3401ꢀ3412 cmꢀ1 were due to the phenolic ꢀOH groups present
in the aromatic rings. The Cu(0) based sulfonated
silica/carbon composites prepared from different natural
organic compounds (Cu(0)ꢀSilCstarꢀSO3H from starch, Cu(0)ꢀ
SilCgluꢀSO3H from glucose; Cu(0)ꢀSilCmalꢀSO3H from
Asym.
1404
1402
1406
1403
1406
Sym.
1104
1100
1105
1102
1106
-OH
3410
3401
3405
3406
3412
Entry
Catalyst
C=C
1
Cu(0)ꢀSilCstarꢀSO3H
1646
2
3
4
5
Cu(0)ꢀSilCgluꢀSO3H
Cu(0)ꢀSilCmalꢀSO3H
Cu(0)ꢀSilCcellꢀSO3H
Cu(0)ꢀSilCchitꢀSO3H
1639
1643
1645
1648
aFTIR was recorded on PerkinꢀElmer FTIR spectrophotometer using KBr
discs.
maltose, Cu(0)ꢀSilCcellꢀSO3H from cellulose, Cu(0)ꢀSilCchit
ꢀ
SO3H from chitosan) showed almost similar FTIR spectra.
The major absorption frequencies are shown in Table 1 and
the FTIR spectra of Cu(0)ꢀSilCcellꢀSO3H is shown in Fig. 1.
Fig. 1 FTIR spectrum of Cu(0)ꢀSilCcellꢀSO3H.
Fig. 1 FTIR spectrum of Cu(0)ꢀSilCcellꢀSO3H.
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The thermal stability of Cu(0)ꢀSilCꢀSO3H was examined by determined by CHNS analysis (Table 3). The results
o
TGA in the temperature range of 30 to 800 C under a static indicated that Cu(0)ꢀSilCstarꢀSO3H contained 0.89 wt%,
atmosphere of nitrogen. The major weight losses of different Cu(0)ꢀSilCgluꢀSO3H contained 0.81 wt%, Cu(0)ꢀSilCmal
ꢀ
Cu(0)ꢀSilCꢀSO3H catalysts are represented in Table 2. The SO3H contained 0.58 wt%, Cu(0)ꢀSilCcellꢀSO3H contained
TGA curve of Cu(0)ꢀSilCcellꢀSO3H (Fig. 2) showed an initial 1.2 wt% and Cu(0)ꢀSilCcellꢀSO3H contained 0.98 wt% of
weight loss below 110 oC which can be ascribed to the sulfur, equivalent to SO3H loading of 0.28, 0.25, 0.18, 0.32
evaporation of water and solvent molecules present in the and 0.30 mmole per gram of the catalyst. Further, the acid
composite. The second weight loss appeared between 310ꢀ densities and the amount of C, H and S present in Cu(0)ꢀSilCꢀ
350 °C, which may be attributed to the decomposition of SO3H as determined by elemental analysis is also presented in
sulfonic acid moieties present in Cu(0)ꢀSilCcellꢀSO3H. Further Table 3. Among the different Cu(0)ꢀSilCꢀSO3H catalysts,
weight loss from 350 to 800 °C was attributed to the bulk Cu(0)ꢀSilCcellꢀSO3H exhibited the highest SO3H density. The
pyrolysis of silica/cellulose skeleton.
most active Cu(0)ꢀSilCcellꢀSO3H was further characterized by
SEM, TEM, XRD, EDX and XPS analysis.
Table 2. Thermogravimetric analysis representing major weight
losses in Cu(0)ꢀSilCꢀSO3Ha
Table 3. Acid densities and amount of C, H and S present in
Cu(0)ꢀSilCꢀSO3Ha
Loss of
residual
Loss of organic
functionality
(oC)
Entry
Catalyst
-SO3H
S
C
H
solvent (oC)
Entry
Catalyst
(mmol
g-1)
(wt
%)
(wt
%)
(wt
%)
1
2
3
4
5
Cu(0)ꢀSilCstarꢀSO3H
Cu(0)ꢀSilCgluꢀSO3H
Cu(0)ꢀSilCmalꢀSO3H
Cu(0)-SilCcell-SO3H
Cu(0)ꢀSilCchitꢀSO3H
100
100
110
108
105
325ꢀ800
318ꢀ670
315ꢀ670
320-800
322ꢀ800
1
2
3
4
5
Cu(0)ꢀSilCstarꢀSO3H
Cu(0)ꢀSilCgluꢀSO3H
Cu(0)ꢀSilCmalꢀSO3H
Cu(0)-SilCcell-SO3H
Cu(0)ꢀSilCchitꢀSO3H
0.28
0.25
0.18
0.32
0.30
0.89 18.2 1.2
0.81 19.1 1.3
0.58 20.4 1.7
1.2
19.6 1.5
aThermal analysis was carried out at a heating rate of 10 oC/min.
0.98 21.5 1.6
aDetermined by elemental analysis.
The shape and surface morphology of the catalyst was
investigated by scanning electron microscopy (Fig. 3). The
SEM micrographs of Cu(0)ꢀSilCcellꢀSO3H showed floccules
like structure with quasi spherical morphology. On the other
hand, the surface of the composite was nonꢀsmooth depicting
its amorphous nature and thus resulted in an increase in its
surface area.
To provide more accurate information on the particle size and
fine structure of Cu(0)ꢀSilCcellꢀSO3H, transmission electron
microscopy (TEM) was carried out. TEM micrographs
revealed the uniform formation of Cu(0) nanoflowers onto the
surface of sulfonated SilCcell material (Fig. 4a). The
sulfonated carbon material appears as light grey particles over
Fig. 2 TGA curve of Cu(0)ꢀSilCcellꢀSO3H.
The amount of sulfonic acid groups grafted onto the surface
of Cu(0) based sulfonated silica/carbon composites was
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patterns of amorphous carbon composed of polycyclic
aromatic carbon sheets oriented in a random manner.29
Fig. 3 SEM images of Cu(0)ꢀSilCcellꢀSO3H.
which black coloured Cu(0) nanorods combined to form
nanoflowers, which can be easily visualised (Fig. 4b). The
average diameter of the Cu(0) nanorods as determined from
TEM analysis was found to be 28 nm (Fig. 4c).
Fig. 5 XRD pattern of Cu(0)ꢀSilCcellꢀSO3H.
The elemental composition of Cu(0)ꢀSilCcellꢀSO3H was
determined by energy dispersive XꢀRay (EDX) analysis. The
EDX spectrum, displayed in Fig. 6, clearly reveals the
presence of C, Si, O, S and Cu present in Cu(0)ꢀSilCcellꢀSO3H.
Further, the weight percentage of Cu in Cu(0)ꢀSilCcellꢀSO3H
was determined by inductively coupled plasma atomic
emission spectroscopy (ICPꢀAES). The results indicated that
Cu content loaded onto Cu(0)ꢀSilCcellꢀSO3H was 3.2 wt%.
Fig. 6 EDX of Cu(0)ꢀSilCcellꢀSO3H.
The electron properties of the species formed on the surface
of Cu(0)ꢀSilCcellꢀSO3H, such as the oxidation state, and the
binding energy of the core electron of the copper metal were
analyzed using XPS. Fig. 7a showed the overall survey
spectrum of Cu(0)ꢀSilCcellꢀSO3H in which peaks
corresponding to carbon 1s (284.9 eV), oxygen 1s (532.2 eV),
silica 2p (105.3 eV) and copper 2p (932.7 eV) are clearly
observed. In addition, the survey spectrum of the catalyst
showed a singlet at around 168 eV for sulphur 2p, which is
associated with SO3H groups.30 Fig. 7b showed the typical
Cu(0) absorptions at 932.7 and 952.1 eV for 2p3/2 and 2p5/2
Fig. 4 TEM images of TGA of Cu(0)ꢀSilCcellꢀSO3H.
The XRD pattern of Cu(0)ꢀSilCCellꢀSO3H exhibited diffraction
peaks corresponding to faceꢀcentered cubic (fcc) Cu.28 Fig. 5
showed three reflection patterns at 2θ = 43.4, 50.5, and 74.2°
which can be indexed as [111], [200] and [220] planes of
copper. All the diffraction peak values along with the dꢀ
spacings match those in standard XRD data for copper
(JCPDS 04ꢀ0836).28 These results indicated that copper metal
in Cu(0)ꢀSilCcellꢀSO3H was present in Cu(0) state and no
peaks of impurity were detected. In addition, a broad
diffraction peak at 2θ = 15ꢀ29° was attributed to the reflection
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respectively, which are consistent with the literature values based sulfonated silica/carbon composite. The high catalytic
for Cu(0).31
activity of Cu(0)ꢀSilCcellꢀSO3H might be due to the fact that
cellulose acts as a nanoreactor for metal nanoparticles32 and
as a result it stabilizes the metal nanoparticles more
effectively. The intraꢀchain hydrogen bonding between
hydroxyl groups and oxygen of the adjoining ring molecules
stabilizes the linkage and results in the linear configuration of
the cellulose chain. It also contains microfibrils with up to 30
nm width that are three dimensionally connected to each
other. The metal nanoparticles can be stabilized in the cavities
of those microfibrils via oxygenꢀmetal electrostatic
interaction, which make cellulose an effective support for the
metal nanoparticles stabilization.32
Table 4. Effect of the Cu(0)ꢀSilCꢀSO3H on the synthesis of
substituted pyridines and nitriles
Entry
Catalyst
Pyridinesa
Nitrilesb
Time Yieldc
(h) (%)
Time Yieldd
(h) (%)
1
2
3
4
5
Cu(0)ꢀSilCstarꢀSO3H
Cu(0)ꢀSilCgluꢀSO3H
Cu(0)ꢀSilCmalꢀSO3H
Cu(0)-SilCcell-SO3H
Cu(0)ꢀSilCchitꢀSO3H
3.5 90
4.5
5
91
86
85
Fig. 7 XPS of Cu(0)ꢀSilCcellꢀSO3H; overall survey spectrum (a), Cu 2p core
level spectrum (b).
4
5
82
80
5
Catalytic testing for the synthesis of polysubstituted
pyridines
2.5 92
90
3.5 93
92
3
4
The catalytic activity of Cu(0)ꢀSilCꢀSO3H was studied for the
synthesis of polysubstituted pyridines via oneꢀpot
condensation of aldehydes, ethyl acetoacetate, malononitrile
aReaction conditions: benzaldehyde (1 mmol), malononitrile (1 mmol), ethyl
acetoacetate (1 mmol) and aniline (1 mmol) and catalyst (0.13 mol% Cu ) at
100 oC under solventꢀfree conditions.
and amines at 100 oC under solventꢀfree conditions. To select bReaction conditions: benzaldehyde (1 mmol), 25% NH3(aq.) (5 mL), 30%
H2O2 (0.1 mL, 2 mmol) and catalyst (0.19 mol% Cu) at 100 oC.
the appropriate Cu(0) based sulfonic acid functionalized
silica/carbon composite, benzaldehyde, malononitrile, ethyl
acetoacetate and aniline were selected as the model substrates
and the reaction was carried out in the presence of Cu(0)ꢀ
SilCstarꢀSO3H, Cu(0)ꢀSilCgluꢀSO3H, Cu(0)ꢀSilCmalꢀSO3H,
Cu(0)ꢀSilCcellꢀSO3H and Cu(0)ꢀSilCchitꢀSO3H at 100 oC under
solventꢀfree conditions (Table 4). It was found that among
the different catalysts screened, Cu(0)ꢀSilCcellꢀSO3H was the
most effective catalyst in terms of reaction time and yield
(entry 4, Table 4). Thus, out of various natural organic
compounds (starch, glucose, maltose, cellulose and chitosan),
cellulose could be the best choice for the preparation of Cu(0)
cIsolated Yields.
dColumn chromatography yields.
Further, to select the appropriate catalyst amount, the model
reaction was also examined by varying the catalyst amounts
such as 0.5 g (0.06 mol% Cu), 0.1 g (0.13 mol% Cu), 0.15 g
(0.19 mol% Cu) and 0.2 g (0.26 mol% Cu). It was found that
0.1 g (0.13 mol% Cu) of Cu(0)ꢀSilCcellꢀSO3H was sufficient
to get the optimum product yield, while further increase in
the catalyst amount did not increase the yield significantly
(entries 8, 10, 11, 12, Table 5). Furthermore, to optimize the
reaction conditions with respect to different solvents and
reaction temperature, the model reaction was examined in
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different solvents such as toluene, acetonitrile, water, ethanol examined and a wide range of pyridine derivatives were
as well as under solventꢀfree conditions. It was found that prepared. The results are summarized in Table 6. Among
higher product yield was obtained when the reaction was various aromatic aldehydes studied, aldehyde possessing
carried out under solventꢀfree conditions in comparison to electronꢀwithdrawing group such as ꢀNO2 gave higher yields
liquid phase conditions (Table 5). The maximum catalytic (product 5g, Table 6) compared to those possessing electronꢀ
activity under solventꢀfree conditions may be attributed to the donating groups (ꢀOCH3, ꢀCH3) (products 5b, 5c, Table 6).
good dispersion of active reagent sites which facilitates better It is pertinent to mention that the positions of these
contact between reactants and the catalyst.33 However, due to substituents had an apparent effect on the reaction. When
the absence of any solvent (as medium), there is no dilution aromatic aldehyde bearing ortho substituted electron
effect and the heat needed for energy of activation is directly withdrawing group (ꢀNO2) was used, a comparatively lower
available to the reactant molecules. Further, to select the yield was obtained (product 5f, Table 6). Moreover, the
optimum reaction temperature, the model reaction was carried reaction with heteroaromatic aldehyde also proceeded well
o
out under solventꢀfree conditions at 80, 90, 100 and 120 C, and gave the corresponding product in good yield (product
and 100 oC was choosen as the optimum reaction temperature 5i, Table 6). Among various substituted aromatic amines, the
in view of the product yield and reaction time (entry 8, Table anilines with electronꢀwithdrawing as well as electron
5).
donating groups also worked well and gave good yields
(product 5j-5o, Table 6). Therefore, the present
methodology offers wider substrate scope for the synthesis of
polysubstituted pyridines under the optimized reaction
conditions.
Table 5. Effect of different solvents, temperature and catalyst
amount on Cu(0)ꢀSilCcellꢀSO3H catalyzed oneꢀpot synthesis of
polysubstituted pyridinesa
Entry
Solvent
Temp.
(oC)
Catalyst
(g)
Time
(h)
Yield
(%)b
The mechanisms of Bronsted acid catalysis as well as the
synergism between metal and Bronsted acid in the field of
organic synthesis have been discussed in detail in the
comprehensive review by Rueping et al.24,34 Accordingly, a
probable mechanism has been proposed (Scheme 2) for the
synthesis of polysubstituted pyridines catalyzed by the
bifunctional Cu(0)ꢀSilCcellꢀSO3H. Initially, aromatic aldehyde
and malononitrile undergo Knoevenagel condensation to give
I. Then, the Michael addition of ethyl acetoacetate to
intermediate I gave intermediate II. Subsequently, aromatic
amine reacts with II to form intermediate III. An
intramolecular cyclization of III (via loss of H2O molecule)
followed by isomerization to give intermediate IV. Finally
oxidative aromatization of IV in the presence of catalyst gave
the final product V. Overall, four new bonds were formed in a
oneꢀpot process to give the polysubstituted pyridines.
1
2
Ethanol
Methanol
reflux
reflux
reflux
reflux
reflux
80
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.05
0.15
0.2
4
5
86
80
45c
55c
35c
85
88
92
92
70
92
92
3
Acetonitrile
Water
7.5
7.5
10
4
5
Toluene
6
Solventꢀfree
Solventꢀfree
Solvent-free
Solventꢀfree
Solventꢀfree
Solventꢀfree
Solventꢀfree
3.5
3.25
2.5
2.5
4
7
90
8
100
9
120
10
11
12
100
100
2.5
100
2.5
aReaction conditions: benzaldehyde (1 mmol), malononitrile (1 mmol), ethyl
acetoacetate (1 mmol) and aniline (1 mmol) at 100 oC under solventꢀfree
conditions.
bIsolated Yield.
cColumn chromatography yields.
After optimizing the reaction conditions, the substrate scope
with respect to various aldehydes and aromatic amines was
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Table 6. Cu(0)ꢀSilCcellꢀSO3H catalyzed oneꢀpot synthesis of polysubstituted pyridines in solventꢀfree conditionsa
R1
R1
CN
O
CN
CHO
1
Cu(0)ꢀSilCcellꢀSO3H
Solvent free, 100 oC
CN
R2
EtO
2
+
R2
O
O
N
N
H
H2N
OEt
3
5
4
Br
OCH3
CH3
O
O
O
O
CN
CN
CN
CN
EtO
EtO
EtO
EtO
N
N
N
N
H
N
N
H
N
N
H
H
5b: 3.5 h, 86%
5c: 3 h, 87%
5d: 3 h, 86%
5a: 2.5 h, 92%
Cl
NO2
NO2
O
O
O
NO2
O
CN
CN
CN
CN
EtO
EtO
EtO
EtO
N
N
H
N
N
H
N
N
H
N
N
H
5h: 3 h, 90%
5f: 3.5 h, 84%
5e: 2.5 h, 90%
5g: 2.5 h, 93%
CH3
NO2
S
O
O
O
O
CN
F
CN
CN
F
CN
F
EtO
EtO
EtO
EtO
N
N
N
N
N
N
H
N
N
H
H
H
5j: 2.5 h, 92%
5k: 3.5 h, 88%
5i: 3.5 h, 86%
5l: 2.5 h, 92%
Br
NO2
CN
O
O
O
F
CN
CH3
CN
CH3
EtO
EtO
EtO
N
N
H
N
N
H
N
N
H
5n: 3 h, 88%
5o: 3 h, 90%
5m: 2.5 h, 94%
aReaction conditions: aldehyde (1 mmol), malononitrile (1 mmol), ethyl acetoacetate (1 mmol) and amine (1 mmol) and Cu(0)ꢀSilCcellꢀSO3H (0.1 g, 0.13
mol% Cu) at 100 oC under solventꢀfree conditions.
bIsolated yield.
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O
O
when the reaction was carried out in the presence of different
oxidants, it was observed that molecular oxygen (O2) gave the
poorest yield (entry 2, Table 7) and the best results were
obtained with H2O2 in terms of time and yield (entry 3, Table
7). Although, good results were obtained with TBHP as well,
however, it has led to decrease in selectivity of the product
formation (entry 4, Table 7). The decrease in yield in case of
TBHP as oxidant may be due to the interaction of the
developed catalyst with tertꢀbutyl alcohol, a byꢀproduct
formed from the decomposition of TBHP which leads to
decrease in activity of the catalyst. However, such kind of
catalyst deactivation or catalyst poisoning is limited using
H2O2 as oxidant and hence results in better conversion than
TBHP.35
Cu(0)
OEt
O
H
C
O
O
Knoevenagel
condensation
Cu(0)
C
O
N
CN
CN
CN
OEt
+
CN
CN
EtO
I
O
II
Cu(0)
Cu(0)ꢀSCcellꢀSO3H
H2N
+
HN
H
N
C
O
O
O
Cu(0)
ꢀH2O
CN
CN
CN
EtO
EtO
EtO
aromatization
N
N
H
N
N
H
O
H
IV
III
V
Scheme 2. Plausible mechanism for the Cu(0)ꢀSilCcellꢀSO3H catalyzed oneꢀ
pot synthesis of polysubstituted pyridines.
Catalytic testing for the synthesis of nitriles
Motivated by the impressive performance of Cu(0)ꢀSilCcell
ꢀ
Table 7. Effect of different oxidants and temperature on
Cu(0)ꢀSilCcellꢀSO3H catalyzed oxidative synthesis of nitriles
in aqueous ammonia at 100 oCa
SO3H for the synthesis of polysubstituted pyridines, we
further explored the catalytic activity of the Cu(0)ꢀSilCꢀSO3H
composites for the synthesis of nitriles. Initially, to optimize
the reaction conditions for the synthesis of nitriles,
benzaldehyde was choosen as the test substrate and the
reaction was carried out in the presence of different catalysts
using H2O2 in aqueous ammonia at 100 oC (Table 4). Again,
it was found that Cu(0)ꢀSilCcellꢀSO3H derived from cellulose
showed the best results in terms of time and yield (entry 4,
Table 4). Further, the effect of catalyst amount on the test
reaction was also examined by varying the catalyst amounts
and the results inferred that the 0.15 g (0.19 mol% Cu) of
Cu(0)ꢀSilCcellꢀSO3H was the sufficient amount required to get
the optimum product yield.
Entry
Oxidant
Temperature Time
Yield
(%)b
NRc
30
(oC)
100
100
100
100
R.T.
60
(h)
5
1
2
3
4
5
6
7
8
ꢀ
O2
5
H2O2
TBHP
H2O2
H2O2
H2O2
H2O2
3.5
3.5
5
93
80
45
4.5
4.0
3.5
85
80
90
120
93
aReaction conditions: benzaldehyde (1 mmol), 25% NH3(aq.) (5 mL), 30%
H2O2 (0.1 mL, 2 mmol) and Cu(0)ꢀSilCcellꢀSO3H (0.15 g, 0.19 mol % Cu) at
100 oC.
bColumn chromatography yields.
cNo reaction.
Furthermore, to optimize the reaction conditions with respect
to different oxidants and reaction temperature, the reaction
with test substrate was carried out under different set of
conditions to obtain the best possible combination. The effect
of various oxidants such as molecular oxygen (O2), Hydrogen
peroxide (H2O2) and TBHP in aqueous ammonia at
temperatures ranging from R.T. to 120 °C was explored with
test substrate (Table 7). Initially, when the reaction was
carried out in the absence of any oxidant at 100 oC, no
product formation took place (entry 1, Table 7). Further,
Further, to select the optimum reaction temperature, the
reaction with test substrate was carried out in aqueous
o
o
ammonia at R.T., 60, 80, 100 and 120 C, and 100 C was
choosen to be the optimum reaction temperature in view of
the product yield and reaction time (entry 3, Table 7). At
room temperature, the yield of the product obtained was very
poor, while the reaction at 60 and 80 oC was little slow, and at
o
120 C, no significant improvement in yield was observed
(entries 5-8, Table 7). With an optimized reaction conditions,
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we then prepared various substituted aryl nitriles to explore activation of H2O2 by coꢀordinating with Cu(0) to form Cu(I)
the substrate scope. As shown in Table 8, a variety of imine complex (II). Then, the activated oxygen of the coꢀ
substituted aldehydes bearing either electronꢀwithdrawing or ordinated H2O2 abstracts hydrogen from the imine to give the
electronꢀdonating groups were oxidatively transformed into final product ‘III’ and an intermediate ‘IV’. Intermediate
corresponding nitriles using H2O2 in aqueous ammonia at 100 ‘IV’ then loses molecule of water and Cu(0) is regenerated.
oC. Various functional groups such as methoxy, methyl,
H2O
Cu(0)
RCHO
bromo, chloro and nitro at ortho, meta or para position in
arylaldehyde were tolerated fairly well and afforded the
desired products in moderate to high yields (products 2b-2h,
Table 8). Further, thiopheneꢀ2ꢀaldehyde and naphthylꢀ1ꢀ
aldehyde were also successfully employed and corresponding
nitriles were obtained in good yields (products 2i-2j).
Cu(0)-SilCcell-SO3H
Cu(0)
H
II
Cu
OH
H
R
IV
O
H O
N
+ R
C
III
2
Aq. NH3
CuI
O
OH
Cu(0)
H
R
H
Table 8. Cu(0)ꢀSilCcellꢀSO3H catalyzed oxidative synthesis of
nitriles using H2O2 in aqueous ammonia at 100 oCa
H
R
H
N
N
II
I
O
H2O2
CN
H
Cu(0)ꢀSilCcellꢀSO3H
Scheme 3. Plausible mechanism for the Cu(0)ꢀSilCcellꢀSO3H catalyzed
NH3 (aq.), H2O2, 100 o
C
R
R
oxidative synthesis of nitriles.
1
2
Recyclability and heterogeneity
CN
CN
CN
In sustainable organic synthesis, the recyclability is an
important parameter. In order to investigate this parameter,
we carried out the recyclability studies of Cu(0)ꢀSilCcellꢀSO3H
in case of 5a, Table 6 and 2a, Table 8. After completion of
the reaction, the catalyst was recovered by simple filteration,
washed with distilled water, dried and reused for subsequent
runs. The results shown in Fig. 8 clearly demonstrate that the
catalyst is recyclable upto 6th run without significant loss of
activity. The heterogeneity of Cu(0)ꢀSilCcellꢀSO3H was further
tested by the hot filtration test to check any possibility of
leaching of Cu(0) metal from the catalyst surface. The
reaction in case of 2a, Table 8 has been carried out in the
presence of Cu(0)ꢀSilCcellꢀSO3H, until the conversion was
45% (1 h) after which the catalyst was filtered off at the
reaction temperature. The reaction was then continued further
under same set of conditions without the catalyst and found
that no significant conversion was observed. The ICPꢀAES
analysis of the used Cu(0)ꢀSilCcellꢀSO3H after 6th run showed
H3CO
H3C
2a: 3.5 h, 93%
2b: 4 h, 91%
2c: 4 h, 92%
CN
CN
CN
Br
Cl
Cl
2f: 4 h, 91%
2d: 3.5 h, 93%
2e: 3.25 h, 93%
CN
CN
O2N
NO2
2h: 4.5 h, 89%
2g: 3 h, 94%
CN
NC
S
2j: 4.5 h, 82%
2i: 4 h, 84%
aReaction conditions: aldehyde (1 mmol), 25% NH3(aq.) (5 mL), 30% H2O2
(0.1 mL, 2 mmol) and Cu(0)ꢀSilCcellꢀSO3H (0.15 g, 0.19 mol% Cu) at 100 oC.
bColumn chromatography yields.
A cyclic mechanism has been proposed (Scheme 3), which
involves the electrophilic activation of aldehyde by Cu(0)ꢀ
SilCcellꢀSO3H followed by the nucleophilic attack of aqueous
ammonia to form an aldimine (I). This step is followed by the
negligible loss of Cu metal from the surface of Cu(0)ꢀSilCcell
ꢀ
SO3H. Further, the amount of SO3H in the used catalyst after
6th run was determined by elemental analysis, showed 0.27
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mmol gꢀ1 of SO3H compared to 0.32 mmol gꢀ1 in the fresh nitriles up to 70% and 75 % respectively (entry 4, Table 9)
catalyst. Furthermore, the XPS analysis of the recovered which implies that both Cu(0) nanoparticles and SO3H
catalyst after six consecutive runs was also carried out, where bronsted acid sites are playing active role in catalysing the
no change of oxidation state of Cu(0) has been confirmed reaction. So, in order to investigate the dual role of the
(Fig. S1, see ESI). Thus, it can be concluded that the catalyst catalyst, we performed the desired reactions in the presence of
is heterogeneous in nature and Cu (0) nanoparticles are Cu(0)ꢀSilCcellꢀSO3H. As expected, the combined use of Cu(0)
efficiently stabilized over the support.
nanoparticles and SilCcellꢀSO3H gave excellent results and the
corresponding products were obtained in 92% and 93%
yields (entry 5, Table 9).
Table 9. Control experiments for the comparison of activity
of Cu(0)ꢀSilCcellꢀSO3H with other catalysts.
Entry
Catalyst
Pyridinesa
Time Yieldc
(h) (%)
Nitrilesb
Time Yieldd
(h) (%)
1
No catalyst
SilCcell
2.5
2.5 traces
NRe
3.5
3.5 traces
NRe
2
3
4
5
Fig. 8 Recyclability of Cu(0)ꢀSilCcellꢀSO3H: Reaction conditions [Pyridines]
:benzaldehyde (1 mmol), malononitrile (1 mmol), ethyl acetoacetate (1
mmol) and aniline (1 mmol), Cu(0)ꢀSilCcellꢀSO3H (0.1 g. 0.13 mol% Cu) at
100 oC under solventꢀfree condition for 2.5 h; [Nitriles]: benzaldehyde (1
mmol), 25% NH3(aq.) (5 mL), 30 % H2O2 (0.1 mL, 2 mmol) and Cu(0)ꢀ
SilCcellꢀSO3H (0.15 g, 0.19 mol % Cu) at 100 oC in 3.5 h.
SilCcellꢀSO3H
Cu(0)ꢀSCcell
2.5
2.5
2.5
65
70
92
3.5
3.5
3.5
60
75
93
Cu(0)ꢀSilCcellꢀSO3H
aReaction conditions: benzaldehyde (1 mmol), malononitrile (1 mmol), ethyl
acetoacetate (1 mmol) and aniline (1 mmol) and catalyst (0.1 g for entries 2
and 3; 0.13 mol% Cu for entries 4 and 5 ) at 100 oC under solventꢀfree
conditions.
bReaction conditions: benzaldehyde (1 mmol), 25% NH3(aq.) (5 mL), 30%
H2O2 (0.1 mL, 2 mmol) and catalyst (0.15 g for entries 2 and 3; 0.19 mol%
Cu for entries 4 and 5 )
cIsolated Yield.
dColumn chromatography yields.
eNo reaction.
Comparison of Cu(0)-SilCcell-SO3H with other catalysts
In order to demonstrate the role of Cu(0)ꢀSilCcellꢀSO3H as
bifunctional catalyst for the oneꢀpot multicomponent
synthesis of polysubstituted pyridines and for the oxidative
synthesis of nitriles, we performed some control experiments
in case of 5a and 2a (Table 6 and 8) and the results are
summarized in Table 9. Initially, no reactions took place
when they were carried out in the absence of catalyst (entry
1, Table 9). Further, reactions in the presence of nonꢀ
sulfonated silica/cellulose catalyst (SilCcell), again gave the
poor results and the corresponding products were obtained in
traces (entry 2, Table 9). However, when we carried out the
reactions in the presence of sulfonated silica/cellulose
(SilCcellꢀSO3H), it has led to significant increase in the yields
(entry 3, Table 9). But, since the yields obtained with
SilCcellꢀSO3H were not satisfactory, we performed the
reactions in the presence of Cu(0) immobilized nonꢀ
sulfonated silica/cellulose [Cu(0)ꢀSilCcell] which had also led
to increase in the yield of polysubstituted pyridines and
Further, in order to study the merits of the current protocol for
the synthesis of polysubstituted pyridines and nitriles, a
comparison of the efficacy of Cu(0)ꢀSilCcellꢀSO3H with some
of the reported catalytic systems in the literature was done.
The results presented in Table 10 clearly depicts that the
present method comparatively affords a truly green process
using benign reaction media along with higher product yields
in shorter reaction time. Further, it is noteworthy to mention
here that Cu(0)ꢀSilCcellꢀSO3H with lower catalyst loading
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Table 10. Comparison of the catalytic activity of Cu(0)ꢀSilCcellꢀSO3H with reported catalytic systems for the synthesis
of polysubstituted pyridines and nitriles
Reaction
Catalyst
Reaction Conditions
Time
(h)
Yield
(%)
Reference
Pyridinesa
FeCl3
EtOH, 70 oC, Catalyst (5 mol
%)
6
8
75
82
36
(Homogeneous)
SnCl2.2H2O
H2O, 60 oC, Catalyst (10 mol
%)
37
Present work
38
(Homogeneous)
Solvent-free, 100 oC, Catalyst
2.5
16
92c
95
Cu(0)-SilCcell-SO3H
(Heterogeneous)
Cu(OTf)2/Bipy
(
0.1 g, 0.13 mol% Cu)
Nitrilesb
OHꢀTEMPO, NH3 (aq), NaOH,
MeCN, 25 oC, Catalyst (10 mol
%)
(Homogeneous)
PolymerꢀPhI(OAc)2
(Heterogeneous)
Sodium dodecylsulfate (SDS),
aq NH4OAc, 70 °C, Catalyst
(1.0 g, 2 mmol).
4
90
39
NH3 (aq), H2O2, 100 oC,
Catalyst (0.15 g, 0.19 mol%
Cu)
3.5
93d
Present work
Cu(0)-SilCcell-SO3H
(Heterogeneous)
aReaction conditions: benzaldehyde (1 mmol), malononitrile (1 mmol), ethyl acetoacetate (1 mmol) and amine (1 mmol).
bReaction conditions: benzaldehyde (1 mmol).
cIsolated Yield.
dColumn chromatography Yield.
exhibits high activity in the synthesis of polysubstituted composite derived from cellulose was found to be most active
pyridines and nitriles under the same reaction conditions and selective under benign reaction media. Further, the
along with the added advantages of catalyst recovery and developed catalyst is recyclable upto 6 consecutive runs
reusability. In addition, the greenness of our protocol is also without significant loss in activity, and exhibits good
evaluated by employing the applications of green chemistry compatibility with a broad range of functional groups.
metrics (Eꢀfactor). The comparison of Eꢀfactor with several Meanwhile, this protocol may complement the existing
other catalyst systems explained the improved greenness of approaches to afford the synthesis of polysubstituted
the present protocol (Table S1, see ESI).
pyridines and nitriles, which are utilized as the key
intermediates in the synthesis of drugs and pharmaceuticals.
Conclusion
Experimental
In conclusion, we have developed bifunctional Cu(0) based
sulfonic acid functionalized silica/carbon composites and
their catalytic activities have been evaluated for the oneꢀpot
synthesis of polysubstituted pyridines and for the oxidative
transformation of aldehydes to nitriles. The bifunctional
General
All the chemicals and solvents used were purchased from
Aldrich Chemical Company or Merck. The H and 13C NMR
1
data were recorded in CDCl3 on Bruker Avance III (400 MHz
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and 100 MHz) and mass spectral data on Bruker Esquires successively washed with H2O (3×5 mL) and ethanol (3×5
3000 (ESI). The FTIR spectra were recorded on PerkinꢀElmer mL), and dried under vacuum at room temperature.
FTIR spectrophotometer. TGA was recorded on Linsesis STA
General procedure for the synthesis of polysubstituted
PTꢀ1000 make thermal analyzer. CHNS analysis was
pyridines
recorded on ThermoFinnigan FLASH EA 1112 series. SEM
To a mixture of aldehyde (1 mmol), ethyl acetoacetate (1
images were recorded using FEG SEM JSMꢀ7600F.
mmol), malononitrile (1 mmol), and amine (1 mmol) in a
Transmission Electron Micrographs (TEM) were recorded on
roundꢀbottom flask (25 mL), Cu(0)ꢀSilCcellꢀSO3H (0.1g, 0.13
Philips CMꢀ200. Xꢀray diffratograms (XRD) were recorded in
mol% Cu) was added, and the reaction mixture was stirred
2 theta range of 10ꢀ80o on a Bruker AXSDB Xꢀray
under solventꢀfree conditions at 100 °C for an appropriate
diffractometer using Cu Kα radiations. EDX analysis was
time (Table 6). After completion of the reaction (monitored
carried out using OXFORD XꢀMAX JSMꢀ7600 and the
by TLC), the reaction mixture was diluted with ethyl acetate
amount of metal in catalyst was determined by ICPꢀAES
and the catalyst was filtered off. The organic layer was
analysis using ARCOS from M/s Spectro, Germany. XPS
washed with water (3×10 mL) followed by brine solution (2 ×
spectra of the catalyst were recorded on a KRATOS ESCA
10 mL) and dried over anhyd. Na2SO4. Finally, the product
model AXIS 165 (Resolution).
was obtained after removal of the solvent under reduced
General procedure for the preparation of Cu(0)-SilC-
pressure followed by crystallization with EtOAcꢀpet. ether.
SO3H
The recovered catalyst was washed with EtOAc (3×10 mL)
Initially, silica/carbon composites were prepared by the followed by double distilled water (3×10 mL) and dried. It
partial carbonization of biomaterials (starch, glucose, maltose, was then reused for subsequent reactions.
cellulose and chitosan) in the presence of silica followed by
sulfonation using sulfonic acid.27 Typically, silica (2 g, K100,
General procedure for the oxidative synthesis of nitriles
from aldehydes
0.063ꢀ0.200 mm) and natural organic compound (2.4 g,
To a mixture of aldehyde (1 mmol), 25% aq. NH3 (5 mL) and
glucose, maltose, cellulose, chitosan or starch) in the ratio of
30 % H2O2 (mL, 2 mmol) in a roundꢀbottom flask (25 mL),
o
1:1.2 in a roundꢀbottom flask (100 mL) were dried at 80 C
Cu(0)ꢀSilCcellꢀSO3H (0.15 g, 0.19 mol% Cu) was added, and
for 6 h, followed by incomplete carbonization at 250 oC under
the reaction mixture was stirred at 100 °C for an appropriate
nitrogen atmosphere for 10 h. These were then sulfonated
time (Table 8). After completion of the reaction (monitored
using concentrated sulfuric acid (15 mL, >96 wt%) on heating
by TLC), the reaction mixture was diluted with ethyl acetate
at 150 oC for 6 h under N2 atmosphere. The composite
and the catalyst was filtered off. The organic layer was
material obtained was then washed repeatedly with hot
washed with water (3×10 mL) followed by brine solution
o
distilled water (>80 C) until sulfate anions were no longer
(2×10 mL) and dried over anhyd. Na2SO4. Finally, the
detected in the filtered water. Sulfonated carbon/silica
product was obtained after removal of the solvent under
o
composites were finally dried in an oven at 100 C for 2 h.
reduced pressure followed by passing through column of
For the preparation of Cu(0)ꢀSilCꢀSO3H, a mixture of
silica gel and elution with EtOAcꢀpet. ether. The recovered
aqueous CuCl2 solution (0.134 g, 1.0 mmol, 3 mL) was added
catalyst was washed with EtOAc (3×10 mL) followed by
to sulfonated silica/carbon composite (1 g) in ethanol (10 mL)
double distilled water (3×10 mL) which was dried and reused
and the reaction mixture was stirred at room temperature for 6
for subsequent reactions.
h. After that, aqueous solution of NaBH4 (1.2 mmol, 5 mL)
was added slowly to the above reaction mixture during 8 h.
Finally, the mixture was filtered, and the residue was
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11. M. A. Alibeik and A. Moaddeli, New J. Chem., 2015,
39, 2116.
Acknowledgements
We gratefully acknowledge the Head, SAIF, IIT Bombay for
SEM, TEM, EDX, CHNS and ICPꢀAES studies; SAIF,
Punjab University Chandigarh for XRD; IICT, Hyderabad for
XPS analysis. Financial assistance from UGC, New Delhi
(SRF to one of the authors, MB and major research project, F
41ꢀ281/2012 SR) is gratefully acknowledged.
12. P. R. Marques, A. L. Perez and A. Corma, Chem.
Commun., 2013, 49, 8160.
13. (a) M. Kidwai, N. K. Mishra, S. Bhardwaj, A. Jahan,
A. Kumar and S. Mozumdar, ChemCatChem, 2010, 2,
1312; (b) L. Ding, A. Tselev, J. Y. Wang, D. N.
Yuan, H. B. Chu, T. P. McNicholas, Y. Li and J. Liu,
Nano Lett., 2009, 9, 800; (c) N. N. Hoover, B. J.
Auten and B. D. Chandler, J. Phys. Chem. B, 2006,
110, 8606.
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Graphical Abstract
Cu(0) onto sulfonic acid functionalized silica/carbon composites as
bifunctional heterogeneous catalysts for the synthesis of polysubstituted
pyridines and nitriles under benign reaction media
Madhvi Bhardwaj, Manmeet Kour and Satya Paul*
Department of Chemistry, University of Jammu, Jammu 180006, India. Fax: +91-191-
2431365; Tel: +91-191-2453969; *Email: pual7@rediffmail.com.
A series of novel and sustainable bifunctional catalysts based on Cu(0) onto sulphonated
silica/carbon composites have been developed for the one-pot synthesis of polysubstituted
pyridines and for the oxidative transformation of aldehydes to nitriles.