Graphene oxide-TiO2 composite: An efficient heterogeneous catalyst for the green synthesis of pyrazoles and pyridines

Article abstract of DOI:10.1039/c5nj03380b

A graphene oxide-TiO₂ composite (GO-TiO₂) has been synthesized and characterized using FT-IR, FT-Raman, XRD, XPS, FESEM, EDX, TEM and N₂ adsorption-desorption. GO-TiO₂ has been found to be a highly efficient and

Full text of DOI:10.1039/c5nj03380b

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DOI: 10.1039/C5NJ03380B
Journal Name
ARTICLE
Graphene oxide-TiO composites: an efficient heterogeneous
2
catalyst for the green synthesis of pyrazoles and pyridines†
a
b
a,
Shweta Kumari, Amiya Shekhar, and Devendra D. Pathak *
0Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Graphene oxide-TiO
Raman, XRD, XPS, FESEM, EDAX, TEM and N
to be highly efficient and recyclable heterogeneous catalyst for the synthesis of pyrazoles and pyridines
in aqueous medium at room temperature.
2
composites (GO-TiO
2
), has been synthesized and characterized by FT-IR, FT-
adsorption-desorption. The GO-TiO has been found
2
2
Introduction
basic conditions, moderate yields, use of expensive catalyst, tedious
Graphene oxide (GO), has aroused considerable interest, due to
owing to its high specific surface area, presence of various oxygen
functionalities, structural defects, high mechanical strength and
excellent electrical conductivity.1 The oxygen functionalities which
are placed in GO have been recognized as anchoring sites for
chemical functionalization, inorganic complexes and nanoparticles
steps, and formation of by-products. The importance of pyrazoles
and pyridines in medicinal chemistry has continually attracted for,
novel protocol.
18
Herein, we report the synthesis of Graphene oxide-TiO
2
composites
(
GO-TiO ), and demonstrate its application as, inexpensive, reusable
2
and efficient heterogeneous catalyst for the synthesis of pyrazoles
and pyridines derivatives in aqueous medium at room temperature in
high yields.
2
immobilization, etc. GO based materials have to found various
applications in nanoelectronics, sensors, super capacitors,
3
,4
semiconductor devices, and photo-catalysis. Recently GO and its
composites have been reported as catalysts for many useful organic
Experimental
5
transformations.
2
Titanium dioxide (TiO ) has been the most famous photocatalyst
Synthesis of GO-TiO
GO was synthesized by the modified Hummer’s method. The GO-
TiO composites was in situ synthesized based on Patra et al.’s
2
composites
6
under UV light. However, it has also been widely used in a variety
of applications such as energy conversion, pollutants degradation
1
9
2
7
and water splitting due to its low cost and high chemical stabilty. In
2
0
work with modifications. The composites was prepared by in situ
reaction of Ti(OiPr) and GO sheets. In brief, 100 mg GO was added
to a 10 mL of distilled water and sonicated for 15 min, and then add
.0 g ammonium chloride and 0.8 g sodium salicylate. The solution
was stirred for 30 min, then 2 mL ammonia solution was added and
the mixture stirred again for 30 min. Ti(OiPr) (5 mmol), was then
recent year much interests has been devoted for developing visible
4
light active TiO
2
by modify TiO
2
2
with nanostructured carbonaceous
8
materials. GO-TiO composites have been used as a heterogeneous
1
photocatalyst for the degradation of pollutants, the water
9
photocatalytic splitting, and antibacterial application. Hybridization
4
of GO-TiO
2
composites have not been much used in organic
taken in 2.5 g isopropyl alcohol and this solution was slowly added
to the GO solution. The pH of the solution was then adjusted to pH =
transformation as a heterogeneous catalysts. Recently Josephine et
2
al. have reported, GO-TiO used as a heterogeneous catalyst for
1
0 by addition of ammonia solution and stirred for 5 h. The mixture
esterification of benzoic acid with dimethyl carbonate.10
o
was hydrothermally heated to at 100 C for 48 h. The resultant solids
were collected by centrifuged. The resultant solid was calcined at
Synthesis of heterocyclic compounds using green approaches has
fascinated many chemists due to easy separable, environmental
friendly and cost-effective nature.11 Pyrazoles and pyridines are
important class of heterocyclic compounds.12 Pyrazole derivatives
are used in medicinal chemistry since they form the nucleus of many
commercially available drugs, for example Acomplia, Viagara,
Celebrex and Zometapina.13 In addition they have also been used as
ligands, chiral catalysts, synthesis of dyes, florescence and
7
2
73 K for 6 h to obtain the desired GO-TiO composites.
General procedure for the synthesis of pyrazoles derivatives
To equimolar ratios of aldehyde (1 mmol), malononitrile (1mmol),
and phenyl hydrazine (1 mmol), were dissolved in water (5 mL) at
room temperature, GO-TiO
mixture was continuously stirred for 10-20 min. The progress of the
reaction was monitored by TLC. After completion, GO-TiO catalyst
removed by filtration. The GO-TiO was further washed with
2
(10 mg) was added as catalyst. Reaction
1
4
luminescence. However, pyridine derivatives are used in anti-
bacterial, anti-prion, anti-hepatitis B virus, and anti-cancer agents.15
A perusal of the literature reveals that several methods reported for
2
2
ethanol (3x5 mL) and then dried under vacuum for reuse. Filtrate
was treated with ethyl acetate (3x10 mL). The combined organic
layers were treated with saturated brine solution and dried over
anhydrous sodium sulphate. The removal of solvent yielded (82-
2
the synthesis of pyrazole derivatives, such as Cu(OAC) , PTSA,
1
6
(
PyPyr)
the synthesis pyridine derivatives, such as ZrOCl
bmim]BF under ultrasound irradiation, Et N, DBU, piperidine,
KF/alumina, K CO /KMnO , ZnCl , silica nanoparticles, boric acid,
2
, Ti(NMe
2
)
2
, Sc(OTf)
3
, Yb(PFO)
3
, and H
2
SO
4
etc and for
2
·8H
2
O/NaNH in
2
[
4
3
9
8%) products. Furthermore, the synthesized pyrazoles products
2
3
4
2
1
were analyzed by H NMR and IR spectra.
1
7
nano MgO Zn(II) and Cd(II) MOFs, etc. However, many of these
reported synthetic protocols suffer from one or more disadvantages,
such as using toxic reagents, prolong reaction time, strong acidic or
Spectral data of selected pyrazoles products
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Journal Name
5
-Amino-1-phenyl-3-phenyl-1H-pyrazole-4-carbonitrile(4a):
IR spectra were recorded on
) δ = 6.80 (t, 1H, Spectrometer) in KBr pellet. The H NMDORI: 1s0p.e1c0t3r9a/Co5fNJo0r3g3a8n0iBc
ArH), 7.00 (d, 2H, ArH), 7.11 (t, 2H, ArH), 7.18 (t, 1H, ArH), 7.45 products were recorded in CDCl on a (Bruker's advance III 400/500
); IR (KBr, cm ): MHz FT-NMR) at room temperature.
a (Perkin-Elmer RXI FT-IR
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1
1
Light yellow solid: H NMR (400 MHz, CDCl
3
3
-1
(t, 2H, ArH), 7.69 (d, 2H, ArH), 7.80 (s, 2H, NH
2
3
5
291, 2366, 1605, 1250.
-amino-3-(2-hydroxyphenyl)-1-phenyl-1H-pyrazole-4-
Results and discussion
carbonitrile (4b):
Yellow solid: H NMR (400 MHz, CDCl
The GO-TiO
Patra et al.’s method. The chemical and structural features of GO
and GO-TiO were elucidated by FT-IR, FT-Raman, PXRD, XPS
FESEM, EDAX and TEM analyses.
2
composite was prepared by slight modification of the
1
3
) δ = 6.89 (t, 1H), 7.13 (d,
2
0
2
H, ArH), 7.21-7.31 (m, 3H, ArH), 7.34 (t, 1H, ArH), 7.47 (d, 1H,
2
2
ArH), 7.85 (d, 1H, ArH), 8.13 (s, 2H, NH ), 10.83 (s, 1H, OH).
5
-Amino-1-phenyl-3-(4-bromophenyl)-1H-pyrazole-4-
carbonitrile (4d):
Light pink solid: H NMR (400 MHz, CDCl
FT-IR spectra of GO and GO-TiO
The chemical changes that occurred during the immobilization of
TiO nanoparticles onto GO were followed by FT-IR. The FT-IR
spectra of GO, and GO-TiO are shown in Fig 1. The FT-IR
spectrum of GO (Fig. 1a) exhibited strong characteristics bands at
2
1
3
) δ = 6.72 (t, 1H, ArH),
7
.11 (d, 2H, ArH), 7.24 (t, 2H, ArH), 7.60 (q, 4H, ArH), 7.85 (s, 2H,
2
-1
2
NH ); IR (KBr, cm ): 3302, 2371, 1592, 1254.
2
5
-Amino-1-phenyl-3-(4-(trifluoromethyl)phenyl)-1H-pyrazole-4-
carbonitrile (4l):
Cream colored solid: H NMR (400 MHz, CDCl
ArH), 7.11 (d, 2H, ArH), 7.23 (t, 2H, ArH), 7.67 (d, 2H, ArH), 7.80
−1
1
035, 1223, 1623, 1731 and 3419 cm , due to the presence of
1
3
) δ = 6.81 (t, 1H,
carboxylic, hydroxyl, phenolic, carbonyl, epoxy, etc. functional
2
1
2
groups in the GO scaffold. The FT-IR spectrum of GO-TiO , was
-1
(d, 2H, ArH), 7.91 (s, 2H, NH
2
); IR (KBr, cm ): 3290, 2361, 1590,
found to be almost identical to the FT-IR spectrum of GO (Fig. 1b).
However, additional sharp peaks observed in FT-IR spectrum of
1
252.
General procedure for the synthesis of pyridines derivatives
A mixture of aromatic aldehydes (1 mmol), thiophenol (1 mmol) and
malononitrile (2 mmol) were dissolved in water (5 mL) at room
−1
GO-TiO
Ti vibrations.
2
at 564, 1002, 779 cm which attributed to Ti-O-Ti, C-O-
2
2
temperature, GO-TiO
mixture was continuously stirred for 1-2 h. The progress of the
reaction was monitored by TLC. After completion, GO-TiO catalyst
removed by filtration. The GO-TiO was further washed with
2
(20 mg) was added as catalyst. Reaction
2
2
ethanol (3x5 mL) and then dried under vacuum for reuse. Filtrate
was treated with ethyl acetate (3x10 mL). The combined organic
layers were treated with saturated brine solution and dried over
anhydrous sodium sulphate. The removal of solvent yielded crude
product, which was purified by chromatography over silica gel G-60
and afforded the desired products (79-89%).
The spectral data of selected synthesized pyridines derivatives are
given below:
2
Figure 1. FTIR spectra of (a) GO and (b) GO-TiO .
2
-Amino-6-(cyclohexa-2,4-dienylthio)-4-phenylpyridine-3,5-
dicarbonitrile (6a):
White solid: 1H NMR (400 MHz, CDCl
FT-Raman spectra of GO and GO-TiO
2
3
) δ = 7.00 (t, 1H, ArH),
The FT-Raman spectra of GO and GO-TiO
2
are shown in Fig. 2. The
show two remarkable
peaks at around 1350, and 1593 cm which were associated with the
7
.13 (d, 2H, ArH), 7.22 (t, 2H, ArH), 7.23 (t, 1H, ArH), 7.39 (d, 2H,
-1
FT-Raman spectrum of GO and GO-TiO
2
2
ArH), 7.48 (d, 2H, ArH), 7.80 (s, 2H, NH ); IR (KBr, cm ): 3450,
−1
3
2
332, 2220, 1642, 1560.
-Amino-4-(4-bromo-phenyl)-6-phenylsulfanyl-pyridine-3,5-
2
3
well-defined D band and G bands, respectively. In additionally,
-1
new peaks are observed at about 149, 407, 517, 630 cm which are
assigned to the Eg, B1g, A1g, and Eg vibrations modes respectively
dicarbonitrile (6b):
1
Yellow solid: H NMR (400 MHz, CDCl
3
) δ = 6.89 (t, 1H, ArH),
2
4
in FT-Raman of GO-TiO
found to be 0.83 and 0.88 for GO and GO-TiO
increase in the ID/IG ratio is indicative of increasing disorder in the
2
(Fig. 2b). The values of ID/IG were
7
.42(d, 2H, ArH), 7.55 (t, 2H, ArH), 7.60-7.65 (m, 2H, ArH) 7.79
-1
2
respectively. The
2
(d, 2H, ArH), 7.89 (s, 2H, NH ). IR (KBr, cm ): 3430, 3356, 2235,
1
650, 1565.
2
5
graphene oxide lattice after functionalization.
Characterizations
The synthesised GO and GO-TiO
ray diffraction (PANalytical Empyrean). Morphology of the GO, and
GO-TiO were observed by Transmission Electron Microscope
TEM Hitachi H-7500), Field Emission-Scanning Electron
Microscopy (FESEM, Supra 55 Carl Zeiss. Germany). The
composition of the GO and GO-TiO were determined by Energy-
2
was characterized by Powder X-
2
(
2
dispersive X-ray spectroscopy (EDAX) analysis on a (Electron
Probe Microscope Oxford X-MAXn). FT-Raman spectra of GO and
GO-TiO
Raman Spectrometer). Study the chemical state variations of the
GO-TiO composites, X-ray photoelectron (XPS) analysis on a PHI
000 Versa Probe II, FEI Inc. Surface area of the GO and GO-TiO
were measured by a Brunauer–Emmett–Teller (BET) surface area
2
was recorded on a (BRUKER RFS 27: Stand alone FT-
2
5
2
Figure 2. FT-Raman spectra of (a) GO and (b) GO-TiO2.
analyzer (Quantumchrome Instrument 3200 Nova E). Electronic Powder XRD spectra of GO and GO-TiO2
absorption spectral analysis was recorded on Shimadzu UV-1800 The PXRD of GO and GO-TiO are given in Fig. 3. The PXRD
2
Spectrophotometer in the wavelength range of 200-800 nm. The FT- exfoliated GO (Fig. 3a) exhibited a broad diffraction peak at 2θ =
2
| J. Name., 2012, 00, 1-3
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o
1
1.64 with a corresponding d spacing of 0.86 nm, which due to Micro-structural features and morphology analysis ofViGewOAraticnledOGnlOine-
presence of functionalities in the basal plane of GO along with TiO
absorbed water molecules causing increase in the interlayer Micro-structural features and morphology of GO and GO-TiO
2
DOI: 10.1039/C5NJ03380B
2
were
2
(Fig. 3b) exhibited new analyzed by FESEM (Fig. 5) and TEM (Fig. 6). The FESEM image
2
6
distance. The PXRD spectrum of GO-TiO
o
o
o
o
o
peaks at 2θ = value of 25.62 , 37.98 , 48.14 , 54.12 , 55.34 were of GO exhibited twisted nanosheets in disordered phase with a lot of
3
2
indexed to (101), (004), (200), (105) and (211) are readily indexed to wrinkles and crumpling features (Fig. 5a). In Fig. 5b depicts the
2
7
the anatase phase of TiO
in PXRD of GO-TiO , probably due to the disrupted layer-stacking sheets in FESEM image of GO-TiO
6a) revealed the nanoscopic features with few numbers of layers.
The Fig. 6b also depicted the fine nanostructures of the GO-TiO
with homogeneous distribution of TiO nanoparticles on the surface
of GO. The EDAX analysis provides detailed chemical compositions
on the GO and GO-TiO (Fig. 7). The GO was found to be mainly
composed of carbon and oxygen (Fig. 7a). The loading of TiO
2
.
The diffraction peak of GO disappeared uniform distribution of TiO
2
nanoparticles on the graphene oxide
2
2
. The TEM images of GO (Fig.
2
8
regularity.
2
,
2
2
2
nanoparticles on GO, changed the elemental composition with the
introduction of titanium, apart from carbon and oxygen (Fig. 7b).
Further, EDAX elemental mapping was performed to understand the
distribution of the TiO
elemental mapping indicated the uniform distribution of Titanium
over GO surface. The BET surface area of the GO and GO-TiO
determined by nitrogen physisorption at 77 K, was about 94.112 and
2
nanoparticles on GO surface (Fig. 8). The
2
,
-1
5
9.956 m²g respectively.
2
Figure 3. PXRD spectra of (a) GO and (b) GO-TiO .
X-ray photoelectron spectra of GO-TiO
In order to study the chemical state variations of the GO-TiO
2
aa
b
2
, X-ray
photoelectron (XPS) analysis was utilized. Fig. 4a showing survey
scan XPS spectrum demonstrates the presence of C, Ti and O species
in GO-TiO
2
composites. Ti2p core level spectrum in Fig. 4b exhibits
two bands located at binding energies 458.8 eV and 464.3
3
/2
1/2
4+
corresponding to Ti(2p ) and Ti(2p ) in the Ti chemical state,
2
9
respectively. The deconvoluted O 1s XPS spectrum of the GO-
TiO has been shown in Fig. 4 c. The binding energy of 530.4 and
32.2 eV are attributed to the O-Ti and HO-Ti bonds respectively
due to growth of TiO on the graphene oxide surface. The
2
2
Figure 5. FEEM image of (a) GO and (b) GO-TiO .
5
2
diminished peaks centered at the binding energies of 531.4 and 533.6
eV are assigned to the C=O and C-OH functional groups,
3
0
respectively. This is consistent with the deconvoluted C 1s spectra
Fig. 4d) showing weaker residual oxygen containing functional
(
group peaks compared to C=C , confirming the partial reduction of
the GO during the composite formation.31
2
Figure 6. TEM image of (a) GO and (b) GO-TiO .
a
b
2
Figure 7. EDAX of (a) GO and (b) GO-TiO .
2
Figure 4. X-ray photoelectron survey spectra of (a) GO-TiO (b) Ti 2p (c)
deconvoluted O 1s and (d) deconvoluted C 1s XPS spectra of GO-TiO .
2
Figure 8. Elemental mapping images of carbon, oxygen and titanium in GO-
TiO
2
.
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ARTICLE
Journal Name
Catalytic application of GO-TiO
The catalytic efficiency of GO-TiO
component reactions of aldehydes, malonitrile and phenyl hydrazine
shown in Scheme 1 and the three-component reactions of reaction of
aldehydes, malononitrile and thiophenol shown in Scheme 2.
2
3
4
5
6
7
8
9
10
TiO
2
(10)
06
0.2
0.2
0.2
0.2
0.2
0.2
0.2
Water
Water
Water
Ethanol
DCM
Acetonitrile
--
35
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GO-TiO
GO-TiO
GO-TiO
GO-TiO
GO-TiO
GO-TiO
GO-TiO
2
2
2
2
2
2
2
(10)
(20)
(10)
(10)
(10)
(10)
(10)
96
2
was explored in the three
DOI: 10.1039/C5NJ03380B
96
84
81
90
72
76
NH2
NC
CHO
DMSO
NHNH2
N
a
Reaction conditions: benzaldehyde (1mmol), malonitrile (1mmol), phenyl
hydrazine (1 mmol).
Isolated yield after work up.
GO-TiO2, (10mg)
N
+
NC
CN
+
Water, RT, 10-20 mim.
b
R
1
2
3
R
4
Table 2. Synthesis of pyrazoles derivatives from aldehydes, malonitrile and
phenyl hydrazine catalyzed by GO-TiO
2
Scheme 1. Synthesis of pyrazoles derivatives using GO-TiO
2
as catalyst.
NH2
R
NC
CHO
NHNH2
N
CHO
SH
NC
CN
GO-TiO2, (10mg)
N
+
NC
CN
+
GO-TiO2, (20mg)
Water, RT, 1-2 h
+
NC
CN
+
Water, RT, 10-20 mim.
R
H2N
N
S
1
2
R
4
3
R
1
2
5
6
NH2
NH2
NH2
Scheme 2. Synthesis of pyridines derivatives using GO-TiO
2
as catalyst.
NC
NC
NC
N
N
N
N
In order to optimize reaction conditions, various parameters such
as, time, solvents and catalyst were studied. Benzaldehyde,
malononitrile and phenyl hydrazine were chosen as model reactants
for the synthesis of pyrazole derivatives at room temperature. These
results are summarized in Table 1 When the reaction was carried out
in absence of catalyst (Table 1, entry 1), no product was formed even
after 08 h at room temperature. The same reaction in presence of 10
mg of GO in water (5 mL) at room temperature for 06 h yielded 30%
of product (Table 1, entry 2). When the same reaction was carried
N
N
OH
4
a, 96%
Cl
4
b, 98%
4c, 92%
NH2
NH2
N
NC
NC
N
N
N
Br
OHC
4
d, 91%
4e, 93%
NH2
out in the presence of TiO
product was yielded (Table 1, entry 3). However, when the reaction
was carried out in the presence of GO-TiO (10 mg) under room
2
(10 mg) in water for 06 h, 35% of
NH2
NH2
NC
NC
NC
N
N
N
N
2
Br
N
N
temperature in water, the desired product was obtained in 96% yield
within 12 minutes (Table 1, entry 4). However, no significant
improvement in the yield of the product was noticed when the
catalyst loading was increased from 10 to 20 mg (Table 1, entry 5).
It indicated that 10 mg of the catalyst was optimum. Among the
various solvents used such as, water, ethanol, dichloromethane,
acetonitrile, neat and dimethyl sulfoxide, water was found to be the
best medium for the synthesis of pyrazole (Table 1, entries 1-10).
The scope of the reaction was further extended to different aromatic
aldehydes with malononitrile and phenyl hydrazine under the
optimized reaction conditions and the products obtained, are
summarized in Table 2. The electronic effect introduced by the
substitution of the aromatic aldehyde had insignificant influence on
the yields of the corresponding pyrazole derivatives. The products
were obtained as white to dark yellow solid and characterized by FT-
O N
2
NO2
OH
4g, 93%
4h, 97%
4
f, 95%
NH2
NH2
NH2
NC
NC
NC
N
N
N
N
N
N
OH
COOH
OH
OH
OCH3
4
i, 90%
4j, 91%
4k, 90%
H2N
N
NH2
NH2
NC
N
NC
OH
NC
N
N
N
N
N
H
F3C
1
1
4l, 93%
4m, 86%
4n, 82%
IR and H NMR. All H NMR of the products exhibited aromatic
resonances between δ 6.71 to 7.89 and -NH resonance between δ
.85 to 8.33. The products, 5-amino-3-(2-hydroxyphenyl)-1-phenyl-
H-pyrazole-4-carbonitrile(4b) and 5-amino-3-(5-bromo-2-
2
7
1
In order to find out the optimum reaction conditions for the
hydroxyphenyl)-1-phenyl-1H-pyrazole-4-carbonitrile (4h) showed a
broad peak at δ 11.01 and 11.36 respectively, due to -OH group
attached to the aromatic ring. The proton of the -CHO group
attached to aromatic ring was observed as sharp singlet at δ 10.05 in
synthesis of pyridines derivatives benzaldehyde, malonitrile and
thiophenol were chosen as model reactants (Table 3). When the
reaction was carried out without catalyst (Table 3, entry 1), no
product was formed after 24 h. When (10 mg) GO as a catalyst
Table 3, entry 2) in water for 24 h at room temperature, no product
was obtained; only starting material was obtained as such. TiO
5
-amino-3-(4-formylphenyl)-1-phenyl-1H-pyrazole-4-carbonitrile
(
1
6
(4e).All data matches well with the literature values.
2
nanoparticles alone (10 mg) under identical conditions, afforded
product in 15% yield (Table 3, entry 3). However, when reaction
Table 1. Optimization of Reaction Conditions for the synthesis of pyrazole
derivatives from benzaldehyde, malonitrile and phenyl hydrazine at room
a
was carried out with 10 mg GO-TiO in water, at room temperature
temperature
2
the desired product was obtained in 72% yield within 2 h (Table 3,
Entry
Catalyst (mg)
Time
h)
Solvents
Yield, %b
entry 4) However, significant improvement in yield of the product
was noticed when the catalyst loading was increased from 10 to 20
mg (Table 3, entry 5). Further increase in amount of catalyst has no
(
1
2
---
GO (10)
08
06
Water
Water
--
30
4
| J. Name., 2012, 00, 1-3
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PNl ee aws eJ do ou rnn oa tl ao df j uC sht emm ai rs gt ir ny s
Journal Name
ARTICLE
Br
effect on the yield of the desired product (Table 3, entry 6). Among
the various solvents used such as DCM, acetonitrile, neat and
DMSO, water was found to be the best solvent for the reactions
View Article Online
DOI: 10.1039/C5NJ03380B
OH
(
Table 3, entries 1–10). After optimization of the reaction conditions,
NC
CN
S
NC
CN
S
NC
CN
we choose a variety of different aldehydes with malononitrile and
thiophenol to discern the catalytic scope and the generality of the
GO-TiO
withdrawing groups, afforded better results as compared to
aldehydes bearing electron donating groups. The products were
obtained as white to brownish solid and fully characterized by FT-IR
H2N
N
H2N
N
H N
2
N
S
6
a, 85%
6b, 83%
6c, 86%
2
(Table 4). Aryl aldehydes, possessing electron
OCH3
CHO
NO2
CN
1
1
and H NMR. All H NMR of the products exhibited aromatic
resonances between δ 6.89 to 7.90 and -NH resonance between δ
.80 to 7.98. The products, 2-amino-4-(2-hydroxyphenyl)-6-
phenylthio)pyridine-3,5-dicarbonitrile (6c), 2-amino-4-(4-
methoxyphenyl)-6-(phenylthio)pyridine-3,5-dicarbonitrile (6d), and
-amino-4-(4-formylphenyl)-6-(phenylthio)pyridine-3,5-
dicarbonitrile (6e) showed a peak at δ 11.01, 3.35 and 10.09 due to -
OH, -CH and -CHO group attached to aromatic ring, respectively.
NC
CN
S
NC
NC
CN
2
H2N
N
H2N
N
S
7
(
H2N
N
S
6
d, 87%
6f, 85%
CF3
6
e, 81%
Br
OH
2
OH
OH
CN
NC
2
CN
NC
NC
CN
3
1
7
These NMR data were in consonance with the earlier reports and
confirms the formation of the desired product.
H N
2
H N
N
S
H2N
N
S
N
S
6
g, 83%
6h, 79%
6i, 89%
Mechanistic considerations
Table 3. Optimization of Reaction Conditions for the synthesis of pyridine
derivatives from benzaldehyde, malonitrile and thiophenol at room The mechanistic aspect of the reaction is ambiguous at present,
a
temperature
Entry
however, a plausible mechanism involving an Ȯ 2 radical has been
proposed (Scheme 3). We speculate that the unpaired electrons
Catalyst (mg)
Time( h)
Solvents
Yield, %b
3
2-34
present at the edge of GO
generate, which subsequently reacts with malononitrile to generate
radical (I ). Condensation of an aldehyde with I leads to the
corresponding Knoevenagel product The thiophenol is
deprotonated by the Ȯ to form thiolate anion (I ). Next, the second
react with the atmospheric oxygen to
1
2
3
4
5
6
7
8
9
---
GO (10)
24
24
24
02
02
02
02
02
02
02
Water
Water
Water
Water
Water
Water
DCM
Acetonitrile
--
DMSO
--
--
1
1
TiO
2
(10)
15
72
85
85
80
79
65
71
2
I .
GO-TiO
GO-TiO
GO-TiO
GO-TiO
GO-TiO
GO-TiO
GO-TiO
2
2
2
2
2
2
2
(10)
(20)
(30)
(20)
(20)
(20)
(20)
2
3
molecule of malononitrile undergoes Michael addition followed by
¯
simultaneous thiolate addition to CN which undergoes oxidative
1
2b
aromatization to provide pyridine derivatives.
1
0
a
Reaction conditions: benzaldehyde (1mmol), malonitrile (2mmol),
thiophenol (1 mmol).
b
Isolated yield after work up.
Table 4. Synthesis of pyridines derivatives from aldehydes, malonitrile and
thiophenol catalyzed by GO-TiO
2
R
CHO
SH
NC
CN
GO-TiO2, (20mg)
Water, RT, 1-2 h
+
NC
CN
+
H2N
N
S
R
1
2
5
6
Scheme 3. Proposed reaction mechanism for the formation of pyridines
derivatives.
Recyclability test
Furthermore, the catalyst was tested for recyclability up to seven
times for the synthesis of pyrazole from benzaldehyde,
malononitrile, and phenyl hydrazine. It is apparent from (Fig. 9) that
the GO-TiO
2
catalyst could be reused up to five cycles without loss
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J. Name., 2013, 00, 1-3 | 5
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Page 6 of 8
ARTICLE
Journal Name
of catalytic activity. However, gradual decline in the catalytic
Jiang, Z. Tao, M. Ji, Q. Zhao, X. Fu and H. Yin, Catal. Commun., 2012,
View Article Online
th
28, 47-51.
activity of GO-TiO
2
was noted after 6 cycle.
DOI: 10.1039/C5NJ03380B
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Figure 9. Recyclability of the catalyst
Conclusions
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In summary, Graphene oxide-TiO
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XPS, FESEM, TEM, EDAX, and N adsorption–desorption. The
GO-TiO has been found to be an efficient heterogeneous catalyst
2 2
composites (GO-TiO ) has been
2
180-2189.
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Email: Shweta@ac.ism.ac.in (Shweta Kumari), amiyashekhar@gmail.com
Amiya Shekhar), ddpathak61@gmail.com (DD Pathak)*
Phone number: +91 9431126250
(
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We are thankful to the CRF ISM, SAIF IIT Madras, SAIF Panjab
University, STIC, Kochi, and IISER Bhopal for providing help in
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New Journal of Chemistry
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DOI: 10.1039/C5NJ03380B
Graphical Abstract
2
The GO-TiO has been found to be highly efficient and recyclable heterogeneous catalyst for the
synthesis of pyrazoles and pyridines in aqueous medium at room temperature.