Organosilane sulfonated graphene oxide in the Biginelli and Biginelli-like reactions

Article abstract of DOI:10.1039/c5nj01741f

Organosilane sulfonated graphene oxides (SSi-GO) have been synthesized by a two-step procedure involving the grafting of graphene oxide (GO) using 3-chloropropyltriethoxysilane (CCPTES) and subsequent oxidation using sulfanilic acid. It has been shown tha

Full text of DOI:10.1039/c5nj01741f

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Organosilane sulfonated graphene oxide in the
Biginelli and Biginelli-like reactions
Cite this:DOI: 10.1039/c5nj01741f
Javad Safari,* Soheila Gandomi-Ravandi and Samira Ashiri
Organosilane sulfonated graphene oxides (SSi-GO) have been synthesized by a two-step procedure
involving the grafting of graphene oxide (GO) using 3-chloropropyltriethoxysilane (CCPTES) and subsequent
oxidation using sulfanilic acid. It has been shown that organosilane sulfonated graphene oxide (SSi-GO)
exhibits a superior catalytic performance to produce pyrimidines in the Biginelli and Biginelli-like reactions.
This stronger acidity corresponds to the cooperative effects of the aryl sulfonic acid groups and other kinds
of acid sites (carboxylic acids). However, the acidic functionalities bonded to the SSi-GO surface are stable
under the catalytic reaction conditions resulting in its efficient reuse.
Received (in Montpellier, France)
7th July 2015,
Accepted 3rd November 2015
DOI: 10.1039/c5nj01741f
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composed of planar and graphene-like aromatic domains with
a hexagonal ring based carbon network in the chair configu-
Introduction
Graphite oxide (GtO) is a carbonaceous layered material con- ration bearing oxygen functional groups.11 GO is a single-layer
sisting of hydrophilic oxygenated graphene sheets (graphene of graphite oxide with a two-dimensional network of sp2- and
oxide sheets). Generally, bulk graphite oxide can be prepared by sp3-hybridized carbon atoms, while an ideal graphene sheet
the oxidative treatment of purified natural graphite powder consists of 100% sp2-bonded carbon atoms.12 The unique
using the modified Hummers method. This involved the use of electronic structure of graphene oxide (GO) is heterogeneous
strong concentrated oxidizing acids (HNO3 and H2SO4) and owing to the presence of mixed sp2 and sp3 hybridizations.13
strong oxidizing salts (potassium permanganate).1 Despite the The availability of oxygen-containing functional groups on the
retention of the layered structure, graphite oxide has much GO sheets allows them to be functionalized with a wide range
lighter color than graphite powder owing to the loss of electronic of organic and inorganic materials in covalent or non-covalent
conjugation afforded by the oxidation. Graphite oxides contain and/or ionic approaches.14 Thus, GO is a significant building
covalently attached oxygen-containing groups such as sp3- block to prepare various functional materials.15 These oxygen
hybridized carbons containing hydroxyl and epoxide functional functional groups afford mild acidic and oxidative properties
groups on above and below each sheet (the basal planes) as for GO,16 and further functionalization can make stronger acid
major components, and sp2-hybridized carbons containing sites on these carbons.17–19 Unique properties of graphene
carbonyl and carboxyl groups located at the edges of the basal oxide (GO) including the 2D structure, high stability and high
planes and hole defects as minor components.2–5 These oxygen surface areas make it a novel type of promising carbocatalysts
functionalities make GtO sheets strongly hydrophilic, therefore which their catalytic performance can be promoted with func-
the mild sonication of graphite oxide (GtO) in both aprotic polar tionalities to both sides of the carbon sheets.20 The function-
solvents and water results monolayer exfoliation to form homo- alized GO has great potential for applications in biosensing,21
geneous and stable aqueous dispersions containing sheets with drug delivery,22–24 bio-analysis,25 gene delivery and bioimaging,26
atomic thickness.6 Indeed, GtO consists of graphene oxide photothermal therapy,27 hydrogen storage,28 transparent film,29
sheets with both covalently bound oxygen and non-covalently high efficiency catalysis,30–32 electronics and optoelectronics,33
bound water between the carbon layers. Therefore, graphene and chemical and biochemical sensors.34 It is because of its
oxide – the oxygenated form of a monolayer graphene platelet – exclusive characteristics such as high mechanical strength,35
should be noticed as an amphiphile with a largely hydrophobic good water dispersibility,36 facile surface modification,37 and
basal plane (polyaromatic islands of unoxidized benzene rings) photoluminescence.38
and hydrophilic edges (–COOH groups).7–10 GO sheets are
A heterocyclic moiety is a key structure in many bioactive
natural and therapeutic products. Nitrogen heterocycles are
important branches of pharmacologically active substances and
they have been utilized as significant precursors in the synthesis
of novel drugs. Out of the five major bases in nucleic acids, three,
i.e., cytosine, uracil and thymine are pyrimidine derivatives
Laboratory of Organic Compound Research, Department of Organic Chemistry,
College of Chemistry, University of Kashan, P.O. Box: 87317-51167, Kashan,
Islamic Republic of Iran. E-mail: Safari@kashanu.ac.ir; Fax: +98-(0)31-5591-2397;
Tel: +98-(0)31-5591-2320
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which are found in DNA and RNA.39,40 Therefore, pyrimidines NMR spectra were recorded on a Bruker DRX-400 spectrometer
have become very important in the world of synthetic organic operated at 400 MHz using CDCl3 as a solvent and TMS as an
chemistry.41 3,4-Dihydropyrimidinones as biologically active interior standard. X-ray diffraction patterns were performed
compounds have been extensively used as drug-like scaffolds on a Holland Philips Xpert X-ray diffractometer with CuK
due to their pharmacological and therapeutic properties.42 Their radiation, (l = 0.154056 nm) at a scanning speed of 21 minÀ1
derivatives such as monastrol, enastron, piperastrol,43 amlodipine from 101 to 1001 (2y). Scanning electron microscopy (SEM) was
and nicardipine44 have been developed as drugs. Biginelli reaction performed on a Quanta 200 SEM operated at 20 kV accelerating
is one of the most efficient and straightforward procedures to voltage. The samples were prepared for SEM by spreading a
obtain the DHPMs including the acid-catalyzed three components small drop containing GO onto a silicon wafer and by drying it
condensation in one-pot. The efficiency of this process is greatly almost completely in air at room temperature for 2 h; it was
limited owing to strong acidic and harsh reaction conditions.45 then transferred onto a SEM conductive tape. The transferred
Thus, new modified routes have been developed to improve the sample was coated with a thin layer of gold before measurement.
efficiency of the Biginelli reaction in the presence of highly active, Sonication was performed in a Shanghai Branson-BUG40-06
stable, and friendly environmental catalysts.
ultrasonic cleaner (with a frequency of 35 kHz and a nominal
Herein, we report the highly efficient activity of organosilane power of 200 W). A circulating water bath (DC2006, Shanghai
sulfonated graphene oxide (SSi-GO) as an effective catalyst to Hengping Apparatus Factory) was adopted with an accuracy of
prepare pyrimidinones (Schemes 1 and 2). First, the sulfonated 0.1 K to keep constant the reaction temperature.
graphene oxide nanosheets are prepared through facile covalent
General procedure for the preparation of graphene oxide (GO)
functionalization with sulfanilic acid. The functionalized GO is
then used as a highly active, selective, reusable and stable catalyst The graphene oxide (GO) nanosheets were prepared from natural
to produce pyrimidinones. The carboxylic acid and sulfonic acid graphite powder using the modified Hummer’s method.1 In a
groups present in SSi-GO are potentially active sites for its typical synthesis process, graphite powder (3 g) and sodium
superior catalytic performance. However, the organosilane nitrate (NaNO3, 1 g) were slowly added to a solution of 98%
sulfonated graphene oxide has not yet been used as a hetero- H2SO4 (46 mL) while stirring in an ice bath at 0–5 1C (15 min).
geneous catalyst in the Biginelli reaction.
Under vigorous stirring, potassium permanganate (KMnO4, 6 g)
was slowly added to the suspension at 10–15 1C for 2 h. The
resulting mixture was stirred continuously at 35 1C for another
30 min. Subsequently, distilled water (138 mL) was slowly added
to the reaction vessel under vigorous stirring, which led to a
color change to yellow and kept the temperature in the range of
95–98 1C for 1 h. Then, the reaction mixture was further diluted
with warm distilled water (200 mL, 40 1C), followed by treated
with 30% hydrogen peroxide (H2O2, 18 mL). The resultant brown
solution was washed using 10% hydrochloric acid (HCl) solu-
tion, after that centrifuged for several times. The brown graphite
oxide was collected by filtration and dried under vacuum at 60 1C
for 24 h. To prepare graphene oxide (GO), 100 mg graphite oxide
was dispersed in distilled water (100 mL) and sonicated in an
ultrasonic bath cleaner (100 W) for hours to exfoliate graphitic
oxide until the solution became clear. Indeed, graphene oxide
(GO) in aqueous solution was generated by the oxidization and
subsequently exfoliation of graphite. Afterwards, the GO solution
was centrifuged for 10 min to remove any unexfoliated graphitic
oxide. The GO powder was obtained after drying in a vacuum
oven at 80 1C for 24 h. Scheme 3 briefly illustrates the used
procedure to synthesize GO.
Experimental
Materials and methods
Chemical reagents were purchased from Merck and Aldrich with
high purity and used as received without further purification. The
completion of reactions was checked by the TLC technique on
silica gel plates in the solvent system petroleum ether–EtOAc
(V/V = 7 : 3). Melting points (1C) were determined in an open-
glass capillary on an Electrothermal MK3 apparatus and are
uncorrected. A Nicolet Magna FT-IR 550 spectrophotometer was
used for recording the IR spectra using potassium bromide
pellets in the range of 400–4000 cmÀ1. The proton and carbon
Scheme 1 SSi-GO-catalyzed synthesis of dihydropyrimidinones.
General procedure for the preparation of organosilane
sulfonated graphene oxide (SSi-GO)
The process for the synthesis of organosilane sulfonated graphene
oxide from graphene oxide was carried in the following manner.
The functionalized, chemically converted graphene nanosheets
(SSi-GO) can be synthesized via the covalent interaction between
GO and sulfanilic acid. In a typical procedure, the functionalization
of GO nanosheets was performed using 3-chloropropyltriethoxy-
silane (CPTES) as the sulfonic acid functional group precursor.
Scheme 2 SSi-GO-catalyzed synthesis of diarylpyrimidinones.
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Scheme 3 Schematic model of the used procedure to synthesize graphene oxide.
The reaction was carried out in toluene at temperature of oven at 80 1C for 24 h. The synthesis of organosilane sulfonated
110 1C under reflux conditions for 24 h, with ratios of 3 : 1 : 2 graphene oxide was described in several steps in Scheme 4.
of toluene, GO and CPTES, respectively. Afterwards, the chloro
General procedure for the preparation of pyrimidinone
derivatives
groups were grafted onto the GO nanosheets to achieve chloro-
functionalized GO (ClSi-GO). The prepared samples were filtered
and washed three times with ethanol to remove the precursor The solution of aromatic aldehyde (1 mmol), ethyl acetoacetate
residue. Samples were then dried at temperature of 80 1C for 8 h. or acetophenone (1 mmol) and urea or thiourea (1.5 mmol) in
Finally, sulfanilic acid (1.5 g) was added to a mixture of ClSi-GO 5 ml of ethanol was stirred and refluxed at 80 1C in the presence
(1 g) and triethylamine (1.2 mL, 8.6 mmol) in 30 mL of toluene. of SSi-GO (0.1 g). The completion of the reaction was determined
The solution was continuously stirred at 110 1C under reflux on TLC plates using ethyl acetate/petroleum ether mixture as a
conditions for 48 h to produce SSi-GO. After the reaction, the mobile phase. After completion, the mixture was cooled to room
mixture was washed with toluene and chloroform for several temperature, then crushed ice was added to precipitate the
times and then collected by a centrifuge and dried in a vacuum product. The yellow solid precipitate was separated by filtration
Scheme 4 Schematic illustration to prepare organosilane sulfonated graphene oxide.
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Table 2 Optimization of amount of urea for the synthesis of DHPMs
through a Buckner funnel and washed with cold water to remove
excess of urea and dried under vacuum. In addition, it was
further purified by recrystallization from hot ethanol to afford
pure pyrimidinones. Then, the filtrate obtained was concentrated
under reduced pressure to recover the catalyst. The recovered
catalyst was washed with water, dried under a vacuum oven at
90 1C for about 3 h and reused for subsequent reaction.
Entry
Urea (mmol)
Yield (%)
1
2
3
4
1
1.5
2
60
94
92
90
2.5
Table 3 Optimization of reaction temperature for the synthesis of DHPMs
Entry
Temperature (1C)
Yield (%)
Results and discussion
1
2
3
4
5
r.t.
60
70
80
100
—
65
79
94
89
The preparation of graphene oxide nanosheets from graphite
powder via the Hummer’s method affords hydrophilic oxygen-
containing functional groups such as carboxyl, epoxy, and
hydroxyl on the surfaces of GO nanosheets, which can stabilize
the dispersion of these sheets in aqueous media.46 Graphene
oxide, which is introduced as oxidized graphene, was modified
by treatment with sulfanilic acid to afford sulfur-containing
acid groups onto the carbon surface.
reaction temperature ranging from 60 to 80 1C (entries 2–4), but
there was not much change when above 80 1C.
Next, the effect of various solvents on the improvement of
reaction was tested. Typically, solvents such as H2O, CH3CN,
DMF, EtOH and CH2Cl2 were selected for comparison. As shown,
the polar solvents such as acetonitrile, H2O and EtOH resulted
good yields, while low yield of the product was obtained using
DMF and CH2Cl2 (Table 4, entries 5 and 6). This may be attributed
to the better solubility of the starting materials in the polar
solvents. In comparison with aprotic solvents, a significant
increase in the yield of the product was obvioused in a shorter
time period when the model reaction was occurred under
solvent-free conditions. The highest yield of the product was
obtained in EtOH as a protic and polar solvent within 20 min,
but the reaction in other solvents afforded low yields in longer
reaction times.
The most important feature of the present reaction is the
recyclability of the catalyst. Subsequently, the recycling and
reusability performance of the SSi-GO nanocatalyst were inves-
tigated using the model reaction under the optimal conditions.
After the catalytic reaction, water was added and the recovered
catalyst was removed from the reaction mixture by filtration
and washed with ethanol, and dried under vacuum at 90 1C
for 3 h. The recovered catalyst was reused five times without
significant loss of catalytic activity. These results show that the
catalyst is very stable. This observation strongly confirms the
high recycling efficiency of the nanocatalyst which is a signifi-
cant property from the economical and environmental points
of view (Fig. 1).
Screening of reaction conditions
The catalytic activity of organosilane sulfonated graphene oxide
was investigated to produce dihydropyrimidinones. Initially, to
evaluate the effect of the catalyst, the reaction of benzaldehyde,
ethyl acetoacetate, and urea in the presence and absence of
organosilane sulfonated graphene oxide nanocatalyst under
reflux conditions was selected as a model reaction. The results
are summarized in Table 1. It could be seen that without any
catalysts no desirable DHPMs product was obtained even after
prolonging the reaction time (Table 1, entry 1). It indicated that
the catalyst is necessary for this reaction. The yield increased
linearly with increase in the amount of the catalyst. Hence,
0.1 g of the catalyst was used in the reaction. It should be noted
that the high catalytic activity of solid acid of SSi-GO was
attributed to the quantity and type of acidic groups and high
surface area. In other experiments, the same reaction was
carried out with different amounts of urea in the presence of the
same quantity of the catalyst (Table 2). Based on these results, the
yield increased with the addition of urea and the optimal amount
of urea was 1.5 mmol. Thus, the best result was obtained with
1.0 : 1.0 : 1.5 molar ratios of aldehyde, 1,3-dicarbonyl compounds,
urea or thiourea, and 0.1 g of SSi-GO.
The potency of the reaction was influenced by temperature.
It was indicated that the reaction did not proceed at room tem-
perature (Table 3, entry 1). Furthermore, it was observed that
the yield of the reaction increased with the increasing of the
The results were tabulated to compare the efficiency of the
present catalyst with some of the reported catalysts for the
Table 1 The effect of various amounts of the catalyst for Biginelli reaction
Table 4 Effect of various solvents on the Biginelli reaction
Entry
Catalyst (g)
Time (min)
Yield (%)
Entry
Solvent
Time (min)
Yield (%)
1
2
3
4
5
—
50
40
20
25
20
—
83
94
92
94
1
2
3
4
5
6
Solvent-free
Water
EtOH
CH3CN
DMF
CH2Cl2
35
40
20
30
35
40
80
84
94
89
65
60
0.05
0.1
0.2
0.3
The yield was calculated according to the limiting factor (aldehyde).
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Table 7 Synthesis of diarylpyrimidin-2(1H)-ones in the presence of the
SSi-GO catalyst under reflux conditions
Mp (1C)
Yield
Entry Aldehyde
Ketone
X
Product Obs.
Lit.
(%)
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
H
H
H
H
O
O
O
O
O
O
O
O
O
O
6a
6b
6c
6e
6f
6g
6h
6i
229–231 228–23065 89
245–246 243–24565 90
258–259 257–25865 93
268–269 267–26966 98
249–251 248–25065 87
260–261 259–26165 92
265–267 266–26765 89
256–258 257–25866 77
270–272 271–27465 83
263–264 264–26565 91
3,4-(OMe)2
3-OMe
4-Cl
4-Me
4-OMe
2-OMe
4-OH
2,4-Cl2
2-Cl
Fig. 1 Reusability of the catalyst for the synthesis of 4a.
Table 5 Comparative study using published methods
6j
6k
10
Time Yield
(min) (%) Ref.
Entry Catalyst
Condition
was also successfully used for acetophenone substrates and
corresponding adducts obtained in good yields. However,
ketones required a longer reaction time (Table 7). The products
were confirmed by comparing their melting points with authentic
1
2
3
PTA@MIL-101
Solvent free, 100 1C
Solvent free, 70 1C
Solvent free, reflux
60
50
75
90
96
88
84
91
93
97
92
92
94
47
48
49
50
51
52
53
54
Cu@PMO-IL
[TEBSA]HSO4
PPA
4
Solvent free, grinding 25
1
5
6
7
8
Nano-g-Fe2O3-SO3H Solvent-free, 60 1C
180
45
45
150
30
20
samples, FTIR and H NMR spectroscopies.
SiO2-BaCl2/SF
Mn@PMO-IL
SiO2-H2PO3
ErCl3
Solvent-free, 85 1C
Solvent free, 70 1C
Solvent-free, 60 1C
Solvent-free, 120 1C
Reflux, 80 1C
Structural characterization of the catalyst
9
55
This
study
The crystalline phases of graphite and graphite oxide samples
prepared were investigated by the X-ray diffraction (XRD)
patterns, as shown in Fig. 2. It can be seen in Fig. 2 that the
natural graphite presented the very strong diffraction peak at
2y = 26.01 with the (002) plane of graphite. The diffraction peak
at around 2y = 43.11 is related to the (100) plane of the hexagonal
structure of carbon.67 The spectrum of graphite oxide after
oxidation for 30 min exhibited the same peak but a little bit
weaker than raw graphite and another peak at 2y = 11.71
appeared. However, after oxidation for 45 min, the peak
becomes even weaker. It can be observed that after com-
plete oxidation, the sharp diffraction peak disappeared in
graphite nanosheets (2y = 26.01), and a new diffraction peak
10
SSi-GO
promotion of the synthesis of DHPMs. The present method was
more efficient according to Table 5.
To investigate the substrates scope, the reactions were
carried out using various aldehydes, 1,3-dicarbonyl compounds
(acetophenone), and urea (or thiourea) catalyzed by SSi-GO under
the optimal conditions. The results are shown in Tables 6 and 7.
The reactions proceeded very efficiently within relatively short
reaction time. The results illustrate that the type and position of
the substituent have no significant effect on the activity of the
catalyst and the reaction yield. These observations confirm
the high efficiency of the nanocatalyst to convert an extensive
range of aldehyde substrates to a series of structurally diverse
pyrimidinones in high purity. Additionally, thiourea was applied
instead of urea to successfully provide the corresponding
3,4-dihydropyrimidin-2(1H)-thiones in good yields (Table 6,
entries 10 and 11). It is essential to note that the methodology
Table 6 Synthesis of 3,4-dihydropyrimidin-2(1H)-one/thiones in the presence
of the SSi-GO catalyst under reflux conditions
Mp (1C)
Time Yield
Entry Aldehyde
X
Product Obs.
Lit.
(min) (%)
1
2
3
4
5
6
7
8
H
4-Cl
4-Me
2-OH
2-Cl
O
O
O
O
O
O
O
O
O
S
4a
4b
4c
4d
4e
4f
4g
4h
4i
200–202 200–20156 20
211–215 209–21157 20
213–216 212–21458 25
201–203 200–20259 25
215–218 216–21859 27
239–240 233–23560 30
215–217 215–21761 20
229–230 229–23162 23
230–232 230–23263 35
216–217 215–21662 30
197–199 199–20064 25
94
96
90
89
91
98
92
94
95
90
93
2-F
Thiophene
3-NO2
4-NMe2
Thiophene
H
9
10
11
Fig. 2 X-ray diffraction patterns of (a) graphite, (b) graphite oxide after
oxidation for 30 min and (c) graphite oxide after oxidation for 45 min.
4j
4k
S
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(2y = 11.71 with 0.8 nm d-spacing corresponds to the (001)
reflection) appeared in graphite oxide, indicating the damage
of the regular crystalline pattern of graphite during the oxida-
tion. The characteristic diffraction peak (001) of graphite oxide
nanosheets and its increased d-spacing is associated to intro-
duce oxygenated functional groups attached on both sides and
edges of carbon sheets as well as water molecules trapped in
the interlayer galleries of hydrophilic graphite oxide.
Fig. 3 shows typical scanning electron microscopy (SEM) images
of the GtO and GO. Indeed, SEM images show the structures
and morphology of graphite oxide before (Fig. 3a and b) and
after (Fig. 3c and d) exfoliation. Compared with graphite oxide
which is a laminar compound with layers stucking together,
graphene oxide exhibits a relatively rough surface and crumpling
features with clear layers. The atomic scale roughness arises
from structural defects (sp3 bonding) generated on the originally
atomically flat graphite oxide sheet. It can be identified that GO
nanosheets are relatively exfoliated and wrinkled and afforded
an increase in the distance between adjacent sheets and
reduction in interaction between sheets. This increased spacing
is considerably different depending on the amount of water
intercalated within the stacked-sheet structure.68 These factors
have potential advantages as the active sites, which can be
easily produced on both sides of the two dimensional graphene
oxide sheets.
The FTIR spectra of graphite, graphite oxide and organosilane
sulfonated graphene oxide were obtained to confirm the presence
of different functional groups on the graphene nanosheets
(Fig. 4). Graphite has two peaks at 3430 cmÀ1 (O–H stretching
vibrations due to adsorbed water) and 1610 cmÀ1 (aromatic
CQC, skeletal vibrations of graphitic domains). Large amounts
of oxygen-containing functional groups are found in the FTIR
spectrum of graphite oxide nanosheets. The strong absorption
bands at 3428, 2923, 1729, 1628, 1385, and 1121 cmÀ1 for
graphite oxide confirm the existence of –OH, C–H, CQO in
COOH, unoxidized graphitic skeletal domains (CQC aromatic)
and the adsorbed water molecules,69 carboxylic C–OH stretching
and C–O stretching vibration functional groups, respectively.70
These results are in good agreement with the structure and
morphology of graphite oxide and confirm the successful
oxidation of graphite. Functionalization with aryl SO3H does
not change the structure of graphite oxide, as shown in Fig. 4c.
The FTIR analysis in Fig. 4c indicates that aryl sulfonic acid
groups were successfully introduced into GO sheets. The strong
absorption peaks at 1159 and 1230 cmÀ1 are assigned to the
symmetric and asymmetric stretching vibrations of SQO in the
–SO3H group, respectively.71 SSi-GO also exhibits additional
bands at 1023 and 692 cmÀ1 which is indicated by S–phenyl
and S–O groups, confirming the presence of aryl SO3H groups
covalently bonded to the graphene sheet. A broad peak at
3228 cmÀ1 corresponded to –OH groups on the surface and
also –SO3H. Additionally, the band stretching from 2888 to
2924 cmÀ1 attributed to the presence of aromatic CH groups
and aliphatic C–H groups of the (CH2)3 chains in the CCPTES
sulfanilic acid precursor.72 The broad bands were also seen
Fig. 3 SEM images (a) and (b) graphite oxide, (c) and (d) graphene oxide
in the regions 3426 and 832 cmÀ1 which are related to aromatic (GO).
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Fig. 4 FT-IR spectra for (a) graphite, (b) graphite oxide, and (c) SSi-GO.
NH and OOP NH groups. The strong absorption peak at
1117 cmÀ1 attributed to the Si–O stretching vibrations.
Energy dispersive spectroscopy (EDS) reveals the presence of
–OSO3H functional groups on SSi-GO. EDX elemental analysis
shows that the element mass ratios of carbon, nitrogen, oxygen,
silicon and sulfur in organosilane sulfonated graphene oxide
are 58.32%, 17.42%, 20.79%, 0.39% and 3.09 wt%, respectively.
The elemental analysis of SSi-GO presented that the calculated
density of the sulfonic acid group on sulfonated graphene oxide is
0.96 mmol gÀ1 of –SO3H based on the sulfur percentage (3.09 wt%).
The O/S atom ratio (6.7 : 1) is higher than 3 : 1 owing to the presence
of residual epoxide, carboxyl and hydroxyl groups on the sulfonated
graphene oxide sheets (Fig. 5).
Fig. 6 Elemental distribution maps in the SSi-GO region shown by EDX.
were densely and homogeneously functionalized with sulfur,
reflecting the uniformity of the modification treatment.
Conclusions
In summary, organosilane functionalized graphene oxides (SSi-GO)
were successfully synthesized and well characterized. The function-
alized chemically modified organosilane graphene oxide was
simply prepared via covalent functionalization with a reactive
surfactant, sulfanilic acid. Hence, this study highlights that
SSi-GO can be a stable, active and efficient carbocatalyst to improve
the yield of pyrimidinones through one-pot multi-component
reaction of aromatic aldehydes, ethyl acetoacetate (or aromatic
ketones) and urea or thiourea under reflux conditions. The strongly
acidic aryl SO3H groups are responsible for the catalytic activity
and high stability of this solid acid catalyst. Moreover, the usage
of SSi-GO as a Brønsted acid and a reusable carbocatalyst can be
extended to other organic reactions.
Furthermore, the carbon, nitrogen, oxygen, silicon and
sulfur distribution maps (Fig. 6) show that the SSi-GO sheets
Acknowledgements
We thanks financial support for this work by the university of
Kashan Research Council (Grant no. 363022/21).
Fig. 5 The EDS spectrum of SSi-GO.
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23 L. Zhang, J. Xia, Q. Zhao, L. Liu and Z. Zhang, Small, 2010,
6, 537.
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