Br?nsted acidic reduced graphene oxide as a sustainable carbocatalyst: A selective method for the synthesis of C-2-substituted benzimidazole

Article abstract of DOI:10.1039/c8nj03257b

Br?nsted acidic reduced graphene acts as an efficient and sustainable carbocatalyst for the selective synthesis of C-2-substituted benzimidazoles under ambient conditions. A massive influx of sulphonic acid group on reduced graphene oxide surface gives graphene sulfonic acid (G-SO₃H), which acts as a Br?nsted acidic catalyst for the synthesis of a series of benzimidazoles under mild conditions. The present methodology is a revamp of the benzimidazole synthesis with broad functional group tolerance in shorter time. G-SO₃H provides an operationally simple, metal-free condition and is amenable to gram-scale production. Pyridine adsorption studies prove the catalytically responsible Br?nsted acidity of the catalyst. The catalyst is highly stable for several cycles without any loss of activity, which is evidenced by the FT-IR, PXRD and TEM characterization of the reused catalyst.

Full text of DOI:10.1039/c8nj03257b

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

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DOI: 10.1039/C8NJ03257B
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Brønsted Acidic Reduced Graphene Oxide as a Sustainable
Carbocatalyst: A Selective Method for the Synthesis of C-2
Substituted Benzimidazole
Received 00th January 20xx,
Accepted 00th January 20xx
Murugan Karthik and Palaniswamy Suresh*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Brønsted acidic reduced graphene acts as an efficient and sustainable carbocatalyst for the selective synthesis of C-2-
substituted benzimidazoles under ambient conditions. A massive influx of sulphonic acid group on reduced graphene oxide
surface gives graphene sulfonic acid (G-SO3H) which act as a Brønsted acidic catalyst for synthesis of a series of
benzimidazole under mild conditions. Present methodology is a revamp of the benzimidazole synthesis with broad
functional group tolerance in a shorter time. G-SO3H provides an operationally simple, metal free condition and amenable
to the gram scale production. The pyridine adsorption studies prove the catalytically responsible Brønsted acidity of the
Catalyst. The catalyst is highly stable for several cycles without loss in its activity which evidenced by the FT-IR, PXRD and
TEM characterization of the reused catalyst.
lack of selectivity and high-temperature conditions which
adversely affect the eco-friendly nature and the economy of
the reaction. Hence it is necessary to develop a catalyst which
Introduction
Benzimidazole is a class of heterocyclic aromatic organic
compound. It starts glows after the discovery of naturally
occurring N-ribosyl dimethylbenzimidazole, which found in
vitamin B12.1 Benzimidazole scaffold offers a unique set of
properties for the designing of bioactive molecules used for
the various pharmaceutical requirement.2 Albeit a number of
methods are available for the synthesis of benzimidazole, the
two principal synthetic strategies such as (a) condensation of
will overcome all types of shortcomings present in the era.
Graphene the material pivotal in nanochemistry. It has a
wide range application in the fields of nanoelectronic devices,
sensors, adsorption, energy storage, and catalysis.23 Its unique
physical and chemical characteristics, including a very high
specific surface area and distinctive structure, renders an ideal
platform to covalently attach a large number of functional
groups with accessible active sites and make it as
indispensable in catalysis.24 The properties of graphene have
tuned by the chemical functionalization using active
intermediates like diazonium radical,25 carbene,26 nitrene,27and
aryn.28 In addition to that hydrothermal treatment also used
for the functionalization of graphene to enhance its properties.
By these processes graphene can be decorated with active
functional groups like amine,29 acids,30 thiols31 and thioether.32
Among graphene functionalized with sulfonic acid (-SO3H) has
received more attention and investigated as a solid catalyst for
a variety of acid catalyzed reactions such as hydrolysis of ethyl
acetate,33 and etherification of glycerol34, in addition, its
application expanded to photocatalysis,35 biosensors,36
electrochemical capacitors,37 pollutant management,38 and
support material for metal nanocomposite.39 In the present
study we explored the synthetic utility of graphene sulfonic
acid as a Brønsted acidic carbocatalyst towards the selective
synthesis of 2-substituted benzimidazole under ambient
condition.
o-arylenediamines with carboxylic acids3
and (b)
dehydrogenation of benzimidazoline intermediates generated
from the condensation of o-arylenediamines with aldehydes in
acidic medium4 are commonly adopted. However, the latter
one is most widely followed due to the ease of accessibility of
aryl aldehydes. In that context, various oxidative reagents and
catalyst such as TEMPO,5 Pb(OAc)4,6 Na2S2O5,7 DDQ,8 MnO2,9
oxone,10 and H2O211 have reported. Also, metal catalyst such as
CuBr,12 CuCl,13 and CuO supported silica,14 were reported.
Readily available Brønsted acids like p-toluenesulfonic
acid15 L-proline16 and various heterogeneous acidic catalyst
,
17
18
such as SiO2-H2SO4, xanthan-H2SO4, solid supported protic
acid,19 Scolecite,20 nanoceria,21 and SDS micelles,22 were
employed to set benzimidazole. However, these methods have
several shortcomings like the use of metal catalysts and
additives, longer time, tedious workup, purification process,
Supramolecular and Catalysis Lab, Dept. of Natural Products Chemistry,
School of Chemistry, Madurai Kamaraj University, Madurai-625021, India.
E-mail: ghemistry@gmail.com, suresh.chem@mkuniversity.org
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Results and discussion
In addition, the elemental mapping by energy dispersive X-ray
spectroscopy (EDS) (Fig.S5) also exhibits the uniform
distribution of sulphur atom on the surface of RGO sheets
which conclusively evident the sulfonation of GO. Finally, the
microstructures of G-SO3H nanosheets was characterized by
TEM analysis, (Fig.1f) which discloses the crumpled sheet-like
morphology, and had good agreement with the images
obtained in SEM. The SEM and TEM images collectively
visualize the sheet like surface morphology of G-SO3H. It
evident the sulfonation does not majorly alter the morphology
of RGO. All the above analytical techniques hands together
support the successful covalent crafting of benzene sulfonic
acid moiety on the reduced graphene oxide sheets.
The graphene sulfonic acid (G-SO3H, 1) used in this work was
prepared by simple diazonium radical chemistry.[30] The
reduced graphene oxide (RGO) obtained through the reduction
of graphene oxide (GO) was reacted with 4-
benzenediazoniumsulfonate
gave
the
corresponding
benzenesulfonic acid functionalized graphene oxide (G-SO3H).
The chemical nature of G-SO3H was characterized by FT-IR
analysis (Fig.1a). The intense broad peak at 3347, 1167, 1110
and 1031 cm-1 are attributed the presence -OH, S-O, S-C and
S=O functional group stretching frequencies of the ‘-SO3H’ of
the benzenesulfonic acid crafted on RGO surface. Another
peak at 2915 cm-1 represents the aromatic C-H of the phenyl
ring attached with sulfonyl moiety. These preliminary data
from FT-IR studies confirm the covalent functionalization of
RGO sheets by benzenesulfonyl moiety.40 further, the
amorphous nature of the as-prepared G-SO3H was examined
by PXRD pattern (Fig.1b). After sulfonation of RGO, a new
broad peak appeared at 24.67o owing to the covalent
incorporation benzenesulfonic acid group on the surface of the
RGO. The appearance of new peak strongly evidences the
formation of graphene sulfonic acid. FT- Raman analysis of G-
SO3H, the ID/IG ratio is higher than (1.044) that of RGO, due to
the covalent attachment of abundant benzenesulfonic acid
groups on the RGO surface. Usually, a red or blue shift of the
band can be used to evaluate the degree of structural changes
during chemical functionalization. Further, the G bands of G-
SO3H shifted from 1586 cm-1 to 1581 cm-1 this observable
redshift strongly supports the covalent anchoring of
benzenesulfonic acid moiety on RGO. SEM image of G-SO3H
(Fig.1e) shown a smooth, silky layered structure that reveals
the sulphonation did not make any distractive changes on the
morphology of RGO sheets. Further XPS spectrum for G-SO3H
of high-resolution survey scan (Fig. 1g) shows the presence of
C1s, O1s, and S2p elemental signals. An S2p peak at 167.6 eV
associated with S-O bond of G-SO3H demonstrate the
functionalization of benzene sulfonic acid group on RGO.37
Table 1 Optimization of G-SO3H catalyzed benzimidazole
synthesisa
Yield b (%)
Temp.
Entry
Solvent
(°C)
RT
4
-
5
-
1
2
3
4
5
6
7
8
Neat
Water
ACN
RT
50
49
RT, 80
80
36, 44
26
64, 56
52
ACN-Water(1:1)
Methanol
Ethanol
Ethanol-Water(3:1)
Hexane
RT, 40
RT, 80
80
22, 40
48 ,-
46
43, 58
34, 82
38
RT, 60
RT, 60
RT, 60
RT, 80
RT, 80
-
-
9
THF
82, 74
41, 44
99 c, 79
40, 40
7, 21
23, 3
-, 20
41, 24
10
11
12
THF-Water (1:1)
1,4-Dioxane
1,4-Dioxane-
Water(1:1)
1,4-Dioxane d
13
RT
-
-
a
Reaction condition: G-SO3H catalyst (10 mg, 18.5 wt %),
benzene-1,2-diamine (0.5 mmol) & benzaldehyde (0.5 mmol)
at RT, 6 h. b HPLC yield. c 2 h. d Without Catalyst.
Fig. 1 a) FT-IR, b) PXRD, c) FT-Raman spectrum, d) EDS, e) SEM image and f) TEM Image, g) XPS of G-SO3H
2 | J. Name., 2012, 00, 1-3
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After unambiguously characterized the structure of the as- good mass transfer of reactants to products. Due to the
prepared G-SO3H ( ), to explore its potential in organic chameleon polarity nature of the 1,4-dioxane, it promotes the
1
synthesis as a catalyst, it has been employed as a carbocatalyst G-SO3H catalysed formation of C-2 substituted benzimidazole.
for the synthesis of medicinally important benzimidazole
under metal-free condition. Initially, to use G-SO3H as a
To enhance the greenness of the reaction, aqueous
Brønsted acid catalyst, a model reaction was carried out using mixture was tried with all solvent system, but the results were
benzene-1,2-diamine (2a) and benzaldehyde (3a), and other not encouraging, and diimine formation was predominant in
reaction controlling parameters such as solvent, temperature, all the cases. (Table-1, entries 4, 7 & 10). Finally, dioxane at
time and catalyst load were also optimized. The observed room temperature was chosen as an optimum medium for the
results are presented in table-1. In the absence of a catalyst G-SO3H catalyzed benzimidazole synthesis. Furthermore,
and solvent, the reaction was inert without any conversion catalyst load has a pivotal role in organic synthesis. Most of
(Table-1, entry 1). To identify a reliable solvent for G-SO3H (
1)
the benzimidazole synthesis involves the stoichiometric
catalyzed benzimidazole synthesis, a series of solvent such as amount of the catalyst. Even though, the solvent and
water, ethanol, methanol, acetonitrile, DMF, hexane, The temperature optimization involve 18.5 wt % (10 mg) of catalyst
and further catalyst load optimization showed that 11 wt % (6
mg) of the G-SO3H is adequate for the complete conversion of
the diamine and aldehyde to the corresponding
benzimidazole, with >99% selectivity and without any by-
Table
2 Graphene Sulfonic Acid Catalyzed Synthesis of
Benzimidazole a
Fig. 2 Catalyst load optimization (b isolated yield)
H
N
H
N
H
N
CH3
N
N
N
tetrahydrofuran, and dioxane were employed. To improve
more benign nature of the carbocatalyst and practice greener
protocols, the reaction was performed using water as a
medium, however, the desired benzimidazole formation was
not encouraging, but in addition, another condensation
4a, 99%, 96%c
4b, 96%
4c, 91%
H
N
H
N
H
N
OCH3
F
OH
N
N
N
4d, 94%
4e, 93%
4f
, 93%
H
N
H
H
N
product, N,N'-(1,2-phenylene)bis(1-phenylmethanimine)
(5)
N
Br
NO2
Cl
was formed. It shown that the catalyst is not selective due to
poor solubility of the precursors in water. (Table-1, entry 2).
Further other polar solvents such as acetonitrile, methanol,
and ethanol were employed to improve the yield and resulted
the moderate amount of benzimidazole along with the
condensation product, both in room and higher temperature
ranges (Table-1, entries 3, 5-6). When moved to non-polar
solvents the catalyst was idle in hexane medium, (Table-1,
entry 8) due to poor dispersion. Meantime, both THF and
dioxane were behaved better than the polar solvents,
however, with THF, a notable diimine formation was observed
(Table-1, entry 9). Among the screened solvents, dioxane
behaves superiorly, and gave benzimidazole selectively as a
sole product with the excellent yield at room temperature and
formation of diimine was circumvented entirely (Table-1, entry
11). Though 1,4-dioxane is a six-membered cyclic diether; it
behaves like cyclohexane and exists in both boat, and chair
forms, among boat form is slightly polar than chair form.
Polarity of the 1,4-dioxane shuttled owing to the structural
flipping which enhances the complete dispersion of Brønsted
acidic reduced graphene oxide (G-SO3H) and facilitates the
N
N
N
4g, 94%
4h, 93%
4i, 88%
HO
H
H
N
H
N
N
CN
N
N
N
Br
CN
4j, 90%
96%
Cl
4l,
4k, 85%
CH3
OH
OCH3
OCH3
H
N
H
N
H
N
N
N
N
N
4m, 96%
4n, 91%
4o, 42%
Cl
H
N
H
N
H
N
NH
N
N
O
N
N
N
4p, 91%
4q, 98%
4r, 97%
H
N
H
H
H
N
N
N
CH3
N
N
N
N
N
4s, 81%
4t, 82%
4u, 98%
4v, 37%
a Reaction condition: G-SO3H catalyst (6 mg, 11 wt %), 0.5
mmol of benzene-1,2-diamine and 0.5 mmol of aldehyde
at RT, 2 h. b isolated yield, c Isolated yield in gram scale.
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Table 3 Catalytic Control Experimentsa
Yieldb (%)
S. No.
Catalyst
4
37
-
5
54
-
1
2
3
4
5
GO
RGO
32
99
7
59
-
p-TSA
G-SO3H
-
G-SO3H-PY
a
Reaction condition: catalyst (6 mg), benzene-1,2-diamine
(0.5 mmol)
HPLC yield.
& benzaldehyde (0.5 mmol) at RT, 2 h,
Fig. 3 FT-IR spectra of G-SO3H and G-SO3H-Py
b
products. Similarly, time optimization results revealed that are compatible with the present methodology and result good
within two hours all the reactants were consumed and to excellent yield of corresponding benzimidazoles (Table 2,
converted into the desired benzimidazole. Furthermore, the 4s- 4u). To demonstrate the potential of the present catalyst,
present catalytic system is highly specific towards the for the preparative purpose, the reaction was also carried out
formation of C-2 substituted benzimidazole, rather than 1,2- on a gram scale, giving the isolated yield (Table 2, 4a) which is
disubstituted benzimidazole even in the presence of excess comparable to those obtained for a small-scale reaction.
equivalence of aldehyde. From the screening results, the as-
Although G-SO3H catalyzes the benzimidazole synthesis
prepared G-SO3H acts as a catalyst for the synthesis of with minimum catalyst load under ambient condition, to
benzimidazole in dioxane using 11 wt % of the catalyst at demonstrate the inherent catalytic activity of G-SO3H, control
ambient temperature. With the mild optimized condition in experiments were carried out with another similar type of
hand, to explore the inherent potential of the G-SO3H catalyst materials such as GO and RGO under the optimized reaction
in synthetic chemistry, the benzimidazole synthesis was condition (Table 3). GO gave only 37 % of the benzimidazole
successfully extended by using a spectrum of aromatic, along with diimine (
heteroaromatic and aliphatic aldehydes with benzene-1,2- owing to the presence of acidic oxygen functionalities.
diamine ( ), which affords the respective products in good to Meantime, RGO was entirely idle under the optimized
excellent yield without any by-products or intermediates such condition (Table 3, entry 2). Commercially available p-
as diimine and disubstituted benzimidazoles. The observed toluenesulphonic acid (Table 3, entry 3) acts as
substrate versatility has been depicted in Table 2. All homogeneous catalyst and yield the mixture of both
aldehydes were efficiently converted into the corresponding benzimidazole and diimine ( ) under the present condition.
5) derivative (54 %) (Table 3, entry 1)
3
a
5
benzimidazoles within the optimized time, among aromatic Though, the above catalysts promote the benzimidazole
aldehydes were found more reactive irrespective of the formation, selectivity is completely lost which enlighten the
substituents electronic nature present on the aryl ring. The uniqueness of the present catalyst
present catalyst works very well for a wide range of para-
substituted aldehydes possessing both electron releasing like
methyl, isopropyl, hydroxyl, and methoxy (Table 2, 4b-4e) and
withdrawing substituents halogens, nitro and nitrile (Table 2,
4f-4j) with good to excellent isolated yield. Meanwhile, a
notable decrease in the yield was observed when the nitrile
group present in the meta-position (Table 2, 4k). Aldehydes
with disubstituent (Table 2, 4l-4n
) were also smoothly
underwent the reaction and gave relatively good yield without
any steric effect. It is noticed that when the nitrogen present
either in aldehyde or diamine’s ring system, which suppress
the reaction and result in moderate yield (Table 2, 4o, 4v),
because of the catalytic poisoning of the pyridine moiety
which hampers the catalytic acidic sites of the G-SO3H.
Aldehydes with
a fused ring system having native,
heteroaromatic and heteroaromatic bicyclic system gave
excellent yield (Table 2, 4p-4r). Aliphatic aldehydes such as
formaldehyde, propionaldehyde and cyclohexane aldehydes
Fig. 4 Reusability of G-SO3H catalyst.
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Fig. 5 (a) PXRD, (b) FT-IR, (c) XPS and (d) HRTEM Image of reused G-SO3H
Furthermore, to ensure the role of the sulfonic acid group Furthermore, to understand the stability and sustainable
present in G-SO3H, pyridine adsorption studies were nature of the G-SO3H, which is recovered by simple filtration
performed. In this typical study, an excess of pyridine was after the reaction finished, then washed, dried under vacuum.
added to the G-SO3H catalyst dispersed in dioxane under Another fresh batch reaction was preceded with the recovered
sonication, and the resultant black precipitate was used as a catalyst under the identical condition. The reuse results
catalyst in benzimidazole synthesis with the same optimized establish that G-SO3H remains equally active and exhibits high
condition. Before the adsorption of pyridine on G-SO3H was stability for five consecutive cycles without any noticeable
confirmed by FT-IR analysis (Fig.3).41 Pyridine is a relatively decay in the activity (Fig.4). The outcome of the recycling
strong base hence, it shows three modes of interaction with behaviour of the present catalyst clearly demonstrates the
solid acids via hydrogen bonding, coordinative bonding, and robustness and sustainable nature of the catalyst. Further, to
chemisorption as a pyridinium ion. This pyridine adsorption confirm the stability of the catalyst, the reused G-SO3H was
study discloses the considerable information about the acidity subjected for further analysis after the reaction.
of G-SO3H by studying changes in the “ring’ vibrations of
pyridine and other bands in the region of 1700 cm-l to 1400
The vacuum dried catalyst was subjected to FT-IR, PXRD,
cm-l. The band at 1540 cm-l indicates the interaction of XPS and HRTEM analyses, to examine the functional group,
pyridine with sulfonic acid group of the G-SO3H catalyst and morphological changes of the catalyst during the reaction
through the formation of positively charged pyridinium ions (Fig. 5). PXRD pattern and FT-IR analysis of the fresh and
chemisorbed on the surface of G-SO3H. The absence of a band reused catalyst are identical without any notable changes even
around 1440-1465 cm-l indicates that no physically or after five consecutive runs. It was supported by the TEM
coordinately bonded pyridine is present. These detailed images of the fifth time reused catalyst where the surface
analyses suggest that the irreversible chemisorption inhibits morphology and microstructures are remains intact. In
the active acidic sites via pyridinium ion formation which addition, the chemical composition of the reused sample was
deliberately explains the catalytic poisoning of G-SO3H by tested by XPS analysis that reveals the sustainable nature of
pyridine and reason outs the lower yield (table 3, entry 5) for the catalyst. Followed by the filtrate was subjected to ICP-OES
the pyridine-based entities in the reaction.
analysis to find any sulphur was leached of after the reaction,
and results show the absence of a detectable amount of
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Table 4 Comparison of Catalytic Selectivity of G-SO3H
Condition
Yield (%)
S. No.
Catalyst
Ref.
Solvent
1,4-Di-oxane
CHCl3
Temp (°C)
Time (h)
4
6
-
1
2
G-SO3H
L-Proline
RT
RT
RT
RT
RT
RT
RT
70
RT
60
2
5
99
-
This work
95
70
90
42
90
92
-
16
19
17
19
19
21
20
40
41
3
Oxalic acid
EtOH: H2O
EtOH: H2O
EtOH
EtOH
H2O
EtOH
EtOH
MeOH
3
14
0
4
5
6
7
8
9
10
Silica sulfuric acid (LiCl)
Montmorillonite K-10
SiO2-HClO4
2
2
2
0.5
1
22
0
Nano ceria
-
Scolecite
Cu-MOF
GO
94
97
81
6
4
-
-
sulphur. This advocacy indicates that there is no markable other existing system which is spectacle by comparison with
change in the chemical and structural morphology of the the reported catalyst. In the present work G-SO3H accord
catalyst even after the fifth use.
specifically mono C-2 substituted benzimidazole without any
other by-product. Meanwhile, other homogeneous catalyst
such as L-proline, oxalic acid and heterogeneous systems such
as silica sulphuric acid, K-10 montmorillonite, SiO2-HClO4, and
nanoceria gave a mixture of mono (C-2 substituted) and 1,2-
disubstituted benzimidazoles, among later one formed as a
predominant product (Table 4, Entries - 2-7). Similarly, with
other heterogeneous solid acid catalyst such as Scolecite, and
metal-based Cu-MOF shown good reactivity, however, they
work with high temperature and long reaction time. (Table 4,
Entry 8-9). Recently GO is also studied in the synthesis of
benzothiazole and benzimidazole but required high
temperature and time with moderate yield. (Table 4, Entry 10).
Present sulfonic acid functionalized graphene exhibits a better
catalytic activity in concerning the synthesis of benzimidazole
in terms of catalyst load, reaction time, temperature and
selectivity. In analogy, G-SO3H is a metal-free, eco-friendly
fanfare catalyst for the selective synthesis of benzimidazole.
Scheme 1 Proposed Mechanism
In accordance with previous report42 and based on the
experimental observations, a plausible mechanism for the G-
SO3H catalyzed synthesis of C-2 substituted benzimidazoles has
been proposed (Scheme 1). In the case of 2-substituted
benzimidazole, the reaction proceeds through imine
intermediate. It is form by the activation of the aldehydic
carbonyl oxygen by the Brønsted acidic sulfonic acid of G-SO3H
and subsequent condensation with one of amine of Benzen-
1,2- diamine to form imine intermediate. Due to the to the
presence of the aromatic system they stabilized on graphene
surface through π- π stacking. Then the lone pair of electron in
the free amine attacks the imine bond lead to cyclization to
yield C-2 substituted benzimidazole.
Conclusions
In summary, a simple, metal free and benign methodology for
the synthesis of C-2 substituted benzimidazole has been
developed using sulfonic acid functionalized graphene as a
solid Brønsted acid catalyst under ambient condition. Catalyst
exhibits good functional group tolerances and selectively,
yields a wide spectrum of C-2 substituted benzimidazoles
under chromatography free pure condition. The pyridine
adsorption studies evidenced the catalytically responsible
sulfonic acid sites. This sustainable carbon-based
organocatalyst is highly stable, and no loss in activity and could
be recovered by simple filtration. The as-prepared G-SO3H
catalyst had shown sustained recyclability over five
consecutive cycles for the synthesis of pharmaceutically
The uniqueness of the as-prepared G-SO3H is rationalized
based on its selectivity in benzimidazole synthesis over the
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essential benzimidazoles. To our delight, this carbocatalyst General Procedure for the Synthesis of Substituted Benzimidazole
may find broad applications in synthetic as well in medicinal
In a typical procedure, 6 mg of the catalyst was dispersed in 1
chemistry.
mL of 1,4-dioxane under sonication. Add 0.5 mmol (1 eq) of
benzene-1,2-diamine in 2 mL of 1,4-dioxane with constant
Experimental
stirring followed by dropwise addition of 0.5 mmol (1 eq) of
benzaldehyde. The course of the reaction was monitored by
General Information
TLC. After completion of the reaction, the catalyst was
recovered by simple filtration and washed twice with
All reagents and starting materials were purchased commercially
methanol and water and dried under vacuum for further use.
from Sigma–Aldrich, Alfa-Aesar, Merck, or Spectrochem and used as
The filtrate was evaporated to dryness under reduced pressure
received without any further purification unless otherwise noted.
to obtain the pure product for NMR and ESI-MS analysis.
The powder X-ray diffraction was recorded in XPERT-PRO
instrument using Cu Kα (λ = 0.1542 nm) radiation operated at a
voltage of 40 kV and a current of 40 mA at room temperature. FT-IR
spectra were recorded in a Shimadzu instrument from 4000 cm-1 to
Conflicts of interest
500 cm-1 with samples being dispersion using KBr pellet technique.
There are no conflicts to declare.
Raman spectra were recorded in Shimadzu laser 630 nm. SEM
analysis was carried out in TESCAN VEGA3 instrument using SE
Acknowledgement
detector and equipped with EDX (AMETEK EDAX, Model Octane
Prime with Active area = 10 mm1) detector. ICP-OES was analysis
We gratefully acknowledge the financial support from the
University Grants Commission, New Delhi, India (UGC grant
No.: 42-291/2013(SR)) Also, we acknowledge Science and
Engineering Research Board SERB), New Delhi, India under
fast-track scheme (Grant No. SR/FT/CS-53/2011) and Council
for Scientific and Industrial Research, New Delhi, India (CSIR
grant No.: 02(0191)14/EMR-II). We thank University Grants
Commission under University with Potential for Excellence
program (UGC-UPE), DST-FIST (for LC-MS) and DST-PURSE (for
FT-IR, SEM, and EDX) for the instrumental facility.
was carried out in Perkin Elmer OPTIMA 5300 DV. Samples for high-
resolution transmission electron microscopy (HRTEM) analysis were
done by dispersing the prepared samples in acetone using
ultrasonication, then deposit a drop of that dispersed solution on
amorphous carbon-coated copper grids. Images were obtained on
TEF 20 TECNAI G2 200kv TEM (FEI). X-ray photoelectron
spectroscopy (XPS) were collected on Omicron Nanotechnology
spectrometer with Al Ka as the excitation source (hm = 1486.6 eV).
All 1H NMR and 13C NMR were measured either in CDCl3 or DMSO-d6
with TMS as an internal standard in 300/75 or 400/100 MHz on a
Bruker spectrometer unless otherwise noted. LC-MS analysis was
carried out using a Thermo Fisher Scientific instrument LTQ Fleet
using C18 column (XTerra - 5µ) with a flow rate of 0.8 mL/min. (0.01
mol ammonium acetate buffer and acetonitrile) at the absorption
wavelength of 254 nm.
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Table of Content
Brønsted Acidic Reduced Graphene Oxide as a Sustainable
Carbocatalyst: A Selective Method for the Synthesis of C-2 Substituted
Benzimidazole
Murugan Karthik, Palaniswamy Suresh*
A selective and sustainable methodology for the synthesis of C-2 substituted
benzimidazole has been demonstrated using benzenesulfonic acid grafted reduced
graphene oxide as a Brønsted acidic carbocatalyst.