A novel bioglycerol-based recyclable carbon catalyst for an efficient one-pot synthesis of highly substituted imidazoles

Article abstract of DOI:10.1016/j.tetlet.2011.12.092

The new bioglycerol-based carbon catalyst acts as an efficient, readily available, and reusable catalyst for the synthesis of 2,4,5-trisubstituted imidazoles/1,2,4,5-tetrasubstituted imidazoles, when aromatic aldehyde, ammonium acetate/amine, and 1,2-diketone are reacted in acetonitrile.

Full text of DOI:10.1016/j.tetlet.2011.12.092

Tetrahedron Letters 53 (2012) 1126–1129
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A novel bioglycerol-based recyclable carbon catalyst for an efficient
one-pot synthesis of highly substituted imidazoles
K. Ramesh a, S. Narayana Murthy a, K. Karnakar a, Y.V.D. Nageswar a, , Kukuma Vijayalakhshmi b,
⇑
Bethala L.A. Prabhavathi Devi b, R.B.N. Prasad b
a Organic Chemistry Division-I, Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad 500607, India
b Centre for Lipid Research, Indian Institute of Chemical Technology, Hyderabad 500607, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The new bioglycerol-based carbon catalyst acts as an efficient, readily available, and reusable catalyst for
the synthesis of 2,4,5-trisubstituted imidazoles/1,2,4,5-tetrasubstituted imidazoles, when aromatic alde-
hyde, ammonium acetate/amine, and 1,2-diketone are reacted in acetonitrile.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 30 August 2011
Revised 19 December 2011
Accepted 22 December 2011
Available online 30 December 2011
Keywords:
Aldehyde
1,2-Diketones
Amine
Ammonium acetate
Multi-component reaction
Acetonitrile
Catalyst
Multi-component reactions1 (MCRs) are one-pot processes,
which emerged as important tools for drug discovery,2 having
advantages such as high atom economy and high selectivity.3
Imidazole represents an important class of compounds in pharma-
ceutical chemistry with potential biological activity and is the core
structural skeleton in many important biological molecules like
histidine, histamine, and biotin as well as several drug moieties4
such as Trifenagrel, Eprosartan, and Losartan (Fig. 1). Numerous
methods have been reported for the synthesis of highly substituted
equipment corrosion, ease of product separation, and reusability.
Cellulose sulfuric acid is one such bio-supported recyclable solid
acid catalyst, effectively used for the synthesis of various imidazole
derivatives.21,22 Mercapto propylsilica (MPS),
a carbon-based
heterogeneous solid catalyst has been reported to catalyze the syn-
thesis of tri-, tetra-substituted imidazoles.23 DABCO, another car-
bon-based catalyst has also been utilized as a mild and efficient
catalyst for the synthesis of highly substituted imidazoles using a
multi component strategy.24 Prabhavathi Devi et al. reported
obtaining similar sulfonic acid-functionalized polycyclic aromatic
carbon catalyst from bioglycerol (biodiesel by-product) and also
from glycerol-pitch (waste from fat splitting industry) by in situ
partial carbonization and sulfonation in a single pot.25 These car-
bon catalysts were demonstrated for their effectiveness for the
esterification and THP protection and deprotection of alcohols
and phenols, respectively.26 Although other solid-supported acid
catalysts like polymer-supported sulfonic acid or silica-supported
sulfonic acid, etc., share similarities like ease of catalyst separation
and product isolation, the present bio glycerol-based acid catalyst
can easily be prepared in the laboratory,27 and is economically via-
ble since the starting material is readily available.
imidazoles by using various catalytic systems such as L
-proline,5
ZrCl4,6 InCl3Á3H2O,7 HClO4–SiO2,8 BF3SiO2,9 K5CoW12O40Á3H2O,10
heteropolyacids,11 silica gel or Zeolite HY,12 silica gel/NaHSO4,13
molecular iodine,14 silica sulfuric acid,15 microwave irradiation,16
ionic liquids,17 NiCl2Á6H2O/Al2O3,18 and CAN.19 However, the above
mentioned methods suffer from one or more disadvantages such as
the use of hazardous organic solvents, expensive moisture-sensi-
tive catalysts, tedious workup conditions, longer reaction times,
or large volumes of catalyst loadings. In recent years carbon-based
solid acid catalysts20 have gained prominence due to their signifi-
cant advantages over homogeneous liquid phase mineral acids
such as increased activity, selectivity, longer catalyst life, negligible
In continuation of our efforts toward exploring the develop-
ment of novel environment friendly methodologies28 which
include the synthesis of heterocyclic compounds, herein, we report
a mild and efficient one-pot protocol for the synthesis of highly
⇑
Corresponding author. Tel.: +91 40 27191654; fax: +91 40 27160512.
E-mail address: dryvdnageswar@gmail.com (Y.V.D. Nageswar).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.12.092

K. Ramesh et al. / Tetrahedron Letters 53 (2012) 1126–1129
1127
Table 3
Synthesis of 1,2,4,5-substituted imidazolesa
S
N
N
N
N
Cl
N
N
NH
HOOC
CHO
NH2
O
Glycerol-based
carbon catalyst
CH3CN, 50-55ºC
N
N
N
N
N
H
Ph
HOH2C
NH4OAc
Ph
O
NMe2
O
COOH
Trifenagrel
Eprosartan
Losartan
Entry Aldehyde Amine
Amine
1,2-
Product
Yieldb
Diketone
(%)
Figure 1. Bio active compounds with imidazole skeleton.
CHO
NH2
O
Ph
Ph
N
N
Ph
Ph
O
1
2
NH4OAc
NH4OAc
82
Bio Glycerol-based
CHO
CHO
O
N
carbon catalyst
CH3CN, 50-55ºC
F
Ph
2 NH4OAc
NH4OAc
Ph
N
H
F
O
CHO
NH2
O
Ph
Ph
N
N
Ph
Ph
Bio Glycerol-based
NH2
O
N
N
O
carbon catalyst
CH3CN, 50-55ºC
Ph
82
Ph
OCH3
NH2
O
OCH3
CHO
CHO
O
O
Ph
Ph
N
N
Ph
Ph
Scheme 1. Synthesis of highly substituted imidazoles.
Ph
Ph
3
4
NH4OAc
NH4OAc
74
77
O
O
N
Ph
Ph
N
N
NH2
NH2
S
S
N
N
CHO
CHO
O
O
Ph
Ph
N
N
Ph
Ph
Ph
Ph
N
5
6
NH4OAc
NH4OAc
73
72
O
O
CH3
Ph
Ph
N
N
N
S
NH2
S
N
Figure 2. SEM images of (a) fresh bioglycerol-based carbon catalyst and (b) bio
glycerol-based carbon catalyst after the fourth cycle.
CHO
H2N
O
Ph
Ph
N
N
Ph
Ph
O
7
NH4OAc
70
Table 1
Optimization conditions for the synthesis of 2,4,5-triphenyl-1H-imidazolea
Yieldb (%)
a
Entry
Solvent
T (°C)
Reaction conditions: aldehyde (1.0 mmol), amine (1.0 mmol), NH4OAc
(1.0 mmol), 1,2-diketone (1.0 mmol), catalyst (10 wt %), CH3CN (10 mL), at 50–
2
2
1
3
4
5
CH3CN
CH3CN
Toluene
Ethanol
Methanol
DMSO
rt
52
84
48
42
28
41
55 °C.
50–55
60–65
60–65
50–55
80–85
b
Isolated yield.
lyst in acetonitrile (Scheme 1). SEM images of bioglycerol-based
carbon catalyst are provided in Figure 2.
In continuation of exploring the possible applications for the
carbon-based solid acid catalyst, here we report a facile and effi-
cient one-pot synthesis of highly substituted imidazoles for the
first time (Scheme 1).
a
Reaction conditions: aldehydes (1.0 mmol), NH4OAc (2.0 mmol), 1,2-diketone
(1.0 mmol), catalyst (10 wt %), solvent (10 mL).
b
Isolated yield.
Table 2
In our initial studies toward the development of this methodol-
ogy, 1,2-diketone (1.0 mmol) was reacted with aldehyde (1.0
mmol) and NH4OAc (2.0 mmol) in acetonitrile and it was found that
no reaction was observed and only starting materials were recov-
ered. It was observed that the same reaction proceeded efficiently
when bioglycerol-based carbon catalyst was used as a catalyst in
acetonitrile at room temperature yielding the corresponding
substituted imidazole in a 52% yield after 8 h. When the same reac-
tion was attempted in acetonitrile at 50–55 °C the reaction pro-
ceeded to completion within 7 h and yielded the corresponding
imidazole derivative in a 84% yield. While evaluating the influence
of different solvent systems for bioglycerol-based carbon-catalyzed
Recyclability of glycerol-based carbon catalyst
Cycles
Yield (%)
Catalyst recovered (%)
Native
84
83
82
81
88
86
85
83
1
2
3
substituted imidazole derivatives for the first time by a multi-com-
ponent reaction involving 1,2-diketone, aldehydes, NH4OAc, and
amine using a novel and recyclable bioglycerol-based carbon cata-

1128
K. Ramesh et al. / Tetrahedron Letters 53 (2012) 1126–1129
Table 4
(a)
Synthesis of 2,4,5-substituted imidazolesa
Glycerol-based
Ph
O
O
Ph
Ph
acidic catalyst
O
NH2
NH2
N
H
Ph
Entry Aldehyde Amine
source
1,2-Diketone
Product
Yieldb
(%)
2NH3
-H2
H
N
O
-2H2O
CHO
O
Ph
N
[A]
[B]
H transfer
Ph
Ph
Ph
1
2
NH4OAc
NH4OAc
84
85
N
Ph
H
O
O
N
CHO
Ph
N
Ph
N
H
Ph
NO2
Ph
N
Ph
H
[C]
O
(b)
NO2
Glycerol-based
acidic catalyst
Ph
OMe
O
O
Ph
Ph
NH2
O
CHO
O
NO2
N
Ph
N
Ph
Ph
Ph
NH4OAc
H
N
H
N
3
4
NH4OAc
NH4OAc
84
81
+
N
O
Ph
H
NO2
MeO
NH2
[D]
[F]
Ph
N
CHO
O
OMe
OMe
Ph
N
H
NH4OAc
at elevated temperature
+
O
O
N
H
CHO
Ph
N
Ph
OH
MeO
NH2
[E]
Ph
N
Ph
H
5
6
NH4OAc
NH4OAc
80
81
O
Scheme 2. Plausible mechanistic pathway for the synthesis of highly substituted
imidazoles using bioglycerol-based carbon catalyst.
OH
CHO
O
Ph
Ph
N
Ph
Ph
N
H
O
washed with ice-cold methanol and dried. The recovered catalyst
was further used in the reaction with the same substrates and
checked for the yields and catalytic activity of the recovered cata-
lyst, as shown in Table 2. It was observed that the yields of highly
substituted imidazoles diminished slightly after two to three recy-
cles. The plausible mechanism for the synthesis of highly substi-
tuted imidazoles in the presence of bioglycerol-based carbon
catalyst can be explained as shown in Scheme 2. In the case of
2,4,5-trisubstituted imidazole the reaction proceeds via diamine
intermediate [A], which is formed by the activation of aldehydic
carbonyl group by the acidic bioglycerol-based carbon catalyst
through intermolecular hydrogen bonding, and condensation of
diamine with 1,2-diketone followed by dehydration to form the
imino intermediate [B], which rearranges to form the desired
product [C]. In the case of tetra substituted imidazoles the reaction
proceeds in the same fashion. Various reactions were conducted to
establish the mechanism of this reaction changing the reactants
and their molar ratios. In the case of the reaction of benzaldehyde
with equimolar amounts of ammonium acetate and 4-methoxy
aniline the intermediate [D] was isolated as an intermediate, which
further reacted with 1,2-dicarbonyl compound to yield the
expected product [F]. When the same reaction was conducted at
elevated temperatures the imino intermediate [E] was also isolated
as a side product (Scheme 2).
CHO
O
Ph
Ph
N
Ph
O
Ph
N
H
O
7
NH4OAc
80
O
CHO
O
Ph
Ph
N
Ph
Ph
Ph
N
H
8
9
NH4OAc
NH4OAc
80
81
O
Ph
CHO
O
Ph
Ph
N
Ph
Ph
N
H
O
O
O
CHO
N
10
NH4OAc
81
N
H
CHO
O
Ph
N
Ph
Ph
11
12
NH4OAc
NH4OAc
81
84
N
H
Ph
Ph
O
O
CHO
N
Ph
Ph
N
H
Acknowledgments
O
Ph
a
Reaction conditions: aldehyde (1.0 mmol), NH4OAc (2.0 mmol), 1,2-diketone
We thank the CSIR, New Delhi, India, for fellowships to S.N.M.,
K.K. and the UGC for fellowship to K.R.
(1.0 mmol), catalyst (10 wt %), CH3CN (10 mL), at 50–55 °C.
b
Isolated yield.
Supplementary data
synthesis of highly substituted imidazoles, the role of different sol-
vents like CH3CN, toluene, ethanol, methanol, and DMSO was exam-
ined. Among these, only CH3CN was found to be the best medium
for attaining optimum yields (Table 1). Several examples illustrat-
ing this simple and practical methodology are summarized in Table
3 and 4. All the products were characterized by 1H, 13C NMR, IR, and
mass spectra and compared with authentic samples.29–31
In all these reactions the glycerol-based carbon catalyst can be
recovered and reused. After the reaction, the reaction mass was
cooled to room temperature and the catalyst was filtered and
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.12.092.
References and notes
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29. General procedure for the synthesis of 2,4,5-substituted imidazole derivatives: To a
stirred solution of acetonitrile (10 mL), aldehyde (1.0 mmol) and bioglycerol-
based carbon catalyst (10 wt %) were added and stirred for 10 min. To this
ammonium acetate (2.0 mmol) followed by 1,2-diketone (1.0 mmol) were
added, after which the reaction mixture was heated at 50–55 °C until
completion of the reaction as indicated by TLC. The reaction mixture was
cooled to room temperature and catalyst was filtered, the solvent was removed
by rotary evaporator. The crude residue was extracted with ethyl acetate
(3 Â 10 mL). The combined organic layers were extracted with water, saturated
brine solution, and dried over anhydrous Na2SO4. The organic layers were
evaporated under reduced pressure and the resulting crude product was
purified by column chromatography by using ethyl acetate and hexane (7:3) as
eluent to give the corresponding substituted imidazole derivative in 78–85%
yield. The identity as well as purity of the product was confirmed by 1H, 13C
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bioglycerol-based carbon catalyst (10 wt %) were added and stirred for
10 min. To this ammonium acetate (1.0 mmol) and amine (1.0 mmol)
followed by 1,2-diketone (1.0 mmol) were added, after which the reaction
mixture was heated at 50–55 °C until completion of the reaction as indicated
by TLC. The reaction mixture was cooled to room temperature and catalyst was
filtered, the solvent was removed by rotary evaporator. The crude residue was
extracted with ethyl acetate (3 Â 10 mL). The combined organic layers were
extracted with water, saturated brine solution, and dried over anhydrous
Na2SO4. The organic layers were evaporated under reduced pressure and the
resulting crude product was purified by column chromatography by using
ethyl acetate and hexane (7:3) as eluent to give the corresponding substituted
imidazole derivative in 70–82% yield. The identity as well as purity of the
product was confirmed by 1H, 13C NMR, and mass spectra.
31. Spectral data of representative examples: 1-(4-fluorophenyl)-2,4,5-triphenyl-
1H-imidazole (Table 3, entry 1): 1H NMR (DMSO-d6, 300 MHz): d 7.54 (d,
J = 6.7 Hz, 1H), 7.46–7.37 (m, 2H), 7.26–7.16 (m, 10H), 7.03–6.91 (m, 6H); 13C
NMR (CDCl3/DMSO-d6, 200 MHz): d 131.0, 129.1, 128.9, 128.6, 128.2, 127.5,
126.9, 114.2.; ESI-MS (m/z): (M+H)+.