DABCO as a mild and efficient catalytic system for the synthesis of highly substituted imidazoles via multi-component condensation strategy

Article abstract of DOI:10.1016/j.tetlet.2010.07.128

A simple and efficient protocol for the synthesis of highly substituted imidazoles is developed through the condensation of 1,2-dicarbonyl compound, aldehyde, and ammonium acetate or amine via multi-component condensation strategy. The present method gives good to excellent yields of substituted imidazoles.

Full text of DOI:10.1016/j.tetlet.2010.07.128

Tetrahedron Letters 51 (2010) 5252–5257
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Tetrahedron Letters
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DABCO as a mild and efficient catalytic system for the synthesis of highly
substituted imidazoles via multi-component condensation strategy
*
S. Narayana Murthy, B. Madhav, Y. V. D. Nageswar
Organic Chemistry Division-I, 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:
A simple and efficient protocol for the synthesis of highly substituted imidazoles is developed through
the condensation of 1,2-dicarbonyl compound, aldehyde, and ammonium acetate or amine via multi-
component condensation strategy. The present method gives good to excellent yields of substituted
imidazoles.
Received 31 May 2010
Revised 19 July 2010
Accepted 22 July 2010
Available online 29 July 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Aldehydes
Ammonium acetate
1,2-Dicarbonyl compounds
DABCO
Multi-component
Condensation
Imidazoles
Aromatic amines
Heterocyclic chemistry is one of the most important topics in
the synthetic organic chemistry arena that covers a wide variety
of potent molecules. Imidazoles and benzimidazoles are structural
scaffolds present as substructures in many natural products which
possess wide spectrum of biological activities covering anti-infla-
matory,1 anti-allergic,2 analgesic,3 and glucagon receptor antago-
nism.4 They are also the key intermediates in the synthesis of
many therapeutic agents. Omeprazole,5 Pimobendan,6 Losartan,
Olmesartan, Eprosartan, and Trifenagrel7 (Fig. 1) are some of the
leading drugs in the market with diverse functionalization around
the imidazole motif. Apart from being biologically active, this
structural core in the recent years has been in other advanced areas
of research such as fluorescence labeling agents,8–10 biological
imaging,11 and chromophores for non-linear optic systems.12 In
view of the numerous biological, pharmacological, and material
properties associated with this five-membered heterocyclic moi-
ety, the development of new synthetic protocols under varied mild
reaction conditions is always a matter of interest.
microwave irradiation,23 and refluxing in acetic acid24 silica sulfu-
ric acid,25 NiCl2Á6H2O/Al2O3,26 ZrCl4,27 ionic liquids,28 CAN,29 and
InCl3Á3H2O.30 However, many of the methods reported above suffer
from one or more disadvantages such as the use of expensive mois-
ture-sensitive metallic reagents, longer reaction times, tedious
separation procedures, and large amount of catalyst loadings
which in turn results in the generation of huge amount of metal
wastes into the environment.
Multi-component reaction (MCRs) is convergent, in analogy to
the convergent synthesis and in contrast to a divergent multi-step
synthesis. These reactions are classified into various ways based on
the number of components involved in the reaction or their intrin-
sic variability. Now-a-days organic chemical syntheses involving
multi-component condensation strategy attained greater value,
as the target molecules are often obtained in a single step rather
S
In the last decade numerous methods have been developed for
the synthesis of highly substituted imidazoles by using various cat-
alytic systems including silica gel or Zeolite HY,13 silica gel/NaH-
SO4,14 molecular iodine,15 K5CoW12O40-3H2O,16 heteropolyacids,17
N
N
HOOC
N
N
H
O
NMe2
HClO4–SiO2,18
L
-proline,19 FeCl3Á6H2O,20 BF3ÁSiO221, and silica-sup-
ported Wells–Dawson acid.22 They can also be obtained by use of
COOH
Trifenagrel
Eprosartan
* Corresponding author. Tel.: +91 40 27191654; fax: +91 40 27160512.
E-mail address: dryvdnageswar@gmail.com (Y.V.D. Nageswar).
Figure 1. Potent multi-substituted imidazole derivatives.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.07.128

S. N. Murthy et al. / Tetrahedron Letters 51 (2010) 5252–5257
5253
CHO
O
N
DABCO
Ph
NH4OAc
2 eq.
Ph
°
C
t-BuOH, 60-65
N
H
O
1 eq.
1 eq.
NH2
CHO
O
N
N
DABCO
-BuOH, 60-65
Ph
NH4OAc
1 eq.
Ph
°
C
t
O
1 eq.
1 eq.
1 eq.
Scheme 1. DABCO-catalyzed synthesis of multi-substituted imidazole derivatives.
than multiple steps which minimize the tedious work-up proce-
dures and environmental hazardous wastes. In a multi-component
reaction the starting materials with desired functionalities reacted
together in a specified path resulting in a complex adduct, incorpo-
rating all the reactants, making it an ideal synthetic setting.31 In
continuation of our efforts toward the development of novel meth-
odologies under green chemical approaches herein we report a
mild, efficient, and facile one-pot synthesis of multi-substituted
imidazole derivatives for the first time by the multi-component
reaction of 1,2-diketone, amine source, and aldehyde using DABCO
as a base (Scheme 1).
plored the efficiency of various bases on the reaction. Among the
bases tested for the condensation of benzaldehyde, ammonium
acetate, and benzil to yield 2,4,5-triphenyl-1H-imidazole are tri-
ethyl amine (75%), piperidine (78%), DBU (81%), and DABCO
(92%). However DABCO gave excellent yields.
In order to determine the most appropriate choice of solvent
system for this DABCO-catalyzed synthesis of substituted imida-
zoles, we have screened the solvents such as methanol, ethanol,
isopropanol, and tert-butanol. Of the solvents tested in the screen-
ing we got the maximum yield of the product in shorter reaction
times, when we use tert-butanol as solvent (Table 2).
In our initial experiments toward the development of this
methodology, we have reacted benzaldehyde (1.0 mmol) with
ammonium acetate (2.0 mmol) and 1,2-diketone derivative
(1.0 mmol) in ethanol resulting in the corresponding substituted
imidazole in lower yields at room temperature. It was observed
that the same reaction proceeded efficiently when we use DABCO
as a catalyst in ethanol at room temperature yielding the corre-
sponding substituted imidazole in 85% yield after 24 h. When we
attempted the same reaction in t-butanol at 60–65 °C the reaction
proceeded to completion within 12 h and yielded the correspond-
ing imidazole in 92% yield.32 Encouraged by this result we have ex-
In order to determine the scope of this reaction, we have syn-
thesized differently substituted 2,4,5-imidazoles and 1,2,4,5-imi-
dazoles by varying differently substituted aldehydes including
both electron-donating and electron-withdrawing groups. It is ob-
served that the reaction gave good yields of products with faster
reaction rate when the aldehyde bearing electron-withdrawing
group is used compared to the aldehydes with electron-donating
groups. The corresponding results are tabulated in Tables 1 and 3.
The plausible mechanism for the synthesis of substituted imida-
zoles in the presence of DABCO involves the initial reaction of DAB-
CO with aldehyde leading to the formation of intermediate (A),
Table 1
Synthesis of 2,4,5-substituted imidazoles by using DABCOa
Entry
1
Aldehyde
Amine source
1,2-diketone
Product
Reported yield [ref]
9019
Yieldb (%)
92
CHO
O
Ph
N
Ph
NH4OAc
Ph
N
H
Ph
Ph
O
O
CHO
N
Ph
F
Ph
N
H
2
3
4
5
NH4OAc
NH4OAc
NH4OAc
NH4OAc
—
81
88
86
89
Ph
O
O
F
CHO
Ph
Ph
N
Ph
OH
Ph
Ph
8919
N
H
O
O
OH
CHO
Ph
Ph
N
Ph
Ph
N
H
—
O
O
Ph
CHO
Ph
Ph
N
Ph
Ph
N
H
—
O
(continued on next page)

5254
S. N. Murthy et al. / Tetrahedron Letters 51 (2010) 5252–5257
Table 1 (continued)
Entry
6
Aldehyde
Amine source
NH4OAc
1,2-diketone
Product
Reported yield [ref]
Yieldb (%)
CHO
O
Ph
N
Ph
O
O
Ph
N
H
Ph
O
—
85
O
CHO
O
Ph
Ph
N
Ph
Ph
N
H
Ph
7
8
NH4OAc
NH4OAc
—
—
88
83
O
O
Ph
CHO
O
F
Ph
N
F
Ph
Ph
O
O
N
H
Ph
Ph
CHO
N
Ph
Ph
9
NH4OAc
NH4OAc
—
84
86
N
H
O
O
Ph
Ph
CHO
N
Ph
7825b
Ph
10
N
H
Ph
Ph
O
O
Cl
Cl
CHO
MeO
N
MeO
OMe
Ph
Ph
11
12
NH4OAc
NH4OAc
—
83
89
O
O
N
H
Ph
Ph
MeO
CHO
N
Ph
NO2
Ph
8719
N
H
Ph
O
NO2
CHO
O
NO2
Ph
N
Ph
Ph
13
14
NH4OAc
NH4OAc
8619
86
82
O
N
H
Ph
Ph
NO2
CHO
Br
O
N
Ph
Ph
Br
8725b
N
H
Ph
O
CHO
O
Ph
Ph
N
Ph
Ph
15
16
NH4OAc
NH4OAc
—
83
84
N
H
O
CHO
O
Ph
Ph
N
Ph
Ph
OMe
8219
N
H
O
OMe
CHO
O
N
17
NH4OAc
9128
87
O
N
H
CHO
CHO
O
Ph
N
18
19
NH4OAc
NH4OAc
—
75
72
Ph
O
N
H
O
O
O
O
N
9328
O
N
H
O
a
Reaction conditions: aldehyde (1.0 mmol), ammonium acetate (2.0 mmol), 1,2-diketone (1.0 mmol), DABCO (0.7 mol %), t-BuOH (10 mL), 60–65 °C, 12–15 h.
Isolated yield.
b

S. N. Murthy et al. / Tetrahedron Letters 51 (2010) 5252–5257
5255
Table 2
which attacked by the amine source gives another intermediate (B)
which in turn reacts with another molecule of DABCO, followed by
a mole of amine source generating intermediate (C), with subse-
quent elimination of DABCO. This intermediate (C) on cyclocon-
densation with 1,2-diketone and 1,5-proton migration leads to
the desired product (Scheme 2). All the products were character-
ized by 1H, 13C NMR, IR, and mass spectra and compared with
authentic samples.33
Screening of solvents for the condensation of benzaldehyde, ammonium acetate,
benzil using DABCO
Entry
Solvent
Time (h)
Yield (%)
1
2
3
4
Methanol
Ethanol
iso-Propanol
tert-Butanol
12
12
12
12
67
80
78
92
Table 3
Synthesis of 1,2,4,5-substituted imidazoles by using DABCOa
NH2
CHO
O
N
DABCO
Ph
NH4OAc
Ph
t
-BuOH, 60-65 °C
N
O
5
1
2
3
4
Entry
1
Aldehyde (1)
Amine (2)
Amine(3)
1,2-diketone (4)
Product (5)
Reported yield [ref]
Yieldb (%)
CHO
NH2
O
Ph
N
Ph
Ph
N
NH4OAc
NH4OAc
Ph
8330
82
O
CHO
CHO
NH2
O
Ph
N
Ph
Ph
2
3
7930
74
77
N
Ph
O
NH2
O
Ph
N
Ph
Ph
N
Ph
O
NH4OAc
—
OMe
NH2
OMe
CHO
O
Ph
N
Ph
Ph
N
Ph
O
4
NH4OAc
—
73
F
F
CHO
NH2
O
Ph
N
Ph
Ph
N
Ph
O
5
NH4OAc
—
72
NO2
H2N
NO2
CHO
O
Ph
N
Ph
Ph
N
Ph
6
NH4OAc
O
9223b
70
a
Reaction conditions: aldehyde (1.0 mmol), ammonium acetate (1.0 mmol), amine (1.0 mmol), 1,2-diketone (1.0 mmol), DABCO (0.7 mol %), t-BuOH(10 mL), 60–65 °C, 12–
15 h.
b
Isolated yield.

5256
S. N. Murthy et al. / Tetrahedron Letters 51 (2010) 5252–5257
NH2
OH2
[B]
NH4OAc
DABCO
N
N
N
NH2
N
[A]
OH
NH2
NH2
C6H5COCOC6H5
Ph
Ph
Intramolecular
N
N
H
N
CHO
Product
N
+ shift
NH4OAc
H
[C]
Scheme 2. Plausible mechanistic pathway for the formation of substituted imidazole scaffold.
C. W. Org. Lett. 2004, 6, 1453; (d) Sparks, R. B.; Combs, A. P. Org. Lett. 2004, 6,
2473; (e) Oskooie, H. A.; Alimohammadi, Z.; Heravi, M. M. Heteroatom Chem.
2006, 17, 699.
In conclusion, we have developed a one-pot multi-component
reaction for the synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetra-
substituted imidazoles catalyzed by DABCO in excellent yields.
This method involves mild reaction conditions, easy work-up,
and cleaner reaction profiles.
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J. M.; Hall, R. F.; Garigipati, R. S.; Bender, P. E.; Erhard, K. F.; Krog, A. J.;
Hofmann, G. A.; Sheldrake, P. L.; McDonnell, P. C.; Kumar, S.; Young, P. R.;
Adams, J. L. Bioorg. Med. Chem. 1997, 5, 49.
Acknowledgment
25. (a) Shaabani, A.; Rahmati, A. J. Mol. Catal. A: Chem. 2006, 249, 246; (b) Shaabani,
A.; Rahmati, A.; Farhangi, E.; Badri, Z. Catal. Commun. 2007, 8, 1149.
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27. Sharma, G. V. M.; Jyothi, Y.; Lakshmi, P. S. Synth. Commun. 2006, 36, 2991.
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Srinivasan, K. V. Tetrahedron 2005, 61, 3539.
S.N.M. and B.M. are grateful to the Council of Scientific and
Industrial Research (CSIR), New Delhi, India for providing
fellowships.
Supplementary data
29. Sangshetti, J. N.; Kokare, N. D.; Kotharkara, S. A.; Shinde, D. B. J. Chem. Sci. 2008,
5, 463.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.07.128.
30. Sharma, S. D.; Hazarika, P.; Konwar, D. Tetrahedron Lett. 2008, 49, 2216.
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32. (a) General procedure for the synthesis of 2,4,5-substituted imidazole derivatives:
to a stirred solution of t-butanol (10 mL), aldehyde (1.0 mmol) and DABCO
(0.7 mol %) 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 60–65 °C until completion of the reaction as
indicated by TLC. The reaction mixture was cooled to room temperature and
the solvent was removed by rotary evaporator. The crude residue was
extracted with ethyl acetate (3 Â 10 mL). The organic layers were washed
with water, saturated brine solution, and dried over anhydrous Na2SO4. The
combined 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 (72–92%) yield. The identity and purity of the product
were confirmed by 1H and 13C NMR spectroscopic analysis; (b) General
procedure for the synthesis of 1,2,4,5-substituted imidazole derivatives: to
a
10. Nakashima, K.; Yamasaki, H.; Kuroda, N.; Akiyama, S. Anal. Chim. Acta 1995,
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stirred solution of t-butanol (10 mL), aldehyde (1.0 mmol) and DABCO
(0.7 mol %) 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 60–65 °C until
completion of the reaction as indicated by TLC. The reaction mixture was
cooled to room temperature and the solvent was removed by rotary
evaporator. The crude residue was extracted with ethyl acetate (3 Â 10 mL).
The combined organic layers were washed with water, saturated brine
solution, and dried over anhydrous Na2SO4. The organic solvent was
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 and purity of the product was confirmed by 1H and 13C NMR
spectroscopic analyses.
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33. Data of representative examples:
2,4,5-Triphenyl-1H-imidazole (Table 1, entry 1): 1H NMR (DMSO-d6, 300 MHz): d
8.05 (d, J = 7.5 Hz, 1H), 7.92 (d, J = 7.5 Hz, 1H), 7.52–7.24 (m, 13H), 12.59 (br s,
1H); IR (KBr): mmax 1584, 1629, 3440 cmÀ1; HRMS m/z calcd for C21H16N2
([M+H]+): 297.1391, found 297.1399.
19. Samai, S.; Nandi, G. C.; Singh, P.; Singh, M. S. Tetrahedron 2009, 65, 10155.
20. Heravi, M. M.; Derikvand, F.; Haghighi, M. Monatsh. Chem. 2008, 139, 31.
21. Sadeghi, B.; Mirjalili, B. B. F.; Hashemi, M. M. Tetrahedron Lett. 2008, 49, 2575.
22. Karimi, A. R.; Alimohammadi, Z.; Amini, M. M. Mol. Divers 2009. doi:10.1007/
s11030-009-9197-x.
2-(4-(Allyloxy)phenyl)-4,5-diphenyl-1H-imidazole (Table 1, entry 6): 1H NMR
23. (a) Usyatinsky, A. Y.; Khemelnitsky, Y. L. Tetrahedron Lett. 2000, 41, 5031; (b)
Balalaie, S.; Hashemi, M. M.; Akhbari, M. Tetrahedron Lett. 2003, 44, 1709; (c)
Wolkenberg, S. E.; Wisnoski, D. D.; Leister, W. H.; Wang, Y.; Zhao, Z.; Lindsley,
(DMSO-d6, 300 MHz):
J2 = 1.1 Hz, 1H), 5.45 (dd, J1 = 17.1 Hz, J2 = 1.5 Hz, 1H), 6.00–6.13 (m, 1H), 7.07
(d, J = 8.8 Hz, 2H), 7.34–7.53 (m, 10H), 8.03(d, J = 8.8 Hz, 2H), 12.52 (br s, 1H);
d 4.61 (d, J = 5.0 Hz, 2H), 5.29 (dd, J1 = 10.3 Hz,

S. N. Murthy et al. / Tetrahedron Letters 51 (2010) 5252–5257
5257
13C NMR (300 MHz, DMSO-d6): d 177.8, 158.3, 145.5, 133.5, 128.3, 127.6, 126.6,
123.2, 117.4, 114.7, 68.1; HRMS m/z calcd for C24H20N2O ([M+H]+): 353.1653,
found 353.1654.
7.62–7.78 (m, 2H), 11.62 (br s, 1H); 13C NMR (300 MHz, CDCl3): d 176.9, 149.3,
146.4, 141.3, 128.9, 128.4, 125.7, 107.7; ESI-MS (m/z): 277 (M++1).
1-(4-Methoxyphenyl)-2,4,5-triphenyl-1H-imidazole (Table 3, entry 3): 1H NMR
(CDCl3, 300 MHz): d 3.76 (s, 3H), 6.72 (d, J = 8.8 Hz, 2H), 6.92 (d, J = 8.8 Hz, 2H),
7.09–7.55 (m, 13H), 8.04 (d, J = 8.8 Hz, 2H); HRMS m/z calcd for C28H22N2O
([M+H]+): 403.1810, found 403.1795.
2-(4-Phenoxyphenyl)-4,5-diphenyl-1H-imidazole (Table 1, entry 7): 1H NMR
(DMSO-d6, 300 MHz):
(300 MHz, DMSO-d6): d 178.5, 157.3, 145.2, 130.9, 130.5, 128.8, 123.9, 120.9,
d
7.05–7.91 (m, 19H), 12.75 (br s, 1H); 13C NMR
118.8, 115.9; HRMS m/z calcd for
C
27H20N2O ([M+H]+): 389.1653, found
1-(4-Fluorophenyl)-2,4,5-triphenyl-1H-imidazole (Table 3, entry 4): 1H NMR
(DMSO-d6, 300 MHz): d 7.25–7.37 (m, 4H), 7.42–7.46 (m, 2H), 7.50–7.63 (m,
9H), 7.72–7.81 (m, 2H), 7.89 (d, J = 6.7 Hz, 2H); HRMS m/z calcd for C27H19FN2
([M+H]+): 391.1610, found 391.1598.
389.1643.
2-(Naphthalen-2-yl)-4,5-diphenyl-1H-imidazole (Table 1, entry 15): d 7.32–7.63
(m, 10H), 7.92–8.02 (m, 5H), 8.25 (d, J = 8.3 Hz, 1H), 8.62 (s, 1H), 12.60 (br s,
1H); 13C NMR (300 MHz, DMSO-d6): d 177.9, 132.9, 132.7, 129.3, 128.5, 128.3,
2,4,5-Triphenyl-1-(1-phenylethyl)-1H-imidazole (Table 3, entry 6): 1H NMR
128.2, 127.8, 126.8, 126.4, 123.7, 123.5; HRMS m/z calcd for
C
25H18N2
(DMSO-d6, 300 MHz): d 1.61 (d, J = 7.2 Hz, 3H), 5.55 (q, J1 = 14.2 Hz,
([M+H]+): 347.1548, found 347.1538.
J2 = 7.1 Hz, 1H), 6.89–7.57 (m, 18H), 8.09 (d, J = 7.2 Hz, 2H); HRMS m/z calcd
4,5-Di(furan-3-yl)-2-phenyl-1H-imidazole (Table 1, entry 19): 1H NMR (CDCl3,
300 MHz): d 6.31–6.33 (m, 2H), 6.79 (d, J = 3.3 Hz, 2H), 7.13–7.51 (m, 5H),
for C29H24N2 ([M+H]+): 401.2017, found 401.2012.