A one-pot, four-component synthesis of N-substituted 2,4-diarylimidazoles

Article abstract of DOI:10.1055/s-0029-1218381

A one-pot, four-component synthesis of 1,2,4-trisubstituted 1H-imidazoles is described. Heating a mixture of a 2-bromoacetophenone, an aldehyde, a primary amine, and ammonium acetate under solvent-free conditions affords functionalized imidazoles in good

Full text of DOI:10.1055/s-0029-1218381

LETTER
3263
A One-Pot, Four-Component Synthesis of N-Substituted 2,4-Diarylimidazoles
S
ynthesis of
N
-Substitu
e
ted 2, 4-Dia
h
rylimidazole
d
s
i Adib,*a Samira Ansari,a Shahzad Feizi,a Jafar Asgarian Damavandi,b Peiman Mirzaeic
a
b
c
Faculty of Chemistry, Teacher Training University, Tehran 15614, Iran
Department of Chemistry, Shahid Beheshti University, Tehran, Iran
Received 26 June 2009
The most common synthetic routes reported for the prep-
aration of imidazole derivatives involve cyclizations
(classified on the basis of the number of ring atoms in each
of the components being cyclized: a) single bond forma-
tion, b) formation of two bonds, from [4+1] or [3+2] atom
fragments, c) formation of three or four bonds), transfor-
mation of an existing heterocyclic ring, including expan-
sion of smaller rings, contraction of larger rings, and
transformation of other five-membered rings.4
Abstract: A one-pot, four-component synthesis of 1,2,4-trisubsti-
tuted 1H-imidazoles is described. Heating a mixture of a 2-bro-
moacetophenone, an aldehyde, a primary amine, and ammonium
acetate under solvent-free conditions affords functionalized imida-
zoles in good to excellent yields.
Key words: 1,2,4-trisubstituted 1H-imidazoles, aldehydes, prima-
ry amines, four-component reactions, cyclizations, heterocycles
Multicomponent reactions (MCR) have become a signifi-
cant part of today’s arsenal of methods in combinatorial
chemistry due to their atom economy, straightforward re-
action design, and the opportunity to construct target com-
pounds by the introduction of several diversity elements
in a single chemical event. Typically, purification of prod-
ucts resulting from MCR is also simple since all the
organic reagents employed are consumed and are incorpo-
rated into the target compound.1 MCR, leading to interest-
ing heterocyclic scaffolds, are particularly useful for the
construction of diverse chemical libraries of ‘druglike’
molecules.
There are several methods for the synthesis of highly sub-
stituted imidazoles. The mostly used methods in the last
decade involve: synthesis via hetero-Cope rearrange-
ment,11 four-component condensation of arylglyoxals,
primary amines, carboxylic acids, and isocyanides on
Wang resin,12 reaction of N-(2-oxo)-amides with
NH4O2CCF3,13 and reaction of N-alkyl-N-(b-keto)amides
with NH4OAc.14 The synthesis of tri- or tetrasubstituted
imidazoles by the condensation of aldehydes, benzils, and
NH4OAc/primary amines is also a well-established proce-
dure.15
As part of our current studies on the development of new
routes in heterocyclic synthesis,16 herein we report a one-
pot and four-component synthesis of 1,2,4-trisubstituted
1H-imidazoles from [2+1+1+1] atom fragments by for-
mation of four bonds. Thus, a mixture of a 2-bromoace-
tophenone (1), a primary amine 2, an aldehyde 3, and
ammonium acetate underwent a novel reaction under sol-
vent-free conditions to produce 1,2,4-trisubstituted 1H-
imidazoles 4 in 80–96% yields (Scheme 1, Table 1). The
reactions were carried out at 130 °C by mixing the four
components, and were complete within two hours to af-
ford the functionalized imidazoles 4. The 1H NMR spectra
of the crude products clearly indicated formation of the
1H-imidazoles 4.17
Imidazoles, an important class of heterocyclic com-
pounds, are of both biological and chemical interest, and
their involvement in catalytic processes at the active sites
of enzymes has been well documented. They are part of a
large number of highly significant biomolecules, such as
the essential amino acid histidine, biotin, and imidazole
alkaloids.2–5 The potency and wide applicability of the im-
idazole pharmacophore can be attributed to its hydrogen
bond donor–acceptor capability as well as its high affinity
for metals present in many protein active sites.6 Some im-
idazole derivatives act as p38 mitogen-activated protein
kinase inhibitors,7a B-Raf kinase inhibitors,7b transform-
ing growth factor b1-type 1 activin receptor-like kinase
inhibitors,7c and cyclooxygenase-2 inhibitors.7d Appropri-
ately substituted imidazoles are extensively used as gluca-
gon receptors,8a CB1 cannabinoid receptor antagonists,8b
modulators of P-glycoprotein-mediated multidrug resis-
tance,8c antibacterials,8d antitumor agents,8e and pesti-
cides.8f Recent advances in green chemistry and
organometallic catalysis has extended the application of
imidazoles as precursors to ionic liquids9 and N-hetero-
cyclic carbenes.10
Ar1
O
N
solvent-free
130 °C, 2 h
Br RNH
+
Ar2CHO NH4OAc
+
+
2
Ar2
Ar1
N
R
3
1
2
4
Scheme 1
The isolated products 4 were characterized on the basis of
1
IR spectroscopy, H NMR and 13C NMR spectroscopy,
SYNLETT 2009, No. 20, pp 3263–3266
x
x
.x
x
.2
0
0
9
mass spectrometry, and elemental analysis. The mass
Advanced online publication: 18.11.2009
DOI: 10.1055/s-0029-1218381; Art ID: D10809ST
spectrum of 4b displayed the molecular ion [M+] peak at
© Georg Thieme Verlag Stuttgart · New York

3264
M. Adib et al.
LETTER
m/z = 355, which was consistent with the adduct structure.
The 1H NMR spectrum of 4b exhibited two sharp singlets
arising from the methylene (d = 5.48 ppm) and the imida-
zole C5–H atom (d = 7.97 ppm), two doublets (d = 7.95
and 8.28 ppm, J = 8.8 Hz) due to the substituted phenyl
ring, along with characteristic signals with appropriate
chemical shifts and coupling constants for ten H-atoms of
NH3
Ar2
O
O
R
N+
Br
N
N
Ar1
Ar1
R
Ar2
1
5
6
Ar2
R
H2N
Ar1
Ar1
1
the other two phenyl moieties. The H-decoupled 13C
O
N
N
– H2O
– H2
NMR spectrum of 4b showed 16 distinct resonances
(CH2, 9 CH and 6 C) in agreement with the adduct struc-
ture.17
Ar2
Ar2
N
Ar1
N
R
8
R
4
7
Table 1 Synthesis of 1,2,4-Trisubstituted 1H-Imidazoles 4a–na
Scheme 2
4
Ar1
R
Ar2
Yield (%)b
terest. Solvent-free conditions, good to excellent yields of
the products, use of simple and readily available starting
materials, and fairly short reaction times are the main ad-
vantages of this method. The simplicity of this method
makes it an interesting and useful alternative to other im-
idazole syntheses.
4a
4b
4c
4d
4e
4f
Ph
Ph
Ph
90
95
91
85
96
85
84
95
93
80
86
90
89
89
Ph
Bn
Bn
Bn
n-Pr
Bn
Bn
Ph
4-O2NC6H4
3-O2NC6H4
4-MeC6H4
4-O2NC6H4
3-ClC6H4
3-MeOC6H4
4-O2NC6H4
4-ClC6H4
4-MeC6H4
Ph
Ph
Ph
Ph
Acknowledgment
Ph
This research was supported by the Research Council of University
of Tehran as a research project (6102036/1/03).
4g
4h
4i
Ph
Ph
References and Notes
Ph
Ph
(1) (a) Multicomponent Reactions; Zhu, J.; Bienaymé, H., Eds.;
Wiley: Weinheim, 2005. (b) Basso, A.; Banfi, L.; Riva, R.;
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Scriven, E. F. V.; Taylor, R. J. K., Eds.; Elsevier Science:
Oxford, 2008, 143–348; and references therein.
4j
Ph
n-Pr
n-Pr
Ph
4k
4l
Ph
4-BrC6H4
4-BrC6H4
4-BrC6H4
Ph
4m
4n
Bn
n-Pr
Ph
Ph
a Compounds 4b,e,h,i precipitated from a 1:1 mixture of acetone–H2O
and then recrystallized from 1:1 of n-hexane–EtOAc; and the other
products were purified by column chromatography (eluent: n-hexane–
EtOAc, 6:1)
b Isolated yields.
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Mechanistically it is reasonable to assume that the first
step could involve condensation of the amine and the al-
dehyde and formation of the imine intermediate 5, fol-
lowed by nucleophilic addition of the imine to the 2-
bromoacetophenone and formation of the iminium inter-
mediate 6, which may undergo addition of ammonia lead-
ing to the aminal intermediate 7. Intramolecular
condensation of the amine moiety with the adjacent carbo-
nyl would yield dihydroimidazole intermediate 8, which
could undergo air oxidation under the reaction conditions
to afford the functionalized imidazole 4 (Scheme 2).
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F.; Kumar, S.; Green, D.; McNulty, D.; Blumenthal, M. J.;
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In conclusion, we have developed a novel, one-pot, and
four-component reaction between 2-bromoacetophe-
nones, primary amines, aldehydes, and ammonium ace-
tate for the preparation of 1,2,4-trisubstituted 1H-
imidazoles of potential synthetic and pharmacological in-
Synlett 2009, No. 20, 3263–3266 © Thieme Stuttgart · New York

LETTER
Synthesis of N-Substituted 2,4-Diarylimidazoles
3265
(8) (a) de Laszlo, S. E.; Hacker, C.; Li, B.; Kim, D.; MacCoss,
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129.08, 129.33 and 129.53 (9 × CH), 134.18, 136.89,
137.40, 141.23, 145.30 and 147.41 (6 × C). MS: m/z
(%) = 355 (87) [M+], 116 (12), 92 (80), 90 (20), 89 (60), 77
(20), 65 (100). Anal. Calcd for C22H17N3O2 (355.40): C,
74.35; H, 4.82; N, 11.82. Found: C, 74.3; H, 4.8; N, 11.7.
1,2,4-Triphenyl-1H-imidazole (4a)
Colorless crystals; mp 88–89 °C (lit.18 92–94 °C). IR (KBr):
1598, 1496, 1361, 1333, 1205, 1101, 1048, 784, 693 cm–1.
1H NMR (300.1 MHz, CDCl3): d = 7.26–7.33 (m, 6 H, 6 ×
CH), 7.39–7.52 (m, 8 H, 8 × CH), 7.93 (d, J = 7.8 Hz, 2 H, 2
× CH). 13C NMR (75.5 MHz, CDCl3): d = 118.56, 125.06,
125.86, 127.03, 128.21, 128.24, 128.47, 128.64, 128.84 and
129.51 (10 × CH), 130.32, 133.87, 138.49, 141.71 and 147.0
(5 × C). Anal. Calcd for C21H16N2 (296.37): C, 85.11; H,
5.44; N, 9.45. Found: C, 85.0; H, 5.5; N, 9.4. MS: m/z
(%) = 296 (7) [M+], 279 (6), 225 (8), 197 (20), 180 (6), 167
(11), 149 (28), 105 (100), 77 (52), 51 (12).
1-Benzyl-2-(3-nitrophenyl)-4-phenyl-1H-imidazole (4c)
Colorless crystals; mp 183–185 °C. IR (KBr): 1605, 1549,
1488, 1463, 1434, 1374, 1345, 1304, 1240, 1207, 1152,
1099, 939, 855, 818, 766, 667 cm–1. 1H NMR (300.1 MHz,
DMSO-d6): d = 5.44 (s, 2 H, CH2), 7.12 (d, J = 7.3 Hz, 2 H,
2 × CH), 7.21–7.43 (m, 6 H, 6 × CH), 7.73 (dt, J = 1.4, 8.0
Hz, 1 H, CH), 7.84 (d, J = 7.7 Hz, 2 H, 2 × CH), 7.95 (s, 1 H,
CH), 8.09 (d, J = 7.6 Hz, 1 H, CH), 8.24 (d, J = 8.2 Hz, 1 H,
CH), 8.38 (d, J = 1.4 Hz, 1 H, CH). 13C NMR (75.5 MHz,
DMSO-d6): d = 50.92 (CH2), 120.27, 123.17, 123.49,
125.06, 127.11, 127.18, 128.18, 128.92, 129.23 and 130.58
(10 × CH), 132.57 and 134.49 (2 × C), 134.71 (CH), 137.33,
141.15, 145.39 and 148.62 (4 × C). MS: m/z (%) = 355 (5)
[M+], 155 (3), 111 (12), 91 (100), 80 (38), 71 (59), 57 (82).
Anal. Calcd for C22H17N3O2 (355.40): C, 74.35; H, 4.82; N,
11.82. Found: C, 74.2; H, 4.9; N, 11.6.
2-(4-Nitrophenyl)-4-phenyl-1-propyl-1H-imidazole (4e)
Colorless crystals; mp 104 °C. IR (KBr): 1595, 1510, 1475,
1384, 1329, 1203, 1100, 1017, 946, 913, 847, 804, 759, 691
cm–1. 1H NMR (300.1 MHz, CDCl3): d = 0.95 (t, J = 7.4 Hz,
3 H, CH3), 1.85 (sext, J = 7.3 Hz, 2 H, CH2CH3), 4.06 (t,
J = 7.2 Hz, 2 H, NCH2), 7.29 (t, J = 7.0 Hz, 1 H, CH), 7.40
(s, 1 H, CH), 7.41 (dd, J = 7.0, 7.5 Hz, 2 H, 2 × CH), 7.84 (d,
J = 7.5 Hz, 2 H, 2 × CH), 7.88 (d, J = 8.6 Hz, 2 H, 2 × CH),
8.35 (d, J = 8.6 Hz, 2 H, 2 × CH). 13C NMR (75.5 MHz,
CDCl3): d = 11.12 (CH3), 24.47 and 49.04 (2 × CH2),
117.72, 123.93, 124.92, 127.16, 128.67 and 129.50 (6 ×
CH(, 133.64, 137.08, 142.18, 145.49 and 147.60 (5 × C).
MS: m/z (%) = 307 (24) [M+], 279 (20), 167 (42), 151 (13),
149 (100), 113 (10), 105 (12), 83 (10), 77 (9), 71 (18), 57
(31), 55 (17), 43 (26). Anal. Calcd for C18H17N3O2 (307.35):
C, 70.34; H, 5.58; N, 13.67. Found: C, 70.3; H, 5.6; N, 13.6.
2-(4-Nitrophenyl)-1,4-diphenyl-1H-imidazole (4h)
Colorless crystals; mp 154 °C. IR (KBr): 1593, 1506, 1418,
1387, 1331, 1200, 1166, 1102, 1073, 974, 913, 849, 745, 692
cm–1. 1H NMR (300.1 MHz, CDCl3): d = 7.28–7.35 (m, 3 H,
3 × CH), 7.45 (dd, J = 7.4, 8.0 Hz, 2 H, 2 × CH), 7.50 (s, 1
H, CH), 7.51–7.53 (m, 3 H, 3 × CH), 7.66 (d, J = 8.6 Hz, 2
H, 2 × CH), 7.91 (d, J = 7.8 Hz, 2 H, 2 × CH), 8.13 (d, J = 8.5
Hz, 2 H, 2 × CH). 13C NMR (75.5 MHz, CDCl3): d = 120.13,
123.54, 125.10, 125.94, 127.50, 128.77, 129.04, 129.08 and
129.98 (9 × CH), 133.27, 136.26, 137.95, 142.70, 144.37
and 147.22 (6 × C). MS: m/z (%) = 341 (6) [M+], 279 (20),
167 (43), 149 (100), 113 (10), 105 (9), 83 (10), 71 (19), 57
(31). Anal. Calcd for C21H15N3O2 (341.37): C, 73.89; H,
4.43; N, 12.31. Found: C, 73.8; H, 4.5; N, 12.2.
(10) (a) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G.
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Mjalli, A. M. M. Tetrahedron Lett. 1996, 37, 751.
(b) Sarshar, S.; Siev, D.; Mjalli, A. M. M. Tetrahedron Lett.
1996, 37, 835.
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Lett. 1998, 39, 8939.
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(15) Das Sharma, S.; Hazarika, P.; Konwar, D. Tetrahedron Lett.
2008, 49, 2216; and references cited therein.
(16) (a) Adib, M.; Mohammadi, B.; Bijanzadeh, H. R. Synlett
2008, 3180. (b) Adib, M.; Sheibani, E.; Bijanzadeh, H. R.;
Zhu, L. G. Tetrahedron 2008, 64, 10681. (c) Adib, M.;
Sayahi, M. H.; Ziyadi, H.; Zhu, L. G.; Bijanzadeh, H. R.
Synthesis 2008, 3289. (d) Adib, M.; Mohammadi, B.;
Bijanzadeh, H. R. Synlett 2008, 177. (e) Adib, M.; Sayahi,
M. H.; Ziyadi, H.; Bijanzadeh, H. R.; Zhu, L. G.
Tetrahedron 2007, 63, 11135. (f) Adib, M.;Aali Koloogani,
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(g) Adib, M.; Sheibani, E.; Abbasi, A.; Bijanzadeh, H. R.
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E.; Mostofi, M.; Ghanbary, K.; Bijanzadeh, H. R.
Tetrahedron 2006, 62, 3435.
(17) 1-Benzyl-2-(4-nitrophenyl)-4-phenyl-1H-imidazole (4b);
Typical Procedure
A mixture of 4-nitrobenzaldehyde (0.302 g, 2 mmol),
benzylamine (0.214 g, 2 mmol), 2-bromo-1-phenylethanone
(0.398 g, 2 mmol), and ammonium acetate (0.231 g, 3 mmol)
was stirred at 130 °C for 2 h, then the reaction mixture was
cooled to r.t. and the product precipitated from a 1:1 mixture
of acetone–H2O and then recrystallized from n-hexane–
EtOAc (1:1) as colorless crystals. Yield: 0.67 g (95%); mp
178 °C. IR (KBr): 1594, 1513, 1502, 1489, 1477, 1446,
1414, 1357, 1330, 1289, 1200, 1176, 1103, 1026, 940, 854,
761, 731, 710 cm–1. 1H NMR (500.1 MHz, DMSO-d6):
d = 5.48 (s, 2 H, CH2), 7.10 (d, J = 7.3 Hz, 2 H, 2 × CH), 7.24
(t, J = 7.6 Hz, 1 H, CH), 7.26 (t, J = 7.5 Hz, 1 H, CH), 7.32
(dd, J = 7.2, 7.7 Hz, 1 H, CH), 7.38 (dd, J = 7.6, 7.8 Hz, 1 H,
CH), 7.84 (d, J = 7.3 Hz, 2 H, 2 × CH), 7.95 (d, J = 8.8 Hz,
2 H, 2 × CH), 7.97 (s, CH, 1 H, CH), 8.28 (d, J = 8.8 Hz, 2
H, 2 × CH). 13C NMR (75.5 MHz, DMSO-d6): d = 50.83
(CH2), 121.23, 124.29, 124.90, 127.05, 127.29, 128.21,
2-(4-Chlorophenyl)-1,4-diphenyl-1H-imidazole (4i)
Colorless crystals; mp 146–148 °C. IR (KBr): 1596, 1489,
1447, 1413, 1240, 1206, 1170, 1087, 1013, 975, 912, 830,
751, 691 cm–1. 1H NMR (300.1 MHz, CDCl3): d = 7.24–7.35
Synlett 2009, No. 20, 3263–3266 © Thieme Stuttgart · New York

3266
M. Adib et al.
LETTER
(21), 227 (65), 192 (26), 165 (39), 139 (30), 105 (75), 89
(27), 77 (72), 51 (32). Anal. Calcd for C21H15ClN2 (330.82):
C, 76.25; H, 4.57; N, 8.47. Found: C, 76.1; H, 4.7; N, 8.3.
(18) Sharma, A. K.; Mazumdar, S. N.; Mahajan, M. P. J. Chem.
Soc., Perkin Trans. 1 1997, 3065.
(m, 5 H, 5 × CH), 7.41–7.47 (m, 8 H, 8 × CH), 7.92 (d,
J = 8.0 Hz, 2 H, 2 × CH). 13C NMR (75.5 MHz, CDCl3):
d = 118.85, 125.06, 125.87, 127.17, 128.51, 128.52 and
128.69 (7 × CH), 128.79 (C), 129.69 and 130.01 (2 × CH),
133.69, 134.52, 138.25, 141.89 and 145.84 (5 × C). MS:
m/z (%) = 332 (45) [M+, 37Cl], 330 (100) [M+, 35Cl], 329
Synlett 2009, No. 20, 3263–3266 © Thieme Stuttgart · New York