Tandem oxidation process using ceric ammonium nitrate: Three-component synthesis of trisubstituted imidazoles under aerobic oxidation conditions

Article abstract of DOI:10.1080/00397910802369661

By combining a ceric ammonium nitrate (CAN)-catalyzed aerobic oxidation of α-hydroxy ketones with a subsequent three-component condensation reaction using aldehydes and ammonium acetate, highly substituted imidazole derivatives can be assembled. This reaction is an important approach for the synthesis of highly substituted imidazole derivatives and was carried out in good to excellent yields in ethanol as an environmentally benign solvent. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910802369661

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Tandem Oxidation Process
Using Ceric Ammonium Nitrate:
Three-Component Synthesis
of Trisubstituted Imidazoles
Under Aerobic Oxidation
Conditions
Ahmad Shaabani a , Ali Maleki a & Maryam Behnam a
a Department of Chemistry , Shahid Beheshti
University , Tehran, Iran
Published online: 15 Feb 2011.
To cite this article: Ahmad Shaabani , Ali Maleki & Maryam Behnam (2008) Tandem
Oxidation Process Using Ceric Ammonium Nitrate: Three-Component Synthesis
of Trisubstituted Imidazoles Under Aerobic Oxidation Conditions, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 39:1, 102-110, DOI: 10.1080/00397910802369661
To link to this article: http://dx.doi.org/10.1080/00397910802369661
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Synthetic Communications1, 39: 102–110, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802369661
Tandem Oxidation Process Using Ceric
Ammonium Nitrate: Three-Component
Synthesis of Trisubstituted Imidazoles Under
Aerobic Oxidation Conditions
Ahmad Shaabani, Ali Maleki, and Maryam Behnam
Department of Chemistry, Shahid Beheshti University, Tehran, Iran
Abstract: By combining a ceric ammonium nitrate (CAN)–catalyzed aerobic
oxidation of a-hydroxy ketones with a subsequent three-component condensation
reaction using aldehydes and ammonium acetate, highly substituted imidazole
derivatives can be assembled. This reaction is an important approach for the
synthesis of highly substituted imidazole derivatives and was carried out in good
to excellent yields in ethanol as an environmentally benign solvent.
Keywords: Aerobic oxidation, ceric ammonium nitrate (CAN), tandem oxidation
process (TOP), three-component reaction, trisubstituted imidazole
INTRODUCTION
Imidazoles are an important class of heterocyclic compounds with an
ability to behave as ligands in metalloenzymes,[1,2] non natural metal
complexes,[3] and highly significant biomolecules, such as the essential
amino acid histidine, biotin, and the pilocarpine alkaloids.[4]
The structures of trisubstituted imidazoles are prevalent in natural
products and pharmacologically active compounds, such as the known
P38 kinase inhibitor I (SB 203580)[5] and cyclooxygenase-2 (COX-2)
inhibitor II (Fig. 1).[6] Furthermore, triarylimidazoles display various
Received June 2, 2008.
Address correspondence to Ahmad Shaabani, Department of Chemistry,
Shahid Beheshti University P. O. Box 19396-4716, Tehran, Iran. E-mail:
a-shaabani@cc.sbu.ac.ir
102

Tandem Oxidation Using CAN
103
Figure 1. Strucures of P38 kinase inhibitor I and COX-2 inhibitor II.
bioactive effects such as herbicidal,[7] fungicidal,[8] analgesic,[9] anti-
inflammatory[10] and antithrombotic activities.[11]
Although there are several papers reporting the synthesis of trisub-
stituted imidazoles using 1,2-diketones,[9–13] there are few reports in the
literature using a-hydroxy ketones as statrting material.[14]
Tandem methodology, especially the tandem oxidation process
(TOP) for heterocyclic synthesis, represents a powerful approach for the
rapid buildup of molecular complexity from potentially simple starting
materials.[15–19]
In continuation of our interest in catalytic–aerobic oxidation condi-
tions for the synthesis of organic heterocyclic compounds,[20–24] herein,
the discovery of ceric ammonium nitrate (CAN)–TOP methodology in
the synthesis of 2,4,5-trisubstituted imidazole derivatives is reported
(Scheme 1).
RESULTS AND DISCUSSION
Although CAN is far superior to many other one-electron oxidants, the
vast majority of CAN-mediated oxidations require more than 2 equiva-
lents of the oxidant for completion of the reaction. This precludes its
use in large-scale transformations. Development of reactions requiring
only catalytic amounts of CAN is therefore very important.[25–27]
Scheme 1. Synthesis of 2,4,5-trisubstituted imidazole derivatives via TOP using CAN.

104
A. Shaabani, A. Maleki, and M. Behnam
As shown in Table 1, to illustrate the need of catalyst, a reaction of
benzoin, benzaldehyde, and ammonium acetate has been studied in var-
ious molar ratios of CAN under air-blowing conditions. The best results
have been obtained with 5 mol% of CAN after 6 h in refluxing ethanol
(entry 1). The yield of reaction with increasing quantity of CAN is not
considerably increased (entry 3). The results also showed that the effi-
ciency and the yield of the reaction in EtOH were higher than those
obtained in other solvents, such as toluene and H2O (entries 4 and 5).
It is interesting to note that in the absence of CAN, the reaction yield
was trace after 6 h (entry 6). Without air blowing, this reaction did not
proceed efficiently, and the reaction yield was only 20% after stirring
for 6 h in refluxing ethanol (entry 7). Therefore, CAN and air are two
important components of the reaction.
The results of aerobic oxidation and CAN-catalyzed condensation reac-
tion of a-hydroxy ketone 1, benzaldehyde 2, and ammonium acetate 3 are
given in Table 2. The procedure gives the products in excellent yields.
To examine the scope and limitations of this reaction, we carried out
the reaction of various aromatic aldehydes including both electron-
donating and electron-withdrawing substituents in ortho, meta, and para
positions of benzaldehyde (Table 2). We found that the reaction proceeds
very efficiently in all cases and the reaction time decreases for aldehydes
caring electron-withdrawing substitutents. The best result was obtained
with 4-bromobenzaldehyde (85% yield, entry 4), and the least yield
(65%) was obtained from salicylaldehyde after stirring for 6h in refluxing
ethanol (entry 8).
Table 1. Effects of molar ratios of CAN and air-blowing conditions for the
synthesis of 4a in various refluxing solventa
Molar ratio
of catalyst
Entry
Solvent
Time (h)
Yieldb (%)
1
2
3
4
0.05
0.01
1.0
0.05
0.05
—
EtOH
EtOH
EtOH
PhCH3
H2O
6
12
6
6
6
75
45
77
30
60
trace
20
5
6c
7d
EtOH
EtOH
6
6
0.05
aBenzoin (1 mmol), benzaldehyde (1 mmol), and ammonium acetate (6 mmol).
bIsolated yields.
cIn the absence of the catalyst.
dWithout air blowing.

Tandem Oxidation Using CAN
105
Table 2. Synthesis of 2,4,5-trisubstituted imidazoles in the presence of CAN
under aerobic oxidation conditions
Entry
R
X
Product
Time (h)
Yielda (%)
1
2
3
4
5
6
7
8
9
10
C6H5
C6H5
C6H5
C6H5
C6H5
H
4a
4b
4c
4d
4e
4f
6
5
5
5
3
3
3
6
4
3
75
65
73
85
78
76
72
65
72
74
4-CH3
4-CH3O
4-Br
3-Cl
4-Cl
3-NO2
2-OH
H
C6H5
C6H5
4g
4h
4i
C6H5
4-CH3O-C6H4
4-CH3O-C6H4
4-CH3
4j
aIsolated yields.
The possible mechanism of the reaction between the a-hydroxy
ketone 1, benzaldehyde 2, and ammonium acetate 3 in the presence
of CAN is shown in Scheme 2. CAN plays dual roles in this reaction:
(i) Ceþ4 moiety oþf CAN catalyzed oxidation of a-hydroxy ketone 1 to
benzil 5. (ii) NH4 moiety of CAN acts as a protic acidcatalyst, which
promotes the condensation of a-diketone 5 with benzaldehyde 2 and
ammonium acetate 3 during the activation process, and reaction times
decreased.
In conclusion, a new, green, and efficient approach has been devel-
oped for the synthesis of 2,4,5-trisubstituted imidazole derivatives using
a-hydroxy ketones, aldehydes, and ammonium acetate in the presence
of a catalytic amount of CAN under aerobic oxidation conditions in
good to excellent yields. This three-component reaction was carried out
in ethanol as an environmentally benign solvent. To the best of our
Scheme 2. The possible mechanism for the synthesis of 2,4,5-trisubstituted
imidazole derivatives via TOP using CAN.

106
A. Shaabani, A. Maleki, and M. Behnam
knowledge, this is the first report of the synthesis of trisubstituted imida-
zoles using CAN in conjunction with aerobic oxidation, and this new
reaction condition is an important alternative to the use of toxic solvents
and catalysts.
EXPERIMENTAL
Melting points were measured on an Electrothermal 9200 apparatus.
Mass spectra were recorded on a Finnigan-MAT 8430 mass spectrometer
operating at an ionization potential of 70 eV. IR spectra were recorded on
a Shimadzu IR-470 spectrometer. 1H and 13C NMR spectra were
recorded on a Bruker DRX-300 Avance spectrometer at 300.13 and
75.47 MHz, respectively. All chemicals were purchased from Fluka,
Merck, and Aldrich and were used without purification. All products
are known compounds. The structures of products were unambiguously
established on the basis of their spectral analysis (IR, 1H, 13C NMR, and
mass spectral data).
Typical Procedure for the Preparation of 2,4,5-Triphenylimidazole (4a)
A mixture of benzoin (1 mmol), benzaldehyde (1 mmol), ammonium acet-
ate (6 mmol), and CAN (0.05 mmol) were taken in a 50-mL, two-necked,
round-bottomed flask equipped with a gas-passing tube. Air was bubbled
at a rate of 5 mL=min, into the reaction mixture while it was stirred for
6 h under reflux in 10 mL of EtOH. After completion of the reaction as
indicated by thin-layer chromatography (TLC; ethyl acetate=n -hexane,
2:1), the reaction mixture was filtered and washed with water
(3 ꢀ 10 mL), and the solid residue was crystallized from ethyl acetateꢂt1o
ꢁ
give pure product 4a as white solid; mp > 250 C. IR (KBr) cm
:
1
3432, 1660, 1550, 1486, 1462. H NMR (DMSO-d6) d: 12.55 (1H, br s),
7.50–7.90 (15H, m). 13C NMR (DMSO-d6) d: 125.1, 126.4, 127.0,
127.7, 128.1, 128.2, 128.3, 128.4, 130.3, 131.0, 135.1, 137.1, 145.3. MS
m=z: 296 (Mþ), 165, 148, 77, 55.
2-(4-Methylphenyl)-4,5-diphenylimidazole (4b)
White solid; mp 230–231 ꢁC. IR (KBr) cmꢂ1: 3405, 3033, 1602, 1493,
1
1449. H NMR (DMSO-d6) d: 12.60 (1H, br s), 7.98 (2H, d, J ¼ 8.4 Hz),
), 7.20–7.56 (12H, m), 2.23 (3H, s). 13C NMR (DMSO-d6) d: 20.8, 125.3,
126.4, 127.0, 127.6, 127.6, 127.6, 128.3, 128.3, 128.6, 129.2, 131.1, 135.2,
136.8, 137.6, 145.5. MS m=z: 310 (Mþ), 190, 165, 155, 103, 89, 77, 55.

Tandem Oxidation Using CAN
107
2-(4-Methoxyphenyl)-4,5-diphenylimidazole (4c)
White solid; mp 230–231 ꢁC. IR (KBr) cmꢂ1: 3402, 3056, 1612, 1492,
1
1450. H NMR (DMSO-d6) d: 12.51 (1H, br s), 8.01 (2H, dt, J ¼ 8.8 Hz,
Hz, 2.0 Hz), 7.52 (4H, d, J ¼ 7.2 Hz), 7.37 (4H, t, J ¼ 7.2 Hz), 7.29 (2H, t,
J ¼ 7.2 Hz), 7.05 (2H, dt, J ¼ 8.8Hz, 2.0 Hz), 3.81 (3H, s). 13C NMR
(DMSO-d6) d: 55.1, 114.0, 122.6, 126.7, 127.3, 127.6, 128.3, 145.6,
159.4. MS m=z: 326 (Mþ), 311, 283, 235, 165, 97, 85, 69, 55.
2-(4-Bromophenyl)-4,5-diphenylimidazole (4d)
White solid; mp > 250 ꢁC. IR (KBr) cmꢂ1: 3400, 3060, 1601, 1482, 1450,
1
1429. H NMR (DMSO-d6) d: 12.79 (1H, br s), 8.04 (2H, d, J ¼ 8.4 Hz),
7.68 (2H, d, J ¼ 8.4 Hz), 7.55–7.23 (10H, m). 13C NMR (DMSO-d6)
d: 121.3, 126.1, 127.0, 127.6, 128.1, 128.3, 128.6, 129.6, 130.8, 131.6,
134.9, 137.3, 144.0. MS m=z: 374 (Mþ ꢂ 1), 295, 190, 165, 147, 89, 77, 55.
2-(3-Chlorophenyl)-4,5-diphenyl-1H-imidazole (4e)
Light yellow solid; mp > 250 C. IR (KBr) cmꢂ1: 3405, 3060, 1605, 1465,
ꢁ
1445, 1382. 1H NMR (DMSO-d6) d: 12.68 (1H, br s), 7.20–8.20 (14H, m).
13C NMR (DMSO-d6) d: 127.0, 126.3, 127.1, 128.2, 128.5, 128.7, 128.8,
128.9, 129.2, 129.4, 130.84, 130.7, 132.2, 135.2, 137.2, 144.5. MS m=z:
330 (Mþ), 256, 165, 147, 129, 111, 97, 69, 55.
2-(4-Chlorophenyl-4,5-diphenylimidazole (4f)
White solid; mp > 250 ꢁC. IR (KBr) cmꢂ1: 3405, 3060, 1603, 1485, 1448,
1
1433. H NMR (DMSO-d6) d: 12.79 (1H, br s), 7.21–7.56 (12H, m), 8.11
(2H, d, J ¼ 8.8 Hz). 13C NMR (DMSO-d6) d: 126.5, 126.2, 127.0, 128.1,
128.3, 128.5, 128.4, 128.7, 129.1, 129.1, 130.81, 130.8, 132.5, 134.9, 137.2,
144.3. MS m=z: 330 (Mþ), 256, 165, 147, 129, 111, 97, 83, 69, 55.
2-(3-Nitrophenyl)-4,5-diphenylimidazole (4g)
Yellow solid; mp > 250 ꢁC. IR (KBr) cmꢂ1: 3382, 3064, 1605, 1519,
1
1348. H NMR (DMSO-d6) d: 13.10 (1H, br s), 8.96 (1H, t, J ¼ 2.0 Hz),
8.50–8.55 (1H, m), 8.15–8.25 (1H, m), 7.78 (1H, t, J ¼ 8.0 Hz),
7.24–7.60 (10H, m). 13C NMR (DMSO-d6) d: 119.4, 122.5, 126.7,
128.0, 128.2, 128.3, 128.6, 129.0, 130.3, 130.5, 131.1, 131.5, 134.7,

108
A. Shaabani, A. Maleki, and M. Behnam
137.5, 143.2, 148.3. MS m=z: 340 (Mþ), 311, 233, 223, 206, 173, 165, 145,
115, 91, 57.
2-(4,5-Diphenyl-1H-imidazol-2-yl)phenol (4h)
Yellow solid; mp 203–204 ꢁC; IR (KBr) cmꢂ1: 3428, 3215, 1602, 1532.
1H NMR (DMSO-d6) d: 12.51 (1H, br s), 7.2–7.52 (10H, m), 6.97–7.01
(2H, d, J ¼ 7.8 Hz), 6.86–6.93 (2H, d, J ¼ 7.5 Hz). 13C NMR (DMSO-d6)
d: 156.5, 145.4, 129.1, 128.3, 127.9, 126.9, 124.7, 119.0, 116.5, 112.8. MS
m=z: 313 (Mþ þ 1), 232, 206, 173, 145, 115, 105, 91, 65.
4,5-Bis(4-methoxyphenyl)-2-phenyl-1H-imidazole (4i)
Yellow solid; mp 202–203 ꢁC; IR (KBr) cmꢂ1: 3325, 1606, 1532, 1436.
1H NMR (DMSO-d6) d: 12.54 (1H, br s), 7.1–7.95 (13H, m), 3.83
(6H, s). 13C NMR (DMSO-d6) d: 158.2, 145.1, 129.1, 128.5, 127.7,
126.9, 124.5, 119.0, 116.5, 111.9, 56. MS m=z: 356 (Mþ), 342, 205, 172,
145, 115, 107, 91, 57.
4,5-Bis(4-methoxyphenyl)-2-p-tolyl-1H-imidazole (4j)
Yellow solid; mp 212–215 ꢁC; IR (KBr) cmꢂ1: 3328, 1605, 1533, 1432.
1H NMR (DMSO-d6) d: 12.57 (1H, br s), 7.1–8.10 (12H, m), 3.85
(6H, s), 2.34 (3H, s). 13C NMR (DMSO-d6) d: 156.5, 145.4, 129.1,
128.3, 127.9, 126.9, 124.7, 119.0, 116.5, 112.8, 57, 23. MS m=z: 370
(Mþ), 341, 203, 172, 145, 115, 103, 91, 55.
ACKNOWLEDGMENT
We gratefully acknowledge the financial support from the Research
Council of Shahid Beheshti University.
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