One-pot synthesis of 2,4,5-trisubstituted imidazoles using [BPy]H 2PO4, an efficient and recyclable catalyst

Article abstract of DOI:10.1080/10826068.2012.719845

[BPy]H₂PO₄ was easily prepared and used as an efficient and recyclable catalyst for the one-pot synthesis of 2,4,5-trisubstituted imidazoles under solvent-free conditions in good to excellent yields. Solvent-free conditions, simple experimental and workup procedures, and the use of a nontoxic, recyclable catalyst of an environmentally benign nature are remarkable features of the procedure.

Full text of DOI:10.1080/10826068.2012.719845

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ONE-POT SYNTHESIS OF 2,4,5-
TRISUBSTITUTED IMIDAZOLES USING
[BPy]H2PO4, AN EFFICIENT AND
RECYCLABLE CATALYST
Yuliang Zhang a & Zhongqiang Zhou a
a Key Laboratory of Catalysis and Materials Science of the State
Ethnic Affairs Commission and Ministry of Education, Hubei
Province, College of Chemistry and Materials Science, South-Central
University for Nationalities , Wuhan , China
Accepted author version posted online: 22 Aug 2012.Published
online: 10 Jan 2013.
To cite this article: Yuliang Zhang & Zhongqiang Zhou (2013) ONE-POT SYNTHESIS OF 2,4,5-
TRISUBSTITUTED IMIDAZOLES USING [BPy]H2PO4, AN EFFICIENT AND RECYCLABLE CATALYST, Preparative
Biochemistry and Biotechnology, 43:2, 189-196, DOI: 10.1080/10826068.2012.719845
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Preparative Biochemistry & Biotechnology, 43:189–196, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 1082-6068 print/1532-2297 online
DOI: 10.1080/10826068.2012.719845
ONE-POT SYNTHESIS OF 2,4,5-TRISUBSTITUTED IMIDAZOLES
USING [BPy]H2PO4, AN EFFICIENT AND RECYCLABLE CATALYST
Yuliang Zhang and Zhongqiang Zhou
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission
and Ministry of Education, Hubei Province, College of Chemistry and Materials Science,
South-Central University for Nationalities, Wuhan, China
& [BPy]H2PO4 was easily prepared and used as an efficient and recyclable catalyst for the
one-pot synthesis of 2,4,5-trisubstituted imidazoles under solvent-free conditions in good to excellent
yields. Solvent-free conditions, simple experimental and workup procedures, and the use of a
nontoxic, recyclable catalyst of an environmentally benign nature are remarkable features of the
procedure.
Keywords 2,4,5-trisubstituted imidazoles, [BPy]H2PO4, solvent-free
INTRODUCTION
The imidazole moiety is a five-membered ring with heterocyclic
nucleus, important due to its use in synthetic chemistry and widespread
biological activity. Imidazoles are substantially used in the synthesis of ionic
liquids[1] that have been given a new approach to green chemistry. In
addition, the imidazole ring system is one of the most important substruc-
tures found in a large number of natural products and pharmacologically
active compounds such as the fungicide ketoconazole,[2] the proton pump
inhibitor omeprazole,[3] the antiulcerative cimetidine,[4] and the hypnotic
agent etomidate.[5]
A literature survey reveals that 2,4,5-trisubstituted imidazoles are gener-
ally synthesized by one-pot, three-component condensation of aromatic
aldehydes, ammonium acetate, benzil, or benzoin in the presence of cata-
lysts such as cellulose sulfuric acid,[6] boric acid,[7] Yb(OTf)3,[8] potassium
aluminum sulfate,[9] ZrOC12 ꢀ 8H2O,[10] NiCl2 ꢀ 6H2O,[11] L-proline,[12]
AcOH,[13–15] glyoxylic acid,[16] sodium bisulfite,[17] UO2(NO3)2 ꢀ 6H2O,[18]
Address correspondence to Zhongqiang Zhou, College of Chemistry and Materials Science,
South-Central University for Nationalities, Wuhan 430074, P. R. China. E-mail: zhou-zq@hotmail.com

190
Y. Zhang and Z. Zhou
SCHEME 1 Synthesis of [BPy]H2PO4 under solvent-free conditions.
MoO3=SiO2,[19] ceric ammonium nitrate,[20] ionic liquid ([Hbim]BF4,[21]
,
tetramethylguanidinium trifluoroacetate[22]), iodine,[23] PEG-400,[24]
KH2PO4,[25] potassium hydrogen sulfate and dibenzo-18-crown-6,[26]
InCl3 ꢀ 3H2O,[27] ammonium metavanadate,[28] and tetrabutyl ammonium-
bromide.[29] The synthetic protocols for imidazoles just described have their
own merit, while some of these suffer from one or more disadvantages, such
as use of organic solvents,[8,9,11,19,23,25] use of toxic catalysts and large
amounts of catalysts that are usually nonrecoverable,[10,20,27] long reaction
times,[29] laborious and complex workup procedures,[28] and the require-
ment of special apparatus.[6,7,16] Exploring a simpler and greener method
for the efficient synthesis of 2,4,5-trisubstituted imidazoles is highly desirable.
Solvent-free organic synthesis has received considerable attention due
to growing worldwide concerns over chemical wastes and future
resources.[30] Ionic liquids have been turned out to be a type of promising
alternative medium=catalysts for various chemical processes.[31] Typical
ionic liquids consist of halogen-containing anions (e.g., BFꢁ4 , PF6ꢁ,
CF3COOꢁ), which some regard as a limit to their ‘‘greenness.’’[32,33]
Recently, halogen-free ionic liquids ([(CH2)4SO3HMIM][HSO4],[34] N-
methyl-2-pyrrolidonium hydrogen sulfate[35]) catalyzing the synthesis of
2,4,5-trisubstituted imidazoles have been reported. Herein, we report a sim-
plified procedure for the preparation of halogen-free ionic liquid
[BPy]H2PO4 (Scheme 1); its use as the novel catalyst in one-pot synthesis
of 2,4,5-trisubstituted imidazoles under solvent-free conditions has also
been investigated (Scheme 2). To the best of our knowledge, such ionic
liquid used for the synthesis of imidazole derivatives has not been reported
in the open literature.
SCHEME 2 [BPy]H2PO4 catalyzed synthesis of 2,4,5-trisubstituted imidazoles.

Efficient and Recyclable Catalyst
191
EXPERIMENTAL
All chemicals were commercial products. Fourier-transform infrared
(FT-IR) spectra were obtained on a Nexus 470 spectrophotometer.
1H-Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker
Avance III 400 with tetramethylsilane (TMS) as internal standard. Melting
points were determined on a melting-point apparatus and were uncorrected.
Preparation of Ionic Liquid [BPy]H2PO4
1-Bromobutane (10.8 mL, 0.1 mol) was added to pyridine (8 mL,
0.1 mol) in a 100-mL three-neck round-bottom flask fitted with a reflux con-
denser. This mixture was stirred with a mechanical stirrer at 90ꢂC for 12 hr
under solvent-free conditions. Then NaH2PO4 ꢀ 2H2O (15.6 g, 0.1 mol) was
added to the three-neck round-bottom flask. The resulting mixture was stir-
red at 90ꢂC for 1 hr. The reaction mixture was cooled to room temperature
and filtered, and the filtrate was dried under vacuum to afford [BPy]H2PO4
as a yellow liquid in 80% yield. IR (KBr, cmꢁ1): 3417, 3037, 2959, 2870,
1630, 1486, 1169, 774, 670. 1H-NMR (DMSO-d6, 400 MHz) d: 0.89 (t,
J ¼ 6.4 Hz, 3H), 1.26–1.28 (m, 2H), 1.89 (s, 2H), 4.68 (s, 2H), 8.18 (s,
2H), 8.63 (s, 1H), 9.22 (s, 2H). 13C NMR (DMSO-d6, 100 MHz) d: 13.8,
19.21, 33.14, 60.93, 128.55, 145.23, 145.95.
General Procedure for the Synthesis of 2,4,5-Trisubstituted
Imidazoles
A mixture of benzil 1 (5 mmol), aromatic aldehyde 2 (5 mmol),
ammonium acetate (10 mmol), and [BPy]H2PO4 (15 mol%) was stirred at
130ꢂC for the appropriate time indicated later in Table 3. The progress of
the reaction was monitored by thin-layer chromatography (petroleum
ether=ethyl acetate at 5=1 as eluent). After completion of reaction, the reac-
tion mixture was cooled to room temperature and diluted with water. The
solid imidazole products that separated out were filtered, washed with water,
dried, and recrystallized from ethanol to get the corresponding 2,4,5-trisub-
stituted imidazoles. All products (3a–3o) obtained are known compounds
and were identified by comparing of their physical data with the reported
ones. The melting points determined during this present work are in agree-
ment with the values reported in the literature (Table 3). Some characteriza-
tion data for selected known products are given next.
2,4,5-Triphenyl-1H-imidazole (3a): IR (KBr, cmꢁ1): 3424, 3041, 2972,
1
1601, 1490, 1461, 1126, 1072, 769, 733, 697. H-NMR (400 MHz, CDCl3=
DMSO-d6) d: 7.29–7.63 (m, 13H), 8.12 (d, J ¼ 7.6 Hz, 2H).

192
Y. Zhang and Z. Zhou
2-(4-Methoxyphenyl)-4,5-diphenyl-1H-imidazole (3c): IR (KBr, cmꢁ1):
3418, 3056, 2963, 1611, 1496, 1446, 1290, 1248, 1177, 1130, 1032, 969,
1
832, 768, 730, 697. H-NMR (400 MHz, CDCl3=DMSO-d6) d: 3.85 (s, 3H),
6.97 (d, J ¼ 8.4 Hz, 2H), 7.27–7.76 (m, 10H), 8.06 (d, J ¼ 8.4 Hz, 2H).
2-(2-Chlorophenyl)-4,5-diphenyl-1H-imidazole (3d): IR (KBr, cmꢁ1):
3426, 3061, 2955, 1601, 1478, 1446, 1126, 1034, 971, 763, 735, 695.
1H-NMR (400 MHz, CDCl3) d: 7.31–7.58 (m, 13H), 8.45 (d, J ¼ 7.6 Hz, 1H).
2-(4-Fluorophenyl)-4,5-diphenyl-1H-imidazole (3e): IR (KBr, cmꢁ1):
3425, 3061, 2955, 1602, 1479, 1446, 1119, 1073, 972, 873, 765, 735, 700.
1H-NMR (400 MHz, CDCl3) d: 7.31–7.58 (m, 12H), 8.45 (d, J ¼ 7.6 Hz, 2H).
2-(3-Methoxyphenyl)-4,5-diphenyl-1H-imidazole (3f): IR (KBr, cmꢁ1):
3434, 3058, 2963, 1592, 1482, 1444, 1240, 1047, 988, 843, 769, 696. 1H-NMR
(400MHz, CDCl3=DMSO-d6) d: 3.89 (s, 3H), 6.91 (s, 1H), 7.34–8.03 (m, 13H).
RESULTS AND DISCUSSION
The preparation of the catalyst [BPy]H2PO4 was made up of a two-step
reaction. The precursor [BPy]Br used was prepared by reaction of pyridine
and 1-bromobutane at 90ꢂC under mechanical stirring for 12 hr. Then one
equivalent of NaH2PO4 ꢀ 2H2O was added and the reaction was carried out
for another hour under the same conditions. The reaction mixture was
cooled to room temperature and filtered; the filtrate was dried under vac-
uum to afford [BPy]H2PO4 as ꢁa yellow liquid at room temperature in 80%
yield. Ionic liquids with H2PO4 as counteranion are generally prepared by
reaction of the precursor and one equivalent of phosphoric acid in organic
solvent.[36] The significant advantages of using sodium bisulfate in place of
phosphoric acid in the preparation of [BPy]H2PO4 are to avoid the release
of corrosive hydrogen bromide and to avoid the use of organic solvent.
In our search for the best experimental reaction conditions in the prep-
aration of 2,4,5-trisubstituted imidazoles, the effect of reaction temperature
was examined using the reaction of benzaldehyde, benzil, and ammonium
acetate in the presence of 10 mol% of [BPy]H2PO4 under solvent-free con-
ditions. As can be seen from Table 1, increasing the reaction temperature
in a range from 100 to 130ꢂC resulted in a decrease in the reaction time.
Further increase in temperature does not lead to an improvement in the
reaction rate. The optimum temperature was at about 130ꢂC. The effect
of the amount of catalyst was also examined. The optimum catalyst loading
for [BPy]H2PO4 was found to be about 15 mol%. When the amount of the
catalyst decreased to 10 mol% from 15 mol% relative to the substrates, the
yield was reduced slightly (Table 1, entries 4 and 7). However, the use of
20 mol% of the catalyst showed the same yield and the same time was
required (Table 1, entry 8).

Efficient and Recyclable Catalyst
193
TABLE 1 Effect of Different Reaction Conditions on [BPy]H2PO4 Catalyzed Synthesis of
2,4,5-Trisubstituted Imidazolesa
Entry
Temperature (ꢂC)
Catalyst (mol%)
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
100
110
120
130
140
130
130
130
10
10
10
10
10
5
65
55
45
40
40
40
40
40
90
88
90
90
91
77
94
94
15
20
aReaction condition: benzaldehyde (5 mmol), benzil (5 mmol), ammonium acetate
(10 mmol) in presence of [BPy]H2PO4 under solvent-free conditions.
bIsolated yield.
From both an environmental viewpoint and an economic viewpoint, it
is highly desirable that the catalyst can be recovered and reused. The reu-
sability of the [BPy]H2PO4 catalyst was also examined. After completion of
the reaction, the reaction mixture was cooled to room temperature and
diluted with water. The solid imidazole products were isolated by simple fil-
tration, washed with water. The combined aqueous filtrate was vacuumed to
remove water and the resulting catalyst was reused directly for the next run.
As shown in Table 2, the recovered catalyst can be reused at least three
additional times in subsequent reactions with almost consistent activity as
well as without any appreciable loss in the yield of the final products.
With these results in hand, we investigated one-pot synthesis of 2,4,5-
trisubstituted imidazoles by three-component condensation of a variety of
aromatic aldehydes (2a–2o), ammonium acetate and benzil (1) at 130ꢂC
in the presence of 15 mol% of [BPy]H2PO4 under solvent-free conditions
(Scheme 2). The results are summarized in Table 3. The aromatic alde-
hydes bearing both electron-donating and electron-withdrawing groups
are desirable substrates for this reaction. In all the cases, the corresponding
TABLE 2 Recycling of the [BPy]H2PO4 Catalysta
Run
Time (min)
Yieldb (%)
1
2
3
4
40
40
40
40
94
94
92
89
aReaction condition: benzaldehyde (10 mmol), benzil (10 mmol),
ammonium acetate (20 mmol), [BPy]H2PO4 (15 mol%) at 130ꢂC
under solvent-free conditions.
bIsolated yield.

194
Y. Zhang and Z. Zhou
TABLE 3 [BPy]H2PO4 Catalyzed synthesis of 2,4,5-Trisubstituted Imidazolesa
Melting Points (ꢂC)
Entry Aldehyde
R
Time (min) Product Yieldb (%) Found
Literature
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
C6H5
40
50
55
50
60
60
70
60
70
90
50
90
50
90
180
3a
3b
3c
3d
3e
3f
3 g
3 h
3i
3j
3k
3 l
3 m
3n
3o
94
84
90
87
92
85
77
93
94
91
87
87
94
91
96
280–282 280–281[24]
231–233 231–233[24]
228–230 230–232[24]
196–198 194–196[12]
187–189 189–190[25]
250–252 252–255[14]
258–260 257–258[25]
260–262 260–262[25]
4-CH3C6H4
4-CH3OC6H4
2-ClC6H4
4-FC6H4
3-CH3OC6H4
4-(CH3)2NC6H4
4-BrC6H4
3-NO2C6H4
2-NO2C6H4
4-ClC6H4
2,4-Cl2C6H3
4-NO2C6H4
2-OHC6H4
4-OHC6H4
>300
>300[24]
233–235 234–235[15]
260–262 260–262[25]
177–178 178–179[24]
229–231 230–232[12]
207–208 203–205[23]
259–261 260–261[23]
aReaction condition: Aromatic aldehyde (5 mmol), benzil (5 mmol), ammonium acetate (10 mmol),
[BPy]H2PO4 (15 mol%) at 130ꢂC under solvent-free conditions.
bIsolated yield.
2,4,5-trisubstituted imidazoles (3a–3o) were obtained in good to excellent
yields in a short time.
CONCLUSION
In summary, we developed a simple and efficient procedure for prep-
aration of room-temperature ionic liquid [BPy]H2PO4. [BPy]H2PO4 was
found to catalyze the one-pot synthesis of 2,4,5-trisubstituted imidazoles
under solvent-free conditions with good to excellent yields in a short time.
Moreover, experimental and workup procedures are simple. In terms of
green chemistry, the catalyst is recyclable for up to four uses with only
negligible loss of activity, rendering the process more economic.
ACKNOWLEDGMENT
We thank the Natural Science Foundation of Hubei Province (grants
2007ABA291, 2008CDB036) for financial support.
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