One-pot synthesis of 2,4,5-trisubstituted imidazole derivatives catalyzed by BTPPC under solvent-free conditions

Article abstract of DOI:10.4314/bcse.v31i1.16

A simple and efficient method for one-pot synthesis of lophine derivatives (2,4,5-trisubstituted imidazoles) by using the benzyltriphenylphosphonium chloride (BTPPC), as a catalyst, under solvent-free conditions is described. BTPPC is an available and inexpensive catalyst; also, it can be easily supplied. This procedure led to the corresponding 2,4,5-trisubstituted imidazoles products in high yields.

Full text of DOI:10.4314/bcse.v31i1.16

Bull. Chem. Soc. Ethiop. 2017, 31(1), 177-184.
 2017 Chemical Society of Ethiopia and The Authors
DOI: http://dx.doi.org/10.4314/bcse.v31i1.16
ISSN 1011-3924
Printed in Ethiopia
SHORT COMMUNICATION
ONE-POT SYNTHESIS OF 2,4,5-TRISUBSTITUTED IMIDAZOLE DERIVATIVES
CATALYZED BY BTPPC UNDER SOLVENT-FREE CONDITIONS
Mohammad Alikarami1* and Mozhgan Amozad2
1Department of Chemistry, Ilam Branch, Islamic Azad University, Ilam, Iran
2Department of Chemistry, Borujerd Branch, Islamic Azad University, Borujerd, Iran
(Received February 10, 2016; revised January 4, 2017)
ABSTRACT. A simple and efficient method for one-pot synthesis of lophine derivatives (2,4,5-trisubstituted
imidazoles) by using the benzyltriphenylphosphonium chloride (BTPPC), as a catalyst, under solvent-free
conditions is described. BTPPC is an available and inexpensive catalyst; also, it can be easily supplied. This
procedure led to the corresponding 2,4,5-trisubstituted imidazoles products in high yields.
KEY WORDS: Lophine derivatives, BTPPC, Solvent-free, One-pot synthesis, Multi component synthesis
INTRODUCTION
Imidazole derivatives are an important class of heterocycles because of their applications in
chemical processes and pharmaceuticals [1]. They have a wide range of biological activities and
are well known analgesics, anti-inflammatory, antipar-asitic, anthelmintic, platelet aggregation
inhibitors and antiepileptic agents [2]. Several methods such as the hetero-Cope rearrangement
[3] and four-component condensation [4] for the synthesis of trisubstituted imidazoles are
reported. In recent years, the synthesis of 2,4,5-trisubstituted imidazoles has been catalyzed by I2
[5], ZrCl4 [6], ionic liquid [7], L-proline [8], microwave irradiation [9], Yb(OPf)3 [10],
InCl3.3H2O [11], NiCl2.6H2O/Al2O3 [12], DABCO [13], magnetic Fe3O4 nanoparticles [14],
nano MgAl2O4 [15], ZrO2-β-cyclodextrin [16], [EMIM]OAc [17], NaH2PO4 [18], N-methyl-2-
pyrrolidone hydrogen sulfate [19], europium triflate [20], sulfated zirconia [21], n-Bu4NBr [22],
silica-supported Preyssler nanoparticles [23], (NH4)6Mo7O24⋅4H2O [24], nano MgO [25], nano
aluminium nitride [26], nano SiO2-supported ferric hydrogen sulfate (FHS) [27] and KSF
supported 10-molybdo-2-vanadophosphoric acid [28]. Although some of the methods are
actually efficient from the synthetic chemist’s points, many of the synthetic protocols for
imidazoles reported above suffer from one or more disadvantages, such as harsh reaction
conditions, poor yields, and prolonged reaction time, use of hazardous and often expensive acid
catalysts. Therefore, the development of efficient, simple, environmentally friendly and high-
yielding methods using new catalysts for the preparation of these compounds is still necessary.
BTPPC is a crystalline quaternary phosphonium salt which finds its application in Wittig
reactions [29] and in phase transfer catalysis [30]. Very recently, we have reported the one-pot
synthesis of dihydropyrimidinones/thiones under solvent-free conditions using BTPPC as a
catalyst [31].
In this paper, we describe an efficient and practical route for the synthesis of lophine
derivatives under solvent free conditions using BTPPC as catalyst (Scheme 1).
__________
*Corresponding author. E-mail: alikarami58@yahoo.com

178
Mohammad Alikarami and Mozhgan Amozad
O
Ph
O
H
Ph
Ph
O
BTPPC (15 mol%)
N
Ph
Cl-
Ph
Ph
Ar
or
NH4OAc
+
+
H
P
+
C
H2
Ph
Ar
Solvent-Free
100 0C
N
OH
O
Ph
Ph
Ph
BTPPC
Scheme 1. Synthesis of lophine derivatives using BTPPC.
RESULTS AND DISCUSSION
In a model reaction, in the presence of the catalyst (15 mol%), the mixture of benzaldehyde (1
o
mmol), benzil (1 mmol) and NH4OAc (2 mmol) as ammonia source, stirred at 100 C under
solvent free conditions. The 2,4,5-triphenyl imidazole are obtained in 92% yield. Various kinds
o
of substituted benzaldehydes were also subjected in the presence of BTPPC at 100 C under
solvent free conditions (Table 1).
Table 1. BTPPC-catalyzed the synthesis of lophine derivativesa.
Entry Ar-CHO
Product
Ph
Time (min)
Yield (%)b
Mp/°Cc
Ref.
Benzyl Benzoin Benzyl Benzoin Found Reported
1
2
C6H5-
10
12
92
90
276- 274-276 8a
278
N
Ph
Ph
N
H
2-MeC6H4-
10
13
89
86
208- 207-208 23
210
Me
N
N
Ph
Ph
N
H
3
2-MeOC6H4-
9
11
88
83
208- 209-211 23
210
MeO
Ph
Ph
N
H
4
5
2-ClC6H4-
Cl
10
9
12
12
89
86
85
81
186 183-184 32
N
Ph
Ph
N
H
2-HOC6H4-
HO
204- 200-203 33
205
N
Ph
N
H
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179
230- 231-232 34
6
7
2-NO2C6H4-
3-MeOC6H4-
O2N
9
11
90
86
85
82
Ph
232
N
N
N
Ph
Ph
N
H
12
14
260- 266-268 35
262
OMe
Ph
Ph
N
H
8
9
3-BrC6H4-
Br
10
10
12
13
89
86
85
81
198- 199-200 36
200
Ph
Ph
N
H
3-HOC6H4-
257- 258-260 8b
258
OH
N
Ph
Ph
N
H
10
11
3-NO2C6H4-
4-MeC6H4-
9
12
12
88
92
83
89
>300 313-315 12
NO2
N
Ph
Ph
N
H
10
231- 234-236 23
232
N
Me
Ph
N
H
Ph
12
13
4-MeOC6H4-
4-ClC6H4-
10
9
12
13
89
90
86
86
228- 229-231 8a
230
N
OMe
Ph
N
H
264- 262-264 8a
266
Ph
N
N
Cl
Ph
N
H
14
15
4-HOC6H4-
4-NO2C6H4-
9
12
12
89
90
85
86
265- 268-270 8a
267
Ph
OH
Ph
N
H
Ph
9
232- 235-238 12
233
N
NO2
Ph
N
H
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Mohammad Alikarami and Mozhgan Amozad
Ph
16
3,4-MeOC6H3-
11
14
89
83
215- 215
217
11
OMe
OMe
N
Ph
N
H
aReaction conditions: benzyl/benzoin (1 mmol), aldehyde (1 mmol), NH4OAc (2 mmol), BTPPC (15 mol%), 100
°C under solvent-free conditions. bIsolated yield. cMelting points are uncorrected.
We found that for aldehydes bearing either electron withdrawing or electron-releasing
substituents in the ortho, meta or para positions; the reaction proceeded very efficiently in all
cases. This procedure provides 2-aryl-4,5-diphenyl imidazoles directly, in relatively short
reaction times and high yields. Furthermore, we used benzoin instead of benzyl and in this case
corresponding products were achieved in good yields. In all cases, complete conversion was
observed after appropriate time and the products were readily isolated in very high yields. A
reasonable reaction mechanism for the BTPPC catalyzed is shown in Scheme 2.
BTPPC catalyst facilitates the formation of diamine intermediate [I] by increasing the
electrophilicity of the carbonyl group of the aldehyde. Intermediate [I], in the presence of
BTPPC, condenses with benzyl or benzoin to form intermediate [II], which in turn rearranges to
the trisubstituted imidazole by a hydrogen shift [1, 5].
+
Cl-
Ph3P CH2Ph
NH4OAc
Ph
Ph
O
Ph
Cl-
Ph
Ph
P
+
C
H2
Ph
Ph
Ph
NH2
Cl-
H
N
O
+
2NH3
-H2O
Ph3P CH2Ph
Ar
NH2
Ar
NH2
O
+
H
O
-H2O
I
Ph3P CH2Ph
Cl-
Ar
H
-H2O
Ph
Ph
Ph
N
H
N
N
[1,5] H shift
Ar
N
H
Ar
Ph
II
Scheme 2. A plausible mechanism for the formation of lophine derivatives.
In summary, a one-pot, multicomponent methodology has been developed for the synthesis
of lophine derivatives catalyzed by BTPPC in high yields. Moreover, easy work-up, clean
reaction profiles, low cost, availability, low toxicity, stable under normal temperatures and
pressures of the catalyst, and short reaction time make this methodology a valid contribution to
the existing processes in the field of 2-aryl-4,5-diphenyl imidazole derivatives synthesis.
EXPERIMENTAL
All the chemicals were obtained from Merck and Fluka Company. The melting points were
obtained using an Electrothermal IA 9100 digital melting point apparatus. NMR spectra were
recorded on a 400 MHz spectrometer using TMS as internal standard.
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181
Preparation of BTPPC. To prepare BTPPC, 10 mmol (3.26 g) of PPh3 and 10 mmol (1.26 g)
of benzyl chloride were carefully dissolved in 15 mL of DMF, and the mixture was stirred at 80
oC. After 2 the resulting white precipitate was collected and washed with Et2O (3×20 mL) and
dried in a desiccator to afford a white solid. M.p. 335 oC [37].
General procedure for preparation of 2-aryl-4,5-diphenyl derivatives. A mixture of
aldehyde (1 mmol), benzyl/benzoein (1 mmol) and NH4OAc (2 mmol), as ammonia source, and
BTPPC (15 mol %) stirred at 100 oC under solvent free conditions. The progress of the reaction
was monitored by TLC. After completion of the reaction, the reaction mixture was dissolved in
ethanol and poured into water. The resulting precipitate was filtered and purified by
recrystallization from ethanol to afford the desired compound in pure form. All products were
identified by comparison of their physical and spectroscopic data with those reported for
authentic samples.
Triphenyl-1H-imidazole (Entry 1). Solid, m.p. = 276-278 oC. 1H NMR (400 MHz, DMSO-d6) δH
(ppm): 7.56-7.22 (m, 15 arom. H), 12.69 (s, 1H, -NH).
2-(4-Methoxylphenyl)-4,5-diphenyl-1H-imidazole (Entry 12). Solid, m.p. = 228-230 oC. 1H
NMR (400 MHz, DMSO-d6) δH (ppm): 3.81 (s, 3H, -OMe), 7.04 (d, J = 9.2 Hz, 2 arom. H),
7.55-7.28 (m, 10 arom. H), 8.02 (d, J = 9.2 Hz, 2 arom. H), 12.52 (s, 1H, -NH).
2-(3,4-Dimethoxylphenyl)-4,5-diphenyl-1H-imidazole (Entry 16). Solid, m.p. 215-217 oC. 1H
NMR (400 MHz, DMSO-d6) δH (ppm): 3.81 (s, 3H, -OMe), 3.85 (s, 3H, -OMe), 7.06 (d, J = 8.0
Hz, 1 arom. H), 7.67-7.21 (m, 12 arom. H), 12.52 (s, 1H, -NH). 13C NMR (100 MHz, DMSO-d6)
δH (ppm): 55.0, 55.0, 109.3, 112.3, 118.3, 123.6, 126.8, 127.5, 128.1, 128.6, 128.8, 129.1, 131.7,
135.7, 137.2, 146.1, 149.2, 149.5.
ACKNOWLEDGEMENTS
Financial support of this work from the Research Council of Islamic Azad University of Ilam is
gratefully acknowledged.
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