One-pot synthesis of tetra-substituted imidazoles on silica gel under microwave irradiation

Article abstract of DOI:10.3987/com-03-9896

A new procedure for synthesis of tetra-substituted imidazoles was developed. A series of imidazole derivatives including six new compounds were synthesized by this procedure via condensation of benzoin, aromatic aldehyde, amine and ammonium acetate in the presence of silica gel under solvent-free microwave irradiation.

Full text of DOI:10.3987/com-03-9896

HETEROCYCLES, Vol. 63, No. 1, 2004, pp. 87 - 93
Received, 28th August, 2003, Accepted, 22nd October, 2003, Published online, 31st October, 2003
ONE-POT SYNTHESIS OF TETRA-SUBSTITUTED IMIDAZOLES ON
SILICA GEL UNDER MICROWAVE IRRADIATION
Yu Xu, Yan-Zhi liu, Lei Rui, Lei Liu, and Qing-Xiang Guo*
Department of Chemistry, University of Science and Technology of China, Hefei
230026, China (e-mail: qxguo@ustc.edu.cn )
Abstract– A new procedure for synthesis of tetra-substituted imidazoles was
developed. A series of imidazole derivatives including six new compounds were
synthesized by this procedure via condensation of benzoin, aromatic aldehyde,
amine and ammonium acetate in the presence of silica gel under solvent-free
microwave irradiation.
INTRODUCTION
Imidazole nucleus is the main component of some important biological molecules, such as histidine,
Vitamin B12, purines, histamine and biotin. It also presents in many natural and synthetic drug molecules,
for example, cimetidine, azomycin and metronidazole.1 Compounds with imidazole ring systems have
many pharmaceutical activities and play important roles in biochemical processes.2
Because imidazoles are versatile intermediates in the manufacture of pharmacologically active
compounds,3 many methods have been developed for the synthesis of substituted imidazoles. Typical
methods include: condensation of diones, aldehydes, primary amines, and ammonia;4 condensation of
benzoin or benzoin acetate with aldehydes, primary amines and ammonia in the presence of copper
acetate;5 four-component condensations of diones, aldehydes, primary amines, and ammonia acetate in
acetic acid under reflux conditions;6 cyclization of sulfonamides with mesoionic 1,3-oxazolium-5-olates;7
condensation of β-carbonyl-N-acyl-N-alkylamines with ammonium acetate in refluxing HOAc;8 and
conversion of N-(2-oxo)amides with ammonium triflouroacetate under neutral conditions;9 iron and
copper catalyzed reaction of benzylamine with carbon tetrachloride.10
Recently, microwave-assisted synthesis in organic chemistry is quickly growing.11 Many organic
reactions proceed much faster and get higher yields under microwave irradiation compared to
conventional heating. Supported reagents on solid surface have been widely employed in organic
synthesis. Reagents impregnanted on solid materials present advantages over the conventional solution

phase reactions, for the good dispersion of active sites leading to improved reactivity and milder reaction
conditions. A solvent-free reaction with microwave irradiation would reduce reaction time, and provide
easier work-up procedures. In contrast to the traditional application of solid phase in synthesis,
microwave technology does not involve the solid phase linking with the reactants and cleaving with the
products. The recycling of the inorganic solid support makes the procedure more environmentally benign.
Two research groups have recently reported a one-pot condensation of benzil, aldehyde, amine and
ammonium acetate on alumina or silica solid support under microwave irradiation.3, 12 It was found that
microwave irradiation and solid support in the solventless reaction considerably shortened the reaction
time and greatly reduced waste production.13
In our laboratory, some imidazole derivatives have been synthesized with microwave assistance.14 It was
interesting to find that use of benzoin instead of benzil as starting material in the condensation also
produced the desired products efficiently (Scheme 1). To our knowledge, the benzils are usually prepared
from benzoins catalyzed by various toxic oxidants, such as thallium nitrate, ytterbium(III) nitrate,
ammonium nitrate-copper acetate, clayfen, ammonium chlorochromate-alumina, nickel acetate, iron(III)
chloride and bismuth(III) nitrate-copper(II) acetate.15 Obviously, direct use benzoin rather than benzil in
the synthesis of imidazoles represents a significant improvement in the syntheses toward to greener
chemistry.
Ph
N
OH
O
Ph
Ph
SiO2
MW
Ph
Ar
NH4OAc
RNH2
ArCHO
N
R
Scheme 1
RESULTS AND DISCUSSION
Tetra-substituted imidazoles were synthesized under microwave irradiation in good yield (Table 1). In
order to avoiding overheating, two 10-minute irradiations were performed. All products, six of them (b, c,
e, f, g, o) were firstly synthesized, were characterized by melting points, and elemental analyses, IR, MS,
1H NMR and 13C NMR spectroscopies.
From Table 1, it can be seen that this procedure could be applied to a broad range of substituted aromatic
and aliphatic amines and aromatic aldehydes. The results indicated that good yields were obtained when
the p-methoxy- and p-methylbenzaldehyde were used as starting materials (Entries a, c, l, n), however,
when p-dimethylaminobenzaldehyde was used, the yield is not so good (Entry o), but still acceptable.
Interestingly, it was found that benzoin could be used in the condensation yielding imidazole in absence
of any oxidizing reagent. A control experiment of the condensation of benzoin, aldehyde, amine and

ammonium acetate under conventional acetic acid reflux conditions was run, no corresponding imidazole
was isolated from the reaction mixture. It was in agreement with the previous finding that an oxidizing
reagent such as Cu(II) was needed in the conventional condensation.5 This finding demonstrated that the
microwave irradiation and silica gel support would play important roles in the reaction.
Table 1. Microwave-assisted synthesis of tetra-substituted imidazoles on silica gel
Entry
Ar
R
Yield (%) / Time(min)
This study Literature[ref]
a
b
c
p-CH3O-C6H4
PhCH2
PhCH2CH2
cyclohexyl
87 / 180 [1]
91 / 10×2*
87 / 10×2
95 / 10×2
-
-
d
e
f
g
h
i
j
k
l
p-HO-C6H4
C6H5
PhCH2
PhCH2CH2
cyclohexyl
Ph
43 / 180 [1]
-
71 / 10×2
65 / 10×2
61 / 10×2
60 / 10×2
86 / 10×2
80 / 10×2
89 / 10×2
75 / 10×2
90 / 10×2
83 / 10×2
-
-
PhCH2
87 / 6 [3]
80 / 20 [12]
80 / 6 [3]
79 / 6 [3]
90 / 6 [3]
68 / 20 [12]
PhCH2CH2
cyclohexyl
Ph
PhCH2
PhCH2CH2
p-Me-C6H4
m
n
cyclohexyl
87 / 6 [3]
-
92 / 10×2
o
p-(CH3)2N-C6H4 m-NO2-C6H4
50 / 10×2
*Irradiation for two 10 min intervals with cooling to room temperature between intervals .
Interestingly, it was reported recently by Balalaie et al.16 that benzoins were oxidized on zeolite A using
microwave irradiation under solvent-free conditions, in which air functioned as oxidant in the conversion.
It was proposed that the zeolite A was significant in the Balalaie’s benzoin oxidation. In our study the
simple silica gel was adequate for a rapid and clean oxidation of the condensation mixture to imidazole,
although at the present stage we cannot conclude whether benzoin is oxidized before condensation or
after partially condensation.
In spite of the fact that one more chemical transformation, such as oxidation has to be involved in the
condensation to imidazole, the yields of the reaction reported herein are as good as those previously
reported (See Table 1).3, 12 Usyatinsky and Khmelnitsky12 reported that a small amount of acetic acid was
needed to accelerate the condensation, however, in our work the acetic acid was not necessary.
Based on the results described above, we can conclude that a facile and environmentally benign one-pot
synthesis of tetra-substituted imidazole derivatives from benzoin, aldehydes, amines and ammonium
acetate was developed.

EXPERIMENTAL
All reported yields are isolated yields after column chromatography. MS spectra were run on a
GCT-CA064 spectrograph. All melting points are uncorrected and were measured on WRS-1A melting
point apparatus. IR spectra were run on a Bruker spectrophotometer and expressed in cm-1(KBr). 1HNMR
13
and CNMR spectra were recorded on FT-NMR Bruker AV-400 (400 MHz) or Bruker AV-300 (300
MHz) in DMSO-d6 with TMS as internal reference. Elemental analysis was performed by the Elementar
Vario EL-III. All the reactions were conducted in a commercial microwave oven. (Galanz WD800B,
2450 Hz, output power 800 W)
Typical procedure for the synthesis of tetra-substituted imidazoles as follows: A mixture of silica gel
(15.4 g) and ammonium acetate (7.7 g, 100 mmol) was ground fully. A solution of benzoin (1.06 g, 5
mmol), aldehyde (5 mmol) and amine (5 mmol) in 20 mL of ethyl acetate was added to the mixture and
mixed thoroughly. The solvent was allowed to evaporate under reduced pressure and the dry residue was
irradiated at 160W (20% power) at 120℃ in 10 min. The mixture was cooled, stirred and irradiated for
another 10 min. The mixture was cooled to rt and extracted with ethyl acetate (3×50 mL). The combined
organic solution was filtered and the solvent was evaporated by a rotary evaporator. The product was
purified by column chromatography.
1-Benzyl-2-(4-methoxylphenyl)-4,5-diphenylimidazole (1a): MS m/z: (M+H)+ 417.1964 (Calcd
417.1967); mp 158-159℃ (ethyl acetate); lit.,1 153-155℃; IR(KBr) ν 2967(C-H), 1609(C=C),
1
1575(C=N), 1500, 1450 cm-1; H-NMR (DMSO-d6, 300 MHz) δH: 3.79 (s, 3H, CH3), 5.14 (s, 2H, CH2),
6.76-7.60 (m, 19H, Ar); 13C-NMR (DMSO-d6, 75 MHz) δC: 47.6, 55.2, 114.0, 123.1, 125.6, 126.1, 126.2,
127.2, 128.1, 128.5, 128.8, 128.9, 129.8, 129.9, 130.7, 130.8, 134.6, 136.6, 137.4, 147.0, 159.6.
2-(4-Methoxylphenyl)-1-phenethyl-4,5-diphenylimidazole (1b): MS m/z: (M+H)+ 431.2128 (Calcd
431.2123); mp126-127℃ (ethyl acetate); IR(KBr) ν 2958(C-H), 1610(C=C), 1575(C=N), 1498, 1459
1
cm-1; H-NMR (DMSO-d6, 300 MHz), δH: 2.53 (t, 2H, J=7.5 Hz , CH2), 3.84 (s, 3H, CH3), 4.05 (t, 2H,
13
J=7.5 Hz , CH2), 6.68-7.60 (m, 19H, Ar); C-NMR (DMSO-d6, 75 MHz), δC: 35.6, 45.8, 55.2, 114.0,
123.5, 126.0, 126.5, 127.9, 128.2, 128.4, 128.8, 129.0, 129.3, 130.0, 130.9, 131.1, 134.7, 136.2, 137.3,
146.6, 159.5; Anal. Calcd for C30H26N2O: C 83.68, H 6.09, N 6.51. Found C 83.56, H 5.89, N 6.41.
1-Cyclohexyl-2-(4-methoxyphenyl)-4,5-diphenylimidazole (1c): MS m/z: (M+H)+ 409.2282 (Calcd
409.2280); mp 191-193℃ (ethyl acetate); IR(KBr) ν 2937(C-H), 1612(C=C), 1541(C=N), 1493, 1443
cm-1; 1H-NMR (DMSO-d6, 300 MHz), δH: 0.55-1.99 (m, 10H, CH2), 3.84 (s, 3H, CH3), 4.00-4.04 (m, 1H,
13
CH), 7.03-8.03 (m, 14H, Ar); C-NMR (DMSO-d6, 75 MHz), δC: 14.1, 20.8, 55.2, 59.8, 113.8, 114.1,
123.1, 125.9, 126.7, 127.9, 128.4, 128.8, 129.1, 131.0, 132.1, 145.6, 159.4.
1-Benzyl-2-(4-hydroxyphenyl)-4,5-diphenylimidazole (1d): MS m/z: (M+H)+ 403.1812 (Calcd
403.1810); mp 258-259℃ (ethyl acetate); lit.,1 134-135℃; IR(KBr) ν 2934(C-H), 1611(C=C),

1543(C=N), 1482, 1451 cm-1; 1H-NMR (DMSO-d6, 300 MHz), δH: 5.12 (s, 2H, CH2), 6.76 -7.46 (m, 19H,
13
Ar), 9.79 (s, 1H, OH); C-NMR (DMSO-d6, 75 MHz) δC 47.6, 115.3, 121.5, 125.6, 126.0, 127.1, 128.1,
128.5, 128.8, 128.9, 129.6, 130.0, 130.8, 134.7, 136.5, 137.5, 147.5, 158.0.
2-(4-Hydroxyphenyl)-1-phenethyl-4,5-diphenylimidazole (1e): MS m/z: (M+H)+ 417.1970 (Calcd
417.1967); mp 238-239℃ (ethyl acetate); IR(KBr) ν 2922(C-H), 1614(C=C), 1542(C=N), 1485, 1443
cm-1; 1H-NMR (DMSO-d6, 300 MHz ), δH: 1.99 (t, 2H, J=7.5 Hz, CH2), 4.02 (t, 2H, J=7.5 Hz, CH2), 6.42
-7.88 (m, 19H, Ar), 9.79 (s, 1H, OH); 13C-NMR (DMSO-d6, 75 MHz), δC: 35.6, 45.8, 115.4, 121.9, 126.0,
126.5, 128.0, 128.3, 128.4, 128.8, 129.1, 130.2, 130.9, 131.2, 134.8, 136.0, 137.5, 147.1, 157.9; Anal.
Calcd for C29H24N2O: C 83.62, H 5.81, N 6.73. Found C 83.55, H 5.73, N 6.55.
1-Cyclohexyl-2-(4-hydroxyphenyl)-4,5-diphenylimidazole (1f): MS m/z: (M+H)+ 395.2122 (Calcd
395.2123); mp 249-251℃ (ethyl acetate); IR(KBr) ν 2934 (C-H), 1609 (C=C), 1542 (C=N), 1494, 1458
1
cm-1; H-NMR (DMSO-d6, 300 MHz), δH: 0.57-1.99 (m, 10H, CH2), 3.99-4.04 (m, 1H, CH), 6.82-7.53
(m, 14H, Ar), 9.81 (s, 1H, OH); 13C-NMR (DMSO-d6, 100 MHz), δC: 14.0, 20.7, 59.7, 115.4, 121.6,
126.3, 126.8, 127.0, 127.3, 127.5, 128.0, 128.3, 128.5, 131.3, 135.4, 136.6, 146.0, 157.8.
2-(4-Hydroxyphenyl)-1,4,5-triphenylimidazole (1g): MS m/z: (M+H)+ 389.1660 (Calcd 389.1654); mp
1
218-220℃ (ethyl acetate); IR(KBr) ν 1608(C=C), 1533(C=N), 1496, 1460 cm-1; H-NMR (DMSO-d6,
13
300 MHz), δH: 6.64-7.48 (m, 19H, Ar), 9.69 (s, 1H, OH); C-NMR (DMSO-d6, 100 MHz), δC: 114.8,
126.3, 128.0, 128.2, 128.3, 128.5, 128.7, 129.0, 129.7, 130.4, 130.5, 131.0, 136.7, 146.3, 157.6.
1-Benzyl-2,4,5-triphenylimidazole (1h): MS m/z: (M+H)+ 387.1885 (Calcd 387.1861); mp 165-167℃
(ethyl acetate); lit.,3 163-164℃; IR(KBr) ν 3037(C-H), 1602(C=C), 1498(C=N), 1480, 1449 cm-1;
13
1H-NMR (DMSO-d6, 300 MHz), δH: 5.16 (s, 2H, CH2), 6.74-7.66 (m, 20H, Ph); C-NMR (DMSO-d6,
100 MHz), δC: 47.6, 125.5, 126.0, 126.2, 127.1, 128.0, 128.4, 128.5, 128.6, 128.7, 128.8, 128.9, 130.1,
130.5, 130.7, 130.8, 134.4, 136.8, 137.2, 147.0; Anal. Calcd for C28H22N2: C 87.01, H 5.74, N 7.25.
Found C 87.04, H 5.70, N 7.09.
1-Phenethyl-2,4,5-triphenylimidazole (1i): MS m/z: (M+H)+ 401.2014 (Calcd 401.2018); mp 161-163℃
1
(ethyl acetate); IR(KBr) ν 2960 (C-H), 1601(C=C), 1498(C=N), 1480, 1444 cm-1; H-NMR (DMSO-d6,
300 MHz), δH: 2.50-2.56 (t, 2H, J=9.0 Hz, CH2), 4.02-4.08 (t, 2H, J=9.0 Hz, CH2), 6.67-7.67 (m, 20H,
13
Ph); C-NMR (DMSO-d6, 100 MHz), δC: 35.5, 45.8, 125.1, 125.9, 126.0, 126.4, 127.9, 128.2, 128.3,
128.5, 2×128.6, 128.7, 128.8, 129.0, 130.8, 130.9, 131.1, 134.5, 137.2, 146.6; Anal. Calcd for C29H24N2:
C 86.96, H 6.04, N 7.00. Found C 86.67, H 5.94, N 6.95.
1-Cyclohexyl-2,4,5-triphenylimidazole (1j): MS m/z: M+ 378.2086 (Calcd 378.2096); mp 168-170℃
(ethyl acetate); lit.,3 167-169℃; IR(KBr) ν 2932 (C-H), 1640(C=C), 1558(C=N), 1503, 1445 cm-1;
1H-NMR (DMSO-d6, 300 MHz), δH: 0.50-1.87 (m, 10H, CH2), 4.37 (m, 1H, CH), 6.75-8.10 (m, 15H, Ph).

1,2,4,5-Tetraphenylimidazole (1k): MS m/z: (M+H)+ 373.1709 (Calcd 373.1705); mp 216-218℃ (ethyl
acetate); lit.,3 216-218℃; IR(KBr) ν 1600(C=C), 1497(C=N), 1480, 1443 cm-1; 1H-NMR (DMSO-d6, 300
13
MHz), δH: 6.80-7.64 (m, 20H, Ph); C-NMR (DMSO-d6, 100 MHz), δC: 126.2, 126.3, 128.0, 128.1,
128.3, 128.6, 129.0, 130.3, 131.0, 131.2, 134.3, 136.6, 136.7, 145.9; Anal. Calcd for C27H20N2: C 87.06,
H 5.42, N 7.53. Found C 86.87, H 5.37, N 7.24.
1-Benzyl-2-(4-methylphenyl)-4,5-diphenylimidazole (1l): MS m/z: (M+H)+ 401.2040 (Calcd 401.2018);
mp 158-160℃ (ethyl acetate); lit.,3 163-166℃; IR(KBr) ν 2926(C-H), 1601(C=C), 1498(C=N), 1484,
1452 cm-1; 1H-NMR (DMSO-d6, 300 MHz), δH: 2.33 (s, 3H, CH3), 5.15 (s, 2H, CH2), 6.74-7.66 (m, 19H,
13
Ar); C-NMR (DMSO-d6, 100 MHz), δC: 20.7, 47.5, 125.5, 126.0, 126.1, 127.1, 127.8, 128.0, 128.3,
128.4, 128.7, 128.8, 129.0, 129.9, 130.5, 130.7, 134.5, 136.7, 137.3, 138.2, 147.1; Anal. Calcd for
C29H24N2: C 86.96, H 6.04, N 7.00. Found C 86.70, H 5.90, N 6.80.
2-(4-Methylphenyl)-1-phenethyl-4,5-diphenylimidazole (1m): MS m/z: (M+H)+ 415.2173 (Calcd
415.2174); mp 124-125℃ (ethyl acetate); IR(KBr) ν 2921(C-H), 1602(C=C), 1500(C=N), 1482, 1451
cm-1; 1H-NMR (DMSO-d6, 300 MHz), δH: 2.40 (s, 3H, CH3), 2.54 (t, 2H, J= 9.0 Hz, CH2), 4.07 (t, 2H, J=
9.0 Hz, CH2), 6.68-7.58 (m, 19H, Ar); 13C-NMR (DMSO-d6, 100 MHz), δC: 20.9, 35.6, 45.8, 126.0, 126.5,
128.0, 128.2, 128.3, 128.4, 128.5, 128.8, 129.0, 129.1, 129.5, 130.9, 131.0, 134.7, 136.3, 137.3, 138.2,
146.7; Anal. Calcd for C30H26N2: C 86.91, H 6.33, N 6.76. Found C 86.68, H 6.25, N 6.60.
1-Cyclohexyl-2-(4-methylphenyl)-4,5-diphenylimidazole (1n): MS m/z: (M+H)+ 393.2356 (Calcd
393.2331); mp 162-164℃ (ethyl acetate); lit.,3 162-164℃; IR(KBr) ν 2929(C-H), 1629 (C=C), 1550
1
(C=N), 1494, 1450 cm-1; H-NMR (DMSO-d6, 300 MHz), δH: 0.55-1.99 (m, 10H, CH2), 2.35 (s, 3H,
CH3), 3.47 (m, 1H, CH), 6.54-7.98 (m, 14H, Ar).
2-(4-Dimethylaminophenyl)-1-(3-nitrophenyl)-4,5-diphenylimidazole (1o): MS m/z: M+ 460.1929 (Calcd
460.1899); mp 238-240℃ (ethyl acetate); lit.,3 167-169℃; IR(KBr) ν 2923(C-H), 1616(C=C),
1551(C=N), 1508, 1497 cm-1; 1H-NMR (DMSO-d6, 300 MHz), δH: 2.97 (s, 6H, CH3), 6.78 -7.92 (m, 18H,
13
Ar); C-NMR (DMSO-d6, 100 MHz), δC: 111.8, 118.3, 121.5, 126.2, 126.9, 127.1, 127.3, 127.8, 127.9,
128.0, 128.2, 128.3, 128.5, 129.4, 129.5, 135.5, 141.7, 146.4, 150.2.
ACKNOWLEDGEMENTS
This research was supported by the Chinese Academy of Sciences, State Ministry of Science and
Technology, and University of Science and Technology of China.
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