Citrate trisulfonic acid: A heterogeneous organocatalyst for the synthesis of highly substituted Imidazoles

Article abstract of DOI:10.1002/jccs.201800015

Citrate trisulfonic acid (CTSA), as a novel recyclable and eco-benign organocatalyst, was employed for the efficient and one-pot synthesis of trisubstituted imidazoles and tetrasubstituted imidazoles using aldehydes, ammonium acetate or aniline, and benzoin, benzyl, or 9,10-phenanthrenequinone under solvent-free conditions providing high to excellent yields. CTSA is easily prepared via the reaction of trisodium citrate and chlorosulfonic acid in high purity. Compared to the conventional procedures, the present method offers several advantages, including high yields, easy work-up, short reaction time, reusability of the catalyst, and simple purification of the products.

Full text of DOI:10.1002/jccs.201800015

Received: 12 January 2018
Revised: 19 July 2018
Accepted: 21 July 2018
DOI: 10.1002/jccs.201800015
A R T I C L E
Citrate trisulfonic acid: A heterogeneous organocatalyst
for the synthesis of highly substituted Imidazoles
Elham Kanaani | Masoud Nasr-Esfahani
Department of Chemistry, Yasouj University,
Citrate trisulfonic acid (CTSA), as a novel recyclable and eco-benign organocata-
Yasouj, Iran
lyst, was employed for the efficient and one-pot synthesis of trisubstituted imidaz-
Correspondence
Masoud Nasr-Esfahani, Department of Chemistry,
oles and tetrasubstituted imidazoles using aldehydes, ammonium acetate or aniline,
and benzoin, benzyl, or 9,10-phenanthrenequinone under solvent-free conditions
providing high to excellent yields. CTSA is easily prepared via the reaction of tri-
Yasouj University, Yasouj 75918-74831, Iran.
Email: manas@yu.ac.ir; m_nasr_e@yahoo.com
sodium citrate and chlorosulfonic acid in high purity. Compared to the conven-
tional procedures, the present method offers several advantages, including high
yields, easy work-up, short reaction time, reusability of the catalyst, and simple
purification of the products.
KEYWORDS
citrate trisulfonic acid, organocatalyst, reusable catalyst, solvent-free,
tetrasubstituted imidazoles, trisubstituted imidazoles
ammonium acetate, and an aldehyde in the presence of a
strong protic acid such as H3PO4,[17] H2SO4,[18] or other cata-
lysts in HOAc[19] under reflux conditions, Wells–Dawson het-
eropolyacid supported on silica (WD/SiO2),[20] silica gel, or
zeolite HY,[21] ionic liquid,[22] InCl3ċ3H2O,[23] Yb(OTf )3,[24]
and KH2PO4.[25] Tetrasubstituted imidazoles have been
obtained by the four-component cyclocondensation of
1,2-diketone, aldehyde, a primary amine, and ammonium ace-
tate using various Lewis or protic acidic reagents such as
BF3-SiO2,[26] silica gel/NaHSO4,[27] InCl3ċ3H2O-MeOH,[23]
1
| INTRODUCTION
Imidazole and its derivatives form an important class of
heterocycles and show a wide variety of biological
activities such as herbicidal,[1] antiallergic,[2] analgesic,[3]
antidepressant,[4] antitubercular,[5] anticancer,[6] anti-inflam-
matory,[7] antifungal,[8] and antiviral.[9] They are present in
important building blocks including histidine[10] and the
related hormone histamine.[11] A number of these ring sys-
tems are used in electronic and optoelectronic devices.[12]
Trisubstituted imidazoles are used in photography as photo-
sensitive compounds.[13] They are also found to possess high
anti-inflammatory activity. Tetrasubstituted imidazoles are
active ingredients in many pharmaceuticals including olme-
sartan, eprosartan, medoxomil, losartan, and trifenagrel.[14]
Imidazole was synthesized for the first time by Heinrich
Debus in 1858 from the reaction between glyoxal, formalde-
hyde, and ammonia.[15] In 1882, poly-substituted imidazole
was synthesized using an aldehyde, a 1,2-dicarbonyl com-
pound, and ammonia.[16] In recent years, a number of proce-
dures for the preparation of trisubstituted and tetrasubstituted
imidazoles have been reported. 2,4,5-Trisubstituted imidaz-
oles are generally synthesized via the three-component con-
densation reaction of α-hydroxyketone or 1,2-diketone,
HPA-EtOH,[28]
,
L-proline,[29] K5CoW12−O40ċ3H2O,[30]
heteropolyacid,[31] zeolite-HY–Cu(NO3)2,[32] and silica-
supported Wells–Dawson acid,[20] hetero-Cope rearrange-
ment,[33] the reaction between 1,2-diketone, a nitrile, and a
primary amine under microwave irradiation,[34] and the reac-
tion of 1,3-oxazolium-5-olates with N-(arylmethylene)
benzenesulfonamides.[35]
Most of the reported literature on the preparation of
2,4,5-triaryl-1H-imidazole has examined benzile as a dike-
tone. In this work, we used both benzil and 9,10- phenan-
threnequinone as diketones or an α-hydroxyketone.
However, most of the existing methods have one or more
disadvantages such as expensive catalysts, long reaction
time, high cost, and cumbersome work-up procedures.
© 2018 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J Chin Chem Soc. 2018;1–7.
http://www.jccs.wiley-vch.de
1

2
KANAANI AND NASR-ESFAHANI
Consequently, a new method that addresses these drawbacks
is of prime importance from both economic and environmen-
tal points of view.
Multicomponent reactions (MCRs) are a useful way to
achieve small molecules with diverse structures. In an MCR,
three or more components are combined together in one step
to produce a final product without the need to isolate any inter-
mediate. Therefore, these reactions reduce the time, save both
energy and raw materials, and are useful for the preparation of
heterocyclic compounds. Here, the use of heterogeneous orga-
nocatalysts can help reduce the amount of toxic waste and
byproducts from chemical processes. This group of catalysts
has several advantages such as simplicity in handling, low
cost, multiple use without loss of efficiency, and simple isola-
tion after the completion of the reaction.[36] In addition, orga-
nocatalysts exhibit low toxicity and no sensitivity toward
moisture or oxygen.[37] Therefore, development of new, effi-
cient, and eco-friendly organic acidic catalysts with the possi-
bility of reuse is of prime importance in organic synthesis.
In continuation of our efforts on the application of solid
acid catalysts, here we report a mild an effective one-pot
synthesis of 2,4,5-trisubstituted imidazoles (Scheme 1) and
1,2,4,5-tetrasubstituted imidazoles (Scheme 2) with high
efficiency in the presence of citrate trisulfonic acid (CTSA)
as a novel organic solid acid catalyst with high catalytic
activity and recoverability under solvent-free conditions.
SCHEME 2 Synthesis of 1,2,4,5-tetrasubstituted imidazoles
SCHEME 3 Synthesis of citrate trisulforic acid
CTSA was characterized by spectroscopic techniques
1
such as IR, 13C NMR, and H NMR. In the IR spectrum of
trisodium citrate (Figure 1a), the absorption band found at
3,394 cm−1 is attributed to the stretching vibration of the
OH group. Two absorption bands at 1,606 and 1,585 cm−1
are due to the carbonyl groups. The absorption peaks in the
region of 1,068–1,442 cm−1 are related to the stretching
vibration of the C–O group and the bending vibration of the
O–H group.
2
| RESULTS AND DISCUSSION
In the IR spectrum of CTSA (Figure 1b), two absorption
bands at 3,500–2,468 and 1,643 cm−1 are assigned to the
stretching and bending vibrations of the OH groups, respec-
tively. The absorption bands in the region between
1,776 and 1,742 cm−1 are attributed to the carbonyl groups.
The absorption bands at 1,226 and 1,060 cm−1 are due to
the stretching vibration of S–OH and the asymmetric stretch-
ing vibration of S=O of the sulforic acid bonds.
2.1 | Characterization of CTSA
Preparation of an eco-friendly and efficient organic acidic
catalyst was attempted, and trisodium citrate was found to
be able to react with chlorosulfonic acid to produce CTSA.
The procedure of the reaction was very simple, so the prod-
uct could be synthesized by a simple filtration (Scheme 3).
The 1H NMR spectrum of CTSA exhibited two doublets
identified as due to the methylene groups (δ = 2.72 and
FIGURE 1 FT-IR spectra of (a) trisodium citrate and (b) citrate
SCHEME 1 Synthesis of 2,4,5-trisubstituted imidazoles
trisulfonic acid

KANAANI AND NASR-ESFAHANI
3
TABLE 1 Catalytic activity evaluation and effect of temperature and
δ = 2.88 ppm), a broad OH due to the alcohol (δ = 3.87
ppm), and two singlets corresponding to the OH groups of
the acid (δ = 12.65 and δ = 12.70 ppm). The proton-
decoupled 13C NMR spectrum of CTSA showed four dis-
tinct resonances, in agreement with the proposed structure.
solvent on the synthesis of multisubstituted imidazolesa
Conditions A and Ba
Yield (%)b
Time (min)
Catalyst
(mol. %)
Entry
Temp.
90
Solvent
—
A
B
A
B
1
2
3
5
10
5
5
5
5
5
5
5
5
5
74
80
96
91
30
43
35
50
32
69
83
90
95
70
77
91
88
32
41
30
40
28
66
70
83
89
41
58
2
90
—
34
47
2.2 | Recovery and reuse of the catalyst
3
90
—
25
34
4
90
—
31
40
Recoverability is one of the main characteristics of a hetero-
geneous catalyst. The recoverability of CTSA was investi-
gated for the model reaction of 4-bromobenzaldehyde,
9,10-phenanthrenequinone, and ammonium acetate. After
completion of the reaction, the separated catalyst could be
5
Reflux
Reflux
Reflux
Reflux
Reflux
40
H2O
EtOH
MeOH
CHCl3
CH3CN
—
360
180
320
380
340
110
40
420
240
380
390
360
120
60
6
7
8
9
ꢀ
recovered after washing with ethanol and drying at 70 C
10
11
12
13
for 3 hr. It can be seen from Figure 2 that the catalytic sys-
tem worked well up to four catalytic runs without any signif-
icant decrease in the product efficiency and its catalytic
activity. Both the fresh and recovered catalysts showed simi-
lar FT-IR spectra, indicating that the structure and morphol-
ogy of the catalyst remained the same after recycling.
70
—
100
—
35
56
120
—
35
40
a
Reaction conditions: benzaldehyde (1 mmol), benzil (1 mmol), ammonium ace-
tate (2 mmol) for trisubstituted imidazole (condition A); and benzaldehyde
(1 mmol), benzil (1 mmol), ammonium acetate (1 mmol), and aniline (1 mmol)
for tetrasubstituted imidazole (condition B).
b
Isolated yields.
2.3 | Effect of solvent, catalyst concentration, and heat
on the synthesis of multisubstituted imidazoles
catalyst did not lead to any improvement in the reaction rate
or yield.
First, in order to optimize the experimental conditions such
as solvent, temperature, and the amount of catalyst for the
synthesis of 2,4,5-trisubstituted imidazoles and 1,2,4,5-
tetrasubstituted imidazoles, the condensation between of
benzaldehyde (1 mmol), benzil (1 mmol), ammonium ace-
tate (2 mmol for trisubstituted imidazole and 1 mmol for tet-
rasubstituted imidazole), and 4-chloroaniline (1 mmol for
tetrasubstituted imidazole) was selected as the model
reaction.
As shown in Table 1, the reaction was screened in etha-
nol, methanol, water, THF, chloroform, and a solvent-free
system, and the best result was obtained after 25 or 34 min
under solvent-free conditions for the preparation of trisubsti-
tuted and tetrasubstituted imidazoles in excellent yields of
96 and 91%, respectively.
After optimization of the catalyst amount and solvent, to
improve the yield and decrease the reaction time, the effect
of temperature was studied. The result indicated that a tem-
ꢀ
perature of 90 C led to improvement in the reaction time
ꢀ
and yield, while above 90 C (Table 1 entries 10–13) the
reaction preceded smoothly.
With the optimized conditions, we proceeded to investi-
gate the general scope and generality of this method using a
wide range of aromatic, heterocyclic, and aliphatic aldehydes
as well as benzil or benzoin, and 9,10-phenanthrenequinone.
As shown in Table 2, an aldehyde (1 mmol), benzil or ben-
zoin (1 mmol), and ammonium acetate (2 mmol) were read-
ily converted to the corresponding 2,4,5-trisubstituted
imidazoles in good to excellent isolated yields over short
reaction times under the optimized conditions. To study the
substituent effect under optimized reaction conditions, a
wide range of aromatic aldehydes containing an electron-
withdrawing (i.e., Cl, Br, and NO2) or electron-donating
(i.e., CH3, OCH3, and OH) group were used. It was found
that aromatic aldehydes having electron-donating groups on
the aromatic ring react more slowly than those with electron-
withdrawing groups (Table 2). In the presence of aliphatic
aldehydes, the reaction resulted in moderate yield at a low
reaction rate (Table 2, entry 12). In the same reactions, ben-
zoin (3) used instead of benzil (2) exhibited lower reactivity
(Tables 2 and 3). Also, the optimized conditions for the
preparation of phenanthro[9,10-d]imidazoles (6) using
9,10-phenanthrenequinone (5) gave products in good to
excellent yields in short reaction times (Table 4).
Thereafter, the amount of the catalyst was evaluated in
the model reaction, and it was found that a quantitative
increase in the amount of CTSA catalyst from 2 to 5 mol%
resulted in higher product yields (Table 1, entries 1–3). It is
to be noted that further increases in the amount of the
90
88
86
84
Yield(%)
82
80
78
76
fresh
1
2
3
4
Run
FIGURE 2 Catalytic recyclability of CTSA

4
KANAANI AND NASR-ESFAHANI
TABLE 2
Preparation of 2,4,5-trisubstituted imidazoles catalyzed by CTSA using benzil (2) or benzoin (3)
Yielda (%)
Benzil (benzoin)
96 (93)
M.p. (ꢀC)
Time (min)
Entry
R
Product
4a
Benzil (benzoin)
25 (35)
Found
Reported
1
C6H5
271–273
240–242
267–270
263–264
173–175
265–266
231–232
232–235
232–234
260–263
233–234
204–206
271–272[38]
241–242[38]
269–271[39]
260–262[40]
174–175[40]
264–266[3]
230–232[41]
233–234[42]
233–235[42]
261–263[3]
233–235[29]
206–208[29]
2
4-NO2C6H4
3- NO2C6H4
4-ClC6H4
2,4- ClC6H3
4-Br C6H4
4-OCH3C6H4
4-CH3C6H4
4-OHC6H4
4-HO-3-EtOC6H3
2-Furyl
4b
4c
24 (33)
92 (91)
3
24 (39)
89 (88)
4
4d
4e
20 (33)
93 (92)
5
26 (32)
88 (86)
6
4f
27 (31)
92 (90)
7
4g
35 (44)
84 (84)
8
4h
4i
37 (45)
81 (80)
9
32 (36)
83 (81)
10
11
12
4j
40 (48)
87 (86)
4k
4l
43 (51)
79 (80)
CH3CH2CH2
56 (62)
74(71)
a
Isolated yields.
A plausible mechanism for the preparation of imidazoles
using aldehyde, benzil, and ammonium acetate is given in
Scheme 4. The first step in this reaction involves the CTSA-
catalyzed formation of diamine intermediate 8 stabilized by
CTSA. Then, the diamine, which is very prone to react with
a diketone, leads to the intermediate 10, which rearranges
TABLE 3 Preparation of 2,4,5-trisubstituted imidazoles catalyzed by CTSA using 9,10-phenanthrenequinone (5)
M.p. (ꢀC)
Found
Entry
R
Product
6a
Time (min)
Yielda (%)
Reported
322–324[45]
337–339[3]
275–276[45]
—
1
2
3
4
5
6
7
8
9
C6H5
22
25
23
25
26
26
33
28
27
94
91
95
89
90
87
88
91
89
323–325
336–339
274–275
224–227
257–259
253–254
268–270
346–348
255–257
4-NO2C6H4
4-ClC6H4
2,4- ClC6H3
4-Br C6H4
4-OCH3C6H4
4-CH3C6H4
4-OHC6H4
5-Br-2-OH C6H4
6b
6c
6d
6e
—
6f
254–256[3]
268–270[3]
348–350[3]
—
6i
6j
6k
a
Isolated yields.

KANAANI AND NASR-ESFAHANI
5
TABLE 4 Catalytic synthesis of 1,2,4,5-tetrasubstituted imidazoles using CTSA and benzil or benzoin
M.p. (ꢀC)
Found
Time (min)
Benzil (benzoin)
34 (39)
Yielda (%)
Entry
R1
R2
Product
7a
Benzil (benzoin)
91 (89)
Reported
1
2
3
4
5
6
7
8
C6H5
4-ClC6H4
C6H5
197–199
184–187
146–149
163–165
151–154
176–178
186–189
288–289
198–199[2]
184–186[3]
149–151[30]
164–166[43]
152–154[44]
176–177[2]
186–188[43]
285–286[43]
4-NO2C6H4
3- NO2C6H4
4-ClC6H4
4-Br-C6H4
4-OCH3C6H4
4-CH3C6H4
4-OHC6H4
7b
39 (47)
87 (85)
4-CH3C6H4
C6H5
7c
40 (47)
87 (88)
7d
40 (50)
88 (89)
C6H5
7e
41 (47)
88 (81)
4-ClC6H4
C6H5
7f
48 (53)
85 (81)
7g
50 (55)
86 (81)
C6H5
7h
42 (53)
85(83)
a
Isolated yields.
SCHEME 4 Possible mechanism for the catalytic synthesis of four highly substituted imidazoles using CTSA
via a [1,5]-sigmatropic proton shift to produce the corre-
sponding imidazole.
3
|
EXPERIMENTAL
On the other hand, the intermediate 8 reacts with benzoin
to produce the intermediate of dihydroimidazole 9, which
undergoes aerobic oxidation to form the fully conjugated
system 10 and consequently rearranges via a [1,5]-
sigmatropic proton shift to give the final product.
3.1 | Chemicals and apparatus
All reagents were purchased from Merck, Fluka, and Aldrich
chemical companies. Melting points were recorded on a
Barnstead Electrothermal (BI 9300) apparatus and are

6
KANAANI AND NASR-ESFAHANI
uncorrected. NMR spectra recorded on a Bruker 400 MHz
Ultrashield spectrometer at 400 MHz (1H) and 100 MHz
(13C), with DMSO-d6 as the solvent and trimethylsilane
(TMS) as the internal standard. IR spectra were recorded on
a JASCO-680 FT-IR spectrometer.
7.88 (dd, 1H, J1 = 8.0 Hz, J2 = 2.8 Hz, aromatic CH), 7.99
(d, 1H, J = 2.8 Hz, aromatic CH), 8.00–8.02 (m, 5H, aro-
matic CH), 8.42 (d, 2H, J = 8.8 Hz, aromatic CH), 8.63 (d,
2H, J = 7.6 Hz, aromatic CH), 13.68 (s, 1H, NH); 13C
NMR (100 MHz, DMSO-d6): 122.19, 125.64, 125.99,
126.13, 126.41, 127.80, 128.18, 129.61, 131.67, 131.87,
136.01, 136.18, 136.21, 148.03; MS (m/z): 363.1
[C21H12Cl2N2]+, 328.1 [C21H13ClN2]+, 294.1 [C21H14N2]+,
193.1 [C14H11N]+, 264.1, 181.2, 164.1, 118.7, 75.1.
3.2 | Preparation of citrate trisulfonic acid
Trisodium citrate was obtainedꢀby drying trisodium citrate
trihydrate in an oven at 250 C for 6 h to remove the
adsorbed water. To chlorosulfonic acid (23.1 mL, 0.3 mol,
34.8 g) in a 250-mL round bottom flask, dried trisodium cit-
rate (25.8 g, 0.1 mmol) was added slowly under vigorous
stirring over a period of 30 min at room temperature. After
the completion of the addition, the reaction mixture was
shaken for 30 min. CTSA was obtained as a light brown
solid after washing with hot methanol. Finally, CTSA was
dried and stored in a capped bottle.
3.6 | 2-(4-Bromophenyl)-1H-phenanthro[9,10-d]
imidazole (6f)
Yellow solid, m.p. 257–259 C; IR (KBr) cm−1: 646, 786,
ꢀ
979, 1,087, 1,151, 1,496, 1,519, 1,614, 3,407; 1H NMR
(400 MHz, DMSO-d6): 7.68–7.69 (m, 4H, aromatic CH),
7.86–7.88 (m, 4H, aromatic CH), 8.32 (d, 2H, J = 8.4 Hz,
aromatic CH), 8.54 (d, 2H, J = 8.0 Hz, aromatic CH), 13.58
(s, 1H, NH); 13C NMR (100 MHZ, DMSO-d6): 120.21,
121.72, 124.87, 125.61, 125.79, 125.80, 129.05, 128.00,
133.67, 133.75, 134.40, 134.50, 135.75, 147.79. MS (m/z):
3.3 | Spectral data of CTSA
−1
ꢀ
M.p. = 190–192 C; IR (KBr, cm ): 439, 592, 873, 1,060,
1,226, 1,643, 1,724, 1,776, 2,468, 2,921, 3,500; 1HNMR
(400 MHz, DMSO-d6): δH 1H NMR (400 MHz, DMSO-d6):
δH 2.72 (d, 2H, J = 15.6 CH2), 2.88 (d, 2H, J = 15.6 Hz,
CH2), 3.87 (br, 1H, OH), 12.65 (s, 2H, S-OH), 12.70 (s, 1H,
S-OH). 13C NMR (100 MHz, DMSO-d6): δC 50.13, 79.41,
188.32, 191.23. MS (m/z): 430.4 [C6H8S3O16]+, 337.1
[C6H8S2O12]+, 382.2 [C6H6S3O13]+, 175.1 [C6H8O6]+, 310.1
[C5H8S2O11], 293.1 [C5H8S2O10], 249, 232, 182, 123, 78.
373.1
[C21H13BrN2]+,
283.2
[C21H17N]+,
207.1
[C15H13N]+, 178.1 [C14H10]+, 294.1, 186.1, 164.1, 146.1,
132.1, 50.1.
3.7 | 4-Bromo-2-(1H-phenanthro[9,10-d]imidazol-2-yl)
phenol (6k)
−1
ꢀ
Brown solid, m.p. 255–257 C; IR (KBr) cm : 615, 754,
1,232, 1,477, 1,590, 1,668, 3,288. 1H NMR (400 MHz,
DMSO-d6): 7.07(dd, 1H, J1 = 6.4 Hz, J2 = 2.4 Hz, aromatic
CH), 7.53 (d, 1H, J = 2.4 Hz, aromatic CH), 7.70 (d, 1H,
J = 6.4 Hz, aromatic CH), 8.02–8.04 (m, 4H, aromatic CH),
8.51 (d, 2H, J = 8.0 Hz, aromatic CH), 8.88 (d, 2H,
J = 7.6 Hz, aromatic CH), 9.19 (brs, 1H, OH), 13.77 (s, 1H,
NH). 13C NMR (100 MHz, DMSO-d6): 115.36, 119.91,
122.21, 122.59, 125.84, 126.22, 126.53, 126.91, 128.56,
129.78, 135.75, 135.88, 148.18, 157.03. MS (m/z): 389.1
[C21H13BrN2O]+, 373.1 [C21H13BrN2]+, 310.1 [C21H14N2O]+,
294.2 [C21H14N2]+, 207.1, 164.1, 150.1, 118.1, 102.1, 50.1.
3.4 | General procedure for the preparation of
multisubstituted imidazoles
A mixture of benzile or 9,10-phenanthrenequinone or
α-hydroxyketone (1 mmol), the corresponding aldehyde
(1 mmol), ammonium acetate (2 mmol for 2,4,5-trisubstituted
imidazoles and 1 mmol for 1,2,4,5-tetrasubstituted imidaz-
oles) or aniline (1 mmol for 1,2,4,5-tetrasubstituted imidaz-
oles), and CTSA (5% mmol) was stirred in an oil bath at
90 ꢀC for a suitable time. After its completion, as indicated by
TLC, hot ethanol (5 mL) was added and the heterogeneous
catalyst was separated by filtration. The solvent was evapo-
rated and the obtained crude product was purified by column
chromatography or recrystallization from ethanol to afford
pure products (Tables 2 and 4). Then, the products were char-
4
| CONCLUSIONS
In this paper, we presented an efficient, fast, and convenient
synthesis of 2,4,5-trisubstituted imidazoles and 1,2,4,5-
tetrasubstituted imidazoles in a one-pot four-component con-
densation of benzyl, benzoin, or 9,10-phenanthrenequinone
with an aldehyde, ammonium acetate, and/or aniline under
solvent-free conditions in the presence of CTSA as a recy-
clable and eco-benign organocatalyst. This method provided
the desired products with short reaction times, high effi-
ciency, low cost, reusability of the catalyst, and simple puri-
fication of the products.
1
acterized by spectroscopic techniques such as IR, H NMR,
and 13C NMR and also by comparison of their melting points
with those reported in the literature. The data of some new
compounds are shown below:
3.5 | 2-(2,4-Dichlorophenyl)-1H-phenanthro[9,10-d]
imidazole (6e)
Milky solid, m.p. 224–227 C; IR (KBr) cm−1: 723, 815,
ꢀ
1,103, 1,457, 1,623, 3,428; 1H NMR (400 MHz, DMSO-d6):

KANAANI AND NASR-ESFAHANI
7
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ACKNOWLEDGMENT
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This work was supported by Yasouj University.
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ORCID
Masoud Nasr-Esfahani
http://orcid.org/0000-0001-5823-4254
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How to cite this article: Kanaani E, Nasr-
Esfahani M. Citrate trisulfonic acid: A heterogeneous
organocatalyst for the synthesis of highly substituted
Imidazoles. J Chin Chem Soc. 2018;1–7. https://doi.
org/10.1002/jccs.201800015