A rapid access to novel and known benzimidazole derivatives using silica chloride as a reusable catalyst

Article abstract of DOI:10.1002/cjoc.201100206

A new application of silica chloride as an easily available and reusable solid acid catalyst for the synthesis of benzimidazole and its derivatives through the condensation of o-phenylenediamines and orthoesters under thermal and solvent-free conditions is described. This novel and eco-friendly method is very cheap and has many advantages including excellent yields, short reaction time, and simple work-up procedure. Copyright

Full text of DOI:10.1002/cjoc.201100206

FULL PAPER
DOI: 10.1002/cjoc.201100206
A Rapid Access to Novel and Known Benzimidazole Deriva-
tives Using Silica Chloride as a Reusable Catalyst
Karami, Bahador*,a
Ghashghaee, Vahidehb
Khodabakhshi, Saeeda
a Department of Chemistry, Yasouj University, Yasouj, Zip Code: 75918-74831, P.O.Box 353, Iran
b Department of Chemistry, Gachsaran Branch, Islamic Azad University, Gachsaran, Iran
A new application of silica chloride as an easily available and reusable solid acid catalyst for the synthesis of
benzimidazole and its derivatives through the condensation of o-phenylenediamines and orthoesters under thermal
and solvent-free conditions is described. This novel and eco-friendly method is very cheap and has many advan-
tages including excellent yields, short reaction time, and simple work-up procedure.
Keywords silica chloride, orthoesters, o-phenylenediamine, benzimidazole
Introduction
Scheme 1 Synthesis of silica chloride
O
Among the wide variety of nitrogen heterocycles
that have been explored for developing pharmaceuti-
cally important compounds, benzimidazoles showed a
relatively high biological activities such as anti-HIV,[1,2]
anti-herpes (HSV-1),[3] anti-influenza,[4] antifungal
agent,[5] and raf kinase inhibitor.[6] The most prominent
benzimidazole compound in nature is N-ribosyldime-
thylbenzimidazole, which serves as an axial ligand for
cobalt in vitamin B12.[7] The discovery of novel syn-
thetic methodologies to facilitate the preparation of
compound libraries is a pivotal focal point of research
activity in the field of modern organic synthesis. Until
now, several methods for the synthesis of benzimidazole
and its derivatives have been reported. In fact, the most
commonly method is the condensation of an ary-
lenediamine with a carbonyl equivalent.[8,9] Also, esters,
lactones and anhydrides could produce benzimidazoles
through the cyclization of amide. However, some of the
reported methods suffer from drawbacks such as the use
of toxic and expensive catalyst or solvent, long reaction
times, low yield of the products, and difficult work-up
procedures. Solid supports have found wide applications
in organic reactions and they have advantages such as
facilitating the work-up of the reaction, and high selec-
tivity accompanied with high yields of the products. It
should be noted that the silica gel is used extensively as
a support in organic chemistry.[10,11] In this work, as can
be seen in Scheme 1, we prepared silica chloride 1 from
the reaction of readily available materials such as sili-
cagel and thionyl chloride.[12]
Reflux
48 h
SiO2
O
+
SOCl2
Si OH
O
O
SiO2
O
Si Cl
+ HCl + SO2
O
1
Considering the wide use of catalyst in chemistry,
we tried to report a new application of silica chloride
(SiO2-Cl) on synthesizing the substituted benzimida-
zoles. Recently, silica chloride was used for several or-
ganic transformations such as one-pot synthesis of 13-
aryl-indeno[1,2-b]naphtha[1,2-e]pyran-12(13H)-ones
(Wu and co-workers),[13] deprotection of trimethylsilyl
ethers to corresponding alcohols (Bamoniri and
co-workers),[14] synthesis of 4-aryl substituted 3,4-dihy-
dropyrimidinones (Kaushik and co-workers),[15]
multi-component synthesis of amidoalkyl-2-naphthols
under sonic conditions (Pasha and co-workers),[16] and
oxidation of urazoles (Zolfigol and co-workers).[17] The
main aims of this research are to obtain a novel class of
benzimidazoles and benzimidazole oximes, and also
introduction of a new application of silica chloride as
solid acid catalyst in organic synthesis.
Results and Discussion
In connection with our studies on synthesis of or-
ganic compounds,[18] we have now found that silica
chloride can be used as an efficient, safe and very cheap
catalyst for the condensation of o-phenylenediamines 2
and orthoesters 3 under thermal and solvent-free condi-
*
E-mail: karami@mail.yu.ac.ir; Tel.: ＋98-7412223048; Fax: ＋98-7412242167
Received August 6, 2011; accepted October 23, 2011.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.20100206 or from the author.
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Karami, Ghashghaee & Khodabakhshi
FULL PAPER
tions to afford benzimidazol derivatives 4 in good to
excellent yields (Scheme 2). Silica chloride is a strong
solid acid so that it can decrease the energy of transition
state of the nucleophilic attack step. The two possible
tautomeric forms of benzimidazole (and of those of its
derivatives possessing a plane of symmetry) are identi-
cal and a definite assignment of structure is possible.[19]
the various amounts of SiO2-Cl according to Table 1
and a wide range of temperatures (Table 2), it was
found that the condensation reaction can be efficiently
carried out by adding 3 mol% of the catalyst at 80 ℃
under solvent-free conditions in a short time span of 12
min. The use of excessive amounts of the catalyst does
not increase the yield and reaction rate.
Scheme 2 Synthesis of benzimidazoles using silica chloride
Table 1 Synthesis of 1H-benzimidazole 4a catalyzed with
Cl
various amounts of SiO2-Cl 1 under solvent-free conditions at 80
℃
G
Entry
Catalyst/mol%
Time/min
Yielda/%
G
SiO2
R2
1
2
3
4
5
—
1
150
30
20
12
9
10b
70
82
90
83
O
R1
(3 mol%)
Solvent-Free, 80 oC
R2
+
N
NH
O
H2N
NH2
O
2
-3R2OH
R2
R1
3
2
3
4
G: H, Me, NO2, Benzoyl
R1 and R2: H, Me, Et
5
a Isolated yields. b Not completed.
A solvent-free or solid state reaction obviously re-
duces pollution, and brings down handling costs due to
simplification of experimental procedure, work up tech-
nique and saving in labour. These would be especially
important during industrial production.
Table 2 Synthesis of 1H-benzimidazole 4a catalyzed with
SiO2-Cl (3 mol%) at several temperatures under solvent-free
conditions
Entry
Temperature/℃
Time/min
432
Yielda/%
1
2
3
4
5
25
40
5b
25
75
90
80
Interest in the environmental control of chemical
processes has increased remarkably during three
decades ago (Over the past three decades) as a response
to public concern about the use of hazardous chemicals.
Therefore, to improve the effectiveness of this method
in preventing chemical waste, it is important to investi-
gate its optimal conditions. For establishing the simple
and suitable conditions to prepare benzimidazole de-
rivatives using SiO2-Cl as a solid acid catalyst, the
treatment of triethyl orthoformate with o-phenylene
diamine was chosen as a model reaction. At first, we
found that in the absence of the catalyst, the reaction did
not proceed even at a high temperature. After examining
288
60
35
80
12
120
10
a Isolated yields. b Not completed.
In order to prove the versatility of this method, after
optimizing the reaction conditions, we have treated dif-
ferent orthoestrs with o-phenylenediamines under sol-
vent-free and thermal conditions. The results are sum-
marized in Table 3.
Table 3 Synthesis of benzimidazole derivatives using SiO2-Cl (3 mol%) at 80 ℃ under solvent-free conditions.
Entry
1
Orthoester
Producta
Time/min
Yieldb/%
m.p./℃ [lit.]c
N
HC(OEt)3
12
90
170—172 [170—172]
N
H
N
Me
2
3
4
MeC(OMe)3
EtC(OEt)3
8
92
85
75
176—178 [172—174]
166—168 [163—165]
N
H
N
Et
N
23
15
N
H
Me
HC(OEt)3
108—110 [112—114]
N
H
960
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A Rapid Access to Novel and Known Benzimidazole Derivatives
Continued
Entry
5
Orthoester
Producta
Time/min
10
Yieldb/%
90
m.p./℃ [lit.]c
Me
Me
N
Me
Et
MeC(OMe)3
198—200 [198—200]
165—167 [160—163]
208—210 [206—208]
225—227 [225—227]
177—179 [177—179]
N
H
N
6
7
8
9
EtC(OEt)3
HC(OEt)3
30
7
85
90
92
85
N
H
O2N
N
N
H
O2N
O2N
N
Me
Et
MeC(OMe)3
4
N
H
N
EtC(OEt)3
28
N
H
a Identified by comparison with authentic samples. b Refers to isolated yields. c Ref. [20].
In another variation, as can be seen in Scheme 3,
with the use of 4-benzoyl-1,2-phenylenediamine 5, three
novel benzimidazole derivatives such as 6, 7, and 8
were synthesized at the same conditions and results ob-
tained are summarized in Table 4. The 1H NMR
(CDC13, 400 MHz) spectrum of 8 exhibited a triplet as
protons of methyl group (δ 1.47), a quartet as protons of
methylene group (δ 3.00). Three multiplet signals
(δ 7.82—7.45) and a singlet signal (δ 7.23—7.27) cor-
responding to the aromatic protons. Also, the resonance
of proton of NH in the 1H NMR spectrum of 8 appeared
at δ 10.80—10.40 as a broad signal. The resonances of
carbonyl groups in the 13C NMR spectrum of 8 ap-
peared at δ 197.62.
Not only the ecological profile (through helping to
decrease hazardous industrial waste), but also the eco-
nomic profile (through the elimination of expensive or-
ganic solvent) is further improved if the catalyst is recy-
clable and reaction conditions are solvent-free. In this
process, as indicated in Figure 1, the recycled catalyst
was used for four cycles during which a little apprecia-
Table 4 Synthesis of new benzimidazole derivatives using
SiO2-Cl (3 mol%) at 80 ℃ under solvent-free conditions
Entry Orthoester Producta Time/min Yieldb/% m.p./℃
1
2
3
4
HC(OEt)3
HC(OMe)3
MeC(OMe)3
EtC(OEt)3
26
30
18
38
85
75
95
90
136—138
136—138
168—170
140—142
6
6
7
8
a Identified by spectral data. b Refers to isolated yields.
ble loss was observed in the catalytic activities.
A plausible mechanism for the reaction is supposed
as shown in Scheme 4. The Si—Cl bond is liable and
can give rise to Lewis acid centers on silica. The Cl is
easily displaced selectively by an ethoxy of orthoester
by a nucleophilic substitution reaction generating a
cationic center on the oxygen which makes a living
group on orthoester and cationic center that subse-
quently is easily attacked by the nitrogen of o-phenilene
diamin to form the dialkoxymethanamine which
Scheme 3 Synthesis of novel benzimidazoles using silica chloride 1
O
O
N
MeC(OMe)3
HC(OEt)3
(i)
Me
N
(i)
N
H
7
N
H
6
NH2
O
O
EtC(OEt)3
(i)
N
NH2
HC(OMe)3
(i)
Et
6
5
N
H
8
(i): Solvent-Free, 80 oC, SiO2-Cl (3 mol%)
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Karami, Ghashghaee & Khodabakhshi
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Scheme 4 Suggested mechanism for the preparation of benzimidazole derivatives by the use of silica chloride 1
R2
R2O
R1
Cl
OR2
R1
H
O
R2
R2
OR2
Cl-
N
OR2
OR2
O
OR2
R1
Cl
SiO2
-
SiO2
SiO2 -R2OH
O OR2
O
+
G
R2O
OR2
Cl-
NH2
SiO2
H
G
NH
Cl
G
-R2OH
SiO2
NH2
H
Cl
R1
N
N
-R2OH
R1
+
OR2
G
Cl
SiO2
N
H
N
H
SiO2
4
benzimidazole oximes 9 and 10 from the reaction of 7
and 8 with hydroxylamine hydrochloride in the presence
of stoichiometric ratio of sodium hydroxide under re-
fluxing ethanol (Scheme 5).
It is worthy noting that some oxime-containing
molecules display biological properties. For example,
furan oximes were found to inhibit DNA, RNA, and
protein synthesis in lipoid leukemia cells.[21] Derivatives
of quinoline oximes were also shown to possess antitu-
mor activity and glucosinolates were suggested as can-
cer preventive agents.[22] It should be mentioned that the
ketoximes exist as two geometric stereoisomers: a syn-
isomer and an anti-isomer. Therefore, the products 9
and 10 were present as a mixture of isomers (each iso-
mer as a syn/anti mixture of oximes), which was diffi-
Figure 1 Recyclability of SiO2-Cl as catalyst for the synthesis
of 4a under solvent-free conditions at 80 ℃. Reaction time＝12
—15 min.
1
cult to identify because in the H and 13C NMR spectra
only small chemical shift differences were observed.
The spectral data shown in experimental section are also
consistent with the abovementioned structures.
cyclizes intramolecularly in the presence of silica chlo-
ride to benzimidazole 4.
Regarding to the importance of novel organic com-
pounds in pharmacological researches, synthetic ma-
nipulations, and industries, in this present work, we de-
cided to synthesize new ketoxime compounds such as
Experimental
The chemicals were purchased from Merck, Fluka
and Aldrich chemical companies. The reactions were
monitored by TLC (silica-gel 60 F254, n-hexane∶ethyl
Scheme 5 Synthesis of novel benzimidazole oximes using silica chloride 1
HO
N
OH
N
N
N
Me
Me
+
N
H
N
H
9 (Syn)
9 (Anti)
.
NH2OH HCl + NaOH
HO
OH
N
N
N
N
+
Et
Et
N
H
N
H
(i): EtOH/H2O, Reflux
10 (Syn)
10 (Anti)
962
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Chin. J. Chem. 2012, 30, 959—964

A Rapid Access to Novel and Known Benzimidazole Derivatives
acetate). IR spectra were recorded on an FT-IR Shima-
dzu-470 Spectrometer and the H NMR spectra were
5-Benzoyl-2-ethyl-benzimidazole 8
1H NMR
1
(CDC13, 400 MHz) δ: 10.70 (s, 1H), 8.06 (s, 1H), 7.78
(dd, J＝8.1 Hz, 3H,), 7.58 (t, J＝6.6 Hz, 2H), 7.47 (t,
J＝7.6 Hz, 2H), 3.02 (q, J＝7.6 Hz, 2H), 1.47 (t, J＝7.6
Hz, 3H); 13C NMR (CDC13, 100 MHz) δ: 197.62,
159.75, 138.30, 132.21, 131.51, 129.97, 128.25, 124.86,
118.16, 114.27, 22.81, 12.32; IR (KBr) ν: 3300—2300,
1650, 1620, 1540, 1475, 1400, 1100, 960, 870,700; MS
(70 eV) m/z: 250.
2-Methyl-benzimidazole-5-yl-(phenyl)methanone
oxime 9 1H NMR (DMSO-d6, 400 MHz) δ: 12.30 (d,
J＝11.2 Hz, 1H), 12.16 (s, 1H), 11.27 (d, J＝20 Hz,
1H), 7.71—7.10 (m, 18H) 11.15 (s, 1H), 2.69 (s, 6H);
13C NMR (DMSO-d6, 100 MHz) δ: 156.38, 156.24,
153.00, 152.88, 138.74, 138.16, 134.82, 132.41, 130.69,
129.87, 129.41, 129.11, 128.82, 128.66, 128.52, 127.79,
126.69, 122.96, 120.85, 117.78, 110.82, 15.26, 15.14;
IR (KBr) ν: 3500—2300, 1620, 1540, 1450, 1320, 1120,
1000, 950, 890,780, 700; MS (70 eV) m/z: 251.
obtained on a Bruker-Instrument DPX-400 Avance 2
model. Mass spectra were recorded on a Shimadzu
Gc-MS QP 100 Ex spectrometer. All of the products
(except novel compounds) were characterized by com-
parison of their spectra and physical data, with those
reported in the literature.
Preparation of silica chloride 1
To an oven-dried (125 ℃, vacuum) sample of sil-
ica-gel 60 (10 g) in a round bottomed flask (250 mL)
equipped with a condenser and a drying tube, was added
thionyl chloride (40 mL) and the mixture in the pres-
ence of CaCl2 as a drying agent was refluxed for 48 h.
The resulting white-grayish powder was filtered and
stored in a tightly capped bottle.
Preparation of benzimidazole derivatives 4 using
silica chloride 1
2-Ethyl-benzimidazole-5-yl-(phenyl)methanone
oxime 10 1H NMR (DMSO-d6, 400 MHz) δ: 12.30 (d,
J＝11.2 Hz, 1H), 12.16 (s, 1H), 11.27 (d, J＝18.8 Hz,
1H), 11.15 (s, 1H), 7.71—7.03 (m, 18H), 2.82 (s, 4H),
1.30 (s, 6H); 13C NMR (DMSO-d6, 100 MHz) δ: 157.78,
156.38, 156.25, 143.70, 138.16, 134.95, 130.73, 129.00,
129.42, 129.09, 128.64, 128.51, 127.78, 126.89, 123.13,
120.63, 119.42, 118.12, 110.83, 22.50, 12.73, 12.59; IR
(KBr) ν: 3350, 3200—2300, 1620, 1540, 1450, 1320,
1120, 1000, 950, 890, 780, 700; MS (70 eV) m/z: 265.
A mixture of o-phenylenediamine 2 (1 mmol), or-
thoester 3 (1.1 mmol) and silica chloride 1 (3 mol%,
0.007 g) was stirred and heated at 80 ℃ in a preheated
oil bath for an appropriate time (Tables 3 and 4). After
completing the reaction as indicated by TLC (ethyl ace-
tate/n-hexan, 1∶2, V/V), the reaction mixture was dis-
solved in hot ethyl acetate and catalyst was separated by
simple filtration. The solvent was evaporated and the
crude product 4 was purified by recrystallization in
ethyl acetate.
Conclusions
Preparation of benzimidazole oxime derivatives 9, 10
A mixture of benzimidazole 7 or 8 (1 mmol),
hydroxylamine hydrochloride (5 mmol), sodium hy-
droxide (5 mmol) 5 mL of 95% ethyl alcohol, and 1 mL
of water is placed in a round-bottomed flask, then the
flask is connected to a reflux condenser, heated to boil-
ing, and refluxed for 5 h. After completion of the reac-
tion (TLC), the precipitate was filtered, thoroughly
washed with water, and dried to afford white solid as
pure products 9 and 10.
In this study, we presented a simple, powerful and
clean method for the synthesis of benzimidazole deriva-
tives (novel and known) by employing the silica chlo-
ride as a reusable solid acid catalyst under solvent-free
conditions. The simple experimental procedure, sol-
vent-free reaction conditions, utilization of an inexpen-
sive and readily available catalyst, short period of reac-
tion and good to excellent yields make this method a
valid contribution to the existing methodologies. Also,
the presence of transformable functionalities in the
novel products makes them potentially valuable from
the aspect of further synthetic manipulations.
Spectral data of novel compounds
5-Benzoyl-benzimidazole 6 1H NMR (CDC13,
400 MHz) δ: 8.22 (s, 1H), 8.18 (s, 1H), 7.84 (t, J＝9.4
Hz, 3H), 7.72 (d, J＝8.4 Hz, 1H), 7.60 (t, J＝7.2 Hz
1H), 7.49 (t, J＝8 Hz, 2H); 13C NMR (CDC13, 100
MHz) δ: 197.31, 157.80, 143.20, 138.11, 132.46, 132.34,
130.05, 128.32, 125.29, 115.12; IR (KBr) ν: 3250—
2300, 1650, 1615, 1450, 1100, 950, 870, 690; MS (70
eV) m/z: 222.
Acknowledgement
The authors gratefully acknowledge partial support
of this work by the Yasouj University Iran.
References
5-Benzoyl-2-methyl-benzimidazole 7 1H NMR
(CDC13, 400 MHz) δ: 8.04 (s, 1H), 7.81 (d, J＝7.6 Hz,
2H), 7.75 (d, J＝8 Hz, 1H), 7.57 (d, J＝7.2 Hz, 2H),
7.47 (t, J＝7.2 Hz, 2H), 2.67 (s, 3H); 13C NMR (CDC13,
100 MHz) δ: 197.51, 154.42, 138.28, 132.24, 131.64,
130.02, 128.27, 124.94, 15.17; IR (KBr) ν: 3200, 2350,
1665, 1620, 1510, 1320, 1020, 820, 720; MS (70 eV)
m/z: 236.
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