Polystyrene-supported pyridinium chloroaluminate ionic liquid as a new heterogeneous Lewis acid catalyst for selective synthesis of benzimidazoles

Article abstract of DOI:10.2298/JSC120401089P

Polystyrene-supported pyridinium chloroaluminate ionic liquid was prepared from the reaction of a Merrifield resin with pyridine followed by reaction with aluminium chloride. This catalyst was used as a new chemoselective Lewis acid catalyst for the exclusive synthesis of 2-substituted benzimidazoles from the reaction of aldehydes with o-phenylenediamines. The catalyst was stable (as a benchtop catalyst) and could easily be recovered and reused without appreciable change in its efficiency.

Full text of DOI:10.2298/JSC120401089P

J. Serb. Chem. Soc. 78 (2) 155–164 (2013)
JSCS–4404
UDC 678.746+547.821+549.456.4:
542.913+547.532+547.78
Original scientific paper
Polystyrene-supported pyridinium chloroaluminate ionic liquid
as a new heterogeneous Lewis acid catalyst for selective
synthesis of benzimidazoles
*
KAVEH PARVANAK BOROUJENI , ASHKAN ZHIANINASAB
and MINA JAFARINASAB
Department of Chemistry, Shahrekord University, Shahrekord 115, Iran
(Received 1 April, revised 27 August 2012)
Abstract: Polystyrene-supported pyridinium chloroaluminate ionic liquid was
prepared from the reaction of a Merrifield resin with pyridine followed by
reaction with aluminium chloride. This catalyst was used as a new chemo-
selective Lewis acid catalyst for the exclusive synthesis of 2-substituted benz-
imidazoles from the reaction of aldehydes with o-phenylenediamines. The cata-
lyst was stable (as a benchtop catalyst) and could easily be recovered and re-
used without appreciable change in its efficiency.
Keywords: ionic liquids; chloroaluminate salts; heterogeneous catalysis; poly-
mer-supported ionic liquids; benzimidazoles.
INTRODUCTION
The most common group of ionic liquids (ILs) are chloroaluminate molten
1–3
salts.
These compounds, composed of mixtures of aluminium chloride with
the corresponding 1,3-dialkylimidazolium or 1-alkylpyridinium chlorides, are li-
quid at or below ambient temperature over a wide range of compositions. The
acidity of the resulting IL can be controlled by varying the relative amounts of
AlCl and organic chloride. Chloroaluminate melts are designated as basic when
3
–
–
the AlCl mole fraction is smaller than 0.5 and the melts contain Cl and AlCl ,
3
4
a Lewis bases. A melt having an AlCl mole fraction of exactly 0.5, where
3
–
AlCl is the only anion present, is referred to as a neutral melt. Finally, an acidic
4
chloroaluminate melt is one in which the AlCl mole fraction is larger than 0.5.
3
–
–
In such acidic melts, Al Cl and Al Cl exist, which act as very strong Lewis
2
7
3
10
2
acids.
Despite the unique physical and chemical properties of chloroaluminate ILs
and their widespread application in organic synthesis, either as solvents or acidic
*Corresponding author. E-mail: parvanak-ka@sci.sku.ac.ir
doi: 10.2298/JSC120401089P
155
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156
BOROUJENI, ZHIANINASAB and JAFARINASAB
catalysts, most of them suffer from one or more of the following drawbacks such
as laborious work-up procedures, difficulty in recovery and recycling, disposal of
spent catalyst, difficulty in handling and corrosion problems. Most importantly,
2
they are extremely hygroscopic and labile towards hydrolysis. Thus, these short-
comings make them a prime target for heterogenization. Although there are seve-
4–13
ral reports on the immobilization of ILs on solid supports,
to the best of our
9,12
knowledge, few examples are known for immobilized chloroaluminate ILs.
The benzimidazole and its derivatives constitute a very important, privileged
class of nitrogen-containing heterocyclic compounds that show various biological
14
15
16
activities, such as anticonvulsant, antidepressant, antihistamine, anti-ul-
17
18
19
20
21
cer,
antihypertensive,
anticancer,
anti-inflammatory,
anthelmintic,
22
23
antiprotozoal and antitumour properties. In addition, benzimidazoles are very
important intermediates in organic reactions. Therefore, their preparations have
24
received increasing attention of synthetic organic chemists and biologists. Usu-
ally, two different approaches have been performed for the preparation of benz-
imidazoles. The first is the coupling of o-phenylenediamines and carboxylic
acids or their derivatives (nitriles, amides, orthoesters and acid chlorides), which
often requires strongly acidic conditions, sometimes combined with high tempe-
25–28
ratures.
Another synthetic approach is the condensation of aldehydes with o-
phenylenediamines, which involves a two-step procedure including the oxidative
cyclodehydration of aniline Schiff bases, which are often generated in situ from
the reaction of an aldehyde and o-phenylenediamine. Various oxidative and cata-
29
30
31
32
lytic reagents, such as Yb(OTf) , I /KI/K CO , In(OTf) , BF ·OEt ,
3
2
2
3
3
3
2
33
34
35
H O /HCl, TsOH–SiO , NaY zeolite, porphyrinatoiron(III) complex sup-
ported on silica, Dess–Martin-periodinane, scolecite, AIKIT-5, WO /
2
2
2
36
37
38
39
x
40
41
42
/ZrO , H O /SiO –FeCl
and Amberlite IR-120 have been employed for
2
2
2
2
3
this purpose. However, many of the above methods suffer from drawbacks such
as strong oxidizing nature, the formation of bis-anil and dihydrobenzimidazoles
as the main side products, involvement of more than one step in the synthesis of
these compounds, the generation of environmentally perilous waste material,
tedious work-up, long reaction times, complicated operations and the use of
moisture-sensitive, expensive, hazardous, difficult to handle or non-reusable
catalysts. In view of these, the search for finding a cost effective, mild and simple
chemoselective protocol especially using heterogeneous catalysts for the syn-
thesis of benzimidazoles is still relevant.
In 2001, Hölderich et al. reported the synthesis and application of silica gel
12
supported chloroaluminate IL as a heterogeneous Lewis acidic IL. This sup-
ported IL was prepared by adding AlCl to imidazolium chloride grafted onto the
3
silica surface. The prepared catalyst was used for Friedel-Crafts alkylations. Un-
fortunately, this catalyst is moisture sensitive and hence demands moisture-free
reactants and also must be handled under a dry atmosphere. Hölderich suggested
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BENZIMIDAZOLE SYNTHESIS
157
that the predominant acidic chloroaluminate species in this immobilized IL was
–
Al Cl .
2
7
In a continuation of research work on the development of synthetic metho-
dologies using solid acid catalysts, herein, the above-mentioned strategy to sup-
port pyridinium chloroaluminate IL on a Merrifield resin, and the employment of
the obtained polymer catalyst as an effective and highly chemoselective hetero-
geneous catalyst for the coupling of aldehydes and o-phenylenediamines to af-
ford 2-aryl and alkylbenzimidazoles (Scheme 1) are reported.
Scheme 1. The coupling reaction of aldehydes and o-phenylenediamines to afford 2-aryl
and alkyl benzimidazoles, catalyzed by pyridinium chloroaluminate IL
supported on a Merrifield resin.
EXPERIMENTAL
Chemicals were either laboratory prepared or purchased from Merck or Fluka. The IR
spectra were run on a Shimadzu model 8300 FT-IR spectrophotometer. The 1H-NMR spectra
were recorded on a Bruker DPX-300 spectrometer using DMSO-d6 as solvent and TMS as the
internal standard. Melting points were determined on a Fisher–Jones melting-point apparatus
and are uncorrected. The capacity of the catalyst was determined by the atomic absorption
technique using a Philips atomic absorption instrument. Monitoring of the reaction progress
and purity determination of the products were accomplished by GLC or TLC on silica-gel
polygram SILG/UV254 plates.
Preparation of poly[styrene-co-1-((4-vinylphenyl)methyl)pyridinium chloroaluminate]
(PS–PyCl–xAlCl3)
In a round bottomed flask (50 mL) equipped with a reflux condenser, a solution of
Merrifield resin (1 g, 2 % divinylbenzene) in pyridine (25 mL) was refluxed for 48 h. After-
wards, the mixture was filtered, washed with distilled water (20 mL) and dried at 80 °C over-
night. Then, 1 g of the obtained resin was added to a solution of AlCl3 (0.5 g) in toluene (10
mL) and stirred under reflux condition for 24 h under an N2 atmosphere. Next, AlCl3 (0.4 g)
was added again and the mixture was stirred for 24 h and filtered and then the excess of AlCl3
was removed by extraction with ethanol using a Soxhlet apparatus.
Typical procedure for benzimidazole synthesis
To a solution of aldehyde (1 mmol) and o-phenylenediamine (1 mmol) in ethanol (5 mL)
was added PS–PyCl–xAlCl3 (0.1 mmol), and the resulting mixture was magnetically stirred at
room temperature. The progress of the reaction was monitored by TLC. After completion of
the reaction, the catalyst was filtered off and washed with EtOH (2×5 mL) and the filtrate was
concentrated on a rotary evaporator under reduced pressure to afford the crude product.
Whenever required, the products were purified by column chromatography on silica gel
(n-hexane/EtOAc) or recrystallization from ethanol. The spent catalyst from different experi-
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158
BOROUJENI, ZHIANINASAB and JAFARINASAB
ments was washed with EtOH and reused without further drying. The analytical and spectral
data of the obtained compounds are given in the Supplementary material to this paper.
RESULTS AND DISCUSSION
Poly[styrene-co-1-((4-vinylphenyl)methyl)pyridinium chloroaluminate], de-
noted as PS–PyCl–xAlCl , was prepared by the procedure shown in Scheme 2. In
3
the first stage, commercially available Merrifield resin (2 % divinylbenzene, 2.1
mmol Cl per gram) was reacted with pyridine to give poly[styrene-co-1-((4-vi-
nylphenyl)methyl)pyridinium chloride], denoted as PS–PyCl. The obtained mate-
rial was a cream solid which was analyzed by elemental analysis to quantify the
percentage loading of the pyridinium moiety by measuring the nitrogen content,
giving 1.89 mmol Py per gram. The PS–PyCl was further treated with excess
amounts of AlCl in refluxing toluene to form PS–PyCl–xAlCl . The resulting
3
3
pale brown solid was reasonably stable to air and moisture and could be kept as a
bench-top catalyst for more than 1 year without appreciable change in its effi-
ciency. It is believed that the hydrophobic nature of the polystyrene protects the
water-sensitive Lewis acid from hydrolysis by atmospheric moisture until it is
suspended in an appropriate solvent where it can be used in a chemical reaction.
The atomic absorption technique gave 3.3 mmol Al per g of the catalyst. Con-
sidering the aluminium and pyridine contents of PS–PyCl–xAlCl , it is clear that
3
the AlCl mole fraction (AlCl /AlCl +Py) in this catalyst was larger than 0.5 and
3
3
3
–
thus it could be imagined that it is the Lewis acid Al Cl (as the predominant
2
7
aluminium species) that plays an important role in the catalytic activity in PS–
2,12
–PyCl–xAlCl .
3
Merrifield resin
(2 % DVB)
PS–PyCl
PS–PyCl–xAlCl3
Scheme 2. Preparation procedure to poly[styrene-co-1-((4-vinylphenyl)methyl)pyridinium
chloroaluminate] (PS–PyCl–xAlCl3).
The successful immobilization of AlCl on the polymer was confirmed by
3
FT-IR spectroscopy. The FT-IR spectra of the Merrifield resin, PS–PyCl and PS–
–PyCl–xAlCl are shown in Fig. 1. The IR spectra of the PS–PyCl showed absorp-
3
–1 43
tion peaks due to the pyridine ring at 3020, 2900, 1640, 1500, 1475, 775 cm .
As can be seen in the spectrum of PS–PyCl–xAlCl , when AlCl was complexed
3
3
–1
with PS–PyCl, new peaks appeared at 575, 500, 430, 385 cm , which can be as-
–
43,44
signed to Al–Cl stretching modes of Al Cl .
2
7
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BENZIMIDAZOLE SYNTHESIS
159
Fig. 1. FT-IR spectra of A) Merrifield resin, B) PS–PyCl and C) PS–PyCl–xAlCl3.
The catalytic activity of PS–PyCl–xAlCl was studied in the synthesis of
3
benzimidazoles by the condensation reaction of different aldehydes (aromatic
and aliphatic) with o-phenylenediamines (Scheme 1). Initially, to optimize the
reaction conditions, the conversion of benzaldehyde (1 mmol) to 2-phenyl-1H-
-benzimidazole with o-phenylenediamine (1 mmol) and PS–PyCl–xAlCl in the
3
presence of a variety of solvents at room temperature was attempted. It was ob-
served that this reaction goes well in ethanol among the commonly used organic
solvents, such as acetonitrile, methanol, 1,2-dichloroethane, dichloromethane and
tetrahydrofuran. Afterwards, to optimize the quantity of PS–PyCl–xAlCl , 0.05–
3
–0.15 mmol of the catalyst was used in the above reaction. It was found that a
0.1:1 mmol ratio of catalyst:benzaldehyde was sufficient to obtain 2-phenyl-1H-
-benzimidazole in 94 % yield within 6 min. Then, under the optimal conditions, a
wide variety of aromatic aldehydes (containing both electron withdrawing and
donating groups) were treated with o-phenylenediamines, and the corresponding
2-substituted benzimidazoles (Scheme 1, I) were obtained exclusively in excel-
lent yields (Table I, entry 1). 2-Naphthaldehyde was also converted to the corres-
ponding benzimidazole in 93 % yield (entry 2). Aliphatic aldehydes required
slightly longer times to produce the corresponding benzimidazoles in moderate
yields (entries 3–6). It was pleasing to observe that even acid-sensitive aldehydes
such as cinnamaldehyde, 2-pyridyl, 2-furyl and 2-thienyl carbaldehyde gave the
corresponding benzimidazoles without the formation of any side products, which
are normally encountered under acidic conditions (entries 7–10). It is noteworthy
that only small activities were observed when PS–PyCl or basic or neutral PS–
45,46
–PyCl–xAlCl were used as the catalyst.
To assess the feasibility of applying
3
this method on the preparative scale, the condensation of benzaldehyde with o-phe-
nylenediamine in a 30-mmol scale in ethanol at room temperature in the presence
of PS–PyCl–xAlCl was performed. The reaction proceeded smoothly and 2-phe-
3
nyl-1H-benzimidazole was obtained in 90 % yield after 15 min.
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BOROUJENI, ZHIANINASAB and JAFARINASAB
TABLE I. PS–PyCl–xAlCl3 catalyzed synthesis of 2-substituted benzimidazoles in the reac-
tions of aldehydes with o-phenylenediamines; ratio of aldehyde:o-phenylenediamine was 1:1.
Unless otherwise noted, all reactions were performed in the presence of 0.1 mmol of PS–
–PyCl–xAlCl3
Entry
1
Aldehyde
R2
–
Time, min
–
Yielda, %
–
X
CHO
1a
1b
1c
1d
1e
1f
X = H
X = H
X = 4-CH3
X = 4-CH3
X = 4-OCH3
X = 4-Cl
H
Me
H
Me
H
6(7)b
10
7
11
6(6)b
9
94(93)b
90
93
91
94(92)b
95
H
1g
1h
1i
1j
1k
1l
1m
1n
1o
2
X = 4-Cl
X = 2-Cl
X = 4-Br
X = 3-NO2
X = 4-NO2
X = 2-OH
X = 4-OH
X = 3-OH
X = 4-CN
Me
H
H
H
H
H
H
H
H
15
10
10
11
13
14
16
16
14
16
90
93
94
92
91
91
93
91
92
93
H
CHO
3
4
5
CH2CHO
CH2CHO
CHO
H
Me
H
21(20)b
30
89(88)b
83
21
89
6
7
CHO
CHO
H
H
30
11(10)b
82
93(90)b
8
H
H
H
11
11
11
94
92
92
CHO
CHO
CHO
N
O
S
9
10
aIsolated yields. All products are known compounds and were identified by comparison of their physical and
spectral data with those of the authentic samples;32,35,36,39,40 bvalue in parenthesis when the reaction was per-
formed in the presence of 1-butylpyridinium chloroaluminate ionic liquid (1-BuPyCl–xAlCl3; x is the mole fr-
action of AlCl3 which is equal to 0.7)46 at 40 °C. Molar ratio of aldehyde:o-phenylenediamine:1-BuPyCl–
–xAlCl3 was 1:1:0.15
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BENZIMIDAZOLE SYNTHESIS
161
In order to confirm the true heterogeneity of the catalytic system (i.e., the
absence of leaching of the AlCl into the reaction mixture), PS–PyCl–xAlCl was
3
3
added to ethanol and the mixture was stirred at room temperature for 2 h. Then,
the catalyst was filtered off and the filtrate was analyzed for its aluminium con-
tent, which showed a negligible release of AlCl . The filtrate was found to be
3
inactive for the condensation reaction of o-phenylenediamines with aldehydes.
These observations indicate that PS–PyCl–xAlCl is stable under the reaction
3
conditions, and there was no leaching of acid moieties during the reactions.
PS–PyCl–xAlCl recovered after a reaction can be washed with ethanol and
3
used again at least five times without any noticeable loss of catalytic activity
(Table II).
TABLE II. Recovery of PS–PyCl–xAlCl3
Use
1
2
3
4
5
Isolated yield, %
94
93
91
91
90
A comparison of the present procedure using PS–PyCl–xAlCl with selected
3
previously known protocols is collected in Table III. As can be seen, in addition
to having the general advantages attributed to solid supported catalysts, PS–
–PyCl–xAlCl has a good efficiency compared to other recently reported cata-
3
lysts in the synthesis of benzimidazoles by the condensation of aldehydes with
o-phenylenediamines.
TABLE III. Comparison of the efficiencies of a number of different reported catalysts with
that of PS–PyCl–xAlCl3 for the synthesis of 2-phenyl-1H-benzimidazole from the conden-
sation of benzaldehyde with o-phenylenediamine
Entry
1
2
3
4
5
6
7
8
Reaction conditions
Time, min
Yielda, %
Yb(OTf)3, solvent free, r.t.
(I2/KI/K2CO3), H2O, 90 °C
In(OTf)3, solvent free, r.t.
BF3⋅OEt2, solvent free, r.t.
(H2O2/HCl), CH3CN, r.t.
TsOH–SiO2, solvent free, 70 °C
NaY zeolite, EtOH, r.t.
30
30
30
30
35
15
48
90
10
55
9329
9030
9531
9032
9733
3234
5035
9536
8937
8938
T(o-Cl)PPFeIIIClb, EtOH, r.t.
9
10
DMPc, dioxane, r.t.
Scolecite, EtOH, 70 °C
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162
BOROUJENI, ZHIANINASAB and JAFARINASAB
TABLE III. Continued
Entry
11
12
13
14
15
Reaction conditions
AIKIT-5, CH3CN, reflux
(WOx/ZrO2), dioxane, 100 °C
FeCl3–SiO2, H2O2, 150 °C
Time, min
Yielda, %
240
300
30
4
9539
9240
9541
9242
94
Amberlite IR-120, solvent free, MW
PS-PyCl-XAlCl3, EtOH, r.t.
6
aIsolated yields; bporphyrinatoiron(III) complex supported on silica gel; cDess-Martin-periodinane
CONCLUSIONS
In conclusion, polymer-supported pyridinium chloroaluminate IL was syn-
thesized as a new heterogeneous Lewis acid catalyst that favourably combines
the properties of ILs and advantages of solid supports. This polymer catalyst has
an activity in the synthesis of 2-substituted benzimidazoles comparable to that of
1-butylpyridinium chloroaluminate IL as far as was tested but offers its own ad-
vantages originating from being supported on a polymeric matrix, enhanced sta-
bility (as a bench-top catalyst), easier handling, high chemoselectivity, simple
product workup, separation and isolation, as well as reuse of the catalyst at least
five times without significant change in its catalytic activity.
SUPPLEMENTARY MATERIAL
Physical and spectral data of the obtained compounds are available electronically from
http://www.shd.org.rs/JSCS/, or from the corresponding author on request.
Acknowledgement. We gratefully acknowledge the partial support of this study by the
Shahrekord University Research Council.
И З В О Д
ЈОНСКА ТЕЧНОСТ ПИРИДИНИЈУМ-ХЛОРOАЛУМИНАТ НА ПОЛИСТИРЕНУ КАО
ЛУИСОВА КИСЕЛИНА И НОВ ХЕТЕРОГЕНИ КАТАЛИЗАТОР ЗА СЕЛЕКТИВНУ
СИНТЕЗУ БЕНЗИМИДАЗОЛА
KAVEH PARVANAK BOROUJENI, ASHKAN ZHIANINASAB и MINA JAFARINASAB
Department of Chemistry, Shahrekord University, Shahrekord 115, Iran
Јонска течност пиридинијум-хлорoалуминат на полистирену добијена je од Мери-
филдове смоле и пиридина, након чега следи реакција са алуминијум-хлоридом. Овај
катализатор је коришћен за хемоселективну катализу као Луисова киселина за добијање
2-супституисаних бензимидазола као јединих производа из реакције алдехида са о-фе-
нилдиаминима. Катализатор је стабилан и може се лако изоловати и поново користити
без уочљивог губитка ефикасности.
(Примљено 1. априла, ревидирано 27. августа 2012)
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164
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