Polystyrene-supported pyridinium chloroaluminate ionic liquid as a new heterogeneous Lewis acid catalyst for Knoevenagel condensation

Article abstract of DOI:10.3184/174751912X13377118207526

Non-hygroscopic polystyrene-supported chloroaluminate ionic liquid was prepared from the reaction of Merrifield resin with pyridine followed by reaction with aluminium chloride. This Lewis acidic ionic liquid is an environmentally friendly heterogeneous c

Full text of DOI:10.3184/174751912X13377118207526

JOURNAL OF CHEMICAL RESEARCH 2012
JULY, 429–431
RESEARCH PAPER 429
Polystyrene-supported pyridinium chloroaluminate ionic liquid as a new
heterogeneous Lewis acid catalyst for Knoevenagel condensation
Kaveh Parvanak Boroujeni* and Mina Jafarinasab
Department of Chemistry, Faculty of Sciences, Shahrekord University, PO Box 115, Shahrekord, Iran
Non-hygroscopic polystyrene-supported chloroaluminate ionic liquid was prepared from the reaction of Merrifield
resin with pyridine followed by reaction with aluminium chloride. This Lewis acidic ionic liquid is an environmentally
friendly heterogeneous catalyst for the Knoevenagel condensation of aromatic and aliphatic aldehydes with ethyl
cyanoacetate. The catalyst is stable (as a bench top catalyst) and can be easily recovered and reused without
appreciable change in its efficiency.
Keywords: ionic liquids, pyridinium chloroaluminate salts, polymer-supported ionic liquids, Knoevenagel condensation
One of the primary groups of ionic liquids is based on a
mixture of aluminium chloride and 1,3-dialkyl-imidazolium or
moisture-sensitive, expensive, hazardous, difficult to handle or
unreusable catalysts. It should also be mentioned that compar-
ing the broad application of basic catalysts, considerably less
attention has been paid to the use of acid catalysts, especially
heterogeneous Lewis acid catalysts, for the Knoevenagel
condensation.
In continuation of our research on the development of
synthetic methodologies using solid acid catalysts, we now
wish to introduce polystyrene-supported pyridinium chloro-
1
,2
1
-alkyl-pyridinium chlorides. The acidity of the resulting
ionic liquid can be controlled by varying the relative amounts
of AlCl and organic chloride. Chloroaluminate melts are
3
designated as basic when the AlCl mole fraction is smaller
3
−
−
than 0.5 and the melts contain the anions Cl and AlCl ,
4
a Lewis base. A neutral melt is referred to at an AlCl mole
3
−
fraction of exactly 0.5, where AlCl is the only anion present.
4
Finally, an acidic chloroaluminate melt is one in which the
aluminate ionic liquid (PS-PyCl-XAlCl ) as an effective
heterogeneous catalyst for the Knoevenagel condensation of
aldehydes with ethyl cyanoacetate (Scheme 1).
3
AlCl mole fraction is larger than 0.5. In such acidic melts, the
3
−
−
anions Al Cl and Al Cl exist, which act as very strong
Lewis acids.
2
7
1
3
10
Poly[styrene-co-(1-((4-vinylphenyl)methyl)pyridinium)
Despite having widespread application in organic synthesis
either as solvents or acidic catalysts, most chloroaluminate
ionic liquids suffer from one or more of the following draw-
backs: laborious work-up procedures, difficulty of recovery
and recycling, disposal of spent catalyst, difficult to handle,
and corrosion problems. Most importantly, they are extremely
chloroaluminate] (PS-PyCl-XAlCl ) was prepared by the pro-
3
cedure shown in Scheme 2. In the first stage, commercially
available Merrifield resin (2% divinylbenzene, 2.1 mmol Cl
per gram) was reacted with pyridine to give the poly[styrene-
co-(1-((4-vinylphenyl)methyl)pyridinium) chloride] (PS-PyCl).
The PS-PyCl was analysed 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
1
hygroscopic and labile towards hydrolysis. Thus, these
shortcomings make them a prime target for heterogenisation.
Although there are several reports on the immobilisation of
PS-PyCl was further treated with excess amounts of AlCl in
3
3
−9
ionic liquids on solid supports, to the best of our knowledge,
very few examples are known for immobilised chloroalumi-
refluxing toluene to form PS-PyCl-XAlCl . The resulting pale
3
brown solid was reasonably stable to air and moisture and
could be kept as a bench top catalyst for more than 1 year with-
out appreciable change in its efficiency. We believe that the
hydrophobic nature of 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
4,6
nate ionic liquids.
The Knoevenagel condensation is one of the most useful
and widely employed methods for carbon–carbon bond forma-
tion in organic synthesis. It is effected by treating a carbonyl
compound with an active methylene compound in the presence
of a catalyst. This reaction produces several important key
products that include nitriles used in anionic polymerisation
and the α,β-unsaturated ester intermediates employed in the
synthesis of several therapeutic drugs and pharmacological
3.3 mmol Al per gram of PS-PyCl-XAlCl . Considering the
3
aluminium and pyridine contents of PS-PyCl-XAlCl , it is
3
clear that the AlCl mole fraction (AlCl /AlCl +Py) in this
3
3
3
10,11
products.
Several types of catalysts were introduced prev-
catalyst is larger than 0.5 and thus we can imagine that it is the
−
iously for Knoevenagel condensation such as the weak bases
ammonia, primary amine and secondary amines and their
Lewis acid Al Cl (as the predominant aluminium species)
2
7
that plays an important role in the catalytic activity in PS-
1
2,13
14
15
16
17
19
1,4
salts, CuCl , ZnCl , [Ni0.73Al0.27(OH) ](CO ) , LaCl ,
PyCl-XAlCl3.
2
2
2
3
0.135
3
1
8
non-cross-linked polystyrene-supported piperazine, zeolite,
amino group immobilised on polyacrylamide, SiO –HClO
and SiO –PPA, amine functionalised MCM-41, ionic
liquids,
functionalised mesoporous silica. However, many of the
reported catalysts suffer from drawbacks such as generation
of environmentally perilous waste material, tedious work-up,
long reaction times, complicated operations, and the use of
For comparison, the FT-IR spectra of the Merrifield resin,
2
0
PS-PyCl, and PS-PyCl-XAlCl are presented in Figure 1. The
2
22
4
3
2
1
IR spectra of PS-PyCl showed absorption peaks due to the
2
2
3−25
26
−1 28
silica based substituted piperidine, and amino
pyridine ring at 3020, 2900, 1640, 1500, 1475, 775 cm . As
2
7
can be seen in the spectrum of PS-PyCl-XAlCl , when AlCl3
3
was complexed with PS-PyCl new peaks appeared at 575, 500,
−1
430, 385 cm , which can be assigned to Al-Cl stretching
−
29
modes of the Al Cl anion.
2
7
Scheme 1
*
Correspondent. E-mail: parvanak-ka@sci.sku.ac.ir

4
30 JOURNAL OF CHEMICAL RESEARCH 2012
Scheme 2
3
Fig. 1 FT-IR spectra of (A) Merrifield resin, (B) PS-PyCl, and (C) PS-PyCl-XAlCl .
In order to explore the catalytic activity of PS-PyCl-XAlCl3,
we studied the Knoevenagel reaction of aromatic and aliphatic
aldehydes with ethyl cyanoacetate (Table 1). The best results
in terms of yield as well as reaction time were obtained at room
temperature in ethanol which proved to be the solvent of choice
among other organic solvents. The optimum molar ratio of
electron withdrawing groups, cleanly and rapidly reacted with
ethyl cyanoacetate to give the corresponding Knoevenagel
products (entry 1a–h). Aliphatic aldehydes needed slightly
more time to produce the corresponding products in good
yields (entries 2–4). Also, 2-pyridyl aldehyde and 2-furalde-
hyde gave the desired products (entries 5 and 6). The reaction
failed to proceed with ketones due to hindered and non
favourable electronic structure (entry 7).
PS-PyCl-XAlCl to aldehyde was found to be 0.1:1. Variously
3
substituted aromatic aldehydes, with electron donating or
To find out whether the reaction takes place in the solid
matrix of PS-PyCl-XAlCl or whetherAlCl simply released in
ethanol is responsible for the Knoevenagel reaction, PS-PyCl-
Table 1 Knoevenagel condensation of aldehydes with ethyl
3
3
a
cyanoacetate in the presences of PS-PyCl-XAlCl
3
XAlCl was added to ethanol and the mixture was stirred at
room temperature for 2 h. When, the catalyst was filtered off
and the filtrate was analysed for its aluminium content, it
Entry Aldehyde
Time Yield M.p./°C or B.p./°C
3
b
/h
/%
/mmHg
Found ReportedRef.
showed negligible release of AlCl . Moreover, the filtrate was
3
found to be inactive for the Knoevenagel reaction of aldehydes
with ethyl cyanoacetate. These observations indicate that
1
a. X= H
0.7
1
94
93
92
93
94
96
97
93
90
90
51
93
49–5030
PS-PyCl-XAlCl is stable under the reaction conditions, and
3
3
0
b. X= 4-CH
3
91.5–92.5
there is no leaching of acid moieties during the reactions.
To explore the selectivity of the present method, equimolar
mixtures of aromatic and aliphatic aldehydes were allowed
to react with ethyl cyanoacetate in the presence of PS-PyCl-
3
3
2
2
c. X= 4-OCH
d. X= 4-Cl
e. X= 2-Cl
3
1.1
0.6
0.6
0.5
0.5
1
1.3
1.3
1.4
1
79
79
90
91
30
50
49–50
0
f. X= 2-NO
2
97
90–1003
171–1723
0
0
g. X= 4-NO
h. X= 4-OH
2
171
174
115
92
XAlCl . As shown in Scheme 3, the catalyst was able to
3
3
172–173
discriminate between aromatic and aliphatic aldehydes.
It is noteworthy that Salunkhe and co-workers used 1-butyl-
pyridinium chloroaluminate ionic liquid as a catalyst for
Knoevenagel condensation, which was moisture sensitive
3
2
2
3
4
5
6
7
Ph-CH=CHCHO
116
87–9031
2
PhCH CHO
3
1
n-PrCHO
90 71–73/3 70–71/3
33
2-pyridyl aldehyde
2-furaldehyde
94
93
5
97
91
–
95–96
32
0.9
2
93
23
and readily hydrolysed. They found that two reactions,
PhCOCH
3
–
Knoevenagel and Michael, competing to give different ratios
of two products, which led to undesirable by-products. It was
pleasing to observe that, we did not observe any side reactions
such as Michael addition or hydrolysis of the ester moiety, due
a
Ratio of aldehyde:ethyl cyanoacetate is 1:1. All reactions were
carried out in EtOH at room temperature in the presence of
0
3
.1 mmol of PS-PyCl-XAlCl .
b
Isolated yields. All products are known compounds and were
identified by comparison of their physical and spectral data
with those of the authentic samples.
to the mild catalytic activity of PS-PyCl-XAlCl and the mild
3
reaction conditions.

JOURNAL OF CHEMICAL RESEARCH 2012 431
Scheme 3
Scheme 4
One notable achievement of PS-PyCl-XAlCl is a one-pot
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). The spent catalyst from different experi-
ments was washed with EtOH and used again without further drying.
3
(
synthesis of biscoumarinyl methane derivatives by a two-
component one-pot domino Knoevenagel-type condensation/
Michael reaction between 4-hydroxycoumarin and aromatic
aldehydes. For example, 4-hydroxycoumarin and benzalde-
hyde were reacted in refluxing toluene in the presence of
PS-PyCl-XAlCl to give the desired 3,3΄-phenylmethylenebis-
3
We gratefully acknowledge the partial support of this study by
the Shahrekord University Research Council.
(
4-hydroxycoumarin) in 90% yield (Scheme 4).
PS-PyCl-XAlCl recovered after a reaction can be washed
3
with ethanol and used again at least five times without any
noticeable loss of catalytic activity (Table 2).
Received 13 April 2012; accepted 19 April 2012
Paper 1201259 doi: 10.3184/174751912X13377118207526
Published online: 26 June 2012
In conclusion, we have synthesised a polymer-supported
pyridinium chloroaluminate ionic liquid as a new heteroge-
neous Lewis acid catalyst that favorably combines the proper-
ties of an ionic liquid and the advantages of a solid support.
This polymer catalyst has an activity in Knoevenagel reactions
comparable to that of 1-butylpyridinium chloroaluminate ionic
liquid as far as we tested but offers its own advantages origi-
nating from being supported on a polymeric matrix, enhanced
stability (as a bench top catalyst), reusability, high chemose-
lectivity, easier handling, and the simplification of product
work-up, separation, and isolation.
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