Catalysis Communications
Short Communication
Synthesis and application of a novel strong and stable supported ionic
liquid catalyst with both Lewis and Brønsted acid sites
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Kaveh Parvanak Boroujeni , Parvin Ghasemi
Department of Chemistry, Shahrekord University, Shahrekord (115), Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Poly(4-vinylpyridine)-supported ionic liquid with both Lewis and Brønsted acid sites was easily prepared
from its starting materials and used as a novel and highly efficient heterogeneous catalytic system for the
synthesis of biscoumarins by two-component one-pot domino Knoevenagel-type condensation/Michael re-
action between various aliphatic and aromatic aldehydes with 4-hydroxycoumarin. The Lewis and Brønsted
acidic sites loading in [P4VPy-BuSO3H]Cl-X(AlCl3) were found to be 2.15 and 0.9 mmol per gram of catalyst,
respectively. The effect of the simultaneous presence of Lewis and Brønsted acid sites was evaluated. The
catalyst was characterized by Fourier transform infrared spectroscopy (FT-IR), thermal gravimetric analysis
(TGA), scanning electron microscopy (SEM), elemental analysis, and atomic absorption technique. The
catalyst is stable (as a bench top catalyst) and reusable.
Received 6 February 2013
Received in revised form 21 March 2013
Accepted 21 March 2013
Available online 29 March 2013
Keywords:
Biscoumarins
Aldehydes
Ionic liquid
One-pot synthesis
Heterogeneous catalysis
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Coumarin and its derivatives are known to possess a wide range of
biological activities including anti-HIV, anti-biotic, anti-fungal,
Brønsted acidic ionic liquids are of special important because they
possess simultaneously the proton acidity and the characteristic
properties of ionic liquid [1]. Among them, SO3H-functionalized
ionic liquids are of particular value, because of their negligible vapor
pressure, outstanding acidity, and tunable solubility for some organic
species [2–4]. Thus, they are designed to replace traditional mineral
liquid acids such as sulfuric acid and hydrochloric acid in chemical
procedures.
Chloroaluminate molten salts are another type of ionic liquids
which have been extensively used as a catalyst or solvent in various
organic transformations [1,5]. These ionic liquids are prepared by
simple addition of AlCl3 to the corresponding 1,3-dialkyl-imidazolium
or 1-alkyl-pyridinium chlorides. The acidity of the resulting ionic
liquid can be controlled by varying the relative amounts of AlCl3
and organic chloride. Chloroaluminate melts are designated as basic
when the AlCl3 mole fraction is smaller than 0.5 and the melts contain
the anions Cl− and AlCl4−, a Lewis base. A neutral melt is referred to at
an AlCl3 mole fraction of exactly 0.5, where AlCl4− is the only anion
present. Finally, an acidic chloroaluminate melt is one in which the
AlCl3 mole fraction is larger than 0.5. In such acidic melts, the anions
Al2Cl7− and Al3Cl1−0 exist, which act as very strong Lewis acids [5,6].
Despite unique physical and chemical properties of chloroaluminate
ionic liquids and their widespread application in organic synthesis,
they are extremely hygroscopic and labile towards hydrolysis.
anti-bacterial, anti-oxidant, anti-cancer, and anti-coagulant [7–10].
Among various derivatives of coumarin, biscoumarins have aroused
considerable interest. They are usually prepared from the reaction
of aldehydes with two equivalents of 4-hydroxycoumarin in the
presence of a catalyst. However, only a few reports were found de-
scribing the synthesis of these compounds. Piperidine [11], tetra-
butylammonium bromide [12], I2 [13], sodium dodecyl sulfate [14],
ionic liquids [15,16], p-dodecylbenzenesulfonic acid/piperidine [17],
NaHSO4 [18], and B(HSO4)3 [19] have been reported as catalysts for
the preparation of biscoumarins so far. Although these methods are
suitable for certain synthetic conditions, there exist some drawbacks
such as low yields, long reaction times, drastic reaction conditions,
require tedious work-up leading to the generation of large amount
of toxic waste, inefficiency of method when aliphatic aldehydes are
used in the reaction, and the use of unrecyclable, hazardous or diffi-
cult to handle catalysts.
It is well known that, both Lewis and Brønsted acid sites were im-
portant for acid-catalyzed reactions and the solid supports or ionic
liquids functionalized with both Lewis and Brønsted acid sites
would become more efficient, which combined the advantages of
Lewis and Brønsted acid sites [1,20,21]. More importantly, the acidity
of these compounds can be controlled by varying relative amounts of
the Lewis and Brønsted acidic component. In this pursuit, and in a
continuation of our work on the development of efficient and envi-
ronmentally benign procedures using poly(vinylpyridine)-supported
reagents and catalysts [22], herein, we now report the synthesis of
poly(4-vinylpyridine-co-1-sulfonic acid butyl-4-vinylpyridinium)
chloroaluminate ([P4VPy-BuSO3H]Cl-X(AlCl3)) as a supported ionic
⁎
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1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.