Ion-pair immobilization of l-prolinate anion onto cationic polymer support and a study of its catalytic activity as an efficient heterogeneous catalyst for the synthesis of 2-amino-4H-chromene derivatives

Article abstract of DOI:10.1007/s11164-015-2302-0

A novel clean and simple technique for the heterogenization of l-proline organocatalyst has been introduced. This procedure is based on non-covalent immobilization of l-proline on the surface of an anion-exchange resin amberlite IRA900OH (AmbIRA900OH) as

Full text of DOI:10.1007/s11164-015-2302-0

Res Chem Intermed
DOI 10.1007/s11164-015-2302-0
Ion-pair immobilization of L-prolinate anion
onto cationic polymer support and a study of its
catalytic activity as an efficient heterogeneous catalyst
for the synthesis of 2-amino-4H-chromene derivatives
Mosadegh Keshavarz1 Nasir Iravani2
•
•
Mohammad Hossein Ahmadi Azqhandi1
Simin Nazari3
•
Received: 13 June 2015 / Accepted: 28 September 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract A novel clean and simple technique for the heterogenization of L-proline
organocatalyst has been introduced. This procedure is based on non-covalent
immobilization of L-proline on the surface of an anion-exchange resin amberlite
IRA900OH (AmbIRA900OH) as an efficient, cheap, and commercially accessible
cationic polymer support. The ion-pair immobilization of L-proline on the surface of
AmbIRA900OH was achieved by the treatment of a MeOH/H2O solution of
L-proline with AmbIRA900OH at 60 °C. L-Proline anions were exchanged with
hydroxyl anions and immobilized via ionic interaction between a carboxylate group
of L-prolinate and quaternary ammonium cations of the cationic amberlite support.
The prepared heterogeneous organocatalyst was well characterized by using FTIR,
TGA, DTG, XRD, and elemental analysis techniques. The prepared heterogeneous
organocatalyst was used as a catalyst for the one-pot, three-component synthesis of
2-amino-4H-chromene derivatives in EtOH at 80 °C. The reactions were efficiently
performed in the presence of the title catalyst with short reaction times and gave the
desired products at high yields. This catalyst can be straightforwardly recovered by
simple filtration and recycled up to eight consecutive runs. Very low leaching of
then organocatalyst into the reaction mixture occurred.
Electronic supplementary material The online version of this article (doi:10.1007/s11164-015-2302-
0) contains supplementary material, which is available to authorized users.
&
Mosadegh Keshavarz
chem.mosadegh@gmail.com
1
2
Department of Gas and Petroleum, Yasouj University, Gachsaran 75918-74831, Iran
Department of Chemistry, Faculty of Science, Gachsaran Branch, Islamic Azad University,
Gachsaran, Iran
3
Department of Chemistry, Sousangerd Branch, Islamic Azad University, Sousangerd, Iran
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M. Keshavarz et al.
Keywords L-Proline Á Anion-exchange resin Á Organocatalyst Á 2-Amino-4H-
chromene Á Heterogeneous catalyst
Introduction
During the past decade, there has been a revolution of organocatalysis in the field of
organic chemistry. Organocatalysts are readily available, much easier to handle than
other catalysts, and free of metal contamination, which is problematic in
pharmaceutical processes. Subsequently, organocatalytic reactions are commonly
considered to be environmentally benign and sustainable processes. Despite these
attractive features, organocatalysts have been rarely applied in industry [1]. This
unpleasant fact can be principally attributed to the unavoidable drawbacks of
homogeneous catalytic processes such as unsatisfactory efficiency, low thermal
stability, and the difficulties in catalyst separation and recycling [2].
One viable approach to address these challenges would be the heterogenization of
organocatalysts. For this reason, the immobilization of organocatalysts has been achieved
through covalent attachment onto supports such as polymers [3–6], silica [7–9], ionic
liquid (IL) [10, 11], ß-cyclodextrin [12], Merrifield resin [13], and magnetite [14].
Although the covalent bond gives robustness to the system and allows a high number of
recycling [15–17], this creativity may induce a partial loss of efficiency due to a lower
mobility of the catalyst that reduces the activity and selectivity compared with their small
molecular parent catalysts. Consequently, higher catalyst loading (both w/w% and mol%)
is required to reach favorable yields [18]. Moreover, laborious synthetic operations are
necessary to reach a covalent immobilization, and substantial structural perturbations to
the parent catalyst skeletons are generally not avoidable in such circumstances.
Noncovalent immobilization strategies recently appeared as creative alternative
solutions with their powers being well-demonstrated in supported organocatalysis.
Several noncovalent immobilization strategies have been used to achieve recoverable
organocatalysts. For example, immobilization via hydrophobic interaction [12, 19],
biphasic immobilization [20, 21], self-supported gel-types, and ion-pair immobilization
[22, 23] have been reported. Among these methods, ion-pair immobilization is a
conceivable way to immobilize this type of catalysts via ion-exchange with cationic or
anionic supports. By this method, anionic organocatalysts can be combined with an
organic cation or cationic supports, and cationic organocatalysts can also be hybridized
with an organic anion or anionic supports leading to noncovalently supported catalysts.
For instance, Itsuno and co-workers reported a novel type of supported chiral
quaternary ammonium comprising an ionic bond between the ammonium moieties
and the polymer anchored sulfonate anions [24, 25]. In 2008, Kaneda and co-workers
prepared the montmorillonite immobilized Macmillan catalyst via cation-exchange
strategy [26]. Layered double hydroxide (LDH) appears as inorganic solid cationic
support. LDH consists of stacks of positively charged metal hydroxide layers and
interlayer anions. Both inorganic and organic anions could be entrapped into the
interlayer. Choudary et al. [27] reported the synthesis of LDH entrapped proline via
anion-exchange method. Imidazolium and its supported forms have been used as
cationic supports for prolinate anion and other anionic organocatalysts [28–32].
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Ion-pair immobilization of L-prolinate anion onto cationic…
The 4H-chromene derivatives have various pharmacological properties such as
anti-coagulant, anti-cancer, anti-HIV, anti-Alzheimer, antitumor, diuretic, anti-
malarial activities, anti-leukemic, antibacterial, emetic, anti-anaphylactic activities,
and spasmolytic [33–38].
Because of these important properties, there has been extensive focus on the
improvement of environmentally friendly methodologies to synthesize 2-amino-4H-
chromene scaffold. The main strategy for the synthesis of chromene derivatives
include the cyclization of an aromatic/aliphatic aldehyde, malononitrile (or ethyl
cyanoacetate), and diverse enolizable C–H activated acidic compounds [39, 40].
Several modified methods have been reported for the synthesis of these derivatives
using different homogeneous or heterogeneous catalysts such as Zn(L-proline)2 [41],
L-proline [42], K2CO3 [43], Na2CO3 under grinding [44], crown ether complex
cation ILs [45], silica-bonded S-sulfonic acid [46] polystyrene-supported basic
dicationic IL [47], [Et3NH][HSO4] [48], CAN [49], sucrose [50], TBAB [51],
4-nitro-2,6-diacetylpyridinebis(2,4,6-trimethylaniline)FeCl2 [52], [BMIm]BF4 [53],
[2-aemim][PF6] [54], DBU [55], and POPINO [56]. However, many proposed
methods for the synthesis of these compounds suffer from disadvantages including
relying on multi-step conditions, the use of toxic organic solvents or catalysts
containing transition metals, tedious work-up procedure, troublesome waste
discarding, high reaction time, and low yields.
In continuation of our research devoted to the applications of ion-exchange resins
for click synthesis of 1,4-disubstituted-1H-1,2,3-triazoles [57, 58], herein we wish
to report ion-exchange resin amberlite IRA900OH (AmbIRA900OH) as an efficient,
cheap, and commercially accessible cationic polymer support for the ion-pair
immobilization of L-proline anion via ionic interaction between a carboxylate group
of L-prolinate and quaternary ammonium cation of the cationic Amb support. This
heterogeneous catalyst was used as an efficient recoverable catalyst for the one-pot,
three-component synthesis of 2-amino-4H-chromene derivatives in ethanol.
Experimental
General
1H NMR spectra of samples were recorded at a Bruker Advanced DPX 400-MHz
spectrometer. X-ray diffraction (XRD) patterns were recorded on a Philips
˚
X’PERT-Pro-MPD diffractometer using Cu Ka radiation (k = 1.542 A). A
continuous scan mode was used to collect 2 h from 5 to 40. Fourier transform
infrared (FT-IR) spectra were obtained as potassium bromide pellets in the range
400–4000 cm-1 using an AVATAR 370 Thermo Nicolet spectrophotometer.
Elemental analyses (C, H, and N) were performed with a Heraeus CHN-O-Rapid
analyzer. The thermogravimetric and differential thermogravimetric (TG–DTG)
analysis was performed on Netsch STA449c. The sample weight was ca. 10 mg and
was heated from room temperature up to 600 °C at 10 °C/min using alumina sample
holders.
123

M. Keshavarz et al.
Preparation of the [Amb]L-prolinate catalyst
Amberlite IRA-900OH (mesh 16–50, 1 g) was suspended in 10 mL of a 1 M
solution of L-proline in MeOH:H2O (1:1). The system was heated at 60 °C for 6 h.
The filtration of the reaction mixture followed by washing with MeOH:H2O (1:1)
(2 9 10 mL) and H2O (2 9 10 mL) afforded [Amb]L-prolinate catalyst. The
prepared catalyst was collected and dried under vacuum.
General procedure for the [Amb]L-prolinate catalyzed multicomponent
synthesis of 2-amino-4H-chromene derivatives
ß-naphthol or enolizable compound (1 mmol), aldehyde (1 mmol), and malonon-
itrile (1 mmol), were placed together in a round-bottom flask containing 5 mL of
EtOH. [Amb]L-prolinate catalyst (0.08 g, 10 mol%) was added to the mixture. The
suspension was magnetically stirred at reflux for an appropriate time according to
(Table 2). After completion of the reaction, as followed by TLC (n-hexane:ethyl
acetate; 3:1), the catalyst was filtered and washed with hot ethanol (2 9 5 mL). The
recovered catalyst was washed with acetone, dried, and stored for other similar
consecutive runs. The filtrate mixture was recrystallized to provide the pure crystals
of 2-amino-4H-chromene derivatives. The products are known compounds and are
characterized by IR and NMR spectroscopy data for new compounds. Their melting
points are compared with reported values.
Results and discussions
Catalyst preparation
The procedure followed to obtain the ion-pair immobilization of L-prolinate anion
on the cationic polymer resin is outlined in Scheme 1. The strategy consists of
building up suitable heterogeneous macroporous polymer-supported L-prolinate
catalyst on the surface of commercially available amberlite IRA-900OH (mesh
16–50). Preparation of heterogeneous polymer-supported L-prolinate catalyst by this
procedure is facile and straightforward. In a typical, procedure AmbIRA900OH was
treated with a solution of 0.01 M L-proline at 60 °C to achieve [Amb]L-prolinate
hybrid.
O
0.01 M L-proline in
O
PS
OH
[Amb]OH
C
PS
N
N
MeOH-H2O (1:1)
°C, 6 h
60
HN
[Amb]L-Prolinate
Scheme 1 Preparation of [Amb]L-prolinate hybrid
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Ion-pair immobilization of L-prolinate anion onto cationic…
Characterization of the catalyst
IR spectra
The ion-pair immobilization of L-prolinate anion on the polymer resin can be
confirmed by characterizing the pure AmbIRA900OH, pristine L-proline, and
[Amb]L-prolinate hybrid using FT-IR spectroscopy, as shown in Fig. 1. The FT-IR
spectrum of pristine L-proline shows characteristic stretching frequencies including:
N–H asymmetric stretching at 3056 cm-1 and carboxylate (COO-) asymmetric and
symmetric stretching at 1622 and 1380 cm-1, respectively (Fig. 1a). These bands
are observed as new peaks in the FT-IR spectrum of [Amb]L-prolinate hybrid when
compared with the FT-IR spectrum of pure AmbIRA900OH (Fig. 1b vs. a). The
carboxylate (COO-) asymmetric and symmetric stretching are presented in [Amb]L-
prolinate and found to shift to lower positions at 1615 and 1375 cm-1, respectively
(Fig. 1b). In addition, the band at 3056 cm-1 corresponding to the asymmetric
stretching vibration of the N–H group in L-proline is also found at 3056 cm-1 in FT-
IR spectrum of [Amb]L-prolinate. All the results from the comparison of FT-IR
spectra encourage us to anticipate that the L-prolinate anion successfully loaded onto
the polymer surface through ionic interaction using ion-pair binding between the
carboxylate group of L-prolinate and quaternary ammonium cation of the cationic
Amb support.
TGA and DTG analysis
Thermogravimetric analysis (TGA) and differential thermal analysis (DTG)
associated with the decomposition profiles of the AmbIRA900OH, pristine
L-proline, and [Amb]L-prolinate hybrid under a nitrogen atmosphere provide further
evidence for the immobilization of L-prolinate anion onto the polymer surface
(Figs. 2, 3). The 100 % weight loss of pristine L-proline appears at 215–250 °C on
Fig. 1 FT-IR spectrum of [Amb]OH (a), [Amb]L-prolinate (b), and pristine L-proline (c)
123

M. Keshavarz et al.
Fig. 2 TGA curve of [Amb]L-prolinate (a), [Amb]OH (b), and pristine L-proline (c)
Fig. 3 DTA curve of [Amb]L-prolinate (a), [Amb]OH (b), and pristine L-proline (c)
the base of its TGA and DTG curves and is assigned to the successive cleavage of
the L-proline at this interval (Figs. 2c, 3c). The TGA curve of pure AmbIRA900OH
shows three weight loss step intervals at 65–100, 150–250, and 370–470 °C. The
first weight loss interval at 65–100 °C is most probably due to a loss of adsorbed
water (weight loss = ca. 11 wt%). The second weight loss interval at 150–250 °C is
presumably assigned to the loss of some functional groups (weight loss = ca.
123

Ion-pair immobilization of L-prolinate anion onto cationic…
19 wt%), and finally the third weight loss interval at 370–470 °C (weight loss = ca.
34 wt%) is presumably assigned to partial polymer decomposition (Fig. 2b).
Figure 3b displays the DTG curve of AmbIRA900OH and is in accordance with the
weight loss steps from its TGA curve. The TGA curve of [Amb]L-prolinate hybrid
displays four weight loss steps including: 60–100, 150–250, 250–370, and
370–470 °C intervals (Fig. 2a). These four weight loss peaks are well distinguished
in the corresponding DTG curve (Fig. 3a). Obviously, in comparison with
AmbIRA900OH, a new decomposition interval is observed in TGA and DTG
curves of [Amb]L-prolinate hybrid (weight loss = ca. 15 wt%). This weight loss is
assigned to the successive cleavage of L-prolinate anion loaded on the surface of the
polymer and also refers to the content of L-prolinate moiety on the Amb cationic
support. The calculation from the TG curve indicated that 1.3 mmol of the
L-prolinate organocatalyst is loaded per 1 g of the [Amb]L-prolinate hybrid. It is
noticeable that the decomposition temperature of L-prolinate anion in [Amb]L-
prolinate hybrid has been increased to 250–370 °C in comparison with the
decomposition temperature of pristine L-proline (215–250 °C). These observations
mean that the thermal stability of the L-prolinate has been increased in comparison
with the pristine L-proline and also explains the carboxylate asymmetric and
symmetric stretching shifts to lower positions in the FT-IR spectrum of [Amb]L-
prolinate hybrid (Fig. 1b). The increased decomposition temperature of the
L-prolinate suggests that the guest/host interaction was done through the ion-pair
exchanges between hydroxyl and L-prolinate anions on the surface of ion-exchange
resin and is an indirect proof for the presence of ion-pair interaction between
L-prolinate anions and quaternary ammonium cations on the surface of cationic
support (Fig. 3a vs. c). The high loading of L-prolinate on the surface of Amb
(15 wt%), together with the unique ion-pair binding behaviors between L-prolinate
and Amb cation, makes the [Amb]L-prolinate hybrid efficient and stable in the
reaction system.
Fig. 4 XRD pattern of pristine L-proline (a) and [Amb]L-prolinate (b)
123

M. Keshavarz et al.
XRD
The crystalline nature of [Amb]L-prolinate hybrid confirms that L-prolinate is non-
covalently supported on the cationic polymer support via ion-pair immobilization. The
main intense diffraction peaks of pristine L-proline based on the standard spectrum
(Fig. 4a) are observed in the XRD pattern of [Amb]L-prolinate hybrid due to the
presence of the L-prolinate on the Amb support thanks to a favorable ion-pair binding
with quaternary ammonium cations of the ion-exchange resin (Fig. 4b vs. a). This
technique gives robustness to the catalytic system and, on the other hand, allows the L-
prolinate organocatalyst to be flexible, mobile, and free on the surface of the polymer at
the same time. Moreover, the thermal stability of the organocatalyst has been improved
by this way. These mentioned advantages are characteristic properties of homogeneous
and heterogeneous catalysts that have been included in [Amb]L-prolinate hybrid.
The one-pot multicomponent synthesis of 2-amino-4H-chromene using
[Amb]L-prolinate catalyst
After preparation and characterization of [Amb]L-prolinate catalyst, its catalytic
activity was investigated in a multicomponent reaction for the synthesis of 2-amino-
4H-chromene derivatives. To optimize the reaction conditions, the reaction of
4-hydroxychomarine, benzaldehyde, and malononitrile was selected as a model to
examine the effect of [Amb]L-prolinate catalyst (2–15 mol%) under a variety of
conditions (Table 1).
Table 1 Investigation of catalytic activity of [Amb]L-prolinate for the synthesis of 2-amino-4H-
chromenes under various conditions
NH2
OH
O
CN
Ph
O
O
O
[Amb]L-prolinate
+
CN
NC
+
Ph
H
Conditions
O
O
Entry
Conditions
Temperature (°C)
[Amb]L-prolinate
Time (min)
Yield (%)a
1
2
3
4
5
6
7
8
9
10
a
Neat
100
2
2
50
50
30
45
35
30
35
25
20
20
50
65
63
65
68
60
70
70
89b
90
CH2Cl2
CH3CN
THF
Reflux
Reflux
65
2
2
DMF
100
2
H2O/DMF
H2O
100
5
Reflux
Reflux
Reflux
Reflux
5
CH3CH2OH
CH3CH2OH
CH3CH2OH
5
10
15
Yield refers to isolated and pure product
b
Optimized reaction condition
123

Ion-pair immobilization of L-prolinate anion onto cationic…
The present optimization studies revealed that the best result was achieved by
carrying out the reaction in the presence of 10 mol% of [Amb]L-prolinate under
reflux in ethanol (Table 1, entry 9). The yield smoothly increased with the catalyst
load up to 10 mol% and use of larger amounts of the catalyst (15 mol%) did not
improve the yield while decreasing the amount of the catalyst led to decreased yield.
Using these optimized conditions, the reaction of enole or enolizable ketones,
malononitrile, and various aromatic aldehydes was explored (Scheme 2).
All the products were cleanly isolated with simple filtration and recrystallization
from hot ethanol. As Table 2 shows, in all the cases, aromatic aldehydes substituted
with either electron-donating or electron-withdrawing groups smoothly underwent
the reaction and gave the target products in good to excellent yields.
For a true heterogeneous catalyst, supported catalyst should not leach into the
reaction mixture and the recyclability of the supported catalyst is also important. To
investigate these properties for the introduced catalyst, the reaction of 4-hydrox-
ychomarine, benzaldehyde, and malononitrile in ethanol was selected again as
model (Table 3).
After completion of the reaction, the mixture was filtered and the recovered
catalyst was washed with acetone and dried before using for next consecutive runs
(seven runs). Almost consistent activity was observed over eight consecutive runs.
From Table 3, It can be seen that [Amb]L-prolinate can be reused up to eight runs
without need to reload and the yield difference between the first and eighth runs is
only 5 %, which indicates that the catalyst efficiency is almost completely
maintained during eight consecutive runs.
The nitrogen content of the fresh and reused catalyst was measured by using
elemental analysis, and the comparison of the nitrogen contents indicated that the
catalyst lost only 3 % of its nitrogen content after eight runs. This is a good proof
for very low leaching account of L-proline organocatalyst from [Amb]L-prolinate
catalyst into the reaction mixture during eight runs and also confirms that the
catalytic ability of [Amb]L-prolinate remained almost completely stable after eight
runs, in agreement with the recyclability study.
NH2
CN
O
OH
Ar
O
O
O
O
O
Ar
O
Ethanol, Reflux
(20-45 min)
CN
NH2
O
+
CN
NC
O
+
Ar
H
O
[Amb]L-prolinate
CN
8 runs
Ar
NH2
OH
O
Scheme 2 Synthesis of 2-amino-4H-chromene derivatives using [Amb]L-prolinate
123

M. Keshavarz et al.
Table 2 Synthesis of 2-amino-4H-chromene derivatives catalyzed by [Amb]L-prolinate
Enol/Enolizable
compound
Time
(min)
Yield
Entry
Aldehyde
Product
(%) a
NH2
O
OH
O
CN
1
20
35
90
85
H
O
O
O
O
O
NH2
O
OH
CN
O
O
H
2
3
4
H3C
O
O
NH2
CH3
O
OH
CN
CN
O
20
30
80
87
H
O
O
O
F
O
O
O
F
NH2
OH
O
H
O
Br
O
O
Br
NH2
OH
O
CN
O
O
H
5
6
7
45
45
40
89
79
83
H3CO
O
O
O
O
O
OCH3
NH2
O
OH
CN
CH3
O
O
H
CH3
O
O
CN
NH2
O
OH
H
O
O
H3C
CH3
O
O
NH2
CH(CH3)2
OH
O
CN
O
8
9
H
25
20
20
90
90
88
O
O
O
NC
O
O
O
NH2
CN
O
OH
CN
H
NO2
O
NO2
O
O
O
O
O
O
O
10
H
CN
O
OCH3
NH2
O
O
O
H
O
O
11
12
35
25
82
89
CN
H3CO
O
F
NH2
O
O
H
CN
F
O
NH2
123

Ion-pair immobilization of L-prolinate anion onto cationic…
Table 2 continued
Br
O
O
O
O
O
O
13
H
30
81
CN
Br
O
NH2
CH3
O
O
O
O
O
O
O
H
14
15
16
35
40
45
83
79
81
CN
H3
C
O
NH2
O
H
H3
C
CN
CH3
O
OCH3
NH2
OCH3
O
H
O
H3CO
CN
OCH3
O
NH2
N
O
O
O
O
O
H
17
35
80
O
CN
N
O
NH2
O
O
OCH3
CN
H
18
19
20
45
35
30
79
84
83
OCH3
O
NH2
NH2
CN
O
H
OH
O
Br
CN
O
NH2
OH
OH
H
O
Br
OCH3
H3CO
O
CN
H
NH2
21
22
45
30
86
87
H3CO
O
OCH3
NO2
O
CN
OH
H
O
NH2
O
NO2
H3
C
CN
OH
OH
NH2
H
H
23
24
35
45
80
89
O
H3
C
H3CO
CN
O
NH2
O
H3CO
a
Yield refer to isolated and pure product
123

M. Keshavarz et al.
Table 3 Recyclability, reusability, and leaching account study of [Amb]L-prolinate
NH2
OH
CN
O
Ethanol, Reflux
O
+
CN
NC
+
Ph
Ph
H
[Amb]L-prolinate
8 runs
O
O
O
O
Run
1
2
3
4
5
6
7
8b
Time (min)
Yield (%)a
a
20
89
20
89
25
88
25
85
25
87
35
86
30
85
30
84
Isolated yield
b
The nitrogen content of the recovered catalyst after eight runs was measured using elemental analysis to
be approximately 3 % less than the nitrogen content of the fresh catalyst
Next, in order to show the superiority of the [Amb]L-prolinate to the
homogeneous L-proline and sodium prolinate, the reaction of 4-hydroxychomarine,
benzaldehyde, and malononitrile in ethanol was selected again as model reaction.
The best results were achieved by using 20 mol% of L-proline and 15 mol% of
sodium prolinate while at the same condition, with 10 mol% of [Amb]L-prolinate
the reaction time was shorter. Moreover, the [Amb]L-prolinate catalyst can be
simply recovered and reused up to eight runs (Table 3) while we failed to recover
and reuse the L-proline and sodium prolinate catalysts.
A proposed mechanism to demonstrate the role of catalyst is shown in Scheme 3.
A Knoevenagel reaction, Michael addition, and intra-molecular cyclization are
Scheme 3 Plausible mechanism for the preparation of chromene derivative using [Amb]L-prolinate
catalyst
123

Ion-pair immobilization of L-prolinate anion onto cationic…
involved simultaneously in the synthesis of 2-amino-4H-chromene derivatives. In
the first step, the [Amb]L-prolinate catalyst abstracts a proton from the active
methylene group of malononitrile leading to formation of carbanion. The formed
carbanion of malononitrile makes a nucleophilic attack on the carbonyl carbon atom
of aromatic aldehydes, followed by loss of a water molecule to form a-cyanocin-
namonitrile derivative. In the second step, [Amb]L-prolinate catalyst reacts with
dimedone and makes an enamine intermediate. Michael addition of enamine
intermediate to Knoevenagel products, followed by ring closing and deprotonation,
furnish 2-aminochromene derivative.
Conclusion
In conclusion, a new heterogeneous organocatalyst based on ion-pair immobiliza-
tion of L-proline on the surface of amberlite hydroxide has been developed. The
non-covalent immobilization strategy was used for the heterogenization of L-proline
via ion-pair immobilization on the surface of amberlite hydroxide. This strategy
made the organocatalyst to be mobile and flexible which not only helped the
supported catalyst to be as powerful as its non-supported form, but also made it
easily recoverable with simple filtration. The novel prepared heterogeneous
organocatalyst efficiently catalyzed the one-pot synthesis of chromene derivatives
using variety of aromatic aldehydes, malononitrile, and enols or enolizable carbonyl
compounds to furnish the corresponding chromenes in good to excellent yields
under green conditions.
Acknowledgments The authors gratefully acknowledge the Research Council of Yasouj University for
financial support.
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