New nano-Fe3O4-supported Lewis acidic ionic liquid as a highly effective and recyclable catalyst for the preparation of benzoxanthenes and pyrroles under solvent-free sonication

Article abstract of DOI:10.1039/c8ra04893b

A novel magnetic nanomaterial-immobilized Lewis acidic ionic liquid was successfully synthesized by the covalent embedding of 3-(3-(trimethoxysilyl)propyl)-1H-imidazol-3-ium chlorozincate (ii) ionic liquid to the surface of Fe₃O₄ nanoparticles. The material was then characterized by FT-IR, SEM, TEM, TGA, ICP-OES, Raman, and EDS. Its performance as a new-generation Lewis acidic catalyst was also examined on the ultrasound-mediated synthesis of benzoxanthenes and pyrroles. Upon completion, the catalyst was simply recovered by an external magnet for multiple reuses without significant lessening of catalytic performance.

Full text of DOI:10.1039/c8ra04893b

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New nano-Fe3O4-supported Lewis acidic ionic
liquid as a highly effective and recyclable catalyst
for the preparation of benzoxanthenes and pyrroles
under solvent-free sonication†
Cite this: RSC Adv., 2018, 8, 35681
Hai Truong Nguyen,
and Phuong Hoang Tran
Ngoc-Phuong Thi Le, Duy-Khiem Nguyen Chau
*
A novel magnetic nanomaterial-immobilized Lewis acidic ionic liquid was successfully synthesized by the
covalent embedding of 3-(3-(trimethoxysilyl)propyl)-1H-imidazol-3-ium chlorozincate (II) ionic liquid to
the surface of Fe3O4 nanoparticles. The material was then characterized by FT-IR, SEM, TEM, TGA, ICP-
OES, Raman, and EDS. Its performance as a new-generation Lewis acidic catalyst was also examined on
the ultrasound-mediated synthesis of benzoxanthenes and pyrroles. Upon completion, the catalyst was
simply recovered by an external magnet for multiple reuses without significant lessening of catalytic
performance.
Received 7th June 2018
Accepted 10th October 2018
DOI: 10.1039/c8ra04893b
rsc.li/rsc-advances
magnetic nanoparticles have been extensively studied on
a variety of principal organic transformations including C–C, C–
Introduction
N, and C–O coupling since their magnetism can enable a much
simplied work-up procedure where nanoparticle-supported
catalysts are promptly separated from the reaction media by
an external permanent magnet rather than ltration and
centrifugation.12,13,20,21 However, in contrast to the widespread
availability of organocatalysts impregnated on solid supports,
the preparation of solid-immobilized ionic liquids has been still
a great challenge. To the best of our knowledge, there have been
only a few reports on the use of magnetic material-supported
ionic liquids in some organic reactions.12,15–17,22–29 Many of
them are the impregnated forms of Brønsted acidic ionic
liquids. Therefore, the demand for designing new magnetic
nanoparticle-supported Lewis acidic ionic liquids has still
remained a topical interest since various organic trans-
formations can be promoted by Lewis acidic species.
Based on the above fundamental understandings, we re-
ported herein the designation of a novel magnetic nanoparticle-
immobilized Lewis acidic ionic liquid and its usage as a prom-
inent catalyst for the preparation of benzoxanthenes and
pyrroles, two important heterocyclic scaffolds commonly
present in various bioactive natural products, agrochemicals,
and pharmaceuticals.30–44 Although a large number of catalytic
procedures have been developed for quantitative synthesis of
these heterocycles, the attempt of searching for a versatile
catalyst fullling all green chemistry criteria, such as high-
turnover property, excellent recyclability, simplicity in
handling and recycling, and the capacity of promoting reactions
under mild condition, has been still an unsolved problem. The
initial results from our screening effort to nd novel ionic
Owing to their exceptional physical and chemical properties,
ionic liquids (ILs) have gained increasing application as green
solvents/catalysts for various organic transformations.1–4
Among them, Lewis acidic ionic liquids (LAILs) have been
intensively studied as efficient alternatives for traditional Lewis
acidic catalysts in many organic syntheses to facilitate higher
yielding transformation to wanted products within short reac-
tion times.5–8 Despite the mentioned preeminence, these LAILs
still have several drawbacks such as the elaborate procedure for
catalyst recovery and product isolation as well as the demand
for the excessive amount of ILs in biphasic systems, which in
turn would raise concerns about economical and toxicological
issues.9,10 As a result, various solid-supported ionic liquids have
been progressively developed to overcome those challenges
thanks to their capability of being handled, separated, and
recycled with ease.11–19 Recently, our group successfully
employed a Brønsted acidic ionic liquid gel as a heterogeneous
catalyst to assist the preparation of bisindolylmethanes.19
Despite the outstanding recyclability, low yields were inevitable
because of the poor dispersion of the ionic liquid gel in the
reaction mixture. Being aware of this shortcoming, relevant
scientic communities are continuously directing their atten-
tion to employ high-surface-area materials as new-generation
catalyst supports for multiple organic reactions. Among them,
Department of Organic Chemistry, Faculty of Chemistry, University of Science, Viet
Nam National University, Ho Chi Minh City 721337, Vietnam. E-mail: thphuong@
hcmus.edu.vn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra04893b
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liquid-based catalysts for certain organic reactions indicated
that magnetic nanomaterial Fe3O4-supported Lewis acidic ionic
liquid could be the best candidate most likely satisfying all
mentioned requirements. Its outstanding catalytic performance
over the multicomponent synthesis of benzoxanthenes and
Paal–Knorr condensation to prepare pyrroles in this paper
promises its prospectively common use as a novel catalyst in
replacement of conventional Lewis acidic ones in organic
synthesis. As far as we have known, this is the rst report on the
synthesis and catalytic reactivity of a magnetic nanoparticle-
immobilized Lewis acidic ionic liquid.
Results and discussion
Characterization of LAIL@MNP
The magnetic nano-Fe3O4-immobilized Lewis acidic ionic
liquid material (LAIL@MNP) was prepared in accordance to the
protocol presented in Scheme 1. Initially, magnetite ferrite
nanoparticle was formed via a co-precipitation method while
the precursor ionic liquid was independently synthesized by the
treatment of (3-chloropropyl)triethoxysilane with imidazole.
Then the outermost surface of MNP was coated with the
prepared ionic liquid. Finally, ZnCl2 was added and the result-
ing mixture was heated under reux for 24 h to afford
LAIL@MNP.
Fig. 1 FT-IR spectra of Fe3O4 (a), primitively prepared LAIL@MNP (b),
and the LAIL@MNP sample after the fifth recovery (c).
The graing of LAIL@MNP was veried by FT-IR analysis.
Fig. 1b and c showed the IR spectra of primitively synthesized
LAIL@MNP and its recycled sample aer the h recycling test,
respectively. As expected, a good resemblance between two
spectra can be seen over a wide range of absorption band,
including peaks at 3430 cmꢀ1 (broad absorption corresponding
to Si–OH group and adsorbed water), 2921 cmꢀ1 (aliphatic C–H
stretch), 1620 cmꢀ1 (C]N stretch of imidazolium ring), and
a cluster of two peaks at 1095 and 1043 cmꢀ1 (Si–O–Si stretch).
These signals were not observed in the spectrum of plain Fe3O4
(Fig. 1a). This indicated that ionic liquid was successfully graf-
ted onto the surface of the Fe3O4 nanoparticle.
Scanning electron microscopy (SEM) image in Fig. 2 showed
the morphology of LAIL@MNP which is depicted as uniform
spherical particles. Transmission electron microscopy (TEM)
conrmed that dark Fe3O4 core coated by grey silica shell has
the average diameter of 10–15 nm (Fig. 2).
The thermostability of the LAIL@MNP was studied by ther-
mogravimetric analysis (TGA) (Fig. 3). The TGA curve indicates
an initial weight loss of 0.74% in the temperature range below
200 ꢁC, most likely corresponding to the evaporation of residual
water from the sample. The entire loss of the graed LAIL was
detected in the region from 220 ꢁC to 400 ꢁC. The percentage of
Fig. 2 SEM image of LAIL@MNP (above) and TEM images (below) of
as-prepared (left) and quinary recycled LAIL@MNP (right).
LAIL, as determined, was approximately 3% of the total solid
matrix. The Raman peak at 480 cmꢀ1 can be interpreted as
stretching vibration of Zn–Cl bond.45 The energy dispersive
spectrum (EDS) showed the presence of C, N, O, Fe, Zn, Si, and
Scheme 1 Synthesis of LAIL@MNP.
35682 | RSC Adv., 2018, 8, 35681–35688
Fig. 3 TGA curve of LAIL@MNP (left) and its Raman spectrum (right).
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Cl elements (Fig. 4). The composition of zinc element in to be favored in acidic media as all metal halides, which are
LAIL@Fe3O4 was determined as 19 890 ppm (0.306 mmol gꢀ1
known as typical Lewis acids, can effectively furnish the ben-
by means of inductively coupled plasma mass spectrometry zoxanthene product in good yields of at least 80% (entry 5–9).
)
(ICP-MS).
Analogously, an array of different ionic liquids possessing
weakly Brønsted acidic imidazolium species can also readily
facilitate the formation of the same annulated product without
any difficulty (entry 1–4). Meanwhile, most of the metal oxides,
because of their predominant Lewis basicity, only exhibited
moderate catalytic performance under the given reaction
condition (entry 10–19). The invention of LAIL@MNP material
having Lewis acidic zinc species immobilized onto magnetic
Fe3O4 nanoparticle not only brought about a remarkable yield
increment but also enable a quite simple work-up step for
product isolation as well as catalyst recovery (entry 21). The
effect of solvent on the studied reaction was also investigated,
showing that low yield of product was observed with polar as
well as nonpolar solvents (Table S2,† entries 22–36).
As can be seen from Table 1, the current method gave access
to the desired product with the same ease as those reported in
the literature. However, LAIL@MNP still gained prominence
over other heterocyclic catalysts owing to its capability to enable
the reaction under solventless condition. Additionally, the
affordability of ZnCl2 also makes the protocol described herein
more advantageous than the others of using expensive
transition-metal catalysts.
With the optimized condition in hand, we then examined
the performance of catalyst over a wide range of aldehydes. As
indicated in Scheme 2, the reaction generally worked well for
saturated aliphatic as well as aromatic aldehydes. On the
contrary, a,b-unsaturated aldehydes such as acrolein and cin-
namaldehyde almost did not undergo the annulation with 2-
naphthol and dimedone even under prolonged reaction time
due to the positive charge delocalization arising from resonance
effect. As expected, butyraldehyde with less steric hindrance at
a-position was more reactive than cyclohexanecarbaldehyde.
Under the same condition, 78% yield of the benzoxanthene
derived from butyraldehyde was recorded, compared to the
yield of 65% noted for the analogue produced from cyclo-
hexanecarbaldehyde. Surprisingly, all benzaldehydes bearing
either electron-donating or electron-withdrawing groups affor-
ded the corresponding products from good to excellent yields
(70–90%). Interestingly, steric hindrance of bulky o-substituents
such as bromo or nitro in aromatic aldehydes did not bring
about any detectable negative impact on the reaction as re-
ported above for aliphatic aldehydes. The structural study of
Synthesis of benzoxanthene derivatives
Initially, the catalytic activity of LAIL@MNP was surveyed in an
ultrasound-assisted solvent-free multicomponent synthesis of
pyran-annulated heterocyclic derivatives from 2-naphthol, dime-
done, and benzaldehyde. As can be seen from Table S1,† signi-
cantly prolonged exposure to ultrasound (120 min) was
necessitated for a quantitative formation of desired product (94%)
at room temperature (entry 11). As expected, performing the
reaction at an elevated temperature of 80 ꢁC can bring the reaction
to completion within 30 min (entry 18). Noticeably, the extremely
poor yield of benzoxanthene was observed in control experiment
where no catalyst was used (entry 23). Surprisingly, the addition of
1 mg of LAIL@MNP to the reaction mixture can give rise to
a tremendous enhancement of reaction yield from 10% to 65%
(entry 24). The transformation of given starting materials to the
expected product can be brought to completion by increasing the
catalyst loading from 1 to 15 mg (Table S1,† entry 18 and 24).
Subsequently, to illustrate the merit of LAIL@MNP, a variety
of other catalysts were also studied for their activity with respect
to the same reaction for the comparative purpose (Table S2†). In
general, the investigated multicomponent reaction seems likely
Fig. 4 EDS spectrum of LAIL@MNP.
Table 1 Comparative effectiveness of the current method versus former works for the three-component synthesis of benzoxanthenes
Entry
Catalytic system
Temp. (ꢁC)
Time (h)
Yield (%)
1
2
3
4
5
6
7
Nano Fe3O4/CS-Ag (15 mg)
Ru@SH-MWCNT (15% wt), EtOH/reux
Nano-WO3-SO3H (19 mg)
W–ZnO (5.6 mol%), EtOH
Amberlyst-15 (200 mg), CH3CN
80
80
100
r.t.
Reux
90
0.5
0.5
1.05
0.17
5
94 (ref. 46)
85 (ref. 47)
90 (ref. 48)
98 (ref. 49)
94 (ref. 50)
89 (ref. 24)
96
AIL@MNP (55 mg), dimedone (2.0 equiv.)
Current work: LAIL@MNP (15 mg), solvent-free sonication
0.75
0.5
80
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Scheme
3
Proposed mechanism for one-pot multicomponent
synthesis of benzoxanthenes.
to this mechanism, the coordination between zinc species and
carbonyl oxygen enhances the electrophilicity of carbonyl
carbon, giving rise to a better nucleophilic attack of 2-naphthol
to benzaldehyde to furnish a,b-unsaturated cyclic ketone
intermediate (A0) aer the loss of water molecule. Then Michael
addition between this intermediate and the enol form of
dimedone followed by the dehydration afforded the expected
product.
Synthesis of pyrrole
The Paal–Knorr synthesis of 2,5-dimethyl-1-phenyl-1H-pyrrole
from aniline and acetonylacetone was chosen as the second
example to examine the catalytic performance of LAIL@MNP
and the results are listed in Table S3.† In the presence of 15 mg
of LAIL@MNP, the reaction proceeded smoothly to provide the
expected pyrrole in quantitative yield of 91% aer 30 min of
sonication.
To exemplify the novelty of LAIL@MNP, the same reaction
was probed over a wide range of catalysts and the results are
presented in Table S4.† Similar to the above discussed multi-
component reaction, Paal–Knorr condensation has been also
known to take place in slightly acidic rather than basic media.
Consequently, all metal halides with Lewis acidic properties can
straightforwardly bring the yield of pyrrole to more than 80%
(entry 4–8), while metal oxides with amphoteric or basic nature
only resulted in poor to moderate formation of the desired
pyrrole under the same condition (entry 9–18). Without a rival,
LAIL@MNP was again proven as the best candidate to induce
a complete transformation to the target pyrrole (entry 20).
The comparison of the present method with formerly re-
ported works was summarized in Table 2. Most obviously, the
fact that LAIL@MNP promoted the synthesis of pyrrole under
solvent-free condition can be easily outlined as the most note-
worthy advantage compared with existing catalysts.
The tolerance of the presently studied Paal–Knorr reaction
towards different functionalities was described in Scheme 4. In
general, the reaction was apparently governed by both elec-
tronic and steric effect. Therefore, p-substituted anilines
bearing an electron-donating (–OMe) or a weakly electron-
withdrawing group (–I) can most easily undergo Paal–Knorr
cyclization to offer corresponding products in quantitative
yields more than 90%. Meanwhile, the analog containing
strongly electron-withdrawing group, p-nitroaniline, can be only
converted to the desired pyrrole with a quite modest yield of
Scheme 2 Reaction scope of the three-component condensation of
2-naphthol, dimedone, and aldehyde in the presence of Fe3O4@SiO2-
IL-ZnxCly.
benzoxanthenes by X-ray crystallography in the previous litera-
ture revealed that the phenyl half is orthogonal rather than
coplanar to the other half to minimize the steric strain of the
whole molecule.51 Therefore, the steric effect of o-substituents
in phenyl moiety is negligible owing to the at structure of
benzene ring. However, this is not the case for an aliphatic half
like cyclohexyl since its uneven conguration always causes
inevitable steric interaction regardless of its spatial arrange-
ment in the molecule.
Based on the current results and previous literature,32,36,52,53
a proposed mechanistic prole for the synthesis of benzox-
anthenes using LAIL@MNP is depicted in Scheme 3. According
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Table 2 Comparison of the present synthesis of pyrrole with previous works in literature
Entry
Catalyst
Temp.a (ꢁC)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
Bi(NO3)3$5H2O (50 mol%), CH2Cl2
I2 (0.1 mmol), THF
Sc(OTf)3 (5 mol%), CH2Cl2
In(OTf)3 (5 mol%), CH2Cl2
[BMIm]Cl
SbCl3/SiO2 (0.1 mmol), hexane
BiCl2/SiO2 (7.5 mmol), hexane
Vitamin B1 (5 mol%), ethanol
a-Zr(KPO4)2 (6 mol%)
Montmorillonite, KSF (1 g), CH2Cl2
PS/AlCl3 (15 mol%), CH3CN
This work: LAIL@MNP, solvent-free sonication
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
Reux
r.t.
10
9
0.5
0.5
3
1
1
1
2
95 (ref. 54)
89 (ref. 55)
77 (ref. 56)
90 (ref. 57)
96 (ref. 58)
91 (ref. 59)
92 (ref. 60)
90 (ref. 61)
78 (ref. 62)
95 (ref. 63)
90 (ref. 64)
91
9
10
11
12
10
1.25
0.5
a
r.t.: room temperature. b Isolated yield.
48% even though the reaction time was extended up to 60 min. also able to exhibit a certain steric effect suppressing the
Shiing the methoxy group to ortho-position with respect to complete transformation to desired products wherein pyrroles
reactive amino site resulted in a diminished yield. The domi- derived from 2-hydroxy-5-methylaniline and 2-hydroxy-5-
nance of steric effect can be unambiguously observed for all o- nitroaniline were isolated in 78 and 63% yield, respectively.
substituted anilines bearing bulky groups, such as methyl, Interestingly, selective pyrrole cyclization can be achieved
chloro, bromo, and phenyl. For instance, the pyrrole derived thanks to the directing effect of present substituents. As illus-
from 2,5-dichloroaniline was formed in much lower yield than trated in Scheme 4, only the amino group at meta-position to
those generated from 3,4- and 3,5-dichloroaniline isomers. nitro substituent was exclusively subjected to the Paal–Knorr
Noticeably, regardless of its small size, o-methoxy group was condensation with acetonylacetone while the other was intact.
Finally, the extension of substrate scope to phenylhydrazines
and triethylenetetramine was also applicable for this method.
A proposed mechanism for the formation of pyrrole cata-
lyzed by LAIL@MNP is depicted in Scheme 5. In a similar
fashion to the above mechanism for the multicomponent
synthesis of benzoxanthene, carbonyl oxygen of acetonylace-
tone is rst coordinated to zinc center to generate a better
electrophilic species which more readily reacts with amine to
give intermediate adduct (C). Following intramolecular nucle-
ophilic cyclization with the loss of two molecules of water
delivers the desired pyrrole.
The recyclability of LAIL@MNP was examined over both
benzoxanthene and pyrrole synthesis under the reported opti-
mized condition (Fig. 5). Upon the completion of each reaction,
the corresponding product was diluted with ethyl acetate (15
mL) and the catalyst was separated by a magnet. The restored
catalyst was washed with ethyl acetate (2 ꢂ 5 mL) and ethanol (3
ꢂ 5 mL). Then it was desiccated in vacuo for 2 h and reutilized in
the next recycling experiments. Nearly quantitative LAIL@MNP
could be recovered from each run. The stability and efficacy of
LAIL@MNP remained constant through ve consecutive recy-
cling tests. No apparent modication in morphology and size of
particles was detected in the TEM image of primitively prepared
Scheme 4 Synthesis of pyrrole derivatives.
Scheme 5 A plausible mechanism for the synthesis of pyrroles.
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the mixture was sonicated at 40 ꢁC for 4 h. The resultant
IL@MNP was washed with dichloromethane and dried at 70 ꢁC
in vacuo. To synthesize LAIL@MNP, a mixture of IL@MNP (1.0
g) and ZnCl2 (1.0 mmol) in ethanol (50 mL) was reuxed for
24 h. Aer cooling to room temperature, the catalyst was
separated by a magnet, washed with ethanol, and dried at
100 ꢁC for 5 h. The LAIL@MNP was characterized by FT-IR,
SEM, TEM, TGA, Raman, and SEM-EDS. The loading amount
of Zn metal was determined based on ICP-OES.
General procedure for one-pot multicomponent reaction
Fig. 5 Recycling tests.
A mixture of benzaldehyde (1.0 mmol, 0.106 g), 2-naphthol
(1.0 mmol, 0.144 g), and dimedone (1.0 mmol, 0.140 g) was
reacted under solvent-free sonication in the presence of
LAIL@MNP (15 mg) at 80 ꢁC for 30 min. Upon completion of the
reaction (monitored by TLC), the resulting mixture was diluted
by ethyl acetate (15 mL) and the solid catalyst was eliminated
from organic solution by a magnet. The catalyst was washed
with ethyl acetate (2 ꢂ 5 mL) followed by ethanol (3 ꢂ 5 mL) and
then reused for next cycles aer drying under vacuum. Mean-
while, the organic solution was dried over MgSO4 and then
concentrated under reduced pressure. The resultant crude
product was recrystallized from ethanol to yield pure benzox-
as well as recovered sample of LAIL@MNP (Fig. 2). The FT-IR
spectra of LAIL@MNP samples also suggested no obvious
change in functionality (Fig. 1).
The primary issue of heterogeneous catalysts is the leaching
of reactive species from solid support, resulting in deactivation
of the catalyst. To evaluate the leaching effect of LAIL@MNP,
the reaction between aniline and acetonylacetone was tempo-
rarily stopped aer 10 min by diluting the ongoing reaction
mixture with a sufficient amount of ethyl acetate. The catalyst
was then removed and the ethyl acetate solution was analyzed
by GC, indicating that 58% yield of pyrrole was formed at that
time. Next, ethyl acetate solvent was decanted and the resultant
free-catalyst reaction mixture was sonicated for further 20 min.
At the end of this stage, the yield of pyrrole only slightly
increased from 58 to 62%, showing that only a trace amount of
zinc species was leached into the reaction mixture during the
reaction course. The leaching level of LAIL@MNP was also
evaluated by ICP-OES whereby the Zn content in reaction
mixture was determined as 1.8 ppm, indicating a negligibly
slight leaching from the solid support.
1
anthene whose structure was veried by H and 13C NMR.
General procedure for Paal–Knorr reaction
A mixture of aniline (1.0 mmol), acetonylacetone (1.2 mmol)
and LAIL@MNP (15 mg) was reacted under solvent-free soni-
cation within an appropriate interval. Upon completion of the
reaction (monitored by TLC and GC), the resulting mixture was
diluted by ethyl acetate (15 mL) and the solid catalyst was
eliminated from the organic solution by a magnet. The catalyst
was washed with ethyl acetate (2 ꢂ 5 mL) followed by ethanol (3
ꢂ 5 mL) and then reused for next cycles aer drying under
vacuum. Meanwhile, the organic solution was dried over MgSO4
and then concentrated under reduced pressure. The crude
product was puried through column chromatography using
hexane–ethyl acetate (9 : 1). The puried pyrrole was then
Experimental
Preparation of magnetic Fe3O4 nanoparticle (MNPs)65
MNP was synthesized by simple co-precipitation of ferrous and
ferric ions in a basic condition medium. Typically, FeCl3$6H2O
(20 mmol) and FeSO4$7H2O (10 mmol) were dissolved in
1
authenticated by H and 13C NMR, GC-MS or HRMS (ESI).
ꢁ
100 mL deionized water. The mixture was stirred at 80 C and
Conclusions
then KOH (12 mmol) was added and stirred continuously within
2 h. The black precipitate, aer being collected by a permanent
magnet, was washed with water (3 ꢂ 100 mL) and ethanol (3 ꢂ
We have designed a novel magnetically separable LAIL@MNP
material which can be used as a green solid Lewis acid catalyst
in various organic reactions. Embedding reactive Lewis acidic
sites onto high-surface area Fe3O4 nanomaterial greatly over-
came the issue of poor dispersion which is common for
heterogeneous catalysts. Owing to this feature, LAIL@MNP can
ꢁ
50 mL). This MNP material was dried at 60–70 C under vacuo
for 1 h.
General procedure for the synthesis of LAIL@MNP
The imidazolium chloride ionic liquid was synthesized pur- exhibit better catalytic performance than conventional Lewis
suant to a procedure reported previously.66 A mixture of 3- acid in acid-mediated organic reactions such as multicompo-
chloroethoxy-propylsilane (5.0 mmol, 1.2 mL) and imidazole nent and Paal–Knorr reactions to synthesize benzoxanthenes
(5.0 mmol, 0.34 g) was stirred at reux for 17 h. The ionic liquid and pyrroles, respectively, in good to excellent yields. Addi-
obtained as a yellowish viscous liquid was diluted in 50 mL of tionally, the efficiency of the protocol was also owing to the use
ethanol–water (1 : 1 volume ratio) solution. To the freshly of ultrasonic irradiation by means of which locally high collapse
prepared suspension of MNP material in 100 mL of 1 : 1 temperature induced by the successive formation and collapse
ethanol–water was added the above ionic liquid solution and of cavitation bubbles in reaction media can extraordinarily
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enhance the reaction rate. Moreover, the catalyst was easily 23 J. Miao, H. Wan and G. Guan, Catal. Commun., 2011, 12, 353–
separated from the reaction mixture by an external magnetic 356.
eld and reused without any signicant loss of catalytic activity 24 Q. Zhang, H. Su, J. Luo and Y. Wei, Green Chem., 2012, 14,
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25 B. Karimi, F. Mansouri and H. Vali, Green Chem., 2014, 16,
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Conflicts of interest
There are no conicts to declare.
27 Z. Wu, Z. Li, G. Wu, L. Wang, S. Lu, L. Wang, H. Wan and
G. Guan, Ind. Eng. Chem. Res., 2014, 53, 3040–3046.
28 H. N. Dadhania, D. K. Raval and A. N. Dadhania, Res. Chem.
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Acknowledgements
This project is nancially supported by Vietnam National
Foundation for Science and Technology Development (NAFOS- 29 Q. Zhang, X.-M. Ma, H.-X. Wei, X. Zhao and J. Luo, RSC Adv.,
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