Nano-Fe3O4 -encapsulated silica particles bearing sulfonic acid groups as a magnetically separable catalyst for the green and efficient synthesis of 14-aryl-14H-dibenzo[a,i]xanthene-8,13-dione derivatives

Article abstract of DOI:10.1007/s11164-015-2073-7

In this research, an efficient and environmentally friendly (green) method for one-pot synthesis of xanthene derivatives was developed in the presence of Fe₃O₄@SiO₂-SO₃H nanoparticles. The functionalization of silica-coated Fe₃O₄ magnetite nanoparticles (Fe₃O₄@SiO₂) using chlorosulfonic acid was afforded by nano-Fe₃O₄ encapsulated-silica particles bearing sulfonic acid (Fe₃O₄@SiO₂-SO₃H) that could be applied as a reusable and efficient heterogeneous nanocatalyst. The multicomponent reactions of 2-hydroxy-1,4-naphthoquinone, β-naphthol and aldehydes were carried out under reflux conditions to obtain some 14-aryl-14H-dibenzo[a,i]xanthene-8,13-dione derivatives. Being a simple procedure with environmental benignness, excellent yields, short reaction times, simple purification and facile catalyst separation are the advantages of this protocol. Fe₃O₄@SiO₂-SO₃H was readily recovered using an external magnet and could be reused several times without significant loss of reactivity. The catalyst was fully characterized by vibrating sample magnetometry (VSM), Fourier transform-infrared (FT-IR) spectrometry, scanning electron microscopy (SEM), X-ray diffraction (XRD) spectroscopy, energy dispersive X-ray spectroscopy (EDAX) and transmission electron microscopy (TEM).

Full text of DOI:10.1007/s11164-015-2073-7

Res Chem Intermed
DOI 10.1007/s11164-015-2073-7
Nano-Fe3O4 -encapsulated silica particles bearing
sulfonic acid groups as a magnetically separable
catalyst for the green and efficient synthesis of 14-aryl-
14H-dibenzo[a,i]xanthene-8,13-dione derivatives
Mohammad Ali Ghasemzadeh1
Mina Azimi-Nasrabad1
•
Received: 23 December 2014 / Accepted: 20 April 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract In this research, an efficient and environmentally friendly (‘‘green’’)
method for one-pot synthesis of xanthene derivatives was developed in the presence
of Fe3O4@SiO2–SO3H nanoparticles. The functionalization of silica-coated Fe3O4
magnetite nanoparticles (Fe3O4@SiO2) using chlorosulfonic acid was afforded by
nano-Fe3O4 encapsulated-silica particles bearing sulfonic acid (Fe3O4@SiO2–SO3H)
that could be applied as a reusable and efficient heterogeneous nanocatalyst. The
multicomponent reactions of 2-hydroxy-1,4-naphthoquinone, b-naphthol and alde-
hydes were carried out under reflux conditions to obtain some 14-aryl-14H-diben-
zo[a,i]xanthene-8,13-dione derivatives. Being a simple procedure with environmental
benignness, excellent yields, short reaction times, simple purification and facile cat-
alyst separation are the advantages of this protocol. Fe3O4@SiO2–SO3H was readily
recovered using an external magnet and could be reused several times without sig-
nificant loss of reactivity. The catalyst was fully characterized by vibratingsample
magnetometry (VSM), Fourier transform-infrared (FT-IR) spectrometry, scanning
electron microscopy (SEM), X-ray diffraction (XRD) spectroscopy, energy dispersive
X-ray spectroscopy (EDAX) and transmission electron microscopy (TEM).
Keywords Fe3O4@SiO2–SO3H Á Magnetite Á Nanoparticles Á Multi-component
reaction Á Core–shell Á Xanthene
&
Mohammad Ali Ghasemzadeh
Ghasemzadeh@qom-iau.ac.ir
1
Department of Chemistry, Qom Branch, Islamic Azad University, Qom,
Islamic Republic of Iran
123

M. A. Ghasemzadeh, M. Azimi-Nasrabad
Introduction
The aim of science and technology in recent years has been to shift toward more
environment friendly and reusable catalysts and sustainable resources in order to
decrease waste and the use of raw materials. The development of new catalysts by
nano-scale design has emerged as a fertile field suitable for research and varied
innovation [1]. Magnetite nanoparticles (MNPs) have been intensively studied
because of their broad application in several fields such as ferrofluids, digital media
recording, targeted drug delivery, magnetic hyperthermia, magnetic resonance
imaging (MRI), etc. [2, 3]. Magnetite, Fe3O4, is also a facile reductant for various
environmental contaminants found in groundwater [4, 5]. However, bare Fe3O4
nanoparticles (NPs) tend to aggregate into large clusters, are easily oxidated in air,
have high dispersion, a loss of magnetism, and agglomeration because of their high
specific area and strong inter particle interaction, limiting their usage. In this regard,
great attention has recently been paid to the preparation of Fe3O4-based NPs with a
core–shell structure [6–9]. Coating MNPs with an amorphous silica layer is a
promising and significant approach in the expansion of MNPs for both basic study
and technological applications [10]. A silica shell on a nanocomposite is chemically
inert and has high stability versus aggregation.
Furthermore, the presence of silanol moieties on the surface can readily be
functionalized through suitable surface modifications, enabling introduction of a
variety of various functionalities [11]. Therefore, the outer shell of silica not only
protects the inner magnetite core from oxidation but also provides sites for surface
functionalization using various groups such as guanidine [12], proline [13], sulfamic
acid [14] and sulfonic acid [15].
Recently, nano-Fe3O4 encapsulated-silica particles bearing sulfonic acid (Fe3-
O4@SiO2–SO3H) NPs were used as an active catalyst in the synthesis of 1,8-dioxo-
octahydroxanthene [16], indazolo[2,1-b]phthalazine-triones and pyrazolo[1,2-b]ph-
thalazine-diones [17], functionalized pyrimido[4,5-b]quinolines and indeno-fused
pyrido[2,3-d]pyrimidines [18] and 3,4-dihydropyrimidinones/thiones [19].
In this research, sulfonic acid-functionalized silica-coated nano-Fe3O4 particles
(Fe3O4@SiO2–SO3H) were prepared as effective acid catalysts in our laboratory
and were used for the synthesis of 14-aryl-14H-dibenzo[a,i]xanthene-8,13-dione. It
could easily be separated from the media using a simple external magnet and reused
several times. The process is more efficient than filtration and centrifugation in
preventing loss of the solid catalyst.
Multicomponent coupling reactions have become significant tools in organic,
combinatorial and medicinal chemistry [20] with their fast production of molecular
complexity and variety with predefined functionality [21].
Recently, three-component reaction of aldehydes, b-Naphthol and 2-hydroxy-1,
4-naphthoquinone has received a great deal of attention in organic synthesis. The
synthesis of xanthene derivatives has received much attention due to their various
range of therapeutic and biological attributes, including antiviral [22, 23],
antibacterial [24, 25] and anti-inflammatory properties [26, 27]. Also, these
123

Nano-Fe3O4 -encapsulated silica particles bearing sulfonic acid…
compounds are applied extensively in laser technologies [28], dyes [29] and as pH-
sensitive fluorescent material for embodiment of biomolecules [30].
Many synthetic methods exist for the synthesis of xanthenes and benzoxanthenes,
including condensation of aldehydes and acetophenones [31], 2-hydroxyaromatic
aldehydes and 2-tetralone [32], and b-naphthol with alkyl or aryl aldehydes [33].
There are some methods reported in the literature for the synthesis of 14-aryl-14H-
dibenzo[a,i] xanthene-8,13-dione involving the three-component reaction of b-
naphthol, 2-hydroxy-1, 4-naphthoquinone with various aldehydes catalyzed by
different catalysts such as poly(4-vinylpyridinium) hydrogen sulfate [34], silica
supported perchloric acid [35], xanthan sulfuric acid [36], silica chloride [37],
Amberlyst-15 [38], acetic acid [39], and sulfuric acid [40].
In view of our interest in sustainable protocols in the synthesis of heterocyclic
compounds using multi-component reactions and nanocatalysts [41–46], here, we
report a new and efficient method for the preparation of 14-aryl-14H-dibenzo[a,i]x-
anthene-8,13-dione derivatives via multi-component synthesis of b-naphthol,
2-hydroxy-1,4-naphthoquinone and various aldehydes using an Fe3O4@SiO2
sulfuric acid core–shell composite (Scheme 1).
Experimental
Chemicals were purchased from Sigma–Aldrich and Merck in high purity. All
materials were of commercial reagent grade and were used without further
purification. All melting points are uncorrected and were determined in capillary
1
tubes on a Boetius melting point microscope. H NMR and 13C NMR spectra were
obtained on a Bruker 400 MHz spectrometer with CdCl3 as the solvent using
tetramethylsilane as an internal standard; the chemical shift values are in d. FT-IR
spectra were recorded on Nicolet Magna 550 IR spectrometer in KBr pellets in the
range of 400–4000 cm-1. The elemental analyses (C, H, N) were obtained from a
Carlo ERBA Model EA 1108 analyzer. Powder XRD was carried out on a Philips
diffractometer (X’pert Company) with mono chromatized Cu Ka radiation
˚
(k = 1.5406 A). Microscopic morphology of products was visualized by SEM
(LEO 1455VP). Mass spectra were recorded on a Joel D-30 instrument at an
ionization potential of 70 eV. Magnetic properties were obtained via VSM using a
BHV-55 vibrating sample magnetometer. Compositional analysis was done by
X-ray energy dispersive analysis (Scheme 2).
Scheme 1 Fe3O4@SiO2–SO3H NPs catalyzed one-pot synthesis of 14-aryl-14H-dibenzo[a,i]xanthene-
8,13-dione derivatives
123

M. A. Ghasemzadeh, M. Azimi-Nasrabad
Scheme 2 Preparation steps for fabricating Fe3O4@SiO2–SO3H
Preparation of Fe3O4 NPs
Fe3O4 NPs were prepared according to the procedure reported by Zhang et al. [47].
To a solution of FeCl2.4H2O (2.5 g) and FeCl3.6H2O (6 g) in 30 ml of deionized
water was added dropwise 1.0 mL of concentrated hydrochloric acid at room
temperature. The solution was added to 300 mL of 1.5 mol L-1 NaOH and then the
solution was stirred vigorously at 80 °C until precipitation. Afterwards, the prepared
magnetic NPs were separated magnetically, washed with deionized water and then
dried at 70 °C for 8 h.
Preparation of Fe3O4@SiO2 NPs
The core–shell Fe3O4@SiO2 microspheres were prepared according to the
previously reported method [48]. Briefly, 1 g of Fe3O4 NPs was treated with 0.5-
M HCl aqueous solution (25 mL) by sonication. After treatment for 10 min, the
magnetite particles were separated and washed with deionized water, and then
homogeneously dispersed in a mixture of ethanol (60 mL), deionized water
(100 mL) and concentrated ammonia aqueous solution (10 mL, 28 wt%), followed
by the addition of tetraethylorthosilicate (TEOS, 0.22 g, 0.144 mmol). After stirring
at room temperature for 2 h, the Fe3O4@SiO2 microspheres were separated using an
external magnet and washed with ethanol and water.
Preparation of Fe3O4@SiO2–SO3H
Fe3O4@SiO2–SO3H MNPs were prepared according to a previously reported
procedure by Kiasat and Davarpanah [17]. A suction flask was equipped with a
constant-pressure dropping funnel. The gas outlet was connected to a vacuum
system through an adsorbing solution functioning as an alkali trap. Firstly,
Fe3O4@SilO2 (1 g) was added to the flask and dispersed in dry CH2Cl2 (10 mL)
ultrasonically for 30 min. Subsequently, chlorosulfonic acid (1 mL) was added
drop-wise to a cooled (ice-bath) solution of Fe3O4@SiO2 (1 g) over a period of
30 min at room temperature. After completion of the addition, the mixture was
stirred for an additional 6 h until complete dissipation of HCl from the reaction
vessel. The resulted MNPs were separated using an external magnet and washed
with ethanol and water before being dried in an oven at 70 °C, giving Fe3O4@SiO2–
SO3H as a brown powder.
The chemical purity of the samples as well as their stoichiometry was tested by
EDAX studies. The EDAX spectrum given in Fig. 1a shows the presence of Fe and O
123

Nano-Fe3O4 -encapsulated silica particles bearing sulfonic acid…
Fig. 1 EDAX spectra of Fe3O4 (a), Fe3O4@SiO2 (b), Fe3O4@SiO2–SO3H (c) NPs
Fig. 2 SEM images of Fe3O4 (a), Fe3O4@SiO2 (b), Fe3O4@SiO2–SO3H (c) NPs
as the only elementary components of Fe3O4 NPs. The EDAX spectrum of
Fe3O4@SiO2 in Fig. 1b shows the elemental compositions are (Fe, Si and O) of core–
shell NPs. The EDAX spectrum of Fe3O4@SiO2-SO3H in Fig. 1c shows the
elemental compositions are (Fe, Si, O and S) of sulfuric acid functionalized silica-
coated MNPs with a core–shell structure.
In order to study the morphology and particle size of Fe3O4 (a), Fe3O4@SiO2
(b) and Fe3O4@SiO2-SO3H (c) NPs, SEM images were prepared (Fig. 2). As shown
in Fig. 2b, Fe3O4@SiO2 NPs retain the morphological properties of Fe3O4 (Fig. 2a),
except for a slightly larger particle size and smoother surface, while silica are
uniformly coated on the Fe3O4 particles to form a silica shell compared to the
Fe3O4@SiO2. The SEM image shown in Fig. 2c demonstrates that Fe3O4@SiO2–
SO3H NPs are nearly spherical, about 30 nm in size.
The structure of Fe3O4 (a), Fe3O4@SiO2(b) and Fe3O4@SiO2 sulfuric acid(c) w-
ere analyzed by XRD spectroscopy (Fig. 3). An XRD diagram of the bare Fe3O4
NPs displayed patterns consistent with the patterns of spinel ferrites described in the
literature (Fig. 3) [14]. The same peaks were observed in the both of the
Fe3O4@SiO2 and Fe3O4@SiO2 sulfuric acid XRD patterns, indicating retention of
the crystalline spinel ferrite core structure during the silica-coating process. The
average MNP core diameter of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2 sulfuric acid
were calculated to be about 18, 25 and 32 nm, respectively, from the XRD results
by Scherrer’s equation.
The FT-IR spectra of Fe3O4 NPs, Fe3O4@SiO2, and Fe3O4@SiO2 sulfuric acid
are shown in Fig. 4. The FT-IR analysis of the Fe3O4@SiO2 and Fe3O4@SiO2
sulfuric acid exhibits two basic characteristic peaks at *3300 cm-1 (O–H
123

M. A. Ghasemzadeh, M. Azimi-Nasrabad
Fig. 3 XRD patterns of Fe3O4 (a), Fe3O4@SiO2 (b), Fe3O4@SiO2–SO3H (c) NPs
Fig. 4 The comparative FT-IR spectra of Fe3O4 (a), Fe3O4@SiO2 (b), Fe3O4@SiO2–SO3H (c) NPs
stretching) and 580 cm-1 (Fe–O vibration) [49]. The band at 1081 cm-1 comes
from the Si–O–Si group. The presence of a saulfonyl group in Fe3O4@SiO2 sulfuric
acid was confirmed by 1217 and 1124 cm-1 bands, which were covered by a
stronger absorption of Si–O bonds at 1081 cm-1 [14]. A wide bond at
2500–3409 cm-1 is due to the stretching of OH groups in the SO3H.
123

Nano-Fe3O4 -encapsulated silica particles bearing sulfonic acid…
The magnetic properties of the samples containing a magnetite component were
studied via VSM at 300 K. Figure 5 shows the absence of the hysteresis
phenomenon and indicates that all of the products have superparamagnetism at
room temperature. The saturation magnetization values for Fe3O4 (a), Fe3O4@SiO2
(b), Fe3O4@SiO2-SO3H (c) were 48.49, 38.16 and 35.82 emu/g, respectively. These
results indicated that the magnetization of Fe3O4 decreased considerably with the
increase of SiO2 and SO3H groups.
The size and morphology of Fe3O4@SiO2–SO3H NPs were analyzed by TEM
(Fig. 6). The results show that these nanocatalysts consist of spherical particles with
a crystallite size of about 30 nm, confirming the results calculated from Scherrer’s
formula based on the XRD pattern.
Fig. 5 Magnetization curves for the prepared Fe3O4 (a), Fe3O4@SiO2 (b), Fe3O4@SiO2–SO3H (c) NPs
Fig. 6 TEM images of Fe3O4@SiO2–SO3H NPs
123

M. A. Ghasemzadeh, M. Azimi-Nasrabad
Typical procedure for the sysnthesis of 14-aryl-14H-dibenzo[a,i]xanthene-8,13-
dione(4a–4p).
A mixture of 2-naphthol (1 mmol) and 2-hydroxynaphthalene-1,4-dione (1 mmol),
aldehyde (1 mmol) and Fe3O4@SiO2–SO3H NPs (0.005 gr) in 2.5 mL of ethanol
and 2.5 mL of water was refluxed at 80 °C. After completion of the reaction, the
reaction mixture was diluted with chloroform (10 mL) and the catalyst was
recovered by using an external magnet. The solvent was evaporated and the
obtained solid was recrystallized using ethanol. All of the products were fully
charactrisized by FT-IR and NMR spectroscopy.
Spectral data of the new products:
14-(4-hydroxyphenyl)-14H-dibenzo[a,i]xanthene-8,13-dione
(4e) Red
solid;
m.p. = 265–267 °C; 1H NMR (400 MHz, CDCl3) 5.20 (1H, s), 5.72 (1H, s),
6.54–6.56 (2H, m), 6.63–6.64 (2H, m), 7.09–7.16 (4H, m), 7.31–7.56 (4H, m),
8.01–8.09 (2H, m). 13C NMR (100 MHz, CDCl3): 38.2, 110.1, 114.2, 116.1, 117.9,
118.6, 122.3, 124.2, 126.1, 126.6, 128.1, 128.9, 129.5, 130.3, 131.7, 135.4, 140.2,
151.8, 152.0, 155.9, 156.0, 159.9, 178.2, 183.0. FT-IR (KBr, cm-1): 3352 (O–H),
1660 (C = O), 1379 (C = C), 1220 (C-O), Anal. Calcd. For C27H16O4: C 80.19, H
3.99. Found C 80.27, H 3.92; MS (EI) (m/z): 404 (M?).
14-(4-isopropylphenyl)-14H-dibenzo[a,i]xanthene-8,13-dione (4l) Yellow powder;
1
m.p. = 190–193 °C; H NMR (400 MHz, CDCl3): 1.2–1.3 (6H, m), 2.74 (1H, s),
5.92 (1H, s), 7.03–7.05 (2H, m), 7.29–7.32 (2H, m), 7.43–7.50 (2H, m), 7.52–7.56
(2H, m), 7.58–7.60 (1H, m), 7.76–7.80 (2H, m), 7.84–7.86 (1H, m), 7.87–7.89 (1H,
m), 8.02–8.04 (1H, m). 13C NMR (100 MHz, CDCl3): 22.8, 32.7, 38.4, 108.5,
112.5, 115.9, 117.9, 118.3, 121.9, 125.4, 127.1, 127.9, 128.1, 129.9, 130.3, 132.7,
134.1, 136.3, 141.3, 144.9, 152.9, 157.1, 160.1, 177.9, 181.1. FT-IR (KBr, cm-1):
1691 (C = O), 1426 (C = C), 1226 (C-O); Anal. Calcd. For C30H22O3: C 83.70, H
5.15. Found C 83.64, H 5.21; MS (EI) (m/z): 430 (M?).
14-(4-formylphenyl)-14H-dibenzo[a,i]xanthene-8,13-dione
(4n) Yellow
solid;
1
m.p = 271–272 °C; H NMR (400 MHz, CDCl3): 5. 90 (1H, s), 7.15 (3H, t), 7.34
(3H, m), 7.45–7.62 (4H, m), 7.77–7.91 (4H, m), 9.81 (1H, s). 13C NMR (100 MHz,
CDCl3): 39.5,113.5, 115.1, 115.3, 116.6, 119.6, 123.1, 124.8, 126.8, 127.3, 128.9,
129.1, 130.5, 131.6, 132.3, 136.2, 142.4, 148.8, 152.8, 158.8, 160.3, 177.6, 184.1,
198.6. FT-IR (KBr, cm-1): 3078, 1692 (C = O), 1595 (C = C), 1232 (C-O); Anal.
Calcd. for C28H16O4: C 80.76, H 3.87. Found C 80.84, H 3.79; MS (EI) (m/z): 416
(M?).
14-(4-thiomethylphenyl)-14H-dibenzo[a,i]xanthene-8,13-dione (4m) Yellow solid;
1
m.p = 283–284 °C; H NMR (400 MHz, CDCl3): 2.32 (3H, s), 5.90 (1H, s), 7.31
(3H, m), 7.43–7.60 (4H, m), 7.76–7.89 (2H, m), 7.98 (3H, m), 8.12 (2H, m). 13C
NMR (100 MHz, CDCl3): 21.4, 35.1, 112.3, 114.7,115.11, 118.1, 121.9, 122.6,
123

Nano-Fe3O4 -encapsulated silica particles bearing sulfonic acid…
125.8, 126.5, 128.2, 129.1, 129.8, 130.3, 131.4, 133.6, 138.2, 140.25, 150.9, 152.75,
156.1, 159.7, 178.7, 182.4. FT-IR (KBr, cm-1): 1662 (C = O), 1367 (C = C), 1200
(C–O) cm-1; Anal. Calcd. for: C28H18O3S: C 77.40, H 4.18, S 7.38. Found: C 77.31,
H 4.28, S 7.31; MS (EI) (m/z): 434 (M?).
Results and discussion
In order to optimize the reaction conditions, the model reaction was carried out by
using 4-chlorobenzaldehyde, b-naphthol and 2-hydroxy-1,4-naphthoquinone under
various conditions (Scheme 3). The reaction conditions were optimized on the basis
of the solvent, catalyst, and different temperatures for synthesis of 14-(4-
chlorophenyl)-14H-dibenzo[a,i]xanthene-8,13-dione (4b).
The influence of the solvent was studied when the model reaction was performed
using Fe3O4@SiO2–SO3H NPs under various solvents and also solvent-free
conditions at different temperatures (Table 1). The best results were obtained
under reflux conditions (Table 1, entries 4–10). As shown in Table 1, in the reflux
conditions, the time of the reaction significantly decreased, but the yield of the
product formation increased in comparison with solvent-free conditions. Table 1
shows that the solvent has a great effect on the acceleration of the reaction. The best
results (97 % yield, 40 min) were obtained in water/ethanol under reflux for this
multicomponent reaction (Table 1, entry 4). The significant results presented in
Table 1 are related to the hydrogen bonding between water/ethanol and substrates
that promote the nucleophilic attack of the reactants.
Then, to show the merit of the present approach in comparison with other
catalysts, the model reaction was performed in the presence of various nanocatalysts
such as CuO, MgO, ZnO, CuI, Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2–SO3H. As
shown in Table 2, the Fe3O4@SiO2–SO3H NPs were the best catalysts with respect
to reaction time and yield of the obtained product.
In continuation of our research, we ran the study model using different amounts
of Fe3O4@SiO2–SO3H nanocatalyst under reflux conditions. No product was
obtained in the absence of the catalyst (Table 3, entry 1). As shown in Table 3, the
optimum concentration of Fe3O4@SiO2–SO3H NPs was obtained at 0.005 g in the
model reaction. Increasing the amount of the catalyst did not change the time and
yield of the reaction (Table 3, entry 5).
Scheme 3 The model reaction for the synthesis of 14-(4-chlorophenyl)-14H-dibenzo[a,i]xanthene-8,13-
dione in the presence of Fe3O4@SiO2–SO3H NPs
123

M. A. Ghasemzadeh, M. Azimi-Nasrabad
Table 1 Preparation of 14-(4-
chlorophenyl)-14H-
Entry
Solvent
T/°C
Time (min)
Yield (%)a
dibenzo[a,i]xanthene-8,13-dione
in different solvents
1
2
Solvent-free
Solvent-free
Water/EtOH
Water/EtOH
EtOH
25
120
100
95
Trace
25
100
3
25
35
4
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
40
97
5
60
60
6
Water
80
54
7
CH3CN
120
140
150
160
45
Reaction conditions: b-naphthol
(1 mmol), 2-hydroxy-1,4-
naphthoquinone (1 mmol) and
4-chlorobenzaldehyde (1 mmol)
8
DMF
38
9
CH2Cl2
35
10
Toluene
30
a
Isolated yields
Table 2 The model reaction
was carried out by various
catalysts
Entry
Catalyst
Time (min)
Yield (%)a
1
2
3
4
5
6
7
Fe3O4
MgO
60
120
75
60
25
65
38
70
85
97
ZnO
CuO
100
80
CuI
Water/ethanol as solvent under
reflux conditions
Fe3O4@SiO2
55
Fe3O4@SiO2–SO3H
40
a
Isolated yields
Table 3 The effect of the
amount of Fe3O4@SiO2–SO3H
on the model reaction
Entry
Catalyst
amount (gr)
Time (min)
Yield (%)a
1
2
3
4
5
None
0.001
0.003
0.005
0.006
240
90
None
30
70
68
Water/ethanol as solvent under
reflux conditions
40
97
40
97
a
Isolated yields
After optimization of the reaction conditions, we used a diversity of aldehydes to
investigate three-component reactions in the presence of Fe3O4@SiO2–SO3H NPs
under reflux conditions. We observed that various aryl aldehydes could be
introduced at a high efficiency and produced high yields of products at a high purity
(-97 % by 1H NMR). When this research was carried out using aliphatic aldehyde
1
including butanal and pentanal, TLC and H NMR spectra of the reaction mixture
showed a combination of the starting materials and numerous products, the yield of
the expected product was very poor. Data in Table 4 show that sterically hindered
aromatic aldehydes required longer reaction times in comparison with p-substituted
aryl aldehydes. In addition, aromatic aldehydes bearing electron-withdrawing
groups such as Cl, Br, and NO2 in the p-position reacted very smoothly, in short
123

Nano-Fe3O4 -encapsulated silica particles bearing sulfonic acid…
Table 4 Fe3O4@SiO2–SO3H NPs catalyzed one-pot synthesis of 14-aryl-14H-dibenzo[a,i]xanthene-
8,13-dione derivatives
Entry
Ar
Product
Time(min)
Yield (%)a
m.p. °C
Lit. m.p.oC
1
C6H5
4a
4b
4c
4d
4e
4f
60
40
55
70
70
65
40
75
70
50
45
65
60
50
91
97
94
90
90
92
95
87
88
96
95
90
92
94
317–319
303–305
290–295
256–258
265–269b
280–282
332–335
280–282
300–303
304–305
303–306
190–195b
283–284b
271–272b
(319–320) [34]
(305–306) [34]
(284–295) [40]
(255–256) [34]
–
2
4-ClC6H4
3
4-BrC6H4
4
4-CH3C6H4
4-HOC6H4
4-MeOC6H4
4-O2NC6H4
2-ClC6H4
5
6
(279–280) [31]
(332–333) [34]
(281–282) [37]
(301–302) [37]
([300) [40]
(304–305) [37]
–
7
4g
4h
4i
8
9
2,4-Cl2–C6H3
4-FC6H4
10
11
12
13
14
4j
3-O2NC6H4
4-(CH3)2CHC6H4
4-MeSC6H4
4-OHCC6H4
4k
4l
4m
4n
–
–
Reaction conditions: aldehyde (1 mmol), 2-naphthol (1 mmol), 2-hydroxy-1,4-naphthoquinone (1 mmol)
and Fe3O4@SiO2–SO3H NPs (0.005 g)
a
Isolated yield
b
New compounds
Table 5 Recoverability of Fe3O4@SiO2–SO3H NPs
Cycle
First
97
Second
95
Third
90
Fourth
88
Fifth
82
Yield (%)a
a
Yields refer to the isolated pure product
reaction times and with high yields while reactants with electron-releasing groups,
such as methoxy and isopropyl, decreased both the rate of the reaction and the yield
of corresponding product, as shown in Table 4.
After completion of the reaction, the reaction mixture was dissolved in chloroform
and the catalyst was then separated magnetically. The Fe3O4@SiO2–SO3H NPs were
washed three to four times with chloroform and methanol and dried at 60 °C for 8 h.
The separated catalyst was used for six cycles with a slightly decreased activity, as
shown in Table 5. In conclusion, we were able to demonstrate that a range of 14-aryl-
14H-dibenzo[a,i] xanthene-8,13-dione derivatives could be obtained by the catalytic
application of Fe3O4@SiO2–SO3H NPs under reflux conditions.
Proposed mechanism
A plausible mechanism for the synthesis of 14-aryl-14H-dibenzo[a,i]xanthene-8,13-
diones using Fe3O4@SiO2–SO3H NPs has been shown in Scheme 4, on the basis of
123

M. A. Ghasemzadeh, M. Azimi-Nasrabad
Scheme 4 The proposed mechanism for the synthesis of 14-aryl-14H-dibenzo[a,i]xanthene-8,13-diones
catalyzed by Fe3O4@SiO2–SO3H NPs
our experimental results together with some literature (Scheme 4) [36, 37]. The
reaction proceeds via a reaction sequence of condensation, addition, cyclization and
dehydration. First, the condensation of aldehyde and 2-naphthol gave the
intermediate (A). The addition of 2-hydroxynaphthalene-1,4-dione to (A) led to
the formation of (B), which, after intermolecular cyclization and dehydration, gave
rise to the desired 14-aryl-14H-dibenzo[a,i]xanthene-8,13-dione derivatives.
Conclusions
We have developed a novel and highly efficient method for the synthesis of 14-aryl-
14H-dibenzo[a,i]xanthene-8,13-dione derivatives by treatment of b-naphthol and
aromatic aldehydes with 2-hydroxy-1,4-naphthoquinone in the presence of
Fe3O4@SiO2–SO3H NPs as the catalyst. This active catalyst is thermally stable,
environmentally friendly (‘‘green’’), inexpensive, and easy to prepare. In addition, it
could be easily separated from the reaction mixture and recycled without any
significant impact on its activity or the reaction yields. The operational simplicity,
high yields, and facile work-up procedures associated with this catalytic process
represent some of the other advantages of this methodology.
Acknowledgments The authors gratefully acknowledge the financial support of this work by the
Research Affairs Office of the Islamic Azad University, Qom Branch, Qom, I. R. Iran.
123

Nano-Fe3O4 -encapsulated silica particles bearing sulfonic acid…
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