MgBr2 supported on Fe3O4@SiO2?~?urea nanoparticle: An efficient catalyst for ortho-formylation of phenols and oxidation of benzylic alcohols

Article abstract of DOI:10.1002/aoc.4006

Urea was successfully immobilized on the surface of chloropropyl-modified Fe₃O₄@SiO₂ core–shell magnetic nanoparticles, then supported by MgBr₂ and acts as a unique catalyst for oxidation of benzylic alcohols to aldehydes and ketones, and ortho-formylation of phenols to salicylaldehydes. The prepared catalyst was characterized by FT-IR, transmission electron microscopy, scanning electron microscopy, X-ray powder diffraction, dispersive X-ray spectroscopy, CHN and TGA. It was found that Fe₃O₄@SiO₂?~?urea/MgBr₂ showed higher catalytic activity than homogenous MgBr₂, and could be reused several times without significant loss of activity.

Full text of DOI:10.1002/aoc.4006

Received: 12 May 2017
DOI: 10.1002/aoc.4006
Revised: 16 June 2017
Accepted: 29 June 2017
F U L L P A P E R
MgBr2 supported on Fe3O4@SiO2 ~ urea nanoparticle: An
efficient catalyst for ortho‐formylation of phenols and
oxidation of benzylic alcohols
Ebrahim Soleimani
| Nadieh Yaesoobi | Hamid Reza Ghasempour
Department of Chemistry, Razi University,
Urea was successfully immobilized on the surface of chloropropyl‐modified
Fe3O4@SiO2 core–shell magnetic nanoparticles, then supported by MgBr2
and acts as a unique catalyst for oxidation of benzylic alcohols to aldehydes
and ketones, and ortho‐formylation of phenols to salicylaldehydes. The
prepared catalyst was characterized by FT‐IR, transmission electron
microscopy, scanning electron microscopy, X‐ray powder diffraction,
dispersive X‐ray spectroscopy, CHN and TGA. It was found that
Kermanshah 67149‐67346, Iran
Correspondence
E. Soleimani, Department of Chemistry,
Razi University, Kermanshah 67149‐
67346, Iran.
Email: e_soleimanirazi@yahoo.com
Fe3O4@SiO2
~
urea/MgBr2 showed higher catalytic activity than
homogenous MgBr2, and could be reused several times without significant
loss of activity.
KEYWORDS
alcohol, formylation, magnetite nanocatalyst, MgBr2, oxidation, salicylaldehyde
1 | INTRODUCTION
nanoparticles Fe3O4 (MNPs‐Fe3O4) have attracted addi-
tional attention due to their convenient isolation and
In recent years, much attention has been focused on het-
erogeneous catalysts due to their potential applications for
replacing homogenous catalysts in organic chemistry and
industry; they exhibit advantages of easy separation of the
catalyst from the reaction medium, minimal corrosion,
simplified recovery, reusability, green chemical processes
and enhanced product selectivity.[1,2] Among heteroge-
neous catalysts, supported catalysts on a solid support
have attracted significant attention due to a number of
advantages, such as their available active sites, stability,
and product separation and their recovery, which are all
important factors in organic chemistry and industry.[3,4]
Supported catalysts are available on different supports,
such as charcoal, alumina, silica and polymers; however,
silica displays many advantageous properties, such as
excellent stability (chemical as well as thermal), no swell-
ing, high surface area, good accessibility, and in addition
organic groups can be robustly anchored to the surface
to provide catalytic center.[5] Recently, magnetic
recovery.[6] It has been reported that the heterogeneous
catalysts supported on MNPs‐Fe3O4 reveal excellent per-
formance in many reactions, including hydrolysis, hydro-
genation, oxidation, carbon–carbon coupling and
reduction.[7]
Salicylic aldehydes are excellent precursors for the
preparation of important classes of organic compounds,
such as coumarins, benzofurans, salen derivatives and
industrially useful metal extractants.[8] Numerous
methods
are
available
for
preparation
of
salicylaldehydes; however, formylation of phenols is an
important reaction for the synthesis of salicylaldehydes.
Moreover, many of the methods involve the use of nox-
ious reagents, harsh reaction conditions, low yields and
lack of regioselectivity.[9] An exception to this is the
formylation of phenols using the acceptable reagents
MgCl2, Et3N and paraformaldehyde, giving high yields
of salicylaldehydes from reactions exclusively at the
ortho‐position.[10]
Appl Organometal Chem. 2017;e4006.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4006

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SOLEIMANI ET AL.
Magnesium bromide (MgBr2) is commercially avail-
2 | RESULTS AND DISCUSSION
able, inexpensive and generates no toxic byproducts on
aqueous work‐up, which would allow us to conduct the
reactions open to the atmosphere using untreated,
reagent‐grade solvents. MgBr2 has found many applica-
tions in synthetic organic transformations as a mild Lewis
acid in Diels–Alder cycloadditions,[11] Cannizzaro reac-
tions,[12] aldol condensation,[13] α‐aminonitrile synthe-
ses,[14] alcohols protection,[15] and thiolysis of
epoxides[16] and ring opening of epoxides.[17]
The use of MgCl2 or MgBr2 has received considerable
attention as an inexpensive, nontoxic, commercially avail-
able catalyst for formylation of phenols. However, in spite
of their potential utility, these homogeneous catalysts
present limitations due to the use of toxic and corrosive
reagents, the tedious work‐up procedure, the necessity of
neutralization of the strong acid media producing unde-
sired washes, long reactions times and high temperature.
Therefore, the discovery of a novel and inexpensive cata-
lyst, which can be easily separated, reused and does not
become contaminated by the products, is of prime impor-
tance. Therefore, we have prepared a magnetic heteroge-
2.1 | Preparation and characterization of
the catalyst (Fe3O4@SiO2 ~ urea/MgBr2)
The Fe3O4@SiO2 ~ urea/MgBr2 nanocatalyst was pre-
pared with a four‐step reaction; initially by preparing iron
oxide as a magnetic core for coating by a silica layer
through sol–gel process. For immobilization of the
organic catalyst on the surface of inorganic support, the
surface was first modified using an appropriate coupling
agent to make covalent bonds with the catalyst. It is
known that organosilanes can act as a linker between
the organic catalyst and the support. In this work,
chloropropyltriethoxysilane was used. Scheme 1 illus-
trates the process of catalyst preparation. A part of the
linker's chloride was replaced by nitrogen of the urea via
an SN2 nucleophilic substitution reaction. Then, MgBr2
was attached onto the surface by interaction between
Mg2+ and electron paired nitrogens from urea. The cata-
lyst preparation steps are illustrated in Scheme 2. To sub-
stantiate this claim, each step of the catalyst preparation
procedure was separated and examined by characteriza-
tion techniques such as FT‐IR to ensure the presence of
new functional groups. Eventually, the nanocatalyst was
studied from different aspects, such as size, size distribu-
tion and morphology, and characterized by different tech-
niques, such as X‐ray powder diffraction (XRD),
transmission electron microscopy (TEM), scanning elec-
tron microscopy (SEM), FT‐IR, dispersive X‐ray spectros-
copy (EDX), CHN and TGA.
neous catalyst of Fe3O4@SiO2
~ urea/MgBr2 by
supporting MgBr2 on Fe3O4@SiO2 ~ urea. To check the
applicability of this catalyst, we chose oxidation of ben-
zylic alcohols and ortho‐formylation of phenols under this
new catalyst.
The oxidation of alcohols is one of the most impor-
tant organic transformation reactions, and studies on
the mild oxidation methods of alcohols with less toxic
reagents have been carried out extensively. Among
these, selective oxidation of benzyl alcohols to benzalde-
hydes with H2O2 on magnetic iron oxides is more
appealing because of its many attractive advantages,
such as low cost, wide availability, good biocompatibil-
The crystalline structure of magnetite NPs before and
after silica coating was identified using the XRD tech-
nique. Figure 1 (see Supporting Information) shows the
ity,
high
stability
and
convenient
magnetic
separation.[18]
Due to above‐mentioned reasons, and as a part of
our ongoing research on new nanomagnetic heteroge-
neous catalysts in the synthesis of organic com-
pounds,[19] we report herein a mild and efficient
approach for the synthesis of salicylaldehydes 3 via
ortho‐formylation of phenols 1 with paraformaldehyde
2 using a catalytic amount of Fe3O4@SiO2 ~ urea/MgBr2
and triethylamine in acetonitrile at 70 °C in high yields
(Scheme 1).
SCHEME
2
The schematic pathway for synthesis of
Fe3O4@SiO2 ~ urea/MgBr2 core–shell NPs
SCHEME 1 Synthesis of salicylaldehydes

SOLEIMANI ET AL.
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FIGURE 1 XRD patterns of: (a) Fe3O4;
(b) Fe3O4@SiO2; (c) Fe3O4@SiO2 ~ Cl; (d)
Fe3O4@SiO2 ~ urea; (e)
Fe3O4@SiO2 ~ urea/MgBr2. The arrow
mark indicates the (101) diffraction peak
due to MgBr2
XRD patterns of these samples. The XRD pattern of Fe3O4
clearly showed six reflection peaks [2θ at 30°, 36.0°, 43°,
54.0°, 57.5° and 62.5° refer to (220), (311), (400), (422),
(511) and (440)] and confirmed the formation of a cubic
spinel ferrite structure, and the crystallite size was calcu-
lated from the Scherrer's formula and found to be an aver-
age diameter of about 20 nm (Figure 1a). Furthermore, no
change was observed in the crystalline structure of the
Fe3O4 core upon coating and immobilization of the mag-
the asymmetric stretching vibration and symmetric
stretching vibration of Si–O–Si. The band at 960 cm−1 is
assigned to the symmetric stretching vibration of Si–OH.
Figure 2c shows the FT‐IR spectrum of Fe3O4@SiO2 ~ Cl.
In Figure 2c, in addition to bands of Fe–O bond, Si–OH
and Si–O–Si, newer bands at 2847 cm−1 and 2926 cm−1
are related to the stretching vibrations of C–H bonds of
CPTES, while peaks at 638 cm−1 and 689 cm−1 are ascribed
to the stretching vibrations of C–Cl bonds of CPTES.
The FT‐IR spectra of Fe3O4@SiO2 ~ urea and
Fe3O4@SiO2 ~ urea/MgBr2 are shown in Figure 3. The
spectra of as‐prepared Fe3O4@SiO2 ~ urea showed a band
due to C = O stretching vibration at 1559 cm−1, N‐C‐N
stretching vibration at 1430 cm−1 and N‐H stretching
vibration at 3410 cm−1. The major peaks of urea over-
lapped with broad and strong bands of Si–O–Si, intermo-
lecular water and moisture (H–OH, bending) in the
samples. Also, this peak appeared in the FT‐IR spectrum
of Fe3O4@SiO2 ~ urea/MgBr2, and showed a new peak
due to MgBr2 at 950 cm−1. For confirmation of the
functionalization processes and the presence of urea and
MgBr2 on the surface, other analyses such as CHN, EDX
and TGA analyses were required.
netite
surface
by
silica
(Figure
1b),
3‐
chloropropyltriethoxysilane (CPTES; Figure 1c), urea
(Figure 1d) and MgBr2 (Figure 1e). After immobilization
of MgBr2 onto the surface of MNPs, the new diffraction
peak appeared at 21.1° and 24.0° attributed to MgBr2.
Figures 2 and 3 (also see Supporting Information) show
the FT‐IR spectra of Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2 ~ Cl,
Fe3O4@SiO2 ~ urea and Fe3O4@SiO2 ~ urea/MgBr2. In
Figure 2a, the bands at 458 cm−1, 574 cm−1 and 620 cm−1
are attributed to the vibration of the Fe–O bond, while
the band at 3421 cm−1 is assigned to the symmetrical
stretching vibration of hydroxyl groups (−OHs), indicating
the presence of some amount of ferric hydroxide in Fe3O4.
The band at 1620 cm−1 is attributed to the bending vibra-
tion of adsorbed water. Figure 2b shows the FT‐IR spec-
trum of Fe3O4@SiO2 core‐shell MNPs. Comparing
Figure 2b with Figure 2a, we found some new absorption
peaks in Figure 2b. The bands at 1089 cm−1 are related to
Transmission electron microscopy was used to observe
the morphology of the four‐stage synthesis of the catalyst.
Figure 4a shows that the NPs have a core–shell structure
with a distinct contrast between silica shells and Fe3O4
FIGURE 2 FT‐IR spectra of Fe3O4,
Fe3O4@SiO2 and Fe3O4@SiO2 ~ Cl

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SOLEIMANI ET AL.
FIGURE 3 FT‐IR spectra of
Fe3O4@SiO2 ~ urea and
Fe3O4@SiO2 ~ urea/MgBr2
FIGURE 4 TEM images of: (a)
Fe3O4@SiO2; (b) Fe3O4@SiO2 ~ Cl; (c)
Fe3O4@SiO2 ~ urea; (d)
Fe3O4@SiO2 ~ urea/MgBr2; and SEM
images of: (e) Fe3O4@SiO2 ~ urea; (f)
Fe3O4@SiO2 ~ urea/MgBr2

SOLEIMANI ET AL.
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cores, implying that silica shells (Fe3O4@SiO2) success-
fully coated the hydrophilic Fe3O4 NPs. The average size
of the cores was found to be ~10–15 nm, and the results
are consistent with the crystallite size of 12 nm for
Fe3O4 NPs obtained from the XRD results. The thickness
of the silica shell was estimated to be about 20–30 nm.
Therefore, the average size of magnetite‐silica core–shell
NPs was determined to be about 50–80 nm. Similarly,
the TEM images of Fe3O4@SiO2 ~ Cl, Fe3O4@SiO2 ~ urea
and Fe3O4@SiO2 ~ urea/MgBr2 are shown in Figure 4b–d,
respectively. These images reveal that the modified mag-
netic silica Fe3O4@SiO2 ~ urea and Fe3O4@SiO2 ~ urea/
MgBr2 NPs are similar in size (~55–85 nm) and are spher-
ical in shape. The SEM image of Fe3O4@SiO2 ~ urea and
Fe3O4@SiO2 ~ urea/MgBr2 in Figure 4e and f shows the
granular and spherical morphology for these NPs. The
average size of ~44–100 nm obtained by SEM analysis is
consistent with the TEM results.
with urea. This indicated the successful attachment of
urea onto the surface of the modified NPs. Also, the ele-
mental analysis of Fe3O4@SiO2 ~ urea/MgBr2 showed a
decrease in content of these elements because the inor-
ganic content of NPs was increased after immobilization
of MgBr2 on the surface. Therefore, elemental analysis
(CHN) of MNPs confirmed the presence of urea and
MgBr2 on the surface.
TGAs of Fe3O4@SiO2 ~ Cl, Fe3O4@SiO2 ~ urea and
Fe3O4@SiO2
~ urea/MgBr2 NPs are presented in
Figure 6 (see Supporting Information). The initial wt%
losses below 150 °C are related to the adsorbed moisture
in the samples. The decomposition temperature, which
started at 460 °C, and continued until 530 °C, is associated
with the decomposition of the organic chains. The organic
species decomposed completely at temperatures higher
than 600 °C, and the residual weights of Fe3O4@SiO2 ~ Cl,
Fe3O4@SiO2 ~ urea and Fe3O4@SiO2 ~ urea/MgBr2 were
90%, 86% and 91%, respectively, at 700 °C. In other words,
the results demonstrate percentages of 10%, 4% and 9% for
organic groups on the surface of Fe3O4@SiO2 ~ Cl,
Fe3O4@SiO2 ~ urea and Fe3O4@SiO2 ~ urea/MgBr2,
respectively. The Fe3O4@SiO2 ~ urea/MgBr2 NPs showed
a lower percentage of organic groups on the surface due to
the presence of MgBr2 on the surface.
Furthermore, the EDX in Figure 5 and Table 1 (see
Supporting Information) confirmed the presence of
MgBr2 and urea on surface Fe3O4@SiO2.
From the elemental analysis (CHN) in Table 2 (see
Supporting Information), it was observed that there was
an increase in carbon content, and in hydrogen and nitro-
gen content after functionalization of the Fe3O4@SiO2 ~ Cl
FIGURE 5 EDX spectra of: (a) Fe3O4@SiO2 ~ urea; and (b) Fe3O4@SiO2 ~ urea/MgBr2

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SOLEIMANI ET AL.
TABLE 1 EDX data of Fe3O4@SiO2 ~ urea and
temperatures
using
identical
amounts
of
Fe3O4@SiO2 ~ urea/MgBr2 (wt%)
Fe3O4@SiO2 ~ urea/MgBr2 for a fixed reaction time of 1 h
in acetonitrile (entries 4–7). The reaction rate increased
as the temperature was raised. At 70 °C, the maximum
yield (97%) was obtained in a reaction time of 1 h (entry
6). The model reaction in acetonitrile at 70 °C was also
studied using different amounts of Fe3O4@SiO2 ~ urea/
MgBr2 (entries 6, 8, 9). The best results were obtained with
0.05 g of Fe3O4@SiO2 ~ urea/MgBr2 (entry 6). Also, when
the reaction was under solvent‐free conditions, the reac-
tion yield was only 51% after 12 h (entry 10). Further work
indicated that the best results were obtained when the
reaction was carried out at 70 °C for 1 h in acetonitrile
using 0.05 g of Fe3O4@SiO2 ~ urea/MgBr2 (entry 6).
With the optimized condition established above, we
next attempted to extend various types of phenols; the
results are summarized in Table 4. In all cases, excellent
yields were obtained.
The recyclability and reusability of the catalyst was
studied by using a model reaction. This heterogeneous
catalyst can be easily recovered with an external magnet
and reused for at least five cycles without a significant
decrease in the catalytic activity and performance
(Figure 7; see Supporting Information).
To show the efficiency of Fe3O4@SiO2 ~ urea/MgBr2 in
comparison with other reported catalysts, we summarized
some of the results for the synthesis of coumarins obtained
by other workers. It is clear from Table 5v that the current
method is simpler, more efficient and less time consuming
forthesynthesisofcoumarinderivatives.Inaddition,inthis
method, this heterogeneous catalyst (Fe3O4@SiO2 ~ urea/
MgBr2) was recovered easily and was reused without a sig-
nificant decrease in the catalytic activity.
Element
(wt%)
Fe3O4@SiO2 ~ urea/
MgBr2
Fe3O4@SiO2 ~ urea
C
2.97
1.54
N
0.75
17.35
–
0.57
14.77
4.55
O
Mg
Si
15.59
63.35
–
11.79
60.69
6.19
Fe
Br
Total
100.00
100.00
TABLE 2 The elemental analysis (CHN) results
Elements
Fe/
SiO2 ~ Cl
Fe/
SiO2 ~ urea
Fe/SiO2 ~ urea/
MgBr2
(mg g−1
)
N
0
30.232
2.071
5.714
37.623
4.430
2.012
23.914
1.584
C
H
2.2 | Catalytic activity of
Fe3O4@SiO2 ~ urea/MgBr2 MNPs in the
synthesis of salicylaldehydes
We chose the reaction of phenol and paraformaldehyde as
a model system for the optimization study (Table 3). First,
we compared the reaction rate in different solvents, such as
water, dichloromethane, toluene and acetonitrile, by mea-
suring the isolated yield using identical amounts of
Fe3O4@SiO2 ~ urea/MgBr2 for a fixed reaction time of 24
h at room temperature (entries 1–4). The desired product
was obtained in acetonitrile and toluene (entries 3 and 4),
but the best result was obtained when acetonitrile was uti-
lized. Next, we studied the model reaction at different
Finally, for further exploration of the scope and limita-
tion of the method, we investigated the oxidation of differ-
ent benzylic alcohols with this new catalyst in the
presence of hydrogen peroxide. Gratifyingly the reaction
FIGURE 6 TGA curves of
Fe3O4@SiO2 ~ Cl, Fe3O4@SiO2 ~ urea and
Fe3O4@SiO2 ~ urea/MgBr2

SOLEIMANI ET AL.
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TABLE 3 Optimization of reaction conditions for synthesis of salicylaldehydea
Entry
Fe3O4@SiO2 ~ urea/MgBr2 (g)
Solvent
Temperature (°C)
Time (h)
Yield%b
1
0.05
H2O
r.t.
12
Trace
2
0.05
0.05
0.05
0.05
0.05
0.05
0.07
0.03
0.05
Dichloromethane
Toluene
r.t.
r.t.
r.t.
50
70
80
70
70
80
12
24
24
1
Trace
23
3
4
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Solvent‐free
38
5
60
6
1
97
7
1
97
8
1
97
9
1
83
10
12
51
aReaction conditions: phenol (1.0 mmol), paraformaldehyde (4 mmol) and triethylamine (1.5 mmol).
bIsolated yields.
led to the formation of the corresponding aromatic alde-
hydes or ketones in high isolated yields (Table 6).
three‐neck flask by ultrasonic treatment for 30 min.
Then, 50 ml deionized water, 3.0 ml TEOS (silica pre-
cursor) and 17.0 ml ammonia (25 wt%) were added
into the mixture at 30 °C under mechanical agitation.
After 12 h, the final product (Fe3O4@SiO2) was col-
lected, washed with ethanol and deionized water, and
dried at 50 °C. The silica‐coated Fe3O4 (Fe3O4@SiO2)
NPs were then functionalized using a post‐modifica-
tion method with CPTES[26] by refluxing in anhydrous
toluene at 110 °C for 24 h (Fe3O4@SiO2 ~ Cl). In brief,
2 g of as‐synthesized Fe3O4@SiO2 NPs was dispersed
in 25 ml of CPTES solution [4%, v/v in dry toluene
(99%)]. The mixture was stirred for 24 h under a reflux
at 110 °C. After cooling, the sample was collected
from the solution using an external magnet, and
washed thoroughly twice with ethanol and once with
distilled water to remove unreacted CPTES. The solid
residue was dried under vacuum conditions at
100 °C for 24 h.
2.3 | Proposed mechanism
A possible mechanism for salicylaldehyde derivatives by
MgCl2 has been proposed previously.[21] Scheme 3 shows
a possible mechanism for this synthesis in the presence
of Fe3O4@SiO2 ~ urea/MgBr2. We expected the mecha-
nism by Fe3O4@SiO2 ~ urea/MgBr2 and MgCl2 to be the
same.
3 | EXPERIMENTAL
3.1 | Preparation of modified silica‐coated
magnetite NPs (Fe3O4@SiO2 ~ Cl)
Magnetite (Fe3O4) NPs were prepared according to the
reported method (co‐precipitation),[24] and coated with
silica by tetraethyl orthosilicate (TEOS) in the presence
of 2‐propanol (sol–gel method).[25] In brief, 0.036 mol
of FeCl3H2O and 0.018 mol of FeCl24H2O were dis-
solved in 20 ml deionized water under nitrogen gas
with vigorous stirring. Then, NH3 (25%) was added
into the solution until the pH of the solution reached
11. Stirring was continued for 1 h at 60 °C. The color
of the bulk solution turned from orange to black
immediately. The magnetite precipitate was separated
from the solution using a magnet, washed several
times with deionized water and ethanol, and left to
dry in air. Then, 0.3 g as‐synthesized Fe3O4 NPs were
mixed with 100 ml isopropyl alcohol in a sealed
3.2 | Immobilization of urea on the
surface of modified silica‐coated magnetite
NPs (Fe3O4@SiO2 ~ urea)
Urea (0.6 g, 10 mmol) was added into a suspension of
Fe3O4@SiO2 ~ Cl (1.0 g), KI (0.16 g, 1 mmol) and
K2CO3 (0.14 g, 1 mmol) in acetonitrile (30 ml). The reac-
tion mixture was refluxed at 80 °C in an oil bath for 24
h. The solid phase was filtered by applying an external
magnet and washed with acetonitrile, followed by dis-
tilled water. Finally, the solid sample was dried at
100 °C for 24 h.

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SOLEIMANI ET AL.
a
TABLE 4 Synthesis of salicylaldehyde derivatives by catalyst Fe3O4@SiO2 ~ urea/MgBr2
Entry
Substrate
Product
Time (h)
Yield (%)b
1
1
97
2
3
4
1.5
1
91
89
90
2.5
5
6
2
1
94
92
7
8
9
1
1
1
74
80
96
aReaction condition: phenol (1.0 mmol) and paraformaldehyde (4 mmol) and triethylamine (1.5 mmol).
bIsolated yields.
FIGURE 7 Reusability of the catalyst
for synthesis of salicylaldehydes (model
reaction: entry 1, Table 4)

SOLEIMANI ET AL.
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TABLE 5 Comparison of the efficiencies of various catalysts used in ortho‐formylation of phenols to salicylaldehydes
Catalyst
Condition
Time (h)
Yield (%)
Reference
[20]
Montmorillonite KSF
Toluene 100 °C,
4
65–70
[21]
[22]
[23]
MgCl2
Acetonitrile, reflux
1.5–44
5–99
SnCl4
Toluene 100 °C, under an argon atmosphere
Toluene 90 °C, HMPA
8
37–62
Up to 40
74–97
CH3CH2MgBr
Fe3O4@SiO2 ~ urea/MgBr2
3
Acetonitrile, 90 °C
1–2.5
This work
ml) and the Fe3O4@SiO2 ~ urea/MgBr2 nanocatalyst
(0.05 g) in acetonirile (5 ml) was heated at 70 °C in a
round‐bottom flask in N2 atmosphere under efficient
stirring. After 1 h, the nanocatalyst was separated from
the reaction mixture by applying an external magnet,
and further washed with ethanol and reused in subse-
quent reactions five times without any loss of activity.
Then, the solvents were evaporated and the crude was
purified by recrystallization over acetone. All products
are known compounds, which were characterized by
FT‐IR and spectral data, and their mp values were com-
pared with literature reports.
3.3 | Anchoring the magnesium bromide
onto the Fe3O4@SiO2 ~ urea
(Fe3O4@SiO2 ~ urea/MgBr2)
A quantity of 1 g of as‐synthesized Fe3O4@SiO2 ~ urea
NPs was dispersed in 10 ml of acetone solution containing
0.19 g of magnesium bromide. The reaction mixture was
stirred for 12 h at room temperature. The solid phase
was filtered by applying an external magnet and washed
twice with deionized water. Then, the solid sample was
dried at 100 °C for 12 h.
3.4 | Oxidation of benzylic alcohols to
aldehydes or ketones in the presence of
Fe3O4@SiO2 ~ urea/MgBr2 as a recyclable
catalyst
3.6 | Characterization techniques
Infrared spectra were recorded using a Ray Leigh Wqf‐
510 FT‐IR spectrophotometer. Morphological study mea-
surements were performed using a TEM (Philips CM10)
operated at an 80 kV electron beam accelerating voltage.
SEM and EDX were performed on a Philips XL‐300
instrument. The sample was sputtered by gold to avoid
undesirable electron charging. XRD was conducted
using a Philips X'pert Pro (PW 3040) X‐ray diffractome-
ter with monochromatic Cu‐Ka radiation (k¼
1.54056 °A, 40 kV, 30 mA). TGAs were investigated
using a TG/DTA6300 instrument, at a heating rate of
20 °C min−1 under nitrogen flow. Elemental analysis
was performed with a CHN machine (Perkin Elmer
To a solution of benzylic alcohol (1.0 mmol) in aqueous
hydrogen
peroxide
(3 ml,
30%)
was
added
Fe3O4@SiO2 ~ urea/MgBr2 nanocatalyst (0.02 g) and
the reaction mixture was stirred at 60 °C. After 2 h, the
nanocatalyst was separated from the reaction mixture
by applying an external magnet, and further washed
with ethanol and reused in subsequent reactions five
times without any loss of activity. Then, the organic
layer was separated and the aqueous layer was extracted
with ethylacetate (2 × 3 ml). The combined ethylacetate
layer was washed with saturated brine (2 × 5 ml) and
dried with anhydrous MgSO4. After evaporation of the
solvent, the residue was purified by column chromatog-
raphy (SiO2, hexane–EtOAc = 3:1) to afford pure car-
bonyl compound. All products are known compounds,
which were characterized by FT‐IR and spectral data,
and their mp values were compared with literature
reports.
1
Series II, 2400). H and 13C NMR spectra were recorded
on a Bruker DRX‐400 spectrometer at 400 and 100 MHz,
respectively. All of the chemicals were purchased from
Fluka, Merck and Aldrich, and used without further
purification.
4 | CONCLUSION
3.5 | Synthesis of salicylaldehyde in the
presence of Fe3O4@SiO2 ~ urea/MgBr2 as a
recyclable catalyst
This work reported on the preparation of
Fe3O4@SiO2 ~ urea/MgBr2 via functionalization of silica‐
coated magnetic NPs with CPTES as a linker for the immo-
bilization of urea and then incorporation of MgBr2 on the
surface of urea by an electron interaction between Mg2+
A solution of the selected phenol (1 mmol), paraformal-
dehyde (4 mmol, 0.12 g), triethylamine (1 mmol, 0.14

10 of 12
SOLEIMANI ET AL.
TABLE 6 Selective oxidation of benzylic alcohol to aromatic aldehyde or ketonea
Entry
Substrate
Product
Time (h)
Yield (%)b
1
2
97
2
1.5
98
3
4
5
6
1.5
2
93
92
93
85
2
2
7
2
91
8
9
2
2
94
93
10
11
1.5
1.5
92
93
aReaction condition: Benzylic alcohol (1.0 mmol), H2O2 (3 mL), Fe3O4@SiO2~Urea/MgBr2 (0.05 g) in acetonitrile solvent.
bIsolated yields.
and N or NH of urea. The core–shell NPs were stable and
reusable, and the non‐toxic and inexpensive
heterogeneous nanocatalyst prepared has great potential
for applications in organic syntheses. The

SOLEIMANI ET AL.
11 of 12
SCHEME 3 Proposed mechanism for
synthesis of salicylaldehydes by
Fe3O4@SiO2 ~ urea/MgBr2
[7] (a) V. Polshettiwar, R. Luque, A. Fihri, H. Zhu, Chem. Rev.
2011, 111, 3036. (b) B. Dam, S. Nandi, A. K. Pal, Tetrahedron
Lett. 2014, 55, 5236.
Fe3O4@SiO2 ~ urea/MgBr2 NPs were developed as a cata-
lyst for oxidation of benzylic alcohols to aromatic alde-
hydes and ketones, and ortho‐formylation of phenols to
salicylaldehydes. The magnetically recoverable catalyst
was found to be capable of repeated reuse without signifi-
cant loss of activity.
[8] (a) V. Lehnert, Tetrahedron 1972, 28, 663. (b) P. Nore, E. J.
Honkanen, Heterocyclic Chem 1980, B, 985. (c) J. F. Larrow, S.
E. Schaus, E. N. Jacobsen, J. Am. Chem. Soc. 1996, 118, 7420.
(d) R. W. Quan, Z. Li, E. N. Jacobsen, J. Am. Chem. Soc. 1996,
118, 8156.
ACKNOWLEDGEMENTS
[9] (a) G. A. Olah, L. Ohannasian, M. Arvanaghi, Chem. Rev. 1987,
87, 671. (b) H. Gilman, R. L. Bebb, J. Am. Chem. Soc. 1939, 61,
109. (c) T. Laird, in Comprehensive Organic Chemistry, (Ed: J.
F. Stoddart) Vol. 1, Pergamon, Oxford 1979 1105.
The authors gratefully acknowledge financial support
from the Iran National Science Foundation (INSF) and
the Research Council of Razi University.
[10] (a) T. V. Hansen, L. Skattebøl, Org. Synth. 2005, 82, 64. (b) Ø. W.
Akselsen, L. Skattebøl, T. V. Hansen, Tetrahedron Lett. 2009, 50,
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ORCID
[11] D. E. Ward, M. S. Abaee, Org. Lett. 2000, 2, 3937.
Ebrahim Soleimani http://orcid.org/0000-0002-5188-8527
[12] M. S. Abaee, R. Sharifi, M. M. Mojtahedi, Org. Lett. 2005, 7,
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N, Ghasempour HR. MgBr2 supported on
Fe3O4@SiO2 ~ urea nanoparticle: An efficient
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