Unmodified Fe3O4 nanostructure promoted with external magnetic field: safe, magnetically recoverable, and efficient nanocatalyst for N- and C-alkylation reactions in green conditions

Article abstract of DOI:10.1007/s11164-017-3243-6

Transition metal compounds have emerged as suitable catalysts for organic reactions. Magnetic compounds as soft Lewis acids can be used as catalysts for organic reactions. In this report, the Fe₃O₄ nanostructures were obtained from Fe²⁺ and Fe³⁺-salts, under an external magnetic field (EMF) without any protective agent. The X-ray photoelectron spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy tools were used to characterize these magnetic compounds. The two-dimensional (2-D, it showed nanometric size in the two dimensions, nanorod structure) Fe₃O₄ compound showed high catalytic activity and stability in N- and C-alkylation reactions. A diverse range of N- and C-alkylation products were obtained in moderate to high yield under green and mild conditions in air. Also the N- and C-alkylation products can be obtained with different selectivity and yield by exposure reactions with EMF. Results of alkylation reactions showed that the presence of Fe(II) and Fe(III) species on the surface of magnetic catalysts (phase structure of magnetic compounds) are essential as very cheap active sites. Also, morphology of magnetic catalysts had influence on their catalytic performances. After the reaction, the catalyst/product(s) separation could be easily achieved with an external magnet and more than 95% of catalyst could be recovered. The catalyst was reused at least four times without any loss of its high catalytic activity for N- and C-alkylation reactions.

Full text of DOI:10.1007/s11164-017-3243-6

Res Chem Intermed
https://doi.org/10.1007/s11164-017-3243-6
Unmodifed Fe O nanostructure promoted
3
4
with external magnetic feld: saꢀe, magnetically
recoverable, and eꢁcient nanocatalyst ꢀor N‑
and C‑alkylation reactions in green conditions
Ezzat Rafee1,2 · Mohammad Joshaghani1,2 · Parvaneh Ghaderi‑Shekhi Abadi2
Received: 13 July 2017 / Accepted: 18 December 2017
©
Springer Science+Business Media B.V., part of Springer Nature 2017
Abstract Transition metal compounds have emerged as suitable catalysts for
organic reactions. Magnetic compounds as soft Lewis acids can be used as catalysts
for organic reactions. In this report, the Fe O nanostructures were obtained from
3
4
2
+
3+
Fe and Fe -salts, under an external magnetic ꢀeld (EMF) without any protec-
tive agent. The X-ray photoelectron spectroscopy, scanning electron microscopy,
and energy dispersive X-ray spectroscopy tools were used to characterize these mag-
netic compounds. The two-dimensional (2-D, it showed nanometric size in the two
dimensions, nanorod structure) Fe O compound showed high catalytic activity and
3
4
stability in N- and C-alkylation reactions. A diverse range of N- and C-alkylation
products were obtained in moderate to high yield under green and mild conditions
in air. Also the N- and C-alkylation products can be obtained with diꢁerent selec-
tivity and yield by exposure reactions with EMF. Results of alkylation reactions
showed that the presence of Fe(II) and Fe(III) species on the surface of magnetic
catalysts (phase structure of magnetic compounds) are essential as very cheap active
sites. Also, morphology of magnetic catalysts had inꢂuence on their catalytic per-
formances. After the reaction, the catalyst/product(s) separation could be easily
achieved with an external magnet and more than 95% of catalyst could be recov-
ered. The catalyst was reused at least four times without any loss of its high catalytic
activity for N- and C-alkylation reactions.
Keywords Fe O · External magnetic ꢀeld · Two-dimensional array · Magnetic
3
4
nanostructure catalyst
*
Ezzat Raꢀee
ezzat_raꢀee@yahoo.com; e.raꢀei@razi.ac.ir
1
2
Department of Inorganic Chemistry, Faculty of Chemistry, Razi University, Tagh-e-Bostan, PO
Box 67346-67149, Kermanshah 67149, Iran
Institute of Nanoscience and Nanotechnology, Razi University, Tagh-e-Bostan, PO
Box 67346-67149, Kermanshah 67149, Iran
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3

E. Raꢀee et al.
Introduction
Due to the low cost of Fe compounds compared to Au, Ag, Pd, Pt, Ru, and Rh com-
pounds, it has attracted particular attention as a catalyst for various organic reactions
[1]. The ꢀrst example of using unprotected magnetite compounds as catalyst was
reported in the four-component Aza-Sakuray reaction [2–7]. Then these catalysts
were applied in N-alkylation reactions [8], trisubstituted imidazoles synthesis [9],
and Sonogashira–Hagihara reactions [10]. These compounds, due to nontoxicologi-
cal features, ease and various preparation processes, being commercially accessible,
and not time-consuming separation of them from reaction mixtures, have attracted
widespread attention as catalyst in the past decades [11, 12]. These properties are a
subject of interest for investigation, especially for large-scale operation in industries.
Among the diꢁerent magnetic compounds, Fe O are used as a catalyst in car-
3
4
bon–carbon double bond isomerization [13], modiꢀed Fischer–Tropsch reactions
14], water–gas shift reactions [15], dehydrogenation [16], and epoxidation [17]
reactions. Catalytic activity of Fe O in these reactions is due to various parameters,
[
3
4
for example, a surface of Fe O (111) terminated by a hexagonal oxygen layer cov-
3
4
ered by one quarter monolayer of iron cations, these Lewis acid centres act as active
centers [18].
Many eꢃcient techniques were developed for preparing micro- and nano-size
Fe O [19]. Recent developments have indicated that the magnetic ꢀeld, in an ele-
3
4
gant way, could be applied for preparation and orientation of magnetic compounds
into nano- or microscale structures [9, 20, 21]. For magnetic compounds, an external
magnetic ꢀeld (EMF) can be manipulated for spin, charge, and orbital degrees of
freedom that lead to rearrangements of electrons and magnetic domains in atomic
and molecular systems [22]. Therefore, these changes aꢁect physical and chemical
reactivity of magnetic compounds [23, 24].
Also, an especially interesting topic is whether such organic reactions may be
controlled by an EMF. A few experimental observations have been reported which
illustrate the possibility of utilizing EMF in the ꢀeld of heterogeneous catalysis. In
an early review from 1975, studies of CuO nanoparticles revealed that a weak mag-
netic ꢀeld can cause a change from antiferromagnetic to ferromagnetic spin ordering
and increase ammonia adsorption [25, 26]. In the presence of a weak magnetic ꢀeld,
CO oxidation on nonmagnetic Pt catalyst supported on carbon coated magnetic Co
particles has been investigated [27]. Also, the activation energy for NO reduction
was found to be reduced under a magnetic ꢀeld, leading to an increase in conversion
rate [28].
According to the above studies, our group decided to apply EMF in the synthe-
sis of Fe O and use this catalyst as a typical heterogeneous Lewis acid in N- and
3
4
C-alkylation reactions. Alkylation reactions during green and mild conditions [29,
3
0], bass-free systems [29], and by non-noble metal catalyst [29, 30] with only H O
2
as byproduct have attracted much attention by our group. Also, to investigate the
eꢁect of EMF on alkylation reaction, the N- and C-alkylation reactions were carried
out under exposure uf EMF in the presence of the magnetic catalyst.
1
3

Unmodifed Fe O nanostructure promoted with external…
3
4
Experimental
Materials
FeCl ·6H O, FeCl ·4H O, commercial Fe O , NaOH, aromatic compounds, amines,
3
2
2
2
3
4
alcohols, solvents, and bases were purchased from Merck, Germany, with analytical
grades and these were used without further puriꢀcation.
Preparation oꢀ magnetic nanostructure catalysts
In a typical procedure [9], FeCl ·6H O (0.06 mol) and FeCl ·4H O (0.03 mol) were
3
2
2
2
dissolved into 5.0 ml of deoxygenated water (by nitrogen gas bubbling). The result-
ing solution was added dropwise into 25 ml NaOH (1.5 M) under vigorous mechani-
cal stirring at room temperature. The overall experimental process was directly per-
formed in a Helmholtz cylinder permanent magnet with a value of 362–526 µT. A
black precipitate formed after 20 min. After synthesis treatment, the resulting black
precipitate was collected, ꢀltered, and washed with deoxygenated distilled water
several times to remove any possible impurities, and ꢀnally dried in a desiccator at
room temperature before characterization. The as-obtained crystalline Fe O sam-
3
4
ple was denoted as the AEMF1–AEMF6 (applied external magnetic ꢀeld 362–526,
respectively) catalyst (Table 1).
For synthesis of AEMF4, a mixture of iron salts in deoxygenated water and
NaOH solution was exposed to EMF (362 µT) for 5 min. Then EMF was removed
and the mixture was stirred for 15 min under vigorous mechanical stirring.
Table 1 Experimental conditions for preparation of diꢁerent magnetic catalysts with various abbrevia-
tions
No.
Intensity of
EMF (µT)
Phase structurea
Fe O /Fe O
Average crystal- Morphologies of Symbol
b
the productsc
Peg-like
lite size (nm)
1
0
34.03
ZEMF (Zero
external mag-
netic ꢀeld)
3
4
2
3
2
3
4
5
6
7
362
432
485
362
362
362
Fe O /FeO
23.10
23.69
23.57
22.22
20.98
17.20
Rod-like
AEMF1
AEMF2
AEMF3
AEMF4
AEMF5
AEMF6
3
4
4
4
4
4
Fe O
Amorphous
Amorphous
Rod-like
Rod-like
Sheet-like
3
Fe O
3
Fe O
3
Fe O
3
Fe O /FeO
3
4
Reaction conditions: FeCl ·6H O (0.06 mol), FeCl ·4H O (0.03 mol), deoxygenated water (5.0 ml),
3
2
2
2
NaOH (1.5 M), time = 20 min
a
According to XRD patterns and FT-IR results reported previously by our group [9]
According to XRD patterns results reported previously by our group [9]
According to scanning electron microscopy (SEM) results reported previously by our group [9]
b
c
1
3

E. Raꢀee et al.
For synthesis of AEMF5, a mixture of iron salts in deoxygenated water and
NaOH solution was stirred for 15 min under vigorous mechanical stirring. Then the
mixture was exposed to EMF (362 µT) for 5 min.
For synthesis of AEMF6, a mixture of iron salts in deoxygenated water and
NaOH solution was exposed to EMF (362 µT) for 5 min. Then EMF was removed
and the mixture was stirred for 5 min under vigorous mechanical stirring. This con-
dition was repeated for two times.
Methods
The magnetic ꢀeld was generated by a Helmholtz cylinder permanent magnet, Iran,
and fuel source. The bore diameter was 3 cm, and an AC magnetic ꢂux density in
the range of 362–526 µT could be imposed. The X-ray photoelectron spectroscopy
(
XPS) was recorded by a Kratos Analytical Axis Ultra, Shimadzu, Japan, with mon-
ochromatic aluminum and magnesium with X-ray source of 1486.6 and 1253.6 eV
and a Concentric Hemispherical Analyzer (CHA). A take-oꢁ angle of 90° was used
on a spot size of 700 µm × 350 µm. The instrument has Ultra High Vacuum (UHV).
The scanning electron microscopy (SEM) has been performed using an AIS2300C
microscope, Japan, with a scanning range from 0 to 20 keV. The energy dispersive
X-ray (EDX) measurement was made with an IXRF model 550i, Japan, attached to
SEM. SEM/EDX samples were prepared by coating of solid particles into a conduc-
tive layer.
General procedure ꢀor the N‑alkylation reaction
In a typical reaction, a suspension of amine (1.0 mmol) and alcohol (1.0 mmol) was
added to a mixture of magnetic catalyst (0.04 g) with 1.5 ml distilled water. The
resulting mixture was heated to 40 °C for an appropriate time under aerobic con-
ditions. In the case of exposure by EMF, the resulting mixture was transferred to
a Helmholtz cylinder permanent magnet and exposed with 362 µT intensity for an
appropriate time. Progress of the reaction was monitored by TLC. After completion
of the reaction, the mixture was cooled to room temperature, and the catalyst was
separated from the product solution using an external magnet, washed with deoxy-
genated distilled water, dried in a desiccator at room temperature, and used for the
next reaction cycle without any pre-treatment. Solvent of the reaction mixture was
evaporated to generate the crude product. The product was concentrated and puriꢀed
by column chromatography on silica-gel using EtOAc/heptane (1:4) as eluent.
General procedure ꢀor the C‑alkylation reaction
In a typical reaction, a suspension of the magnetic catalyst (0.004 g), alcohol
(
1.0 mmol), and aromatic compound (2.5 ml) were mixed in a reaction ꢂask. The
reaction mixture was stirred at 60 °C for an appropriate amount of time. In the case
of exposure by EMF, the resulting mixture was transferred to a Helmholtz cylin-
der permanent magnet and exposed with 432 µT intensity for an appropriate time.
1
3

Unmodifed Fe O nanostructure promoted with external…
3
4
Progress of the reaction was monitored by TLC. After the complete conversion, the
reaction mixture was allowed to cool to room temperature. The catalyst was sepa-
rated from the product solution using an external magnet, washed with deoxygen-
ated distilled water, dried in a desiccator at room temperature and used for the next
reaction cycle without any pretreatment. Solvent of the reaction mixture was evapo-
rated to generate the crude product. The product was concentrated and puriꢀed by
column chromatography on silica-gel using EtOAc/heptane (1:3) as eluent.
Results and discussion
General procedure ꢀor optimization oꢀ the N‑alkylation reaction conditions
We initially conducted the reaction of aniline and benzyl alcohol as a model reac-
tion. A series of parameters were optimized in this reaction (Table 2).
With 0.02 g AEMF1, four diꢁerent temperatures were evaluated in the model
reaction (Table 2, entries 1–4). Increase of the temperature from 40 to 80 °C shows
no signiꢀcant eꢁect on the yield of the corresponding product. Besides, N-alkyla-
tion reaction was uncompleted at room temperature. Then other reactions were per-
formed at 40 °C. The use of 0.04 g of the catalyst increased the reaction yield up to
8
5% after 20 min (Table 2, entries 2 and 5). Besides, in the absence of the catalyst,
the reaction exhibited very low yield although in a longer reaction time (Table 2,
entry 6). The base employed in the reaction also seems to be important, in the pres-
ence of NaOH, low yield of corresponding product was obtained (Table 2, entry
7
). However, the relevant yield was obtained when the alkylation reaction was car-
ried out in base-free conditions (Table 2, entry 8). Interestingly, similar results were
obtained in polar or nonpolar solvents (Table 2, entries 8, 9), which implies there are
no relevant solvent polarity eꢁects under these conditions. When the reaction was
carried out in solvent-free condition, low yield was obtained at a high reaction time
(
Table 2, entry 10). Equimolar amounts of aniline and benzyl alcohol (1.0 mmol)
resulted in excellent yield of the product compared with other ratios (Table 2,
entries 8, 11, 12). We tested the catalytic eꢃcacy of other magnetic catalysts in the
N-alkylation reaction with optimized reaction conditions (Table 2, entries 13–19). It
seems that AEMF1 and AEMF6 as catalysts showed the best performance (Table 2,
entries 11, 17). Because the synthetic procedure of the AEMF6 is tedious and not
technically simple [15], AEMF1 was chosen as the best catalyst (Table 2, entry 11).
According to the above results, the phase structure of magnetic nanocompounds
(
Table 1) greatly inꢂuences their catalytic performance. The alkylation reaction
using ZEMF due to presence Fe O (Table 1, entry 1), which presents distribution
2
3
Fe(III) on a surface, is not suitable (Table 2, entry 18). However, the alkylation reac-
tion using Fe O and FeO in magnetic catalysts (AEMF1–AEMF6, Table 1, entries
3
4
2
–7), which have their surface layer terminated purely by oxygen atoms, gave a sig-
niꢀcant result (Table 2, entries 11, 13–17) [7, 31]. These results pointed out that
the reaction seemed to be more catalysed by Fe(II) compared to Fe(III) centres [32,
3
3]. Also, the morphology of magnetic nanocatalysts [9] may be inꢂuenced by their
catalytic performance due to the diꢁerence in adsorption/desorption of reaction
1
3

E. Raꢀee et al.
Table 2 Optimization of the N-alkylation reaction conditions
NH2
OH
N
H
Catalyst
+
Entry PhNH :PhCH OH Catalyst (g)
Temp. (°C) Solvent Base
Time (min) Yield (%)a
2
2
(
molar ratio)
t
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:1
2:1
1:1
1:1
1:1
1:1
1:1
1:1
AEMF1/(0.02) r.t.
AEMF1/(0.02) 40
AEMF1/(0.02) 60
AEMF1/(0.02) 80
AEMF1/(0.04) 40
Water
Water
Water
Water
Water
Water
Water
Water
Toluene
–
Water
Water
Water
Water
Water
Water
Water
Water
NaO Bu 20 (2 h)
0 (51)
70
78
80
85
t
NaO Bu 20
t
NaO Bu 20
t
NaO Bu 20
t
NaO Bu 20
t
–
40
NaO Bu 20 (60)
0 (19)
19 (59)
71 (93)
90
10 (51)
40 (96)
0 (42)
35 (93)
0 (58)
20 (85)
20 (67)
50 (95)
35 (85)
AEMF1/(0.04) 40
AEMF1/(0.04) 40
AEMF1/(0.04) 40
AEMF1/(0.04) 40
AEMF1/(0.04) 40
AEMF1/(0.04) 40
AEMF2/(0.04) 40
AEMF3/(0.04) 40
AEMF4/(0.04) 40
AEMF5/(0.04) 40
AEMF6/(0.04) 40
NaOH
20 (50)
20 (30)
20
–
–
–
–
–
–
–
–
–
–
–
0
1
2
3
4
5
6
7
8
20 (60)
20 (30)
20 (2 h)
20 (50)
20 (4 h)
20 (50)
20 (50)
20 (30)
20 (60)
ZEMF/(0.04)
40
Reaction conditions: Base (1.0 mmol); 1.5 ml solvent
a
Isolated yield
molecules, involving substrates, intermediates, and target product on diꢁerent crys-
tal facets of magnetic catalysts surfaces with the diꢁerent morphologies [34–38].
General procedure ꢀor optimization oꢀ the C‑alkylation reaction conditions
Benzene and benzhydrole were selected as a model substrate for the C-alkylation
reaction. A series of parameters were optimized in this reaction (Table 3).
With 0.002 g AEMF1, three diꢁerent temperatures were evaluated in the model.
When the reaction was performed at 60 °C, the expected product was aꢁorded a
9
2% yield after 30 min (Table 3, entries 1–3). By using 0.003 and 0.004 g of the
AEMF1 as catalyst, the product was obtained in excellent yield (Table 3, entries
4
and 5). It seems that when the amount of the catalyst increased, reaction time
1
3

Unmodifed Fe O nanostructure promoted with external…
3
4
Table 3 Optimization of the C-alkylation reaction conditions
OH
Catalyst
+
Heat
Entry
Catalyst (g)
Temp. (°C)
Catalyst type
Time (min)
Yield (%)a
1
2
3
4
5
6
7
8
9
0.002
0.002
0.002
0.003
0.004
–
0.004
0.004
0.004
0.004
0.004
0.004
30
60
90
60
60
60
60
60
60
60
60
60
AEMF1
AEMF1
AEMF1
AEMF1
AEMF1
None
AEMF2
AEMF3
AEMF4
AEMF5
AEMF6
ZEMF
50
30
25
20
10
10 h
30
30
15
40
10
25
81
92
93
94
96
–
68
40
91
89
90
50
1
1
1
0
1
2
Reaction conditions: benzhydrol, (1.0 mmol) and benzene, (2.5 mL)
a
Isolated yield
gradually decreased. Therefore, 0.004 g of the AEMF1 was selected as the best
catalyst loading in further investigations (Table 3, entry 5). In the absence of the
catalyst, no product was formed after 10 h (Table 3, entry 6). In the later step, diꢁer-
ent catalysts were used in the model reaction. According to the results, it seems that
the best C-alkylation yield was obtained for the AEMF1 catalyst while the AEMF2
and AEMF3 catalysts exhibited low activity and selectivity compare with AEMF1
(
Table 3, entries 7, 8). Besides, the AEMF4 and AEMF5 were evaluated as cata-
lyst (Table 3, entries 9, 10), but the results were all inferior to the AEMF1. In the
case of AEMF6, which shows 90% product yield after 10 min, the reaction is not
technically simple (Table 3, entry 11) [9]. On the other hand, magnetic-free cata-
lyst (ZEMF) exhibited low activity which indicates an inevitable role of EMF as a
promoter in the activity of the catalytic sites (Table 3, entry 12) [32, 33]. According
to the above results, again the nature of the catalysts has a dominating eꢁect on the
yield of the products [32, 33].
According to above results, the AEMF1 catalyst showed the best performance
in N- and C- alkylation reactions (Tables 2, 3). Therefore, the reusability, catalytic
mechanism, and structural properties of AEMF1 were investigated here.
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E. Raꢀee et al.
Reusability oꢀ the AEMF1 in the N‑ and C‑alkylation reactions
For a heterogeneous catalyst, it is important to examine its ease of separation, recov-
erability, and reusability. The reusability of the AEMF1 catalyst was investigated in
the N- (Table 2, entry 11) and C-alkylation (Table 3, entry 5) reactions, and experi-
ments were properly scaled up. After each run, the catalyst was separated from mix-
ing reaction by using an external magnet, washed with deoxygenated distilled water,
dried in a desiccator at room temperature and used for the next reaction cycle with-
out any pre-treatment. An almost consistent activity was observed for the next four
consecutive cycles (Fig. 1). We consider that loss of the catalyst during the separa-
tion process after four successive runs of N- and C-alkylation reactions were 14 and
1
5.5 wt%, respectively (compared with the amount of fresh catalyst used in the ꢀrst
run).
Mechanism oꢀ the N‑ and C‑alkylation reactions
The plausible catalytic mechanism for the synthesis of corresponding N- and
C-alkylation products in a model reaction in the presence of AEMF1 is proposed
in Scheme 1. According to previous results, surfaces of the magnetite Fe O and
3
4
FeO are terminated by a hexagonal oxygen layer covered by a one quarter monolayer
of iron cations [7, 31]. These Lewis acid centers on the surface as the active sites
for chemisorption could catalyze the organic reaction [7, 9, 18]. The strong Lewis
acid centers on the surface of AEMF1 could activate the hydroxyl group in benzyl
alcohol or benzhydrol [39] which was attacked by the aniline or benzene, following
generation of the ꢀnal N- and C-alkylation products [40].
Characterization oꢀ the AEMF1 catalyst
The surface chemical states of the active sites in AEMF1 were investigated using a
typical high and low energy-scan XPS analysis (Fig. 2a, b). The XPS spectra indicate
the presence of Fe(II) and Fe(III) octahedral species (Fig. 2b) [9, 41]. These metal-
lic sites on the surface of AEMF1 could activate the hydroxyl group of alcohols in
N- and C-alkylation reactions. Therefore, active alcohols can be directly attacked by
aniline or aromatic compounds for N- and C-alkylation reactions, respectively.
Fig. 1 Recycling eꢃciency
of the AEMF1 for the N- and
C-alkylation reactions
1
3

Unmodifed Fe O nanostructure promoted with external…
3
4
Scheme 1 Plausible mechanism of the N- and C-alkylation reactions in the presence of the AEMF1
catalyst
Fig. 2 XPS analysis of AEMF1 with a low and b high resolution spectra
SEM images in Fig. 3a–c reveal that AEMF1 displays uniformly rod-like net-
works with an average diameter around 100–200 nm. For the crystal growth pro-
cess of nanorods Fe O under EMF, the clusters were ꢀrst grown to the aggrega-
3
4
tion nanocrystals as a bed (substrate), then the oriented nanocrystals aggregated into
nanorods through Ostwald ripening on this bed along with the direction of EMF
because EMF made an anisotropic media for the growth of the crystals. According
to the EDX elemental mapping images for a single nanorod Fe O structure, Fig. 3d
3
4
shows uniform distribution of Fe and O in the nanorod structure.
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E. Raꢀee et al.
Fig. 3 SEM images of AEMF1 in a, b low resolution, c high resolution, and d EDX elemental mapping
data of a single nanorod Fe O structure
3
4
Synthesis oꢀ various N‑alkylation products
To evaluate the scope and generality of this new protocol, various amines and alco-
hols were tested (Table 4). We started by changing the electronic nature of the sub-
stituent on amine. Electron donating groups at the para-position of the arylamines
aꢁorded high yields in comparison with electron withdrawing groups at the ortho,
meta, and para-position (Table 4, entries 2–9). It seems that formation of a hydrogen
bond between the amine and nitro present on the 2-nitro-phenylamine inhibits the
program of the reaction compared to 4-nitro-phenylamine (Table 4, entries 4, 6). In
the case of a sterically demanding group of the arylamine, yield of the correspond-
ing product was low at the same reaction time (Table 4, entry 10). Benzylamine was
converted to its respective secondary amine in only 70% yield at a high reaction
time (Table 4, entry 11); however, reaction with propan-2-amine did not occur over
1
2 h (Table 4, entry 12). Besides, butan-1-amine reacted within 120 min to give the
desired product in 15% yield (Table 4, entry 13).
The reaction using diꢁerent 4-substituted electron donating and withdrawing
group alcohols with aniline gave the same results (Table 4, entries 14–17). Forma-
tion of hydrogen bond between the hydroxide and nitro group on the 2-nitrobenzyl
alcohol does not shows a signiꢀcant eꢁect on the yield of corresponding N-alkyla-
tion product compared to 4-nitrobenzyl alcohol (Table 4, entries 17, 18). Reaction of
aniline with alkyl alcohols, occurred in high reaction times (Table 4, entries 19, 20).
We next turned our attention to the N-alkylation reactions of heterocyclic aro-
matic and secondary amines with diꢁerent alcohols. At ꢀrst, pyridin-2-amine, pyri-
din-3-amine, and 4-aminopyridine with benzyl alcohol reacted eꢃciently to give a
1
3

Unmodifed Fe O nanostructure promoted with external…
3
4
Table 4 N-alkylation of the various amines and heterocyclic aromatic amines with alcohols
R
R
R'
R'
NH
2
OH
N
H
AEMF1
+
water
a
Entry
1
Product
Time (min)
30
Yield (%)
Ref.
[42]
95
N
H
2
3
45
60
91
39
[43]
[44]
N
H
O
N
H
O N
2
4
5
12 h
60
28
82
[45]
[45]
N
H
NO2
N
H
NO2
6
7
60 (2 h)
61 (74)
81
[45]
[46]
N
H
Br
60
N
H
1
3

E. Raꢀee et al.
Table 4 (continued)
a
Entry
Product
Time (min)
60
Yield (%)
69
Ref.
Cl
F
8
[47]
[48]
N
H
9
60 (2 h)
39 (50)
N
H
N
1
1
0
1
H
60 (2 h)
48 (80)
51 (70)
[49]
[44]
N
H
120 (12 h)
1
1
2
3
N
12 h
-
-
H
N
H
120 (12 h)
15 (30)
91
[46]
[42]
4
0
1
1
1
4
5
6
N
H
4
0
94
92
[42]
[42]
N
H
H CO
3
6
0
N
H
Cl
6
0
78
[42]
1
7
N
H
O N
2
1
3

Unmodifed Fe O nanostructure promoted with external…
3
4
Table 4 (continued)
a
Entry
Product
Time (min)
Yield (%)
Ref.
[50]
6
0
70
NO2
1
8
N
H
3
3
h
60
58
70
[51]
[52]
[47]
1
2
9
0
N
H
h
N
H
3
h
H
N
2
1
N
3
3
h
h
71
82
[53]
[47]
H
N
N
2
2
2
2
3
4
N
H
N
1
6
2 h
0
-
-
H
N
N
NO2
N
N
95
[54]
H
N
2
5
good yield of corresponding products (Table 4, entries 21–23). The position of this
extra nitrogen atom seems to have no inꢂuence on the results. An ortho-nitro substi-
tuted of pyridin-3-amine did not give a yield of the product even over a long reaction
1
3

E. Raꢀee et al.
Table 4 (continued)
a
Entry
Product
Time (min)
Yield (%)
Ref.
H
N
1
2 h
-
-
N
N
N
2
6
NH
1
1
20
70
-
[55]
-
N
O
2
2
7
8
2 h
N
O
Cl
9
0
90
[56]
N
O
2
9
O N
2
Reaction conditions: AEMF1, (0.04 g); amine and heterocyclic aromatic amines, (1.0 mmol); alcohol,
(
1.0 mmol); 1.5 mL water at 40 °C
Isolated yield; all products are identiꢀed by comparing of their spectral data with those of the authentic
a
samples [42–56]
time, possibly due to formation of hydrogen bond between the nitro and amine group
for the 3-nitropyridin-2-amine and steric reasons (Table 4, entry 24). Besides, the
existence of two nitrogen atoms on the six membered ring of the amine was aꢁected
on the yield and reaction time of alkylation reaction (Table 4, entry 25). However,
9
H-purin-6-amine and benzyl alcohol did not react over 12 h (Table 4, entry 26). We
considered morpholine as a suitable candidate for further evaluation of the inꢂuence
of AEMF1 on the N-alkylation of a cyclic secondary amine. Our catalytic system
showed moderate activity for the N-alkylation of morpholine with benzyl alcohol
(
Table 4, entry 27). However, reaction of morpholine with 4-chlorobenzyl alcohol
failed to produce the desired product (Table 4, entry 28). The alkylation reaction
1
3

Unmodifed Fe O nanostructure promoted with external…
3
4
between morpholine and 4-metoxybenzyl alcohol showed high yield (Table 4, entry
9).
Ethylene glycol and benzene-1,2-diamine are interesting test substrates as amine
2
and alcohol, because these contain double functional groups, both susceptible to the
N-alkylation reaction. In the case of ethylene glycol as alcohol source, the product
of ꢀrst N-alkylation (Scheme 2a, a1) was obtained after 2 h. Products of secondary
N-alkylation (Scheme 2a, a2) were not observed even after 24 h at 40 °C. In the case
of benzene-1,2-diamine as amine source, the ꢀrst N-alkylation product (Scheme 2b,
b1) was obtained after 5 h, but the product of secondary N-alkylation (Scheme 2b,
b2) was not observed, even after 24 h at 40 °C.
According to previous reports, yield and selectivity of the products at organic
reactions are aꢁected by exposure reaction ambience by EMF [25–28]. There-
fore, we tried to increase the selectivity and yield of the diamine products in the
N-alkylation reactions. Therefore, the above alkylation reactions were studied under
exposure of EMF (362 µT) as modiꢀed reaction conditions. However, this reaction
system oꢁers an eꢁective method for the synthesis of diamine products (a2 and
b2) with suitable yield and selectivity after 8 h (Scheme 2a and b). Under EMF,
preferential and selective adsorption/desorption of reaction molecules (substrates,
intermediates, and target products) on diꢁerent sites of magnetic particles surface
occurred [25–28].
(
a)
(
b)
Scheme 2 Alkylation of a aniline with ethylene glycol and b m-benzenediamines with benzyl alcohol
using the AEMF1 catalyst in the presence and absence of EMF [57–59]
1
3

E. Raꢀee et al.
Table 5 C-alkylation of various alcohols with aromatic compounds
OH
R
R"
R
R"
R'
AEMF1
+
ο
6
0
C
R=NO , Cl, CH , OCH
2
3
3
R'=H, Ph
b
c
Entry
1
Product
Time (min)
40
Yield (%)
60
Ref.
[60]
[61]
O2N
Cl
2
30
60
3
4
30
20
80
80
[62]
[61]
1
a
H3CO
5
10
96
91
[63]
[64]
1b
1
0
6
7
1
c
1
0
89
[38]
H3CO
1
3

Unmodifed Fe O nanostructure promoted with external…
3
4
Table 5 (continued)
b
c
Entry
Product
Time (min)
Yield (%)
92
Ref.
20
[64]
8
30
88
[64]
9
3
0
0
74
90
[65]
[65]
1
0
1
Cl
3
1
Br
Reaction conditions: the AEMF1, (0.004 g); alcohol, (1.0 mmol); aromatic compound, (2.5 mL);
T = 60 °C
a
Isolated yield
b
All products are known and identiꢀed by comparing of their spectral data with those of the authentic
samples [38, 60–65]
Synthesis oꢀ various C‑alkylation products
To evaluate the scope and generality of this new protocol, various alcohols and
aromatic compounds were employed as a substrate for the C-alkylation reaction
(
Table 5). Alcohols with both electron withdrawing and donating groups showed a
diꢁerence in reactivity. For example, low yields were observed with electron with-
drawing substituents on the benzyl alcohol (Table 5, entries 1, 2) compared with
alcohols containing electron donating groups (Table 5, entries 3–5). However,
according to these results, benzhydrol is a suitable selection for more investiga-
tions in C-alkylation reaction with various aromatic compounds. The C-alkylation
benzhydrol with benzene, toluene, and methoxy benzene gave nearly the same
and excellent yields at similar reaction times (Table 5, entries 5–7). p-Xylene and
mesitylene obtain corresponding products in longer reaction times probably due
to the steric eꢁect (Table 5, entries 8, 9). It seems that the existence of electron
1
3

E. Raꢀee et al.
75%
OH
+
1a'
OH
OH
3
0%
Under EMF
+
1
1
b'
c'
+
4
0%
Scheme 3 Investigation of the EMF eꢁect on the C-alkylation reactions
withdrawing groups on the benzene reduced yield of the corresponding product and
took a longer reaction time compared to the benzene as substrate (Table 5, entries
1
0, 11).
Toward an investigative eꢁect of EMF on the C-alkylation reaction, some of
alkylation reactions reported in Table 5 entries 3, 5, 6 were selected and carried out
under EMF (432 µT). Yield for the 1b′ and 1c′ products (Scheme 3) were decreased
(
compared to 1b and 1c, entries 5, 6 of Table 5) at 20 min of reaction time without
any progress (Fig. 4). The reaction proꢀles of these reactions are shown in Fig. 4.
According to the results, the reaction rates under EMF were higher at the start of the
reaction. The EMF can lead to activation of the magnetic catalyst for the ꢀrst occur-
rence of the reaction but, due to the agglomeration of the magnetic catalyst during
reaction, its surface area decreases. These phenomena may be the basic reason for
lower activity of the magnetic catalyst in the C-alkylation reaction [9, 66, 67].
100
8
0
0
0
0
1
1
1
1
1
1
a'
a
6
4
2
b'
b
c'
c
0
0
10
20
30
Time (min)
Fig. 4 Reaction proꢀles of the C-alkylation of benzhydrol and benzene (1a′, green dash line), benzhy-
drol and toluene (1b′, blue dash line), p-tolyl-methanol and benzene (1c′, red dash line), and under expo-
sure EMF of benzhydrol and benzene (green solid line), benzhydrol and toluene (blue solid line), p-tolyl-
methanol and benzene (red solid line) during 30 min tracking in the presence of AEMF1 catalyst. (Color
ꢀ
gure online)
1
3

Unmodifed Fe O nanostructure promoted with external…
3
4
Conclusion
In summary, Fe O synthesized under EMF as a promoter and a resister. N- and
3
4
C-alkylation reactions catalyzed using a cheap, simple, and non-toxic nanorod
Fe O catalyst. Under these conditions, alkylation reactions had high activity and
3
4
selectivity under green and mild conditions. Also, alkylation reactions were carried
out under EMF to investigate the EMF eꢁect. The Fe O catalyst was analyzed using
3
4
the XPS, SEM, and EDX analyses. It seems that a special phase structure and the
morphology of Fe O compound have very important eꢁects on its catalytic activity.
3
4
Fe O catalyst with high stability was recovered and reused very easily in N- and
3
4
C-alkylation reactions. The simple recyclability makes this catalyst suitable for con-
tinuous industrial processes.
Acknowledgements The authors thank the Razi University Research Council for support of this work.
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