Magnetic g-C3N4 nanocomposite-catalyzed environmentally benign aminolysis of epoxide

Article abstract of DOI:10.1007/s11164-017-3176-0

A magnetic graphitic carbon nitride (g-C₃N₄) nanocomposite was prepared and used as a novel magnetically retrievable nanocatalyst for efficient ring opening of epoxides with aromatic amines. A variety of aryl and alkyl epoxides were

Full text of DOI:10.1007/s11164-017-3176-0

Res Chem Intermed
DOI 10.1007/s11164-017-3176-0
Magnetic g-C3N4 nanocomposite-catalyzed
environmentally benign aminolysis of epoxide
Tahereh Soleymani Ahooie1 Najmedin Azizi1,2
•
•
Mohammad Mahmoudi Hashemi1 Issa Yavari1
•
Received: 26 July 2017 / Accepted: 28 August 2017
Ó Springer Science+Business Media B.V. 2017
Abstract A magnetic graphitic carbon nitride (g-C3N4) nanocomposite was pre-
pared and used as a novel magnetically retrievable nanocatalyst for efficient ring
opening of epoxides with aromatic amines. A variety of aryl and alkyl epoxides
were examined under the mild reaction condition and the corresponding b-amino
alcohols were obtained in good to excellent yields under solvent-free condition.
Using the Fe3O4 as a low-priced magnetic support to immobilize an active material
g-C3N4 can provide a better result than using traditional catalysts with respect to the
efficiency, yield, easy work-up, short reaction time, possibility of regeneration, and
ease of applicability .
Keywords Epoxide Á Magnetic g-C3N4 Á Ring opening Á Aminolysis
Introduction
Carbon-based nanocomposite materials can provide a better result than using
traditional catalysts with respect to applications in energy, pharmaceutical, [1, 2]
magnetic resonance imaging, separation technique, optical application and
especially in catalytic performances [3–5]. Over the past two decades, the rapid
progress of nanomaterials- and nanostructures-based technology has enabled
researchers to fabricate devices on the molecular and nanoscale to a great extent
[6, 7]. Among the nanostructure materials, graphitic carbon nitride (g-C3N4) has
&
Najmedin Azizi
azizi@ccerci.ac.ir
1
2
Department of Chemistry, Science and Research Branch, Islamic Azad University, Tehran, Iran
Chemistry and Chemical Engineering Research Center of Iran, P.O. Box 14335-186, Tehran,
Iran
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T. S. Ahooie et al.
been recognized as an excellent and promising catalyst for a variety of applications
due to several advantages such as low cost, non-toxicity, and excellent thermal and
chemical stability [8–12].
Epoxides are the most fascinating targets in synthetic chemistry as useful
electrophiles, due to the advantage of their predictable highly regioselective ring-
opening reactions [13]. Furthermore, they are novel therapeutic agents, natural
products [14] and active biologically compounds [15]. They usually undergo a
variety of transformations with a broad range of nucleophiles which is due to their
rich chemistry caused by a reactive moiety of the strained oxirane, and play a
considerable role in the development of modern organic chemistry as well as in the
total synthesis of natural products [16]. Nucleophilic ring opening of epoxides in the
presence of various catalysts or promoters has been extensively studied in the
literature [17–35].
Regarding our interest in the ring-opening reactions of epoxide using green
media/catalytic systems [36–39], we here report a solvent-free ring opening of
symmetrical and unsymmetrical epoxides with aromatic amines to prepare b-amino
alcohols in the presence of new heterogeneous magnetic g-C3N4 nanocatalysts.
Experimental
Materials and methods
All chemicals and solvents were obtained from commercially available sources and
1
all products were confirmed by HNMR, FT-IR spectroscopy and mass spectrom-
1
1
etry. H NMR spectra were recorded on a 500- or 300-MHz HNMR, 13C NMR
125.7- and 75-MHz NMR spectrometer using DMSO-d6 as a solvent, and chemical
shifts have been expressed in (ppm) 40 downfield from TMS. Melting points were
recorded on a Buchi 535 melting point apparatus and are uncorrected. FT-IR spectra
were determined on a Bruker Vector-22 infrared spectrometer using KBr disks.
Preparation of magnetic g-C3N4 catalyst
Bulk g-C3N4 was prepared by directly calcining melamine in air, with 2 g melamine
being placed in a muffle furnace with an alumina crucible with a loose cover. Then,
the programmed temperature was set to 550 °C for 3 h with a ramp rate of
5 °C min-1. After heat treatment, a light yellow powder was obtained [42]. In the
next step, Fe3O4/g-C3N4 was prepared by reported methods [43]. The g-C3N4
(500 mg) was dispersed in 300 mL of ethanol/water (1:1) using sonication for 4 h at
room temperature. Then, FeCl3Á6H2O (1.838 g, 0.0216 mol) and FeCl2Á4H2O
(0.703 g, 0.0108 mol) were dissolved in the solution using sonication and 10 mL of
ammonia solution (NH4OH) (28 wt%) was quickly injected into the mixed solution.
Under nitrogen atmosphere, the reaction mixture was stirred for 2 h at 90 °C, and
cooled to room temperature, washed with water and ethanol, separated magneti-
cally, and dried overnight at 60 °C under vacuum.
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Magnetic g-C3N4 nanocomposite-catalyzed environmentally…
General procedure
To a dry test tube containing a magnetic bar, 75 mg magnetic g-C3N4, amine
(1 mmol) and epoxide (1 mmol) were added and the mixture was stirred under
heating at 60 °C. When the reaction was completed, as indicated by TLC, or GC,
the crude reaction mixture was diluted with ethyl acetate or diethyl ether (10 mL)
and magnetic g-C3N4 was removed with external magnet. The organic solvent was
evaporated under vacuum and the crude products were purified by flash column
chromatography using silica gel or recrystallization from ethanol or diethyl ether to
give pure products. All products are known compounds and the characterizations of
these compounds were identical to literature reports.
2-(Phenylamino)cyclohexanol (3a)
FT-IR (KBr) m = 3398, 2960, 1606,1345, 1100 cm-1 1H NMR (500 MHz,
;
CDCl3): d 7.20–7.15 (m, 2H), 6.72–6.64 (m, 3H), 3.32–3.07 (m, 2H), 2.95–3.01
(brs, 2H), 2.10–2.08 (m, 2H), 1.74–1.64 (m, 2H), 1.40–1.22 (m, 3H), 1.09–0.98 (m,
1H); 13C NMR (CDCl3, 125 MHz): d 147.9, 128.8, 118.1,114.1, 73.9, 59.8, 33.1,
31.4, 25.1, 24.2.
2-(4-methoxyphenylamino)cyclohexanol (3b)
FT-IR (KBr) m = 3380, 2945, 1600,1158, 1068 cm-1 1H NMR (500 MHz,
;
CDCl3): d 6.72–6.59 (m, 4H), 3.67 (s, 3H), 3.28–2.78 (m, 2 H), 2.95–2.72 (m,
3H), 2.00–1.99 (m, 2H), 1.71–1.64 (m, 2H), 1.34–0.90 (m, 4H); 13CNMR (CDCl3,
125 MHz): d 152.4, 140.9, 116.1, 113.9, 74.1, 61.3, 55.1, 33.0, 31.2, 25.4,24.1
2-(3-chlorophenylamino)cyclohexanol (3c)
1H NMR (500 MHz, CDCl3): d 7.10–6.69 (m, 4H), 3.28–2.78 (m, 2 H), 2.95–2.72
(m, 3H), 2.00–1.99 (m, 2H), 1.71–1.64 (m, 2H), 1.34–0.90 (m, 4H).
2-(4-i-propylphenylamino)cyclohexanol (3d)
1H NMR (500 MHz, CDCl3): d 7.05 (d, J = 7.8 Hz, 2H), 6.66 (d, J = 7.8 Hz, 2H),
3.56 (brs, NH), 3.01–2.58 (m, 3H), 2.01–1.82 (m, 2H), 1.50–1.25 (m, 7H).
2-(2-tolylamino)cyclohexanol (3e)
1H NMR (CDCl3, 500 MHz): d 7.15–6.70 (m, 4 H), 3.42–3.40 (m, 1 H), 3.22–3.14
(m, 1 H), 2.13 (s, 3 H), 2.14–2.11 (m, 2 H), 1.80–1.71 (m, 2 H), 1.45–1.10–1.05 (m,
4H); 13C NMR (CDCl3, 125 MHz): d 143.9, 130.1, 126.9, 122.8, 116.71, 110.8,
73.5, 58.2, 32.9, 30.6, 24.1, 23.2, 16.6.
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T. S. Ahooie et al.
2-(4-bromophenylamino)cyclohexanol (3f)
1H NMR (500 MHz,CDCl3): d 7.12 (d, J = 7.8 Hz, 2H), 6.68 (d, J = 7.8 Hz, 2H),
3.36–3.10 (m, 2H), 2.64 (brs, NH), 2.10–2.02 (m, 2H), 1.70–1.64 (m, 2H),
1.35–1.01 (m, 4H), 13C NMR (125 MHz, CDCl3): d 145.1, 131.8, 115.2, 109.1,
73.05, 60.0, 32.9, 31.8, 24.1, 23.7.
2-(2-tolylamino)cyclohexanol (3g)
1H NMR (500 MHz, CDCl3): d 7.01 (d, 2H, J = 7.8 Hz), 6.55 (d, 2H, J = 7.8 Hz),
3.24–2.94 (m, 4H), 2.14 (s, 3H), 2.00–1.92 (m, 2H), 1.64–1.56 (m, 2H), 1.29–1.14
(m, 3H), 0.91–0.83 (m,1H); 13C NMR (CDCl3, 125 MHz): d 145.1, 129.2, 127.3,
113.4, 73.8, 60.0, 32.8, 31.2, 25.3, 24.1, 20.1.
2-(N-methyl-N-phenylamino)cyclohexanol (3h)
1H NMR (500 MHz, CDCl3): d 7.12 (t, 2H, J = 8.3 Hz), 6.84 (d, 2H, J = 8.3 Hz),
6.76 (t, 1H, J = 7.2 Hz), 3.62–3.59 (m, 1H), 3.39–3.36 (m, 1H), 2.71 (s, 3H), 2.70
(s,1H), 2.17–2.14 (m, 1H), 1.73–1.65 (m, 3H), 1.40–1.20 (m, 5H); 13C NMR
(CDCl3, 125 MHz):d 151.4, 129.1, 118.5, 115.6, 70.0, 67.0, 33.4, 31.1, 26.1, 25.5,
24.4
1-Phenoxy-3-(phenylamino)propan-2-ol (3ia)
1H NMR (500 MHz, CDCl3): d 7.39–7.34 (m, 2H), 7.29–7.24 (m, 2H), 7.08–7.04
(m, 1H), 7.00–6.97 (m, 2H), 6.86–6.81 (m, 1H), 6.76–6.73 (m,2H), 4.32–4.28 (m,
1H), 4.10–4.04 (m, 2H), 3.62 (brs, 1H), 3.47 (dd, 1H, J1 = 13.0 Hz, J2 = 4.3 Hz),
3.33 (dd, 1H, J1 = 13.0 Hz, J2 = 7.32 Hz);13C NMR (CDCl3, 100 MHz): d
158.5,148.0, 129.7, 129.5, 121.4, 118.3, 114.7, 113.5, 70.1, 68.8, 46.9.
Results and discussion
Initially, in order to investigate the ring-opening reaction of epoxide using a new
and greener nanocatalyst, magnetically active g-C3N4 has been synthesized without
the generation of any hazardous product in the simplest manner, as reported in the
literature [40], and the formation of the nanocomposite was confirmed by scanning
electron microscopy (SEM) and Fourier transformed infrared (FT-IR).
The powder FT-IR pattern (Fig. 1) reveals that the characteristic bands in
400–3200 cm-1 region with peaks appearing at 11,167, 1302, 1478 and 1660, cm-1
are attributed to either trigonal C–N(–C)–C or bridging C–NH–C units. The broad
peak around at 3135 cm-1 can be ascribed to the stretching mode of the amino
groups (N–H, NH2). Additionally, the characteristic sharp peak at 785 cm-1 is
attributed to the vibrations of the striazine ring. In addition, a broad peak from 650
to 760 cm-1 550 to 650 cm-1 corresponding to Fe–O proves the existence of Fe3O4
in the composite.
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Magnetic g-C3N4 nanocomposite-catalyzed environmentally…
Fig. 1 FTIR spectra of the nanocomposite
The morphologies of the synthesized nanocomposite were examined by SEM and
EDX images. Figure 2 shows the SEM images of the pristine Fe3O4/g-C3N4
nanocomposite. The pristine g-C3N4 showed an irregular sheet-like structure which
is overlapped. Further, the morphology of the Fe3O4/g-C3N4 composite exhibited an
irregular distribution of Fe3O4 NPs throughout the surface ofthe g-C3N4 sheets.
(Figure 2). Individual EDS elemental maps clearly detected the presence of Fe, O,
C, and N which indicated that the Fe3O4 nanoparticles are deposited on the surface
of the g-C3N4 sheets (Fig. 3).
Next, using well-characterized magnetically active g-C3N4 (75 mg), the ring-
opening reaction between aniline (1.0 mmol) and cyclohexene oxide (1.0 mmol)
was selected as the probe reaction to determine the catalytic activity of magnetic
g-C3N4 and to optimize the reaction conditions. The results are summarized in
Table 1. When the model reaction proceeded at room temperature in the presence of
Fe3O4/g-C3N4 and under these conditions, we observed the formation of the desired
b-amino alcohols 3a with 40% yield after 2 h (Table 1, entry 1). A repeat of this
experiment using 140 mg of nanocatalyst resulted in a 58% yield under otherwise
identical conditions (Table 1, entry 2). The chemical yield of 3a jumped to 92%
when the reaction temperature had been increased to 60 °C (Table 1, entry 3). To
reveal this unprecedented reaction, we embarked on studying the feasibility of this
novel nonocomposite. It was observed that g-C3N4 (Table 1, entry 4) or Fe3O4
(Table 1, entry 5) alone as the nanocatalyst are not effective for this nucleophilic
ring-opening process and a conversion of 68% and 45% towards the target product
has been realized, respectively. This impressive boost in catalytic activity is
expected to be the result of a synergitic effect of g-C3N4 and Fe3O4 while preserving
the high catalytic rates.
When a similar experiment was carried out in an organic solvent such as ethyl
acetate, acetonitrile, dichloromethane, ethanol and isopropanol as well as water at
60 °C, (Table 1, entries 6–11) unsatisfactory formations of the product were
observed. The reaction did not take place without a catalyst and only starting
123

T. S. Ahooie et al.
Fig. 2 SEM image of the nanocomposite
materials were obtained (Table 1, entry 12). Therefore, the optimized reaction
condition was 75 mg of magnetic g-C3N4 under solvent-free condition at 60 °C for
the subsequent experiments.
Under the optimum reaction condition, to elaborate the generality and scope of
our protocol, we have also investigated the aminolysis of a variety of sterically,
electronically, and functional arylamines with cyclohexene oxide. The results are
summarized in Table 2. It was observed that sterically, electronically and
functionally diverse arylamines reacted with cyclohexane oxide without any
difference, and the corresponding products were obtained in good to excellent yields
(Table 2). Furthermore, the reactions were strostereo selective and the trans
stereochemistry of the ring products were determined from the coupling constants of
1
the C–H protons to the heteroatoms in the H NMR spectra.
Further investigations were carried out using various alkyl/aryl-epoxides such as
phenyl-2,3-epoxypropyl ether, isopropyl-2,3-epoxypropyl ether, 1,2-epoxy butane,
allyl-2,3-epoxypropyl ether and styrene oxide, with aryl amines (Table 3). Aryl
amines with different substituents (e.g., 4-MeO, 4-Cl, 4-Br and 4-Me) at the phenyl
123

Magnetic g-C3N4 nanocomposite-catalyzed environmentally…
Fig. 3 The EDX image of the nanocomposite
ring proceed smoothly in the presence of magnetic g-C3N4 to afford the
corresponding 2-amino alcohols in good to excellent yield with good regioselec-
tivity (Table 3). The regioselectivity in the reaction for unsymmetrical epoxides is
governed by both steric and electronic effects. Aliphatic oxiranes underwent
cleavage by aromatic amines with the preferential attack at the less substituted
carbon atom. These results suggest that the steric hindrance effect should still be the
predominant factor affecting the regioselectivity. In the cases of the styrene oxide,
an a-attack was preferred as the major product due to the formation of a carbocation
intermediate (Table 3).
The combination ease of preparation and reuse of magnetic g-C3N4 as novel
reaction media and catalyst are expected to contribute to the development of a novel
protocol for the simple and rapid preparation of 2-amino alcohol derivatives. The
recycling of magnetic g-C3N4 was examined using the reaction of aniline and
cyclohexene oxide under optimized conditions. After the reaction was completed,
ethyl acetate (10 mL) was added to the reaction mixture, shaken vigorously and the
magnetic g-C3N4 be isolated by an external magnet from the products, followed by
the usual work-up and chromatography. The magnetic g-C3N4 was dried under
vacuum and reused for the next batch and recycled again (Fig. 4).
Although there are no mechanistic insights into the precise role of Fe3O4/C3N4 in
this process, based on the experimental results, synchronous reaction mechanism
has been suggested (Fig. 5).
In general, the ring opening of epoxides can occur more frequently under acidic
conditions, and the addition of the nucleophile is considerably accelerated due to the
reversible formation of the more reactive epoxide in the presence of metal catalysts,
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T. S. Ahooie et al.
Table 1 Optimization of model reaction
Entry
Solvent
Temp. (°C)
Yield (%)a
1
2b
Neat (no solvent)
Neat (no solvent)
Neat (no solvent)
Neat (no solvent)
Neat (no solvent)
H2O
r.t.
r.t.
60
60
60
60
60
60
40
60
60
40
58
92
68
45
40
64
62
74
65
72
3
4c
5d
6
7
CH3CO2C2H5
C2H5OH
8
9
CH2Cl2
10
11
CH3CH2OHCH3
CH3CN
aIsolated yields
b140 mg of nanocatalyst
cYield in the presence of g-C3N4
dYields in the presence of Fe3O4
Lewis or Bronsted–Lowry acids [40]. However, the general acid- or base-catalyzed
proposed mechanisms to activate the epoxide ring-opening reaction for the Fe3O4/
C3N4 were certainly unexpected. This result can possibly be rationalized by p-
stacking interactions and hydrogen bonding between the epoxide and the C3N4
surface. The mode of action is believed to be through the hydrogen bonding
interaction between N–H and the epoxide already suggested for graphene-catalyzed
ring-opening reactions [27] and organocatalyzed ring opening of epoxides [41].
However, the introduction of Fe3O4 on the surface of the g-C3N4 increases the
acidity of N–H (Fig. 5), which forms strong hydrogen bonds with hydrogen bonding
donor motifs such as epoxide. Thus, we propose that the mode of activation of the
epoxide ring can be attributed to the synergistic contribution of Fe3O4 and g-C3N4
on the basis of the hydrogen bonding between the N–H bond and epoxide (Fig. 5).
Finally, the comparison between the catalytic activity of the aminolysis of
styrene oxide with aniline (in terms of turnover number) over our nanocomposite
and those reported in the literature can be found in Table 4. Turnover number for
the nanocomposite was calculated per mole of C3N4, considering the fact that the
actual catalyst was the C3N4.
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Magnetic g-C3N4 nanocomposite-catalyzed environmentally…
Table 2 Ring opening reaction of cyclohexene oxide with various arylamines
Entry
1
ArNH2
Product (3a-g)
Yield (%)a
92
Time (min.)
60
Reported Ref.
34
NH2
OH
N
H
3a
2
94
60
36
OH
NH2
OMe
N
H
3b
OMe
NH2
3
4
80
74
120
120
30
32
Cl
OH
N
H
Cl
3c
3d
5
6
76
84
120
60
24
28
3e
OH
NH2
Br
Br
N
H
3f
7
8
90
82
60
60
28
27
3g
3h
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T. S. Ahooie et al.
Table 3 The ring opening of various aryl and alkyl epoxides with amines using magnetic g-C3N4
Entry Product
1
Time (min) Yield (%)a Ratio a:b Reported Ref.
60
94
80:20
28
2
3
120
90
84:16
19
120
120
78
80
88:12
93:07
19
24
4
5
6
60
60
95
76
20:80
19:81
23
17
7
120
92
90:10
17
8
9
120
60
85
92
85:15
82:18
18
19
a 1H NMR yield
92
92
92
90
90
Fig. 4 Recyclability study of
the magnetic C3N4
100
80
60
40
20
0
1
2
3
4
5
Number of Cycles
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Magnetic g-C3N4 nanocomposite-catalyzed environmentally…
Fig. 5 Proposed reaction
mechanism
Fe3O4
H
O
N
N
N
N
N
N
N
N
H
N
N
N
N
N
N
N
N
N
N
g-C3N4 sheet
Table 4 Comparison between the catalytic activity of the nanocomposite with the reported catalysts
Entry
Catalyst
Yield (%)
Turnover no.
Ref.
1
2
3
4
5
6
Meso-Zr-beta
Zr-Nano ZSM-5
Ti-SBA-16
95
96
86
95
92
95
810
566
936
4750
–
40
44
45
Zn(BF4)2
30
Graphene oxide
Fe3O4@C3N4
27
1165
This work
Conclusion
In summary, we have described a green, novel and reusable magnetic g-C3N4
catalyst for the aminolysis of epoxides with aromatic amines under solvent-free
conditions. The notable features of this procedure are the mild reaction conditions,
high conversions, short reaction times, economic viability of the reagents, simple
experimental procedure and product isolation.
Acknowledgements Financial support of this work by Chemistry and Chemical Engineering Research
Center of Iran is gratefully appreciated.
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