Synthesis and characterization of a novel nano-Fe3O4-copoly[(styrene/acrylic acid)/grafted ethylene oxide and its application as a magnetic and recyclable phase-transfer catalyst in the preparation of β-azido alcohols and β-nitro alc

Article abstract of DOI:10.1007/s11164-015-2043-0

A facile method was developed for the synthesis of β-azido alcohols and β-nitro alcohols in the presence of a novel nano-Fe₃O₄-copoly[(styrene/acrylic acid)/grafted ethylene oxide (nano-Fe₃O₄-PS-Co-[PAA-g-PEG])

Full text of DOI:10.1007/s11164-015-2043-0

Res Chem Intermed
DOI 10.1007/s11164-015-2043-0
Synthesis and characterization of a novel nano-Fe3O4-
copoly[(styrene/acrylic acid)/grafted ethylene oxide
and its application as a magnetic and recyclable phase-
transfer catalyst in the preparation of b-azido alcohols
and b-nitro alcohols
1
2
Ali Reza Kiasat1 Mina Daei Seyyed Jafar Saghanezhad
•
•
Received: 2 January 2015 / Accepted: 2 April 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract A facile method was developed for the synthesis of b-azido alcohols and
b-nitro alcohols in the presence of a novel nano-Fe3O4-copoly[(styrene/acrylic
acid)/grafted ethylene oxide (nano-Fe3O4-PS-Co-[PAA-g-PEG]) as a phase-transfer
catalyst in water. The catalyst was characterized with IR, SEM, and TGA. This
procedure offers several advantages, including excellent regioselectivity, high
yields, short reaction times, a recyclable catalyst, easy separation of the catalyst
through an external magnet, and easy workup.
Keywords Ring opening reaction Á Epoxide Á b-Azido alcohols Á b-Nitro alcohols Á
Magnetic nanocatalyst Á Phase-transfer catalyst (PTC)
Introduction
Due to the difficulties in separating homogeneous catalysts from products and
reaction solvents, recycling of these catalysts is complicated and costly, especially
when noble and/or toxic metal complexes are used. As such, heterogeneous
catalysts have been proposed as a solution. However, during the recovery and
recycling of the catalyst from these immobilized systems, similar problems are
encountered. Therefore, an efficient technique for facile catalyst separation was
needed, and for this reason, magnetic separation was introduced.
Magnetic nanoparticles have now been extensively employed as alternative
catalyst supports given their high surface area, resulting in high catalyst loading
&
Ali Reza Kiasat
akiasat@scu.ac.ir
1
2
Chemistry Department, Faculty of Sciences, Shahid Chamran University,
61357-4-3169, Ahvaz, Iran
ACECR-Production Technology Research Institute, Ahvaz, Iran
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A. R. Kiasat et al.
capacity, high dispersion, outstanding stability, and convenient catalyst recycling.
Magnetic separation renders the recovery of catalysts from liquid-phase reactions
much easier than cross-flow filtration and centrifugation. As the catalysts are usually
immobilized on the surface of the magnetic nanoparticles, easy access of reactants
to the active sites can also be achieved [1–5].
Phase-transfer catalysis (PTC) is a very useful approach that typically involves
simple experimental operations, mild reaction conditions, inexpensive and environ-
mentally benign reagents and solvents, and large-scale reactions [6]. A practical
limitation to the phase-transfer method, however, is that many of the catalysts used
promote the formation of stable emulsions [7]. Immobilization of the phase-transfer
catalyst on an insoluble polymeric matrix can provide a simple solution to this problem.
Poly(ethylene glycol) (PEG) and its derivatives are interesting catalysts and
solvents. PEGs are inexpensive and thermally stable, have almost negligible vapor
pressure, and are environmentally benign media for chemical reactions [8].
b-Azido alcohols are compounds of interest in organic synthesis as either precursors
of vicinal amino alcohols or in the chemistry of carbohydrates, nucleosides, lactams,
and oxazolines [9]. Generally, azidohydrins are prepared through the ring opening of
epoxides using different azides in suitable solvents. Although the classical protocol
uses sodium azide and ammonium chloride, the azidolysis reaction requires a long
reaction time (12–48 h), and the azidohydrin is often accompanied by isomerization,
epimerization, and rearrangement of products [10].
Although a few reagents and catalysts have recently been reported for the conversion
of epoxides to b-azido alcohols, [11–18] disadvantages such as long reaction times, low
product yields, difficulty in preparation and/or storage of reagents or catalysts, tedious
workup, and in most cases, low regioselectivity, clearly necessitate the introduction of
new methods for such functional group transformations.
Given the importance of ring-opening reactions of epoxides [19], and in
continuation of our efforts in the synthesis of novel catalysts and their application in
organic synthesis [20–23], Fe3O4-PS-Co-[PAA-g-PEG] was introduced as a novel
heterogeneous phase-transfer catalyst for the one-pot preparation of b-azido
alcohols under reflux conditions (Scheme 1).
Experimental
General
All commercially available chemicals were purchased from Fluka and Merck
companies and used without further purification. Products were characterized by
Scheme 1 One-pot preparation of b-azido alcohols using Fe3O4-PS-Co-[PAA-g-PEG] as catalyst
123

Synthesis and characterization of a novel…
their physical constant and comparison with authentic samples. Reaction monitoring
was accomplished by thin-layer chromatography (TLC) on silica gel polygram SIL
G/UV254 plates. The IR spectra were recorded on a Bomem MB-Series 1998 FTIR
spectrophotometer using KBr pellets for the samples and the catalyst in the range of
1
4000–400 cm-1. H and 13C NMR spectra were recorded in CDCl3 and DMSO-d6
on a Bruker Avance DPX 400 MHz spectrometer using tetramethylsilane (TMS) as
the internal standard. The SEM analyses were carried out using a LEO 1455 VP
scanning electron microscope operating at 1–30 kV.
General procedure for the preparation of nano-Fe3O4
Nano-Fe3O4 was prepared using an improved chemical co-precipitation method [24].
A 500-mL round-bottom flask was charged with FeCl2Á4H2O (3.1736 g, 16 mmol)
and FeCl3Á6H2O (7.57 g, 28 mmol) and dissolved in 320 mL of deionized water. The
solution was stirred under nitrogen at 80 °C for 1 h, and 40 mL NH3ÁH2O (25 %) was
rapidly injected into the reaction mixture, which was again stirred under a nitrogen
atmosphere for 1 h, and then cooled to room temperature. The precipitated particles
were washed five times with hot water and separated by magnetic decantation.
Finally, the nano-Fe3O4 was dried under a vacuum at 70 °C.
General procedure for the preparation of cross-linked copoly(styrene/
acrylic acid)/grafted ethylene oxide
A three-neck round-bottom flask was charged with styrene (104.4 mmol), acrylic acid
(75.8 mmol), divinyl benzene (2.1 mmol), and 10 mL H2O. The flask was equipped
with a reflux condenser and a nitrogen atmosphere while it was stirred using a
mechanical stirrer. After purging N2 for 30 min, the mixture was refluxed for 12 h in
an oil bath at 70 °C. The white copolymer [cross-linked copoly(styrene/acrylic acid)]
was washed with water, methanol and acetone, and was dried at 70 °C for 4 h.
In the next step, the obtained cross-linked copoly(styrene/acrylic acid) (1.6 g)
and polyethylene glycol 300 (40 mL) were charged in a round-bottom flask, and 3–4
drops of sulphuric acid (98 %) were added. The mixture was refluxed for 15 h. After
cooling to room temperature, the grafted copolymer [cross-linked copoly(styrene/
acrylic acid)/grafted ethylene oxide] was filtered using a Bu¨chner funnel and
washed several times with plenty of water and acetone. The cross-linked Co
(styrene/acrylic acid)-g-PEG, after drying at 70 °C, was 1.97 g, which is equal to
0.19 g PEG per 1 g of the copolymer.
General procedure for the preparation of nano-Fe3O4@copoly[(styrene/
acrylic acid)/grafted ethylene oxide
In a 50-mL round-bottom flask, Fe3O4 (0.1 g) was dispersed in 20 mL acetone and
sonicated for 45 min. In another flask cross-linked Co (styrene/acrylic acid)-g-PEG
(0.7 g) and 20 mL acetone was similarly sonicated. The two flasks were merged in a
123

A. R. Kiasat et al.
100-mL flask and sonicated for 45 min, and this mixture was refluxed for 2 h. After
evaporation of the acetone, the catalyst [nano-Fe3O4@copoly[(styrene/acrylic
acid)/grafted ethylene oxide] was dried at 70 °C for 2 h. The resulting nanocomposite
was stirred in sulfuric acid 1 M to remove any bare nano-Fe3O4 and was washed
several times with water and methanol and dried in vacuum for an additional 2 h.
General procedure for the preparation of b-azido alcohols in the presence
of nano-Fe3O4-PS-Co-[PAA-g-PEG]
A 25-mL RBF was charged with epoxide (1 mmol), sodium azide (2 mmol, 0.13 g),
nano-Fe3O4-PS-Co-[PAA-g-PEG] (0.02 g), and 5 mL deionized water and refluxed
in an oil bath for specific lengths of time (5–120 min). Completion of the reaction
was indicated by TLC [TLC ethyl acetate/n-hexane (2:5)]. After completion of the
reaction (as indicated in Table 2), the catalyst was separated utilizing an external
magnet. The product was extracted with diethyl ether (3 9 10 mL), and the organic
phase was concentrated and dried using CaCl2. After evaporation of the solvent
under reduced pressure using a rotary evaporator, the b-azido alcohols were
obtained with 78–99 % yield.
General procedure for the preparation of b-nitro alcohols in the presence
of nano-Fe3O4-PS-Co-[PAA-g-PEG]
A 25-mL RBF was charged with epoxides (1 mmol), sodium nitrite (10 mmol,
0.69 g), nano-Fe3O4-PS-Co-[PAA-g-PEG] (0.02 g) and 5 mL deionized water and
refluxed in an oil bath for specific lengths of time (15–45 min). Completion of the
reaction was indicated by TLC [TLC ethyl acetate/n-hexane (2:5)]. After completion
of the reaction (as indicated in Table 3), the catalyst was separated using an external
magnet. The product was extracted with diethyl ether (3 9 10 mL), and the organic
phase was concentrated and dried using CaCl2. After evaporation of the solvent
under reduced pressure using a rotary evaporator, the b-nitro alcohols were obtained
with 70–90 % yield.
Selected spectrum
1-Azido-3-allyloxypropan-2-ol (Entry 3, Table 2) IR (neat): mmax N3(2103 cm-1);
1H NMR (400 MHz, CDCl3): d: 3.45–3.54 (m, 2H), 3.8 (m, 1H), 3.96–4 (m, 2H),
4.18 (s br, 1H), 3.7 (m, 2H), 5.2 (dd, 1H), 5.3 (dd, 1H), 5.8 (dd, 1H). 13C NMR
(100 MHz, CDCl3): d: 53.37, 69.87, 71.32, 72.07, 117.36, 134.36 ppm.
Results and discussion
Due to the importance of PTCs in organic reactions and the need to recycle the
catalyst, we designed a recyclable PTC capable of enhancing nucleophilic ring-
opening reactions. Among PTCs, polyethylene glycol (PEG) is an efficient solvent
123

Synthesis and characterization of a novel…
and also a nonionic surfactant that has terminal hydroxyl groups capable of
immobilizing on solid supports. Thus, as shown in the SEM images, Fe3O4
nanoparticles 50 nm in size were prepared (Fig. 1).
The Fourier transform infrared spectroscopy (FTIR) spectrum of nano-Fe3O4
shows its characteristic Fe–O peak at 570 cm-1. A broad peak at 3400 cm-1 is
evident, which is due to OH groups on the surface of the nanoparticles (Fig. 2).
In the next step, the random copolymer of styrene/acrylic acid was prepared via
suspension free-radical polymerization of styrene (57.3 %) and acrylic acid
(41.5 %) in the presence of 1.2 % divinylbenzene (DVB) as cross-linking agent.
Deionized water was used as the polymerization medium and dibenzoyl peroxide as
radical initiator (Scheme 2).
The next step involved the grafting of PEG 300 to the prepared copolymer. Two
methods were investigated. In the first method, copoly(styrene/acrylic acid) was
refluxed in the presence of SOCl2 so that carboxylic acid groups were transformed
into acid chloride. After refluxing for 2 h, the excess thionyl chloride was distilled
off and PEG 300 was added (Scheme 3, reaction A).
Fig. 1 SEM images of nano-Fe3O4
Fig. 2 FTIR spectrum of nano-Fe3O4
123

A. R. Kiasat et al.
CH2 CH
COOH
dibenzoyl peroxide
H2O,70oC
CH2 CH
Ph
CH
Ph
CH
CO2H
H2C
+
H2C
CH
+
H2C
Scheme 2 Preparation of copoly(styrene/acrylic acid) via suspension free-radical polymerization
COCl
P
E
A
B
G
-O
H
l2
C
O
S
COOH
C
O
O
PEG
PEG-OH
H+
Scheme 3 Grafting PEG 300 onto copoly(styrene/acrylic acid) via A and B methods
Fig. 3 TGA diagram of copoly(styrene/acrylic acid)/grafted PEG
In the second method, direct esterification was used for immobilization of PEG
300 onto the copolymer in the presence of sulphuric acid as catalyst (Scheme 3,
reaction B).
A comparison of the two methods revealed that method A caused the copolymer
to be brownish and jelly-like, and some decomposition was seen; therefore, we
selected method B. Based on gravimetric analysis, 0.37 g of PEG was grafted onto
1.6 g of copolymer.
To evaluate the thermal stability of the grafted copolymer, thermogravimetric
analysis (TGA) was conducted. As shown in the TGA diagram, the grafted
copolymer is stable up to 400 °C (Fig. 3).
123

Synthesis and characterization of a novel…
Fig. 4 FTIR spectrum of copoly[(styrene/acrylic acid)/grafted PEG]
Fig. 5 FTIR spectrum of nano-Fe3O4-PS-Co-[PAA-g-PEG]
FTIR spectrum of the copoly(styrene/acrylic acid)/grafted PEG showed stretch-
ing peaks at 1722 cm-1 due to ester groups and a broad peak at 3453 cm-1 due to
OH groups of grafted PEG (Fig. 4).
Finally, the nano-Fe3O4 was coated with the PEG-grafted copolymer in acetone
via sonication, refluxing, and evaporation.
FTIR spectrum of nano-Fe3O4-PS-Co-[PAA-g-PEG] showed a characteristic
peak of nano-Fe3O4 and also the grafted copolymer peaks (Fig. 5).
123

A. R. Kiasat et al.
Fig. 6 TGA diagram of nano-Fe3O4-PS-Co-[PAA-g-PEG]
Fig. 7 SEM images of nano-Fe3O4-PS-Co-[PAA-g-PEG]
The TGA diagram of the catalyst is shown in Fig. 6. The catalyst is stable up to
390 °C.
SEM images of the magnetic catalyst were also recorded (Fig. 7).
The X-ray diffraction pattern of nano-Fe3O4 and the nanocomposite was recorded
(Fig. 8). We can see that the sites and intensity of the diffraction peaks are
consistent with the standard pattern for JCPDS Card No. (79 - 0417) Magnetite–
synthetic [29].
After characterizing nano-Fe3O4-PS-Co-[PAA-g-PEG] (Fig. 9) using FTIR, SEM
and TGA, we evaluated its catalytic activity as a phase-transfer catalyst in the ring
opening of epoxides.
First, a model reaction of 2,3-epoxypropyl phenyl ether and sodium azide in the
presence of the nanocomposite in 5 mL of deionized water was conducted under
123

Synthesis and characterization of a novel…
Fig. 8 X-ray diffraction pattern of the nano-Fe3O4 (a) and nanocomposite (b)
reflux conditions. According to TLC, only one spot was seen after 50 min.
Optimization of the amount of reactants and the catalyst showed that 2 mmol of
sodium azide and 0.02 g of the catalyst completed the reaction in the minimum
length of time (Table 1).
Subsequently, under optimal conditions, 1 mmol epoxide: 2 mmol azide: 0.02 g
nanocomposite and 5 mL deionized water under reflux conditions, the generality
and synthetic scope of this coupling protocol were demonstrated by synthesizing a
series of b-azido alcohols (Table 2) in 5–120 min and 78–99 % yield.
Regioselectivity in the ring opening reaction of epoxides is relevant to the
reaction mechanism, specifically to electronic and steric effects. The azide anion
attacks the less crowded carbon of all epoxides but styrene oxide. With styrene
oxide, despite having a phenyl group as an electron-withdrawing group, the
electronic effect precedes the steric effect such that 2-azido-2-phenyl ethanol is the
major product, with 83 % yield (Table 2).
After the success of nano-Fe3O4-PS-Co-[PAA-g-PEG] in the preparation of b-
azido alcohols, we decided to explore its catalytic activity in the preparation of b-
nitro alcohols. Subsequently, the ring opening of epoxides (1 mmol) was conducted
with sodium nitrite (10 mmol) using nano-Fe3O4-PS-Co-[PAA-g-PEG] (0.03 g) as
123

A. R. Kiasat et al.
Fig. 9 Structure of nano-
Fe3O4-PS-Co-[PAA-g-PEG]
HO
O
O
O
HC
COOH
O
C
CH2
H
O
COOH
HC
C
Ph
H2C
CH
CH2
Ph
H2C
HC
H2C
CH2
CH
H2C
O
C
O
Ph
C
H
H2C
O
HOOC
CH
O
O
OH
Table 1 Optimization of reaction conditions for the preparation of b-azido alcohol from 2,3-e-
poxypropyl phenyl ether, in the presence of the nanocatalyst in aqueous medium
Entry
NaN3 (mmol)
Nanocomposite (g)
Temp (°C)
Time (min)
1
2
3
4
5
6
7
1.5
2
0.01
0.01
0.01
0.02
0.02
0.03
0.02
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
r.t.
65
55
3
50
3
35
2
35
2
35
2
240
the magnetic PTC under the above optimized reaction condition. All epoxides
reacted almost equally well to afford corresponding b-nitro alcohols in high to
excellent yields (Scheme 4; Table 3).
We can postulate that the nanocomposite facilitates the nucleophilic attack of the
azide anion by chelating the cation by PEG (Fig. 10).
Given the importance of green chemistry and the recycling of solvents and
catalysts, the recyclability of the nanocomposite was assessed in the reaction of 2,3-
epoxypropyl phenyl ether. The result is shown in Fig. 11. The activity of the
nanocomposite was slightly decreased after reuse five times.
123

Synthesis and characterization of a novel…
Table 2 Preparation of b-azido alcohols from corresponding epoxides and sodium azide in the presence
of the nanocomposite in aqueous medium under reflux conditions
O
OH
Fe3O4 -PS -Co-[PAA-graft PEG](0.03 g)
H2O, reflux, 25 min
+ NaNO2
PhO
PhO
NO2
10 mmol
1 mmol
Scheme 4 Ring opening of epoxides with nitrite using nano-Fe3O4-PS-Co-[PAA-g-PEG]
To demonstrate the superiority of nano-Fe3O4-PS-Co-[PAA-g-PEG] over the
reported catalysts, the reaction of 2,3-epoxypropyl phenyl ether was considered as a
representative example (Table 4). While in all of these cases, comparative yields of
the desired product were obtained following the nano-Fe3O4-PS-Co-[PAA-g-PEG]-
123

A. R. Kiasat et al.
Table 3 Preparation of b-azido alcohols from corresponding epoxides and sodium nitrite in the presence
of the nanocomposite in aqueous medium under reflux conditions
Fig. 10 Chelating sodium by
PEG which is immobilized on
the nanocomposite
CH
C
H2C CH CH2
Ph
H
Na+
O
O
O
C
O
O
O
O
O
H
O
-
N3
catalyzed procedure (except entry 1), all of the reported procedures required long
reaction times (except entries 5 and 6) or high catalyst loading (entries 5 and 6).
These results clearly demonstrate that nano-Fe3O4-PS-Co-[PAA-g-PEG] is an
equally or more efficient catalyst for this reaction.
123

Synthesis and characterization of a novel…
99
99
98
Yield, %
98
98
97
96
95
94
97
96
1
2
Yield, %
3
4
5
No. of Runs
Fig. 11 Reusability of the nanocomposite in the reaction of 2,3-epoxypropyl phenyl ether and sodium
azide
Table 4 Comparison of azidolysis of 2,3-epoxypropyl phenyl ether with other methods reported in the
literature
Entry NaN3 (mmol) Catalyst
Time (h) Yield (%) Ref.
1
2
3
4
5
6
7
8
a
1.33
1.1
5
b-cyclodextrin/1.13 g
CeCl3Á7H2O/0.187 g
PTCa/0.1 g
AMPb/(10 mol%)
PEG-g-Dowex resin/0.1 g
MPTCc/0.2 g
5
45
99
94
96
96
85
90
99
[25]
3
[11]
12
3–5
0.75
0.5
1
[12]
1.5
1.2
3
[26]
[19]
[27]
2
–
[28]
2
Fe3O4-PS-Co-[PAA-graft PEG]/0.02 g 0.5
This work
Cross-linked poly acrylamide
b
Ammonium-12-molybdophosphate (NH4)3[PMo12O40
]
c
Multi phase-transfer catalyst
Conclusions
In conclusion, A novel nano-Fe3O4@copoly[(styrene/acrylic acid)/grafted ethylene
oxide was synthesized and its application as a magnetic and recyclable phase-
transfer catalyst in the preparation of b-azido alcohols and b-nitro alcohols was
investigated. The experimental results showed that the regioselective ring opening
of epoxides in the presence of azide and nitro anions in aqueous medium led to the
corresponding products in 5–120 min and 78–99 % yield. In addition to the
recyclability of the catalyst via an external magnet, the short reaction times, high
product yield, regioselectivity, facile workup, and eco-friendly and green nature of
the procedure are notable advantages of this method.
Acknowledgments We are grateful to the Research Council of Shahid Chamran University for financial
support.
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A. R. Kiasat et al.
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