Facile synthesis of an organic-inorganic nanocomposite, PEG-silica, by sol-gel method; Its characterization and application as an efficient catalyst in regioselective nucleophilic ring opening of epoxides: Preparation of β-azido alcohols and β-cyanohydrin

Article abstract of DOI:10.1016/j.crci.2013.07.008

The sol-gel method was used for the synthesis of a PEG-silica hybrid. In order to introduce PEG into the cavities of silica gel, first, the bis(3-trimethoxysilylpropyl)-polyethylene glycol precursor was synthesized by the reaction of 3-chloropropyltrimeth

Full text of DOI:10.1016/j.crci.2013.07.008

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CRAS2C-3797; No. of Pages 7
C. R. Chimie xxx (2013) xxx–xxx
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Comptes Rendus Chimie
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Full paper/M e´ moire
Facile synthesis of an organic–inorganic nanocomposite,
PEG–silica, by sol–gel method; its characterization and
application as an efficient catalyst in regioselective
nucleophilic ring opening of epoxides: Preparation of
b
-azido alcohols and b-cyanohydrins
Ali Reza Kiasat *, Simin Nazari, Jamal Davarpanah
Chemistry Department, College of science, Shahid Chamran University, Ahvaz 61357-4-3169, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
The sol–gel method was used for the synthesis of a PEG–silica hybrid. In order to introduce
PEG into the cavities of silica gel, first, the bis(3-trimethoxysilylpropyl)-polyethylene
glycol precursor was synthesized by the reaction of 3-chloropropyltrimethoxysilane with
alkoxides formed on the PEG terminals. The organic–inorganic hybrid silica was then
synthesized by hydrolysis and polycondensation of the precursor under mild acidic
conditions. The characteristics results of FT–IR, XRD and TGA confirmed the coexistence of
silica and PEG networks. The catalytic ability of this heterogeneous catalyst to the
Received 13 March 2013
Accepted after revision 11 July 2013
Available online xxx
Keywords:
PEG–silica hybrid
Sol–gel
regioselective ring opening of epoxides by azide and cyanide anions in H
investigated.
2
O was also
Ring opening
Epoxides
ß 2013 Acad e´ mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
b
-Azido alcohols
b
-Cyanohydrins
1
. Introduction
The study of inorganic–organic hybrid materials has
According to the nature of the links and interactions
between the organic and inorganic parts, the hybrids can
be divided into two main different classes [6]: the first
class would be composed of the molecule-based composite
materials, in which the organic and inorganic components
are linked together through strong chemical interactions
(covalent, ion-covalent, or coordination bonds), and the
second one belongs to the hybrid systems, in which one of
the components (usually the organic) is entrapped within a
network of the other. For the latter, only weak interactions,
such as hydrogen bonding, Van der Waals forces or weak
static effects, operate [7,8]. These materials with tailored
properties are adequate for use as adsorbent synthesis [9],
gas separation techniques [10], catalysis [11–14], mole-
cular recognition [15,16], nanoreactors [17], and biological
uses [18,19].
showed considerable attraction due to their potential for
the development of new material with designed properties
[
1,2]. Hybrid materials are in the edge of two different
chemical worlds, since they not only combine the
advantages of both organic components (flexibility, low
dielectric constant, low density and process ability) and
inorganic ones (rigidity, strength, durability, and heat
resistance), but also often exhibit exceptional properties
that exceed what would be expected for a simple mixture
of the components [3–5].
Silica is a widely involved inorganic material in the
synthesis of inorganic–organic hybrid materials. On the
*
Corresponding author.
E-mail address: akiasat@scu.ac.ir (A.R. Kiasat).
1
631-0748/$ – see front matter ß 2013 Acad e´ mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.crci.2013.07.008
Please cite this article in press as: Kiasat AR, et al. Facile synthesis of an organic–inorganic nanocomposite, PEG–silica, by
sol–gel method; its characterization and application as an efficient catalyst in regioselective nucleophilic ring opening of
epoxides: . . . C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.07.008

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A.R. Kiasat et al. / C. R. Chimie xxx (2013) xxx–xxx
other hand, poly(ethylene glycol) is the most useful
polymer in terms of its water solubility, body reservation,
and biological safety. It has been used as a catalyst or (and)
solvent in many organic transformations [20–22]. The
main purposes of the present work are sol–gel synthesis,
structural characterization and potential applications of
PEG–silica hybrids as new phase transfer catalysts for
Then, the system was stirred at 60 8C for 1 h to complete
the terminal alkoxides formation of PEG and cooled again
to 0 8C. To the mixture, 3-chloropropyltrimethoxysilane
(5.97 g, 30 mmol) was added and stirred at 60 8C for 90 h.
After removing of the solvent under reduced pressure, 7.5 g
of deionized water and 33.75 g of 2 M HCl solution were
added, then the mixture is stirred at 40 8C for 24 h under an
argon atmosphere. The resulting mixture was then
transferred into a Teflon-lined autoclave and heated at
rapid, efficient and regioselective formation of
alcohols and -cyanohydrins.
b-azido
b
100 8C for 72 h under static conditions. The obtained
mixture was first thoroughly washed with deionized
water/ethanol solvent and then dried at room tempera-
ture.
2
. Experimental
.1. General
Epoxides and other chemical materials were purchased
2
2.3. Typical procedure for the preparation of
b-azido alcohols
and -cyanohydrins in water catalyzed by PEG–silica hybrid.
b
from Fluka and Merck in high purity. Products were
characterized by comparison of their physical data (IR, H
1
13
To a magnetically stirred mixture of the epoxide
1.0 mmol) and nucleophilic reagents (NaN or NaCN)
2 mmol) in H O (5 mL), PEG–silica hybrid (0.2 g) was
NMR, and C NMR spectra) with those of known samples.
NMR spectra were recorded in CDCl on a Bruker Advance
DPX 400 MHz instrument spectrometer using TMS as the
internal standard. FT–IR spectra were recorded on a
BOMEM MB-Series 1998 FT–IR spectrometer.
The morphology was examined by SEM using a Philips
XL30 scanning electron microscope. The DTA curve of the
PEG–silica hybrid was recorded on a BAHR, SPA 503 at
(
(
3
3
2
added. The resulting mixture was stirred at 90 8C for the
appropriate time (0.5–2.5 h). After completion of the
reaction as indicated by TLC [using n-hexane/ethyl acetate
(
5:1)], the insoluble catalyst was filtered off and the filtrate
extracted with diethyl ether (2 Â 10 mL). The organic
phase was dried over calcium chloride, and evaporated in
vacuo to give the product in 78–86% isolated yields.
À1
heating rates of 10 8C min
.
2.2. Preparation of PEG–silica hybrid particle
3. Results and discussion
To a solution of PEG-300 (3 g, 10 mmol) in toluene
(
(
20 mL), NaH (1.44 g, 60 mmol) suspended in toluene
10 mL) was added at 0 8C under nitrogen atmosphere.
The synthetic strategy for the PEG–silica hybrid by sol–
gel method is presented in Scheme 1. To introduce PEG into
Scheme 1. Synthesis of the PEG–silica hybrid. Color available online.
Please cite this article in press as: Kiasat AR, et al. Facile synthesis of an organic–inorganic nanocomposite, PEG–silica, by
sol–gel method; its characterization and application as an efficient catalyst in regioselective nucleophilic ring opening of
epoxides: . . . C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.07.008

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3
À1
4
2
60–465 cm , respectively. The peaks at 2928 and
À1
881 cm presented the CH
2
groups in the PEG backbone.
À1
The peaks at 1350 and 1440 cm are shown in those of
PEG with slight shifts. The spectral shifts occurring by a
strong interaction between the two components indicate
the hybrid nature. Through the FT–IR spectra, the
successful introduction of PEG onto the silica surface
was verified.
Fig. 2 shows powder X-ray diffraction (XRD) patterns of
the hybrid material prepared through a covalent self-
assembly process via a sol–gel technology. The broad peak
around 228 in the XRD patterns is ascribed to amorphous
silica [24,25].
The introduction of PEG onto the silica network was
also confirmed through thermogravimetric analysis (TGA)
and differential thermal analysis (DTA). The weight loss by
changing temperature was measured to estimate the rate
of thermal degradation. Fig. 3a shows TGA the thermogram
of the PEG–silica hybrid particles. The TGA curves of the
hybrid particles show a weight loss of about 55% at around
610 8C. The PEG–silica hybrid material loses most of the
weakly bonded water molecules below 300 8C. At higher
Fig. 1. FT–IR spectra (a) of silica, and (b) of the PEG–silica hybrid.
the network structure of the silica matrix, first, the bis(3-
trimethoxysilylpropyl)-polyethylene glycol precursor was
synthesized by the reaction of 3-chloropropyltrimethox-
ysilane with the alkoxides formed on the PEG terminals.
The organic–inorganic hybrid silica, PEG–silica, was then
synthesized by hydrolysis and polycondensation of the
precursor under mild acidic conditions (Scheme 1).
The chemical structure of PEG–silica hybrid was
characterized using FT–IR, XRD, SEM, and DTA analyses.
The FT–IR spectra of the prepared PEG–silica hybrid
(
Fig. 1) showed bands corresponding to the structural units
À1
of the solid network. The broad band near 3400 cm is
assigned to the SiO–H stretching of surface silanols. In
addition, the peak at 1635 cm is also assigned to the
À1
adsorption of moisture on the material surface [23]. The
intensities of these peaks in PEG–silica hybrid were
reduced compared to silica, which indicated the enhanced
hydrophobicity of the PEG–silica hybrid surface. The
À1
intense and broad band at 1100–1050 cm
and the
À1
shoulder at around 1200 cm are attributed to the Si–
O–Si asymmetric stretching vibrations of the dense silica
network. In addition, the Si–O–Si symmetric stretching
vibrations and its bending mode appeared at 800 and
Fig. 2. XRD pattern of the PEG–silica hybrid.
Fig. 3. TGA (a) and DTA (b) curves of the PEG–silica hybrid.
Please cite this article in press as: Kiasat AR, et al. Facile synthesis of an organic–inorganic nanocomposite, PEG–silica, by
sol–gel method; its characterization and application as an efficient catalyst in regioselective nucleophilic ring opening of
epoxides: . . . C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.07.008

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Scheme 2. Preparation of b-azido alcohols and b-cyanohydrins catalyzed
by the PEG–silica hybrid.
addition, the SEM images show the presence of voids that
can be attributed to the silica network. SEM micrographs of
the synthesized nanocomposite also show that the
particles have a spongy structure and a uniform distribu-
tion of the hybrid matrices.
We examined the catalytic ability of PEG–silica hybrid
for nucleophilic ring opening of epoxides in H
ring opening of phenyl glycidyl ether was investigated
with NaN in the presence of PEG–silica hybrid. TLC
analysis of the reaction mixture interestingly showed that
this catalyst acted very efficiently in H O, and that 0.2 g of
2
O. Initially,
3
2
the catalyst was enough to convert 1 mmol of different
epoxides, carrying electron-donating or -withdrawing
groups, to their corresponding
b-azido alcohols in high
yields. It is noteworthy that no evidence of the formation of
diols as a by-product of the reaction was observed (Scheme
2
).
As shown in Table 1, using the optimized reaction
conditions (3:1 molar ratio of NaN :epoxide and 0.2 g of
3
the catalyst), styrene oxide afforded the product of the
nucleophilic attack at the benzylic position as the major
product, while 2-alkyl epoxides gave the products just
formed by the cleavage of the CH
observations demonstrated that in the former case, the
product was generated through the formation of
stabilized benzylic cation during the reaction and that in
the second case, they were formed by predominant attack
of the azide ion on the less hindered carbon of the epoxide.
The structures of the products were established from their
2
–O bond. These
Fig. 4. SEM image (a) of unmodified amorphous silica and (b) of the PEG–
a
silica hybrid.
temperatures, around 300–400 8C, the unreacted PEG
monomers are released and the PEG polymer near the
surface of hybrid particles is also decomposed. Finally, the
PEG polymer is lost in the core of the particles at around
1
13
spectroscopic ( H and
C
NMR) data. Furthermore,
cycloalkyl epoxide reacted
in the presence of
PEG–silica hybrid to afford the corresponding -azido
cyclohexene oxide as
a
6
00–700 8C. Considering that the free PEG decomposes
N 3
smoothly in an S 2 fashion with NaN
completely at 610 8C, its temperature of complete decom-
position is delayed by 300 8C compared to that of the free
PEG polymer.
The DTA curve of the PEG–silica hybrid also shows that
most of the degradation occurred around 620 8C (Fig. 3b),
which is rather high compared to conventional PEG, which
generally starts decomposing at around 400 8C [24]. It is
believed that the thermal stability of PEG bounded to silica
is largely improved because:
b
alcohol in high yield. The configuration of the ring opening
product was found to be trans from the coupling constants
1
of the ring protons in the H NMR spectrum.
It was noted that PEG–silica hybrid did not suffer from
extensive mechanical degradation and was quantitatively
recovered simply by filtration and washing with H
MeOH. It could be reused several times.
2
O and
With this promising results in hand and establishing
the advantages of the PEG–silica hybrid as a phase transfer
ꢀ
ꢀ
the interaction between PEG and silica is very strong
and;
the silica matrix prevents the transfer of heat to the PEG
polymer located inside this network [26,27].
catalyst, we focused our attention to the ring opening of
–
epoxides with another anion, CN in H
2
O. Thus, different
types of oxiranes carrying activated and deactivated
groups were cleanly, easily and efficiently converted into
the corresponding
the optimized reaction condition for N
b-cyanohydrins in good yields under
–
3
. The scope and
The morphology of the samples was observed by
generality of this process is illustrated with several
examples and the results are summarized in Table 1.
It should be pointed out that in the absence of the
catalyst, the reaction was sluggish and a considerable
amount of starting material was recovered unchanged. In
scanning electron microscopy. Fig. 4 shows SEM images
of unmodified amorphous silica [28] and of PEG–silica
hybrid material. The separated and amorphous hybrid
particles were observed for the PEG–silica hybrid. In
Please cite this article in press as: Kiasat AR, et al. Facile synthesis of an organic–inorganic nanocomposite, PEG–silica, by
sol–gel method; its characterization and application as an efficient catalyst in regioselective nucleophilic ring opening of
epoxides: . . . C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.07.008

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5
Table 1
Nucleophilic ring opening of epoxides with azide and cyanide anions in water catalyzed by our polyethylene glycol–silica hybrid.
a
b
Entry
1
Epoxides
Product(s)
Yield (%)
86
O
N3
OH
OH
c
(90:10)
+
+
N3
Ph
OH
Ph
Ph
Ph
90
NC
CN
OH
Ph
2
3
4
5
6
89
O
HO
PhOCH2
PhOCH2
HO
N3
93
PhOCH2
CN
92
O
HO
CH =CHCH OCH
2
2
2
CH =CHCH OCH
2
N3
2
2
90
HO
CH =CHCH OCH
CN
2
2
2
87
O
HO
(
CH ) CHOCH
3 2 2
N3
(
CH ) CHOCH
3 2
2
86
HO
CN
(
CH ) CHOCH
3
2
2
80
O
HO
CH (CH ) CH OCH
2
3
2 2
2
CH (CH ) CH OCH
N3
3
2 2
2
2
93
HO
CN
CH (CH ) CH OCH
3
2 2
2
2
81
OH
O
N3
85
OH
CN
OH
d
7
85
O
N3
Please cite this article in press as: Kiasat AR, et al. Facile synthesis of an organic–inorganic nanocomposite, PEG–silica, by
sol–gel method; its characterization and application as an efficient catalyst in regioselective nucleophilic ring opening of
epoxides: . . . C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.07.008

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Table 1 (Continued )
a
b
Entry
8
Epoxides
Product(s)
Yield (%)
e
40
OH
N3
O
a
Products were identified by comparison of their physical and spectral data with those of authentic samples.
b
c
Isolated yields.
According to GC analysis.
In the presence of pure PEG-300.
In the presence of pure silica.
d
e
phase transfer reactions. It can be emphasized that the
reaction is clean from economical and environmental
points of view, since using water as the solvent is more
favorable than using organic solvents.
Acknowledgements
We are grateful to the Research Council of Shahid
Chamran University for financial support.
Appendix A. Supplementary data
Scheme 3. Postulated roles of the PEG–silica hybrid for the nucleophilic
ring opening of epoxides.
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.crci.2013.07.008.
addition, the reaction medium was contaminated by diol.
The reaction was also performed in the presence of pure
silica (Table 1, entry 7) and pure PEG-300 (Table 1, Entry 8).
As shown in Table 1, although the best results were
obtained in the presence of PEG–silica nanocomposite and
pure PEG-300, in aqueous conditions, the PEG–silica
nanocomposite is recoverable and suitable.
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epoxides: . . . C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.07.008

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Please cite this article in press as: Kiasat AR, et al. Facile synthesis of an organic–inorganic nanocomposite, PEG–silica, by
sol–gel method; its characterization and application as an efficient catalyst in regioselective nucleophilic ring opening of
epoxides: . . . C. R. Chimie (2013), http://dx.doi.org/10.1016/j.crci.2013.07.008