Greener and facile aqueous regioselective synthesis of vicinal azidoalcohols using silica-bound 3-((polyethyleneglycol)ethyl)-8-methyl-1H- imidazol-3-ium bromide as a recyclable catalyst

Article abstract of DOI:10.1007/s13738-013-0257-x

An environmentally benign, aqueous synthesis of vicinal azidoalcohols by the regioselective nucleophilic ring opening of epoxides with azide anion using silica-bound 3-((polyethyleneglycol)ethyl)-8-methyl-1H-imidazol-3-ium bromide as an effective heteroge

Full text of DOI:10.1007/s13738-013-0257-x

J IRAN CHEM SOC
DOI 10.1007/s13738-013-0257-x
ORIGINAL PAPER
Greener and facile aqueous regioselective synthesis of vicinal
azidoalcohols using silica-bound 3-((polyethyleneglycol)ethyl)-8-
methyl-1H-imidazol-3-ium bromide as a recyclable catalyst
•
Ali Reza Kiasat Neda Ayashi
•
Mehdi Fallah-Mehrjardi
Received: 2 November 2012 / Accepted: 25 March 2013
Ó Iranian Chemical Society 2013
Abstract An environmentally benign, aqueous synthesis
of vicinal azidoalcohols by the regioselective nucleophilic
ring opening of epoxides with azide anion using silica-
bound 3-((polyethyleneglycol)ethyl)-8-methyl-1H-imida-
zol-3-ium bromide as an effective heterogeneous phase
transfer catalyst is described. Short reaction time, simple
work-up procedure, high yield, reusability, and use of an
eco-friendly catalyst are some of the striking features of the
present protocol.
Obviously, water does not dissolve most of the organic
reactants, but that fact has been recognized as a benefit on
rates and selectivities of several transformations [4].
Moreover, the addition of a phase transfer agent enables to
improve an intimate contact with an organic layer [5–7].
One practical limitation to the phase transfer method,
however, is that many of the catalysts used promote the
formation of stable emulsions [8]. Immobilization of the
phase transfer catalyst on an insoluble matrix can provide a
simple solution to this problem. This technique would have
considerable advantages. Not only the catalyst recovery
and product isolation would be greatly simplified, but also
owing to the three-phase nature of the system, continuous
flow methods could be employed in making the technique
particularly attractive for industrial applications.
Keywords 1,2-Azidoalcohol Á Phase transfer catalyst Á
Regioselective Á Ring opening Á Epoxide
Introduction
1,2-Azidoalcohols are versatile intermediates in organic
synthesis, since they are important precursors of b-ami-
noalcohols and vicinal diamines, which are present in
numerous natural products [9]. Furthermore, their impor-
tance is significant in the chemistry of carbohydrates and
nucleosides [10].
The concept of green chemistry has been playing an
important role in recent years for meeting the fundamental
scientific challenges of protecting the living environment
[1]. Again, as a result of global environmental legislation
on chemical process industries, aqueous environment has
been currently receiving considerable attention in organic
chemistry because water is abundant in nature, has virtually
no cost, and is the safest among all available solvents, thus
leading to environmentally benign chemical processes [2,
3].
The simple and the most straightforward synthetic
method for the preparation of 1,2-azidoalcohols involves
the regioselective ring opening of epoxides with sodium
azide under various reaction conditions [11]. These epox-
ide opening reactions are, however, generally carried out
under either acidic or basic conditions [11, 12] and they
usually require high temperatures and/or long reaction
times, and side reactions, such as isomerization, epimer-
ization and rearrangements may be induced by the alkaline
conditions. Furthermore, high temperatures are detrimental
not only to certain functional groups, but also in the con-
trol of regioselectivity. A variety of promoters have been
reported to assist epoxide ring-opening reactions with
sodium azide under mild conditions [13–18]. Since
A. R. Kiasat (&) Á N. Ayashi
Chemistry Department, College of Science, Shahid Chamran
University, 6135743169 Ahvaz, Iran
e-mail: akiasat@scu.ac.ir
M. Fallah-Mehrjardi
Marine Chemistry Department, Khorramshahr University
of Marine Science and Technology, Khorramshahr, Iran
123

J IRAN CHEM SOC
N3
120° C overnight. After cooling, CH2Cl2 (20 ml) was
added, the solution was filtered, and the solvent was
removed to obtain polyethylene glycol monobromide
(26 g, 87 %).
O
OH
(II)
SiO2-PEG-ImBr
NaN3
+
OH
N3
H2O/ reflux
0.5-2.5 h
+
R
R
R
(I)
80-98%
Preparation of silica chloride [27]
Scheme 1 Synthesis of 1,2-azidoalcohols from epoxides in the
presence of phase transfer catalysis
To the dried activated silica gel (20 g) was added dropwise
freshly distilled SOCl2 (45 mL) under nitrogen atmosphere
and at room temperature. Evolution of copious amounts of
HCl and SO2 occurred instantaneously. After stirring for
another 4 h, the excess unreacted thionyl chloride was
distilled off and the resulting grayish silica chloride was
flame dried, stored in air-tight container and used as such
for the reactions.
1,2-azidoalcohols have become increasingly useful and
important in drugs and pharmaceuticals, the development
of simple, efficient and environmentally friendly processes
for their synthesis is desirable.
In continuation of our studies on the use of phase transfer
catalysts in organic synthesis [19, 20], specially in the ring
opening of epoxides [21–24], very recently we introduced
silica-bound 3-((polyethyleneglycol)ethyl)-8-methyl-1H-
imidazol-3-ium bromide as phase transfer catalyst in cya-
nohydrine synthesis [25]. We now wish to report here the
synthetic applicability of this catalyst for regioselective ring
opening of epoxides with azide anion in water (Scheme 1).
Immobilization of polyethylene glycol monobromide
on silica gel, SiO2-PEG-Br [25]
To a well-stirred silica chloride (11 g) in dry CH2Cl2
(40 mL) was added dropwise HO-PEG-Br (5 g) under
nitrogen atmosphere and at room temperature. HCl was
instantaneously evaluated. After stirring for another 3 h,
the obtained SiO2-PEG-Br was removed by filtration and
the product was washed several times by acetone
(3 9 20 mL) and dried.
Experimental
Chemicals were purchased from Fluka, Merck and Aldrich
Chemical Companies. Polyethylene glycol (600) was
heated at 80 °C under vacuum for 30 min before use to
remove traces of moisture. Yields refer to isolated crude
products. NMR spectra were recorded in CDCl3 on a
Bruker Avance DPX 400 MHz instrument spectrometer
using TMS as internal standard. The purity determination
of the products and reaction monitoring were accomplished
by TLC on silica gel poly Gram SILG/UV 254 plates.
Preparation of SiO2-PEG-ImBr [25]
A mixture of SiO2-PEG-Br (14 g) and 1-methyl imidazole
(8.2 g, 0.1 mol) was heated with stirring at 80° C in an oil
bath for 48 h. The inorganic–organic graft copolymer was
washed with Et2O (20 mL) and then with MeCN (20 mL)
and dried.
Preparation of polyethylene glycol monochloride,
HO-PEG-Cl [26]
General procedure for the preparation of
1,2-azidoalcohols in water
To a solution of polyethylene glycol 600 (30 g, 0.1 mol OH)
and pyridine (7.9 g, 0.1 mol) in 500 mL of toluene, thionyl
chloride (8 g, 0.07 mol)wasslowlyadded withstirringover a
period of half an hour. The mixture was then refluxed for
about 6 h. After cooling and filtering off the pyridine
hydrochloride salt, the solvent was removed in vacuo. The
residue was dissolved in CH2Cl2 and treated with activated
alumina. The process was repeated twice. The CH2Cl2 solu-
tion was filtered and evaporated to dryness. The yield was
28 g (90 %) of polyethylene glycol monochloride.
To a 25-mL round-bottomed flask were added 0.2 g of
SiO2-PEG-ImBr, 1 mmol of epoxide, 2 mmol of sodium
azide (0.65 g) and 5 mL of water. The mixture was stirred
at 90 °C. Progress of the reaction was monitored by TLC
using hexane:AcOEt (5:1). After completion of the reac-
tion, the catalyst was recovered for reuse by filtration and
the product was extracted with Et2O (3 9 10 mL). The
solvent was dried with CaCl2 and evaporated under
reduced pressure. The desired 1,2-azidoalcohols were
obtained in high isolated yields (80–98 %).
Preparation of polyethylene glycol monobromide,
HO-PEG-Br [26]
Selected spectroscopic data
2-Azido-2-phenyl-1-ethanol IR (neat): vꢀ N3 (2,102 cm-1);
1H-NMR (400 MHz, CDCl3): d 2.98 (s, 1H), 3.22 (m, 2H),
A mixture of HO-PEG-Cl (28 g, 45 mmol) and sodium
bromide (15.5 g, 150 mmol) was heated in an oil bath at
123

J IRAN CHEM SOC
4.56 (m, 1H), 7.14–7.33 (m, 5H). 13C-NMR (100 MHz,
CDCl3): d 66.17, 68.14, 127.33, 128.45, 129.01, 136.76.
1-Azido-3-phenoxypropan-2-ol IR (neat): vꢀ N3
(2,105 cm-1); 1H-NMR (400 MHz, CDCl3): d 2.06 (d,
2H), 2.69–2.81 (m, 1H), 4.05 (d, 2H), 4.32 (s, 1H),
6.91–6.97 (m, 2H), 7.01–7.04 (m, 1H), 7.29–7.33 (m, 2H).
13C-NMR (100 MHz, CDCl3): d 53.72, 68.82, 69.59,
115.14, 121.39, 129.93, 158.80.
2,800–2,950 cm-1 and 1,400–1,500 cm-1, respectively. In
addition, the presence of imidazolium unit was confirmed
by the bands at 1,619 cm-1 (imidazole –C=N bending) and
1,567 cm-1 (imidazole ring stretching). Therefore, the
successful introduction of PEG-ImBr onto silica surface
was verified through the FT-IR spectra.
The thermogravimetric analysis (TGA) curve of the
SiO2-PEG-ImBr showed mass loss of the organic materials
as they decompose upon heating (Fig. 2). The initial
weight loss up to 126 °C is due to the removal of physi-
cally adsorbed water and surface hydroxyl groups. The
weight loss of about 18 % between 150 and 420 °C may be
associated to the thermal decomposition of PEG-ImBr. On
the basis of this observation, the well grafting of PEG-ImBr
on the MNPs was also verified.
1-Azido-3-butoxypropan-2-ol IR (neat): vꢀ N3
(2,103 cm-1); 1H-NMR (400 MHz, CDCl3): d 0.87 (t, 3H),
1.22–1.27 (m, 2H), 1.61–1.64 (m, 2H), 3.12 (s, 1H),
3.49–3.52 (m, 2H), 3.67 (t, 2H), 3.80 (d, 2H), 3.99–4.02
(m, 1H). 13C-NMR (100 MHz, CDCl3): d 15.18, 19.28,
32.47, 54.67, 68.77, 71.55, 71.67.
2-Azidocyclohexan-1-ol IR (neat): vꢀ N3 (2,096 cm-1);
1H-NMR (400 MHz, CDCl3): d 1.22–1.34 (m, 4H),
1.72–1.78 (m, 2H), 2.00–2.07 (m, 2H), 2.50 (s, 1H),
3.10–3.22 (m, 1H), 3.28–3.42 (m, 1H). 13C-NMR
(100 MHz, CDCl3): d 24.38, 24.48, 24.51, 33.14, 62.14,
73.79.
To determine the amount of imidazolium bromide in
SiO2-PEG-ImBr, 0.01 g of the catalyst was immersed into
20 ml of 0.1 mol L-1 aqueous HNO3 solution. The bro-
mide ions were determined by Volhard’s method. The
amount of bromide equaled by imidazolium units was
2.85 mmol g-1. The amount of imidazolium units was also
determined by CHN analysis method (2.667 mmol g-1
)
Results and discussion
(Fig. 3) which is in good agreement with the obtained
result from the Volhard’s titration method.
A general synthetic route for the preparation of the silica-
bound 3-((polyethyleneglycol)ethyl)-8-methyl-1H-imida-
zol-3-ium bromide, SiO2-PEG-ImBr, is presented in
Scheme 2.
To start our investigation, we studied different solvent
effects on the course of the reaction of styrene oxide
(1 mmol) with azide anion (2 mmol) promoted by SiO2-
PEG-ImBr (0.2 g). The reaction was carried out in dieth-
ylether, chloroform, acetonitrile, dichloromethane, water
and ethanol. From the results given in Table 1, it fallows
that the best solvent for this reaction is water. Although,
The FT-IR spectra of silica gel and SiO2-PEG-ImBr are
shown in Fig. 1. In the SiO2-PEG-ImBr spectrum (b), the
C–H stretching and bending bands can be observed at
Scheme 2 Representative
preparation of silica-bound
3-((polyethyleneglycol)ethyl)-8-
methyl-1H-imidazol-3-ium
bromide
O
SOCl2/pyridine
O
NaBr
180oC
HO
HO
nCH2CH2OH
nCH2CH2Br
HO
nCH2CH2Cl
toluene
HO-PEG-Cl
O
HO-PEG-Br
CH2Cl2
N2/ 3 h
O
SiO2
O
nCH2CH2Br
SiO2-PEG-Br
N2
SiO2
OH + SOCl2
SiO2
Cl
r.t. / 4h
1-methyl imidazole
80oC/ 48 h
O
NCH3
+
SiO2
O
nCH2CH2
N
Br-
SiO2-PEG-ImBr
123

J IRAN CHEM SOC
Table 1 The effect of solvent on reaction of styrene oxide with NaN3
(2 mmol) in the presence of SiO2-PEG-ImBr
Entry
Solvent
Condition
Time (h)
Result
1
2
3
4
5
6
7
Et2O
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
r.t.
3
Not completed
Not completed
Not completed
Completed
CHCl3
CH2Cl2
EtOH
CH3CN
H2O
3
3
2.5
1.5
0.5
3
Completed
Completed
H2O
Not completed
Si-O-H
O
O
Si
O
Fig. 1 FT-IR spectra of a silica, and b SiO2-PEG-ImBr
O
O
O
Na+
-
O
N3
Si-OH
+
O
MeN
N
-
N3
Scheme 3 Catalytic activity of silica-bound catalyst
This organic–inorganic hybrid catalyst contains three
different units linked together and we believed each unit
has especial role in the ring opening of epoxides by azide
anion. It seems that polyethylene glycol units in SiO2-PEG-
ImBr encapsulate alkali metal cations, much like crown
ethers, and these complexes cause the anion to be activated.
The 1-methylimidazol-3-ium units introduced ionic liquid
property to the catalyst. In addition, silanol hydroxy groups
at the silica gel surface, probably, facilitate the ring
opening of the epoxide by hydrogen bonding (Scheme 3).
Fig. 2 TGA curve of SiO2-PEG-ImBr
acetonitrile can also be used as a solvent in this reaction,
water is an excellent solvent in terms of cost, availability,
and environmental impact and shorter reaction times.
Fig. 3 CHN analysis of SiO2-
PEG-ImBr
123

J IRAN CHEM SOC
To ascertain the scope and limitation of the present
reaction, different types of epoxides carrying activating and
deactivating groups were cleanly, easily and efficiently
converted to the corresponding 1,2-azidoalcohols in water
at 90 °C in high isolated yields (Table 2). The structure of
each product was confirmed from its analytical and spectral
(IR, 1H and 13C-NMR) data and by direct comparison with
an authentic sample [10e,12d].
1,2-azidoalcohol with preferential nucleophilic attack at
benzylic position and was produced 2-phenyl-2-azidoethanol
as sole product (Table 3, entry 1). Epoxides carrying elec-
tron-withdrawing groups reacted under similar reaction
conditions with azide anion and their corresponding 1,2-az-
idoalcoholswereproducedwithpreferentialattackatthenon-
benzylic terminal carbon of the epoxide (Table 2, entries
2–5). Furthermore, cyclic epoxides, such as cyclohexene
oxide reacted smoothly in SN2 fashion to afford the corre-
sponding 1,2-azidoalcohols (Table 2, entries 6–8). The ste-
reochemistry of the ring-opening products was found to be in
the trans-configuration as determined from the coupling
constants associated with the 1H NMR spectral.
The regioselectivity of the ring opening of epoxides is
dependent on the mechanism of the reaction and particularly
on steric and electronic factors. Azide anion underwent the
reaction with styrene oxide in the presence of SiO2-PEG-
ImBr in a regioselective manner to give the corresponding
Table 2 Conversion of
epoxides to 1,2-azidoalcohols
with NaN3 (2 mmol) in the
presence of SiO2-PEG-ImBr in
water at 90 °C
Time
(h)
Yields
(%)
Entry
1
Substrate
Product(s)
O
N3
0.5
1
98
98
OH
Ph
Ph
OH
O
2
3
N3
PhOCH2
PhOCH2
OH
O
1.5
0.5
2.5
93
90
80
N3
CH2=CHCH2OCH2
(CH3)2CHOCH2
CH2=CHCH2OCH2
OH
O
4
5
6
N3
(CH3)2CHOCH2
OH
O
N3
CH3(CH2)2CH2OCH2
CH3(CH2)2CH2OCH2
OH
O
1.5
85
N3
OH
O
N3
7
8
2
91b,c
a
Products were identified by
comparison of their physical
and spectral data with those of
authentic samples
OH
90 b,c
O
2.5
b
3 mmol NaN3 was used
N3
c
Purified by column
chromatography
123

J IRAN CHEM SOC
Table 3 Comparison of azidolysis of some epoxides with different catalysts
Entry
1
Epoxide
Catalyst/reagent/solvent
Time (h)
Yield (%)
Reference
Zr(OTf)4 (0.1 mmol)/TMGA/CH3CNa
Polymeric PTC (0.1 g)/NaN3/H2Ob
LiClO4 (1.5 mmol)/NaN3/CH3CN
Mg(ClO4)2 (1.5 mmol)/NaN3/CH3CN
NH4Cl (2.2 mmol)/NaN3/MeOH-H2O
CeCl3 (0.5 mmol)/NaN3/CH3CN-H2O
SiO2-PEG-ImBr (0.2 g)/NaN3/H2Oc
Zr(OTf)4 (0.1 mmol)/TMGA/CH3CNa
Polymeric PTC (0.1 g)/NaN3/H2Ob
LiClO4 (1.5 mmol)/NaN3/CH3CN
Mg(ClO4)2 (1.5 mmol)/NaN3/CH3CN
NH4Cl (2.2 mmol)/NaN3/MeOH-H2O
CeCl3 (0.5 mmol)/NaN3/CH3CN-H2O
SiO2-PEG-ImBr (0.2 g)/NaN3/H2Oc
Zr(OTf)4 (0.1 mmol)/TMGA/CH3CNa
Polymeric PTC (0.1 g)/NaN3/H2Ob
LiClO4 (1.5 mmol)/NaN3/CH3CN
Mg(ClO4)2 (1.5 mmol)/NaN3/CH3CN
NH4Cl (2.2 mmol)/NaN3/MeOH-H2O
CeCl3 (0.5 mmol)/NaN3/CH3CN-H2O
SiO2-PEG-ImBr (0.2 g)/NaN3/H2Oc
2
12
5
88
94
97
96
95
99
98
84
95
92
78
94
89
98
84
89
95
94
92
98
85
[28]
[29]
[30]
[30]
[30]
[31]
–
O
PhOCH2
2
12
3
1
2
3
a
42
6
[28]
[29]
[30]
[30]
[30]
[31]
–
O
5
Ph
2
20
3
0.5
42
8
[28]
[29]
[30]
[30]
[30]
[31]
–
O
36
24
36
3
1.5
TMGA: 1,1,3,3-tetramethylguanidinium azide
Polymeric PTC: poly[N-(2-aminoethyl)acrylamido]-trimethyl ammonium iodide
Present method
b
c
The reusability of the catalyst is important for the large-
Recyclability of catalyst
scale operation and industrial point of view. Therefore, the
recovery and reusability of SiO2-PEG-ImBr was examined.
The catalyst was separated and reused after washing with
water and methanol and drying at 70 °C. The reusability of
the catalyst was investigated in the ring opening of styrene
oxide (Fig. 4). The results illustrated in Fig. 1 showed that
the catalyst can be reused five times with consistent yield.
The color of the catalyst remains same even after the fifth
cycle.
100
90
80
In order to show the merit of our procedure for the
synthesis of 1,2-azidoalcohols, we have shown the advan-
tages of the present procedure by comparing our results
with those previously reported in the literature (Table 3).
70
1
2
3
4
5
No. of cycles
Fig. 4 Recyclability of the catalyst
Conclusion
In conclusion, in this study we have introduced a simple,
rapid and mild procedure for the regioselective preparation
of 1,2-azidoalcohols using a new phase transfer catalyst
bound to silica gel in water. The advantages of the present
protocol, such as being an environmentally friendly alter-
native, relatively short reaction times, simplicity in
operation, high regioselectivity, and high yield of products,
make this new process an attractive alternative to current
methodologies.
Acknowledgments We are grateful to the Research Council of
Shahid Chamran University for financial support.
123

J IRAN CHEM SOC
17. A.R. Kiasat, M. Fallah-Mehrjardi, J. Iran. Chem. Soc. 6, 542
(2009)
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