Polyethylene glycol functionalized magnetic dicationic ionic liquids as a novel catalyst and their application in ring opening of epoxides in water

Article abstract of DOI:10.1002/jccs.201400383

A simple and environmentally benign protocol for the aqueous synthesis of 1,2-azidoalcohols via regioselective ring opening of 1,2-epoxides using PEG-MDIL as a novel magnetic phase transfer catalyst is described. The catalyst was studied by UV spectroscop

Full text of DOI:10.1002/jccs.201400383

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JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Polyethylene Glycol Functionalized Magnetic Dicationic Ionic Liquids as a Novel
Catalyst and Their Application in Ring Opening of Epoxides in Water
Bijan Mombeni Godajdar* and Sare Mombeni
Department of Chemistry, Islamic Azad University Omidiyeh Branch, Omidiyeh, Iran
(Received: Sept. 13, 2014; Accepted: Feb. 11, 2015; Published Online: Apr. 17, 2015; DOI: 10.1002/jccs.201400383)
A simple and environmentally benign protocol for the aqueous synthesis of 1,2-azidoalcohols via
regioselective ring opening of 1,2-epoxides using PEG-MDIL as a novel magnetic phase transfer catalyst
is described. The catalyst was studied by UV spectroscopy, IR spectroscopy, and thermogravimetric anal-
ysis. The reactions occur in water and furnish the corresponding azidoalcoholes in high yields. No evi-
dence for the formation of by-product and the products were obtained in pure form without further purifi-
cation.
Keywords: Azidoalcohol; Ring opening; Epoxide; Phase transfer catalyst.
INTRODUCTION
The development of simple and efficient chemical
method used in various fields of organic chemistry, and has
also found widespread industrial applications.
processes or methodologies for the synthesis of biologi-
cally active compounds in water is one of the major chal-
lenges for chemists. Furthermore, using water as a solvent
offers many advantages, such as safe, very cheap, readily
available, and environmentally benign solvent.1 Although
today’s environmental consciousness imposes the use of
water as a solvent on both industrial and academic chem-
ists, however, organic solvents are still used instead of wa-
ter for mainly two reasons. First, most organic substances
are insoluble in water, and as a result, water does not func-
tion as a reaction medium. Second, many reactive sub-
strates, reagents, and catalysts are decomposed or deacti-
vated by water.2
In recent yeas, the design of efficient and recoverable
phase-transfer catalysts has become an important issue for
reasons of economic and environmental impact. In particu-
lar, PEG based dicationic ionic liquid has considerable ad-
vantages, including easy catalyst recovery and product iso-
lation, and employment of a continuous flow method ow-
ing to the two-phase nature of the system, which make the
technique attractive for industrial applications. Although
many phase-transfer catalysts are known, functionalized
ionic liquids such as polyethylene glycol functionalized
dicationic ionic liquids are practically important and used
in many of organic reactions. High polarity and ability to
dissolve organic and inorganic compounds increases the
reaction rate and selectivity higher than with traditional
methods has been reported.5-8
One of the most important strategies to overcome this
limitation is the utilization of phase transfer catalyst such
as ionic liquid (IL).
Recently, magnetic ionic liquids are attracting in-
creasing interest as environmentally benign reaction media
for various organic synthesis, catalysis, etc.9-11 Further,
magnetic ionic liquid offers several advantages, such as ex-
tremely low volatility, high thermal stability, non flamma-
bility and high ionic conductivity. These advantages and
nontoxic nature of magnetic ionic liquid show its potential
as a catalyst in organic synthesis. It is also reported that
magnetic ionic liquids could be separated from other sol-
vents by a combination of magnetic field and conventional
methods such as filtration, ultracentrifugation, and adsorp-
tion.12,13
In 1971, Starks introduced the term “phase-transfer
catalysis” to explain the critical role of tetraalkylammo-
nium or phosphonium salts (Q+X-) in the reactions be-
tween two substances located in different immiscible
phases.3
Since then, the chemical community has witnessed an
exponential growth of phase-transfer catalysis as a practi-
cal methodology for organic synthesis. The advantages of
this method are its simple experimental procedures, mild
reaction conditions, inexpensive, environmentally benign,
and the possibility of conducting large-scale preparations.4
Nowadays, it appears to be the most important synthetic
Epoxides are often used as starting materials and in-
* Corresponding author. Tel: +986114448041; Email: bmombini@gmail.com
Supporting information for this article is available on the www under http://dx.doi.org/10.1002/jccs.201400383
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Azidolysis of Epoxides Catalyzed by PEG-MDIL
termediates in organic synthesis partly because their open-
ing leads to vicinal azidoalcohols are an important class of
organic compounds and they serve as precursors in the syn-
thesis of vicinal aminoalcohols, carbohydrates, nucleo-
sides,14-16 lactames,17 and oxazolines.18 Optically active
b-azido alcohols are of great significance as potential pre-
cursors for non-racemic aziridines and b-amino alco-
hols.19,20 The latter compounds are not only a common
structural component in a vast group of naturally occurring
and synthetic molecules, but also can be widely used as
versatile chiral building blocks and chiral catalysts in or-
ganic synthesis.21 In particular, chiral 2-amino-1-aryleth-
anols are important structural elements in pharmaceuticals
such as-or-adrenergic blockers and agonists in the treat-
ment of cardiovascular disease, cardiac failure, asthma and
glaucoma.22
tionic ionic liquid (PEG-MDIL) was synthesized as shown
in Scheme 1. Polyethylene glycol dicholorid was prepared
in high yield following a literature method.32 They were
further treated with two equivalents of 1-methylimidazole,
respectively, under neat reaction conditions, to form di-
cationic cholorid bridged by polyether linkage chains in
high yields. With the exception of one compound contain-
ing one ether linkage chain (solid), the diimidazolium
cholorid derivatives is sticky colorless liquid.
Synthesis of PEG-MDIL
Scheme 1
The most common method for the preparation of
azidohydrins is the ring opening of epoxides by using dif-
ferent azides in suitable solvents. The reactions often car-
ried out under either alkaline or acidic conditions and sev-
eral different methods have been devised in order to ob-
tain the direct azidolyses of epoxides in the presence of
sodium azide.23 Under these conditions, azidolyses are
usually carried out over a long reaction times and azi-
dohydrin is often accompanied by isomerization,
epimerization, and rearrangement of products.24 Unfortu-
nately, some of these methods are not always fully satis-
factory and suffer from disadvantages such as these me-
thods require hazardous or toxic reagents high reaction
temperature or relative long reaction times, difficulty in
preparation and/or storage of reagents or catalysts, diffi-
culty in work-up and isolation of products or low regio-
selectivity. Consequently, it seems that there is still a need
for development of newer methods that proceed under
mild and economically appropriate conditions. In order to
overcome some of these limitations, a number of alterna-
tive procedures have been reported over the past few
years using a variety of catalysts.25-31
In the latest step, the anions of imidazolium based
dicationic room temperature ionic liquid, Cl-, were easily
-
changed with FeCl4 anions by the simple mixing with
FeCl3 under neat conditions.
Due to the paramagnetic nature of the Polyethylene
glycol functionalized magnetic dicationic ionic liquid, nu-
clear magnetic resonance technique could not be used to
confirm its structure. Instead, UV, IR and Raman spectro-
scopes were used to characterize the PEG-MDILstructure.
The UV–vis absorption spectra of PEG-MDIL in
acetonitrile consist of two main bands at 310–425 nm and
240–300 nm (see Fig. 1a). The shorter wavelength band is
attributed to the p-p* transition in the imidazole ring, and
the longer wavelength band was found to be due to the n-p*
transition.
As a part of our program aiming at developing selec-
tive and environmental friendly methodologies for the
preparation of fine chemicals and in continuation of our in-
terest in magnetic ionic liquid promoted organic reactions,
in this paper, we report on the evaluation of ring opening
epoxide reaction in the H2O-PEG-MDIL system.
RESULTS AND DISCUSSION
PEG-MDIL spectra exhibited absorption bands in the
visible region at 534, 473 nm which are characteristic for
Polyethylene glycol functionalized magnetic dica-
J. Chin. Chem. Soc. 2015, 62, 404-411
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Article
Godajdar and Mombeni
-
the FeCl4 anion (Fig. 1b).
-
39%). The second event corresponded to the loss of FeCl4
The thermal gravity analysis curve of catalyst under a
nitrogen atmosphere at a heating rate of 10 °C min–1 is
shown in Figure 2. There were three main steps of weight
loss, and the decomposition events took place at 220 °C,
310 °C and 400 °C. On the base of weight changes, the first
process PEG-MDIL spectra exhibited absorption bands
was attributed to the loss of ether linkage (found 37% calc.
(found 40% calc. 42%). The weight loss (about 15%) dur-
ing 400 °C to 500 °C was attributed to the decomposition of
1-methylimidazole.
To characterize the PEG-MDIL, we used the FT-IR
spectrum. The FT-IR spectrum of native Cl-PEG-Cl and
PEG-MDIL sample are shown in Fig. 3.
There are peaks at about1165 cm-1, which were as-
signed to the characteristic absorption of N–CH2 in func-
tionalized PEG-MDIL. The absorption bands at 3152 and
3013 cm-1 (imidazolium CH stretching modes), presented
in the inset of Fig. 3 demonstrate modification of the PEG.
The absorption at 2911cm-1 is usually assigned to C-H
stretching of the polyether linkage chains. The absorption
observed at 1571 cm-1 is also characteristic of the imida-
zolium ring and is assigned to imidazolium ring stretch-
ing.
In a typical experiment, phenyl glycidyl ether, and
NaN3 was selected. The effect of the amount of PEG-MDIL
on the reaction time was studied by varying the quantity of
ionic liquid (0, 0.05, 0.1, 0.15, and 0.2 gr/1 mmol phenyl
glycidyl ether). The reactions were carried out under simi-
lar reaction conditions. It was observed that, with an in-
crease in the proportion of ionic liquid, the azidolysis of
phenyl glycidyl ether increases. The highest conversion of
epoxide was obtained when the amount of ionic liquid used
was 0.05 gr./1 mmol epoxide.
The effect of the reaction temperature on the reaction
time of phenyl glycidyl ether azidolysis was investigated at
Fig. 1. Visible spectrum of PEG-MDIL (200-400 nm)
(a), UV spectra of PEG-MDIL, PEG-DIL, and
Cl-PEG-Cl (400-700 nm) (b).
Fig 3. Comparison between FT-IR spectra of PEG-
MDIL (b) and Cl-PEG-Cl (a).
Fig. 2. TGA of PEG-MDIL.
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Azidolysis of Epoxides Catalyzed by PEG-MDIL
Table 1. Efficiency of solvents on azidolysis of epxidesa
reaction temperatures ranging from 25 to 90 °C. The results
show that the suitable reaction temperature is 90 °C.
The reaction was carried out in diethyl ether, n-hex-
ane, acetonitrile, dichloromethane, ethyl acetate and water.
From the results given in Table 1, it fallows that the best
solvent for this reaction is water. Thus, water is an excellent
solvent in terms of cost, availability, and environmental im-
pact and shorter reaction times.
Entry
Solvent
Time (min)
Yield (%)
1
2
3
4
5
6
Et2O
CH2Cl2
n-hexane
EtOAc
DMSO
H2O
180
180
180
180
20
20
25
20
40
85
94
15
a 2,3-Epoxypropyl phenyl ether (1 mmol), NaN3 (3 mmol), and
0.05 gr of PEG-MDIL were used under reflux conditions.
Epoxides bearing activated and deactivated groups
were quickly and efficiently converted to the virtually pure
azidohydrine and b-hydroxy azidoalcoholes products in
high isolated yields (Table 2, Scheme 2). No evidence for
the formation of a-azidoalcoholes as by-products of the re-
actions was observed and the products were obtained in
pure form without further purification. Furthermore, cyclic
epoxides, such as cyclohexene oxide reacted smoothly in
SN2 fashion to afford the corresponding 1, 2 azidoalcohols
(Table 2, entries 3). The stereochemistry of the ring open-
ing products was found to be in the trans-configuration as
determined from the coupling constants associated with the
1H NMR spectral resonances of the ring protons.
probably, facilitates the ring opening of epoxide (Scheme
3).
Plausible mechanism for the ring opening of
epoxide catalyzed by PEG-MDIL.
Scheme 3
Synthesis of vicinal azidoalcohols from
epoxides in the presence of PEG-MDIL.
Scheme 2
The success of the above reactions prompted us to in-
vestigate the recyclability of catalyst. We carried out our
study by using the reaction 2,3-epoxypropyl phenyl ether
with NaN3 and under optimal conditions as a model study.
The aqueous phase was then subjected to distillation at 80
°C under reduced pressure (10 mm Hg) for 4 h to recover
the PEG-MDIL almost completely. It was found that the
catalyst could be reused five times with slightly decreasing
catalytic activity (Fig. 4).
In the next step, the scope and efficiency of the cata-
lyst were explored under the optimized reaction conditions
for the substituted aromatic compounds, unsaturated ali-
phatic compounds, and aromatic heterocyclic compounds
(Table 2, entries 8-10).
All the products were characterized and identified by
1
comparison of their spectral data (IR, H NMR and 13C
The advantage of using this magnetic phase transfer
catalyst for the synthesis of 1,2-azido-alcohols using azide
ion is shown by comparing our results with those previ-
ously reported in the literature (Table 3).
NMR) with those of authentic samples. After completion of
the reaction (Table 2), the mixture was cooled and organic
phase was extracted with diethyl ether. Evaporation of or-
ganic solvent the desired products.
PEG-MDIL showed remarkable reactivity as a Lewis
acid reagent and considerably accelerated the reactions. It
seems that polyethylene glycol unitts in PEG-MDIL encap-
sulate 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 prop-
EXPERIMENTAL
Material and Methods: Melting points were measured on
1
an Electrothermal 9100 apparatus and are uncorrected. H &
13CNMR spectra were recorded on a Bruker Advanced DPX 400
MHz instrument spectrometer using TMS as the internal standard
in CDCl3. IR spectra were recorded on a BOMEM MB-Series
1988 FT-IR spectrometer. Raman spectroscopy were recorded on
-
erty to the catalyst. In addition, FeCl4 groups at the IL,
J. Chin. Chem. Soc. 2015, 62, 404-411
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