Microwave-assisted one pot-synthesis of amino acid ionic liquids in water: Simple catalysts for styrene carbonate synthesis under atmospheric pressure of CO2

Article abstract of DOI:10.1039/c3cy00769c

A novel variety of ionic liquids based on naturally occurring amino acids is expeditiously synthesized in water using microwave energy. The amino acid ionic liquids (AAILs) exhibit eminent catalytic activities towards the synthesis of styrene carbonate from styrene oxide and carbon dioxide at atmospheric pressure. The synergistic interaction of the hydrogen-bonding groups with the nucleophile in the AAIL is believed to be the key factor behind the catalytic cycloaddition. Among the various kinds of AAILs tested, the basic AAILs were found to be the most efficient owing to the presence of extra amino groups that could activate the carbon dioxide molecule by formation of a carbamate salt. The AAILs showed appreciable reusability over four cycles without compromising the selectivity towards styrene carbonate synthesis and hence represents an easily synthesizable series of eco-friendly catalysts for CO₂ fixation. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3cy00769c

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Microwave-assisted one pot-synthesis of amino
acid ionic liquids in water: simple catalysts for
styrene carbonate synthesis under atmospheric
pressure of CO2
Cite this: Catal. Sci. Technol., 2014,
4, 963
Kuruppathparambil Roshith Roshan, Tharun Jose, Dongwoo Kim,
*
Kathalikkattil Amal Cherian and Dae Won Park
A novel variety of ionic liquids based on naturally occurring amino acids is expeditiously synthesized in
water using microwave energy. The amino acid ionic liquids (AAILs) exhibit eminent catalytic activities
towards the synthesis of styrene carbonate from styrene oxide and carbon dioxide at atmospheric
pressure. The synergistic interaction of the hydrogen-bonding groups with the nucleophile in the AAIL is
believed to be the key factor behind the catalytic cycloaddition. Among the various kinds of AAILs tested,
the basic AAILs were found to be the most efficient owing to the presence of extra amino groups that
could activate the carbon dioxide molecule by formation of a carbamate salt. The AAILs showed
appreciable reusability over four cycles without compromising the selectivity towards styrene carbonate
synthesis and hence represents an easily synthesizable series of eco-friendly catalysts for CO2 fixation.
Received 4th October 2013,
Accepted 13th December 2013
DOI: 10.1039/c3cy00769c
www.rsc.org/catalysis
Introduction
The relevance of an effective mitigation of excess CO2 from the
environment is undoubtedly clear to the scientific community
around the globe because of CO2’s obvious detrimental impact
on the environment. The direct sequestration of CO2 such as
by carbon capture and storage (CCS) techniques, employed
directly from industrial emission outlets to underground
aquifer sinks, is a widely practiced methodology adopted for
addressing the exponential increase in atmospheric CO2 con-
centrations. However, the physico-chemical properties of CO2,
such as its non-toxicity, non-flammability, ease of storage, and
abundant nature, evoked a new window of chemical explora-
tions for its utilization, whereby serious and dynamic research
has been devoted to the successful transformation of CO2 into
valuable industrial products. Although currently the mitigation
of CO2 emissions by this route is limited, the employment of
CO2 as a C1 feedstock could replace the currently employed
hazardous C1 materials such as COCl2 and CO in a large num-
ber of organic transformations. Cyclic carbonates, synthesized
by the cycloaddition of epoxides and CO2 (Scheme 1), are
excellent aprotic solvents with low odor and toxicity and have
been employed as electrolytic solvents for lithium-ion batteries
and monomers for polymer synthesis.1a–e,2
Scheme 1 Synthesis of styrene carbonate.
Of the many catalytic systems deployed, ionic liquids3a–h
and metal complex systems4a–d stand out as the most dis-
tinguished and promising tailor-made, task-specific cata-
lysts. This description of ionic liquids is attributed to the
ease of tunability of their physical and chemical properties
by a meticulous choice of cationic and anionic species.
However, the definition of ionic liquids is quite ambiguous;
they are neoteric solvents with green properties such as
non-volatility, liquidity over a wide range of temperatures,
high thermal stability, and incombustibility. The typical for-
mulation of an ionic liquid catalyst includes quaternary
ammonium, imidazolium, pyridinium, and phosphonium
cations, as well as a wide variety of anions such as halides,
hexafluorophosphates, tetrafluroborates, and nitrates, and
these have been intensively studied for their potential
towards CO2–epoxide cycloaddition.3a–g Various experimental
and theoretical studies have shown that ionic liquids with
strongly nucleophilic anions, in association with hydroxyl
and carboxylic groups, furnish a better yield of cyclic car-
bonates by means of playing a synergistic role. Hence, there
Division of Chemical and Biomolecular Engineering, Pusan National University,
Busan 609-735, Republic of Korea. E-mail: dwpark@pusan.ac.kr;
Fax: +82 51 512 8563; Tel: +82 510 2399
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has been a growing interest in the rational design and syn-
thesis of ionic liquids equipped with hydrogen-bonding
groups from various starting materials such as alkylimi-
dazoles, followed by the post-synthetic tethering of hydroxyl
and/or carboxylic moieties to these ionic liquids.3b,3g How-
ever, the claim of ionic liquids to be fully “green” is still
open to debate since the relative abundance and non-toxicity
of the starting materials is a pre-requisite for green chemistry.
Recently, bio-renewable materials such as lactates, sugar
substitutes, and amino acids have been identified as ideal
precursors for ILs from both economical and environmental
perspectives.5a–p These attempts tend to capitalize the poten-
tial design capacity of the biological materials for the deve-
lopment of task-specific eco-friendly ILs.
In this context, utilization of natural amino acids (AA) as
ionic liquid precursors (Scheme 2) becomes the crescendo of
designer ionic liquid synthesis, since AA represent one of the
most versatile class of natural, bio-renewable, and non-toxic
raw materials from nature's toolbox. The additional advan-
tage of amino acid ILs (AAILs) is the presence of multiple
hydrogen-bonding groups, which eliminates the need for
post-synthetic covalent tethering of functional moieties. By
virtue of their excellent and easy tunability, amino acids can
make a direct contribution to the search for ‘greener’ sol-
vents with desirable physicochemical properties.
According to the tenets of ‘green’ chemistry, the efficiency
of a chemical synthesis is not only evaluated on the basis of
the abundance and non-toxicity of the raw material, but the
reaction time, energy requirement, and the protocols
involved are also critically monitored. Recently, the ability of
aqueous microwave chemistry to induce organic transforma-
tions has attracted increased interest owing to reduced reac-
tion times, lower environmental impact, and minimal
formation of side products.5a–g Since dielectric heating of
polar molecules is the key to efficient microwave heating,
water is the best solvent for microwave-assisted chemical
Scheme 3 Synthesis of amino acid ionic liquids.
reactions. A number of studies have already reported the use
of microwaves for ionic liquid synthesis in solvent-free and
aqueous media.5f,6g The capacity of ionic environments to
generate internal pressure and promote the interaction of
reactants in solvent cavities renders the use of microwave
energy for ionic liquid synthesis an expeditious approach.
Hence, we have attempted the one-pot synthesis of cationic
AAILs in water using microwave energy for time spans as
short as 2 minutes (Scheme 3). We have also investigated the
catalytic activity of these AAILs towards the cycloaddition of
CO2 to styrene oxide at atmospheric pressure.
Results and discussion
Until now, the two approaches adopted for the synthesis of
AAILs have used amino acids either as anions of commer-
cially available IL cations such as alkyl imidazolium5b–d or as
cations by means of simple acidification followed by metathe-
sis with different anions.5e–h Most work has focused on the
former approach, wherein nearly all the natural amino acids
have been successfully converted into anionic species, whose
physicochemical properties have been studied. In particular,
a few works demonstrated the possibility of using amino
acids as cation centers for a wide variety of anions that were
successfully used as chiral ligands in chiral ligand-exchange
capillary electrophoresis and as catalysts for Diels–Alder type
reactions,5f etc. With respect to CO2 fixation, Gong et al.5g
successfully developed an L-proline-based ionic liquid under
reflux conditions in acetonitrile for 48 h and they efficiently
catalyzed the synthesis of cyclic carbonates under atmo-
spheric pressures of CO2 within 24 h. Most recently, our
group7a has theoretically proposed and experimentally vali-
dated the synergistic catalytic activity of the iodide nucleo-
philes and –COOH entities of the quaternized glycine catalyst
in the CO2–epoxide cycloaddition, via density functional cal-
culations. A similar betaine system7b was also found to be an
effective catalyst in cyclic carbonate synthesis under moder-
ate reaction conditions. However, a systematic study of the
catalytic behaviors of other cationic AAILs in the CO2–epoxide
cycloaddition remains underexplored, despite the fact that
amino acids are the monomer units of the most efficient bio-
logical catalysts, the enzymes.
Since natural amino acids exhibit diverse chemistries by
virtue of their different side-chain functionalities and
Scheme 2 Amino acid precursors.
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hydrogen-bonding groups, we deemed it worthwhile to inves-
tigate the catalytic ability of various AAILs based on their
molecular structure. Assuming glycine (a) to be the standard
amino acid, (by virtue of its simple structure and small size)
the other amino acid (AA) precursors were classified into four
groups: (b) AAs with hydrophobic side chains, (c) AAs with
polar uncharged side chains, (d) acidic and (e) basic
(Scheme 2). These AAILs were easily synthesized in water by
means of acidification with HI in a one-pot synthetic proce-
dure using microwave energy in 2 minutes (Scheme 3). The
catalytic behavior of the AAILs towards the cycloaddition of
styrene oxide to carbon dioxide is shown in Table 1. All the
reactions were carried out at 120 °C for 6 h under solvent free
conditions. No product (styrene carbonate) was detected in
the absence of a catalyst (entry 1). Neither glycine (entry 2)
nor HI (entry 3) resulted in a conversion higher than 5%
when used alone under the employed reaction conditions.
However, the amino acid ionic liquid Gly–HI (entry 4)
achieved a conversion of 34% with selectivity for styrene
carbonate (SC) in excess of 97% under atmospheric pressure
of CO2. This reaction demonstrated the synergistic catalytic
activities of the hydrogen-bonding groups and the anion of
the AAIL. The three AAILs that had different hydrophobic
side chains (Trp, Tyr, Ala) resulted in different conversion
rates of SO under the same reaction conditions. While Ala–HI
exhibited a SO conversion of 29% (entry 5), Tyr–HI that
possessed a hydroxyl group in the side chain resulted in 52%
conversion (entry 6), and the –NH containing Trp–HI pro-
duced even better conversion of 73% (entry 7). The hydroxyl-
containing polar uncharged side chain AAILs, Ser–HI and
Thr–HI achieved respective conversions of 61% and 52%
(entries 8 and 9). The acidic AAILs, i.e., Asp–HI, and Glu–HI,
also resulted higher conversion rates of SO with high selectiv-
ities (entries 10 and 11). All the basic AAILs afforded a rela-
tively higher SO conversion (more than 70%), with His–HI
being the most effective catalyst. While Lys–HI (entry 12) and
Arg–HI (entry 13) exhibited 79% and 70% SO conversion,
respectively, His–HI (entry 14) exhibited a SO conversion of
almost 92% (TON = 328), maintaining the high selectivity
induced by other AAILs. All the above AAILs maintained
selectivities of more than 97%. The fact that hydrogen-
bonding entities can accelerate epoxide conversion is consis-
tent with the obtained results (entries 4 and 5 vs. 6, 8 and 9).
The increased catalytic activities of Ser–HI (entry 8) and
Thr–HI (entry 9) and the acidic AAILs (entries 10 and 11) that
contained additional –OH and –COOH groups, respectively,
also corroborated the active catalytic role of hydrogen-
bonding groups of AAILs in the SO–CO2 cycloaddition. To the
best of our knowledge, this is the highest catalytic activity
exhibited by any AAIL in a CO2–epoxide cycloaddition under
the employed reaction conditions. The previously reported
proline-based ionic liquid5g exhibited a SO conversion of
>77% (TON = 84) only with triethylamine as co-catalyst
(1 mol% catalyst, 90 °C, 1 atm CO2, 24 h), whereas the
proline ionic liquid alone yielded only 12% SO conversion.
Although a direct comparison between the various efficient
ionic liquids for cyclic carbonate synthesis is tedious owing
to the differences in the reaction conditions, we noticed that
a
Table 1 Catalytic test runs towards synthesis of styrene carbonate from styrene oxide and CO2
Catalyst
Conversionb (%)
Selectivityb (%)
TON
1
None
Gly
HI
Gly–HI
Ala–HI
Tyr–HI
Trp–HI
Ser–HI
Thr–HI
Asp–HI
Glu–HI
Lys–HI
—
—
3
—
—
81
97
98
97
99
98
98
93
95
97
98
99
98
99
98
—
—
96
—
—
10
2c
3
4c
34
29
52
73
61
52
42
49
79
70
92
96
99
79
64
66
99
121
103
185
260
217
185
149
175
282
250
328
63
62
84
24.8
26.4
1980
5d
6d
7d
8d
9e
10e
11f
12f
13g
14g
15h
16i
17j
18k
19l
20m
Arg–HI
His-HI
CBImBr3h
HEMImBr3b
Pro4,4Br/Et3N5g
[(salen)Al]2O/Bu4NBr4b
Zn–salphen complex–2/Bu4NBr4c
Al–cat–1/Bu4NBr4d
a
b
Catalyst amount: 0.1 mmol (0.28 mol%), 35 mmol SO, 120 °C, 0.1 MPa, 6 h. From GC using toluene as internal standard (side product is
c
d
styrene-1,2-diol). AAIL synthesized from neutral amino acid. AAIL synthesized from neutral amino acid with hydrophobic amino acid chain.
e
f
g
AAIL synthesized from neutral amino acid with polar side chain. AAIL synthesized from acidic amino acid. AAIL synthesized from basic
amino acid. h AAIL synthesized from 1.5 mol%, 120 °C, 1.3 MPa, 2 h. AAIL synthesized from 1.6 mol%, 125 °C, 2 MPa, 1 h. AAIL synthesized
from 1 mol%, 90 °C, 0.1 MPa, 24 h. AAIL synthesized from 2.5 mol%, co-catalyst 2.5 mol%, 25 °C, 0.1 MPa, 3 h. AAIL synthesized from
2.5 mol%, co-catalyst 1 mol%, 45 °C, 1 MPa, 18 h, 5 mL CH2Cl2. AAIL synthesized from 0.05 mol%, co-catalyst 0.25 mol%, 70 °C, 10 MPa,
i
j
k
l
m
18 h, 0.5 MEK (methylethylketone) co-solvent.
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AAILs furnished higher TON values compared to the other
ionic liquid systems with hydrogen bonding functionalities
(Table 1, entry 14 vs. 15 and 16). However, these
organocatalysts were inferior to the inorganic metal com-
plexes, which excelled in their activities under very mild reac-
tions (Table 1, entry 18, 19, and 20).
The probable reason for the higher catalytic activities of
basic AAILs compared to those of acidic AAILs can be attrib-
uted to the presence of additional amine moieties in the side
chains of basic AAILs. While the –COOH groups and –OH
groups of the catalytic species are believed to activate the
epoxide –O atom towards the ring opening reaction,8a–k the
nucleophilic –NH groups are thought to have an increased
ability to interact with the non-reactive CO2 molecules. The
fact that aqueous alkyl amine solutions are industrially
employed as CO2-absorbing agents corroborates the concept
that the –NH moiety effectively forms a carbamate salt by
means of a nucleophilic attack on the carbon atom of the car-
bon dioxide, which results in the activation of the otherwise
non-reactive CO2 molecule.
Since His–HI exhibited the highest catalytic activity in SC
synthesis, we extended the histidine-based AAIL synthesis
by using methyl and ethyl iodide in aqueous media in a
microwave reactor (Scheme 4). Table 2 lists the catalytic
activities exhibited by the His–MeI and His–EtI ionic liquids
during SC synthesis under solvent-free conditions. The
activity with methyl and ethyl substituted histidine ionic liq-
uids wasn't much different. To gather insight into the role
of anions, chloride and bromide-possessing histidine ionic
liquids similar to His–HI were synthesized and the catalytic
activity of these ionic liquids are shown in Table 2, entries
4 and 5. As is evident from entries 1–3, all the iodide
anion-possessing histidine ionic liquids (His–HI, His–MeI,
and His–EtI) resulted in extremely high conversion/yields of
more than 90%, irrespective of the length of the alkyl chain
involved. The reactions were carried out under an atmo-
spheric pressure of CO2 at 120 °C over 6 h using 0.28 mol%
of the catalyst.
Since the catalytic activity exhibited by bromide and
chloride-containing histidine ionic liquids was limited to
64% and 27%, respectively (entries 4 and 5), which was sig-
nificantly lower than the SC yields obtained with histidine
ionic liquids that had iodide as the counter anion. This dem-
onstrated the superior ability of the iodide anion to effectuate
the synergy with the hydrogen-bonding groups of the amino
acid cation compared with the bromide and chloride anions.
Optimization studies carried out with the His–MeI ionic
liquid under various experimental conditions of catalyst con-
centration, temperature, and reaction time are discussed
below. Fig. 1 shows the dependence of SO conversion on the
amount of His–MeI catalyst. The catalytic activity gradually
increased on going from 0.1 mol% to 0.28 mol% of the
His–MeI catalyst and reached near-saturation at 0.28 mol%
of His–MeI, where the SO conversion reached 95% with 99%
SC selectivity.
Temperature also has a very strong influence on the
His–MeI catalysis of SO conversion to SC. Fig. 2 shows the
increase in the catalytic rate with increasing temperature
between 80 °C and 140 °C. An almost negligible yield of SC
was obtained at 80 °C; however, the catalytic activity
increased exponentially at temperatures above 100 °C,
reaching a maximum at 120 °C with 95% SO conversion and
SC selectivity in excess of 99%. These results were attributed
to the decrease of viscosity of the catalyst system at elevated
temperatures, thereby resulting in increased mobility of the
active species. This in turn facilitated more effective interac-
tions of the catalyst with the substrate molecules, overcoming
the energy barrier of the reaction. Interestingly, the His–MeI
system maintained its selectivity even at high temperatures
of 140 °C. The dependence of SC yield and selectivity on the
reaction time was also evaluated (Fig. 3), where the catalytic
activity was found to be optimum at 6 h.
Scheme 4 Synthesis of histidine–MeI ionic liquid (His–MeI).
Table 2 Role of anions and alkyl chainsa
Entry
Catalyst
SO conversionb (%)
SC selectivitya (%)
1
2
3
4
5
His–MeI
His–EtI
His–HI
His–HBr
His–HCl
95
92
93
64
27
99
99
99
98
96
a
Reaction conditions: SO
4
ml (35.02 mmol), catalyst amount
0.1 mmol, 120 °C, 0.1 MPa,
ethyliodide, X = iodide (I), bromide (Br), chloride (C). From GC
using toluene as internal standard.
6 h. MeI = methyliodide, EtI =
b
Fig. 1 Dependence of SC yield on catalyst amount. Reaction conditions:
SO = 35.5 mmol, 120 °C, 0.1 MPa, 6 h. Selectivity of SC >99%.
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of SO was found to decrease in consecutive cycles during
reuse, the selectivity remained almost unchanged (>95%)
demonstrating the efficacy of the ‘green’ catalyst from a
renewable source.
Trials to find out the pressure dependence of His–MeI
mediated catalysis of SO–CO2 coupling and the catalytic activ-
ity towards a few other epoxides led us to some interesting
results. While SO efficiently underwent cycloaddition to CO2
within the pressure range of 0.1 MPa to 1.2 MPa, the cycload-
dition tendency of the other epoxides were different. At
120 °C in 6 h with 0.28 mol% of catalyst the reactivity of vari-
ous epoxides towards cyclic carbonate formation followed the
order: styrene oxide ≫ allyl glycidyl ether > propylene oxide
> epichlorohydrin ≫ cyclohexene oxide at atmospheric pres-
sures of CO2 (Table 3). However, with a gradual increase in
the CO2 pressure, the various epoxides responded in different
quantities to the coupling reaction such that propylene oxide
(entry 3) and allyl glycidyl ether (entry 2) reached higher con-
version rates of 81% and 93%, respectively, at 0.6 MPa pres-
sure with very high selectivity towards the corresponding
cyclic carbonates. While all the epoxides attained excellent
conversion at 1 MPa, the internal epoxide cyclohexene oxide
(entry 5) remained hard for effective coupling reaction with
CO2 throughout the entire pressure studies due to its high
Fig. 2 Effect of temperature in the His–MeI catalyzed cycloaddition.
Reaction conditions: SO = 35.5 mmol, cat.amt = 0.28 mol%, 0.1 MPa,
6 h. Selectivity of SC >99%.
steric hindrance,
a fact well known to the research
community.9a,b We surmise this difference in cyclic carbon-
ate yield from different epoxide substrates at various pres-
sures to the difference of CO2 solubility in the corresponding
reaction mixtures, which may have directly influenced the
rate of carbamate formation. Since styrene oxide gave consis-
tently higher cycloaddition rates throughout the CO2 pres-
sures applied, CO2 solubility in SO should be higher than
that in other epoxides, even under pressures as low as
1 atmosphere.
Fig.
3 Time variant study of SC synthesis using His–MeI catalyst
systems. Reaction conditions: SO = 35.5 mmol, cat.amt = 0.28 mol%,
120 °C, 0.1 MPa. Selectivity of SC >99%.
The mechanism of the cycloaddition of CO2 to epoxides
has been studied elaborately previously, both experimentally
and theoretically.8a–k When this reaction proceeds under the
influence of an ionic catalyst, such as an ionic liquid, a
nucleophilic attack on the least hindered carbon atom of
the epoxide by the anion initiates the reaction, followed by
the cycloaddition of CO2. In this study, neither HI (Table 1,
entry 3) nor glycine (Table 1, entry 2) catalyzed the CO2–
epoxide coupling to any significant extent, whereas Gly–HI
(entry 4) appreciably catalyzed the reaction, which demon-
strated the synergistic interaction of the iodide anion with
the hydrogen-bonding groups of the glycine cation. From
Table 2, entries 3–5, the anion was found to have a sub-
stantial effect on the catalytic rate, as a strongly nucleo-
philic leaving group (iodide anions) resulted in high SC
yields when compared to the other halide ions (bromide
and chloride). The comparison between the different kinds
of AAILs employed revealed that AAILs with an additional
basic moiety produced better yields of cyclic carbonates.
The role of –NH groups in activating the CO2 molecule has
been already conclusively reported by several studies.8d,k In
summary, a plausible mechanism of the His–MeI catalyzed
Since AAILs are generated in aqueous media, the recovery
of the catalyst is easily achieved by separation using water
followed by drying at 60 °C for 24 h (vacuum). Gratifyingly,
the recovered catalysts exhibit moderately good reusability
towards styrene carbonate synthesis under the optimized reac-
tion conditions (Fig. 4). Although the percentage conversion
Fig. 4 Reusability performance of His–MeI ionic liquid in SC synthesis,
35.5 mmol SO, 0.28 mol% catalyst, 120 °C, 6 h, 0.1 MPa CO2.
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Table 3 Pressure dependence of various epoxides on cyclic carbonate using His–MeI catalysta
Product yieldb (%) at various pressures of CO2 (MPa)
Entry
1
Epoxide
Product
0.1
12
0.6
1.0
99
71
2
3
4
56
16
95
—
93
81
98
12
98
99
98
29
5
a
b
His–MeI 0.28 mol%, 35.5 mmol epoxide, 120 °C, 6 h. From GC using toluene as internal standard.
cycloaddition of CO2 to epoxide is shown in Scheme 5. The
–COOH group of the catalyst hydrogen bonds with the oxy-
gen atom of the epoxide, weakening the more probable β
C–O bond of the epoxide.8b,g,h Then, the iodide nucleophile
attacks the β carbon atom and forms a C–I bond. Mean-
while, the electrophilic C atom of the CO2 molecule inter-
acts with the amine moiety of the imidazole ring to form a
carbamate salt.8d,k Thereafter, the oxy anion of the carba-
mate salt attacks the β carbon atom of the epoxide when
the iodide anion leaves the loop. Cycloaddition follows this
step, producing cyclic carbonate and regenerating the cata-
lyst. It is worthwhile to mention that the α C atom of the
epoxide is also prone to undergo the same events as those
of the β carbon since the energy barriers do not differ
that much.8l
Conclusions
We have utilized a simple and efficient method for synthesiz-
ing ionic liquids from bio-renewable amino acids using
microwave energy and water (reaction time 2 minutes) for the
synthesis of styrene carbonate from CO2 and styrene oxide at
atmospheric pressure under solvent-free conditions. All the
synthesized AAILs exhibited promising catalytic activity
through the synergy between the hydrogen-bonding groups
and the nucleophiles of the AAILs. The iodide ion was found
to be the most compatible nucleophile for an efficient
catalysis. The catalysts displayed appreciable recyclability with
respect to styrene oxide conversion and styrene carbonate
selectivity. We believe that the development of microwave-
assisted approaches that employ the most environmentally
benign solvent, water, and very short reaction times meets
the requirements of sustainable chemistry for synthesizing
task-specific ionic liquids. The facile activation of water
molecules using microwaves demonstrates the utility of
yet another synergistic approach for the development of
eco-compatible methodologies for the synthesis of chemically
significant materials such as ionic liquids.
Experimental
All reagents and solvents (except DS water) were pure analyti-
cal grade materials purchased from commercial sources
(Sigma Aldrich Co. South Korea) and were used without
Scheme 5 Mechanism of cycloaddition of CO2 with epoxides using
His–MeI catalyst.
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further purification, unless stated otherwise. CO2 (99.9%)
was obtained from MS Gas Corporation, Korea. A gas chro-
matograph (HP 6890 A) equipped with a HP-1 capillary col-
umn was used to determine the concentration of the various
cyclic carbonates formed. The peak of the cyclic carbonate
was identified with an authentic sample. The 1H-NMR spec-
tra were recorded in D2O on a Varian 500 MHz instrument
with TMS as an internal standard. DMSO was used as the sol-
vent for 13C-NMR (75 MHz).
L-Glutamic acid (Glu–HI). 1H-NMR: δ = 8.42 (s, 3H), 4.21
(t, 3H), 2.32 (m, 2H), 2.42 (t, 2 H) ppm. 13C-NMR: δ = 169.84,
171.23, 53.61, 31.26, 38.14 [yellow viscous liquid at RT].
L-Serine (Ser–HI). 1H-NMR: δ = 8.45 (s, 3H), 4.17 (t, 3H),
3.83 (d, 2 H) ppm. 13C-NMR: δ = 169.68, 59.52, 52.54 [chrome
yellow viscous liquid at RT].
1
L-Threonine iodide (Thr–HI). H-NMR: δ = 8.3 (s, 3H), 4.12
(t, 3H), 3.71 (m, 1H), 1.21 (d, 3H) ppm. 13C-NMR: δ = 169.75,
63.71, 58.56, 20.11 [pale yellow viscous liquid at RT].
L-Lysine iodide (Lys–HI). 1H-NMR: δ = 8.13 (s, 3H), 3.87
(t, 3H), 3.1 (t, 2 H), 1.94 (m, 2H), 1.74 (m, 2H), 1.50 (m, 2H)
ppm. 13C-NMR: δ = 172.7, 55.8, 33.9, 24.7, 29.0, 42.6 [dark
yellow solid at RT].
Typical procedure for preparation of AAILs
Synthesis of His–HI ionic liquid. 0.1mol of L-histidine was
dissolved in 5 ml distilled water. One molar equivalent of HI
(55%) was added dropwise. The reaction mixture was
irradiated with microwave pulses (100 W) for 2 minutes. The
solution turned chrome yellowish and the solid collected
using a rotary evaporator. After drying in vacuo at 70 °C for
10 h, a pale yellow solid was obtained. All the other AA–HI
ionic liquids were synthesized as above. 1H-NMR: δ = 8.34
(s, 3 H), 7.71 (s, 1 H), 7.01 (s, 1 H), 4.1 (t, 1 H), 3.3 (q, 1 H),
3.1 (q, 1 H) ppm. 13C-NMR δ = 174.19, 136.58, 132.47, 117.03,
55.07, 28.43.
L-Arginine iodide (Arg–HI). 1H-NMR: δ = 8.16 (s, 3H),
4.16 (t, 3H), 3.2 (t, 2 H), 1.55 (m, 2H), 1.78 (m, 2H) ppm.
13C-NMR: δ = 173.7, 158.1, 55.7, 41.6, 28.5, 24.6 [dark
brown viscous liquid at RT].
Coupling reaction
All the reactions were carried out in a 25 mL stainless-steel
batch reactor with a magnetic stirrer at 800 rpm. In a typical
batch reaction process, a pre-determined amount of the cata-
lyst was charged into the reactor containing 35.5 mmol of SO.
The reaction was carried out under a preset pressure of carbon
dioxide at different temperatures. After the completion of the
reaction, the reactor was cooled to RT and the products were
identified by gas chromatograph (Agilent HP 6890 A) equipped
with a capillary column (HP-5, 30 m × 0.25 μm) using a flame
ionized detector.
Synthesis of histidine–alkyl halide ionic liquid. i) N,N,N-
trimethylhistidine iodide (His–MeI). 0.1mol of L-histidine was
dissolved in 5 ml distilled water. Three molar equivalents of
methyl iodide was added and the reaction mixture was
irradiated with microwave pulses (100 W) for 2 minutes. The
solution turned pale yellow after a few seconds and rotary
1
evaporation yielded a light yellow viscous fluid. H-NMR: δ =
8.26 (s, 1H), 7.69 (s, 1 H), 7.1 (s, 1 H), 4.0 (t, 1 H), 3.3 (q, 1 H),
3.8 (s, 9H), 3.1 (q, 1 H) ppm. 13C-NMR: δ = 172.20, 134.11,
127.92, 118.53, 80.06, 53.07, 25.43.
Reusability tests
After the reaction, 10 ml each of distilled water and
dichloromethane was added to the reaction mixture. The
organic layer and aqueous layer were separated using a sepa-
rating funnel. The aqueous layer, upon concentrating using
the rotary evaporator followed by drying under vacuum for 10
h at 60 °C, yielded the ionic liquids. Coupling reactions were
performed again with the recovered ionic liquids as men-
tioned above.
ii) N,N,N-triethylhistidine iodide (His–EtI) was synthesized
by dissolving the L-amino acid (0.1 mmol) in an ethanol/
water (1 : 1) mixture followed by the addition of the respective
alkyl halide (0.3 mol) and irradiation with microwave (100 W)
for 2 minutes. After the reactor was cooled, ether separation
was performed. Upon concentration using rotary evaporation,
a highly viscous yellow ochre liquid resulted. The liquid was
extracted with methanol and concentrated again to yield a
pure ionic liquid.
1H-NMR: δ = 8.42 (s, 1H), 7.49 (s, 1H), 7.1 (s, 1 H), 4.0
(t, 1 H), 3.35 (q, 1 H), 3.8 (s, 9H), 3.1 (q, 1 H), 1.1 (t, 9H) ppm.
13C-NMR: δ = 174.60, 136.23, 115.92, 117.53, 77.65, 52.5, 20.43.
Notes and references
1
(a) M. Aresta and A. Dibenedetto, Dalton Trans., 2007, 2975;
(b) M. North, R. Pasquale and C. Young, Green Chem.,
2010, 12, 1514; (c) M. Yoshida and M. Ihara, Chem.–Eur. J.,
2001, 10, 2886; (d) T. Sakakura, J. C. Choi and H. Yasuda,
Chem. Rev., 2007, 107, 2365; (e) A.-A. G. Sheikh and
S. Sivaram, Chem. Rev., 1996, 96, 951.
NMR spectra of AA–HI ionic liquids
Glycine iodide (Gly–HI). 1H-NMR: δ = 8.13 (s, 3H), 3.63
(s, 2H) ppm. 13C-NMR: δ = 171.05, 41.37 [pale yellow viscous
liquid at RT].
2
3
T. Sakakura and K. Kohno, Chem. Commun., 2009, 1312.
(a) H. Y. Ju, J. Y. Ahn, M. D. Manju, K. H. Kim and
D. W. Park, Korean J. Chem. Eng., 2008, 25, 471; (b) J. Sun,
S. Zhang, W. Cheng and J. Ren, Tetrahedron Lett., 2008, 49,
3588; (c) H. Kawanami, A. Sasaki, K. Matsui and
Y. Ikushima, Chem. Commun., 2003, 896; (d) E. H. Lee,
J. Y. Ahn, M. M. Dharman, D. W. Park, S. W. Park and
L-Alanine iodide (Ala–HI). 1H-NMR: δ = 8.32 (s, 3H), 4.13
(t, 3H) ppm. 13C-NMR: δ = 169.98, 52.32, 20.12 [yellow viscous
liquid at RT].
1
L-Aspartic acid iodide (Asp–HI). H-NMR: δ = 8.42 (s, 3H),
4.18 (t, 3H), 2.61(d, 2H) ppm. 13C-NMR: δ = 171.84, 173.14,
38.14, 52.64 [yellow viscous liquid at RT].
This journal is © The Royal Society of Chemistry 2014
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View Article Online
Paper
I. Kim, Catal. Today, 2008, 131, 130; (e) J. Sun, S. Ichiro,
Catalysis Science & Technology
6
(a) V. Polshettiwar and R. S. Varma, Aquous microwave
assisted chemistry, RSC Green Chem. Ser., no. 7, Royal
Society of Chemistry, 2010; (b) R. S. Varma and
V. V. Namboodiri, Pure Appl. Chem., 2001, 73, 1309; (c)
H. Hu, H. Yang, P. Huang, D. Cui, Y. Peng, J. Zhang, F. Lu,
J. Lian and D. Shi, Chem. Commun., 2010, 46, 3866; (d)
R. M. Palou, Mol. Diversity, 2010, 14, 3; (e) A. Aupoixa,
B. Pegot and G. V. Thanh, Tetrahedron, 2010, 66, 1352; (f)
M. Messali and S. A. Ahmed, Green Sustainable Chem.,
2011, 1, 70; (g) L Zhai, Q. Zhong, C. He and J. Wang,
J. Hazard. Mater., 2010, 177, 803.
S. Fujita and M. Arai, J. Organomet. Chem., 2005, 690, 3490;
(f) J. Peng and Y. Deng, New J. Chem., 2001, 25, 639; (g)
T. Y. Shi, J. Q. Wang, J. Sun, M. H. Wang, W. G. Cheng and
Suo-Jiang Zhang, RSC Adv., 2013, 3, 3726; (h) L. Han,
S. J. Choi, M. S. Park, S. M. Lee, Y. J. Kim, M. I. Kim, B. Liu
and D. W. Park, React. Kinet. Mech. Catal., 2012, 106, 25.
(a) A. Decortes, A. M. Castilla and A. W. Kleij, Angew. Chem.
Int. Ed., 2010, 49, 9822; (b) J. Meléndez, M. North and
R. Pasquale, Eur. J. Inorg. Chem., 2007, 3323; (c) A. Decortes,
M. M. Belmonte, J. B. Buchholza and A. W. Kleij, Chem.
Commun., 2010, 46, 4580; (d) C. J. Whiteoak, N. Kielland,
V. Laserna, E. C. E. Adán, E. Martin and A. W. Kleij, J. Am.
Chem. Soc., 2013, 135, 1228.
4
5
7
8
(a) J. Tharun, G. Mathai, R. Roshan, A. C. Kathalikkattil,
K. Bomi and D. W. Park, Phys. Chem. Chem. Phys., 2013, 15,
9029; (b) Y. Tsutsumi, K. Yamakawa, M. Yoshida, T. Ema
and T. Sakai, Org. Lett., 2010, 12, 5728.
(a) X. Mua, L. Qi, H. Zhang, Y. Shen, J. Qiao and H. Maa,
Talanta, 2012, 97, 349; (b) K. Fukumoto, M. Yoshizawa and
H. Ohno, J. Am. Chem. Soc., 2005, 127, 2398; (c) J. Kagimoto,
K. Fukumoto and H. Ohno, Chem. Commun., 2006, 2254; (d)
J. Zhang, S. Zhang, K. Dong, Y. Zhang, Y. Shen and X. Lv,
Chem.–Eur. J., 2006, 12, 4021; (e) G. H. Tao, L. He, N. Sun
and Y. Kou, Chem. Commun., 2005, 3562; (f) G. H. Tao,
L. He, W. S. Liu, L. Xu, W. Xiong, T. Wang and Y. Kou,
Green Chem., 2006, 8, 639; (g) Q. Gong, H. Luo, J. Cao,
H. Zhang, W. Wang and X. Zhou, Aust. J. Chem., 2012, 65,
381; (h) H. Rong, W. Li, Z. Chen and X. Wu, J. Phys. Chem. B,
2008, 112, 1451; (i) X. Chen, X. Li, A. Hua and F. Wangb,
Tetrahedron: Asymmetry, 2008, 19, 1; (j) M. J. Earle,
P. B. McCormac and K. R. Seddon, Green Chem., 1999, 1, 23;
(k) J. Pernak, I. Goc and I. Mirska, Green Chem., 2004, 6, 323;
(l) E. B. Carter, S. L. Culver, P. A. Fox, R. D. Goode, I. Ntai,
M. D. Tickell, R. K. Traylor, N. W. Hoffman and J. H. Davis,
Chem. Commun., 2004, 630; (m) J. Pernak, F. Stefaniak and
J. Weglewski, Eur. J. Org. Chem., 2005, 650; (n) K. Fukumoto,
M. Yoshizawa and H. Ohno, J. Am. Chem. Soc., 2005, 127,
2398; (o) S. T. Handy, M. Okello and G. Dickenson, Org.
Lett., 2003, 5, 2513; (p) W. L. Bao, Z. M. Wang and Y. X. Li,
J. Org. Chem., 2003, 68, 591; (q) H. Clavier, L. Boulanger,
N. Audic, L. Toupet, M. Mauduit and J. C. Guillemin, Chem.
Commun., 2004, 1224.
(a) T. Takahashi, T. Watahiki, S. Kitazume, H. Yasuda and
T. Sakakura, Chem. Commun., 2006, 1664; (b) L. Han,
H. J. Choi, S. J. Choi, B. Liu and D. W. Park, Green Chem.,
2011, 13, 1023–1028; (c) S. Liang, H. Liu, T. Jiang, J. Song,
G. Yang and B. Han, Chem. Commun., 2011, 47, 2131; (d)
J. Sun, J. Wang, W. Cheng, J. Zhang, X. Li, S. Zhang and
Y. She, Green Chem., 2012, 14, 654; (e) L. Song, Z. F. Zhang,
B. Han, S. Q. Hu, W. J. Li and Y. Xie, Green Chem., 2008, 10,
1337; (f) J. Shi, J. Song, J. Ma, Z. Zhang, H. Fan and B. Han,
Pure Appl. Chem., 2013, 85, 1633; (g) K. R. Roshan,
G. Mathai, J. Kim, J. Tharun, G. A. Park and D. W. Park,
Green Chem., 2012, 14, 2933; (h) J. Tharun, G. Mathai,
A. C. Kathalikkattil, R. Roshan, J. Y. Kwak and D. W. Park,
Green Chem., 2013, 15, 1673; (i) J. Ma, J. Liu, Z. Zhang and
B. Han, Green Chem., 2012, 14, 2410; (j) T. Jose, Y. Hwang,
R. Roshan, S. Ahn, A. C. Kathalikkattil and D. W. Park, Catal.
Sci. Technol., 2012, 2, 1674; (k) K. R. Roshan, J. Tharun,
A. C. Kathalikkattil, D. W. Kim, B. Kim and D. W. Park, Appl.
Catal., A, 2013, 467, 17; (l) F. C. Gmez, G. Salassa, A. W. Kleij
and C. Bo, Chem.–Eur. J., 2013, 19, 6289.
9
(a) D. J. Darensbourg, S. J. Lewis, J. L. Rodgers and
J. C. Yarbrough, Inorg. Chem., 2003, 42, 581; (b)
M. Taherimehr, S. M. Al-Amsyar, C. J. Whiteoak, A. W. Kleij
and P. P. Pescarmona, Green Chem., 2013, 15, 3083.
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