Design of task-specific ionic liquids for catalytic conversion of CO 2 with aziridines under mild conditions

Article abstract of DOI:10.1016/j.cattod.2012.04.006

A series of polyethylene glycol (PEG)-functionalized ionic liquids (ILs) were developed as recyclable and efficient catalysts for selective synthesis of 5-aryl-2-oxazolidinones from aziridines and CO₂ without addition of any organic solvents or additives. In particular, high yields, chemo- and regio-selectivities of oxazolidinones were attained when BrDBNPEG ₁₅₀DBNBr (DBN: 1,5-diazabicyclo[4.3.0]non-5-ene) was used as the catalyst, presumably due to activation of CO₂ by the ether linkage in the PEG backbone, and stabilization of the ring-opened species of aziridine by the delocalized cation BrDBNPEG₁₅₀DBN⁺. Furthermore, the catalyst could be reused for over four consecutive cycles without appreciable loss of catalytic activity and selectivity. The effects of catalyst structure and various reaction parameters on the catalytic performance were also investigated in detail. It was demonstrated that the catalyst worked well for a variety of aziridines producing the corresponding oxazolidinones in good yields and excellent regio-selectivities. Therefore, this solvent-free process could thus represent an environmentally friendly approach for ILs-catalyzed conversion of CO₂ into value-added chemicals.

Full text of DOI:10.1016/j.cattod.2012.04.006

Catalysis Today 200 (2013) 2–8
Contents lists available at SciVerse ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Design of task-specific ionic liquids for catalytic conversion of CO2 with
aziridines under mild conditions
Ya-Nan Zhao, Zhen-Zhen Yang, Si-Hang Luo, Liang-Nian He∗
State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of polyethylene glycol (PEG)-functionalized ionic liquids (ILs) were developed as recyclable and
efficient catalysts for selective synthesis of 5-aryl-2-oxazolidinones from aziridines and CO2 without
addition of any organic solvents or additives. In particular, high yields, chemo- and regio-selectivities of
oxazolidinones were attained when BrDBNPEG150DBNBr (DBN: 1,5-diazabicyclo[4.3.0]non-5-ene) was
used as the catalyst, presumably due to activation of CO2 by the ether linkage in the PEG backbone,
and stabilization of the ring-opened species of aziridine by the delocalized cation BrDBNPEG150DBN+.
Furthermore, the catalyst could be reused for over four consecutive cycles without appreciable loss of
catalytic activity and selectivity. The effects of catalyst structure and various reaction parameters on the
catalytic performance were also investigated in detail. It was demonstrated that the catalyst worked
well for a variety of aziridines producing the corresponding oxazolidinones in good yields and excellent
regio-selectivities. Therefore, this solvent-free process could thus represent an environmentally friendly
approach for ILs-catalyzed conversion of CO2 into value-added chemicals.
Received 6 February 2012
Received in revised form 27 March 2012
Accepted 6 April 2012
Available online 10 May 2012
Keywords:
Polyethylene glycol
Ionic liquids
Carbon dioxide
Aziridine
Oxazolidinone
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
and dual-component systems, for example, organo-antimony
and -tin compounds/hexamethyl phosphoramide (HMPA) [34],
With the increasing environmental concerns about global
warming associated with emission of greenhouse gas, development
of highly efficient methods for chemical fixation of CO2 has been
drawing more and more attention over the last decades in organic
synthesis and chemical industry [1–4]. As well known, as an easily
available and renewable carbon resource, CO2 has the advantages of
being nontoxic, abundant, and economical [5,6]. Besides, oxazolidi-
nones as important heterocyclic compounds find a large application
as intermediates [7–10] and chiral auxiliaries [11–13] in organic
synthesis. Thus, the coupling reaction of CO2 with aziridines to
afford oxazolidinones represents one of the few green and efficient
commercial routes converting CO2 into value-added chemicals, as
depicted in Scheme 1. On the other hand, this cycloaddition proce-
dure utilizing CO2 as a feedstock is more attractive compared with
other processes including carbonylation of amino alcohols with
phosgene, or carbon monoxide [14–17], and reaction of propar-
gylamines or propargylic alcohols with CO2 [18–22].
salenCr(III)/4-dimethylamino pyridine (DMAP) [35], or phe-
nol/DMAP [36]. In particular, polymer-supported active compo-
nents (ammonium salts [37], amino acids [38], ILs [39]) have been
developed as efficient heterogeneous catalysts for the synthesis
of oxazolidinones. In addition, the cycloaddition of aziridines to
CO2 also proceeded smoothly under electrochemical reaction con-
ditions in the presence of nickel complex catalyst [40]. Although
significant advances have been made, there are still some draw-
backs, such as the need of co-catalyst or co-solvent, high catalyst
loading, along with toilsome purification of product and a limited
substrate scope in most of the above-mentioned cases. There-
fore, developing a low cost, easily prepared, thermally stable and
efficient single component catalyst for the conversion of CO2 to
oxazolidinone is still desirable.
To our best knowledge, ILs have distinctive properties such
as high thermal stability, negligible vapor pressure, high load-
ing capacity, easy recyclability and diversiform structure/property
modulation [41]. In recent years, ILs have attracted great attention
as they permit mutual advantages of both the homogeneous and
the heterogeneous catalysis [42–45]. Kawanami and co-workers
firstly reported a biphasic supercritical CO2 (scCO2)/ILs catalysis by
employing symmetric tetraalkyl ammonium salts for the reaction
of aziridines with CO2 in the presence of iodine in high yield of 98%
within 5 min [28]. Recently, Bhanage’s group [39] reported a kind
of polymer supported diol-functionalized ILs for coupling of CO2 to
aziridines providing 5-aryl-2-oxazolidinones. Our group has also
In the past decades, numerous catalysts have been developed
for the cycloaddition reaction between CO2 and aziridines, such as
single-component catalysts including alkali metal halide [23–25],
tetraalkylammonium halide system [26], iodine [27,28], amino
acids [29,30], zirconyl chloride [31] and especially ILs [32,33],
∗
Corresponding author. Tel.: +86 22 2350 3878; fax: +86 22 2350 3878.
E-mail address: heln@nankai.edu.cn (L.-N. He).
0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2012.04.006

Y.-N. Zhao et al. / Catalysis Today 200 (2013) 2–8
3
2.2. Materials
O
O
R1
N
R1
R1
Cat.
Aziridines [37] were synthesized according to the published
procedures. CO2 with a purity of 99.99% was commercially avail-
able. The other organic and inorganic compounds from Tianjin
Guangfu Fine Chemical Research Institute were used without fur-
ther purification except for the solvents, which were distilled by
the known method prior to use.
+ CO2
O
N
O
N
+
R2
R2
R2
2
1
3
Scheme 1. Synthesis of oxazolidinones from aziridines and CO2.
developed Lewis base-derivated protic ILs as highly efficient and
recyclable homogeneous catalysts for the selective synthesis of 5-
aryl-2-oxazolidinones without addition of any organic solvents or
additives [32,33].
2.3. Characterization
1H NMR spectra were recorded at Bruker 300 or 400 spectrom-
eter in CDCl3 or D2O and CDCl3 (7.26 ppm) or D2O (4.80 ppm) were
used as internal reference, 13C NMR were recorded at 75 MHz or
100.6 MHz in CDCl3 or D2O and CDCl3 (77.0 ppm) was used as
internal reference. ESI-MS was recorded on a Thermo Finnigan
LCQ Advantage spectrometer in ESI mode with a spray voltage
of 4.8 kV (see Supplementary data). GC analyses were performed
on Shimadzu GC-2014, equipped with a capillary column (RTX-5,
30 m × 0.25 ␮m) using a flame ionization detector.
Interest in employing PEG stems from its distinctive proper-
ties, such as inexpensive, thermally stable, almost negligible vapor
pressure, toxicologically innocuous and environmentally benign
characterization. More importantly, PEG could be regarded as
a CO2-philic material. In other words, “CO2-expansion of PEG”
effect could lead to changes in the physical properties of the
liquid phase mixture including lowered viscosity and increased
gas/liquid diffusion rates, and could assumedly facilitate CO2
conversion with increasing activity [46–48]. Previously, PEG6000
(MW = 6000 Da)-supported ammonium salts have been utilized as
an efficient and recyclable catalyst for the cycloaddition reaction
of aziridines to CO2 [37]. In addition, BrTBDPEG150TBDBr (TBD:
1,5,7-triazabicyclo[4.4.0]dec-5-ene) has been already proved to be
highly efficient and recyclable catalyst for the synthesis of cyclic
carbonates from CO2 and epoxides, presumably due to activation
of epoxides assisted by hydrogen bonding and activation of CO2 by
the ether linkage in the PEG backbone or through the formation
of carbamate species with the secondary amino group in the IL’s
cation on the basis of in situ FT-IR study under CO2 pressure [49].
Herein, we would like to report a process for the synthesis of 5-
aryl-2-oxazolidinones by employing a series of PEG-functionalized
ILs as efficient and recyclable catalysts, as shown in Scheme 2.
Indeed, BrDBNPEG150DBNBr displayed high catalytic activity for
the synthesis of oxazolidinones from CO2 in the absence of any
additional organic solvent or additive. High yields, chemo- and
regio-selectivities of 5-aryl-2-oxazolidinones were obtained under
the optimal reaction conditions. More importantly, the catalyst
could be stable during the reaction and can be reused for over four
successive cycles without loss of efficiency.
2.4. Preparation of XPEG150X (X = Cl, Br, I)
2.4.1. Synthesis of 1,2-bis(2-chloroethoxy)ethane (ClPEG150Cl)
[50]
The mixture of triethylene glycol (PEG150) (0.28 mol) and pyri-
dine (0.61 mol) in benzene (250 mL) was heated to reflux, and then
thionyl chloride (0.61 mol) was added dropwise over 3 h. The mix-
ture was kept on refluxing for another 16 h and then cooled to room
temperature. Then hydrochloric acid (6.3 mL, 11.8 M) diluted with
water (50 mL) was added dropwise in 15 min. The upper benzene
solution was separated, and after evaporation under reduced pres-
sure, the target compound ClPEG150Cl was obtained as a clear oil.
ClPEG150Cl: colorless liquid, yield 60%, 1H NMR (CDCl3, 400 MHz)
3.77 (t, 3J = 6.0 MHz, 4 H), 3.70 (s, 4 H), 3.64 (t, 3J = 6.0 MHz, 4 H).
2.4.2. Synthesis of 1,2-bis(2-bromoethoxy)ethane (BrPEG150Br)
[51]
To a stirred mixture of PEG150 (0.13 mol) and pyridine (0.51 mol)
at 0 ◦C, phosphorus tribromide (0.103 mol, distilled) was added
dropwise over 30 min. The resulting suspension was heated at 60 ◦C
for 4 h, and then the mixture was poured into ice-water (30 mL). The
lower organic layer was washed with water (5× 20 mL), and then
was dried with MgSO4. Finally, the target compound BrPEG150Br
was purified through distillation under reduced pressure as a light
yellow liquid. BrPEG150Br: light yellow liquid, yield 50%, 1H NMR
(400 MHz, CDCl3) 3.83 (t, 3J = 6.4 MHz, 4 H), 3.69 (s, 4 H), 3.48 (t,
3J = 6.4 MHz, 4 H).
2. Experimental
2.1. Cations
Experiments using compressed gas CO2 are potentially haz-
ardous and must only be carried out by using the appropriate
equipment and under rigorous safety precautions.
O
N
N
N
N
N
N
N
N
N
N
N
N
O
2
2
Br
Br
Br
Br
Br
Br
BrDMImPEG150DMImBr
BrMImPEG150MImBr
BrDMIm(CH2)4DMImBr
NH
HN
N
N
N
N
N
N
O
O
O
N
N
N
Br
N
N
Br
N
2
2
2
X
X
Br
Br
X = Cl, Br, I
XDBNPEG150DBNX
BrTBDPEG150TBDBr
BrDBUPEG150DBUBr
Scheme 2. Structures of the PEG-functionalized ionic liquids used in this study.

4
Y.-N. Zhao et al. / Catalysis Today 200 (2013) 2–8
Table 1
PEG functionalized-ILs promoted oxazolidinone synthesis.a
Et
N
Et
N
O
O
Ph
Ph
Ph
Et
N
PEG-functionalized ILs
Et
Et
+ CO2
O
N
+ O
N
+
+
N
Ph
N
Ph
Ph
Ph
Et
Et
1a
5a
2a
3a
4a
.
Entry
Catalyst
Conv. (%)b
Yield (%)b,c
Regio-sel.d
1
2
3
4
5
6
7
8
9
None
PEG150
KBr
56
52
67
63
86
62
85
94
91
96
97
95
8
16
8
97:3
96:4
96:4
94:6
98:2
98:2
98:2
96:4
96:4
97:3
92:8
97:3
D201e
7
BrDMImPEG150DMImBr
BrDMIm(CH2)4DMImBr
BrMImPEG150MImBr
BrDBUPEG150DBUBr
BrTBDPEG150TBDBr
BrDBNPEG150DBNBr
ClDBNPEG150DBNCl
IDBNPEG150DBNI
64
51
60
68
80
86
80
73
10
11
12
a
Reaction conditions: 1a (1 mmol, 0.147 g), catalyst loading (1 mol%, relative to aziridine), CO2 (3 MPa), 100 ◦C, 2 h.
Determined by GC with biphenyl as an internal standard.
Total yield of 2a and 3a.
b
c
d
e
Molar ratio of 2a to 3a.
D201: a kind of ion exchange resin, that is, polystyrene with N+(CH3)3 Cl− as a functional group (exchange capacity, 3.8 mmol g−1; degree of crosslinking of bead, 4%;
bead diameter, 0.3–1.25 mm), supplied from the Chemical Plant of Nankai University, Tianjin, China.
2.4.3. Synthesis of 1,2-bis(2-iodoethoxy)ethane (IPEG150I) [52,53]
2.4.3.1. Synthesis of 1,2-bis(2-tosylethoxy)ethane (TsPEG150Ts). To a
round-bottom flask with a magnet, 400 mL of pyridine and 0.5 mole
of PEG150 was added. After cooling to <0 ◦C in an ice bath, 1.0 mole
of p-toluenesulfonyl chloride (TsCl) was next added to the reaction
flask in several portions, keeping the reaction temperature <0 ◦C.
Near the completion of this addition, a slurry was formed. Stir-
ring was continued for another 1.5 h. Next the slurry was poured
into a solid ice and water mixture where the TsPEG150Ts was solid-
ified and pyridine hydrochloride salt dissolved. After filtration,
TsPEG150Ts was recrystallized from ethanol. TsPEG150Ts: white
solid, yield 75%, 1H NMR (400 MHz, CDCl3) 7.79 (dd, 3J = 8.4 MHz,
4 H), 7.34 (dd, 3J = 8.4 MHz, 4 H), 4.13 (t, 3J = 4.8 MHz, 4 H), 3.65(t,
3J = 4.8 MHz, 4 H), 3.52 (s, 4 H), 2.44 (s, 6 H).
to room temperature and was then cooled to −10 ◦C overnight,
during which time yellow oil was formed. The excess toluene was
decanted. The resulting product was washed with dry Et2O for five
times and then dried in vacuo for 24 h.
2.6. General procedure for cycloaddition reaction of aziridines
with CO2
The cycloaddition of the aziridine with CO2 were carried
out in a 25 mL stainless steel autoclave. Aziridine (1 mmol) and
BrDBNPEG150DBNBr (5.2 mg, 1 mol% relative to aziridine) were
charged into the reactor at room temperature. CO2 was introduced
into the autoclave and then the mixture was stirred at prede-
termined temperature for 15 min to reach the equilibration. The
pressure was then adjusted to the desired pressure and the mixture
was stirred continuously. When the reaction finished, the reactor
was cooled in ice-water and CO2 was ejected slowly. An aliquot
of sample was taken from the resultant mixture and dissolved
in dry CH2Cl2 for GC analysis using a flame ionization detector.
The residue was purified by column chromatography on silica gel
(eluting with 8:1 to 1:1 petroleum ether/ethyl acetate) to afford
the product. 3-Ethyl-5-(4-chlorophenyl)oxazolidin-2-one White
solid; 1H NMR (400 MHz, CDCl3) ı 1.17 (t, 3J = 7.3 Hz, 3 H), 3.30–3.43
(m, 2 H), 3.69–3.76 (m, 1 H), 3.92 (t, 3J = 8.7 Hz, 1 H), 5.44 (t,
3J = 8.0 Hz, 1 H), 7.27–7.38 (m, 4 H); 13C NMR (100 MHz, CDCl3) ı
12.6, 38.9, 51.5, 73.6, 126.9, 129.1, 134.7, 137.4, 157.4; ESI-MS calcd.
for C11H12ClNO2 225.67, found 451.64 (2M+H)+.
2.4.3.2. Synthesis of 1,2-bis(2-iodoethoxy)ethane (IPEG150I). The
1,2-bis(2-tosylethoxy)ethane (TsPEG150Ts) (13.0 mmol) was dis-
solved in acetone (50 mL), and KI (50.0 mmol) was added to the
solution. The mixture was stirred at 40 ◦C for 4 days. The solvent
was then removed under reduced pressure, and water (10 mL) was
added to the residue. The mixture was extracted with CH2Cl2 (4×
50 mL), and then the combined extracts were washed with brine,
dried over anhydrous Na2SO4, filtered, and concentrated under
reduced pressure. Purification by silica gel column chromatogra-
phy (CH2Cl2) gave the product IPEG150I. IPEG150I: yellow liquid,
yield 90%, 1H NMR (CDCl3, 400 MHz) 3.77 (t, 3J = 6.8 MHz, 4 H), 3.67
(s, 4 H), 3.27 (t, 3J = 6.8 MHz, 4 H).
General procedure for catalyst recycling: The product was sepa-
rated by distillation under reduced pressure. Then, the substrate, i.e.
2.5. Preparation of PEG-functionalized ILs [54]
toluene
X
X
HBase
X
BaseH
X
2Base
+
O
O
2
2
Base = DMIm, DBU
DBN, MIm, TBD
XPEG150
X = Cl, Br, I
X
In
a typical reaction, DBN (20.0 mmol) and BrPEG150Br
(20.0 mmol) were added to dry toluene (2 mL) in a round-bottomed
flask under N2. The mixture was heated under reflux under N2 for
24 h, upon two layers were formed. The flask was allowed to cool
1-ethylene-2-phenylaziridine (1 mmol) was added to the residual
for the next cycle as the same procedure as the first run.

Y.-N. Zhao et al. / Catalysis Today 200 (2013) 2–8
5
Fig. 1. Optimization of reaction conditions and catalyst recyclability. 1a conversion and yield of (2a + 3a) were determined by GC using biphenyl as an internal standard.
Reaction conditions: 1a (1 mmol), BrDBNPEG150DBNBr (5.2 mg, 1 mol% relative to aziridine); (a) CO2 pressure (3 MPa), 2 h; (b) temperature (120 ◦C), 2 h; (c) temperature
(120 ◦C), CO2 pressure (3 MPa); (d) temperature (120 ◦C), CO2 pressure (3 MPa), 2 h.
3. Results and discussion
efficiently than Cl−, I− (Br− > Cl− > I−) (entries 10–12), probably
due to the appropriate leaving ability and nucleophilicity [56,57].
In summary, BrDBNPEG150DBNBr was identified as the most
effective IL catalyst, and thus chosen as the model catalyst for
further investigation. 96% Conversion of 1a and 86% yield of the
oxazolidinone was obtained with high regio-selectivity of 2a:3a
molar ratio: 97:3 (entry 10), alongside with small amounts of
1,4-diethyl-2,5-diphenyl-piperazine (4a) and 1,4-diethyl-2,3-
diphenyl-piperazine (5a) being detected by GC–MS and 1H NMR
(see Supplementary data). Besides, catalytic activity of alkali
metals such as KBr and ion exchange resin e.g. D201 (polystyrene-
supported-Me3NCl functional group) was found to be much lower
than PEG-functionalized ionic liquids (entry 3 vs. 10, 4 vs. 11). In
addition, commonly used catalysts including ammonium iodide
[24] or alkali metals halide [26] with high catalytic loading (NH4I,
1 equiv.) are difficult to be recycled and organic solvents are also
needed.
3.1. Activity of various catalysts
In the preliminary study, the synthesis of oxazolidinones from
CO2 and 1-ethylene-2-phenylaziridine (1a) was carried out in the
presence of a series of PEG-functionalized ILs derived from 1,2-
dimethylimidazole (DMIm), 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU), DBN, 1-methylimidazole (MIm) and TBD under identi-
cal reaction conditions (catalyst loading, 1 mol%; CO2 pressure,
3 MPa; 100 ◦C; 2 h). As shown in Table 1, almost no reaction
occurred in the absence of any catalyst (entry 1, Table 1). Only
52% conversion of 1a together with 16% yield of 2a was obtained
using non-functionalized PEG150 as catalyst (entry 2). Notably,
the results revealed that the PEG-functionalized ILs could be
active catalytic species and the cation of the IL has remarkable
impact on the catalytic performance (entries 5–10). Particularly,
the DMImPEG150DMIm2+ cation with CO2-philic ether linkage
showed higher activity, whereas, the cation containing alkyl group
[DMIm(CH2)4DMIm2+] gave much lower activity (entries 5 vs. 6).
Those delocalized cations which could stabilize the ring-opened
structure of the aziridine (as the intermediate, see the proposed
mechanism), thus gave moderate yield of oxazolidinone under the
reaction conditions. The catalytic efficiency increased in the fol-
3.2. Influences of different parameters
Subsequently, the influence of reaction parameters on the
reaction was also evaluated, as illuminated in Fig. 1a–c. It is
worth mentioning that variation of the reaction temperature, CO2
pressure or reaction time has almost no influence on the regio-
selectivity, i.e. molar ratio of 2a to 3a, which keeps over 97:3 (molar
ratio 2a:3a) in all the cases using BrDBNPEG150DBNBr as the cata-
lyst.
Obviously, the catalytic activity was sensitive to the reaction
temperature. As shown in Fig. 1a, both 1a conversion and yields
of oxazolidinones (2a + 3a) were increased with the reaction tem-
perature increasing from 60 to 120 ◦C. In particular, moderate
lowing order with Br− as the counter anion: MImPEG150MIm2+
DMImPEG150DMIm2+ < DBUPEG150DBU2+ < TBDPEG150TBD2+
<
<
DBNPEG150DBN2+, which is probably relative to the basicity of
the cations (MIm < DMIm < DBN < DBU < TBD [55]) and their steric
hindrance (DBU > TBD > DBN). Then the anion (Cl−, Br−, I−) effect on
the reaction outcome was also evaluated with DBNPEG150DBN2+
as the counter cation. As a result, Br− was performed more

6
Y.-N. Zhao et al. / Catalysis Today 200 (2013) 2–8
Table 2
Various oxazolidinones synthesis catalyzed by BrDBNPEG150DBNBr.a
O
O
R1
N
BrDBNPEG150DBNBr
R1
R1
+ CO2
O
N
O
N
+
R2
R2
R2
1
2
3
.
Entry
R1, R2
Time (h)
Conv. (%)b
Yield (%)b,c
Regio-sel. (%)d
1
2
3
4
5
6
7
8
9
Et, Ph
2
2
2
2
2
24
4
48
48
48
97
95
95
94
96
88
89
84
81
89
68
93
77
60
97:3
99:1
99:1
96:4
97:3
99:1
99:1
99:1
98:2
97:3
Et, p-Me-Ph
Et, p-Cl-Ph
n-Pr, Ph
n-Bu, Ph
i-Pr, Ph
>99
95
>99
94
79
61
i-Bu, Ph
c-Hex, Ph
c-Hex, p-Cl-Ph
c-Hex, p-Me-Ph
10
a
Reaction conditions: Aziridine (1 mmol), BrDBNPEG150DBNBr (5.2 mg, 1 mol%), CO2 (3 MPa), 120 ◦C, 2 h.
Determined by GC with biphenyl as an internal standard.
Total yield of 2a and 3a.
b
c
d
Molar ratio of 2a and 3a.
catalytic activity could be still attained even at room tempera-
word, too high CO2 pressure may dilute the aziridines and catalysts,
thus resulting in lower reaction efficiency. Therefore, suitable CO2
pressure would be 3 MPa. Furthermore, the influence of reaction
time on the reaction was also examined. As shown in Fig. 1c, 2 h
is needed for the reaction to give a good yield and selectivity of
oxazolidinones.
ture. On the other hand, excellent results were also obtained when
the reaction temperature was further increased to 140 ◦C, which
also could experimentally prove the excellent thermal stability of
BrDBNPEG150DBNBr. Accordingly, the appropriate reaction tem-
perature would be 120 ◦C.
On the other hand, the influence of CO2 pressure on the reac-
tion was investigated under identical reaction conditions (Fig. 1b).
Pressure has great influence on the reaction outcome with variation
of CO2 pressure from 0.5 to 7 MPa. Obviously, the conversion has
almost no change when the pressure increasing from 0.5 to 7 MPa.
However, the total yield of 2a and 3a was sensitive to the CO2 pres-
sure and the yield of 2a and 3a decreased by further increasing
the pressure over 3 MPa. Excessive CO2 pressure may cause a low
concentration of aziridines in the vicinity of the catalyst. In other
As well known, the stability and reusability of the catalyst are
two key factors to identify whether it has potentially practical
application in industry. To test the catalyst reusability, the reaction
was performed under optimal reaction conditions, viz. aziridine,
1 mmol; catalyst loading, 1 mol%; CO2 pressure, 3 MPa; 120 ◦C; 2 h.
The catalyst (BrDBNPEG150DBNBr) was recovered after the prod-
uct was separated by distillation under reduced pressure from
the reaction mixture. The recovered catalyst was directly used for
the next run without further purification under the same reaction
N
O
N
O
N
O
R1
R1
O
N
R2
a
O
R2
Br
3
Br
N
N
Br
N
Br
N
N
N
O
2
R1
N
R1
N
O
O
N
O
O
R2
R2
6
a
1
b
Br
BrDBNPEG150DBN
N
O
N
O
O
R1
R1
R2
O
R2
N
b
O
Br
2
Br
N
N
Scheme 3. The proposed mechanism for the cycloaddition reaction of CO2 with aziridines.

Y.-N. Zhao et al. / Catalysis Today 200 (2013) 2–8
7
conditions. As illustrated in Fig. 1d, no significant decrease in either
Appendix A. Supplementary data
the yield or selectivity of oxazolidinones was detected after five
successive cycles.
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.cattod.2012.04.006.
3.3. Other substrates
References
The generality and utility of this protocol were examined by
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optimal conditions. The results are listed in Table 2. It was found
that the reaction of aziridines bearing alkyl groups (entries 1, 4, 5,
Table 2) at the nitrogen atom proceeded smoothly and >80% yield
of oxazolidinones was achieved under the reaction conditions of
120 ◦C, 3 MPa CO2 pressure within 2 h. However, longer reaction
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come (entry 2 vs. 3). However, an electron-withdrawing group on
the benzene ring showed higher activity than an electron-donating
group when R1 is cyclohexyl group (entry 9 vs. 10).
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4. Conclusions
In
summary,
the
PEG-functionalized
ILs
such
as
BrDBNPEG150DBNBr, proved to be high efficient catalysts for
the cycloaddition of CO2 to aziridines under solvent-free condi-
tions. The ILs used in this study represent stable, easily synthesized,
cheap, extremely robust and environmentally benign catalysts,
which can effectively active CO2 through the PEG backbone in
the cation part of the ILs. The catalyst could be recycled for
over four times without loss of yield and selectivity. Therefore,
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Acknowledgements
This work was financially supported by the National Natural Sci-
ence Foundation of China (Grant No. 21172125), Ministry of Science
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