Polymer supported diol functionalized ionic liquids: An efficient, heterogeneous and recyclable catalyst for 5-aryl-2-oxazolidinones synthesis from CO2 and aziridines under mild and solvent free condition

Article abstract of DOI:10.1016/j.molcata.2011.10.007

Polymer supported diol functionalized ionic liquids (PS-DFILXs) were investigated as an efficient, heterogeneous and recyclable catalyst for coupling of carbon dioxide (CO₂) to aziridines providing high conversion with excellent regioselectivit

Full text of DOI:10.1016/j.molcata.2011.10.007

Journal of Molecular Catalysis A: Chemical 351 (2011) 196–203
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Polymer supported diol functionalized ionic liquids: An efficient, heterogeneous
and recyclable catalyst for 5-aryl-2-oxazolidinones synthesis from CO2 and
aziridines under mild and solvent free condition
Rahul A. Watile, Dattatraya B. Bagal, Krishna M. Deshmukh, Kishor P. Dhake, Bhalchandra M. Bhanage∗
Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai 400019, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Polymer supported diol functionalized ionic liquids (PS-DFILXs) were investigated as an efficient, het-
erogeneous and recyclable catalyst for coupling of carbon dioxide (CO2) to aziridines providing high
conversion with excellent regioselectivity towards 5-aryl-2-oxazolidinones under mild and solvent
free conditions. The developed methodology was found to be applicable for wide variety of 1-alkyl-
2-arylaziridines producing the corresponding 5-aryl-2-oxazolidinones with good yields and excellent
regioselectivity. The hydroxyl groups present on vicinal carbon of PS-DFILX proved crucial for higher
efficiency of catalyst. In addition, the catalyst could be reused for four consecutive recycles without
significant loss in its catalytic activity and selectivity. Hence, the developed process represents environ-
mentally benign chemical fixation pathway of CO2 for synthesis of 5-aryl-2-oxazolidinones under mild
condition.
Received 26 August 2011
Received in revised form 3 October 2011
Accepted 8 October 2011
Available online 14 October 2011
Keywords:
Carbon dioxide
Aziridines
5-Aryl-2-oxazolidinones
Heterogeneous catalysts
Polymer supported diol functionalized
ionic liquids
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
functionalized polyethylene glycol [24], zirconyl chloride [25],
polystyrene supported amino acid [26] and [C4DABCO] Br [27].
Entrapment of greenhouse gas CO2 into valuable chemicals has
received a great attention so as to limit the increasing concentra-
tion of atmospheric CO2. In respect, exhaustive research has been
made to understand the importance of utilization of CO2 in organic
synthesis [1–4]. One of the most attractive synthetic goals starting
from CO2 is synthesis of five-membered oxazolidinones possessing
wide application as intermediates [5–8], chiral auxiliaries [9,10]
and building blocks for biologically active pharmaceutical agents
[11–13]. Therefore, the development of greener and efficient cat-
alyst for synthesis of oxazolidinones under mild conditions has
attracted much more attention.
From the prospective of green chemistry, synthesis of 2-
oxazolidinones utilizing CO2 as a feedstock is more attractive
in comparison to other processes like carbonylation of 1,2-
amino alcohols with phosgene or CO [14–17] and carboxylative
cyclization of propargylamines or propargylic alcohols with CO2
[18–21]. Several catalysts are been reported for synthesis of oxazo-
lidinones from CO2 and aziridines, using (salen)-Cr(III)/DMAP
[22], phenol/DMAP [23], quaternary ammonium bromide
However, most of the catalysts required harsh reaction conditions
(8.0 MPa of CO2 pressure, 100–140 ◦C temperature, longer reaction
time and higher catalyst/substrate ratio) with tedious work up
procedure. Hence, the development of environmental benign,
thermo-stable, heterogeneous recyclable catalyst for efficient
synthesis of oxazolidinones from CO2 and aziridines under mild
reaction condition is still desirable.
In recent years, ionic liquids (ILs) based heterogeneous catalysis
has attracted a great attention as they permit mutual advantages
of both homogeneous as well as heterogeneous catalysts [28–31].
In comparison to pure Lewis acidic ILs, IL-based heterogeneous
catalyst offers additional advantage like decrease in the amount
of ionic liquids used with ease of separation and efficient cata-
lyst recovery [32–34]. In context, we envisage that the presence of
vicinal hydroxyl groups on ionic liquid cation moiety have great
potential to accelerate the reaction in forward direction. This is
because of its strong chelating ability through hydrogen bonding
with nitrogen atom of aziridines thus enabling the ring opening
of aziridine. Han et al. made a similar observation wherein the
activity of cellulose/KI was increased due to the presence of vicinal
hydroxyl groups for ring opening reaction of epoxide to cyclic car-
bonate effectively [35]. This important observation encouraged us
to develop an IL-based heterogeneous catalyst for cycloaddition of
CO2 and aziridines which would function at ambient temperature
∗
Corresponding author. Tel.: +91 22 3361 1111/2222; fax: +91 22 3361 1020.
E-mail addresses: bhalchandra bhanage@yahoo.com,
bm.bhanage@ictmumbai.edu.in (B.M. Bhanage).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.10.007

R.A. Watile et al. / Journal of Molecular Catalysis A: Chemical 351 (2011) 196–203
197
Scheme 1. Schematic representation for the preparation of polymer-supported diol functionalized ionic liquids as catalyst.
and pressure conditions. To best of our knowledge, this is the first
report where an IL-based heterogeneous catalyst was efficiently
employed to catalyze the reaction of aziridines with CO2.
were added to 100 mL round bottom flask and refluxed for 24 h.
On completion, the reaction mixture was cooled to room temper-
ature. It was then filtered and residue obtained was washed with
toluene, 10% HCl, water and methanol sequentially, followed by
drying under reduced pressure to afford the supported ionic liquid
catalyst PS-IM (loading of ionic liquid was determined by elemental
analysis). The catalyst was further characterized by FT-IR analysis
to examine the attachment of ionic liquid.
In present communication, we report the synthesis and cat-
alytic activity of PS-DFILX as a highly efficient, heterogeneous and
recyclable catalyst with mechanistic understanding of coupling
reaction of CO2 and aziridines under mild reaction condition. The
catalyst exhibited significant catalytic activity providing good to
excellent yield of the desired product with excellent chemo and
regioselectivity at room temperature. Detailed studies revealed
that the hydroxyl groups on the vicinal carbons of PS-DFILX plays
a vital role for remarkable efficiency of the catalyst (Scheme 2).
2.2.2. Step-2: Preparation of polymer supported diol
functionalized ionic liquids (PS-DFILXs)
Secondly, PS-DFILXs were prepared from PS-IM and cor-
responding halide substituted diol. In
a typical preparation,
3-bromopropane-1,2-diol (54 mmol), PS-IM (10 g) and acetonitrile
(70 mL) were added into a 125 mL three-necked flask equipped
with a magnetic stirrer and was heated at 80 ◦C for 24 h with vig-
orous stirring. On completion of reaction, the reaction mixture
was cooled to room temperature. The upper phase was decanted
carefully and the solid residue was washed with ethyl acetate
(3× 20 mL). Further, the solid was dried under vacuum at 60 ◦C
for 12 h to provide the polymer supported 1-(2,3-dihydroxyl-
propyl)-imidazolium bromide (PS-DHPIMBr) (Scheme 1). Based on
the similar procedure, the polymer supported 1-(2,3-dihydroxyl-
propyl)-imidazolium chloride (PS-DHPIMCl) was synthesized.
2. Experimental
2.1. Materials
All chemicals and reagents were purchased from firms of
repute with their highest purity available and used without further
purification/pre-treatments. Polymer supported diol functional-
ized ionic liquid were prepared according to the procedure reported
in literature [36] with slight modification.
2.2. Typical experimental procedure for the preparation of
polymer-supported diol functionalized ionic liquids
2.3. Characterization of polymer supported diol functionalized
ionic liquids (PS-DFILXs)
2.2.1. Step-1: Preparation of imidazolium-loaded polymeric
support (PS-IM)
Merrifield peptide resin (2% cross linked, 2.3 mmol Cl/g, Aldrich)
(10.0 g, 53.4 mmol), imidazole (53.5 mmol) and toluene (70 mL)
To confirm the immobilization of DHPIMBr on a polymer,
Fourier transform infrared (FT-IR) spectra were recorded on a
Scheme 2. Carboxylation of aziridine with CO2.

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13C NMR (100 MHz, CDCl3) 158.06, 139.07, 129.03, 128.89,
125.63, 74.44, 52.30, 44.34, 31.53, 27.45, 26.6, 22.6, 14.12.
GC–MS (EI) m/z (%) = 247(42) [M]+, 156(21.0), 132(100),
104(53.0), 43(36.0).
PerkinElmer FT-IR spectrophotometer with anhydrous KBr as stan-
dard (Thermo Electron Co.). The amount of diol functionalized
ILs attached on the (chloromethyl polystyrene resin) CMPS was
determined by elemental analysis (Thermo finnigan) PS-DHPIMBr
(loading, 3.2 mmol/g), PS-DHEIMCl (loading, 2.8 mmol/g). Solid-
state NMR was carried out for 13C nuclei. Thermo gravimetric
analysis (TGA) was carried out using TGA-SDT (Q600 V8.2 Build
100) in a nitrogen atmosphere between 25 ◦C and 600 ◦C at a heat-
ing rate 5 ◦C/min.
(v) 3-Cyclohexyl-5-phenyloxazolidin-2-one (Table 1, entry 10)
1H NMR (300 MHz, DMSO-d6) ı = 7.31–7.46 (m, 5H), 5.53
(dd, J = 8.7, 72 Hz, 1H), 3.92 (t, J = 8.9 Hz, 1H), 3.57 (m, 1H),
3.26 (m, 1H), 0.91–1.94 (m, 10H).
GC–MS (EI) m/z (%) = 245.90(12.7) [M+1]+, 245(67) [M]+,
201(18), 163(37), 157(35), 120(38), 104(100), 91(65.0),
55(80), 45(84), 41(57).
2.4. General procedure for carboxylation of aziridines with CO2
(vi) 3-Butyl-5-(4-(tert-butyl) phenyl) oxazolidin-2-one (Table 1,
entry 12)
In a typical experimental procedure, coupling of aziridines with
CO2 was carried out in a 100 mL stainless steel autoclave reac-
tor with a mechanical stirrer. Aziridine (1 mmol) and catalyst
(1.5 mol %) were charged into the reactor at room temperature.
CO2 gas was introduced into the autoclave and then pressure was
adjusted to desired pressure (5 MPa) and the mixture was stirred
(550–600 rpm) continuously at room temperature for mentioned
time period. On completion of reaction, the reactor was cooled
in ice-water and CO2 was ejected slowly. The reaction mixture
was analyzed by GC (PerkinElmer, Clarus 400) equipped with a
flame ionization detector (FID) and a capillary column (Elite-1,
30 m × 0.32 mm × 0.25 ␮m). The residue was purified by column
chromatography on silica gel (eluting with 10:1 to 1:1 petroleum
ether/ethyl acetate) to afford the product. The products further ana-
lyzed by 1H and 13C spectra recorded on NMR spectrometer (Varian
300) using TMS as internal standard and by GC–MS (Shimadzu QP
2010) which are consistent with those reported in the literature
[24–27].
1H NMR (300 MHz, DMSO-d6) ı = 7.41–7.46 (m, 4H), 5.47
(m, 1H), 3.91 (t, J = 8.9 Hz, 1H), 3.39 (dd, J = 8.9, 7.4 Hz, 1H),
3.18 (t, J = 7.0 Hz, 2H), 1.39–1.53 (m, 4H), 1.17–1.35 (m, 9H),
0.93 (m, 3H).
(vii) 5-Phenyloxazolidin-2-one (Table 1, entry 1)
GC–MS (EI) m/z (%) = 162.96(16) [M]+, 107(100), 91(13),
89(15), 45(31).
(viii) 3-Methyl-5-phenyloxazolidin-2-one (Table 1, entry 2)
GC–MS (EI) m/z (%) = 177(4) [M]+, 176(29.40), 133(24.6),
132(55), 91(23), 85(10.7), 43(100).
2.5.2. Characterization of aziridines
(i) 1-Methyl-2-phenylaziridine (Table 1, entry 2)
1H NMR (300 MHz, DMSO-d6) ı = 7.07–7.39 (m, 5H), 2.37
(m, 3H), 2.24 (m, 1H), 1.72 (d, J = 3.4 Hz, 1H), 1.61 (d, J = 6.4 Hz,
1H).
GC–MS (EI) m/z (%) = 133(10) [M]+, 132(100), 91(39),
42(12).
(ii) 1-Butyl-2-phenylaziridine (Table 1, entry 6)
1H NMR (300 MHz, DMSO-d6) ı = 7.31–7.40 (m, 2H),
7.23–7.30 (m, 3H), 2.31–2.54 (m, 3H), 1.88 (d, J = 2.3 Hz, 1H),
1.17 (d, J = 6.4 Hz, 1H), 1.31–1.61 (m, 4H), 0.99 (m, 3H).
GC–MS (EI) m/z (%) = 175.90(12) [M]+, 174(72.8), 132(100),
133(30), 118(73.8), 98(45.5), 91(97.5), 65(14.1).
(iii) 1-Ethyl-2-phenylaziridine (Table 1, entry 3)
GC–MS (EI) m/z (%) = 147(12.5) [M]+, 146(100), 118(25),
91(88), 77(11.3), 65(15.7).
(iv) 2-Phenyl-1-propylaziridine (Table 1, entry 4)
GC–MS (EI) m/z (%) = 161(8.6) [M]+, 160(62.60), 118(47.7),
91(100), 65(10.7).
(v) 1-Tert-butyl-2-phenylaziridine (Table 1, entry 9)
GC–MS (EI) m/z (%) = 175(6.8) [M]+, 174(34.1), 118(100),
91(94.5), 57(18.3), 41(15.0).
(vi) 1-Hexyl-2-phenylaziridine (Table 1, entry 7)
GC–MS (EI) m/z (%) = 203(21.8) [M]+, 132(50.4), 126(26.6),
117(31.6), 114(19.6), 91(46.4), 86(23.7), 73(55.9), 72(66.9),
44(70.8), 43(100).
(vii) 1-Cyclohexyl-2-phenylaziridine (Table 1, entry 11)
GC–MS (EI) m/z (%) = 201(16.4) [M]+, 200(68.4), 174(10),
119(14.6), 118(100), 91(96), 55(21.4), 41(18.3).
(viii) 1-Butyl-2-(4-(tert-butyl) phenyl) aziridine (Table 1, entry 12)
GC–MS (EI) m/z (%) = 231(3.8) [M]+, 230(26.5), 229.95(94.7),
215(10), 187(94.7), 188(36.4), 160(41.5), 132(83.3),
118(53.0), 117(39.5), 98(56), 57(100), 41(50).
2.5. Spectral data of selected products
2.5.1. Characterization of oxazolidinones
(i) 3-Ethyl-5-phenyloxazolidin-2-one (Table 1, entry 3)
1H NMR (300 MHz, DMSO-d6) ı = 7.34–7.65 (m, 5H), 5.51
(m, 1H), 3.93 (t, 1H), 3.43 (t, 1H), 3.15 (m, 2H), 1.15 (t, 3H).
13C NMR (100 MHz, CDCl3) 157.57, 138.76, 131.83, 128,
126.9, 69.72, 51.15, 38.73, 12.41.
GC–MS (EI) m/z (%) = 191(55.0) [M]+ 175(7.9), 146(39),
132(22), 105(22.2), 91(43.2), 77(22.1), 65(14), 57(100),
42(57.9).
(ii) 5-Phenyl-3-propyloxazolidin-2-one (Table 1, entry 4)
1H NMR (300 MHz, DMSO-d6) ı = 7.34–7.65 (m, 5H), 5.52
(m, 1H), 3.92 (m, 1H), 3.32 (m, 1H), 3.26 (m, 2H), 1.57 (m, 2H),
0.91 (m, 3H).
13C NMR (100 MHz, CDCl3) 158.10, 139.02, 132.09, 128,
125.59, 69.93, 52.23, 45.09, 20.75, 11.18.
GC–MS (EI) m/z (%) = 205(7.2) [M]+, 204(51.0), 114(13),
132(100), 117(15), 105(41.0), 91(29.0), 70(38), 43(43),
42(33).
(iii) 3-Butyl-5-phenyloxazolidin-2-onen (Table 1, entry 6)
1H NMR (300 MHz, DMSO-d6) ı = 7.33–7.46 (m, 5H), 5.51
(m, 1H), 3.94 (t, J = 8.9 Hz, 1H), 3.41 (t, 1H), 3.19 (t, J = 7.0,
4.3 Hz, 2H), 1.40 (m, 2H), 1.29 (d, J = 7.6 Hz, 2H), 0.79–0.95
(m, 3H).
13C NMR (100 MHz, CDCl3) 158.03, 139.01, 128.99, 128.85,
125.60, 74.42, 52.25, 44, 29.49, 19.93, 13.79.
GC–MS (EI) m/z (%) = 219(55.7) [M]+, 133(10.0), 134(100),
117(13.7), 105(37.4),103(30.0), 91(20.3), 84.5(25.7), 78(10).
(iv) 3-Hexyl-5-phenyloxazolidin-2-one (Table 1, entry 7)
1H NMR (300 MHz, DMSO-d6) ı = 7.33–7.45 (m, 5H), 5.55
(dd, J = 8.7, 7.2 Hz, 1H), 3.94 (t, J = 8.7 Hz, 1H), 3.41 (m, 1H),
3.18 (td, J = 7.1, 2.1 Hz, 2H), 1.47 (t, J = 6.8 Hz, 2H), 1.12–1.35
(m, 6H), 0.77–0.93 (m, 3H).
2.5.3. Catalyst reusability
The reaction was carried out as mentioned above in typical
experimental procedure. After completion of reaction, the reaction
mixture was cooled to room temperature and the insoluble cata-
lyst was recovered by filtration technique. It was then thoroughly
washed with acetone to remove all traces of product or reactant
if present and was dried in oven at 80–85 ◦C for 1 h. It was then

R.A. Watile et al. / Journal of Molecular Catalysis A: Chemical 351 (2011) 196–203
199
Table 1
Carboxylative cyclization of various aziridines with CO2 using PS-DHPIMBr as catalyst.a.
c
Entry
Substrates
Time (h)
Conversion (%)b
Selectivity A:B (%)
TOF (h−1
)
1
3
99
95:5
21.9
2
3
100
92:8
22.2
3
4
3
3
100
100
97:3
99:1
22.2
22.2
5
3
99
99:1
22.2
6
7
3
3
100
99
99:1
97:3
22.2
22.0
8
8
15
65
99:1
97:3
1.2
3.6
9d
12
10
12
8
20
67
95:5
95:5
1.1
5.6
11d
12
3
99
98:2
22.0
a
Reaction condition: aziridines (1 mmol), catalyst (PS-DHPIMBr) (1.5 mol%), CO2 pressure (5 MPa), temperature (room temperature, 25 ◦C).
Conversion based on GC analysis.
b
c
Turnover frequency (TOF): sum of moles of A and B produced per mole of catalyst per hour.
d
Temperature (110 ◦C).

200
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Fig. 1. Solid state 13C NMR of PS-DHPIMBr catalyst.
further used for recyclability study. The same procedure was fol-
lowed for consecutive recycling.
In addition, the thermal stability of catalyst was investi-
gated using a TGA analysis under nitrogen atmosphere between
25 and 400 ◦C at a heating rate 5 ◦C/min. The results revealed
that PS-DHPIMBr could tolerate about 304.9 ◦C with little
loss in weight (Please refer to supplementary information,
Fig. S2).
3. Results and discussion
3.1. Characterization of polymer supported diol functionalized
ionic liquids (PS-DFILXs)
3.2. Application of (PS-DFILX) catalyst for
5-aryl-2-oxazolidinones synthesis
The novel imidazolium-based ionic liquids with diol functional-
ity possessing different anions were chemically immobilized using
the commercial available polystyrene resin via covalent bonds. The
resulting heterogeneous catalysts (PS-DHPIMX) and polystyrene
resin were characterized by FT-IR analysis. As shown in supple-
mentary data (Fig. S1, 1b and 1c), the PS-DHPIMX displays a typical
strong peak corresponding to O–H stretching frequency centred
at about 3302 cm−1. In comparison with polystyrene resin, newly
appeared C–H stretching (2923.57 and 2846 cm−1) and imida-
zolium ring stretching bands (1618 and 1454.07 cm−1) are the
characteristic features for presence of IL components in the hetero-
geneous catalysts. In supplementary information (Fig. S1, 1a) the
typical peak centred at 1265 cm−1 corresponding to the stretching
frequency of the functional group CH2Cl which disappears in the
spectra of PS-DHPIMBr (Fig. S1, 1b) and PS-DHPIMCl (Fig. S1, 1c)
suggesting the complete modification of CMPS. This fact provides
an evidence for chemical immobilization of the active catalyst on
the CMPS support.
To investigate the effects of molecular composition (diol func-
tionality and halide anion) on cycloaddition reaction, the polymer-
supported 1-(2-3-di-hydroxyl-propyl)-imidazolium-bromide (PS-
DHPIMBr) and polymer-supported 1-(2-3-di-hydroxyl-propyl)-
imidazolium chloride (PS-DHPIMCl) were screened for the
oxazolidinones synthesis (Scheme 2). We observed that PS-
DHPIMBr provided 98% yield with high selectivity of 97% for
‘A’ whereas PS-DHPIMCl provided 96% yield with appreciable
selectivity of 95% for ‘A’. These slight variations observed in
catalytic activity of PS-DHPIMBr and polymer-supported 1-(2-3-
di-hydroxyl-propyl)-imidazolium chloride (PS-DHPIMCl) might be
due to differences in nucleophilicity of the anions. The control
experiments were carried out in absence of catalyst keeping other
reaction parameters constant, however no yield of desired prod-
uct was obtained signifying that PS-DFILX was responsible for the
respective transformation and it was observed that the typical TOF
of the catalyst PS-DHPIMBr is 22 h−1. The catalytic activity of PS-
DHPIMBr was comparatively higher and hence was selected for
further study.
To investigate the efficiency of grafting reaction, a solid-state 13
C
NMR analysis of PS-DHPIMBr was carried out. As shown in Fig. 1,
the chemical shifts at 124 and 140 ppm corresponds to the three
imidazole ring carbon atoms. The signal at 53 ppm is attributed to
the carbon atoms connecting on imidazole ring and other carbon
atoms gave peaks from 25 to 47 ppm.
In order to optimize the reaction conditions, initial studies
were conducted using PS-DHPIMBr as a choice of catalyst for syn-
thesis of 5-aryl-2-oxazolidinones from 1-butyl-2-phenylaziridine

R.A. Watile et al. / Journal of Molecular Catalysis A: Chemical 351 (2011) 196–203
201
investigated (Fig. 2c). The reaction was performed at room
temperature and 5 MPa pressure. The result demonstrates that
the reaction proceeded rapidly within 2 h while the yield
exceeded up to 97% after completion of 3 h. It is note-
worthy to mention that the time required to achieve the
remarkable yield of desired product with appreciable selectiv-
ity was very less in comparison to other reported heterogeneous
catalysts such as polystyrene-supported amino acids [26] indi-
cating that the developed catalytic protocol was very efficient
for above transformation. Hence, the final optimized reaction
parameters for the synthesis of 5-aryl-2-oxazolidinones were
aziridine (1 mmol), catalyst loading (1.5 mol%), CO2 pressure
(5 MPa), temperature (room temperature, 25 ◦C) and reaction
time (3 h).
Further to demonstrate the utility and general applicability of
developed protocol, the cycloaddition reactions of CO2 with a vari-
ety of aziridines were studied under optimized reaction conditions
(Table 1, entries 1–12). The catalyst was found to be enormously
active providing corresponding 5-aryl-2-oxazolidinones with good
to excellent yields with excellent regioselectivity. Especially, with
increase in the alkyl chain on nitrogen of aziridine excellent yield of
expected product with appreciable regioselectivity were obtained
(Table 1, entries 2–7) while increasing steric hindrance of R1 group
led to a lower yield of desired product (Table 1, entries 8 and 10). The
problem of steric hindrance could be overcome with increasing the
temperature of reaction to obtain appreciable yields of expected
products (Table 1, entries 9 and 11). The above results indicated
that PS-DHPIMBr was an effective catalyst for the cycloaddi-
tion reaction of aziridines and CO2 under mild and solvent free
conditions.
3.3. Reaction mechanism
Several research groups reported that hydrogen bonding can
activate the ring-opening reaction of oxygen analogue of aziridines
[37,38]. In present study, PS-ILs with diol functional groups demon-
strated better catalytic activity and selectivity for ring opening
of aziridines. It was interesting to study the reasons for high
efficiency of catalyst for cycloaddition reactions (Table 2, entries
1–8). We wondered that hydroxyl groups might play a key role
in promoting the reaction. To investigate the effect of hydroxyl
groups on cycloaddition reaction, the experiments of coupling
reaction of CO2 and aziridines were conducted using KI as cata-
lyst in presence of several compounds bearing hydroxyl groups
(Table 2, entries 2–6). It was observed that the diols with two
hydroxyl groups on the vicinal carbons were more capable to
promote the reaction in forward direction rather than water,
monohydroxyl alcohols and other diols (Table 2, entries 3–4). Sim-
ilar results were obtained using KBr as a co-catalyst for present
study (Table 2, entry 7). This strongly signifies that the vicinal
hydroxy group is favourable for the cycloaddition reaction. In addi-
tion, it is well known that the two hydroxyl groups in 1,2-diol
can form ring species with nitrogen atoms by hydrogen bonding
where 6-membered or 7-membered ring species are most stable
[39–42].
On the basis of these findings and previous reports [24–27] we
proposed a plausible mechanism for the chemical fixation of CO2
reaction (Scheme 2). The coupling reaction is initiated by polar-
isation of C-N bond of aziridines through hydrogen bonding of
hydroxyl groups of ILs which acts as a Lewis acidic site (Fig. 3). This
activates the aziridine ring (I) and further leads to the ring opening
via two different pathways (path ‘a’ and ‘b’). It mainly depends on
the nature of R1 group present on N-atom of aziridines (step II) fol-
lowed by cyclization leading to oxazolidinones and regeneration of
the catalyst. The main product A could originate from ring-opening
of the aziridine at the most hindered carbon, as in general three
Fig. 2. Optimization of reaction parameters for oxazolidinones synthesis.
and CO2 as a model reaction. Various reaction parameters such
as catalyst screening, catalyst loading, reaction temperature and
time were studied and the results obtained are summarized
(Fig. 2).
In an effort to determine the optimum concentration of the cat-
alyst, we studied the catalyst loading ranging from 0.5 to 2 mol%
where increase in initial catalyst concentration up to 1.5 mol%
has increased the yield of product while further increase cata-
lyst loading had no profound effect on the yield of the desired
product (Fig. 2a). The effect of the CO2 pressure on the cat-
alytic activity of PS-DHPIMBr was studied at room temperature
in a pressure range of 2-8 MPa and the results are shown in
Fig. 2b. We observed that increasing the pressure from 2 MPa
to 5 MPa increased the selectivity for ‘A’ and yield of the reac-
tion while further increase in pressure led to slight decrease in
the selectivity for ‘A’ and yield of the reaction. The effect of
reaction time on the yield of 5-aryl-2-oxazolidinones was also

202
R.A. Watile et al. / Journal of Molecular Catalysis A: Chemical 351 (2011) 196–203
Table 2
The effect of various co-catalysts and diol functional IL on the synthesis oxazolidinones synthesis.a.
Entry
Catalyst
Conversion (%)b
Regioselectivity (%)
A
B
C
D
1
None
CH3OH
≤10
67
100
100
99
≤1
77
97
98
89
86
97
–
–
10
3
2
6
7
3
–
–
6
–
7
2c
3c
4c
5c
6c
7d
8e
HOCH2CH2OH
Propane-1,2-diol
HO(CH2)3OH
HO(CH2)4OH
HOCH2CH2OH
HOCH2CH2OH
–
2
4
–
–
–
3
3
–
–
99
100
≤10
a
Reaction conditions: aziridine (1 mmol), CO2 (5 MPa), room temperature (25 ◦C), time (3 h).
Conversion based on GC analysis.
Co-catalyst used like KI (15 mg).
Co-catalyst used like KBr (15 mg).
b
c
d
e
Without using co-catalyst.
Fig. 3. The proposed reaction mechanism for cycloaddition of aziridines and CO2 catalyzed by PS-DHPIMBr.
member heterocyclic rings opens by this route [43]. Overall the
role of the hydroxyl groups on vicinal carbon atom was proposed
to involve initial activation of the aziridine and stabilization of the
ring-opened (II) and carbamate intermediates formed (III) during
the reaction which reduced the time as well as temperature of
reaction.
3.4. Catalyst reusability
Stability and reusability of the catalysts are the two key issues
that identify their potential and practical application for indus-
trial scale. From economical point of view catalyst reusability was
investigated for four consecutive recycles (Fig. 4). During the

R.A. Watile et al. / Journal of Molecular Catalysis A: Chemical 351 (2011) 196–203
203
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2011.10.007.
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