Regioselective synthesis of 5-aryl-2-oxazolidinones from carbon dioxide and aziridines using Br- Ph3+PPEG 600P+Ph3Br- as an efficient, homogenous recyclable catalyst at ambient conditions

Article abstract of DOI:10.1016/j.tetlet.2011.09.056

Polyethylene glycol functionalized phosphonium salt (Br^- Ph ₃⁺PPEG₆₀₀P⁺Ph₃Br ^-) was found to be an efficient, homogenous, recyclable catalyst for coupling of CO₂ with a variety of aziridines producing corresponding 5-aryl-2-oxazolidinones with good yields and excellent regioselectivity under relatively mild and solvent free conditions. Furthermore, the catalyst was effectively recycled for four consecutive cycles without any significant loss in its catalytic activity and selectivity.

Full text of DOI:10.1016/j.tetlet.2011.09.056

Tetrahedron Letters 52 (2011) 6383–6387
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Tetrahedron Letters
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Regioselective synthesis of 5-aryl-2-oxazolidinones from carbon dioxide
and aziridines using BrÀPh3 PPEG600P+Ph3BrÀ as an efficient, homogenous
+
recyclable catalyst at ambient conditions
⇑
Rahul A. Watile, Dattatraya B. Bagal, Yogesh P. Patil, Bhalchandra M. Bhanage
Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai 400 019, India
a r t i c l e i n f o
a b s t r a c t
Polyethylene glycol functionalized phosphonium salt (BrÀPh3+PPEG600P+Ph3BrÀ) was found to be an
efficient, homogenous, recyclable catalyst for coupling of CO2 with a variety of aziridines producing
corresponding 5-aryl-2-oxazolidinones with good yields and excellent regioselectivity under relatively
mild and solvent free conditions. Furthermore, the catalyst was effectively recycled for four consecutive
cycles without any significant loss in its catalytic activity and selectivity.
Article history:
Received 23 August 2011
Revised 12 September 2011
Accepted 13 September 2011
Available online 28 September 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Carbon dioxide
Aziridines
5-Aryl-2-oxazolidinones
Homogeneous
Polyethylene glycol functionalized
phosphonium salt
Stabilizing and reducing atmospheric carbon dioxide (Green
house gas) levels have emerged as a pressing global issue for miti-
gating the effects of climate change. In addition, due to its non-
toxic, abundant, and renewable carbon resource, development of
commercially viable routes to basic chemicals that employ carbon
dioxide as the starting material becomes of practical value for syn-
thetic chemists.1 In particular, one of the attractive routes for the
chemical fixation of CO2 is to efficiently convert CO2 into five-
membered heterocycles such as oxazolidinones. The importance
of 5-membered oxazolidinones has been emphasized in organic
synthesis. They have been widely used as chiral synthons, chiral
auxiliaries, and as synthetic intermediates in several asymmetric
transformations.2,3 Moreover, some substituted oxazolidinones
are also being used as building blocks for the synthesis of biologi-
cally active compounds.4
Although oxazolidinones have been known for their great vari-
ety of applicability, merely only few preparative methods exist for
the target compounds from C1 sources including amino alcohols
with phosgene and CO,5 propargylamines, or propargylic alcohols
with CO26, insertion of CO2 into the aziridines, etc. Synthesis of
oxazolidinones utilizing CO2 as a feedstock is more attractive in
comparison to other processes. In this regard, numerous homoge-
neous catalysts were reported for the synthesis of oxazolidinones
from CO2 and aziridines such as alkali metal halide,7 iodine,8 natu-
rally occurring amino acids,9(salen)-Cr(III)/DMAP,10 phenol/
DMAP,11 quaternary ammonium bromide functionalized polyeth-
ylene glycol,12 zirconyl chloride,13 polystyrene supported amino
acid,14, and [C4DABCO]Br.15 Although significant advances have
been made, still there are some disadvantages like requirement
of high reaction temperature, longer reaction time, toxic organic
solvents, and co-catalysts to achieve higher yields. Hence, develop-
ment of an efficient, environmental benign, thermo-stable, recycla-
ble catalyst for the synthesis of oxazolidinones from CO2 and
aziridines under milder reaction condition is still desirable.
In recent years, ionic liquids (ILs) have attracted significant
attention from the scientific community due to its distinctive prop-
erties such as low viscosity, high thermal stability, negligible vapor
pressure, high loading capacity, ease in recyclability, and as envi-
ronmentally benign solvents.16 Various chemical reactions with
high conversion and selectivity have been smoothly performed
using ILs as a catalyst or reaction medium.17 We previously re-
ported that polyethylene glycol functionalized phosphonium salt
derived ILs can efficiently catalyze the coupling of carbon dioxide
with epoxide for the synthesis of cyclic carbonate.18a The wide
application of PEG in organic synthesis is closely related to its
broad range of solubility. We envisage the phosphonium halide
as a catalytically active species which is covalently grafted onto
highly CO2-philic polymer PEG, expected to enhance the catalytic
activity for coupling of carbon dioxide with aziridine to afford
2-aryl oxazolidinones synthesis. We were delighted to find that
⇑
Corresponding author. Tel.: + 91 223361 1111/2222; fax: +91 223361 1020.
E-mail addresses: bm.bhanage@gmail.com, bm.bhanage@ictmumbai.edu.in,
bhalchandra_bhanage@yahoo.co (B.M. Bhanage).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.09.056

6384
R. A. Watile et al. / Tetrahedron Letters 52 (2011) 6383–6387
O
O
R2
N
R2
R2
+
Br-Ph3 PPEG600P+Ph3Br-
O
N
N
O
+
CO2
+
50 oC, solvent free
R1
R1
R1
A
B
up to 98% yield,
99 % selectivity
Scheme 1. Synthesis of 2-oxazolidinones from aziridines and CO2.
polyethylene glycol functionalized phosphonium salt provided
excellent catalytic activity toward the selective synthesis of 5-
aryl-2-oxazolidinones from aziridine and CO2 under relatively
milder reaction condition. Our strategy is also called as ‘mono-
phase reaction, two-phase separation’.18b
10 wt % of catalyst provided excellent yield (98%) and selectivity
(99%) toward the desired product while further increase in the cat-
alyst loading had no significant effect on catalyst activity and selec-
tivity (Table 1, entries 1–4). The effect of the reaction temperature
ranging from 25 to 100 °C on the cycloaddition reaction of aziridine
catalyzed by BrÀPh3+PPEG600P+Ph3BrÀ at 5 MPa CO2 pressure was
investigated (Table 1, entries 5–7). We observed that at room tem-
perature (25 °C), 77% conversion with excellent regioselectivity of
desired product (99%) was obtained after the reaction time of
16 h (Table 1, entry 5). The yield of oxazolidinones increased with
increasing temperature up to 50 °C, while on further increase in
reaction temperature the yield and selectivity decreased consider-
ably due to the formation of side products.
Next, we studied the effect of CO2 pressure on the conversion of
product using BrÀPh3+PPEG600P+Ph3BrÀ as a catalyst at 50 °C in the
pressure range of 2–7 MPa (Table 1, entries 8–11). The 5-aryl-2-
oxazolidinones yield increases in the pressure range of 5–6 MPa
and remained unchanged with the further increase in pressure.
The yield of 3-butyl-5-phenyl-2-oxoazolidinone was maximum at
5 MPa CO2 pressures (98%) after 6 h (Table 1, entry 2). In addition,
we also performed the reaction at 2 MPa CO2 pressure at 50 °C
where the 3-butyl-5-phenyl-2-oxoazolidinone yields 72% with a
Herein, we report the use of BrÀPh3+PPEG600P+Ph3BrÀ as a
highly efficient catalyst for the synthesis of 5-substituted-2-
oxazolidinone under mild reaction conditions, without the need
of any additives providing high yield or excellent regioselectivity
(Scheme 1). In addition, the catalyst was effective for the synthesis
of a wide range of substrates under solvent-free conditions.
To optimize the reaction conditions, the reaction of 1-butyl-2-
phenylaziridine with CO2 in the presence of BrÀPh3+PPEG600P+
Ph3BrÀ as a catalyst was chosen as a model reaction for the synthe-
sis of 5-aryl-2-oxazolidinones. Various reaction parameters such as
catalyst loading, effect of CO2 pressure, reaction temperature, and
time were studied and the results obtained are summarized in
Table 1.
The concentration of the catalysts has a strong influence on the
yield of 5-aryl-2-oxazolidinones from CO2 and aziridine. The con-
centration of catalyst was optimized by employing the reaction
with 5, 10, 15, and 20 wt % of catalyst. It was observed that
Table 1
a
Effect of reaction parameters on the synthesis of 2-oxazolidinones from CO2
O
O
n-Bu
Br-Ph3+PPEG600-P+Ph3Br-
n-Bu n-Bu
N
O
N
N
O
+
CO2
50 oC, solvent free
+
Ph
Ph
Ph
A
B
Entry
Catalyst loading (wt %)
Temp (°C)
Time (h)
Pressure (MPa)
Conv.b (%)
Selectivity A:B (%)
Catalyst loading
1
2
3
4
5
50
50
50
50
6
6
6
6
5
5
5
5
63
97
98
98
91:9
98:2
99:1
99:1
10
15
20
Effect of temperature
5
6
7
10
10
10
rt
80
100
16
6
6
5
5
5
77
94
92
99:1
97:3
90:10
Effect of pressure
8
9
10
11
10
10
10
10
50
50
50
50
6
6
6
16
7
6
4
2
90
97
82
72
96:4
99:1
93:7
95:5
Effect of time
12
13
14
10
10
10
50
50
50
12
3
2
5
5
5
96
74
62
97:3
97:3
92:8
a
Reactions and conditions: 1-butyl-2-phenyl aziridine (1 mmol), catalyst, BrÀPh3+PPEG600P+Ph3BrÀ (10 wt %), CO2 (5 MPa), 50 °C, time (6 h).
Determined by GC analysis.
b

R. A. Watile et al. / Tetrahedron Letters 52 (2011) 6383–6387
6385
reaction time of 16 h (Table 1, entry 11), indicating that the
reaction could be conducted effectively under milder reaction
conditions. Furthermore, the effect of reaction time on the
5-aryl-2-oxazolidinones conversion was studied (Table 1, entries
12–14). The yield of oxazolidinones increased with increasing time
and approached to 100% conversion after 6 h (Table 1, entry 2).
Table 2
Reaction of various aziridines with CO2 using BrÀPh3+PPEG600P+PPh3BrÀ as catalyst under solvent-free conditionsa
O
O
R2
Br-Ph3+PPEG600P+Ph3Br-
R2
R2
N
O
N
O
N
CO2
+
+
50 oC, solvent free
R1
R1
R1
B
A
Entry
1
Substrates
Time (h)
6
Conversion (%)
99
Yieldb (%)
Selectivity A:B (%)
95:5
N
H
80
N
2
3
4
5
6
6
6
6
100
100
100
99
89
95
95
89
92:8
97:3
99:1
99:1
N
N
N
N
6
7
8
6
6
8
100
99
93
98
10
99:1
99:3
99:1
N
N
15
N
N
9c
10
12
12
8
65
20
67
57
15
59
97:3
95:5
95:5
11c
N
N
12
13
6
6
99
99
91
95
96:4
98:2
N
a
b
c
Reactions and conditions: aziridines (1 mmol), catalyst (10 wt %), CO2 pressure (5 MPa), temperature (50 °C).
Isolated yield.
Temperature (110 °C).

6386
R. A. Watile et al. / Tetrahedron Letters 52 (2011) 6383–6387
R2
N
R1
I
R2Ph3P
N
PEG
-
-
PPh3Br
Br
R1
PPh3
PEG
Ph3P
II
-
Br
-
-
Br
PEG
Ph3P
R2
PPh3Br
PPh3
PEG
2Ph3P
R
N
O
or
N
R1
O
R2
R1
O
N
R2
(a)
Br
N
Br
R1
III
(b) Disfavoured
O
R1
A
CO2
Ph3P
PEG
PPh3 Br-
Br
Figure 1. Plausible reaction mechanism.
polarization of the C–N bond of aziridines through phosphonium
cation which acts as a Lewis acidic site and this step activates
the aziridine ring (Fig. 1, step I). Subsequently, the nucleophilic at-
tack of bromide anion (BrÀ) leads to the ring opening of aziridines
providing the two different intermediates as represented as inter-
mediate a and b mainly depending on the nature of the R2 group
present on the N-atom of aziridines (Fig. 1, step II). Insertion of
CO2 in to P–O bond to obtain carbamate salt could be stabilized
by the phosphonium cation of BrÀPh3+PPEG600P+Ph3BrÀ and fol-
lowed by cyclization leading to oxazolidinones and regeneration
of the catalyst. The main product A could originate from the ring
opening of aziridine at the most hindered carbon. Generally the
19,20
three-membered heterocyclic ring opens by this route
and
Figure 2. Catalyst reusability. aReactions and condition: aziridines (1 mmol),
catalyst (10 wt %), pressure (5 MPa), temperature (50 °C).
hence remains the most favorable route for the formation of
oxazolidinones.
In an effort to craft the synthetic protocol more economical it is
necessary to investigate the recyclability of the catalyst
21
(BrÀPh3+PPEG600P+Ph3BrÀ).
In each cycle, BrÀPh3+PPEG600P+Ph3
Hence, the final optimized reaction parameters for the synthesis of
5-aryl-2-oxazolidinones were aziridine (1 mmol), catalyst loading
(10 wt %), CO2 pressure (5 MPa), temperature (50 °C), and reaction
time (6 h).
BrÀ could be easily recovered and reused for the subsequent re-
cycle. The yield of 5-aryl-2-oxazolidinones was consistent up to
four consecutive recycles without any significant loss in its cata-
lytic activity thus indicating high stability and activity of devel-
To broaden the scope and generality of the developed proto-
col, we screened variety of aziridines for oxazolidinones synthe-
sis under optimized reaction conditions (Table 2, entries 1–13).
The reaction of 2-phenylaziridine with CO2 furnishes 80% yield
of the corresponding desired product with good regioselectivity
(Table 2, entry 1) while 1-methyl-2-phenylaziridine provided rel-
atively similar yield (89%) of the desired products (Table 2, entry
2). With increase in the alkyl chain on nitrogen of aziridine,
excellent yield of expected product with appreciable regioselec-
tivity was obtained (Table 2, entries 3–7). Increase in the steric
hindrance of R2 group at nitrogen atom for substrate has low-
ered the yield of desired products (Table 2, entries 8 and 10).
However, it was overcome with increase in the temperature
and time which substantially increased the yield of desired prod-
uct (Table 2, entries 9 and 11).
oped catalyst (Fig. 2). Therefore, immobilization of
a
phosphonium salt on a soluble polymer (e.g., PEG600) provides an
alternative pathway to demonstrate the recycling of homogeneous
catalyst.
In conclusion, we have developed a highly efficient, homoge-
neous, and recyclable catalytic protocol for the synthesis of 5-
aryl-2-oxazolidinones from CO2 and aziridine with high conversion
and excellent regioselectivity. To the best of our knowledge this is
the first time, the coupling reactions using BrÀPh3+PPEG600P+
Ph3BrÀ as a homogeneous recyclable catalyst under mild reaction
conditions has been demonstrated without any significant loss in
its catalytic activity. The developed methodology was effectively
applicable for several aziridines affording good to excellent yield
of desired products demonstrating a broad application of the
methodology. Hence, the present methodology will sound to be
viewed as a useful contribution for the utilization of CO2 in the
synthesis of industrially important chemicals.
On the basis of earlier reports and the experiment results,12,18
we proposed a plausible reaction mechanism for the cycloaddition
of CO2 with aziridine (Fig.1). The coupling reaction is initiated by

R. A. Watile et al. / Tetrahedron Letters 52 (2011) 6383–6387
6387
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The author (R.A.W.) is greatly thankful to the Council of Scien-
tific and Industrial Research (CSIR, India) for providing Junior Re-
search Fellowship (JRF).
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Supplementary data
Supplementary data (experimental procedures and character-
ization data of selected aziridines, oxazolidinones and copies of
NMR spectra) associated with this article can be found, in the on-
line version, at doi:10.1016/j.tetlet.2011.09.056.
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21. In a typical experimental procedure, coupling of CO2 with aziridines was
carried out in a 100 mL stainless steel autoclave reactor with a mechanical
stirrer. The reactor was charged with aziridine (1 mmol) and catalyst
(10 wt %) at room temperature. CO2 gas was introduced into the autoclave
and pressure was adjusted to desired pressure (5 MPa) and the mixture was
stirred (550–600 rpm) continuously for desired time period. After completion
of reaction, the reactor was cooled to room temperature and the remaining
CO2 was carefully vented and then reactor was opened. The product was
extracted in diethyl ether (10 mL Â 1). The solvent was evaporated in vacuum
to obtain the crude product which was then purified by column
chromatography silica gel, (100–200 mesh size) with petroleum ether/ethyl
acetate (PE–EtOAc, 80:20) as eluent to afford pure product. The reaction
mixture was analyzed by GC (Perkin–Elmer, Clarus 400) equipped with a
flame ionization detector (FID) and
a
capillary column (Elite-1, 30 m Â
0.32 mm  0.25
l
m). The products were confirmed by 1H NMR (Varian
300 MHz NMR Spectrometer), 13C NMR spectra (100 MHz) and GC–MS
(Shimadzu GC–MS QP 2010) (Rtx-17, 30 m  25mmID, film thickness
0.25 lm df) (column flow-2 mL/min, 80 °C to 240 °C at 10°/min rise.) which
were consistent with those reported in the literature.
[Monophase reaction, two-phase separation: The catalyst has some solubility
in the product. The catalyst was precipitated from the reaction mixture using
antisolvent (e.g., diethyl ether) and thus the catalyst phase was used further
for next recycle run. On the other hand the products were extracted in the
diethyl ether phase.]