Water as an efficient medium for the synthesis of cyclic carbonate

Article abstract of DOI:10.1016/j.tetlet.2008.11.034

Herein, we report a novel method for the synthesis of cyclic carbonate in water. By tuning the amount of water, cycloaddition of CO₂ to epoxide in aqueous medium leads to cyclic carbonates with moderate to excellent yields and high selectivities. In addition, the presence of water could remarkably improve the activity of ionic liquids by which the turnover frequency of the reaction is about 4-5 times higher in the presence than in the absence of water. The relationship between the higher catalytic reactivity and water was proposed.

Full text of DOI:10.1016/j.tetlet.2008.11.034

Tetrahedron Letters 50 (2009) 423–426
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Water as an efficient medium for the synthesis of cyclic carbonate
Jian Sun , Junyi Ren , Suojiang Zhang a,*, Weiguo Cheng a
a,b
c
a
Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China
Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China
Dalian Polytechnic University, Dalian 116034, PR China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Herein, we report a novel method for the synthesis of cyclic carbonate in water. By tuning the amount of
Received 29 September 2008
Revised 7 November 2008
Accepted 10 November 2008
Available online 17 November 2008
2
water, cycloaddition of CO to epoxide in aqueous medium leads to cyclic carbonates with moderate to
excellent yields and high selectivities. In addition, the presence of water could remarkably improve the
activity of ionic liquids by which the turnover frequency of the reaction is about 4–5 times higher in the
presence than in the absence of water. The relationship between the higher catalytic reactivity and water
was proposed.
Keywords:
Water
Ó 2008 Elsevier Ltd. All rights reserved.
Carbon dioxide
Ionic liquid
Cyclic carbonate
Cyclic carbonates, such as ethylene carbonate and propylene
carbonate, are important species in organic chemistry. They have
attracted the attention of many chemists, not only because they
can serve as excellent aprotic polar solvents and extensively as
intermediates in the production of pharmaceuticals and fine chem-
icals, but also because their current synthesis methods (Scheme 1)
herein, we decided to develop a new method for preparing cyclic
carbonate in water.
Using propylene oxide (PO) as a model substrate, the compari-
2
son experiments of cycloaddition of CO to PO were carried out un-
2
5
der the cases with and without water. The results were listed in
Table 1. In the presence of water, interestingly, PO conversion and
PC yield are considerably increased, and almost all the employed
Lewis base catalysts show high activities. When KI and KBr are
used, the obtained turnover frequencies (TOFs) are surprisingly
about 54 and 236 times higher than those without water, respec-
are one of the most attractive synthetic protocols utilizing CO
2
as
an attractive C1 building block.1
,2
Numerous catalysts including ionic liquid (IL) have been devel-
oped for this transformation.3 However, the above catalysts gen-
erally suffer from low catalytic activity, water or air sensitivity, and
the need for toxic co-solvent. In addition, methods for cyclic car-
bonate preparation are traditionally in anhydrous systems, which
is a critical reaction condition for eliminating unavoidable water
in applications.
–23
+
+
+
3 4
tively. The activity order of cations is PPh Bu > Bu N , [bmim] >
+
+
K > Na (entries 1, 2, 4, 7, and 12). A longer alkyl chain phospho-
nium halide, such as butyl-triphenylphosphonium bromide
3 3
(PPh BuBr) or hexyl-triphenylphosphonium bromide (PPh HeBr),
still exhibits higher activity and selectivity than a shorter chain
in the presence of water (Table 1, entries 12, 15, and 16). The activ-
We recently demonstrated that hydroxyl group when added to
traditional ionic liquid catalysts could efficiently catalyze the reac-
À
À
À
À
À
À
ity of anions varies in the order I > Br > Cl > PF , BF (entries 7–
6
4
2
4
À
À
À
À
tion and yield high cyclic carbonates without Lewis acid. Since
water is a green, cheap, and hydroxyl group-containing solvent,
11), while it is Br > Cl > I > PF , BF without water. The results
show that the activity of I is very sensitive to water. Because of
the low nucleophilic nature of PF6 and BF4 anions, low PO
conversions and PC selectivities are obtained using 1-butyl-3-
6 4
À
À
À
9c
O
methyl-imidazolium-hexafluorophosphate ([bmim]PF
tyl-3-methyl-imidazolium tetrafluoroborate ([bmim]BF
6
), and 1-bu-
) in the
O
+
CO2
O
O
4
R
presence of water (entries 10 and 11). Due to high water sensitivity
of zinc halides, their binary catalytic systems exhibited unsatisfac-
tory results and lost their activities quickly in aqueous reaction
system (Table 1, entries 17 and 18). Because of its good perfor-
R
Scheme 1. Synthesis of cyclic carbonate from CO
2
and epoxide.
3
mance (entries 14, 19, and 20), PPh BuI was chosen for further
*
Corresponding author. Tel./fax: +86 10 8262 7080.
E-mail address: sjzhang@home.ipe.ac.cn (S. Zhang).
experiments.
0
040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.11.034

4
24
J. Sun et al. / Tetrahedron Letters 50 (2009) 423–426
Table 1
Comparison experiments of cycloaddition of CO
2
to PO under the cases with and without watera
Entry
Catalyst
With waterb
Y (%)
Without waterb
Y (%)
À1
À1
X (%)
TOF (h
)
X (%)
TOF (h )
1
2
3
4
5
6
7
8
9
NaBr
KBr
KI
40
60
78
95
70
95
94
64
97
3.3
10
96
72
100
95
97
1.2
78
99.6
80
30
52
68
86
46
88
87
42
63
0.8
1
Trace
Trace
Trace
54
42
26
50
44
51
—
1.7
52
44
24
49
59
2.7
97
22
8
1.6
2
0.6
111
127
53
104
90
105
—
5.4
108
131
49
100
122
7
199
48
16
108
142
175
98
185
180
75
184
0.7
1.2
177
90
194
173
182
0.5
142
192
150
0.3
56
44
27
52
46
53
Trace
2.7
54
45
25
50
Bu
4
Bu
4
Bu
4
NBr
NCl
NI
[bmim]Br
[bmim]Cl
[bmim]I
[bmim]BF
[bmim]PF
90
1
1
1
1
1
1
1
1
1
1
2
0
1
2
3
4
5
6
7
8
9
0
4
Trace
Trace
88
51
95
85
89
Trace
70
93
6
PPh
PPh
PPh
PPh
PPh
3
3
3
3
3
BuBr
BuCl
BuI
EtBr
HeBr
61
3.5
100
24
10
c
d
e
f
ZnBr
ZnBr
2
2
/PPh
3
HeBr
PPh
PPh
3
BuI
BuI
3
72
a
Reaction conditions: PO (0.2 mol), H
X: PO conversion; Y: isolated PC yield; TOF: mole of synthesized PC per mole of catalyst per hour.
2
O (0.067 mol), catalyst (1 mmol), CO
2
pressure (2 MPa), 125 °C, 1 h.
b
c
d
e
f
ZnBr
ZnBr
2
(0.5 mmol).
(0.165 mmol), PPh HeBr (0.67 mmol).
3
2
After ten times reuse.
CO pressure (0.5 MPa).
2
In order to further investigate the effect of the amount of water
in tailoring the synthesis rate of PC, experiments were carried out
in which the ratio of H O/PO was increased from 0 to 2. As shown
in Figure 1, only 26% of PO conversion is observed using PPh BuI by
itself in the absence of water after 1 h. This might be caused by the
is another important chemical in organic chemistry. In general, to
obtain both high PC yield and high selectivity, the ratio of 0.33 is
optimal.
2
3
Additional experiments showed that the amount of water could
remarkably affect the synthesis rate of cyclic carbonate. For exam-
ple, the results of PC yield versus reaction time are listed in Figure
2. It can be observed that in the absence of water (A), it takes more
than 4.0 h to obtain only 57.0% PC yield with low TOF value
difficult ring-opening of epoxide with Lewis base alone,5
,8,10
while
with the presence of water, the activity of PPh BuI is sensitively
3
improved. In a low-ratio region (from 0 to 0.33), an increase in ra-
tio could result in remarkable increase in both PC yield and PO con-
version. Within this region, PC selectivity keeps above 97%. And the
maximal PC yield (97%) obtained at the ratio of 0.33 is about 4
times higher than that without water (25%). While further increas-
ing the amount of water from 0.33 to 2 causes a decrease in the
yield and selectivity of PC though PO conversion keeps at 100%.
PC selectivity decreases quickly from 97% to 63% correspondingly.
It can be explained by the side reaction between water and PO to
produce corresponding chemical, 1,3-propylene glycol (PG), which
À1
(24 h ). At a low amount of water (B), the synthesis rate of PC is
about 2 times faster compared to that without water. The highest
synthesis rate obtained in the ratio of 0.33 (E) could be more than 7
times faster than that of curve (A) in 0.5 h. When the ratio is above
1:3, for example, H O/PO = 1:1 (F), PC yield is decreased but the PC
2
synthesis rate is still increased. At this amount of water, PC yield
could reach its maximal value within 0.5 h. From the above results,
2
water accelerated the cycloaddition reaction of CO to PO, by
which the reaction could proceed more smoothly with the pres-
1
8
6
4
2
0
00
0
100
1
00
E
D
F
80
8
6
4
2
0
0
0
0
C
B
6
4
2
0
0
0
0
0
PO conversion
PC yield
PC selectivity
A
0
0
0
30
60
90
120
150
180
210
240
0
.0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
Time (min)
H O/PO (molar ratio)
2
Figure 2. PC yield versus reaction time with different amounts of water . Molar
ratio of H O/PO: (A) 0, (B) 1:20, (C) 1:10, (D) 1:5, (E) 1:3, (F) 1:1. Reaction
conditions: PO (0.2 mol), PPh BuI (1 mmol), 2 MPa, 125 °C.
Figure 1. Effect of the amount of water on the synthesis of propylene carbonate.
Reaction conditions: PO (0.2 mol), PPh BuI (1 mmol), 2 MPa, 125 °C, 1 h.
2
3
3

J. Sun et al. / Tetrahedron Letters 50 (2009) 423–426
425
ence of more water content. However, in order to balance both
Table 3
a
Effect of various solvents on the synthesis of PC
high PC yield and high synthesis rate, 1:3 is the optimal ratio to
be considered, which is consistent with the results in Figure 1. It
can also be found that at the ratio of 0.33, increasing reaction time
beyond 0.5 h up to 4.0 h makes but a slight enhancement in PC
yield. In this case, more than 99% PC selectivity and 96% PC yield
Entry
Solvent
Resultsb
S (%)
À1
X (%)
TOF (h
)
1
2
3
4
5
6
7
8
9
Water
Phenol
100
96
95
94
81
79
75
36
35
34
31
30
18
16
97
95
92
97
99
98
97
>99
>99
99
>99
>99
>99
>99
194
185
174
181
161
154
145
72
71
68
61
61
À1
could be obtained with about 380 h TOF value.
Acetic acid
PG
Ethanol
2-Propanol
PEG
DMF
PC
Acetone
Acetonitrile
Cyclohexane
DMC
Cycloadditions of CO
2
to other epoxides in water were also
examined using PPh BuI catalyst at 125 °C and 2 MPa. As shown
3
in Table 2, the reactions proceed smoothly for all the substrates
used in the presence of water. High epoxide conversions as well
as P92% cyclic carbonate selectivities are obtained under the de-
sired reaction conditions (entries 1–5). Aromatic 1e and aliphatic
10
1
1
1
1
2
3
1
a, 1c, and 1d epoxides are preferred substrates for the reaction.
Epoxide 1f exhibits the lowest activity of the epoxides surveyed
as expected from the higher hindrance that originated from the
two rings. However, the yields obtained without water are about
36
32
14
Dichloromethane
a
Reaction conditions: PO (0.2 mol), solvent (1.2 mL, 25 °C), PPh
3
BuI (1 mmol),
3
–4 times lower than those in the presence of water. For example,
the synthesis of ethylene carbonate (EC) from ethylene oxide (EO)
and CO could be completed within 0.5 h at 110 °C with 98% selec-
tivity (Table 2, entry 1) in the presence of water (H O/EO = 0.33,
2
MPa, 125 °C, 1 h.
b
X: PO conversion; S: PC selectivity; TOF: mole of synthesized PC per mole of
2
catalyst per hour.
2
molar ratio), while only 30% EO conversion could be obtained
under the same reaction conditions without water. If we further
increase the molar ratio of H O/EO up to 1:1 (entry 1), high EO
2
conversion as well as high EC selectivity could also be achieved.
When 1e is substituted for 1a, similar tendency could also be
observed (entry 4).
the same conditions (entries 2–7). But, the results obtained are
lower than that in water (entry 1). On the contrary, PO conversions
are little improved in the presence of non-OH group containing
solvents, such as N,N-dimethylformamide (DMF), PC, acetone, ace-
tonitrile, cyclohexane, dimethyl carbonate (DMC), and dichloro-
To understand the role of the OH group in accelerating the reac-
tion, other solvents with or without OH groups were also tested
methane (CH
2 2
Cl ) (entries 8–14). Although these polar solvents
are commonly used to improve the catalytic activities of catalysts
in the previous literatures (e.g., Mg–Al mixed oxides, SmOCl,
15
4c
substituting for water for the synthesis of PC with PPh
3
BuI. It can
2
2
23
be seen from Table 3 that the activity of PPh BuI can also be greatly
3
TBD/MCM-41, and Cr-salen ), herein, their functions in acceler-
ating the activity of catalyst are still limited. In addition, some of
them are toxic and expensive. From the viewpoint of application
and green chemistry, developing cheap, efficient, and safe solvent
in catalysis is another requirement aspect. The above results indi-
cate that Brønsted acidic solvents, especially OH group containing
solvents, are considerable promoters to improve the activity of the
improved in the presence of OH groups containing chemicals, such
as phenol, acetic acid, propylene glycol (PG), ethanol, 2-propanol,
and polyethylene glycol (PEG with molecular weight 400). Both
high PO conversion and high PC selectivity can be realized under
Table 2
catalyst for the synthesis of PC.
a
Synthesis of other cyclic carbonates in the presence of water
2b,12,24
Based on the above results and the previous reports,
a
Entry
Epoxide
T/t (°C/h)
110/0.5
125/1
Resultsb
Y (%)
mechanism portraying the probable sequence of events is shown
in Scheme 2. It was proposed that water (acidic site) and the bro-
mine anion of Lewis base (basic site) coordinately attacked the dif-
ferent parts of epoxide firstly. The coordination of the H atom of
water molecule with the O atom of epoxide through a hydrogen
Yc (%)
28
S (%)
1
2
O
95/89d
98/93d
1
a
O
O
21
20
95
97
98
H C
3
1
c
H
O
H
H
3
4
125/1
125/1
87
O-
H C
3
O
H
O
+
3
LB'
X
1
d
step1
R
O
CO2
R
20
83/85d
96/92d
step1
step 2
1
e
H
O
O
H
O-
O
LB
O
O
5
125/2.5
6
47
96
O
+
step 3
LB'
O
1f
R
X
a
R
Reaction conditions: epoxide (0.2 mol), H
2
O (0.067 mol), PPh
3
BuI (1 mmol),
2
MPa, 125 °C.
b
-
-
-
X: epoxide conversion; Y: isolated yield; S: selectivity.
In the absence of water.
LB=LB'X (Lewis base), X=Br , I , Cl
c
d
H
2
O (0.2 mol).
Scheme 2. Proposed mechanism for the reaction in water.

4
26
J. Sun et al. / Tetrahedron Letters 50 (2009) 423–426
bond resulted in the polarization of C–O bonds, and the halide
anion made the nucleophilic attack on the less sterically hindered
b-carbon atom of the epoxide at the same time. As a result, the ring
of the epoxide was opened easily (step 1). Then, the interaction
5. (a) Xiao, L. F.; Li, F. W.; Peng, J. J.; Xia, C. G. J. Mol. Catal. A: Chem. 2006, 253, 265;
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(
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7.
8.
9.
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2
occurred between the oxygen anion and CO , forming an alkylcar-
bonate anion (step 2) which would be transformed into a cyclic
carbonate by the intramolecular substitution of the halide in the
next step (step 3). In the reaction, water played a similar function
like a Lewis acid on the ring-opening of epoxide, by which the Le-
wis base could show excellent activity in the absence of Lewis acid.
From the above experiments (Table 3, entries 2–7), other solvents
containing OH groups might also play the similar role in accelerat-
ing the activity of the catalyst.
10. Sun, J.; Wang, L.; Zhang, S. J.; Li, Z. X.; Zhang, X. P.; Dai, W. B.; Mori, R. J. Mol.
Catal. A: Chem. 2006, 256, 295.
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2
In conclusion, the synthesis of cyclic carbonates from CO and
terminal epoxides in water was reported using a single Lewis base
catalyst. The coupling reactions proceeded smoothly with moder-
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cant role in determining the selectivity in addition to the activity of
the catalyst. It was proposed that water and the anion of Lewis
base played a synergic epoxyl ring-opening effect in accelerating
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3
the reactions. Among the catalysts investigated, PPh BuI was the
best effective Lewis base for the synthesis of cyclic carbonate under
the mild conditions (125 °C, 2.0 MPa, and 1.0 h), and showed high
stability in activity and selectivity.
1
20. Daniel, G. L.; Brent, H. S. J. Catal. 2005, 232, 386.
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Acknowledgments
22. Barbarini, A.; Maggi, R.; Mazzacani, A.; Mori, G.; Sartori, G.; Sartorio, R.
Tetrahedron Lett. 2003, 44, 2931.
2
2
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This work was supported by the National Science Fund of China
for Distinguished Young Scholar (20625618), National 863 Pro-
gram of China (2006AA06Z317), and National Natural Science
Foundation of China (20876162).
25. Cycloaddtion procedure for the reaction of propylene oxide (PO) with CO : All the
2
coupling reactions were conducted in a 100 ml stainless-steel reactor equipped
with a magnetic stirrer and automatic temperature control system. A typical
reaction was carried out as follows: in the reactor, an appropriate CO
2
(
0
ꢀ1.0 MPa) was added to a mixture of PO (14.0 mL, 0.2 mol), H
2
O (1.2 mL,
.067 mol), and catalyst (1 mmol) at room temperature. Then, the temperature
References and notes
was raised to 125 °C with the addition of CO2 from a reservoir tank to maintain
a constant pressure (2.0 MPa). After the reaction had proceeded for a desirable
time, the reactor was cooled to 5 °C in an ice-water bath, and the remaining
1
2
.
.
Parvulescu, V. I.; Hardacre, C. Chem. Rev. 2007, 107, 2615.
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2
CO was removed slowly. After reaction, the catalyst was separated by
distillation under vacuum and reused for further recycling experiment. The
crude product mixture was dried over anhydrous sodium sulphate, and then
subjected to column chromatography using a 6:1 petroleum ether/EtOAc
eluent system on silica gel (200–300 mesh) to give cyclic carbonate as a
colorless liquid. The isolated products were analyzed by NMR and Agilent
6890/5973B GC–MS equipped with a MS detector using acetophenone as the
internal standard.
3
4
.
.
2
007, 271, 284; (c) Yasuda, H.; He, L. N.; Sakakaura, T. J. Catal. 2002, 209, 547;
(
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