Efficient and practical organocatalytic system for the synthesis of?cyclic carbonates from carbon dioxide and epoxides: 3-hydroxypyridine/tetra-n-butylammonium iodide

Article abstract of DOI:10.1016/j.tet.2017.01.018

An efficient and practical organocatalytic system comprising 3-hydroxypyridine and tetra-n-butylammonium iodide was developed for the synthesis of cyclic carbonates from carbon dioxide and epoxides under mild conditions (1?atm CO₂, 25–60?°C) wi

Full text of DOI:10.1016/j.tet.2017.01.018

Accepted Manuscript
Efficient and practical organocatalytic system for the synthesis of cyclic carbonates
from carbon dioxide and epoxides: 3-hydroxypyridine/tetra-n-butylammonium iodide
Xiangyong Wang, Lin Wang, Yingying Zhao, Koichi Kodama, Takuji Hirose
PII:
S0040-4020(17)30031-5
10.1016/j.tet.2017.01.018
TET 28388
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 30 November 2016
Revised Date: 4 January 2017
Accepted Date: 6 January 2017
Please cite this article as: Wang X, Wang L, Zhao Y, Kodama K, Hirose T, Efficient and practical
organocatalytic system for the synthesis of cyclic carbonates from carbon dioxide and epoxides: 3-
hydroxypyridine/tetra-n-butylammonium iodide, Tetrahedron (2017), doi: 10.1016/j.tet.2017.01.018.
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Graphical Abstract
ACCEPTED MANUSCRIPT
Efficient and Practical Organocatalytic System for Synthesis of Cyclic Carbonates from Carbon Dioxide
and Epoxides: 3-Hydroxypyridine/TBAI
Xiangyong Wang,a Lin Wang,a Yingying Zhao,a Koichi Kodamaa and Takuji Hirose*a
Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama 338-8570, Japan.
*Corresponding author E-mail: hirose@apc.saitama-u.ac.jp; Fax: +81 48-858-3522.
An efficient and practical organocatalytic system comprising 3-hydroxypyridine and tetra-n-butylammonium iodide was
developed for the synthesis of cyclic carbonates from carbon dioxide and epoxides under mild conditions (1 atm CO2, 25−60
°C) without organic solvent. By comparing with related hydroxypyridine derivatives, the effects of the hydroxyl group, the
acidity and the steric factor, were discussed. Study on the mixtures of CO2 and N2 in various ratios indicated that the yield
depends on CO2 content, due to the solubility of CO2 in the reaction mixture. The system was also shown not to be deteorated
by the presence of H2O, air or O2.
OH
cat.
O
TBAI
1 atm
CO2
O
O
O
N
+
no solvent, 25 - 60 oC
R
R
R = alkyl, aryl

ACCEPTED MANUSCRIPT
Efficient and practical organocatalytic system for the synthesis of
cyclic carbonates from carbon dioxide and epoxides:
3-hydroxypyridine/tetra-n-butylammonium iodide
Xiangyong Wang,a Lin Wang,a Yingying Zhao,a Koichi Kodamaa and Takuji Hirose*a
Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo,
Sakura-ku, Saitama 338-8570, Japan.
*Corresponding author E-mail: hirose@apc.saitama-u.ac.jp; Fax: (+) 81-48-858-3522.
Abstract: An efficient and practical organocatalytic system comprising 3-hydroxypyridine and
tetra-n-butylammonium iodide was developed for the synthesis of cyclic carbonates from carbon dioxide and
epoxides under mild conditions (1 atm CO2, 25−60 °C) without organic solvent. By comparing with related
hydroxypyridine derivatives, the effects of the hydroxyl group, the acidity and the steric factor, were discussed.
Study on the mixtures of CO2 and N2 in various ratios indicated that the yield depends on CO2 content, due to the
solubility of CO2 in the reaction mixture. The system was also shown not to be deteorated by the presence of
H2O, air or O2.
Keywords: Carbon dioxide; Cyclic carbonate; 3-Hydroxypyridine; Tetra-n-butylammonium iodide;
Hydrogen bond.
1

ACCEPTED MANUSCRIPT
Efficient and practical organocatalytic system for the synthesis of cyclic carbonates from
carbon dioxide and epoxides: 3-hydroxypyridine/tetra-n-butylammonium iodide
Xiangyong Wang,a Lin Wang,a Yingying Zhao,a Koichi Kodamaa and Takuji Hirose*a
Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura-ku,
Saitama 338-8570, Japan.
*Corresponding author E-mail: hirose@apc.saitama-u.ac.jp; Fax: (+) 81-48-858-3522.
Abstract: An efficient and practical organocatalytic system comprising 3-hydroxypyridine and
tetra-n-butylammonium iodide was developed for the synthesis of cyclic carbonates from carbon dioxide and
epoxides under mild conditions (1 atm CO2, 25−60 °C) without organic solvent. By comparing with related
hydroxypyridine derivatives, the effects of the hydroxyl group, the acidity and the steric factor, were discussed.
Study on the mixtures of CO2 and N2 in various ratios indicated that the yield depends on CO2 content, due to the
solubility of CO2 in the reaction mixture. The system was also shown not to be deteorated by the presence of
H2O, air or O2.
Keywords: Carbon dioxide; Cyclic carbonate; 3-Hydroxypyridine; Tetra-n-butylammonium iodide;
Hydrogen bond.
1. Introduction
Current CO2 emissions into the earth’s atmosphere have raised serious environmental and societal
concerns in connection with the effect of CO2 on global warming as a major greenhouse gas. As the
main sources of CO2 emission are the combustion of carbonaceous materials and animal metabolism, the
reduction of CO2 production has become an important and challenging research topic.1,2 As an abundant,
nontoxic, nonflammable, easily available, and renewable carbon resource, CO2 is a very attractive
carbon feedstock.3-8 Therefore, the chemical fixation of CO2 into valuable organic chemicals has
attracted much attention as a goal of environmental protection and sustainable development. One of the
most successful examples is the synthesis of cyclic carbonates from CO2 and epoxides. These carbonates
are valuable chemicals that can be used as polar aprotic solvents, electrolytes for lithium secondary
batteries, and synthetic intermediates for pharmaceuticals and polymers.9-13
In past decades, various homogeneous and heterogeneous catalysts have been developed for coupling
reactions between CO2 and epoxides, including metal oxides,14,15 modified molecular sieves,16,17 alkali
metal salts,18 organometallic complexes,19,20 and ionic liquids.21-26 However, many of these catalytic
systems require high temperatures, high CO2 pressures, or toxic polar solvents and the reaction under
milder conditions is still an active research topic.
Many metal-based catalytic systems have been developed for the synthesis of cyclic carbonates. In
recent years, on the other hand, the number of organocatalytic systems has been drastically increasing
because organocatalysts offer several advantages, being inexpensive, robust, nontoxic, and
bench-stable.27 Until now, several organocatalysts that contain functionality such as hydroxy groups,28-33
amino groups,34 and carboxy groups35 have been well recognized for playing important roles frequently
combined with a halide salt as a co-catalyst. Such organocatalysts activate an epoxide and/or CO2 to
2

ACCEPTED MANUSCRIPT
facilitate the nucleophilic ring-opening of the epoxide by the halide ion and/or following addition
reaction with CO2. Recently, we developed an organocatalytic system composed of
2-hydroxymethylpyridine and tetra-n
-butylammonium iodide (TBAI).36 We proposed that
2-hydroxymethylpyridine serves as a bifunctional catalyst and its hydroxyl group (hydrogen-bonding
donor) and nitrogen atom (Lewis base) activate the epoxide and coordinate to CO2, respectively.37,38 As
a new organocatalyst with a stronger hydrogen-bonding donor group, herein, we report
3-hydroxypyridine/TBAI as a new catalyst system for the synthesis of cyclic carbonates under mild and
economical conditions such as solvent-free, atmospheric pressure of CO2, moderate temperature
(Scheme 1).
OH
cat.
O
TBAI
1 atm
CO2
O
O
O
N
+
no solvent, 25 - 60 oC
R
R
R = alkyl, aryl
Scheme 1. Synthesis of cyclic carbonates from CO2 and epoxides.
2. Results and discussion
2.1. Reaction with hydroxypyridine–TBAI systems
Although the application of various organic bases to CO2 activation or absorption has been
reported,39-44 we have shown that pyridine is not a good catalyst for the cycloaddition of CO2 and
epoxides in the presence of TBAI at ambient pressure.36 However, as mentioned above, hydroxy groups
are good activators of epoxides due to hydrogen bond formation.28-33,36 Therefore, hydroxypyridines
were expected to be alternative catalysts for the cycloaddition of CO2 and epoxides. We studied three
hydroxypyridine isomers
with tetra- -butylammonium halide as a co-catalyst, to form a new catalytic system for the synthesis of
cyclic carbonate from CO2 and epoxides.
1−3 (Scheme 2) as simple and easily available organocatalysts, in combination
n
OH
OH
N
OH
N
N
1
2
3
pKa 11.65
pKa 8.75
pKa 11.12
Scheme 2. Structures of hydroxypyridines and pKa values of the hydroxy groups.
Catalysts 1−3 were screened with the same amount of TBAI as a co-catalyst using 1,2-epoxyhexane (4a) as a
substrate at 40 °C under atmospheric pressure of CO2 for 24 h. The results are summarized in Table 1. Good to
excellent yields of 4-butyl-1,3-dioxolan-2-one (4b) were obtained, with no by-products detected in the
experiments (Table 1, entries 1–3). As reported previously,36 pyridine was ineffective under the same conditions
(Table 1, entry 4). The effect of the hydroxyl group, therefore the hydrogen bonding ability, is evident in the
3

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reaction between CO2 and an epoxide, as mentioned previously.28-33,36 When 2 and TBAI were used individually,
both gave poor yields (Table 1, entries 5 and 6). Thus, the high catalytic activity was realized by the cooperative
effect of hydroxypyridines and TBAI.28,29,31,36
Kleij et al. worked on the combinations of various phenols and TBAI for the cycloaddition of CO2
and epoxides in methyl ethyl ketone.29 In their work, simple phenol afforded only 15% yield from 4a
under 10 bar at 45 °C while pyrogallol gave 100% yield (93% isolated yield) under the same condition
and 54% yield under 2 bar. Comparing with the present result of
2 (95% isolated yield under 1 atm and
40 °C; Table 1, entry 2), the solvent-free system seems effective for the phenol−TBAI system. In fact we
found both phenol and pyrogallol gave comparable or surpassing yield (97% isolated yield) under the
present conditions, that is in combination TBAI with no solvent (entries 12 & 13).
On the other hand, Shi et al. studied the effect of
found that high yield of 98% is achieved with
pressure and temperature (3.57 MPa, 120 °C) but only 20% is obtained using pyridine.33 It is also
interesting that they observed -toluenesulfonic acid, much stronger acid, and DMAP gave poor yield of
10% under the same condition. We tried the combination of and DMAP to obtain almost no carbonate
p
-methoxyphenol combined with several bases and
N,N-dimethylaminopyridine (DMAP) under high CO2
p
2
(Table 1, entry 7). Under the present reaction conditions (1 atm CO2, 40 °C),
combination with TBAI.
2 works very efficiently in
Table 1. Reaction of 1,2-epoxyhexane with CO2 using various catalystsa.
O
O
Catalyst
O
O
+
CO2
nBu
nBu
4a
4b
Entry
Catalyst
Co-catalyst Yieldb (%)
1
2
1
nBu4NI
nBu4NI
nBu4NI
nBu4NI
-
nBu4NI
DMAP
nBu4NI
nBu4NI
nBu4NBr
nBu4NCl
nBu4NI
nBu4NI
67
95
86
10
6
2
3
3
4
pyridine
5
2
-
6
9
7
2
0
8
2-methyl-3-hydroxypyridine
95
83
82
36
97
97
9
2,4-dimethyl-3-hydroxypyridine
10
11
12
13
2
2
phenol
pyrogallol
a
o
Reaction conditions: 4a (5.0 mmol), 40 C, CO2 (1 atm), 24 h, Catalyst (5.0 mol%), Tetrabutylammonium
halide (5.0 mol%), Solvent-free. b Isolated product obtained after chromatographic purification.
4

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70
2
60
50
40
30
20
3
1
10
0
0
1
2
3
4
Time (h)
Figure 1. Comparative kinetics in the formation of 4b from 4a and CO2 using
conditions: 4a (5.0 mmol), CO2 (1 atm), catalyst (5.0 mol%), TBAI (5.0 mol%), 40
1
–
3
as catalysts. Reaction
C.
°
Interestingly, the reaction yield varied depending on the structure of the hydroxypyridine:
the best result, followed by and then (Table 1, entries 1–3). To study the intrinsic catalytic activity in
detail, kinetic studies of the catalytic conversion of CO2 and 4a to 4b were carried out using , or
and TBAI as co-catalysts for 4 h. The resulting kinetic curves of three different catalysts are shown in
Figure 1 and summarized in Table S1. Catalyst showed the highest catalytic activity, with reaction
rates in the order , affording the TON of 7.8 for in the initial 2 h while those of and were
2 afforded
3
1
1
,
2
3
2
2
>
3
>
1
2
1
3
1.5 and 3.3, respectively. Such a difference can be explained from the acidity of their hydroxyl groups.
Shi et al. reported that this reaction was catalyzed well by phenols with pKas between 7.15 and 10.64.33
The pKa of
highest yield. It is known that
S1),46 which lowers their acidity and therefore the catalytic ability of the hydroxyl groups.32 It is still
interesting that weaker acids, and with pKa values higher than 11, have good catalytic activities. The
2
meets this acidity requirement (Scheme 2),45 which is a principal reason that
and tautomerize to the corresponding pyridones 1’ and 3’ (Scheme
2 afforded the
1
3
1
3
tautomer forms containg the NH structure might contribute the reaction more or less.
Two kinds of methylated 3-hydroxypyridines were studied to further study the effect of hydroxyl
group. While 2-methyl-3-hydroxypyridine afforded the same high yield (95%) as
2 (Table 1, entry 8),
the yield of the product (83%) became lower more than 10% by 2,4-dimethyl-3-hydroxypyridine (entry
9). The results clearly suggest the importance of hydrogen bonding of the hydroxy group; steric
hindrance of the methyl groups introduced at 2- and 4-positions of
formation of a hydrogen bond with the epoxide.
2 disturbs the hydroxy group in
2.2 Optimization of the reaction conditions
In the synthesis of cyclic carbonates from CO2 and epoxides, it has often been proposed that the
halide ion opens the activated epoxide ring through a nucleophilic reaction.30,34-36,47,48 In order to study
the effect of the halide ion, tetra-
n
-butylammonium bromide (TBAB) and tetra-
n-butylammonium
chloride (TBAC) were used in combination with
2
; the results were also summarized in Table 1. The
activity of the catalysts increased in the order: TBAC < TBAB < TBAI (entries 2, 10, and 11), however
this is inconsistent with the order of nucleophilicity of halide anions (Cl– > Br– > I–), which has been
observed in some systems.30,47 It is probable that the activity is corresponding with leaving ability of the
halide ions (Cl– < Br– < I–), as noted previously.30,34-36,48
5

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100
95
90
85
80
75
70
0
1
2
3
4
5
Catalyst (mol%)
Figure 2. Effect of the amount of
2
/TBAI (1:1) on the yield of 4b. Reaction conditions: 4a (5.0 mmol),
C.
CO2 (1 atm), (1.0−5.0 mol%), TBAI (1.0−5.0 mol%), 24 h, 40
2
°
The effect of amount of the binary catalyst
2 and TBAI (1:1) was evaluated, with the results shown in
Figure 2. Even when the catalyst loading was reduced to 1 mol%, the yield of 4b was as high as 71%,
demonstrating the high capability of this catalytic system. Catalytic activity gradually increased with the
catalyst amount, and the yield increased to 94% using a 4 mol% catalyst loading.
100
95
90
85
80
75
70
65
60
0
5
10
15
20
25
30
Reaction time (h)
o
Figure 3. Time dependence of the synthesis of 4b by 2/TBAI. Reaction conditions: 4a (5.0 mmol), 40 C, CO2
(1 atm), 2 (5.0 mol%), TBAI (5.0 mol%).
The reaction time course was also examined at 40 °C with a 5 mol% loading of 2/TBAI, with the
results shown in Figure 3. The reaction proceeded quickly in the first few hours, then gradually slowed,
to afford a 95% yield after 24 h. The effect of reaction temperature was studied and summarized in
Table S2. It was shown that the yield substantially increased with increase of the reaction temperature
from 25 to 40 °C, whereas there seemed minor increase in catalytic activity at 60 °C.
6

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2.2. Cycloaddition of CO2 to various epoxides
According to the results mentioned above, cycloaddition reactions of CO2 with various epoxides in
the presence of /TBAI (5 mol%) were studied under atmospheric pressure and at 25–60 C depending
2
°
on the reactivity of the epoxide; the results are summarized in Table 2. All products are known and the
analytical data match those for the compounds prepared in the earlier study.36,40 The catalytic system
effectively converted mono-substituted aliphatic epoxides into the corresponding cyclic carbonates in
yields of 82–96% at 25 °C without solvent (Table 2, entries 1–6). In entry 5, product precipitation
lowered the yield by preventing continuous stirring of the reaction mixture. Epoxides with longer alkyl
groups gave slightly lower yields (Table 2, entries 6 and 7), which could be improved (>90%) at higher
temperatures (40 °C). The relatively low yield using styrene oxide at 40 °C was probably due to the
presence of the aromatic ring, which makes the ring-opening step more favorable on Cα (most
substituted carbon), unlike the apparent preference for Cβ (least substituted carbon) observed for other
epoxides (Table 2, entries 1–7), resulting in a less nucleophilic alkoxide intermediate, which causes CO2
activation (insertion) to be kinetically more difficult than for aliphatic epoxides.49 Thus, a higher
temperature (60
°
C) was necessary to achieve a 95% yield. Cyclohexene oxide exhibited the lowest
C (Table 2, entry 9), probably owing to the greater steric hindrance compared
reactivity, even at 60
°
with monosubstituted epoxides.28
This catalytic system was also applied for the coupling reaction of enantiomerically pure epoxides and
CO2. When (
carbonate was 92% ee at 40
unexpectedly, further decrease to 64% ee was observed for (
S
)-styrene oxide was transformed to the corresponding carbonates, the ee value of the
C and it lowered to 84% ee at 60 C (Table 2, entry 10). On the other hand,
)-glycidyl benzyl ether even at 40 °C
°
°
R
(entry 11), which demands further refinement of the present system in the next stage.
7

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Table 2. Reactions of various epoxides with CO2 using
2
as catalyst and TBAI as co-catalysta.
Entry
Epoxide
Product
Yieldb (%)
O
O
O
O
1
96
Cl
Cl
O
O
2
O
O
2
93
2
O
O
O
O
3
4
5
91
86
82
O
O
O
O
O
O
O
O
O
O
O
O
Ph
O
Ph
O
O
82
91c
O
6
7
H3C
5
H3C
5
O
O
78
95c
O
nBu
O
n
Bu
O
86c
95d
O
O
O
8
9
Ph
Ph
O
14d
O
O
O
O
84c (92% ee
93d (84% ee
)
O
O
O
10
)
Ph
Ph
O
O
O
O
11
89c (64% ee
)
O
Ph
O
Ph
a
Reaction conditions: Epoxide (5.0 mmol), CO2 (1 atm),
2
(5.0 mol%), TBAI (5.0 mol%), solvent-free,
C. d 60
C.
24 h, 25
°
C. b Isolated product obtained after chromatographic purification. c 40
°
°
2.3. Effect of water and other gas components in CO2
8

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The ultimate goal of this research was the direct fixation of CO2 in combustion gases, which can
contain other gas components, including water. Under such conditions, other gas components could
affect the reaction and/or catalytic system. For practical applications, it is important to study the effect of
water and other gas components on the present system.
First, the effect of H2O on the 2/TBAI catalytic system was studied using 4a in the presence of H2O
(1–5 mmol). As shown in Figure 4, the yield was unaffected by addition of a small amount of water up
to the amount of the substrate and 20 times more than the catalyst. Moisture in the feed gas, therefore,
does not appear to impair the application of this catalytic system to fix CO2 gas at 40
CO2.
°C under 1 atm of
100
90
80
70
60
50
40
30
20
10
0
1
2
3
4
5
H2O (mmol)
Figure 4. Effect of water on the catalytic activity of
2
/TBAI for the synthesis of 4b. Reaction
conditions: 4a (5.0 mmol), CO2 (1 atm),
2
(5.0 mol%), TBAI (5.0 mol%), H2O (variable), 24 h, 40 C.
°
100
80
60
40
20
0
0
20
40
60
80
100
CO2 purity
9

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Figure 5. Effect of CO2 purity in feed gas (CO2/N2) on catalytic activity for the synthesis of 4b
.
Reaction conditions: 4a (5.0 mmol), gas mixture (1 atm),
2 (5.0 mol%), TBAI (5.0 mol%), 24 h, 40 °C.
CO2 purities were 9.93%, 39.9%, 67.1%, and >99.99% (CO2/N2, v/v). See text for data of 93% pure
CO2.
100
80
60
40
20
0
N2
O2
Air
Minor component
Figure 6. Effect of a minor component in CO2 on the catalytic activity of 2/TBAI for 4b synthesis. Reaction
conditions: 4a (5.0 mmol), CO2/minor gas (93/7 (v/v); 1 atm), 2 (5.0 mol%), TBAI (5.0 mol%), 24 h, 40 °C; the
minor gas is air, N2 or O2.
In most studies, highly pure CO2 (>99%) was used for the cycloaddition reaction with epoxides. The
content of CO2 in the feed gas is an important factor in practical applications, and the effect of a second
gas component was investigated using CO2/N2 mixtures with various CO2 volume fractions (9.93%,
39.9%, 67.1%, and >99%). As shown in Figure 5 and Table S3, the yield of carbonate increased with the
increase of CO2 content. On the other hand, it was also found that prolonged reaction time was not such
effective (Table S3, Entries 1 and 5 vs. 2 and 6). It is partly because light sensitive TBAI decomposed
during the reaction. In fact the reaction in the dark improved the yield more (Entries 3 & 7).
In order to gain a deeper insight into the effect of a second gas component, O2 or air (7%, v/v) was
added to CO2. Figure 6 shows that the yields were not affected by O2 or air, indicating that these minor
components do not deteriorate the reaction. These results mean that the catalytic system has good
resistance to oxidation in the presence of up to 7% O2. To check the reliability of the manual mixing
method for air and O2, the same procedure was also applied to N2 (7%, v/v; Figures 5 and 6).
3. Conclusions
A new efficient catalytic system (3-hydroxypyridine,
for the synthesis of cyclic carbonates by coupling CO2 with epoxides without solvent, under atmospheric
pressure, and at low to moderate temperatures, 25 60 C. The effects of hydroxyl group, the pKa and
2/tetra-n-butylammonium iodide) was presented
–
°
hydrogen bonding ability, were discussed by comparing 2-hydroxypyridine, 4-hydroxypyridine, and
2,4-dimethyl-3 hydroxypyridine: catalytic activity was significantly influenced by the acidity and the
steric environment of its vicinity. Moreover, the organocatalytic system exhibited excellent stability in
the cycloaddition reaction of CO2 with epoxides, even in the presence of minor non-toxic components
10

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such as water, air and O2. This highly active and stable organocatalytic system is expected to provide
practical applications for CO2 fixation.
4. Experimental
All commercially available starting materials and solvents were purchased and used as received,
unless stated otherwise. Carbon dioxide (99.99% pure) and CO2−N2 mixtures (CO2 purities: 67.1%,
39.9%, and 9.93%) were used as received without further purification and drying prior to use (Tokyo
Koatsu Yamazaki Co. Ltd.). The purities of N2 and O2 were 99.9% and 99.5%, respectively.
In order to prepare the mixtures of CO2 (93 ± 2%, v/v) with N2, O2 or air (7 ± 2%, v/v), the minor gas
component was introduced into the balloon via syringe, as follows: pure CO2, N2 and O2 were
introduced into separate balloons (~4.2 L). Then, 300 mL of CO2 was removed from the CO2 balloon
and 300 mL of N2 or O2 taken from the corresponding balloon were introduced into the CO2 balloon via
syringe. Ambient air was also introduced into the CO2 balloon in the same way.
Reactions were monitored using thin-layer chromatography using 0.25-mm E. Merck silica gel coated
on aluminum plates (60F-254). Chemical yields refer to the pure isolated carbonate.
1H and 13C NMR spectra were recorded on Bruker AV-300 or AV-500 spectrometers. The chemical
shifts of the products were reported in ppm with reference to Me4Si as the internal standard in CD3OD.
4.1. General procedure for the synthesis of cyclic carbonates from CO2 and epoxides
A 5 mL Schlenk flask was charged with epoxide (5 mmol), hydroxypyridine (0.25 mmol), and
tetrabutylammonium halide (0.25 mmol), and a CO2 balloon (20 cm of diameter) was connected to the
flask (1 atm). The total volume of the gas phase, including the connection, was approx. 4.2 L. The
reaction mixture was set to the relevant temperature and stirred for 24 h. The reaction mixture was
allowed to cool to room temperature and the product was isolated by silica gel column chromatography
using a mixture of hexane and ethyl acetate (1:1–2:1, v/v) as an eluent.
In order to examine the effect of water on catalytic activity, the above procedure was used, but with
the known amounts of distilled water added into the reactor.
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