Effective synthesis of cyclic carbonates from carbon dioxide and epoxides by phosphonium iodides as catalysts in alcoholic solvents

Article abstract of DOI:10.1016/j.tetlet.2013.10.068

Phosphonium iodides effectively catalyzed the reaction of CO₂ and epoxides under mild conditions such as ordinary pressure and ambient temperature in 2-propanol, and the corresponding five-membered cyclic carbonates were obtained in high yields.

Full text of DOI:10.1016/j.tetlet.2013.10.068

Tetrahedron Letters 54 (2013) 7031–7034
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Effective synthesis of cyclic carbonates from carbon dioxide
and epoxides by phosphonium iodides as catalysts in alcoholic
solvents
⇑
Naoto Aoyagi, Yoshio Furusho, Takeshi Endo
Molecular Engineering Institute, Kinki University, 11-6 Kayanomori, Iizuka, Fukuoka 820-8555, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Phosphonium iodides effectively catalyzed the reaction of CO2 and epoxides under mild conditions such
as ordinary pressure and ambient temperature in 2-propanol, and the corresponding five-membered cyc-
lic carbonates were obtained in high yields.
Received 5 August 2013
Revised 9 October 2013
Accepted 15 October 2013
Available online 23 October 2013
Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
Keywords:
Phosphonium iodide
Carbon dioxide
2-Propanol
Cyclic carbonate
Epoxide
OMe
Introduction
The increasing concentration of carbon dioxide (CO2) in the
atmosphere is partly responsible for the climate changes, while
CO2 is also regarded as a cheap, green C1 resource.1 CO2 is an envi-
ronmentally friendly chemical reagent and is especially useful as a
phosgene substitute.2 One of the most important processes is
incorporation of CO2 into epoxides to give five-membered cyclic
carbonates,3 which are widely used as starting compounds for
polycarbonate derivatives,4 aprotic polar solvents,5 electrolytes,6
and lithium ion batteries.7 Accordingly, a wide range of CO2 incor-
poration reactions for the synthesis of cyclic carbonates have been
developed, and we have recently reported that the cyclic amidine
hydroiodide effectively catalyzed the reaction of CO2 and epoxides
under mild conditions such as ordinary pressure and ambient tem-
perature, and the corresponding five-membered cyclic carbonates
were obtained in moderate to high yields.8 However, the synthesis
of functionalized cyclic amidine salts was limited by their strongly
basic nature. Additionally, linear9 and cyclic10 amidines are, in
general, sensitive to moisture. For example, 1,5-diazabicyclo-
[3.4.0]non-5-ene (DBN) is quantitatively hydrolyzed by using
2–25 equiv of water at room temperature over 12 h.10a On the
other hand, phosphonium salts are stable in protic solvents and
P
P
P
MeO
OMe
1a
1b
1c
N
P
O
P
P
N
N
O
O
1d
1e
1f
Chart 1. Phosphine derivatives used in this study.
can often be purified by recrystallization from water and/or -alco-
hol.11 Phosphonium salts are also used as effective catalysts for the
synthesis of cyclic carbonates from epoxides and CO2.12 However,
most of these reactions require several tens of atmospheric pres-
sure (20–80 atm) and/or high temperature with the exception of
several metal catalyst/phosphonium co-catalyst systems. During
the course of our research on the synthesis of cyclic carbonates
⇑
Corresponding author. Tel.: +81 948 22 9210; fax: +81 948 21 9032.
E-mail addresses: tendo@moleng.fuk.kindai.ac.jp, tendo@me-jsr.fuk.kindai.ac.jp
(T. Endo).
0040-4039/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.10.068

7032
N. Aoyagi et al. / Tetrahedron Letters 54 (2013) 7031–7034
O
O
Catalyst (5 mol%)
CO2
(1 atm)
O
O
+
Org. Solv.
25 or 45 °C
R
R
2a-d
3a-d
a: R = CH2OPh, b: R = ClCH2, c: R = CH2=C(CH3)CO2CH2, d: R = n-Bu
Org. Solv. = 2-propanol, 1-methoxy-2-propanol, etc
Scheme 1. Synthesis of cyclic carbonates from epoxides and CO2.
using various phosphonium salts and solvents, we have found that
tertiary and quaternary phosphonium iodides efficiently catalyze
the cyclic carbonate-forming reactions from CO2 and epoxides in
2-propanol (Chart 1). Herein, we report a novel efficient synthetic
method of cyclic carbonates from CO2 and epoxides by using a
combination of phosphonium iodides and secondary alcohols
under mild conditions such as ordinary pressure and ambient
temperature (Scheme 1).
Figure 1. Partial 1H NMR spectra (400 MHz, 25 °C) of a solution of 2a (110
with 2-propanol (55 mol) (a) or methanol (55 mol) (b) in CDCl3 (550 L).
lmol)
l
l
l
hydroxyl and the epoxy groups, which were suggested by the 1H
NMR spectrum of 2a with alcohols in CDCl3 (Fig. 1). It was found
that the OH-proton signal of 2-propanol shifted from 1.41 ppm to
a lower magnetic field of 1.50 ppm at ambient temperature despite
being diluted with CDCl3. Hydroxyl protons of other alcohols (eth-
anol, isobutyl alcohol, and tert-butanol) shifted similar to 2-propa-
nol. Although the OH-proton signal of methanol also shifted from
1.01 ppm to a lower magnetic field of 1.09 ppm, the product yield
in methanol was much lower than that obtained in 2-propanol.
This could be accounted for by methanol’s stronger ability to sol-
vate the iodide anion than 2-propanol,13 which hampers the nucle-
ophilic attack of the iodide anion to the epoxy group (this point is
discussed later in Table 4).
Next, we investigated the effect of the counter anions of 1a salts
on the carbonate formation from 2a and CO2 in 2-propanol at
1 atm and ambient temperature (Table 2). Several phosphonium
chlorides catalyze the reaction of epoxides and CO2 under rela-
tively severe conditions such as 30 atm and 160 °C or supercritical
conditions.12d However, neither triphenylphosphine hydrochloride
(1aÁHCl) nor methyltriphenylphosphonium chloride (1aÁMeCl)
gave much cyclic carbonate (2a) at 1 atm and 25 °C for 24 h
(entries 1 and 2). In contrast, the hydrobromide (1aÁHBr) and the
phosphonium bromide (1aÁMeBr) resulted in increased yields of
32% and 37%, respectively (entries 3 and 4), when the hydroiodide
(1aÁHI) and the phosphonium iodide (1aÁMeI) were used as the cat-
alyst, the yield of 3a was remarkably increased up to 93% and 97%,
Results and discussion
First, we examined the effect of solvent on the carbonate forma-
tion from the phenyl glycidyl ether (2a) and CO2 using the hydroio-
dide salt (1aÁHI) at 1 atm and ambient temperature (Table 1).
Tetrahydrofuran (THF), an aprotic polar solvent, gave 3a in a lower
yield than that obtained in bulk (entries 1 and 2), although 1,8-
diazabicyclo[5.4.0]undec-7-ene hydroiodide (DBUÁHI) gave a high
yield of 3a in THF under same conditions.8 Toluene and chloroform,
aprotic non-polar solvents, gave also 3a in low yields (entries 3 and
4), and the yield of 3a remarkably decreased in 1-methyl-2-pyrro-
lidinone (NMP) and dimethyl sulfoxide (DMSO) (entries 5 and 6).
These results suggest that neither the aprotic solvents nor 1aÁHI
can activate the epoxy ring. On the other hand, protic polar sol-
vents significantly improved the reactivity of 2a with CO2 in the
presence of 1aÁHI. Simple primary alcohols such as methanol and
ethanol gave 3a in low yields (entries 7 and 8). In contrast, the
use of 2-methylpropyl alcohol resulted in an increased yield of
68%, and the yield was remarkably increased up to 93% when
2-propanol was used as the solvent (entries 9 and 10). Tertiary
butyl alcohol gave 3a in a low yield, which was attributed to the
interference of the activation of the epoxy ring in consequence of
the bulky tert-butyl group (entry 11). The protic solvents can
activate the epoxide via hydrogen bonds formed between the
Table 1
Effect of solvent for the synthesis of 2a by using 1aÁHI under ambient conditionsa
O
Table 2
O
Effect of anion moiety of catalyst for the synthesis of 2a under ambient conditionsa
O
1a•HI
O
(5 mol%)
O
O
O
+
CO2
Org. Solv.
(1 atm)
1a•RX
(5 mol%)
O
O
25 °C, 24 h
O
2a
3a
Yieldb (%)
O
O
+
CO2
(1 atm)
2-Propanol
25 °C, 24 h
Entry
Solvent
2a
3a
1
2
3
4
5
6
7
8
Bulk
THF
43
12
20
30
2
Entry
Catalyst
Yieldb (%)
Toluene
CHCl3
NMP
1
2
3
4
5
6
7
8
1aÁHCl
1aÁMeCl
1aÁHBr
1aÁMeBr
1aÁHI
2
7
32
37
93
97
DMSO
MeOH
EtOH
Not detected
4
26
68
93
57
9
10
11
i-BuOH
i-PrOH
t-BuOH
1aÁMeI
PPh3
Not detected
Not detected
P(O)Ph3
a
a
Reaction conditions: 1 mmol of 2a, 0.05 mmol of 1aÁHI, 0.2 mL of solvent, 25 °C
Reaction conditions: 1 mmol of 2a, 0.05 mmol of 1aÁRX, 0.2 mL of 2-propanol,
under 1 atm of CO2 for 24 h.
25 °C under 1 atm of CO2 for 24 h.
b
Determined by 1H NMR.
b
Determined by 1H NMR.

N. Aoyagi et al. / Tetrahedron Letters 54 (2013) 7031–7034
7033
Nucleophilicity in
Table 3
Effect of structure of catalyst for the synthesis of 2a under ambient conditionsa
ROH
protic conditions
O
O
I
> Br > Cl
O
PhO
PhO
1a- f•MeI
(5 mol%)
O
O
O
B
+ X HOR
O
O
+
CO2
(1 atm)
2-Propanol
25 °C, 24 h
Catalyst
Solvent
ROH
HOR
B+
O
B+•X
PhO
HOR
2a
3a
(B = MePh3P ) (excess)
X
O
Entry
Catalyst
Yieldb (%)
O
1
2
3
4
5
6
1aÁMeI
1bÁMeI
1cÁMeI
1dÁMeI
1eÁMeI
1fÁMeI
97
87
84
89
87
O
O
O
O
CO2
PhO
PhO
B+
X
Product yield (ROH = IPA)
I (97%) > Br (37%) > Cl (7%)
Leaving ability
Br > Cl
I
>
Not detected
a
Reaction conditions: 1 mmol of 2a, 0.05 mmol of 1a–fÁMeI, 0.2 mL of 2-propa-
Scheme 2. A plausible mechanism for catalyzed synthesis of cyclic carbonates from
epoxides and CO2.
nol, 25 °C under 1 atm of CO2 for 24 h.
b
Determined by 1H NMR.
of the ring-opening reaction. In fact, the product yields decreased
with increasing acceptor number (AN) of solvents, that is, IPA
(33.8) < MeOH (41.3) < HFIP (63.0), which are known to be propor-
tional to the solvation ability.13
respectively (entries 5 and 6). The use of phosphine ligands with-
out counter anion gave no cyclic carbonate (entries 7 and 8). The
catalytic activity is thus highly affected by the counter anion of
the catalysts.
A plausible mechanism for the catalytic synthesis of carbonate
is shown in Scheme 2. Initially, the protic solvent activates the
epoxide, and at the same time solvates the halide anion of the cat-
alyst through hydrogen bonds. Then, the ring-opening intermedi-
ate is formed by the nucleophilic attack of the halide anion on
the activated epoxy group, and subsequent nucleophilic attack on
CO2 leads to the alkylcarbonate anion. Finally, the ring-closure
through the elimination of the halide anion gives the cyclic carbon-
ate. In the reaction, the protic solvents play two opposite roles, that
is, activation of the epoxy ring via hydrogen bonds and diminishing
the reactivity of the halide anion through solvation. The latter is
the dominant factor for the catalytic activity, which can be roughly
indicated by the acceptor numbers. The counter anion of the cata-
lysts is another important factor of the catalytic activity, which was
in the order of iodide > bromide > chloride. This could be ac-
counted for by the nucleophilicity of halide anions in protic sol-
vents at the ring-opening step as well as the leaving ability at
the ring-closing step, both in the order of IÀ > BrÀ > ClÀ.
Furthermore, we examined various phosphonium iodides
(1a–fÁMeI) in the carbonate formation from 2a and CO2 in 2-propa-
nol at 1 atm and ambient temperature (Table 3). Catalytic activities
of the phosphonium salts bearing substituents more electron-rich
than 1aÁMeI (1b–dÁMeI) were slightly less than that of 1aÁMeI (en-
tries 1–4). The phosphonium iodide with dimethylamino groups
resulted in more or less the same yield as 1b–dÁMeI (entry 5). In
contrast, the phosphonium iodide with phenoxy substituents
(1fÁMeI) gave no cyclic carbonate (entry 6). Thus, the methyltri-
phenylphosphonium iodide (1aÁMeI) was the best catalyst among
the phosphonium salts investigated here (1a–fÁRX).
Next, the carbonate formation of 2a with CO2 was carried out in
several secondary alcohols and other solvents using 1aÁMeI
(Table 4). As in the case of 1aÁHI, THF and methanol gave 2a in
low yields (entries 1 and 2), and the yield of 2a was not obtained
in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) (entry 3). In contrast,
both 1-methoxy-2-propanol and 2-propanol resulted in almost
quantitative yields (entries 4 and 5). The reaction efficiency was
thus significantly affected by the structures of the secondary alco-
hols (entries 3–5). These results could be explained in terms of the
solvation of the halide anions. The protic solvents activate the
epoxide via hydrogen bonds as described above, while they
strongly solvate iodide anion, thereby diminishing the reactivity
Finally, we examined the carbonate-forming reactions of other
epoxides with 1aÁMeI and 1-methoxy-2-propanol at 25 or 45 °C
(Table 5).14 The carbonate (3a) was isolated quantitatively by short
Table 5
Synthesis of various cyclic carbonates by using 1aÁMeI under ambient conditionsa
O
1a•MeI
O
(5 mol%)
O
Table 4
+
CO2
(1 atm)
Effect of solvent on the synthesis of 2a by using 1aÁMeI under ambient conditionsa
O
1-Methoxy-2-propanol
25 or 45 °C, 24-48 h
R
R
O
2a-d
3a-d
O
O
1a•MeI
a: R = PhOCH2, b: R = ClCH2, c: R = CH2=C(CH3)CO2CH2, d: R = n-Bu
O
(5 mol%)
O
O
+
CO2
(1 atm)
Entry
Epoxide
Temp (°C)
Time (h)
Yieldb (%)
Org. Solv.
25 °C, 24 h
1
2
3
4
5
6
7
8
2a
2b
2b
2c
2c
2d
2d
2d
25
25
25
25
25
25
25
45
24
24
36
24
36
24
48
24
99 (99)c
90
2a
3a
98 (97)c
96
Entry
Solvent
Yieldb (%)
1
2
3
4
5
THF
MeOH
(CF3)2CHOH
35
30
>99 (>99)c
65
Not detected
99
97
86
(MeOCH2)(CH3)CHOH
(CH3)2CHOH
92 (91)c
a
Reaction conditions: 1 mmol of 2a–d, 0.05 mmol of 1aÁMeI, 0.2 mL of 1-meth-
a
Reaction conditions: 1 mmol of 2a, 0.05 mmol of 1aÁMeI, 0.2 mL of solvent,
oxy-2-propanol, 25 or 45 °C under 1 atm of CO2 for 24–48 h.
b
25 °C under 1 atm of CO2 for 24 h.
Determined by 1H NMR.
Determined by 1H NMR.
Isolated yields are in parentheses.
b
c

7034
N. Aoyagi et al. / Tetrahedron Letters 54 (2013) 7031–7034
Churkina, G.; Cramer, W.; Denning, A. S.; Field, C. B.; Friedlingstein, P.; Goodale,
C.; Heimann, M.; Houghton, R. A.; Melillo, J. M.; Moore, B., III; Murdiyarso, D.;
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Steffen, W. L.; Wirth, C. Nature 2001, 414, 169–172.
column chromatography on silica gel (entry 1). Epichlorohydrin
(2b) and glycidyl methacrylate (2c) containing electron withdraw-
ing groups on the carbon atom next to the epoxy group that were
as reactive as 2a (entries 2–5), while the epoxide bearing hydrocar-
bon substituents, 1,2-epoxyhexane (2d), had a lower reactivity
than those of other epoxides (entries 6 and 7). When the reaction
temperature was raised from 25 to 45 °C, the conversion of 2d
increased and the cyclic carbonate (3d) was isolated in high yield
(entry 8). Other reported reactions using protic compounds for
the activation of epoxide require several tens of atmospheric
pressure (20–40 atm) and high temperature (80–140 °C).12e,15
In conclusion, we have demonstrated that the phosphonium
iodides catalyzed effectively the reactions of CO2 and epoxides in
secondary alcohols and the corresponding cyclic carbonates are
obtained in reasonable yields under mild conditions such as ordin-
ary pressure and 25–45 °C. The catalytic activity is highly affected
by the counter anions of the phosphonium catalysts; the iodides
catalyze efficiently the carbonate-forming reactions, in contrast
to the bromide and chloride counterparts that show low reactivity.
The choice of the solvent is also important. The protic solvents with
low acceptor numbers can activate the epoxy group without
diminishing the nucleophilicity of the halide anions through strong
solvation.
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Supplementary data
Supplementary data (1H and 13C spectra of isolated compounds
and experimental data of phosphonium halides) associated with
this article can be found, in the online version, at http://dx.doi.
org/10.1016/j.tetlet.2013.10.068.
14. General
procedure
for
five-membered
cyclic
carbonate
(2a–d):
Methyltriphenylphosphonium iodide (20.2 mg, 0.05 mmol) was added to a
solution of epoxide (1 mmol) in 1-methoxy-2-propanol (0.2 mL). The
atmosphere inside the flask was replaced with CO2 (balloon, ca. 1 atm), and
the reaction mixture was stirred at 25 or 45 °C. After 24–48 h, the mixture was
cooled to rt, and evaporated in vacuo. The residue was purified by silica gel
column chromatography (eluent: CHCl3) to give the corresponding cyclic
carbonate (91–99%).
References and notes
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