Amidine-mediated delivery of CO2 from gas phase to reaction system for highly efficient synthesis of cyclic carbonates from epoxides

Article abstract of DOI:10.1039/b916235f

A novel, efficient synthesis of cyclic carbonates from the reaction of epoxides and gaseous CO₂ under mild conditions (1 atm, rt to 45 °C) was achieved in reasonable yields (65-83%) by using N- methyltetrahydropyrimidine (MTHP) that facilitated efficient delivery of CO ₂ from the gas phase to the reaction system.

Full text of DOI:10.1039/b916235f

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Amidine-mediated delivery of CO2 from gas phase to reaction system
for highly efficient synthesis of cyclic carbonates from epoxides
Balaka Barkakaty, Kazuhide Morino, Atsushi Sudo and Takeshi Endo*
Received 7th August 2009, Accepted 6th October 2009
First published as an Advance Article on the web 29th October 2009
DOI: 10.1039/b916235f
A novel, efficient synthesis of cyclic carbonates from the
reaction of epoxides and gaseous CO2 under mild condi-
tions (1 atm, rt to 45 ◦C) was achieved in reasonable yields
(65–83%) by using N-methyltetrahydropyrimidine (MTHP)
that facilitated efficient delivery of CO2 from the gas phase
to the reaction system.
This intrinsic nature of MTHP prompted us to design a new
system for converting epoxides into the corresponding carbon-
ates under milder temperature conditions and at atmospheric
pressure, which relies on utilization of MTHP that can naturally
capture CO2 molecules from the gas phase and deliver them to
the reactive site in a reversible cycle (Scheme 2).
Introduction
Innovation of novel methods for easy and economical chemical
fixation of CO2 and its use as a C1 resource for synthesis
of valuable target molecules is the need of this century for
the sustenance of life on earth.1 Recently, the synthesis of
five-membered cyclic carbonates from epoxides using CO2 has
gained much attention, owing to its increased importance
as a building block in polymer synthesis, electrolytes, apro-
tic polar solvents and precursors for pharmaceuticals.2 For
this reaction, various catalysts, including metal complexes,3
smectites,4 titanosilicates,5 zeolites,6 metal oxides,7 organic
bases,8 alkali metal halides9 and ionic liquids,10 have already
been developed and reported. However, high pressure and/or
high temperature are important pre-requisites for high efficiency
in these catalytic systems, and some of them suffer from
the formation of poly(carbonate) as a by-product. Endo and
coworkers have reported that alkali metal salts such as lithium
bromide catalyzed the reaction at 1 atm pressure to give five-
membered cyclic carbonates in high yields and without the
formation of poly(carbonate).9 However, in order to achieve
a satisfactory rate of reaction, heating the system to 100 ◦C
was required. In other words, catalysis of the reaction at
1 atm and at near ambient temperature is still a highly chal-
lenging endeavour. Recently, Endo and coworkers have reported
a reversible system for efficient trap-and-release of CO2(g)
by N-methyltetrahydropyrimidine (MTHP) and its analogous
functionality attached to polymer side chains (Scheme 1).11
Scheme 2 Delivery of CO2 by amidine and its insertion reaction with
epoxide.
Herein, we report our achievement of a highly efficient
utilization of CO2 at low temperature (<45 ◦C) and under
1 atm, which was facilitated by participation of MTHP as a
“CO2 deliverer”.
Results and discussion
For the present study, two monosubstituted epoxides, n-butyl
glycidyl ether (1a), glycidyl phenyl ether (1b), and one sterically
hindered disubstituted epoxide, cyclohexene oxide (2), were
employed as substrates (Scheme 3). The reactions were carried
out at 25 or 45 ◦C, which were much lower than the previously
Scheme 1 Reversible fixation-release of CO2 by amidine (MTHP).
Molecular Engineering Institute, Kinki University, 11-6 Kayanomori,
Iizuka, Fukuoka, 820-8555, Japan.
E-mail: tendo@mol-eng.fuk.kindai.ac.jp; Fax: +81-948-22-7210;
Tel: +81-948-22-7210
Scheme 3 Amidine catalyzed synthesis of cyclic carbonates from
epoxides under mild conditions.
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Table 1 Synthesis of cyclic carbonates from epoxidesa
Table 2 Reusability of MTHP–LiBr system
Substrate/ Amount of
Entry
Recycle number
Yield/%a
Entry product
MTHP/mol%
Time/h T/◦C
Yield/%b
1
2
3
Fresh
Recycle 1
Recycle 2
100
46
22
1
2
1a/3a
1a/3a
1a/3a
1b/3b
1b/3b
1b/3b
2/4
0
6
6
6
25
25
25
25
25
25
45
45
45
45
45
48
100 (77)
0
45
35
100 (83)
12
25
68
10
10
0
10
10
0
10
10
10
10
3c
4d
5d
6d
7
a Determined by 1H NMR.
96
24
96
72
12
24
48
72
markedly. The yield of 4 reached 25% after 12 h (entry 8),
followed by a smooth progress of the reaction to achieve 95%
yield at 72 h (entries 8–11).
8
9
10
11
2/4
2/4
2/4
84
95 (65)
However, the catalytic efficiency of the catalysts showed a
gradual decrease when recycled (Scheme 4, Table 2). This might
be attributed to the gradual decrease of LiBr concentration dur-
ing the process of product separation by filtration and repeated
washing with diethyl ether. Since Li salts are known to have
a high affinity for carbonate and ether compounds,12 succesive
formation of carbonate products followed by further treatment
with diethyl ether probably removes substantial amounts of the
required LiBr catalyst from the reaction system, resulting in
lower yields of the desired products when the same loadings of
catalysts were reused.
2/4
a Conditions: [epoxide]0 : [MTHP] : [LiBr] = 100 : 10 : 25. b Determined
by 1H NMR. Isolated yields are shown in parentheses. c Reaction carried
out in the absence of LiBr. d Reaction carried out in NMP, the initial
concentration of epoxide was 1 M.
reported temperature (100 ◦C)9 for the efficient transformation
of epoxide into the corresponding carbonate by using lithium
bromide as catalyst under 1 atm CO2. The results are summa-
rized in Table 1.
The reaction of 1a was studied first. As a reference experiment,
the reaction was performed in the absence of MTHP (entry 1);
when a bulk mixture of 1a and LiBr (25 mol%) was stirred at
room temperature in a reaction vessel equipped with a 1 atm
CO2 balloon, 1a was slowly converted into the corresponding
cyclic carbonate 3a, and after 6 h, its yield, determined by
1H NMR analysis of the resulting mixture, had reached 48%. On
the other hand, the addition of 10 mol% MTHP under similar
conditions remarkably improved the efficiency of the reaction
to allow complete conversion of 1a into 3a after 6 h (entry 2).
However, use of LiBr was mandatory for the reaction; in its
absence, the reaction fails to proceed (entry 3).
Scheme 4 Synthesis of butylglycidyl carbonate from recycled catalysts.
Conclusion
This remarkable acceleration was also observed on adding
MTHP in the reaction of epoxide 1b (entry 4 vs. entry 5). In
this case, use of a solvent was required, because the intrinsically
high crystallinity of the corresponding carbonate product 3b
prevented efficient progress of the reaction in the bulk state.
N-Methylpyrrolidone (NMP) was used as a suitable solvent
which allowed the reaction to proceed in the homogeneous
system.9 In the absence of MTHP, only 45% yield of the
corresponding cyclic carbonate 3b was achieved, even though
the reaction time was prolonged to 96 h (entry 4). In contrast,
by adding MTHP, the reaction was greatly accelerated to achieve
100% yield of 3b under the same reaction conditions (entry 6).
These successful results at low temperature encouraged us
further to apply the MTHP-mediated system for the reaction of
cyclohexene oxide 2 (Table 1, entries 8–11). So far, transforma-
tion of such 1,2-disubstituted epoxides into the corresponding
carbonates by conventional systems has been rather difficult due
to steric hindrance.3d,3g,8c,10a,10e For example, when cyclohexene
In summary, a highly efficient synthesis of cyclic carbonates
from the corresponding epoxides was designed and achieved
by utilizing only a catalytic amount of natural CO2 fixer like
MTHP along with the conventional LiBr catalyst. This system
broadens the scope of such reactions by outlining a key concept
to overcome the high activation barrier which is faced by these
reactions when using conventional methodologies employing
high temperature and high pressure techniques. The reported
method can be operated successfully at atmospheric pressure
and at 25–45 ◦C, which allows less energy-demanding and more
environmentally friendly chemical fixation of CO2 into valuable
compounds. The successful results imply that addition of MTHP
as a “CO2 deliverer” might be a versatile strategy to improve
the efficiency of various reaction systems which use CO2 as a
reactant.
Experimental
◦
oxide 2 was heated in NMP (at 100 C, for 24 h) under 1 atm
a) The experimental procedure for the synthesis of 3a
CO2, the reaction was quite slow, leading to only 35% yield of
the carbonate 4. This poor efficiency implied the intrinsic low
reactivity of 2, and, in fact, its bulk reaction at 45 ◦C in the
absence of MTHP was also very slow, giving the corresponding
carbonate 4 in only 12% yield after 72 h (entry 7). In contrast,
by adding MTHP, the efficiency of the reaction was improved
CO2(g) from a gas balloon (purchased from AS ONE, code
9-086-01) was allowed to react with 10 mol% of MTHP (25 mg,
0.25 mmol) at room temperature for 25 min, after which the
liquid MTHP turned into white solid. Then, 25 mol% of LiBr
(55 mg, 0.64 mmol) followed by 2.55 mmol of n-butyl glycidyl
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ether (1a, 0.36 ml) were added to the reaction mixture and the
Notes and references
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This research was financially supported by the JSR Corporation.
We are thankful to Dr. Bungo Ochiai from Yamagata University
for providing us with MTHP to carry out our initial investiga-
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44 | Green Chem., 2010, 12, 42–44
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