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40
Published on the web February 25, 2012
Remarkably Efficient Catalysts of Amidine Hydroiodides for the Synthesis
of Cyclic Carbonates from Carbon Dioxide and Epoxides under Mild Conditions
Naoto Aoyagi, Yoshio Furusho, and Takeshi Endo*
Molecular Engineering Institute, Kinki University, 11-6 Kayanomori, Iizuka, Fukuoka 820-8555
(
Received December 12, 2011; CL-111185; E-mail: tendo@moleng.fuk.kindai.ac.jp)
Hydroiodides of amidines effectively catalyzed the reaction
of CO2 and epoxides under mild conditions such as ordinary
pressure and ambient temperature, and the corresponding 5-
membered cyclic carbonates were obtained in moderate to high
yields.
The increasing concentration of carbon dioxide (CO ) in the
2
atmosphere is partly responsible for climate change, while CO2
1
is also regarded as a cheap, green C1 resource. One of the most
Scheme 1. Synthesis of cyclic carbonates from epoxides and
CO2.
important processes is incorporation of CO into epoxides to
give five-membered cyclic carbonates, which are widely used as
starting compounds for various materials. A wide range of CO2-
incorporation catalysts for the synthesis of cyclic carbonates
2
2
Table 1. Effect of counter anions of amidinium salts on the
synthesis of the cyclic carbonate 3a from 2a and CO2 (1 atm) in
the presence of amidinium halides (5 mol %) at 25 °C for 24 h
3
4
have been developed, including alkali metal salts, onium salts,
5
6
metal complexes, ionic liquids, and so on. However, most
of these catalytic systems require high pressure and/or high
temperature for achieving high efficiency with the exception of
several expensive metal catalysts. In addition, some of them
suffer from the formation of poly- and oligocarbonates as by-
product. We have recently reported that the LiBr-catalyzed
reaction of CO2 and epoxides is highly accelerated in the
presence of N-methyltetrahydropyrimidine (MTHP) as a CO2
carrier, and the corresponding cyclic carbonates are obtained in
Entry
Catalyst
Yield/%c
1
2
3
4
5
1a¢HOAc
1a¢HCl
1a¢HBr
1a¢HI
<1
4.0
33
95
N.D.e
moderate to high yields under mild conditions (1 atm, r.t. to
7
d
4
5 °C). However, this system requires large amounts of LiBr as
HI
well as MTHP for obtaining the carbonates in reasonable yields.
Over the course of our program to develop CO2-capturing
materials based on amidines such as MTHP, we have found that
a[2a] = 5.0 M. MeTHF: 2-Methyltetrahydrofuran. cDeter-
b
mined by 1H NMR. dNMP (1-Methyl-2-pyrrolidinone) was
8
e
used as solvent. Not detected.
hydroiodides of amidines efficiently catalyze the cyclic carbon-
ate-forming reactions from CO and epoxides (Chart 1). Herein
MeTHF (2-methyltetrahydrofuran) at 1 atm and ambient temper-
ature (Table 1). It has recently been reported that the acetate
(1a¢HOAc) and the chloride (1a¢HCl) catalyze the reaction of
2
we report a novel efficient synthetic method of cyclic carbonates
from CO2 and epoxides by using a catalytic amount of
hydroiodides of amidines9 under mild conditions such as
ordinary pressure and ambient temperature (Scheme 1).
propylene oxide and CO under relatively severe conditions such
2
6
d
as 10 atm and 140 °C. However, neither 1a¢HOAc nor 1a¢HCl
gave much cyclic carbonate (3a) at 1 atm and 25 °C for 24 h
(
Entries 1 and 2). In contrast, the use of the bromide (1a¢HBr)
resulted in an increased yield of 33%, and the yield was
remarkably increased up to 95% when the iodide (1a¢HI) was
used as the catalyst (Entries 3 and 4). The catalysis is thus highly
affected by the leaving ability of the counter anion moiety of the
amidinium salt. Therefore, the leaving of iodide anion at the
ring-closure step is most likely to be the rate-determining step
for the reaction. A plausible mechanism for the catalytic
synthesis of carbonates consists of nucleophilic attack of the
halide anion to the epoxy group followed by carbonate
formation with CO2 and subsequent ring-closing reaction
Chart 1. Amidines and amines used in this study.
(
Scheme 2). The catalytic cycle does not involve activation of
First, we investigated the effect of the counter anions of the
salts of 1a (DBU: 1,8-diazabicyclo[5.4.0]undec-7-ene) on the
carbonate formation from phenyl glycidyl ether (2a) and CO2 in
the epoxide by the acidic moiety of the catalyst, which was
confirmed by the H NMR spectrum of a solution of 2a and
1a¢HI in CD3CN that showed no chemical shift change of the
1
Chem. Lett. 2012, 41, 240241