L. Ji et al. / Molecular Catalysis 446 (2018) 124–130
125
Table 1
The synergistic effect of organic bases and ionic liquids for the cycloaddition of CO2
and epichlorohydrin.[a]
Entry
Ionic liquids
Organic bases
Yield[b]
[%]
1
2
3
4
5
6
7
8
BnBimBr
–
–
50
20
96
82
95
53
93
91
72
80
81
85
78
DEA
DEA
DEA
DEA
DEA
DEBA
MIm
DBU
TEA
BnBimBr
BmimBr
DBnimBr
BnMBimBr
BnBimBr
BnBimBr
BnBimBr
BnBimBr
BnBimBr
BnBimBr
BnBimBr
9
10
11
12
13
n-BA
DABCO
Im
[a] Reaction conditions: Epichlorohydrin (10 mmol), CO2 (0.1 MPa), organic base
(0.1 mmol), ionic liquids (0.1 mmol), 80 ◦C, 3 h. [b] GC yield.
Scheme 1. Mechanism of cycloaddition of CO2 and epoxides by different catalytic
system a) ionic liquids; b) organic bases; c) our proposed dual catalysts.
gas chromatography to determine yield using dodecane as internal
standard. The product was purified by chromatography on silica
gel and characterized structurally by NMR spectroscopy.
Herein, we present a study on the combination of imidazolium
ionic liquids and organic bases as synergistic catalytic system for
the cycloaddition of CO2 and epoxides for the first time (Scheme 1c).
In this catalytic system, ionic liquids and organic bases are able to
activate epoxides and CO2, respectively. The dual activation facili-
tates the reaction to proceed under mild conditions. Experimental
and theoretical methods on molecules were applied to investigate
the synergistic mechanisms of ionic liquids/secondary amines or
ionic liquids/tertiary amines.
2.3. Computational methods
The structural optimization by DFT calculations was carried out
at the M06-2x/6–31 + G* level using G09 program package [12,13].
The frequency analyses were then performed at the M06-2X/6-
the minima and transition states on potential surfaces. The intrin-
sic reaction coordinate calculations were conducted to verify that
the transition structures connect the correct reactants and prod-
ucts in the forward and reverse directions. The solvent effect on
the solute was evaluated using the SMD [14,15] model in G09. All
the calculated Gibbs free energies ꢀGsol were evaluated under the
experimental conditions with the temperature 353 K and pressure
0.1 MPa. More details about the optimized structures are provided
in Supporting Information.
2. Experimental
2.1. General
1-Benzyl-3-butylimidazolium
bromide
(BnBimBr),
1,3-dibenzylimidazolium bromide (DBnimBr), 1-benzyl-2-methy-
3-butylimidazolium bromide (BnMBimBr) were synthesized
according to the literature with minor modification (see Sup-
porting Information). 1-Butyl-3-methylimidazolium bromide
(BmimBr) was supplied by the Centre for Green Chemistry and
Catalysis, LICP, CAS. CO2 was supplied by Doumaoai Factory
with a purity of 99.9%. Epichlorohydrin, epibromohydrin, 2-
(phenoxymethyl)oxirane ethylene oxide, propylene oxide, styrene
oxide and 1,2-hexene oxide were purchased from TCI. The other
reagents were supplied by Sinopharm. All chemicals were used
without further purification.
NMR spectra were recorded on Bruker Ascend400 instruments
with tetramethylsilane as the internal standard. GC analysis was
performed by using a Shimadzu GC–14 B equipped with a capillary
column DM-1701 (60 m-0.32 mm-0.25 mm) equipped with a flame
ionization detector. FTIR spectra were recorded on a Nicolet nexus
670 instrument.
3. Results and discussion
3.1. Synergistic catalysis of organic bases and ionic liquids
To investigate the effect of intermolecular synergistic catalytic
for the cycloaddition of CO2 and epichlorohydrin. When BnBimBr
carbonate was obtained after 3 h under atmospheric CO2 pressure
(entry 1, Table 1). If DEA was used alone, a poor yield of 20% was
obtained (entry 2, Table 1). However, the combination of BnBimBr
and DEA gave excellent yield of 96% (entry 3, Table 1), indicating the
also observed in the combination of other ionic liquids and DEA
as well. DBnimBr and BmimBr gave yields of 95% and 82%, respec-
tively (entry 4–5, Table 1). However, BnMBimBr, in which the C2-H
of the imidazolium ring was replaced by methyl group, provided a
lower yield of product (53%) (entry 6, Table 1). These results indi-
cated that C2-H of imidazolium was also crucial for the reaction.
NMR spectroscopy and DFT calculations demonstrated the interac-
tion between epoxide and C2-H through hydrogen bonding which
led to the activation of epoxide (Fig. S1, S2). The similar observa-
tion has been reported by us and others [16]. Various organic bases
2.2. Typical procedure for the cycloaddition of CO2 and epoxide
Typically, epichlorohydrin (0.925 g, 10 mmol), BnBimBr
(0.029 g, 0.1 mmol) and DEA (0.007 g, 0.1 mmol) were mixed
together and put into a 30 mL stainless steel autoclave equipped
with a magnetic stirrer. The reaction was carried out under
atmospheric pressure of CO2 at 80 ◦C. After the completion of
reaction, the autoclave was cooled to room temperature. In order
to remove the ionic liquid, Et2O (40 mL) was added to the reaction
mixture and filtered. The filtrate was subsequently analyzed by