The effective synthesis of propylene carbonate catalyzed by silica-supported hexaalkylguanidinium chloride

Article abstract of DOI:10.1039/b504822b

We have described that both homogeneous and a silica-supported hexaalkylguanidinium chloride were effective catalysts for CO₂ fixation to carbonate without any solvent under mild reaction conditions (4.5 MPa, 120 °C, 4 h), the silica-supported

Full text of DOI:10.1039/b504822b

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P A P E R
The effective synthesis of propylene carbonate catalyzed by
silica-supported hexaalkylguanidinium chloride
Haibo Xie,a Haifeng Duan,b Shenghai Lib and Suobo Zhang*a
a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied
Chemistry, Graduate School of Chinese Academy of Sciences, Changchun, 130022, China.
E-mail: sbzhang@ciac.jl.cn; Fax: þ86-431-5685653; Tel: þ86-431-5262347
b College of Chemistry, Jilin University, Changchun, 130023, China
Received (in Montpellier, France) 7th April 2005, Accepted 24th June 2005
First published as an Advance Article on the web 28th July 2005
We have described that both homogeneous and a silica-supported hexaalkylguanidinium chloride were
effective catalysts for CO2 fixation to carbonate without any solvent under mild reaction conditions
(4.5 MPa, 120 1C, 4 h), the silica-supported hexaalkylguanidinium chloride showing the great advantage that
it could be recycled easily at least 5 further times without any obvious decrease in its catalytic activity,
after simple filtration.
1. Introduction
2. Experimental
The chemical fixation of carbon dioxide into industrially useful
compounds has been given much attention in recent years due
to the following two reasons. Firstly, carbon dioxide is re-
garded as the primary greenhouse effect gas, therefore, devel-
oping an efficient way to reduce carbon dioxide in our
environment is intriguing work. Secondly, as an inexpensive,
non-toxic and non-flammable gas, carbon dioxide has been
considered an attractive C1 building block for producing useful
organic compounds.1
GC was carried out on a Hewlett-Packard III 5720 instrument.
1H and 13C NMR spectra were determined on a Bruker
spectrometer (300 MHz) with TMS as the internal standard.
Mass spectra were measured on a Hewlett-Packard HP-5989A
apparatus. Elemental analyses were obtained on a VarioEL
analyzer. FTIR spectra were obtained with a Bio-Rad Digilab
Division FTS-80 spectrometer. Melting points were deter-
mined on a RY-1 melting point apparatus. All the epoxides
were used after distillation over CaH2.
The formation of cyclic carbonate via the cycloaddition of
epoxides and carbon dioxide, which is one of the routes for the
chemical fixation of CO2, has been investigated widely, since
the cyclic carbonates show interesting applications as polar
aprotic solvents, as precursors for polycarbonate materials,
and in general, as intermediates in organic synthesis.2 Numer-
ous catalyst systems, including alkali metal salts,3 metal com-
plexes,4 MgO,5 Mg–Al mixed oxides,6 ionic liquids,7
biopolymer-supported zinc chloride,8 NaI/PPh3/PhOH,9
DMAP/PhOH10 and homogeneous/heterogeneous guanidine
catalyst systems,11 have been developed for this transforma-
tion. While the advances have been significant, all suffer from
low catalyst stability and activity, air sensitivity, the need for
co-solvents or co-catalysts, or the requirement for high pres-
sures and/or high temperatures. Hence the exploration of
highly efficient, easily separated and recycled catalyst systems
for this transformation, under mild conditions, still remains a
challenge.
2.1 The synthesis of ionic liquids13
The ionic liquids 1a, 1d, 1e, 2a, 2b and 2c were synthesized
according to the literature. The ionic liquids 1b and 1c were
synthesized by anion exchange between ionic liquid 1a and the
corresponding inorganic salt (Scheme 1).
Recently, we demonstrated that hexabutylguanidinium
chloride (HBGCl) is an effective catalyst for the reaction of
styrene oxide with CO2 to afford cyclic carbonate at 60 1C and
atmospheric pressure.12 The high catalytic efficiency may be
due to the Lewis acidic guanidinium cation activating the ring
opening of epoxides with electrophiles. Due to the great
interest in this topic, the efforts made to obtain a better catalyst
separation, and to make the catalyst recovery simpler, here we
present a study of an effective coupling of CO2 and propylene
oxide to give cyclic carbonate, in the presence of a silica-
supported pentabutylpropylguanidinium chloride (PBGSiCl)
catalyst.
Scheme 1
Synthesis of 1c. A mixture of HBGCl (4.32 g, 10 mmol) and
KI (3.32 g, 20 mmol) in acetone (10 mL) was stirred at room
temperature for 24 h and a white salt (KCl) precipitated out.
After the filtration of the solid, further KI (3.22 g, 20 mmol)
was added to the solution and it was stirred at room tempera-
ture for 24 h in order for the exchange to complete. Finally
after the filtration of the solid, the solution was concentrated in
a rotary evaporator and then diluted with dichloromethane.
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 5
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 1 9 9 – 1 2 0 3
1199

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The solution was again filtered and concentrated in a rotary
evaporator. The concentrated solution was evaporated to
dryness in a vacuum drying oven (0.1 mmHg and 80 1C),
yielding 5.18 g (99%) of a white solid of 1c. The ionic liquid 1b
was prepared by the same procedure.
Scheme 2
Ionic liquid 1a. mp: 142–144 1C. dH (300 MHz, CDCl3) 0.96
(18 H, t, J ¼ 6.8 Hz), 1.30–1.45 (18 H, m), 1.74–1.78 (6 H, m),
3.06–3.12 (6 H, m) and 3.13–3.38 (6 H, m). dC (100 MHz,
CDCl3) 12.98, 19.35, 28.88, 48.82 and 163.34. Calc. C, 69.52;
H, 12.51; N, 9.73; Cl, 8.24%; Found C, 69.23; H, 12.85; N,
cetyltrimethylammonium bromide (CTAB) in the respective
molar composition 0.9 : 0.1 : 114 : 8 : 0.12 (Scheme 2). The
resulting mixture was heated to 80 1C for 48 h and a white
precipitate obtained after filtration. The material was washed
with acidic ethanol repeatedly and then extracted in a Soxhlet’s
extractor by ethanol for 48 h to remove the surfactant. The
material was finally dried at 110 1C for 24 h. The incorporation
of the organic entities within the silica network was monitored
by FT-IR and 13C CP-MAS spectroscopy and quantified by
elemental analysis. The obtained mesoporous silicas, contain-
ing covalently-linked guanidinium ionic liquid, had an average
pore diameter of 3.6 nm, and whose surface area and porous
volume (determined by nitrogen adsorption) were 627.5 m2 gꢀ1
and 0.57 mL gꢀ1 respectively. FTIR (diffuse reflectance) n
9.70; Cl, 8.22%. IR (KBr, n) 1540 cmꢀ1 (C N).
Q
Ionic liquid 1b. mp: 132 1C. dH (300 MHz, CDCl3) 0.96 (18
H, t), 1.30–1.44 (18 H, m), 1.73–1.78 (6 H, m), 3.06–3.12 (6 H,
m) and 3.13–3.38 (6 H, m). Calc. C, 63.04; H, 11.35; N, 8.83;
Br, 16.78%; Found C, 62.95; H, 11.40; N, 8.95; Br, 16.70%. IR
(KBr, n) 1540 cmꢀ1 (C N).
Q
Ionic liquid 1c. mp: 138 1C. dH (300 MHz, CDCl3) 0.96 (18
H, t), 1.30–1.45 (18 H, m), 1.74–1.78 (6 H, m), 3.06–3.12 (6 H,
m) and 3.13–3.38 (6 H, m). Calc. C, 57.37; H, 10.33; N, 8.03; I,
24.27%; Found C, 57.26; H, 10.22; N, 8.15; I, 24.37%. IR
Si–O–Si 1088.7, n C N 1546.0 (guanidinium) and n C–H
Q
2938.4. Found C, 18.81; H, 5.67; N, 2.57; Cl, 3.93%. Active site
(KBr, n) 1540 cmꢀ1 (C N).
Q
concentration ¼ 1.12 mmol guanidinium gꢀ1 of support.
Ionic liquid 1d. dH (300 MHz, CDCl3) 0.89 (6 H, t), 1.17–1.66
(20 H, m) and 3.20–3.42 (12 H, m). IR (KBr, n) 1540 cmꢀ1
2.3 General procedure for the homogeneous process
(C N). MS m/z 284 (A1, 100%). Calc. C, 63.85; H, 11.89;
Q
A mixture of propylene oxide (5.8 g, 100 mmol) and hexabu-
tylguanidinium halide (0.5–1.5 mmol) was charged in a 100 mL
high pressure stainless steel reactor, an excess of CO2 with
respect to the epoxide was introduced (2.5–4.5 MPa), and the
mixture heated at 90–120 1C for 2–6 h. The reaction mixture
was then cooled to room temperature and the product purified
by distillation.
N, 13.15; Cl, 11.11%; Found C, 63.55; H, 11.95; N, 13.04;
Cl, 11.46%.
Ionic liquid 1e. dH (300 MHz, CDCl3) 0.96 (6 H, t), 1.37 (4 H,
m), 1.54 (4 H, m) and 3.09–3.23 (16 H). IR (KBr, n) 1540 cmꢀ1
(C N). MS m/z 228 (A1, 100%). Calc. C, 59.20; H, 11.39; N,
Q
15.94; Cl, 13.47%; Found C, 59.05; H, 11.43; N, 15.82; Cl,
13.70%.
2.4 General procedure for the PBGSiCl catalytic process
Ionic liquid 2a. dH (300.1 MHz, CDCl3) 3.87 (4 H, s, N–CH2
–CH –N), 3.28 (2 H, t, J ¼ 7.4 Hz, –CH –N ), 3.03 (9 H, s,
A mixture of epoxide (100 mmol) and PBGSiCl (1.34 g, 1.5
mmol catalyst) was charged in a 100 mL high pressure stainless
steel reactor, an excess of CO2 with respect to the epoxide
introduced (0.1 or 4.5 MPa), and the mixture heated to 120 1C
for 2–20 h. The reaction mixture was then cooled to room
temperature and ethyl acetate added. The catalyst was re-
moved by filtration and the products purified by a silica gel
column using hexane/ethyl acetate (v/v, 1 : 5). All products
Q
2
2
2
CH –N C(NCH ) ), 1.55–1.65 (2 H, m, –CH –), 1.25–1.35 (2
3
Q
3 2
H, m, –CH2–) and 0.90 (3 H, t, J ¼ 7.3 Hz, CH3–C). dC (75.4
MHz, CDCl3) 164.5, 53.2, 50.4, 38.9, 37.7, 30.1, 20.0 and 14.1.
MS m/z 184.2 (A1, 100%) (A1 ¼ [BuMeN1 C(CH N
Q
3
CH2)2]).
1
were characterized by H NMR, IR and elemental analysis.
Ionic liquid 2b. dH (300.1 MHz, CDCl3) 3.79 (4 H, s, N–CH2
–CH –N), 3.29 (2 H, t, J ¼ 7.5 Hz, –CH –N ), 3.04 (9 H, s,
Q
2
2
2
CH –N C(NCH ) ), 1.62–1.66 (2 H, m, –CH –), 1.31–1.36 (2
Q
3
3 2
2.5 General procedure for the blank experiment
H, m, –CH2–) and 0.95 (3 H, t, J ¼ 7.3 Hz, CH3–C). MS m/z
184.2 (A1, 100%) (A1 ¼ [BuMeN1 C(CH NCH ) ]). IR
A mixture of propylene oxide (100 mmol) and silica gel
(purchased from Aldrich, surface area 500 m2 ꢀ1, average
g
Q
3
2 2
(KBr, n) 1619 (N C) and 2961 (CH –N).
Q
3
pore diameter 6 nm, containing many silanol groups on the
walls of the pores) (1.34 g) was charged in a 100 mL high
pressure stainless steel reactor, an excess of CO2 with respect to
the epoxide introduced (4.5 MPa), and the mixture heated at
120 1C for 4 h. The reaction mixture was then cooled to room
temperature and ethyl acetate added. The silica gel was re-
moved by filtration and the product obtained after distilling
the solvent.
Ionic liquid 2c. dH (300.1 MHz, CDCl3) 3.77 (4 H, s, N–CH2–
CH –N), 3.28 (2 H, t, J ¼ 7.5 Hz, –CH –N ), 3.03 (9 H, s,
Q
2
2
CH –N C(NCH ) ), 1.59–1.69 (2 H, m, –CH –), 1.28–1.38 (2
Q
3
3 2
2
H, m, –CH2–) and 0.96 (3 H, t, J ¼ 7.3 Hz, CH3–C). MS m/z
184.2 (A1, 100%) (A1 ¼ [BuMeN1 C(CH NCH ) ]). IR
Q
3
2 2
(KBr, n) 1619 (N C) and 2961 (CH –N).
Q
3
2.2 The synthesis of PBGSiCl
3. Results and discussion
The triethoxysilylated pentabutylpropylguanidinium chloride
was prepared according to the literature.14 The silica material
containing covalently-linked pentabutylpropylguanidinium
halides were prepared from alkaline mixtures containing tetra-
ethoxysilane (TEOS), triethoxysilylated pentabutylpropylgua-
nidinium chloride, water, ammonia and the surfactant
3.1 The effect of reaction parameters on the cycloaddition of
CO2 to propylene oxide with homogeneous guanidinium salts
At first, a variety of homogeneous catalysts of guanidinium
salts (Scheme 1) were prepared. Their catalytic activities for the
cycloaddition of CO2 to propylene oxide were investigated to
1200
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 1 9 9 – 1 2 0 3

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Table 1 The effect of the structure of guanidinium salts on the
cycloaddition of CO2 to propylene oxide with homogeneous HBGCla
The effect of the amount of 1a on the reaction was also
investigated. The results (Table 2, entries 1–4) indicate that the
yield was improved by the increase in the amount of catalyst.
The yield is only 80.5% when the catalyst amount is 0.5 mmol
(0.5% mol to substrate), and it can be up to 100% using 1.5
mmol catalyst under otherwise the same conditions. To our
knowledge, this is the best result obtained when using non-
metal Lewis acid species to catalyze this reaction. Kawanami
et al. reported that 1-octyl-3-methylimidazolium tetrafluoro-
borate can catalyze quantitatively the coupling of CO2 to
propylene carbonate in 5 min at 100 1C by applying super-
critical CO2 (14 MPa); however, only 71% yield and 78%
selectivity were attained under 6 MPa at 100 1C after 2 h.7c
Entry
Ionic liquid
Anion
Time/h
Yield (%)b
1
2
3
4
5
6
7
8
a
1a
1b
1c
1d
1e
2a
2b
2c
Cl
Br
I
3
3
3
3
3
6
6
6
100
92
90
85
80
95
99
55
Cl
Cl
I
BF4
PF6
Reaction conditions: Propylene oxide 100 mmol, ionic liquid 1.5%
mmol to the substrate, temperature 110 1C, CO2 pressure 4.5 MPa.
b
Isolated yield; the selectivity is 499% based on GC.
3.2 The cycloaddition of CO2 with epoxides to give cyclic
carbonates catalyzed by PBGSiCl catalyst
obtain structural information on the grafting of the guanidi-
nium units. As shown in Table 1, the cations and anions of the
guanidinium units have a strong effect on catalytic activities.
Essentially, the halide anions show a considerable activity
among the anions, probably because halide anions exhibit a
moderate nucleophilicity and high leaving ability. The order of
the activity of hexabutylguanidinium halide salts was found to
be Clꢀ 4 Brꢀ 4 Iꢀ (Table 1, entries 1–3). It is known that the
nucleophilicity of an anion depends on the solvent used, the
order of the nucleophilicity of halides in SN2-type reactions in
aprotic solvent being Clꢀ 4 Brꢀ 4 Iꢀ.15 Thus, the order of
activity of halide anions is consistent with the order of nucleo-
philicity. In this case, the propylene oxide and carbonate act as
an aprotic solvent. Among the various guanidinium salts,
hexabutylguanidinium chloride, 1a, shows the highest activity.
This could be ascribed to its structural features. Two refer-
ences14,16 have reported the structural features of 1a based on
1H, 13C and 15N NMR spectroscopic studies, suggesting the
existence of an electron-deficient state at the central carbon
atom surrounded by the three nitrogen atoms. It is postulated
that the epoxide is activated along the electron-deficient gua-
nidinium central carbon axis, not unlike Lewis acid activation.
On the other hand, the bulkiness of hexabutylguanidinium ion
makes the electrostatic interaction between the cation and the
anion weaker, which renders the counter anion more nucleo-
philic.
The PBGSiCl catalyst was prepared by the direct hydrolysis–
polycondensation of TEOS with triethoxysilylated pentabutyl-
propylguanidinium chloride under alkaline conditions. The
optimum homogeneous reaction conditions were used for the
cycloaddition of CO2 with propylene oxide using PBGSiCl as
catalyst. As shown in Table 3, after being supported, the
catalytic activity of hexaalkylguanidinium halide made almost
no obvious change (Table 2, entry 1; Table 3, entry 2).
However in 2003, Barbarini et al.11 reported that the reactivity
of guanidine catalyst supported on MCM-41 was very much
lower than that of homogeneous guanidine catalyst. This may
be attributed to the high activity of hexaalkylguanidinium
halide and a different catalytic mechanism.
A series of epoxide substrates were examined for the synth-
esis of the corresponding carbonates in the presence of
PBGSiCl catalyst at 120 1C, 4.5 MPa and without any solvent
(Table 3, entries 2–5, 8–10). All the corresponding carbonates
were successfully synthesized from each epoxide in good yield
and with excellent selectivity, except for the cyclohexene oxide.
In order to investigate the effect of the silanol groups on the
silica gel, a blank experiment was carried out under the same
conditions. The result (entry 1) shows that no cyclocarbonate
was obtained when using the unfunctionalised silica molecular
sieve as the catalyst, demonstrating that the observed catalysis
is due to the hexaalkylguanidinium chloride linked onto the
Besides the effect of the guanidinium’s structure, the influ-
ence of several parameters have been studied with catalyst 1a,
so as to optimize the performance of the guanidinium salt’s
catalytic system. As shown in Table 2, the catalytic system is
quite sensitive to reaction temperature and pressure; when the
temperature was decreased from 120 1C to 90 1C and the CO2
initial pressure was decreased from 4.5 MPa to 2.5 MPa, the
yields were decreased (entries 3, 5–8). Hence, 120 1C and 4.5
MPa were the optimum reaction temperature and pressure
respectively.
Table 3 The cycloaddition of CO2 with epoxides to give cyclic
carbonates, catalyzed by silica-supported hexaalkylguanidinium chlo-
ridea
Entry
1d
Substrate
Time/h
4
Yield (%)b
—
Selectivity (%)c
—
2
3
4
4
2
4
100
92
499
499
499
94
Table 2 The effect of reaction parameters on the cycloaddition of CO2
to propylene oxide with homogeneous HBGCla
5
4
95
499
6e
4
20
65
85
499
Catalyst
Entry amount/mmol MPa
Pressure/ Temperature/ Time/ Yield
(%)b
7e
499
1C
h
8
9
4
95
499
1
2
3
4
5
6
7
8
a
1.5
1.5
1.0
0.5
1.0
1.0
1.0
1.0
4.5
4.5
4.5
4.5
4.5
4.5
4.5
2.5
110
110
110
110
120
90
3
2
3
3
3
6
3
3
100
94.5
95.3
80.5
100
4
4
95
57
499
499
10
85.5
91.5
90.0
a
Unless otherwise indicated, the reaction conditions are as follows:
Oxides 100 mmol, PBGSiCl 1.34 g (1.5 mmol catalyst), 1.5% mmol to
100
110
b
the substrate, temperature 120 1C, CO2 pressure 4.5 MPa. Isolated
yield. The selectivity is 499% based on GC. Using the unfunctio-
nalised silica as catalyst. The pressure of carbon dioxide is 0.1 MPa.
b
c
d
Reaction conditions: Propylene oxide 100 mmol. Isolated yield; the
selectivity is 499% based on GC.
e
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 1 9 9 – 1 2 0 3
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Table 4 Catalyst recycling studies in the coupling of carbon dioxide
with propylene oxidea
Entry
Recycle no.
Yield (%)b
Selectivity (%)c
1
2
3
4
5
6
a
Fresh
100
100
98
499
499
499
499
499
499
1
2
3
4
5
99
98
97
Reaction conditions: Propylene oxide 0.1 mol, PBGSiCl 1.34 g,
b
temperature 120 1C, time 4 h, pressure 4.5 MPa. Isolated yield.
Based on GC.
c
silica and not to the silanol group. It is quite noteworthy that
the PBGSiCl catalyst is also very efficient for the cycloaddition
of styrene oxide and carbon dioxide under these mild condi-
tions without any solvent, and that the yield is up to 95%
(entry 5). It is well known that styrene oxide, due to the low
reactivity of its b-carbon, has a lower activity compared to
propylene oxide. Furthermore, decreasing the CO2 pressure
down to 0.1 MPa at a constant temperature of 120 1C and time
of 4 h leads to a remarkable decrease in yield, down to 65%
(entry 6). If we prolong the reaction time to 20 h, we can also
obtain satisfactory yields up to 85% (entry 7).
Scheme 3 A proposed mechanism for the addition of CO2 to epoxide,
catalyzed by HBGCl.
3.3 The possibility of recycling PBGSiCl catalyst
cussion, the mechanism for the cycloaddition of epoxides to
CO2, catalyzed by HBGCl, has been postulated in Scheme 3.
The following steps are involved: Firstly, the epoxide is co-
ordinated to the Lewis acid site (the central carbon) of
hexabutylguanidinium chloride to form complex A, the Clꢀ
anion then preferentially makes a nucleophilic attack on the
lesser substituted site of the coordinated epoxide; this is
followed by ring opening, producing an oxy-anion species B;
next, a CO2 molecule is coordinated to the complex through
interaction of the Oꢀ and C1, resulting in the formation of C
and D in equilibrium; finally, the cyclocarbonate is produced
by intramolecular cyclic elimination, releasing the catalyst for
recycling.
One of our greatest motivations is to get a better catalyst
separation and make the catalyst recovery simpler. The effi-
ciency of PBGSiCl catalyst in the cycloaddition of CO2 to
propylene oxide is shown in Table 4 by the possibility of
recycling the catalyst for a further 5 cycles, without there being
any obvious decrease in catalytic activity after simple filtration,
washing with ethyl acetate and immediate reuse. In 1998,
Sheldon et al. described that the catalysis observed during
some heterogeneous liquid oxidation processes was due to
homogeneous catalysis by small amounts of leached catalyst.17
In order to verify whether there is a similar phenomena
occurring during our process, we undertook the same test by
filtering the catalyst at reaction temperature, washing it with
hot ethyl acetate and then immediately reusing it. The results
were consistent with our initial results, which further proved
that heterogeneous hexaalkylguanidinium chloride was res-
ponsible for the observed catalysis in the present study.
4. Conclusion
We have demonstrated that under mild reaction conditions (4.5
MPa, 120 1C, 4 h) both homogeneous and a silica-supported
hexaalkylguanidinium chloride were effective catalysts for
fixation of CO2 to carbonate without solvent, and that the
silica-supported catalyst had the great advantage of being
easily recycled at least 5 times without any obvious decrease
in its catalytic activity, after simple filtration. The possible
mechanism was also discussed. We realized that the hexaalk-
ylguanidinium chloride acted as a bi-functional catalyst during
this transformation due to its strong steric and electrophilic
effects.
3.4 Mechanism consideration
Taking account of the reaction mechanism for the cycloaddi-
tion of CO2 with epoxides to give cyclic carbonates catalyzed
by an onium salt catalyst, i.e. tetrabutylammonium bromide
salt, it has been proposed that this reaction involves the ring
opening of the epoxide by means of a nucleophilic attack by the
anion, yielding an oxy-anion species. This then reacts with CO2
and the resulting cyclic carbonate subsequently cyclizes so that
the catalytic activity is mostly based on the nucleophilicity and
leaving ability of the anion.7f Considering the high catalytic
activity of HBGCl during this transformation and the special
structure of it, we can speculate that its better performance
could be ascribed to the cooperation of the nucleophilic attack
of the anion (Lewis base), and the Lewis acid activation of the
hexabutylguanidinium cation to the epoxide. This is a similar
catalytic mechanism to that of metal complex cata-
lysts,3a,5,6,7e,18 and it has been proposed that this reaction
promotes both Lewis base activation of CO2 for the nucleo-
philic attack at epoxide or the Lewis base nucleophilic attack at
epoxide directly and the Lewis acid activation of epoxide. It is
also called a ‘‘bi-functional catalyst system’’. Based on the
experimental results obtained and the above mentioned dis-
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N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 1 9 9 – 1 2 0 3
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