DBU/benzyl bromide: An efficient catalytic system for the chemical fixation of CO2 into cyclic carbonates under metal- and solvent-free conditions

Article abstract of DOI:10.1039/c5cy01892g

A simple, mild, and inexpensive catalytic system consisting of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and benzyl bromide was developed for the cycloaddition of epoxides with ambient CO₂ under metal-free and solvent-free conditions. Moreover,

Full text of DOI:10.1039/c5cy01892g

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ARTICLE
Organic base/benzyl bromide: an efficient catalytic system for
chemical fixation of CO into cyclic carbonates under metal- and
solvent-free conditions
2
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
Lin Wang, Koichi Kodama and Takuji Hirose *
DOI: 10.1039/x0xx00000x
www.rsc.org/
A simple, mild, and inexpensive catalytic system consisting of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and benzyl
bromide was developed for the cycloaddition of epoxides with ambient CO
Moreover, a wide range of epoxides was converted to the corresponding cyclic carbonates with good-to-excellent yields
81-95%).
2
under metal-free and solvent-free conditions.
(
1
quaternary ammonium or phosphonium salts, and N-
0
11
1
2
Introduction
heterocyclic carbenes have been investigated as both single
catalysts and co-catalysts to promote the cycloaddition of
The exhaustion of carbon-based feedstocks from fossil fuels
1
will occur in decades to come. Thus, employing renewable
epoxides to CO . However, these systems often suffer from
2
low catalyst activity, the presence of solvents and toxic metals,
the need for harsh reaction conditions, and/or expensive
multistep catalyst preparation.
resources is a key technology for a sustainable society. CO
fixation and conversion techniques show great promise for the
recycling of CO into valuable chemicals due to the fact that
CO is an abundant, sustainable, nontoxic, and renewable C1
2
2
To overcome the thermodynamics of CO , the use of organic
2
2
2
bases has recently emerged. Among these materials,
superbases, which provide new opportunities for CO2
activation, are attracting ever-increasing attention in green
feedstock for organic synthesis. However, many challenges
must be overcome to increase its value as a chemical
feedstock, because the inherent kinetic and thermodynamic
1
3,14
chemistry.
conversion of CO
by the groups of Shi (T= 100-120
Organic bases have recently been used in the
2
stability of CO hampers the development of high-performance
3
2
into cyclic carbonates works such as those
catalysts that effect CO
2
activation and transformation.
1
4a,b
°
C, pCO = 35 atm),
Zhang
T= 120 °C, pCO = 20 atm), and Chung (T= 80 °C, pCO = 1
2
4e
2
The development of an efficient chemical process for the
chemical fixation of this greenhouse gas into value-added
organic chemicals under mild, metal-free reaction conditions is
currently a very attractive prospect from the viewpoint of
1
4d
(
2
1
atm) . However, high CO pressures or metallic ions are still
2
required. Thus, metal-free synthetic methods using CO under
2
low pressure (optimally at 1 atm) are still highly desired.
Recently, we reported the efficient benzyl bromide/DMF-
sustainability and green chemistry. Many efficient methods for
4
have emerged, but the atom-efficient
transformation of CO
addition of CO
2
catalyzed conversion of CO and epoxides to cyclic carbonates
2
2
to epoxides to form cyclic carbonates is one of
1
5
under mild and metal-free conditions (1 atm, 120 °C), but to
realize the reaction at low temperature is still a great
the most effective strategies. Cyclic carbonates are widely
applied in industry and commerce. They are used as
challenge.
monomers
in
polymerization
reactions
(producing
Herein, we present a one-pot process for the combination of
benzyl halides and organic bases as catalytic systems for the
polycarbonates and polyurethanes), green polar aprotic
solvents, fuel additives, constituents of oils and paints, and
electrolytes in lithium-ion batteries. They are also important
cycloaddition of epoxides to ambient CO . The reaction affords
2
5
five-membered cyclic carbonates without the need for organic
solvents or metal catalysts. This procedure has a broad
substrate scope, and represents an efficient, simple, mild, and
cost-effective route to the synthesis of cyclic carbonates from
CO2.
intermediates for pharmaceuticals in biomedical applications.
6
Many types of catalysts, including alkali metal halides,
8
metalloporphyrin or metallosalen complexes, ionic liquids,
7
9
a.
School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura-
ku, Saitama 338-8570,Japan. E-mail: hirose@apc.saitama-u.ac.jp.
Footnotes relaꢀng to the title and/or authors should appear here.
Results and discussion
†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
To investigate the effects of organic bases and halides on
the CO
2
conversion, several typical organic bases (Scheme S1,
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Journal Name
see ESI) and halides were selected and explored. The bases times of 22 and 24 h give almost the same yield of 95%, but a
were
1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU),
1,5- slightly lower yield of 89% is observed when the reaction time
1,4- is 20 h (Fig. 1; entries 3, 8, and 9, Table 1). Hence, 22 h was
N,N- chosen as the optimum reaction time. Moreover, the
diazabicyclo[4.3.0]non-5-ene
diazabicyclo[2.2.2]octane
(DBN),
(DABCO),
dimethylaminopyridine (DMAP), pyridine (Py), triethylamine selectivity of the reaction for the desired product remained
(TEA), imidazole (Im), and N-methylimidazole (MIm).
above 99% throughout.
a
Table 1 Optimization of reaction conditions
O
1
00
80
O
Catalyst
O
O
Cl
+
1 atm CO
2
Solvent free
Cl
b
Entry
Catalytic system (mol)
5% DBU
T(°C) Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
65
65
65
65
65
45
25
65
65
24
24
24
24
24
24
24
22
20
48
0
6
4
2
0
0
0
0
5% PhCH
5% PhCH
5% PhCH
2
2
2
Br
Br + 5% DBU
95
96
64
85
54
95
89
Br + 10% DBU
Br + 5% DBU
2.5% PhCH
2
5% PhCH
5% PhCH
5% PhCH
5% PhCH
2
2
2
2
Br + 5% DBU
Br + 5% DBU
Br + 5% DBU
Br + 5% DBU
0
4
8
12
16
20
24
Time (h)
a
b
Reaction conditions: epichlorohydrin (6 mmol), CO
2
(99.999%, balloon).
a
Isolated yield
Fig. 1 Dependence of product yield on reaction time
a
Reaction conditions: epichlorohydrin (6 mmol), PhCH
0.3 mmol), CO (99.999%, balloon), isolated yield.
2
Br (0.3 mmol), DBU
First, the superbase DBU was employed in combination with
benzyl bromide to systematically study the effect of various
(
2
a
parameters on the model reaction of epichlorohydrin and CO
2
Table 2 The effect of organic bases on the cycloaddition
b
c
d
to produce the cyclic carbonate (chloromethyl)ethylene
carbonate. The corresponding results are summarized in Table
Entry
Base
pKa
T(°C)
65
65
65
65
65
65
65
Yield (%)
TON
1
2
3
4
5
6
7
8
DBU
DBN
DMAP
DABCO
Pyridine 12.5
TEA
MIm
Im
24.3
23.7
18.0
(8.7)
95
91
83
54
85
84
81
75
19.0
18.2
16.6
10.8
17.0
16.8
16.2
15.0
1. At ambient CO
2
pressure, 5 mol% DBU alone afforded the
corresponding cyclic carbonate in 48% yield (entry 1, Table 1),
which might be due to the nucleophilic capability of DBU and
1
6
2
its CO activation effect. When only benzyl bromide was used
18.8
(7.1)
(7.0)
as the catalyst, negligible yield was observed (entry 2).
However, a combination of DBU and benzyl bromide realized a
significant enhancement in catalytic activity (95%, entry 3)
compared to that of each component individually, indicating
the highly positive synergistic effect of DBU and benzyl
bromide. No increase in the product yield was observed upon
increasing the amount of DBU from 5 to 10 mol% (entries 3
and 4). However, an obvious decrease in the yield was
observed when the amount of benzyl bromide decreased to
65
a
Reaction conditions: epichlorohydrin (6 mmol), PhCH Br (0.3 mmol, 5
b
(99.999%, balloon). The pKa value of the conjugated
2
mol%), time 22h, CO
2
1
4d,g c
acid in acetonitrile or water (in parentheses)
d
runs). TON: turnover number (in 22 hours).
Isolated yield (average of
2
Table 2 shows the effects of the different organic bases
employed on the reaction in the presence of 5 mol% benzyl
bromide. It can be seen that DBU is the most effective among
the organic bases (entry 1, Table 2). Heldebrant et al. reported
that organic bases with a lower pKa value possess a lower
2
.5 mol% (64%, entry 5), indicating that 5 mol% of DBU/benzyl
bromide was the optimal catalyst loading. Additionally, upon
lowering the temperature to 45 C, a moderate yield (85%,
entry 6) was obtained. However, further decrease of the
temperature from 45 C to 25 C leads to an unsatisfactory
C was selected as the optimal
°
1
6a
negative Gibbs free energy to activate CO
the ability of the organic bases to form carbamate salts with
CO is in accordance with the order of the pKa values. Notably,
2
.
Consequently,
°
°
yield (54%, entry 7). Thus, 65
°
temperature.
2
the results in Table 2 are largely consistent with this order.
However, the presence of DABCO affords an unsatisfactory
result (entry 4), which might be related to its high steric
hindrance due to its rigid spherical shape. The results indicate
that the strength of the base is a key factor for its catalytic
activity in the reaction, and that steric hindrance of the base is
also an important factor.
In addition, the time dependence of product yield was also
evaluated. As shown in Fig. 1, it can be seen that reaction time
has a significant influence on the reaction yield, and that the
cycloaddition reaction proceeds rapidly within the first 10 h,
during which time the product yield increases from 0 to 74%.
Further extending the reaction time results in relatively little
improvement, because the concentration of starting materials
decreases gradually with reaction time. As a result, reaction
2
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Based upon previous mechanistic studies on related excess amount of PhCH
ARTICLE
2
Br (7.5 mol%; DBU: 5 mol%), no
1
4a,17,19
systems
and the above results, we propose a plausible significant increase in the reaction rate was observed. This
mechanism, as shown in Scheme 1. It is known that benzyl further indicates the important role of the initially formed
+
-
halides can react with organic bases easily to form quaternary complex of DBU and PhCH
onium salts. In the present DBU/PhCH Br system, the cycle, that is, the ring-opening reaction is through nucleophilic
) is initially formed and attack of its bromide ion on the epoxide rather than sole
2
Br (Bn-DBU Br (A)) in the catalytic
2
+
-
aminidinium salt Bn-DBU Br
(A
catalyzes the ring-opening reaction through nucleophilic attack PhCH
2
Br.
of its bromide ion at the less sterically hindered β-carbon atom
of the epoxide, which results in oxy anion species
Simultaneously, DBU coordinates reversibly with CO
B.
1
4f
60
2
,
affording carbamate salt
Subsequently, nucleophilic attack by
alkylcarbonate anion Finally, the ring-closing of
intermediate affords the corresponding cyclic carbonate and
regenerates the catalyst; thus the catalytic cycle is completed.
According to this mechanism, when salt is formed from
DABCO and CO , the approach of should be hindered due to
C
, affording an activated form of CO
2
.
2.5 mol% A
5
0
5
mol%
B
B
on forms
C
10 mol% C
40
D
.
D
30
C
20
2
B
10
its ball-like shape, leading to the lower yield observed in this
system.
0
0
20
40
60
80
100 120 140 160 180 200
Reaction Time (min)
DBU
PhCH
2
Br
+
a
Fig. 2 Effect of DBU concentration on the reaction .
O
O
O
a
O
O
Reactions were carried out under
1
2
atm CO without solvent.
R
R
o
Br 5 mol%, T = 65 C, isolated yield. Line A:
Epichlorohydrin 6 mmol, PhCH
2
R
2
n[DBU] = 2.5 mol%, y = 0.1893x, R = 0.9997; line B: n[DBU] = 5 mol%, y =
2
0.2569x, R = 0.9988; line C: n[DBU] = 10 mol%, y = 0.2810x, R = 0.9986.
2
O
C
Bn
Bn
O
O
O
N
N
N
N
Br
N
N
O
R
A
50
Br
R
D
2.5 mol% A
5
mol%
B
Br
40
O
CO
2
7.5 mol% C
Bn
DBU
N
N
30
B
Scheme 1. Proposed mechanism for the coupling reaction of
epoxide with CO catalyzed by PhCH Br and DBU.
20
2
2
10
To investigate the reaction mechanism in detail, kinetic
studies of the catalytic conversion of CO and epichlorohydrin
to cyclic carbonate were carried out using PhCH Br and DBU as
0
2
2
-20
0
20
40
60
80 100 120 140 160 180 200
co-catalysts. As shown in Fig. 1, the product yield and reaction
time almost have a linear relationship in the first 4 hours. Thus,
Reaction Time (min)
a
the first 3 hours were chosen to investigate the reaction. The Fig. 3 Effect of DBU concentration on the reaction .
a
resulting kinetic curves of three different concentrations of
Reactions were carried out under 1 atm CO
2
without solvent. Epichlorohydrin 6
DBU on the reaction are shown in Fig. 2. Notably, the mmol, DBU 5 mol%, T = 65 C, isolated yield. Line A: n[PhCH Br] = 2.5 mol%, y =
concentration of DBU has a positive effect on the reaction rate, 0.1422x, R2 = 0.9952; line B: n[PhCH Br] = 5 mol%, y = 0.2569x, R2 = 0.9988; line
indicating the important step of DBU coordinates reversibly C: n[PhCH Br] = 7.5 mol%, y = 0.2499x, R2 = 0.9971.
with CO to afford carbamate salt in the proposed
mechanism (Scheme 1). However, when using the excess
amount of DBU (10 mol%; PhCH Br: 5 mol%), no drastic above mechanism indicate that the role of the halide anion is
increase in the reaction rate was observed (line B and line C). important to the reaction rate. It is known that the order of
o
2
2
2
2
C
The present results (entries 1, 3, and 5, Table 1) and the
2
-
-
This behaviour may indirectly indicate that the step of forming nucleophilicity is Cl > Br , while the order of leaving ability is
-
-
CO
determining step in the proposed mechanism. Additionally, is important. The proposed mechanism also indicates that the
the kinetic curves of three different concentrations of PhCH Br nucleophilicity of and the leaving ability of halide anions
on the reaction were also investigated (Fig. 3). It can be clearly from are important for the catalytic activity. Hence, we
seen from Fig. 3 that the concentration of PhCH Br plays a investigated the effect of the halide anions on the catalytic
significant role in the reaction. However, when using the activity, and the results are summarized in Table 3.
2
activated species by DBU (C, Scheme 1) is not the rate- Br > Cl . Thus, the balance between the two conflicting factors
2
A
D
2
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ARTICLE
Table 3 The effect of halides on the cycloaddition
Journal Name
a
with CO
2
poorly due to its steric hindrance, was converted to
C, which
b
d
Entry Halide
Yield %
95
71
92
83
TON
19.0
14.2
18.4
16.6
17.2
13.6
5.4
the corresponding carbonate 9b in 31% yield at 95 °
was confirmed exclusively the cis isomer by H NMR and
1
13
C
1
2
3
4
5
6
7
8
PhCH
2
Br
1
8
PhCH Cl
NMR spectra.
2
t
p- BuPhCH
2
Br
4-Nitrobenzyl bromide
α-Bromodiphenylmethane
CH CH CH CH Br
Table 4 Cycloaddition of CO
a
PhCH Br/DBU
2
2
to various epoxides catalyzed by
86
68
3
3
2
2
2
2
2
2
O
CH
CH
CH
CH
I
27
5
% PhCH
2
Br / DBU
O
c
1 atm CO2
+
O
O
nBu
4
NBr
72 (58)
14.4
6
5oC, Solvent-free
R
a
Reaction conditions: epichlorohydrin (6 mmol), DBU (0.3 mmol, 5 mol%),
b
C, time 22h. Isolated yield (average of 2
R
1
a - 10a
1b - 10b
CO
2
(99.999%, balloon), T= 65
°
O
O
O
O
c
runs). without of DBU. TON: turnover number (in 22 hours).
d
O
O
O
O
O
O
O
O
-
-
CH3
3
CH
3
5
It can be seen that the activity increased in the order Cl < Br
entries 1 and 2, Table 3). Additionally, the benzyl bromide
CH
3
Cl
(
d
b
b
c
b
1
b 82
2b 85
3b 55 88
4b 95
derivatives with an electron-donating group gave better
results than those with an electron-withdrawing group (entries
O
O
O
O
O
O
O
O
O
O
O
O
3
, 4, and 5), presumably due to electron-withdrawing groups
O
O
2
b
Ph
Ph
decreasing the leaving ability of the bromide anion from the
aminidinium center. Notably, in the presence of benzyl
b
b
b
5
b 81
6b 85
O
7b 94
O
8b 82
O
bromide, the reaction yield was better than those in the
t
presence of p- BuPhCH Br and α-bromodiphenylmethane
O
O
O
O
O
O
O
O
O
2
O
O
(entries 1, 3, and 5), indicating that the bulkiness of the
Ph
Ph
b
Ph
e
b
formed quaternary onium salts also affected the reaction.
When the arylmethyl halide is replaced by an alkyl halide, the
yield decreases significantly (entries 6 and 7). For comparison
with the ammonium salts formed using arylmethylbromide
(R)-8b 83
(98% ee)
(R)-10b 89
(84% ee)
(R)-10b 35
c
cis-9b 31
(99% ee)
a
Reaction conditions: epoxide (6mmol), binary catalyst (5 mol%), CO
b
99.999%, balloon), reaction time 22 h, isolated yield. T = 65
2
c
C. T = 95
(
d
°
°
C.
derivatives, nBu NBr was employed as the bromide anion
4
e
Experiment reacted in 6M toluene, T = 45
C.
°C. DBU as a sole catalyst, T =
donor under the same conditions as the model reaction (entry
6
5 °
8). However, the yield was significantly lower than that using
benzyl bromide as the bromide anion source, which is
presumably due to the electrostatic interaction between the
bromide anion and the ammonium center decreasing with the
DBU-Bn
Br- exchange
1
7
H
H
O
H
bulkiness of the cation. Thus, the nucleophilicity of the
R
bromide anion is weaker for nBu NBr than the salts (Bn-
4
Br
+
-
H H
O
5% DBU/PhCH
2
Br
DBU Br ).
In order to examine the substrate scope of the present dual
DBU-Bn
Br
H
R
DBU-Bn
(R)
O
O
catalyst system, a wide range of epoxides were investigated
under the optimal reaction conditions (Table 4). The results
show that diverse epoxides are converted to the
corresponding cyclic carbonates under ambient conditions
with good-to-excellent yields. Among them, epichlorohydrin
H
H
H
R
H
H
H
R
Br
Br
DBU-CO
2
DBU-CO2
O
O
O
O
H
O
H
O
H
H
H
4
a
gave the best result, probably due to the electron-
withdrawing properties of the chlorine atom. Similarly, the
epoxides with an oxymethylenemoiety (6a 7a, and 10a, Table
) afforded higher yields than those with alkyl groups. These
R
Br
R
H
Br
,
O
O
4
substituents resulted in facilitated nucleophilic attack at the
epoxide ring carbon atoms. Additionally, the other functional
groups on the epoxide were stable in the reaction, indicating
the outstanding efficiency of the catalytic system. The
O
O
H
O
H
O
H
H
H
R
H
R
rentention (R)
invertion (S)
path A
major
path B
minor
epoxides with large steric hindrance (2a, 3a, and 8a) were also
converted to the corresponding carbonates in good yields. Due Scheme
2
. Possible reaction pathways for the partial
to the higher steric hindrance of epoxide 3a compared to the conversion of configuration.
other epoxides, a higher reaction temperature (95 C) was
necessary to obtain a higher product yield. On the other hand,
cyclohexene oxide, which is known to perform cycloaddition applied in the coupling of CO2 and enantiomerically pure
°
This commercially inexpensive catalytic system can also be
4
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epoxides. (R)-8a was transformed to (R)-8b in 98% 3.5 Hz, 1H). C NMR (125 MHz, CDCl
ARTICLE
1
3
3
): 154.8, 74.7, 67.0, 44.6. All
enantiomeric excess (ee), while (R)-10b was obtained with the data are consistent with that previously reported. See the
some loss in optical purity (84% ee) (Table 4). It can be Supporting Information for characterization data of other
concluded that reaction at the β-carbon of epoxide seems compounds.
favorable (Scheme 2, path A). Epoxide (R)-10a has lower
potential steric hindrance and higher reactivity than (R)-8a
;
Notes and references
thus, as for (R)-10b, the ee loss may be attributed to the
nucleophilic attack on the α-carbon of the epoxide by the
bromide anion, which may also be followed by a bromide-
bromide ion exchange process to eventually afford the (S)-10b
1
E. A. Quadrelli, G. Centi, J.L. Duplan and S. Perathoner,
ChemSusChem, 2011, , 1194.
(a) J. Louie, Curr. Org. Chem., 2005,
4
2
9, 605; (b) B.H. Xu, J. Q.
1
9
Wang, J. Sun, Y. Huang, J. P. Zhang, X. P. Zhang and S. J.
Zhang, Green Chem., 2015, 17, 108; (c) L. J. Murphy, K. N.
Robertson, R. A. Kemp, H. M. Tuononend and J. A. C.
Clyburne, Chem. Commun., 2015, 51, 3942.
product (Scheme 2, path B). When only DBU was used as a
catalyst, (R)-10b was obtained in 35% yield, but with retention
of stereochemistry (>99% ee), which indicates that direct
nucleophilic attack of carbamate salt
C
on the epoxide
3
4
T. Sakakura, J. C. Choi and H. Yasuda, Chem. Rev., 2007, 107,
2
365.
(a) I. Omae, Coord. Chem. Rev., 2012, 256, 1384; (b) X.-B. Liu
and J. Darensbourg, Chem. Soc. Rev., 2012, 41, 1462; (c) C.
proceeds exclusively at the less sterically hindered carbon
1
2a,b
atom.
This further demonstrates the possible role of the
bromide anion in path B for the ring-opening process.
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1
6
Conclusions
5
In summary, a simple, inexpensive and efficient binary
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2
3
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2
epoxides and CO under metal- and solvent-free conditions. A
diverse range of epoxides was transformed into the
corresponding cyclic carbonates with good-to-excellent yields
(
81-95%) under very mild conditions (65
°
C, 1 atm CO
2
).
6
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Furthermore, optically pure epoxides were also investigated,
and a plausible mechanism for the coupling reaction of
epoxide with CO
2
was proposed. The proposed protocol
7
8
provides an example of an easily handled, commercially viable,
and environmentally benign alternative for chemical fixation of
2
CO into 5-membered cyclic carbonates.
Experimental section
9
1
All starting materials and solvents commercially available were
purchased at the highest quality from Sigma-Aldrich or Wako
and used as received unless otherwise indicated. Chemical
1
yields refer to the pure isolated substances. H (500 MHz) and
1
3
C (125 MHz) NMR spectra were obtained using a Brucker AV- 11 (a) Q. W. Song, L. N. He, J. Q. Wang, H. Yasuda and T.
Sakakura, Green Chem., 2013, 15, 110; (b) Y. P. Ren and J. J.
Shim, ChemCatChem, 2013, , 1344.
5
00 (500 MHz) spectrometer. The chemical shifts of the
products were reported in ppm with reference to Me Si as the
internal standard in CDCl solution. Enantiomeric excesses of
5
4
1
1
1
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3
the carbonates were determined by HPLC analyses with a
Daicel Chiralcel OD-3 with detection at 254 nm.
2
009, 48, 4194.
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Representative procedure for the cyclic carbonate formation
(
4b): In a 20 mL two-neck flask, epichlorohydrin 4a (6 mmol, 0.555
g), DBU (5 mol%, 0.046g) and PhCH Br (5 mol%, 0.051g) were added
under N gas and then stirred at 65 C for 22 h under an
atmosphere of CO (99.999%, balloon). After completion, the
reaction mixture was purified by column chromatography
hexane:ethyl acetate = 1:1) to afford the desired cyclic carbonate
4
2
Luo, H. R. Li and S. Dai, Green Chem., 2010, 12, 2019; (c) X.
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Chem., 2014, 16, 3071; (e) U. R. Seo and Y. K. Chung, Adv.
o
2
2
(
0
.778g (yield: 95.0%).
1
3
H NMR (500 MHz, CDCl ): 5.12−5.01 (m, 1H), 4.69−4.55 (m, 1H),
4
.48−4.35 (m, 1H), 3.89 (dd, J = 12.5, 4.0 Hz, 1H), 3.77 (dd, J = 12.5,
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DOI: 10.1039/C5CY01892G
ARTICLE
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DOI: 10.1039/C5CY01892G
Graphical Abstract
Organic base/benzyl bromide: an efficient catalytic system for chemical fixation of CO
carbonates under metal- and solvent-free conditions
2
into cyclic
O
O
5 mol% PhCH Br
2
CO
2
+
O
O
o
5
mol% DBU,65 C
Yield: 81-95%
R
(
1 atm)
R
R: alkyl, aryl
Solvent-free
No metal complexes
Operationally simple
At ambient pressure
A simple, mild and inexpensive catalytic system consisting of 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) and benzyl bromide was
developed for the cycloaddition of epoxides to ambient CO under metal-free and solvent-free conditions. Moreover, a wide range of
2
epoxides was converted to the corresponding cyclic carbonates with good-to-excellent yields (81-95%). The proposed protocol provides an
example of an easily handled, practical, and environmentally benign alternative for chemical fixation of CO2 into 5-membered cyclic
carbonates.