Synthesis of cyclic carbonates from CO2 and epoxides catalyzed by low loadings of benzyl bromide/DMF at ambient pressure

Article abstract of DOI:10.1039/c4cc06791f

An efficient, metal-free catalytic system for the conversion of CO₂ and epoxides to cyclic carbonates under mild conditions with good-to-excellent yields (57-99%) was developed. A possible reaction mechanism involving the electrophilic activation of epoxides by benzyl cations and nucleophilic activation of CO₂ by DMF is proposed. This journal is

Full text of DOI:10.1039/c4cc06791f

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COMMUNICATION
Synthesis of Cyclic Carbonates from CO
2
and Epoxides Catalyzed by
Low Loadings of Benzyl Bromide/DMF at Ambient Pressure
a
a
b
a
a
a
Lin Wang, Li Lin, Guangyou Zhang, Koichi Kodama, Mikio Yasutake, and Takuji Hirose*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
An efficient, metal-free catalytic system for the conversion of
CO and epoxides to cyclic carbonates under mild conditions
and readily available compounds and without employing high
2
50 CO pressure.
2
with good-to-excellent yields (57−99%) was developed. A
possible reaction mechanism involving the electrophilic
activation of epoxides by benzyl cations and nucleophilic
Many organic reactions are known in which a combination of
simple compounds forms a reactive intermediate or a new reagent
like in the Swern oxidation and the Vilsmeier reaction. We
noticed that a benzyloxysulfonium ion is formed from dimethyl
sulfoxide (DMSO) and benzyl halide (BnX) in the Kornblum
1
0
activation of CO
2
by DMF is proposed.
5
6
6
5
0
5
1
4
In the last two decades, much attention has been paid to CO
2
oxidation (Scheme 1). If the benzyl cation could be transferred
to epoxide and allow for the formation of an oxonium ion species,
because it is the largest source of greenhouse gases and its
1
emission should be reduced. Moreover, in recent years, CO
2
has
it would undergo reaction with CO
CO
by N,N-dimethylformamide (DMF),13d,15 a combination of
DMSO and DMF was expected to further enhance the reaction.
2
. Considering the activation of
1
2
2
3
3
4
4
5
0
5
0
5
0
5
been regarded as nontoxic, nonflammable, renewable,
inexpensive, and an abundant C1 feedstock for organic
2
2
synthesis. However, many challenges need to be overcome to
O
S
increase its value as a chemical feedstock because of its low
reactivity and high thermodynamic stability.3
From the standpoint of environmental protection and resource
S
Br
Ph
Br
Ph
O
Scheme 1 Benzyloxysulfonium Ion from BnBr and DMSO in Kornblum
Reaction.
utilization, chemical fixation of CO
organic chemicals under mild reaction conditions is an attractive
approach. Many efficient routes for chemical CO fixation have
been developed.4 As 100% atom-economical conversion, the
cycloaddition of epoxides to CO affording five-membered cyclic
carbonates is one of the most promising pathways. Cyclic 70 halide and a small excess amount of N,N-dimethylformamide
2
to obtain value-added
Herein, we report an easy and mild procedure for the
2
cycloaddition of epoxides to ambient CO
2
affording five-
membered cyclic carbonates using a catalytic amount of benzyl
2
(
DMF) as a recyclable solvent. This procedure has a broad
carbonates are widely used as polar aprotic high-boiling solvents,
precursors of polymers, fuel additives, electrolytes for lithium-ion
batteries, and important intermediates for pharmaceuticals and
substrate scope. The ring opening of epoxide can be attributed to
the electrophilic activation of epoxides by benzyl cations and the
5
fine chemicals. Until now, numerous catalysts including
nucleophilic activation of CO
2
by DMF.
6
7
quaternary ammonium or phosphonium salts, alkali metal
75
The reaction of styrene oxide (SO) to CO was used as the
2
8
9
10
halides, ionic liquids, metalloporphyrin, or metallosalen
model reaction to systematically study the effect of various
parameters on the reaction outcome (Table 1). At ambient CO2
pressure, the reaction in DMSO afforded the corresponding cyclic
carbonate (entry 1, Table 1). Although the yield was low (6%, 24
complexes, and N-heterocyclic carbenes12 have been developed
to facilitate the reaction of an epoxide with CO . However, these
systems, often require additives or cocatalysts and suffer from
11
2
low catalyst reactivity/stability, high catalyst loading, use of toxic 80 h), the promising result encouraged us to further investigate
metals, high cost, and harsh reaction conditions; thus, there are
significant safety considerations, in particular, on an industrial
scale. Recently, on the other hand, many metal-free systems have
been investigated from resource and environmental concern6a,7,9,13
benzyl cation as an important intermediate. Because no reaction
occurred in the absence of BnBr (entry 2), BnBr was confirmed
to be a neutral and mild catalyst for the formation of cyclic
carbonates from epoxides and CO . Recently, the nucleophilic
2
1
3d
8
5
2
activation of CO by DMF was reported and this type of
but most of them were carried out under high CO
2
pressure.
Synthetic methods using CO under low pressure (ideally at 1 atm)
2
activation seemed appropriate for our study. As shown in entry 3,
when DMSO and DMF were used as a mixed solvent system, the
yield improved by about 10 times (59%), indicating that the
1
3
are highly desirable.
Hence, simple, metal-free catalytic systems with enhanced
properties remain to be developed for the efficient conversion of
nucleophilic activation of CO
present system. When only DMF was used as the solvent (entry
), the yield further improved to 73% with no detectable
2
by DMF was also feasible in the
9
0
CO
2
to cyclic carbonates under mild reaction conditions.
with epoxides,
4
Therefore, during our study on the reaction of CO
2
we focused on developing a catalyst system using inexpensive
formation of by-products, indicating that DMF could activate
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[journal], [year], [vol], 00–00 | 1

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CO
BnBr (entry 5) and in toluene (entry 6), revealed that the
cycloaddition of SO to CO proceeded with assistance from BnBr
2
and BnBr in this reaction. The control experiments, without
activating the epoxide and generating a benzyl-oxonium species
(B).
On the other hand, CO
2
is activated by DMF to form C13d,15
2
and DMF. Notably, this change makes it easier to recycle the
solvent given that DMF has a lower boiling point than DMSO.
which then reacts with B to afford intermediate D. According to
50 the mechanism, the efficient ring-opening reaction is attributed to
both the electrophilic activation of epoxide by benzyl cation and
5
Table 1 Influence of Various Parameters on the Cycloaddition of CO
2
to
2
the nucleophilic activation of CO by DMF. Two possible
a
Styrene Oxide (SO).
mechanisms can account for formation of the product. First the
benzyl ether oxygen of D may attack the carbonyl carbon,
55 forming a carbonate oxonium species and liberating DMF. Then a
O
O
2
CO , solvent
+
2
PhCH Br
O
O
10
-
Temperature
bromide ion (Br ) cleaves the O-Bn bond regenerating BnBr.
Ph
-
Ph
Alternatively, the O-Bn bond of D may be cleaved by Br , thus
regenerating BnBr. Simultaneously, an alkoxide anion is liberated
and undergoes cyclization to form the carbonate and release DMF.
Entry PhCH
2
Br
Yield
o
Solvent
T ( C)
b
(mol%)
(%)
60
In both senarios, smooth cyclization of D and regeneration of
DMF are important for negligible by-product formation.
1
2
3
4
5
6
7
8
5
0
5
5
0
5
5
5
DMSO (5 mL)
DMSO (5 mL)
120
120
120
120
120
120
100
80
6
0
DMF (2.5 mL)/DMSO (2.5 mL)
DMF (5 mL)
59
73
0
DMF (5 mL)
Toluene (5 mL)
DMF (5 mL)
0
62
0
DMF (5 mL)
a
Reaction conditions: SO (5 mmol, 0.6008 g), CO
2
(99.999%, balloon),
b
time 24 h. Isolated yield.
Temperature was also found to have significant effect on this
reaction (entries 4, 7, and 8). When the temperature was
decreased to 100 °C, a lower yield of 62% was obtained with 5
mol% of BnBr (entry 7), and no conversion was observed at
1
5
8
0 °C (entry 8).
H3C
H3C
O
Bn
Br
BnBr+ DMF
A
N
O
20
H
O
R
O
O
1
Figure 1 H NMR spectra of pure DMF (A) and a mixture of benzyl
R
bromide and DMF (molar ratio=1:1) (B). (298 K, Dual NMR cell. The
DMF
6
samples were placed in the inner cell, while DMSO-d was placed in the
2
5
Bn
O
65 outer cell as the external standard.)
Bn
O
Br
R
O
Br
H3C
O
H
N
O
R
The reaction conditions were optimized and the results are
summarized in Table 2. The reaction yield was improved when
DMF and SO were stirred at 120 C for 4 h (86%, entry 2) rather
than at room temperature for 30 min (73%, entry 1). The
H3C
B
D
O
O
3
0
H3C
O
H
DMF + CO2
N
70
2
nucleophilic activation of CO by DMF seems to be important in
H3C
C
this reaction. It can be seen that 1 mol% of BnBr was sufficient as
a catalyst. Moreover, the amount of DMF was successfully
decreased to 5 equiv. without affecting the yield (86%, entry 3).
When DMF was decreased to 3 equiv., the yield also decreased
(29%, entry 4), but a longer reaction time afforded a satisfactory
result (81%, entry 5). Thus, DMF is expected to work as a solvent
Scheme 2 A Plausible Mechanism for the Coupling Reaction of Epoxide
with CO Catalyzed by BnBr and DMF.
2
Based on the present and previous results,13d the plausible
catalytic mechanism is shown in Scheme 2. Similar to the
formation of a benzyl cation in the reaction with DMSO (Scheme
75
3
4
4
5
0
5
2
for CO as well.
With the above results in hand, BnCl was investigated to
compare the effect of the halogen atom (81%, entry 6). Under the
80 same reaction conditions, the yield decreased slightly (vs. entry
3), while being comparable to the reaction with 1 mol% BnBr
1
), probably DMF also interacts with BnBr to generate iminium
salt A. The interactions between BnBr and DMF were studied by
1
the H NMR spectrum of their 1:1 mixture at room temperature
1
(
Figure 1). All the H NMR signals of DMF (a, b, and c) in the
(
entry 5). This is mainly due to the lower reactivity of BnCl
mixture shifted up-field relative those of pure DMF (from  =
.04, 2.969, and 2.804 (Figure 1A) to 7.44, 2.180, and 2.175
Figure 1B), respectively), indicating that BnBr was activated by
DMF. The benzyl group of A transfers as a benzl cation,
-
-
because Cl constitutes a poorer leaving group than Br .
8
(
85

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a
a
Table 2 Optimization of Reaction Conditions.
Table 3 Synthesis of Cyclic Carbonates from CO
2
and Epoxides.
55
O
O
1
mol% PhCH
2
Br
Br
O
CO2
20 o
t
O
or p- BuC
6
H
4
CH
2
+
PhCH X + DMF
O
O
2
+
1 atm CO
2
O
O
1
C
5 eq. DMF, 120 o
Ph
C
R
5
Ph
R
1
a - 8a
1b - 8b
Entry PhCH
2
X (mol%)
Br (5)
Br (5)
Br (1)
Br (1)
Br (1)
Cl (1)
DMF
Method
Yield (%)b
O
O
O
O
1
2
3
4
5
6
PhCH
PhCH
PhCH
PhCH
PhCH
PhCH
2
2
2
2
2
2
~13 eq. [5 mL]
~13 eq. [5 mL]
5 eq.
a
b
b
b
c
b
73
86
86
29
81
81
O
O
O
O
O
CH
3
CH
3
CH3
1b
3
2b
5b
5
3b
3 eq.
c
c
c
7
5
79
82
85
3 eq.
d
d
7
8
5 eq.
O
O
O
a
Reaction conditions: SO (5 mmol, 0.6008 g), CO
2
(99.999%, balloon).
O
O
O
O
O
O
Method a: After DMF and SO were stirred at room temperature for 30
min under CO , BnBr was added and stirred for 23.5 h at 120 C. Method
b: After DMF and SO were stirred at 120 C for 4 h under CO , benzyl
O
Cl 4b
2
6b
2
82c
1
0
2
74c
57c
halide was added and stirred for 20 h at 120 C. Method c: The reaction
8d
59d
7
time was 48 h (4 h + 44 h) under the same reaction conditions as those of
O
O
O
b
method b. Isolated yield.
O
O
O
O
O
O
O
Ph 7b
Ph 8b
Ph (R)-8b
1
5
The solvent effect on the reaction was further investigated for
some amide-type solvents and the results are summarized in
Table S1 (see ESI). All commercially available amides, DMAc,
9
99d
0c
86c
92d
85 (69 %ee)c
91 (63 %ee)d
a
N-formylmorpholine,
N-methylpyrrolidone,
and
N-
Reaction conditions: epoxide (5 mmol except for 1a (10 mmol) and 2a,
formylpiperidine, afforded styrene carbonate in moderate-to-high 60 4a, and 5a (6 mmol)), benzyl bromide (1 mol%), DMF (5 equiv.), CO
2
b
2
2
3
3
4
4
5
0
5
0
5
0
5
0
yields (47−83%), but DMF gave the best result (86% yield). It is
interesting that N,N,N,N-tetramethylurea also resulted in a good
(99.999%, balloon), temperature 120 °C, total reaction time 24 h.
c
d
Isolated yield. By BnBr. By p-tert-butylbenzyl bromide.
yield of 71% (entry 6). The results support the mechanistic role of
the amide-type carbonyl group, as shown in Scheme 2.13d
When p-tert-butylbenzyl bromide was used, a further decrease in
optical purity (63 %ee) was observed despite higher chemical
yield. On the other hand, negligible racemization of (R)-8b (from
6
5
The structural effect of the catalyst was investigated for some
benzyl bromides and halomethane derivatives, and the results are
summarized in Table S2 (see ESI). Although the reactions of all
benzyl bromides afforded styrene carbonate in high yields
6
9 %ee to 67 %ee) was confirmed at 120 C for 24 h. Based on
these results, the reaction at the less-hindered side of epoxide
seems favorable as shown in Scheme 2. The stabilization of the
positive charge at the tertiary 2-position of B is expected to be the
main cause of the decrease in optical purity by direct attack of
(>80%), benzyl bromides with an electron-donating group gave
70
better results than those with an electron-withdrawing group or a
branched structure (entries 2, 4, and 5 vs. 3, 6 and 7).
Interestingly, some halomethane derivatives, bromoacetonitrile,
chloroacetone, and N-chloromethylphthalimide, also resulted in
good yields of 72−88% (entries 8−10); however, no reaction
occurred using benzyl cyanide (entry 11).
1
3d
activated CO
2
and thus, more reactive p-tert-butylbenzyl
bromide resulted in less regioselectivity (entry 9, 63 %ee).
Conclusions
To evaluate the substrate scope of the catalytic system, various
epoxides were reacted with CO using 1 mol% BnBr and p-tert-
2
75
In summary, a new metal-free system for the conversion of
CO to cyclic carbonates was developed using commercially
2
available and inexpensive materials: 1 mol% of benzyl halide as
the nucleophilic catalyst and 3-5 equiv. of DMF as the recyclable
solvent and activator. Under ambient CO , the catalyst system
2
smoothly converted diverse terminal epoxides up to 99% yields.
Further study is currently underway to develop more economical
butylbenzyl bromide in 5 equiv. of DMF at 120 °C as the optimal
conditions. As shown in Table 3, most of the substrates were
smoothly converted to the corresponding cyclic carbonates in
good-to-excellent yields up to 99%. As shown in Table 3, p-tert-
butylbenzyl bromide resulted in better yields for all the substrates
studied.
80
and mild procedures for the cycloaddition of CO
our laboratory.
2
to epoxides in
Under the optimal conditions disubstituted epoxides such as
cyclohexene oxide and trans-stilbene oxide were not converted to
cyclic carbonates, presumably owing to the high steric
Notes and references
a
1
6
hindrance. This steric effect was more likely to hinder the
nucleophilic attack to the epoxide rather than the coordination of 85 Graduate School of Science and Engineering, Saitama University, 255
Shimo-Okubo, Sakura-ku, Saitama 338-8570, Japan
benzyl cation. Comparing to the previous metal-free catalytic
b
School of Chemistry and Chemical Engineering, University of Jinan,
1
3c,13d
systems under atmospheric CO
2
pressure,
the present system
Jinan 250022, China.
: Corresponding author. E-mail: hirose@apc.saitama-u.ac.jp
90 Fax: (+)81 488583522.
gave comparable yields and scope of various cyclic carbonates
using with low loading of inexpensive catalyst.
*
Enantiomerically pure epoxide, (R)-8a, was transformed to
(
R)-8b with some ee loss in 85% yield (entry 9, 69 %ee) by BnBr.

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†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
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