Direct synthesis of cyclic carbonates from olefins and CO2 catalyzed by a MoO2(acac)2-quaternary ammonium salt system

Article abstract of DOI:10.1039/c1gc15549k

A highly efficient synthesis of cyclic carbonates from olefins and CO ₂ has been achieved by the use of a molybdenyl acetylacetonate (MoO₂(acac)₂)-quaternary ammonium salt catalytic system with tert-butyl hydroperoxide as an oxidant through a one-pot multistep process. This simple and cheap method can be applied to various olefins, such as 1-octene, 1-hexene, allyl chloride, cyclohexene and styrene, affording the highest yields in comparison with the data reported previously except for styrene. A plausible mechanism is proposed based on the results.

Full text of DOI:10.1039/c1gc15549k

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PAPER
Direct synthesis of cyclic carbonates from olefins and CO2 catalyzed by a
MoO2(acac)2-quaternary ammonium salt system†
Fawang Chen, Tao Dong, Tiegang Xu, Xiaofang Li* and Changwen Hu*
Received 13th May 2011, Accepted 3rd June 2011
DOI: 10.1039/c1gc15549k
A highly efficient synthesis of cyclic carbonates from olefins and CO2 has been achieved by the use
of a molybdenyl acetylacetonate (MoO2(acac)2)–quaternary ammonium salt catalytic system with
tert-butyl hydroperoxide as an oxidant through a one-pot multistep process. This simple and
cheap method can be applied to various olefins, such as 1-octene, 1-hexene, allyl chloride,
cyclohexene and styrene, affording the highest yields in comparison with the data reported
previously except for styrene. A plausible mechanism is proposed based on the results.
to the epoxide formed in one pot, is a more economically
Introduction
viable process because of the low-priced and easily available raw
The chemistry of carbon dioxide (CO2) has received much
material olefins compared with the epoxides and the avoidance
attention from both scientific and practical perspectives because
of the separation of the epoxide after its synthesis in the
CO2 is not only one of the greenhouse gases but also an
first step (Scheme 1). Although the oxidative carboxylation of
inexpensive, nontoxic, nonflammable, naturally abundant and
olefins has been known for a long time, since 1962,9 however,
biorenewable C1 resource.1 The synthesis of cyclic carbonates
only a few reports are known thus far about such a useful
(CCs), which are excellent aprotic polar solvents, substrates
process in contrast to extensive studies on the cycloaddition
for small molecule synthesis, additives, antifoam agents for
reaction of CO2 and epoxide.10–19 At first, some known catalysts
antifreeze, and plasticizers, is one of the most promising
previously reported for the epoxidation of olefins were tried
technologies for chemical fixation and utilization of CO2.2 Until
as active catalysts for the oxidative carboxylation process.11–12
now, a number of syntheses of CCs from CO2 have been
Subsequently, in view of the remarkable catalytic reactivity
described over the last 30 years.3 Among them, a classical and
industrial way is a cycloaddition reaction of CO2 with epoxide. A
of quaternary ammonium salts in the cycloaddition reaction
of CO2 with the epoxide, Arai et al. found that quaternary
remarkable variety of active initiators/catalysts,3 such as metal
ammonium halides or imidazolium salts were suitable catalysts
and quaternary ammonium salts,4 phthalocyanine and salen
and/or solvents for the oxidative carboxylation of styrene using
complexes,5 ionic liquids,6 metal oxides7 and immobilized molec-
ular catalysts8 have been reported for this transformation so far.
tert-butyl hydroperoxide (TBHP) as oxidant.13 Under optimum
reaction conditions, tetrabutylammonium bromide (TBAB) was
However, the toxicity and the complicated production process
the most effective catalyst and the yield of styrene carbonate
of epoxide, coupled with its high cost, hamper its widespread
(SC) could reach 39%. The yield of SC in this one-pot reaction
use as an original material and clearly decrease the number of
was significantly increased up to 70% by the TBAB/organic
applications for CC. The search for an environmentally benign
base 1,8-diazabicylco[5.4.0]undec-7-ene (DBU) system using
and economical process for the synthesis of CCs from CO2 is
therefore of significant importance and interest.
H2O2 as an oxidant in water, reported by Li et al.14 In order
The direct synthesis of CCs from olefins and CO2, which
can be regarded as an oxidative carboxylation process in
combination with two sequential reactions of epoxidation of
the olefin and the subsequent cycloaddition reaction of CO2
Key Laboratory of Cluster Science of Ministry of Education of China,
Beijing Institute of Technology, Beijing, 100081, P. R. China.
E-mail: cwhu@bit.edu.cn, xfli@bit.edu.cn; Fax: 86-10-68912631;
Tel: 86-10-68912631
† Electronic supplementary information (ESI) available: Details of
catalysts preparation, IR spectra for catalysts and GC, NMR spectra
for cyclic carbonates. See DOI: 10.1039/c1gc15549k
Scheme 1 The synthesis of cyclic carbonates via the oxidative carboxy-
lation of olefins.
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to further improve the conversion of olefins to CC, some
combined catalytic systems consisting of excellent epoxidation
catalysts and quaternary ammonium salts were investigated
for this oxidative carboxylation process, using mainly a one-
pot single step protocol.15–18 Arai and Sun demonstrated that
supported Au catalysts–zinc bromide/TBAB system or nano-
gold catalyst supported R201 resin containing an ammonium
salt or amino group could significantly increase the activity
using TBHP as an oxidant.15 For example, a 53% yield of SC was
obtained by Au/Fe(OH)3–ZnBr2/TBAB.15b He and coworkers
reported a sodium phosphotungstate–TBAB catalytic system
for the facile synthesis of SC with a 57% yield using H2O2
as an oxidant.16 Liu et al. reported a direct synthesis of
propylene carbonate with a yield of 48% by a titanosilicate–
quaternary ammonium halide system using H2O2 as an
results reported by Sheldon and Van Doorn shown that all these
molybdenum catalysts gave rise to a common compound, a 1,2-
diol complex MoO2(OCHRCH2O)2, irrespective of the starting
material.21c Therefore, our efforts in the end were mainly focused
on MoO2(acac)2, which was easily prepared and commercially
available, as a catalyst for the epoxidation of olefins using
anhydrous TBHP as an oxidant.22 TBAB was chosen as an
effective cycloaddition catalyst.3,4b The simple aliphatic olefin
1-octene was used as a model substrate. Then the one-pot
synthesis of 1-octene carbonate from 1-octene and CO2 by the
MoO2(acac)2–TBAB system was carried out, employing both
the single step and multistep protocols, and the representative
results were summarized in Table 1.
At first, the epoxidation of 1-octene to 1-octene oxide
(OO) and the cycloaddition of CO2 with OO were separately
investigated by MoO2(acac)2 (1) and TBAB in order to know the
compatibilities of these catalysts in terms of chemical stabilities
in the oxidative carboxylation process in one pot. Consistent
with the results reported previously,21 in the presence of
5.5 mmol of TBHP, 5 mmol of 1-octene was almost completely
converted (conversion > 98%) by 0.1 mol% of MoO2(acac)2
after 1 h at 100 ◦C without additional solvent, affording an 86%
yield of OO (Table 1, Entry 1). In contrast, TBAB was inactive
for the epoxidation reaction under the same conditions (Table
1, Entry 2). In addition, adding a catalytic amount of TBAB
(5 mol%) to the MoO2(acac)2/TBHP system markedly pre-
vented the epoxidation reaction proceeding and the expected
product 1-octene carbonate was not obtained, suggesting
strongly that TBAB had a negative effect on the epoxidation
reaction in the MoO2(acac)2/TBHP system (Table 1, Entry 3).
This is maybe due to the complete consumption of the
MoO2(acac)2/TBHP system by excess TBAB since the bromide
ion can catalyze the unwanted and non-productive decomposi-
tion of tert-butyl hydroperoxide and MoO2(acac)2 to generate
O2, tert-butyl alcohol, and MoO2Br2.21,23 In the cycloaddition
reaction, MoO2(acac)2 was inactive as a catalyst (Table 1, Entry
4), while TBAB afforded a quantitative yield of CC, almost
99%, in the presence of 1-octene epoxide and CO2 (Table
1, Entry 5). The presence of the MoO2(acac)2/TBHP system
had a slight influence on the catalytic performance of TBAB
in the cycloaddition of CO2 with 1-octene oxide (Table 1,
Entry 6), consistent with the previous result that VO(acac)2
and an onium salt could efficiently catalyze the cycloaddition
reaction of trimethylene oxide and CO2 to produce trimethylene
carbonate.24 As a result, when a one-pot single step synthesis
of 1-octene carbonate was carried out under the same reaction
conditions in the presence of 3 MPa CO2 at 100 ◦C for 2 h, the
MoO2(acac)2/TBHP system showed good conversion of olefin
to give high yield of OO, ca. 85%, whereas no CC was obtained
(Table 1, Entry 7). TBAB alone and the combination of the
MoO2(acac)2/TBHP–TBAB system under the same conditions
gave disappointing results by yielding only trace amounts of OO
and 1-octene carbonate since the MoO2(acac)2/TBHP system
was used up by the reaction with TBAB in the latter case
(Table 1, Entries 8–9). Interestingly, a yield of ca. 83% of
the desired product CC could be achieved by changing to a
multistep protocol in which CO2 and TBAB were subsequently
added into the same reactor after the epoxidation of the olefin
catalyzed by the MoO2(acac)2/TBHP system in 1 h, much higher
oxidant.17
A dioxo(tetraphenylporphyrinato)ruthenium(VI)–
quaternary onium salt catalytic system with O2 as the oxidant
was recently developed by Jing as an excellent catalyst for
the direct synthesis of CCs at ambient temperature under low
pressure, giving a 76% yield for SC.18 In some cases, however,
the poor compatibilities and chemical stabilities between these
two kinds of catalysts in one pot cannot be avoided, resulting in
undesirable yield or long reaction time. In 2007, Yokoyama et al.
reported a one-pot synthesis of SC from styrene and CO2 by the
use of methyltrioxorhenium (MTO)–Zn[emim]2Br4/[bmim]BF4
(emim = 1-ethyl-3-methylimidazolium, bmim = 1-butyl-3-
methylimidazolium) with urea hydrogen peroxide as an oxidant
in a multistep protocol, affording SC with an 83% yield.19
However, the complicated catalyst systems limited their wide
uses in industry. Therefore, the search for a simple and highly
efficient industrial catalyst for the direct synthesis of CCs from
olefins and CO2 is still of great importance from an industrial
point of view.
Selective epoxidation of olefins by alkyl hydroperoxide cat-
alyzed by d0 transition metal complexes (Mo(VI), V(V), and
Ti(IV)) has been known as the most important industrial
process for the manufacture of propylene oxide.20 Although
the molybdenum complex is the most active catalyst for the
epoxidation reaction using TBHP as the oxidant,20,21 however,
it has never been used as catalyst for the one-pot synthesis
of CC from olefins. Herein, we report the direct synthesis of
CCs with very high yield from olefins and CO2, catalyzed by a
MoO2(acac)2–quaternary ammonium salt catalytic system using
TBHP as an oxidant through a one-pot multistep process. This
simple, cheap, highly-efficient method can be applied to various
olefins, such as 1-octene, 1-hexene, allyl chloride, cyclohexene
and styrene, affording the highest yields in comparison with the
data reported previously except styrene.
Results and discussion
Oxidative carboxylation of 1-octene catalyzed by a
MoO2(acac)2/TBHP–TBAB system
The abilities of molybdenum complexes to catalyze the epox-
idation of olefins with organic hydroperoxide as an oxidant
were first discovered by Smith in a patent issued in 1956.21a
Since then, a lot of efforts have been made to develop these
efficient catalysts based on the epoxidation mechanism. The
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Table 1 The oxidative carboxylation of 1-octene catalyzed by the MoO2(acac)2/TBHP–TBAB systema
Yield (%)b
OO
Entry
Catalyst
Temp. (◦C)
Time (h)
CC
Typec
1d
2d
3d
4e
5e
6e
7
1
100
100
100
140
140
140
100
100
1
1
1
1
1
1
2
2
86
2
—
—
—
<1
99
97
—
2
E
E
E
C
C
C
O-S
O-S
O-S
O-M
O-M
O-M
O-M
O-M
O-M
TBAB
1/TBAB
1
3
—
TBAB
1/TBAB
1
TBAB
1/TBAB
1
TBAB
1/TBAB
1/TBAB
1/TBAB
1/TBAB
< 1
—
85
4
5
8
9
100
2
5
10
11
12
13
14
15
100/140f
100/140f
100/140f
100/120f
100/100f
100/140f
1/1g
1/1g
1/1g
1/1g
1/1g
2/1g
81
—
7
< 1
< 1
10
30
< 1
83h
74h
55h
81h
a Conditions: 1-octene 5 mmol, TBHP 5.5 mmol, 1 (MoO2(acac)2) 0.1 mol%, 5 mmol, TBAB 5 mol%, 0.25 mmol, P(CO2) 3 MPa, unless otherwise
noted. b Determined by GC. c E, epoxidation reaction; C, CO2 cycloaddition reaction; O-S, one-pot single step process; O-M, one-pot multistep
process. d Without CO2. e 1-octene oxide as starting material, 5 mmol. f The former is the reaction temperature for epoxidation reaction and the latter
is the reaction temperature for cycloaddition reaction. g The former is the reaction time for epoxidation reaction and the latter is the reaction time for
cycloaddition reaction. h Selectivity is close to the yield of cyclic carbonate.
Table 2 The oxidative carboxylation of 1-octene catalyzed by
than the yields obtained by MoO2(acac)2 and TBAB alone
(Table 1, Entries 10–12). As a result, the one-pot multistep
synthetic protocol was viewed as a more attractive and practical
route for the oxidative carboxylation of 1-octene and CO2
catalyzed by the MoO2(acac)2–TBAB catalytic system. The yield
of CC was sensitive to the reaction temperature and time. With
the decreasing temperature in the cycloaddition process from
140 to 100 ◦C, the yield of CC gradually decreased from 83% to
55% (Table 1, Entries 12–14). An extension of the epoxidation
reaction time resulted in the decrease of OO yield because of its
ring-opening reaction, thus leading to a decreased yield of CC
(Table 1, Entry 15).
MoO2(acac)2/TBHP–various cycloaddition catalystsa
Yield (%)b
Entry
Cycloaddition Catalyst
OO
CCc
1
2
3
4
5
6
7
8
(n-C16H33)(CH3)3NBr
(n-C2H5)4NBr
16
12
< 1
19
< 1
81
2
45
84
80
80
86
80
82
80
79
63
73
83
64
83
3
81
42
< 1
4
6
< 1
3
4
5
(n-C4H9)4NBr
(n-C4H9)4NCl
(n-C4H9)4NI
(n-C4H9)4NHSO4
[bmim]Br
[bmim]Cl
[bmim]BF4
pyridine
N,N-dimethylaminopyridine
DBU
ethylenediamine
1,2-diaminopropane
[(n-C7H15)4N]6SiW11CoO39
[(n-C7H15)4N]6GeW11MnO39
Oxidative carboxylation of 1-octene by MoO2(acac)2/TBHP-
various cycloaddition catalysts through a one-pot multistep
process
9
10
11
12
13
14
15d
16d
On the basis of the above results, the MoO2(acac)2/TBHP–
TBAB system showed very good activity and selectivity for the
direct synthesis of 1-octene carbonate from 1-octene and CO2 in
a one-pot multistep process. In view of the large-scale application
of the MoO2(acac)2/TBHP system in industry, a lot of highly
efficient cycloaddition catalysts, mainly based on quaternary
ammonium salts, ionic liquids, organic bases, and transition-
metal-substituted Keggin-type polyoxometalates, have been in-
troduced into the MoO2(acac)2/TBHP system in order to get
a novel catalytic system for the one-pot synthesis of CC. The
representative results are illustrated in Table 2.
5
a Conditions: 1-octene
5 mmol, TBHP 5.5 mmol, MoO2(acac)2
0.1 mol%, 5 mmol, 100 ◦C, 1 h; cycloaddition-catalyst 5 mol%,
0.25 mmol, 140 ◦C, P(CO2) 3 MPa, 1 h, unless otherwise noted.
b Determined by GC. c Selectivity is close to the yield of cyclic carbonate.
d Cycloaddition-catalyst 0.1 mol%, 5 mmol.
Arai and Sun had reported that the catalytic performances
of quaternary ammonium salts in the cycloaddition reaction
of CO2 and epoxide were significantly dependent on the nucle-
ophilicities of the anions because their nucleophilic attack to a
carbon atom of epoxide can result in ring opening of epoxide to
generate a oxygen anion as active species to active CO2.13 A quick
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screening of various quaternary ammonium salts was carried
out in the second step of direct synthesis of 1-octene carbonate
under 3 MPa CO2 at 140 ◦C for 1 h (Table 2, Entries 1–6). The
results indicated that the kinds of anions and cations involved in
quaternary ammonium salts really did have some effect on the
yield of CC. The cation of quaternary ammonium salts played
an important role in the second step of one-pot synthesis of CC.
The bulkiness of the tetrahedral ammonium ion [NR4]+ forces
the halogen anion away from the cation, thus making the anion
more nucleophilic.4b,13 As a result, the more bulky [(n-C4H9)4N]+
cation showed higher activity than the less sterically demanding
[(n-C16H33)(CH3)3N]+ and [(n-C2H5)4N]+ cations under the same
conditions (Table 2, Entries 1–3). Similar to TBAB, quaternary
ammonium salts containing strongly nucleophilic halide ions,
such as (n-C4H9)4NCl and (n-C4H9)4NI also exhibited the same
or slightly low activities for the one-pot multistep synthesis of
CC from 1-octene under the same conditions (Table 2, Entries
3–5). The decreasing tendency of their activities corresponded
with the decreasing nucleophilicity of the halide anion in the
order I- > Br- > Cl-. Whereas, (n-C4H9)4NHSO4 with the weakly
Unfortunately, several ammonium bases and transition-
metal-substituted polyoxometalates, which were reported as
active catalysts for the cycloaddition of CO2 and epoxides to
produce CCs,3,25 afforded unsatisfactory yields of CCs, maybe
due to the absence of the strongly nucleophilic halide ion or
coordination with tert-butyl alcohol formed in the first step
in the one-pot synthesis of CCs (Table 2, Entries 10–16).
Thus, quaternary ammonium salts and ionic liquids containing
strongly nucleophilic halide anions were found to be effective
catalysts in the second step of the one-pot synthesis of CC.
Oxidative carboxylation of various olefins by
MoO2(acac)2/TBHP-TBAB system
To explore the utility of the MoO2(acac)2/TBHP–TBAB system
in a one-pot multistep synthesis of cyclic carbonates, other
olefins were then investigated and the results are presented in
Table 3. Compared with the previous catalyst systems,9–19 the
MoO2(acac)2/TBHP–TBAB system demonstrated its superior-
ity in the highly efficient conversion of most olefins to CCs (Table
3, Entries 1–10). Under 3 MPa CO2, the MoO2(acac)2/TBHP-
TBAB system catalyzed the oxidative carboxylation of 1-hexene,
affording 1-hextene carbonate with a moderate yield of ca. 49%
(Table 3, Entry 2). With further increase of the epoxidation
reaction time or the amount of the oxidant TBHP, slightly
higher yields of 1-hexene carbonate (ca. 56–58%) were obtained
(Table 3, Entries 4–5). Allyl chloride carbonate and cyclohexene
carbonate were produced in 45% and 33% yields, respectively,
under the same conditions; however, with decreasing the epoxi-
dation temperature from 100 ◦C to 70 ◦C or with increasing the
cycloaddition reaction time from 1 h to 6 h, the yields of allyl
chloride carbonate and cyclohexene carbonate were significantly
increased up to 80% and 84%, respectively, which are the highest
yields reported in the literature (Table 3, Entries 5–8). The only
exception was styrene (Table 3, Entries 9–10). Only a 68% yield
of styrene carbonate was achieved using this one-pot multistep
protocol and the by-product benzaldehyde was simultaneously
detected. Although a 68% yield of styrene carbonate was higher
than those of most of catalyst systems,9–17 it was inferior to the
-
nucleophilic HSO4 anion was almost inactive for the oxidative
carboxylation of 1-octene (Table 2, Entry 6).
Similarly, ionic liquids with strongly nucleophilic halide ions,
such as [bmim]Br and [bmim]Cl were also effective for the direct
synthesis of CC from 1-octene, producing CCs with 83% and
42% yields, respectively (Table 2, Entries 7–8). In contrast,
[bmim]BF4 could not convert OO to CC under the same
conditions (Table 2, Entry 9). Such a result was similar to the
MTO–Zn[emim]2Br4/[bmim]BF4 catalytic system during one-
pot multistep synthetic process,19 although [bmim]BF4 alone
was active for the cycloaddition of CO2 and epoxide in a
long reaction time.6 A possible reason for the better catalytic
performance of quaternary ammonium salts compared with that
of the ionic liquids may be ascribed to the different cations in the
structure. The planer cation [bmim]+ in the ionic liquid bound
the anion more tightly than the [NBu4]+ (Bu, butyl) cation in the
quaternary ammonium salt, which resulted in the relatively low
nucleophilicity of the halide anion in the ionic liquid (Table 2,
Entries 4 and 8).4b,13
Table 3 The oxidative carboxylation of various olefins by the MoO2(acac)2/TBHP–TBAB systema
Entry
Olefin
Time (h)
Yieldb (%)
Yield reportedc (%)
Ref
1
2
3
4
5
6
7
8
1-Octene
1-Hexene
1-Hexene
1-Hexene
Allyl chloride
Allyl chloride
Cyclohexene
Cyclohexene
Styrene
1/1
1/1
2/1
2/1d
1/1
1/1e
1/1
1/6
1/1
—
83
49
56
59
45
80
33f
84
68g
—
78
47
—
—
57
—
31
—
83
76h
18
16
—
—
18
—
18
—
19
18
9
10
Styrene
a Conditions: olefin 5 mmol, TBHP 5.5 mmol, MoO2(acac)2 0.1 mol%, 5 mmol, TBAB 5 mol%, 0.25 mmol, 100 ◦C for epoxidation reaction and
140 ◦C for cycloaddition reaction, P(CO2) 3 MPa, unless otherwise noted. b Yield of CC is determined by GC. Selectivity is close to the yield of
cyclic carbonate. c The highest yield reported previously. d TBHP 7 mmol. e Reaction temperature for epoxidation reaction is 70 ◦C. f 53% Yield of
cyclohexene oxide is detected. g 13% Yield of benzaldehyde is detected. h Only one example has 100% selectivity for CC.
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83% yield offered by MTO–Zn[emim]2Br4/[bmim]BF4 using a
multistep protocol and the 100% selectivity obtained by a Ru
catalyst,18,19 maybe due to the attack of TBHP on the epoxide
formed and cleavage of the C–C bond in the first step.20,21 These
results strongly indicated that the MoO2(acac)2/TBHP–TBAB
system in a one-pot multistep protocol could efficiently promote
oxidative carboxylation of various olefins to prepare CCs,
which was probably ascribed to avoiding interaction between
catalysts themselves and those of the catalyst and intermediate
product, resulting from the addition of catalyst at a different
stage.
anion Br- on the less hindered carbon atom of the epoxide,
which leads to opening of the epoxide ring and generation
of an oxygen anion species. The nucleophilic attack of the
newly formed oxygen anion species on the carbon atom of CO2
affords the key intermediate NBu4OCOOCHR2CH2Br. After
the intramolecular cycloaddition reaction, the corresponding
CC is formed in combination with TBAB being regenerated as
the active species in the catalytic cycle.3,4b
Remarkably, the MoO2(acac)2–TBAB catalytic system can be
recycled by distilling all of the organic solvents, including tert-
butyl alcohol, toluene, and CC consecutively after the whole
reaction, since both of the catalysts are solid and they were
left in the bottle. Moreover, their diverse solubilities in ethanol
separate them completely. For example, the yield of the first
recycle of about 80% was slightly lower than the previous result
(83%) using 1-octene as a model olefin. A 78% yield of CC was
obtained in the second recycle. Furthermore, the by-product
tert-butyl alcohol can be converted into TBHP by reacting with
H2O2,30 thus olefins, CO2 and H2O2 were actually consumed
during the whole process.
Possible reaction mechanism
Since the one-pot synthesis of cyclic carbonate through a
multistep process is viewed as the combination of two separate
reactions catalyzed by their respective catalysts, it is apparent
that the mechanism of oxidative carboxylation of olefin over
catalyst MoO2(acac)2/TBHP-TBAB consists of the mechanism
of the epoxidation reaction catalyzed by MoO2(acac)2 and the
mechanism of the following cycloaddition reaction catalyzed
by TBAB. Due to the industrial importance, the mechanism
of the epoxidation reaction catalyzed by MoO2(acac)2 has been
studied by several groups. Some represented results have been
obtained,26 for example, the alkylperoxo mechanism proposed
by Chong and Sharpless27 and the metalladioxolane mech-
anism suggested by Mimoun.28 Recently, theoretical calcula-
tions indicate that the alkylperoxo mechanism in which an
oxygen is directly transferred from a hydroperoxide species
to olefin is more energetically favorable.29 Thus, a plausible
mechanism is proposed based on these results and presented in
Scheme 2.
Conclusions
In summary, a simple, cheap, and highly-efficient preparation
of cyclic carbonates from olefins and CO2 has been achieved
by the MoO2(acac)2/anhydrous TBHP–TBAB system through
a one-pot multistep process. Compared with other catalytic
systems, the present catalytic system, which is easy to prepare
and commercially available, can convert various olefins into
the corresponding cyclic carbonates in the highest yields in
comparison with the data previously reported, except for styrene.
From an economic point of view, the direct synthesis of cyclic
carbonates from olefins and CO2 catalyzed by this low-cost and
highly-efficient MoO2(acac)2/TBHP–TBAB system employing
the multistep protocol would be of great potential in industry
for the environmentally benign chemical fixation and utilization
of CO2. The investigation of the optimization and application
of the MoO2(acac)2/TBHP–TBAB system in industry is under
way in our laboratory.
Experimental
Materials and instrumentation
Cyclohexene oxide (98%), 1-octene oxide (97%), styrene oxide
(97%) were purchased from Aldrich without further purifica-
tion. Chloropropylene (98%), 1-hexene oxide (97%), 1-octene
(99%), 70% TBHP aqueous solution, and [(n-C4H9)4N]HSO4
(99%) was purchased from J&K Chemical Ltd. Cyclohexene
(99%), 1-methylimidazolium (99%) and biphenyl (99%) were
purchased from Alfa. Epichlorohydrin, styrene, and other
reagents and chemicals with analytical purity were purchased
from Sinopharm Chemical Reagent Co., Ltd, P. R. China. CO2
(99.99%) was purchased locally.
Fourier-transform infrared spectra (FT-IR) of the samples
were recorded on the Thermal Nicolet 670 FT-IR over the
range of 4000–400 cm-1 using the KBr pellet technique at room
temperature. 1H NMR and 13C NMR spectra were recorded on
a Bruker spectrometer (400 MHz and 100.6 MHz) in CDCl3.
Scheme 2 The proposed mechanism of the oxidative carboxylation of
olefins catalyzed by the MoO2(acac)2/TBHP–TBAB system.
In the epoxidation reaction, the hydroperoxide is suggested to
coordinate to the metal center through the distal or terminal
oxygen, rather than the proximal oxygen, to form the key
intermediate alkylperoxo M–OOR. The coordination of the
distal oxygen atom is reasonable from steric considerations and
the fact that its complexation to the metal center will activate it.
Subsequently, the olefin is coordinated with the alkylperoxo M–
OOR proceeding with a three-membered transition state, and
then the epoxide is produced after complexation and oxygen
transfer.28,30 In the following cycloaddition reaction, the coordi-
nation of the [n-Bu4N]+ cation of TBAB with the oxygen atom of
the resulting epoxide will promote the nucleophilic attack of the
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GC analyses were performed on a Kechang GC-9800, equipped
with a flame ionization detector (FID) and a SE-30 capillary
column using nitrogen as a carrier gas.
J = 8.1 Hz), 4.74–4.65 (1 H, m); 13C NMR (CDCl3, 101 MHz,
ppm): d 14.0, 22.4, 24.3, 28.8, 31.5, 33.8, 69.4, 77.1, 155.2.
Styrene carbonate. 1H NMR (CDCl3, 400 MHz, ppm): d
4.34–4.38 (1 H, m), 4.80–4.84 (1 H, t, J = 8.4 Hz), 5.68–5.72 (1
H, t, J = 8.0 Hz), 7.37–7.48 (5 H, m); 13C NMR (CDCl3, 101
MHz, ppm): d 71.2, 77.9, 125.9, 129.3, 129.7, 135.9, 154.9.
Preparations of catalysts and oxidant
MoO2(acac)2 was synthesized according to the published
procedure.31 [bmim]BF4, [bmim]Br and [bmim]Cl were synthe-
sized according to the literature.32 Transition-metal-substituted
Keggin-type polyoxometalates were synthesized following the
published process.24,33 All these catalysts were determined by IR
and were consistent with the literature values.31–33
Anhydrous TBHP in toluene was synthesized from refluxing
70% TBHP aqueous solution and toluene following the liter-
ature procedure.22 TBHP concentrations were determined by
iodometric titration according to the reference method21d,22 and
was detected to be 3.25 M.
Recyclability of catalyst and regeneration of TBHP
With respect to recyclability of the present catalyst and regen-
eration of TBHP, the one-pot multistep reaction was performed
on a mole scale. The catalytic reaction was carried out in a
1 L stainless-steel autoclave equipped with a magnetic bar. The
reactor was charged with MoO2(acac)2 (1 mmol, 0.33 g), 1-
octene (1 mol, 112 g), anhydrous TBHP in toluene solution
(1.1 mol, 340 mL, 3.25 mol L-1), and then the reactor was
heated to 100 ◦C for 1 h. After the epoxidation reaction
was finished, the autoclave was cooled to room temperature,
and was charged TBAB (0.05 mol, 16.10 g). CO2 was then
pumped into the reactor and the pressure was adjusted to 3
MPa by a CO2 boosting pump. The autoclave was heated to
140 ◦◦C for 1 h. After the reaction, the autoclave was cooled
to 0 C in an ice-water bath in order to release the remaining
CO2 slowly. The mixture was distilled of all of the organic solvent
consecutively by vacuum. Then the solid was extracted by a small
amount of ethanol. MoO2(acac)2 was obtained by filtration and
TBAB was obtained by recrystallization from ethanol in low
temperature after concentration.
Catalytic reactions and analyses
The catalytic reaction was carried out in a 25 mL stainless-steel
autoclave equipped with a magnetic bar. In the one-pot single
process, the reactor was charged with an appropriate amount
of catalysts MoO2(acac)2 and TBAB, olefin, TBHP at room
temperature and CO2 was pumped in. The reactor was then
heated to a desired temperature, and the pressure was adjusted
to the desired pressure by a CO2 boosting pump.
In the one-pot multistep process, the reactor was charged with
the appropriate amounts of catalyst MoO2(acac)2, olefin and
TBHP, and then was heated to the desired temperature. After
the epoxidation reaction was finished, the autoclave was cooled
to room temperature, and was charged with the appropriate
amount of catalyst TBAB. CO2 was then pumped into the
reactor and the pressure was adjusted to the desired pressure
by a CO2 boosting pump.
The by-product tert-butyl alcohol was collected individually
and transferred to a 3-necked flask. The same amount of 70%
H2SO4 was added to the flask and then the flask was placed in
an ice-water bath. The same amount of 30% H2O2 solution was
slowly added. The mixture was stirred overnight. The organic
liquid was extracted by toluene and distilled by vacuum to afford
anhydrous TBHP in toluene.
In both processes, the autoclave was cooled to 0 ◦C in an ice-
water bath after the reaction in order to release the remaining
CO2 slowly. The reaction products were determined by gas
chromatograph (GC). The mixture was identified by the absolute
retention time compared to pure samples, and further purified by
column chromatography. The yield and selectivity of products
were calculated by peak area using an interstandard technology
from 1H NMR and 13C NMR spectra.
Acknowledgements
This work was financially supported by the Natural Science
Fund Council of China (20731002, 10876002, 20974014), the
111 Project (B07012), the Program of Cooperation of the Beijing
Education Commission (20091739006).
Chloropropylene carbonate. 1H NMR (CDCl3, 400 MHz,
ppm): d 3.73–3.88 (m, 2 H), 4.40–4.43 (dd, J = 6.0, 9.0 Hz, 1
Notes and references
H), 4.60–4.64 (dd, J = 8.0, 9.0 Hz, 1 H), 5.02–5.07 (m, 1 H); 13
NMR (CDCl3, 101 MHz, ppm): d 44.1, 67.0, 74.5, 154.5.
C
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