Cr(salophen) Complex Catalyzed Cyclic Carbonate Synthesis at Ambient Temperature and Pressure

Article abstract of DOI:10.1021/acscatal.6b01386

The combination of a chromium(III) salophen bromide complex and tetrabutylammonium bromide is shown to catalyze the reaction between terminal epoxides and carbon dioxide at ambient temperature and 1 bar carbon dioxide pressure and between internal epoxide

Full text of DOI:10.1021/acscatal.6b01386

Research Article
pubs.acs.org/acscatalysis
Cr(salophen) Complex Catalyzed Cyclic Carbonate Synthesis at
Ambient Temperature And Pressure
Jose A. Castro-Osma,*,† Katie J. Lamb,‡ and Michael North*,‡
́
†Universidad de Castilla-La Mancha, Departamento de Química Inorgan
Química Avanzada (ORFEO-CINQA), Instituto Regional de Investigacion
‡Green Chemistry Centre of Excellence, Department of Chemistry, The University of York, Heslington, York YO10 5DD, U.K.
́
ica, Organ
́ ́
ica y Bioquímica-Centro de Innovacion en
́
Científica Aplicada-IRICA, 13071 Ciudad Real, Spain
S
* Supporting Information
ABSTRACT: The combination of a chromium(III) salophen
bromide complex and tetrabutylammonium bromide is shown
to catalyze the reaction between terminal epoxides and carbon
dioxide at ambient temperature and 1 bar carbon dioxide
pressure and between internal epoxides and carbon dioxide at
80 °C and 10 bar carbon dioxide pressure to form cyclic
carbonates. The optimal conditions involve the use of 1.5−2.5
mol % of both the chromium(III) salophen bromide complex
and tetrabutylammonium bromide and result in the formation
of cyclic carbonates in 57−92% isolated yields after a reaction time of 24 h. Under these conditions, no polycarbonate formation
is observed except when cyclohexene oxide is used as substrate. The reactions were found to proceed with retention of epoxide
stereochemistry. A study of the reaction kinetics revealed that the chromium(III) complex and tetrabutylammonium bromide
react together to form a six-coordinate anionic chromium complex which is the actual catalyst, and a catalytic cycle is proposed
which explains the experimentally observed results.
KEYWORDS: chromium, salophen, cyclic carbonate, epoxide, carbon dioxide
carbonate4), largely due to the release of the ring strain
INTRODUCTION
■
associated with the three-membered ring within the epoxide.
The chemical utilization of carbon dioxide is currently
However, carbon dioxide does not react spontaneously with
undergoing a significant resurgence of interest due to the
epoxides; a suitable catalyst is required, and depending upon
realization that carbon capture and utilization can provide a
the choice of catalyst (and reaction conditions), the reaction
cost-effective complement to carbon capture and storage and
can be controlled to form either a cyclic carbonate5,6 (the
that carbon dioxide is an inexpensive and sustainable alternative
thermodynamic product of the reaction) or an aliphatic
carbon source for the chemicals industry.1,2 There are two main
polycarbonate6,7 (the kinetic product of the reaction). These
challenges in carbon dioxide chemistry. The first of these is that
are both important reactions, with cyclic carbonate production
carbon dioxide has a very negative heat of formation (ΔHf° =
−394 kJ mol−1);3 thus, there are rather few reactions of carbon
dioxide that have negative enthalpies or Gibbs energies of
reaction. The second challenge is that very few reactions of
carbon dioxide occur spontaneously; therefore, effective
catalysts have to be developed even for reactions which do
have negative enthalpies and Gibbs energies of reaction.
These challenges are highlighted in the reaction between
carbon dioxide and epoxides (Scheme 1). This reaction is
exothermic (ΔHr = −144 kJ mol−1 for the reaction between
carbon dioxide and ethylene oxide to form ethylene
having been commercialized over 50 years ago8 and cyclic
carbonates now having a range of applications including as
polar aprotic solvents,9,10 electrolytes for lithium ion batteries,11
monomers for polymer synthesis,12 and intermediates for the
synthesis of other chemicals.13 Aliphatic polycarbonate
production is just in the process of being commercialized,7d,14
as aliphatic polycarbonates have the potential to replace
aromatic polycarbonates15 and can also be used in the
production of polyurethanes.16
Even though the synthesis of cyclic carbonates from epoxides
and carbon dioxide is a highly exothermic reaction, commercial
processes still operate at high temperatures and pressures and
use highly purified carbon dioxide.17 This is because the
catalysts used commercially are quaternary ammonium18 or
phosphonium19 salts, which are inexpensive but not very
Scheme 1. Reaction between Carbon Dioxide and Epoxides
Received: May 17, 2016
Revised: June 21, 2016
© XXXX American Chemical Society
5012
DOI: 10.1021/acscatal.6b01386
ACS Catal. 2016, 6, 5012−5025

ACS Catalysis
Research Article
effective catalysts. As a result, the commercial synthesis of cyclic
carbonates is currently a net carbon dioxide emitter rather than
consumer. Over the last 9 years we have developed bimetallic
aluminum salen complexes such as 1 (Chart 1) as much more
zinc35,37,43 as catalysts for cyclic carbonate synthesis under
the appropriate reaction conditions. However, in our previous
work21b we found that mononuclear salen complexes of
aluminum, chromium, cobalt, and manganese were not effective
catalysts under mild reaction conditions (1 bar of CO2 pressure
and room temperature). In view of the excellent results
reported using zinc salophen complexes such as 6 (Chart 2),44
Chart 1. Aluminum-Based Catalysts for Cyclic Carbonate
Synthesis
Chart 2. Metal salophen Complexes for Cyclic Carbonate
Synthesis
we therefore decided to investigate the use of the salophen
ligand with a metal in the +3 oxidation state and herein report
the results of a study using chromium salophen complexes as
catalysts.
RESULTS AND DISCUSSION
■
The condensation reaction of substituted salicylaldehydes and
various o-phenylenediamines gave several salophen ligands with
different steric and electronic properties. Subsequent reaction
with chromium(II) chloride under an argon atmosphere and
oxidation by air afforded the respective chromium(III)
salophen complexes in excellent yield, as shown in Scheme 2.
Complexes 7a,e have previously been reported and used as
catalysts for the copolymerization of oxetanes and carbon
dioxide.45 The X-ray structure of complex 7a has previously
been reported,46 and the infrared and mass spectra of
compounds 7a−k confirmed their structures (see the
Experimental Section and Supporting Information).
effective catalysts for cyclic carbonate synthesis.20 The
combination of complex 1 and a quaternary ammonium or
phosphonium halide cocatalyst (2.5 mol % of each catalyst
component) is capable of converting a wide range of terminal
epoxides into the corresponding cyclic carbonates at room
temperature (or below) and 1 bar carbon dioxide pressure,21 in
some cases with kinetic resolution of the epoxide.22 Recently,
we have shown that complex 1 with no halide cocatalyst can
catalyze the synthesis of cyclic carbonates from terminal
epoxides and carbon dioxide under optimal reaction con-
ditions.23 Reactions catalyzed by complex 1 were also shown to
be compatible with the unpurified carbon dioxide produced by
methane combustion in a membrane-based oxyfuel combustion
system.24 When the temperature was raised to 50 °C and the
carbon dioxide pressure to 25 bar, even compressed air could
be used as the carbon dioxide source, and at 60 °C and 10 bar
carbon dioxide pressure, internal epoxides could be converted
into the corresponding cyclic carbonates.25 Subsequently, one-
component26 2 and immobilized27 3 catalysts were prepared
and used in both batch and gas-phase flow reactors.
Immobilized catalysts 3 were shown to be compatible with
waste carbon dioxide present in power station flue gas.28
Recently, we have been seeking to extend our studies of
cyclic carbonate synthesis away from aluminum salen
complexes and have demonstrated that bimetallic aluminum
acen complexes29 such as 4 and aluminum heteroscorpionate
complexes30 such as 5 also give highly active catalysts for cyclic
carbonate synthesis at room temperature and 1 bar carbon
dioxide pressure. However, we31 and others32 were also
interested in exploring the use of metals other than aluminum.
Previous work has reported the use of salen complexes of
chromium,33,34 cobalt,33,35,36 copper,35,37 magnesium,33a man-
ganese,38 nickel,39 ruthenium,40 tin,41 titanium,42 and
Complex 7a was first tested as a catalyst for the formation of
styrene carbonate (9a) from styrene oxide (8a) and carbon
dioxide under solvent-free conditions, as shown in Scheme 3,
and the results are shown in Table 1. Control experiments
(Table 1, entries 1 and 2) showed that neither tetrabutylam-
monium bromide nor chromium salophen complex 7a
displayed significant catalytic activity in the absence of the
other catalyst component. This is in line with previous work
using chromium salen derived catalysts, where the reaction only
occurred in the presence of a cocatalyst.33,34 The combination
of complex 7a and tetrabutylammonium bromide, tetrabuty-
lammonium iodide, or bis(triphenylphosphoranylidene)-
ammonium bromide as cocatalyst was found to be a highly
efficient catalyst system (Table 1, entries 5, 7, and 9).
Tetrabutylammonium fluoride, tetrabutylammonium chloride,
4 - d i m e t h y l a m i n o p y r i d i n e , a n d b i s -
(triphenylphosphoranylidene)ammonium chloride were also
investigated as cocatalysts but were found to be less effective
(Table 1, entries 3, 4, 8, and 10). The trend found for the
influence of the halide counterion in the cocatalyst on the
catalyst activity was Br ≈ I > Cl > F (Table 1, entries 3−9).
This suggests that the optimal results are obtained when the
halide is a good nucleophile and also a good leaving group, so
that the anion can effectively ring open the epoxide and be
displaced to allow the formation of the cyclic carbonate.
5013
DOI: 10.1021/acscatal.6b01386
ACS Catal. 2016, 6, 5012−5025

ACS Catalysis
Research Article
Scheme 2. Synthesis of Chromium salophen Complexes 7a−k
Scheme 3. Synthesis of Cyclic Carbonates 9a−j using
Complexes 7a−k
Table 2. Conversion of Epoxides 8a−j into Cyclic
a
Carbonates 9a−j using Catalyst 7a and Bu4NBr
b
c
entry
epoxide
temp (°C) conversion (%)
yield (%)
1
8a (R = Ph)
25
0
100
d
93
71
84
88
79
91
64
72
73
50
85
89
2
8b (R = Me)
3
8c (R = Et)
25
25
25
25
25
25
25
25
50
50
100
100
100
100
67
4
8d (R = Bu)
5
8e (R = Oct)
6
8f (R = CH2Cl)
8g (R = CH2OH)
8h (R = CH2OPh)
8i (R = 4-ClC6H4)
8j (R = 4-BrC6H4)
8i (R = 4-ClC6H4)
8j (R = 4-BrC6H4)
7
8
77
Doubling the concentration of tetrabutylammonium bromide
(Table 1, entry 6) did not significantly increase the conversions.
The synthesis of 10 cyclic carbonates 9a−j derived from
terminal epoxides 8a−j was investigated using 2.5 mol % of
complex 7a and tetrabutylammonium bromide at room
temperature and 1 bar carbon dioxide pressure (except for
propylene oxide 8b, which was used at 0 °C due to its
volatility) to investigate whether complex 7a provided a good
catalyst system before further optimization (Table 2 and
Scheme 3). To our delight, catalyst 7a was able to convert a
wide range of terminal epoxides into their corresponding cyclic
carbonates in good to excellent yields at ambient temperature
and pressure.
9
80
10
11
12
60
100
100
a
Reactions carried out at 1 bar CO2 pressure for 24 h using 2.5 mol %
b
of complex 7a and 2.5 mol % of Bu4NBr cocatalyst. Determined by
c
1H NMR spectroscopy of the crude reaction mixture. Yield of pure
d
isolated cyclic carbonate. The volatile nature of epoxide 8b meant
that conversion could not be determined.
salicylaldehyde moieties 7a,c,d and the unsubstituted salicy-
laldehyde catalyst 7b gave excellent conversions after 24 h and
very similar conversions after 3 and 6 h (Table 3, entries 1−4).
This trend can be related to the absence of any steric effect.
Complexes 7e−f were synthesized to study the electronic
influence of the substituents on the aromatic rings. It is
apparent that electronic effects have a significant impact on the
activity of the catalyst (Table 3, entries 5 and 6). If an electron-
donating methoxy group is introduced to the catalyst, the
catalytic activity increases. However, strongly electron with-
After epoxides 8a−j were tested using complex 7a as catalyst
and in view of the potential of this chromium(III) salophen
chloride based system, a catalyst optimization study was carried
out using tetrabutylammonium bromide as cocatalyst and
styrene oxide as substrate at room temperature and 1 bar
pressure of carbon dioxide. In order to increase the activity of
the catalyst, the substituents on the aromatic rings of the
t
salophen ligand were varied. Catalysts with Bu groups on the
a
Table 1. Synthesis of Carbonate 9a Catalyzed by Complex 7a
b
c
d
c
e
c
entry
cocat.
conversion in 3 h (%)
TOF (h−1
)
conversion in 6 h (%)
TOF (h−1
)
conversion in 24 h (%)
TOF (h−1
)
1
2
0
0
0.00
0.00
0.40
1.33
4.93
5.46
4.53
2.40
4.13
0.00
0
0
0.00
0.00
0.60
1.33
4.00
4.33
3.53
1.93
3.13
0.00
0
1
0.00
0.02
0.57
1.03
1.67
1.67
1.55
1.03
1.52
0.03
Bu4NBr
3
Bu4NF
3
9
34
62
100
100
93
62
91
2
4
Bu4NCl
10
37
41
34
18
31
0
20
60
65
53
29
47
0
5
Bu4NBr
6
Bu4NBr (5 mol %)
Bu4NI
7
d
8
PPNCl
e
9
PPNBr
f
10
DMAP
a
b
Reactions carried out at room temperature and 1 bar CO2 pressure for 24 h using 2.5 mol % of catalyst and 2.5 mol % of cocatalyst. Conversion
c
determined by 1H NMR spectroscopy of the crude reaction mixture. TOF = (mol of product)/((mol of catalyst) time).
d
e
f
Bis(triphenylphosphoranylidene)ammonium chloride. Bis(triphenylphosphoranylidene)ammonium bromide. 4-Dimethylaminopyridine.
5014
DOI: 10.1021/acscatal.6b01386
ACS Catal. 2016, 6, 5012−5025

ACS Catalysis
Research Article
a
Table 3. Synthesis of Carbonate 9a Catalyzed by Complexes 7a−k
b
c
d
c
e
c
entry
cat.
conversion (%)
TOF (h−1
)
conversion (%)
TOF (h−1
)
conversion (%)
TOF (h−1
)
1
7a
7b
7c
7d
7e
7f
31
31
30
21
45
7
4.13
4.13
4.00
2.80
6.00
0.93
1.60
1.60
2.80
3.60
4.00
51
49
53
41
69
13
26
27
43
47
52
3.40
3.27
3.53
2.73
4.60
0.87
1.73
1.80
2.87
3.13
3.47
93
92
1.55
1.53
1.67
1.53
1.67
0.68
1.42
1.20
1.58
1.67
1.62
2
3
100
92
4
5
100
41
6
7
7g
7h
7i
12
12
21
27
30
85
8
72
9
95
10
11
7j
100
97
7k
a
b
d
Reactions carried out at room temperature and 1 bar CO2 pressure for 24 h using 2.5 mol % of catalyst and 2.5 mol % of Bu4NBr cocatalyst.
c
1
Conversion determined by H NMR spectroscopy of the crude reaction mixture after 3 h. TOF = (mol of product)/((mol of catalyst) time).
e
Conversion determined by 1H NMR spectroscopy of the crude reaction mixture after 6 h. Conversion determined by 1H NMR spectroscopy of the
crude reaction mixture after 24 h.
drawing substituents such as NO2 (7f) on the aromatic ring
decrease the activity. This trend can be explained by
considering the interaction between the Lewis acidic chromium
complex and the bromo-alkoxide intermediate formed by
reaction of the epoxide with tetrabutylammonium bromide.
Electron-donating substituents on the salophen ligand will
weaken the chromium−alkoxide bond and thus facilitate the
carbon dioxide insertion step. In contrast, electron-withdrawing
substituents will strength the chromium−alkoxide bond and
make its reaction with carbon dioxide more difficult.
Chart 3. Counterion Optimization for Chromium salophen
Complexes
The introduction of substituents at R1 had a less marked
effect (Table 3, entries 7−9). Complex 7g with 2,3-
diaminonapthalene as the diamine backbone showed slightly
lower activity than complex 7b derived from 1,2-diaminoben-
zene. The introduction of an electron-donating methyl group to
the R1 position (7h) gave lower activity in comparison to 7b,
but a substitution by Cl (7i) gave a catalyst with slightly higher
productivity for the synthesis of styrene carbonate than 7b after
24 h.
As complexes 7c,e had given complete conversion of styrene
oxide to styrene carbonate in 24 h at room temperature and 1
bar of carbon dioxide pressure (Table 3, entries 3 and 5), the
synthesis of catalysts 7j,k was undertaken, combining the
optimal aldehydes and the optimal diamine. However, these
catalysts were found to be less active than catalysts 7c,e after 24
h (Table 1, entries 10 and 11). Therefore, catalyst 7e was
chosen as the best catalyst, as it gave higher conversions after 3
and 6 h than catalyst 7c.
To further optimize the composition of the catalyst, the
effect of altering the counterion present in the most active
catalyst (complex 7e) was investigated (Chart 3). The catalytic
activity of complexes 7e,l−o was found to vary depending upon
the counterion with the order of reactivity bromide > chloride
> acetate > iodide > tosylate (entries 1−5, Table 4). In general,
the more nucleophilic the counterion, the more active the
catalyst system for cyclic carbonate formation. A similar trend
was seen in chromium(III) salen complexes by Darensbourg,46
where a higher catalytic activity was obtained when a more
nucleophilic azide counterion was used instead of a chloride
counterion of the catalysts. The trend is broken, however, by
the iodide counterion (entry 3, Table 4), probably due to the
large size of the iodide counterion, hindering the reaction.
Having optimized the composition of the catalyst, the
synthesis of cyclic carbonates 9a−j from terminal epoxides 8a−
j was studied using 2.5 mol % of both complex 7l and
tetrabutylammonium bromide at room temperature and 1 bar
carbon dioxide pressure for 24 h. This study allowed the
catalytic efficiency of complexes 7a,l to be compared for a range
of epoxides, as there may be a dependence of the catalytic
activity on the solubility of the catalyst in the epoxide. The
results of this study are shown in Table 5. In general, catalysts
7a,l gave very similar conversions (compare Tables 1 and 5),
but complex 7l gave better results with epoxides 8g,j, which
gave the lowest conversions in reactions catalyzed by complex
7a. Therefore, complex 7l was felt to be the more generally
applicable catalyst and was used for further studies. Notably, the
catalysts were compatible with aliphatic, aromatic, and
functionalized substrates containing halogen and hydroxyl
groups. There was also no apparent influence of epoxide size
on catalyst activity. This may be connected to the stability of
the salophen ligand in complexes 7a,l and the ease of epoxide
5015
DOI: 10.1021/acscatal.6b01386
ACS Catal. 2016, 6, 5012−5025

ACS Catalysis
Research Article
a
Table 4. Synthesis of Carbonate 9a Catalyzed by Complexes 7e,l−o
b
c
d
c
e
c
entry
cat.
conversion (%)
TOF (h−1
)
conversion (%)
TOF (h−1
)
conversion (%)
TOF (h−1
)
1
2
3
4
5
7e
7l
45
44
28
34
4
6.00
5.87
3.73
4.53
0.53
69
71
46
57
7
4.60
4.73
3.07
3.80
0.47
100
100
100
100
42
1.67
1.67
1.67
1.67
0.70
7m
7n
7o
a
b
d
Reactions carried out at room temperature and 1 bar CO2 pressure for 24 h using 2.5 mol % of catalyst and 2.5 mol % of Bu4NBr cocatalyst.
c
1
Conversion determined by H NMR spectroscopy of the crude reaction mixture after 3 h. TOF = (mol of product)/((mol of catalyst) time).
e
Conversion determined by 1H NMR spectroscopy of the crude reaction mixture after 6 h. Conversion determined by 1H NMR spectroscopy of the
crude reaction mixture after 24 h.
Table 5. Conversion of Epoxides 8a−j into Cyclic
dioxide pressure for 24 h (Scheme 4) and initial experiments
with cyclohexene oxide 10a showed that under these conditions
the complex 7l and tetrabutylammonium bromide loadings
could be reduced from 2.5 to 1.5 mol % without having a major
effect on the conversion (82 and 78%, respectively). However,
further reduction of the loading of complex 7l and
tetrabutylammonium bromide to 0.5 mol % resulted in the
conversion decreasing to just 20%.
a
Carbonates 9a−j using Catalyst 7l and Bu4NBr
b
c
entry
epoxide
temp (°C) conversion (%)
yield (%)
1
8a (R = Ph)
25
0
100
d
92
57
86
81
82
78
72
71
78
49
91
89
2
8b (R = Me)
3
8c (R = Et)
25
25
25
25
25
25
25
25
50
50
100
95
4
8d (R = Bu)
5
8e (R = Oct)
89
Disubstituted cyclic carbonates 11b−f were synthesized in
moderate to good yield from their corresponding internal
epoxides 10b−f and carbon dioxide using 1.5 mol % of both
complex 7l and tetrabutylammonium bromide at 50 °C and 10
bar carbon dioxide pressure under solvent-free conditions for
24 h (Table 6). It is worth noting that, when cyclohexene oxide
10a was used, poly(cyclohexene carbonate) was obtained in
72% yield (Table 6, entry 1), as previously reported by
Darensbourg and co-workers.34 However, no polycarbonate
formation was observed when other epoxides were used; rather,
the cyclic carbonate product was obtained with a selectivity
higher than 99%. Under these reaction conditions, cyclo-
pentene oxide 10b was a good substrate, giving cyclic carbonate
11b in 66% yield (Table 6, entry 2). The cis and trans cyclic
carbonates 11c,d were isolated in low yield (Table 6, entries 3
and 4), with almost complete retention of stereochemistry.
Stilbene oxide 10e gave no conversion at 50 °C, as it is a solid
under these reaction conditions. Sterically hindered epoxide 10f
was converted into the cyclic carbonate product 11f in 52%
yield.
6
8f (R = CH2Cl)
8g (R = CH2OH)
8h (R = CH2OPh)
8i (R = 4-ClC6H4)
8j (R = 4-BrC6H4)
8i (R = 4-ClC6H4)
8j (R = 4-BrC6H4)
100
86
7
8
71
9
100
71
10
11
12
100
100
a
Reactions carried out at 1 bar CO2 pressure for 24 h using 2.5 mol %
b
of complex 7l and 2.5 mol % of Bu4NBr cocatalyst. Determined by
c
1H NMR spectroscopy of the crude reaction mixture. Yield of pure
d
isolated cyclic carbonate. The volatile nature of epoxide 8b meant
that conversion could not be determined.
coordination to mononuclear complexes bearing a planar
salophen ligand.
In order to expand the substrate scope, the synthesis of cyclic
carbonates from internal epoxides and carbon dioxide was
investigated using the catalyst system comprised of complex 7l
and tetrabutylammonium bromide. Even though internal
epoxides are less reactive than terminal epoxides, successful
catalysts for these substrates have been recently develo-
ped.30c,32,47 In view of this lower reactivity, reactions with
internal epoxides were carried out at 50 °C and 10 bar carbon
By increasing the reaction temperature to 80 °C and keeping
the catalyst concentration at 1.5 mol %, epoxides 10b−f were
successfully converted into cyclic carbonates 11b−f in 60−89%
Scheme 4. Synthesis of Polycyclohexene Carbonate 11a or Cyclic Carbonates 11b−f using Complex 7l and
Tetrabutylammonium Bromide as a Catalyst System
5016
DOI: 10.1021/acscatal.6b01386
ACS Catal. 2016, 6, 5012−5025

ACS Catalysis
Research Article
catalyzed by complex 7l and tetrabutylammonium bromide.
Analysis of the NMR spectra of cyclic carbonates 11a−g
revealed that cyclic carbonate formation catalyzed by complex
7l and tetrabutylammonium bromide occurs with retention of
the epoxide stereochemistry.30,32,47,48 These results are in good
agreement with the general mechanism proposed for the
synthesis of cyclic carbonates catalyzed by a combination of a
Lewis acid (including chromium metal complexes) and a
nucleophile as catalyst system5,33,34 in which the epoxide
undergoes two substitution reactions at the less hindered
carbon atom.47,48
To further study the reaction mechanism, a kinetic study of
the synthesis of styrene carbonate 9a catalyzed by catalyst 7l
and tetrabutylammonium bromide was carried out at 50 °C and
1 bar carbon dioxide pressure under solvent-free conditions.
Samples were taken from the reaction mixture every 30 min
and analyzed by high-performance liquid chromatography
(HPLC) to determine the conversion of styrene oxide 8a
into styrene carbonate 9a. We have previously shown that,
under these conditions, the reaction follows zero-order kinetics
in the early stages of the reaction, since the epoxide acts as a
substrate and reaction solvent, but as the reaction proceeds the
reaction follows first-order kinetics as the cyclic carbonate
becomes the main species in the reaction mixture.23,30c
The general rate equation for this reaction is shown in eq 1.
Since the concentrations of carbon dioxide, complex 7l, and
tetrabutylammonium bromide do not change during the
reaction, eq 1 may be written as eq 2. As mentioned previously,
in the early stages of the reaction, the concentration of styrene
oxide 8a does not change; therefore, the kinetics can be fitted
to zero-order kinetics and the rate equation can be written as eq
3. However, as the reaction proceeds, the reaction becomes first
order in epoxide 8a concentration and the rate equation may be
expressed as eq 4.
Table 6. Conversion of Epoxides 10a−f into
Poly(cyclohexene carbonate) 11a or Cyclic Carbonates
a
11b−f using Catalyst 7l and Bu4NBr
yield at
yield at
conversion at
50 °C
conversion at
b
80 °C
b
c
c
entry substrate 50 °C (%)
(%)
80 °C (%)
(%)
d
d
d
d
1
2
3
4
5
6
10a
10b
10c
10d
10e
10f
78
72
e
72
80
76
66
20
19
0
93
e
85
f
21, 63
25, 60
89
f
e
e
0
98
100
58
52
87
a
Reactions carried out at 10 bar CO2 pressure for 24 h using 1.5 mol
b
% of catalyst 7l and 1.5 mol % of Bu4NBr cocatalyst. Conversion
1
determined by H NMR spectroscopy of the crude reaction mixture
c
d
after 24 h. Yield of pure isolated cyclic carbonate. Poly(cyclohexene
e
carbonate) was obtained. The volatile nature of the epoxide meant
that conversion could not be determined. Using 3.0 mol % of Bu4NBr.
f
yield (Table 6). Once again, poly(cyclohexene carbonate) was
obtained in 76% yield with a high content of carbonate linkages
(>99%) and PCHC/CHC ratio (>99%) when cyclohexene
oxide 10a was used as substrate (Table 6, entry 1).
1
Poly(cyclohexene carbonate) was characterized by H NMR
and 13C NMR and Gel Permeation Chromatography (GPC)
(see the Supporting Information). 13C NMR confirmed that the
poly(cyclohexene carbonate) 11a obtained is atactic, as both
isotactic (153.7 ppm) and syndiotactic (153.0−153.3 ppm)
diads were observed. GPC showed that the polycarbonate
prepared using chromium complex 7l as catalyst at 80 °C and
10 bar CO2 had Mn = 20518 and Mw = 23202 (Mw/Mn = 1.13)
relative to polystyrene standards (see the Supporting
Information).
rate = k[8a]a[CO2]b[7l]c[Bu4NBr]d
(1)
rate = kobs[8a]a where kobs = k[CO2]b[7l]c[Bu4NBr]d
(2)
rate = k0,obs where k0,obs = k0[8a]a[CO2]b[7l]c[Bu4NBr]d
On the other hand, the cyclic carbonate product was
obtained when epoxides 10b−f were used. As expected, at 80
°C the catalyst system displays higher activity, resulting in an
increase in the yield of the corresponding cyclic carbonate
(Table 6, entries 2−6). Under these reaction conditions,
cyclopentene carbonate 11b was isolated in 85% yield (Table 6,
entry 2). When the same reaction conditions were applied to
epoxides 10c,d, the cis and trans cyclic carbonates 11c,d were
isolated in poor yield. However, when the tetrabutylammonium
bromide loading was increased to 3 mol %, the yields of cyclic
carbonates 11c,d increased to 60−63% (Table 6, entries 3 and
4). A 97:3 ratio of cis and trans isomers of cyclic carbonate 11c
was observed from cis epoxide 10c, while complete retention of
stereochemistry of the trans isomer of cyclic carbonate 11d
from trans epoxide 10d was observed. trans-Stilbene oxide 10e
was successfully converted into trans-1,2-diphenylethylene
carbonate 11e in 89% isolated yield at 80 °C (Table 6, entry
5). Finally, epoxide 10f was converted into its corresponding
cyclic carbonate product 11f in 87% yield under these reaction
conditions.
(3)
rate = k1,obs[8a] where k1,obs = k1[CO2]b[7l]c[Bu4NBr]d
(4)
The order with respect to the concentration of complex 7l
and tetrabutylammonium bromide were studied by carrying out
two sets of reactions at four different concentrations of complex
7l or tetrabutylammonium bromide, keeping the concentration
of the other catalyst component constant.49 First, the order
with respect to the [Bu4NBr] was studied using [7l] = 175 mM
and [Bu4NBr] = 175−437 mM. The log [Bu4NBr]/log k0,obs
and log [Bu4NBr]/log k1,obs plots had slopes of 0.96 and 1.05,
respectively, indicating that the reaction is first order with
respect to the concentration of tetrabutylammonium bromide
(d = 1).49 This was also confirmed by plotting k0,obs or k1,obs
versus [Bu4NBr] ,which also showed a good fit to a straight line
that passed through the origin (Figure 1).
The order with respect to [7l] was first studied with
[Bu4NBr] = 175 mM and [7l] = 175−437 mM. The slope
obtained when plotting log [7l]/log k1,obs was 0.09 (Figure 2)
indicated that the reaction is zero order with respect to the
concentration of catalyst 7l under conditions where [Bu4NBr]
The use of internal epoxides 10a−f as substrates gave us the
opportunity to gain insight into the reaction mechanism and
study the stereochemistry of the synthesis of cyclic carbonates
5017
DOI: 10.1021/acscatal.6b01386
ACS Catal. 2016, 6, 5012−5025

ACS Catalysis
Research Article
°C and 1 bar pressure of carbon dioxide may be expressed as
shown in eq 5 during the early stages of the reaction or as
shown in eq 6 during the latter stages of the reaction.
rate = k0,obs where k0,obs = k0[8a][CO2][7l][Bu4NBr]
(5)
rate = k1,obs[8a] where k1,obs = k1[CO2][7l][Bu4NBr]
(6)
From the kinetic experiments it is apparent that the
chromium salophen complex and tetrabutylammonium bro-
mide interact to form a new 1:1 species, the concentration of
which depends upon the concentration of the limiting catalyst
component. Darensbourg and co-workers have shown by X-ray
crystallography that chromium salen chloride compounds form
six-coordinate complexes in the presence of 2 equiv of anions
Figure 1. Plot of the observed first-order rate constant versus
[Bu4NBr]. Reactions were carried out at 50 °C under solvent-free
conditions with [7l] = 175 mM and [Bu4NBr] = 175−437 mM. Data
shown are the average of two experiments, with the individual
experiments being used to indicate the error bars.
50
such as Cl−, CN−, N3 , and NCO−. By extension of this
reasoning to our chromium salophen system, Bu4NBr could act
as an anion source to form the six-coordinate [Cr(salophen)-
Br2]− complex 12 (Scheme 5). This would explain the need for
an excess of Bu4NBr to study the order with respect to the
concentration of complex 7l in our chromium salophen/
Bu4NBr catalyst system.
−
Experiments were conducted in which tetrabutylammonium
bromide was added to a solution of catalyst 7l, to determine if
the observation of a Br−Cr−Br system was possible.
Tetrabutylammonium bromide was added to a solution of the
catalyst in dichloromethane in 0.5 mol equiv aliquots up to 5
mol equiv, and the solution was analyzed via UV−vis and FT-
IR spectroscopy.
UV−vis analysis showed an increase in absorbance at
approximately 418 and 506 nm on addition of tetrabutylam-
monium bromide, with the increase in absorbance plateauing
after addition of 3.5 mol equiv of tetrabutylammonium
bromide. A slight shift in peak position also occurred for
both peaks, from 506 to 512 nm and 423 to 413 nm,
respectively. This suggests that a change in the structure of the
catalyst is occurring during the addition of from 0.5 to 3.5 mol
equiv of tetrabutylammonium bromide (Figure 4).
Figure 2. Plot of the log(k1,obs avg) against log[7l]. Reactions were
carried out at 25 °C under solvent-free conditions with [Bu4NBr] =
175 mM and [7l] = 175−437 mM. Data shown are the average of two
experiments, with the individual experiments being used to indicate
the error bars.
< [7l] (c = 0).49 However, when the kinetic study was repeated
with [Bu4NBr] = 437 mM and [7l] = 87−350 mM ([Bu4NBr]
> [7l]), the slopes obtained when plotting log [7l]/log k0,obs
and log [7l]/log k1,obs were 0.81 and 1.07, respectively,
indicating that the reaction is first order with respect to the
concentration of catalyst 7l (c = 1).49 This was confirmed by
plotting k0,obs and k1,obs versus [7l], which also showed a good
fit to a straight line that passed through the origin (Figure 3).
Consequently, the rate equation for the synthesis of styrene
carbonate 9a from styrene oxide 8a catalyzed by complex 7l
and tetrabutylammonium bromide with [Bu4NBr] > [7l] at 25
In the FT-IR spectra, a new peak is present at 697 cm−1
which is not visible in the spectra of tetrabutylammonium
bromide or catalyst 7l alone (Figure 5). The peak appears after
1.5 equiv of tetrabutylammonium bromide is added and shows
no major changes after addition of 3−3.5 equiv. A peak at 730
cm−1 also grows in intensity during the addition of
tetrabutylammonium bromide and appears at a shift slightly
different than that seen in pure tetrabutylammonium bromide
(738 cm−1).
Taking into account that cyclic carbonate formation occurs
with retention of the epoxide stereochemistry and that the
reaction kinetics indicate a first-order dependence on the
concentration of the catalyst and Bu4NBr, the mechanism
shown in Scheme 6 is proposed. On addition of 1 equiv of
Bu4NBr to a Cr(salophen) bromide complex, the six-coordinate
bis-bromide complex [Cr(salophen)Br2]− is formed. Upon
addition of the epoxide to the catalyst system an equilibrium is
established between the bis-bromide complex 12 and the
epoxide adduct Cr(salophen)Br·(epoxide). It has been reported
that this equilibrium is shifted toward the neutral epoxide
adduct in pure epoxide solution.50 The noncoordinated
bromide attacks the less hindered carbon of the epoxide with
the first inversion of the stereochemistry. Then, carbon dioxide
is inserted into the chromium−oxygen bond to form a metal
Figure 3. Plot of the observed first-order rate constant versus [7l].
Reactions were carried out at 25 °C under solvent-free conditions with
[Bu4NBr] = 437 mM and [7l] = 87−350 mM. Data shown are the
average of two experiments, with the individual experiments being
used to indicate the error bars.
5018
DOI: 10.1021/acscatal.6b01386
ACS Catal. 2016, 6, 5012−5025

ACS Catalysis
Research Article
Scheme 5. Formation of a [Cr(salophen)Br2]− Complex in the Presence of Bromide Anions
Scheme 6. Proposed Mechanism for the Synthesis of Cyclic
Carbonates from Epoxides and Carbon Dioxide Catalyzed by
Complex 7l and Tetrabutylammonium Bromide as a Catalyst
System
Figure 4. UV−vis spectra on addition of tetrabutylammonium
bromide to a solution of 7l in dichloromethane.
tetrabutylammonium bromide displays excellent catalytic
activity at room temperature and 1 bar pressure of carbon
dioxide under solvent-free conditions. The substrate scope was
evaluated, and the catalyst system displayed a broad substrate
scope catalyzing cyclic carbonate formation from alkyl, aryl, and
functionalized terminal epoxides as well as internal epoxides in
good to excellent yields. Cyclic carbonates were synthesized
with a selectivity higher than 99% except when cyclohexene
oxide 10a was used as substrate. In this case, poly(cyclohexene
carbonate) was obtained in good yield.
carbonate which ring closes with a second inversion to afford
the cyclic carbonate with overall retention of the epoxide
stereochemistry.
CONCLUSIONS
■
Chromium salophen complexes 7a−o have been developed as
highly active catalysts for the synthesis of cyclic carbonates from
terminal epoxides and carbon dioxide in the presence of
tetrabutylammonium bromide as a cocatalyst. The catalyst
system formed by the combination of complex 7l and
The reaction kinetics of the catalytic process were studied,
and the reaction was found to be first order with respect to the
Figure 5. IR spectra obtained on addition of tetrabutylammonium bromide to a solution of 7l in dichloromethane.
5019
DOI: 10.1021/acscatal.6b01386
ACS Catal. 2016, 6, 5012−5025

ACS Catalysis
Research Article
concentration of complex 7l and tetrabutylammonium bro-
mide. From the kinetic study, we can conclude that both
catalyst components interact to form the six-coordinate
[Cr(salophen)Br2]− complex 12, which is the catalytically
active species. On the basis of the experimental results and the
kinetic study, a possible mechanism for the synthesis of cyclic
carbonates catalyzed by complex 7l and tetrabutylammonium
bromide has been proposed.
Even though there are many catalyst systems that have been
reported in the literature for cyclic carbonate synthesis, most of
them require the use of high temperature and pressure, which
emits more carbon dioxide than is used. We have shown that
chromium salophen complex 7l is an efficient catalyst for the
synthesis of cyclic carbonates even from challenging epoxides
under mild reaction conditions.
then cooled to room temperature and further diluted to 100
mL with ultrapure water in a 100 mL volumetric flask. Any
further dilutions were performed using ultrapure water if
required. The samples were analyzed with an Agilent 7700x
ICP-MS, using nickel sample and Skinner cones, and the
analysis was run under helium. In terms of sampling, the sample
was taken up for 60 s, stabilized for 40 s, and washed for 60 s
(5% HCl for 30 s and 2% HNO3 washes for 30 s). Each sample
was run three times, and the overall mean value of chromium in
ppm or ppb was obtained.
General Synthesis of Chromium salophen Complexes
7a−k. salophen ligand (1.0 equiv) was dissolved in dry THF
(50 mL) under argon. Then, anhydrous CrCl2 (1.0 equiv) was
added and the mixture was stirred for 24 h under argon. Air was
bubbled through the flask for another 24 h. Then, Et2O was
added and the organic layer was washed with a saturated
solution of ammonium chloride (2 × 25 mL) and brine (2 × 50
mL). The organic layer was dried (Na2SO4), and the solvent
was removed under reduced pressure to give chromium
salophen complexes 7a−k as red−brown solids (yield: 69−
97%).
EXPERIMENTAL SECTION
■
All chemicals were provided by Alfa Aesar, Acros Organics,
Sigma-Aldrich, or Fisher Scientific. Carbon dioxide was
purchased from BOC and used without further purification.
All reaction solvents were obtained from Fisher Scientific unless
otherwise stated and were HPLC grade wherever possible. For
reactions which required anhydrous conditions, all glassware
was predried in a heated oven (∼120 °C), flame-dried while in
vacuo, and then kept under a nitrogen or argon atmosphere.
Dry solvents were predried and purified using a Pure Solv MD-
7 solvent purification system.
Chromium Complex 7a. Yield: 0.3785 g, 97%. MS (ESI):
calcd for [C36H46CrN2O2]+, 590.2959; found, 590.2972, MS
(LIFDI): calcd for [C36H26CrClN2O2]+, 625.27; found, 625.27.
IR neat (selected absorbances): 2652, 2904, 2867 (C−H alkyl)
1602 (CN), 1524 (CC aromatic), 1198 (C−O), 1171
(C−N), 748 (C−H) cm−1. Mp: >350 °C. ICP-MS: expected
Cr 8.3%, found Cr 8.4%.
1H and 13C NMR spectra were obtained from a Jeol ECS-400
or Jeol ECX-400 NMR spectrometer (400 MHz) at room
temperature. 1H NMR spectra were referenced using the
CHCl3 peak of CDCl3 at 7.26 ppm or TMS at 0.00 ppm, and
13C NMR spectra were referenced using the middle peak of the
triplet CDCl3 peak at 77.00 ppm. MS analysis was run using a
Bruker microTOF MS, twinned with an Agilent series 1200 LC
for ESI spectrometry, or a Water GCT Premier MS, twinned
with an Agilent (HP) 7890A GC for EI and LIDFI mass
spectrometry. IR spectra of samples were obtained neat using a
PerkinElmer UATR Two FT-IT spectrometer. Spectra were
analyzed using the PerkinElmer software “Spectrum”. UV−vis
spectra of samples were obtained in solution using a Shimadzu
UV-1800 spectrometer and quartz UV−vis cells (1 cm).
Melting points of samples were measured using a Stuart SMP3
melting point apparatus (25−350 °C). High-performance
liquid chromatography was performed on an Agilent 1220
instrument fitted with a diode-array detector using a Chiralcel
OD column (25 cm by 4.6 mm), using hexane/isopropyl
alcohol (80/20 v/v) as eluent and a flow rate of 1.00 mL/min.
XRF analysis was performed using a Horiba XGT-7000 XRF
spectrometer with a Rh X-ray generator. Analyses of samples
and standards were performed with no X-ray filter, a preset
time of 60 s, and an X-ray voltage of 30 keV with a beam size of
1.2 mm. The calibration curves were created by mixing together
standards of known Cr and halide concentrations, making into
pellets after mixing together with a pestle and mortar, and
comparing the intensity readings obtained to the known
concentrations. All samples and standards were analyzed eight
times. The average value of the eight runs was used in
consequent calculations. Quantification of chromium in catalyst
samples was performed via ICP-MS. Samples were prepared by
first dissolving the desired sample in 5 mL of TraceSELECT
solvent grade nitric acid (Sigma-Aldrich) in a sample vial. The
sample was then heated to 110 °C for 3 h with a glass cover on
top of the vial to avoid sample evaporation. The samples were
Chromium Complex 7b. Yield: 0.9282 g, 91%. MS (ESI):
calcd for [C20H14CrN2O2]+, 336.0455; found, 366.0463, MS
(LIFDI): calcd for [C20H14CrN2O2]+, 336.05; found, 336.05.
IR neat (selected absorbances): 1606 (CN), 1537 (CC
aromatic), 1189 (C−O), 1150 (C−N), 749 (C−H) cm−1. Mp:
> 350 °C. ICP-MS: expected Cr 12.9%, found Cr = 12.6%.
Chromium Complex 7c. Yield: 0.180 g, 76%. MS (ESI):
calcd for [C28H30CrN2O2]+: 478.1707; found, 478.1715. Mass
Spec LIDFI: Calc: [C28H30CrClN2O2]+: 513.14; found, 513.15.
IR neat (selected absorbances): 2953 (C−H alkyl), 1601 (C
N), 1385 (CC aromatic), 1185(C−O), 1026 (C−N), 748
(C−H) cm−1. Mp: >350 °C. ICP-MS: expected Cr 10.1%,
found Cr 9.8%.
Chromium Complex 7d. Yield: 0.5738 g, 95%. MS (ESI):
calcd for [C28H30CrN2O2]+, 478.1707; found, 478.1698. MS
(LIFDI): calcd for [C28H30CrClN2O2]+, 513.1; found, 513.18.
IR neat (selected absorbances): 2956, 2918, 2649 (C−H alkyl),
1614 (CN), 1579 (CC aromatic), 1260 (C−O), 1180
(C−N), 746 (C−H) cm−1. Mp: >350 °C. XRF: expected ratio
Cr/Cl = 1/1, found ratio Cr/Cl = 1/0.97. ICP-MS: expected
Cr 10.1%, found Cr 10.2%.
Chromium Complex 7e. Yield: 0.0501 g, 90%. MS (ESI):
calcd for [C30H34CrN2O4]+, 538.1928; found, 538.1918. MS
(LIFDI): calcd for [C30H34CrClN2O4]+, 573.16; found, 573.18.
IR neat (selected absorbances): 2917 (C−H alkyl), 1601 (C
N), 1532 (CC aromatic), 1361 (C−O from C−O-Me), 1211
(C−O), 1050 (C−N), 744 (C−H) cm−1. Mp: >350 °C. XRF:
expected ratio Cr/Cl 1/1, found ratio Cr/Cl 1/0.96.
Chromium Complex 7f. Yield: 0.1055 g, 90%. MS (ESI):
calcd for [C28H28CrClN4O6]+: 603.1098; found, 603.1205. IR
neat (selected absorbances): 2958, 2921, 2854 (C−H alkyl),
1607 (CN), 1587 (N−O) 1523 (CC aromatic), 1323
(C−O), 1285 (N−O) 1199 (C−N), 744 (C−H) cm−1. Mp: >
350 °C. ICP-MS: expected Cr 12.9%, found Cr = 12.6%.
5020
DOI: 10.1021/acscatal.6b01386
ACS Catal. 2016, 6, 5012−5025

ACS Catalysis
Research Article
Chromium Complex 7g. Yield: 0.3118 g, 85%. MS (ESI):
calcd for [C24H16CrN2O2]+, 416.0622; found, 416.0634. MS
(LIFDI): calcd for [C24H16CrClN2O2]+, 451.03; found, 451.04.
IR neat (selected absorbances): 2917, 2849 (C−H alkyl), 1604
(CN), 1530 (CC aromatic), 1197 (C−O), 1150 (C−N),
747 (C−H) cm−1. Mp: >350 °C.
Chromium Complex 7h. Yield: 0.1632 g, 69%. MS (ESI):
calcd for [C22H18CrN2O2]+, 394.0768; found, 394.0756. MS
(LIFDI): calcd for [C22H18CrClN2O2]+, 429.05; found, 429.08.
IR neat (selected absorbances): 2917, 2846 (C−H alkyl), 1592
(CN), 1461 (CC aromatic), 1259 (C−O), 1027 (C−N),
751 (C−H) cm−1. Mp: >350 °C.
Chromium Complex 7i. Yield: 0.268 g, 75%. MS (ESI):
calcd for [C20H12Cl2CrN2O2]+, 433.9676; found, 433.9653. MS
(LIFDI): calcd for [C20H12Cl2CrN2O2]+, 433.97; found,
433.98. IR neat (selected absorbances): 2918, 2849 (C−H
alkyl), 1607 (CN), 1530 (CC aromatic), 1148 (C−O),
1029 (C−N), 752 (C−Cl) cm−1. Mp: >350 °C. XRF: expected
ratio Cr/Cl 1/3, found ratio Cr/Cl 1/3.3.
Chromium Complex 7j. Yield: 0.0332 g, 82%. MS (ESI):
calcd for [C28H28Cl2CrN2O2]+, 546.0928; found, 546.0913. MS
(LIFDI): calcd for [C28H28Cl3CrN2O2]+, 581.06; found,
581.08. IR neat (selected absorbances): 2917 (C−H alkyl),
1596 (CN), 1530 (CC aromatic), 1187 (C−O), 1147
(C−N), 870 (C−Cl), 749 (C−H) cm−1. Mp: >350 °C. XRF:
expected ratio Cr/Cl 1/3, found ratio Cr/Cl 1/2.9.
saturated aqueous sodium acetate solution (55 mL) to give
compound 7n as a dark red solid (0.0872 g, 83%). MS
(LIFDI): calcd for [C32H37CrN2O6]+, 597.21; found, 597.20.
MS (LIFDI): calcd for [C30H34CrN2O4]+, 538.19; found,
538.19. IR neat (selected absorbances: 2930 (C−H alkyl), 1602
(CN), 1531 (CC aromatic), 1360 (C−O from C−O−
Me), 1207 (C−O), 1058 (C−N), 748 (C−H) cm−1. Mp: >350
°C. XRF showed no chlorine present.
Synthesis of Chromium salophen Complex 7o.
Complex 7e (0.0790 g, 0.138 mmol) was dissolved in dry
acetonitrile (2 mL) to form a dark red solution. Toluene-p-
sulfonate (0.0520 g, 0.175 mmol) was dissolved in acetonitrile
(4 mL) to form a white clear solution. The toluene-p-sulfonate
solution was added slowly to the complex 7e solution and was
stirred under argon with the round-bottom flask covered with
foil for 18 h. The reaction mixture was filtered through Celite
and washed thoroughly with acetonitrile (15 mL). The solution
was concentrated in vacuo to afford complex 7o as a dark red
solid (0.0752 g, 77%). MS (LIFDI): calcd for
[C30H34CrN2O4]+, 538.19; found, 538.24. MS (LIFDI): calcd
for [C37H41CrN2O7S]+, 709.20; found, 709.17. IR neat
(selected absorbances): 2951 (C−H alkyl), 1601 (CN),
1534 (CC aromatic), 1358 (C−O from C−O−Me), 1150
(C−O), 1057 (C−N), 794 (C−H) cm−1. Mp: >350 °C. XRF
showed no chlorine present.
General Procedure for Catalyst Synthesis of Cyclic
Carbonates at 1 bar Pressure. An epoxide (1.66 mmol),
catalyst 7a or 7l (0.025 mmol), and Bu4NBr (8 mg, 0.025
mmol) were placed in a sample vial fitted with a magnetic
stirrer bar and placed in a large conical flask. Cardice pellets
were added to the conical flask, which was then fitted with a
rubber stopper pierced by a deflated balloon. The reaction
mixture was stirred at 25 °C for 24 h. The conversion of
epoxide to cyclic carbonate was determined by analysis of a
Chromium Complex 7k. Yield: 0.0156, 83%. MS (ESI):
calcd for [C30H32Cl2CrN2O4]+, 606.1140; found, 606.1134. MS
(LIFDI): calcd for [C30H32Cl3CrN2O4]+, 643.11; found,
643.08. IR neat (selected absorbances): 2919, 2850 (C−H
alkyl), 1597 (CN), 1530 (CC aromatic), 1357 (C−O
from C−O−Me), 1159 (C−O), 1057 (C−N), 822 (C−Cl),
778 (C−H) cm−1. Mp: >350 °C. XRF data: expected ratio Cr/
Cl 1/3, found ratio Cr/Cl 1/2.8. ICP-MS: expected Cr 8.1%,
found Cr 8.2%.
1
sample by H NMR spectroscopy. Cyclic carbonates 9a−j are
Synthesis of Chromium salophen Complex 7l.
Complex 7e (0.0756 g, 0.132 mmol) was dissolved in
CH2Cl2 (15 mL), forming a dark red solution, and was stirred
with saturated aqueous sodium bromide solution (55 mL) in a
round-bottom flask. The solvents were stirred vigorously for 3
h. The red organic layer was then extracted from the clear
aqueous layer, dried (Na2SO4), filtered, and concentrated in
vacuo to afford the product as a dark red solid (0.0345 g, 42%).
MS (LIFDI): calcd for [C30H34BrCrN2O4]+, 617.11; found,
617.09. MS (LIFDI): calcd for [C30H34CrN2O4]+ , 538.19;
found, 538.21. IR neat (selected absorbances): 2953 (C−H
alkyl), 1601 (CN), 1533 (CC aromatic), 1359 (C−O
from C−O-Me), 1208 (C−O), 1055 (C−N), 751 (C−H)
cm−1. Mp: >350 °C. XRF showed no chlorine present.
Synthesis of Chromium salophen Complex 7m. The
synthesis of 7m was carried out in a manner identical with that
for 7l, using complex 7e (0.0816 g, 0.1421 mmol) and a
saturated aqueous sodium iodide solution (55 mL) to give
compound 7m as a dark red solid (0.0489 g, 52%). MS
(LIFDI): calcd for [C30H34ICrN2O4]+, 665.10; found, 665.09.
MS (LIFDI): calcd for [C30H34CrN2O4]+, 538.19; found,
538.19. IR neat (selected absorbances): 2923 (C−H alkyl),
1602 (CN), 1533 (CC aromatic), 1359 (C−O from C−
O−Me), 1207 (C−O), 1057 (C−N), 751 (C−H) cm−1. Mp:
>350 °C. XRF showed no chlorine present.
all known compounds, and the spectroscopic data of samples
prepared using catalysts 7a,l were consistent with those
reported in the literature.21,23,30,44,47,48
Styrene Carbonate (9a). Purification by flash column
chromatography with hexane/EtOAc (6/4) gave a white solid
(251 mg, 92%): mp 53−55 °C (lit.21,23,30 mp 50−51 °C); H
1
NMR (400 MHz, CDCl3) δ 7.30−7.19 (m, 5H, Ph), 5.52 (t, J =
8.0 Hz, 1H, OCH), 4.64 (t, J = 8.0 Hz, 1H, CH2), 4.16 (t, J =
8.0 Hz, 1H, CH2); 13C NMR (100 MHz, CDCl3) δ 154.7 (C
O), 135.6 (ArC), 129.4 (ArCH), 128.9 (ArCH), 125.7
(ArCH), 77.8 (OCH), 70.9 (OCH2); IR (neat, cm−1): ν
3068, 3039, 2981, 2925, 1773, 1553; HRMS (ESI+) calcd for
C9H8O3 [M + Na]+ 187.0366, found 187.0363.
Propylene Carbonate (9b). Purification by flash column
chromatography with hexane/EtOAc (8/2) gave a colorless
1
liquid (114 mg, 57%): H NMR (400 MHz, CDCl3) δ 4.87−
4.79 (m, 1H, OCH), 4.52 (t, J = 8.0 Hz, 1H, CH2), 3.99 (t, J =
8.0 Hz, 1H, CH2), 1.46 (d, J = 6.4 Hz, 1H, CH3); 13C NMR
(100 MHz, CDCl3) δ 155.0 (CO), 73.5 (OCH), 70.6
(OCH2), 19.2 (CH3); IR (neat, cm−1): ν 2987, 2924, 1782;
HRMS (ESI+) calcd for C4H6O3 [M + Na]+ 125.0209, found
125.0213.
1,2-Butylene Carbonate (9c). Purification by flash column
chromatography with hexane/EtOAc (8/2) gave a colorless
1
liquid (237 mg, 86%): H NMR (400 MHz, CDCl3) δ 4.69−
Synthesis of Chromium salophen Complex 7n. The
synthesis of 7n was carried out in a manner identical with that
for 7l, using complex 7e (0.1007 g, 0.1754 mmol) and a
4.62 (m, 1H, OCH), 4.52 (t, J = 8.0 Hz, 1H, CH2), 4.08 (t, J =
8.0 Hz, 1H, CH2), 1.86−1.71 (m, 2H, CH2), 1.02 (t, J = 8.0 Hz,
CH3); 13C NMR (100 MHz, CDCl3) δ 155.1 (CO), 78.0
5021
DOI: 10.1021/acscatal.6b01386
ACS Catal. 2016, 6, 5012−5025

ACS Catalysis
Research Article
(OCH), 69.0 (OCH2), 26.9 (CH2), 8.5 (CH3); IR (neat, cm−1)
ν 2938, 2942, 2885, 1781; HRMS (ESI+) calcd for C5H8O3 [M
+ Na]+ 139.0366, found 139.0363.
CH2); 13C NMR (100 MHz, CDCl3) δ 154.5 (CO), 135.8
(ArC), 134.2 (ArC), 129.5 (ArCH), 127.2 (ArCH), 77.2
(OCH), 71.0 (OCH2); IR (neat, cm−1) ν 2964, 2912, 2342,
1960, 1866, 1789; HRMS (ESI+) calcd for C9H7ClO3 [M + H]+
220.9976, found 220.9975.
4-Bromostyrene Carbonate (9j). Purification by flash
column chromatography with hexane/EtOAc (8/2) gave a
white solid (357 mg, 89%): mp 72−73 °C (lit.21,23,30 mp 68−
69 °C); 1H NMR (400 MHz, CDCl3) δ 7.59 (d, J = 8.0 Hz, 2H,
ArH), 7.24 (d, J = 8.0 Hz, 2H, ArH), 5.64 (t, J = 8.0 Hz, 1H,
CH), 4.80 (t, J = 8.0 Hz, 1H, CH2), 4.30 (t, J = 8.0 Hz, 1H,
CH2); 13C NMR (100 MHz, CDCl3) δ 154.5 (CO), 134.8
(ArC), 132.5 (ArCH), 127.4 (ArCH), 123.9 (ArC), 77.2 (CH),
70.9 (CH2); IR (neat, cm−1) ν 2963, 2910, 1786; HRMS
(ESI+) calcd for C9H7BrO3 [M + Na]+ 264.9471, found
264.9470.
1,2-Hexylene Carbonate (9d). Purification by flash column
chromatography with hexane/EtOAc (6/4) gave a colorless
1
liquid (194 mg, 81%): H NMR (400 MHz, CDCl3) δ 4.71−
4.65 (m, 1H, OCH), 4.51 (t, J = 8.0 Hz, 1H, CH2), 4.06 (t, J =
8.0 Hz, 1H, CH2), 1.83−1.63 (m, 2H, CH2), 1.47−1.29 (m,
4H, 2 × CH2), 0.91 (t, J = 8.0 Hz, 3H, CH3); 13C NMR (100
MHz, CDCl3) δ 155.1 (CO), 77.0 (OCH), 69.3 (OCH2),
33.5 (CH2), 26.3 (CH2), 22.2 (CH2), 13.7 (CH3); IR (neat,
cm−1): ν 2959, 2933, 2873, 1786; HRMS (ESI+) calcd for
C7H12O3 [M + Na]+ 167.0679, found 167.0678.
1,2-Decylene Carbonate (9e). Purification by flash column
chromatography with hexane/EtOAc (8/2) gave a colorless
1
liquid (272 mg, 82%): H NMR (400 MHz, CDCl3) δ 4.72−
General Procedure for Synthesis of Cyclic Carbonates
at 10 Bar Pressure. An epoxide 10a−f (1.7 mmol), catalyst 7l
(0.025 mmol), and Bu4NBr (8 mg, 0.025 mmol) were placed in
a multipoint reactor with a magnetic stirrer bar, and an
autoclave was pressurized at 10 bar pressure of carbon dioxide.
The reaction mixture was stirred at 50 or 80 °C for 24 h.
Conversion of epoxide to cyclic carbonate was then determined
by analysis of a sample by 1H NMR spectroscopy. The
remaining sample was filtered through a plug of silica, with
CH2Cl2 as eluent to remove the catalyst. The eluent was
evaporated in vacuo to give either the pure cyclic carbonate or a
mixture of cyclic carbonate and unreacted epoxide. In the latter
case, the mixture was purified by flash chromatography using a
solvent system of first hexane, then hexane/EtOAc (9/1), then
hexane/EtOAc (3/1), and then EtOAc to give the pure cyclic
carbonate. Cyclic carbonates 11a−f are all known compounds,
and the spectroscopic data for samples prepared using catalyst
7l were consistent with those reported in the literature.25,30,47,48
Poly(cyclohexene carbonate) (11a). Obtained as a light
orange solid (207 mg, 76%): 1H NMR (400 MHz, CDCl3, 298
K): δ 4.95−4.50 (bs, 2H, CHO), 2.20−1.80 (bs, 2H, CH2),
1.80−1.20 (bs, 6H, CH2); 13C{1H} NMR (100 MHz, CDCl3,
298 K) δ 153.8, 77.1, 29.7, 23.1.
4.65 (m, 1H, OCH), 4.51 (t, J = 8.0 Hz, 1H, CH2), 4.06 (t, J =
8.0 Hz, 1H, CH2), 1.84−1.62 (m, 2H, CH2), 1.48−1.26 (m,
12H, 6 × CH2), 0.87 (t, J = 8.0 Hz, CH3); 13C NMR (100
MHz, CDCl3) δ 155.0 (CO), 77.0 (OCH), 69.4 (OCH2),
33.8 (CH2), 31.7 (CH2), 29.2 (CH2), 29.1 (CH2), 29.0 (CH2),
24.3 (CH2), 22.6 (CH2), 14.0 (CH3); IR (neat, cm−1): ν 2924,
2855, 1794, 1551, HRMS (ESI+) calcd for C11H21O3 [M + H]+
201.1485, found 201.1487; calcd for C11H21O3 [M + Na]+
223.1305, found 223.1308.
3-Chloropropylene Carbonate (9f). Purification by flash
column chromatography with hexane/EtOAc (6/4) gave a
white solid (176 mg, 78%): mp 67−69 °C (lit.21,23,30 mp 68−
1
69 °C); H NMR (400 MHz, CDCl3) δ 4.98−4.92 (m, 1H,
OCH), 4.59 (t, J = 8.0 Hz, 1H, OCH2), 4.41 (t, J = 8.0 Hz, 1H,
OCH2), 3.79−3.71 (m, 2H, CH2Cl); 13C NMR (100 MHz,
CDCl3) δ 154.0 (CO), 74.2 (OCH), 67.0 (OCH2), 43.7
(CH2Cl); IR (neat, cm−1) ν 2966, 2925, 1780, 663; HRMS
(ESI+) calcd for C4H5ClO3 [M + Na]+ 158.9819, found
158.9815.
Glycerol Carbonate (9g). Purification by flash column
chromatography with hexane/EtOAc (6/4) gave a colorless
1
liquid (141 mg, 72%): H NMR (400 MHz, CDCl3) δ 4.85−
4.77 (m, 1H, CH), 4.53 (t, J = 8.3 Hz, 1H, OCH2), 4.45 (dd J =
9.3, 5.8 Hz, 1H, OCH2), 4.00 (ddd J = 12.7, 5.7, 3.1 Hz, 1H,
CHOH), 3.68 (ddd J = 9.3, 7.6, 4.1 Hz, 1H, CHOH), 2.01 (dd
J = 7.0, 5.7 Hz, 1H, CHOH); 13C NMR (100 MHz, CDCl3) δ
155.1 (CO), 76.4 (OCH), 65.7 (OCH2), 61.7 (CH2OH); IR
(neat, cm−1): ν 3385, 2899, 1795; HRMS (ESI+) calcd for
C8H15O6 [M + EtOAc]+ 207.0863, found 207.0865.
cis-1,2-Cyclopentene Carbonate (11b). Obtained as a white
solid (181 mg, 85%): mp 31−33 °C (lit.25,30,47,48 mp 29−30
°C); 1H NMR (400 MHz, CDCl3, 298 K) δ 5.15−5.02 (m, 2H,
CHO), 2.23−2.08 (m, 2H, CH2), 1.90−1.57 (m, 4H, CH2);
13C{1H} NMR (100 MHz, CDCl3, 298 K) δ 155.4, 81.8, 33.1,
21.5; IR (neat, cm−1) 2971, 2874, 1786; HRMS (ESI+) calcd
m/z 151.0366 [M + Na]+, found 151.0368.
3-Phenoxypropylene Carbonate (9h). Purification by flash
column chromatography with hexane/EtOAc (8/2) gave a
white solid (229 mg, 71%): mp 97−98 °C (lit.21,23,30 mp 94−
95 °C); 1H NMR (400 MHz, CDCl3) δ 7.31 (t, J = 8.0 Hz, 2H,
m-Ph), 7.02 (t, J = 8.0 Hz, 1H, p-Ph), 6.91 (d, J = 8.0 Hz, 2H, o-
Ph), 5.06−5.00 (m, 1H, OCH), 4.64−4.52 (m, 2H, OCH2),
4.24 (dd, J = 11.0, 4.0 Hz, 1H, CH2), 4.15 (dd, J = 11.0, 4.0 Hz,
1H, CH2); 13C NMR (100 MHz, CDCl3) δ 157.7 (CO),
154.6 (ArC), 129.7 (ArCH), 122.0 (ArCH), 114.6 (ArCH),
74.1 (OCH), 66.8 (OCH2), 66.2 (CH2); IR (neat, cm−1) ν
2924, 1788, 1598; HRMS (ESI+) calcd for C10H10O4 [M +
Na]+ 217.0471, found 217.0474.
4-Chlorostyrene Carbonate (9i). Purification by flash
column chromatography with hexane/EtOAc (6/4) gave a
white solid (299 mg, 91%): mp 66−68 °C (lit.21,23,30 mp 68−
69 °C); 1H NMR (400 MHz, CDCl3) δ 7.35 (d, J = 8.0 Hz, 2H,
ArH), 7.24 (d, J = 8.0 Hz, 2H, ArH), 5.59 (t, J = 8.0 Hz, 1H,
CH), 4.73 (t, J = 8.0 Hz, 1H, CH2), 4.23 (t, J = 8.0 Hz, 1H,
cis-2,3-Butene Carbonate (11c).25,30,47,48 Obtained as a
colorless liquid in a 97/3 mixture of cis and trans isomers (121
mg, 63%): 1H NMR (400 MHz, CDCl3, 298 K) δ 4.81 (m, 2H,
CH), 1.31 (6H, d, J 6.1 Hz, CH3); 13C{1H} NMR (100 MHz,
CDCl3, 298 K) δ 154.3, 75.9, 14.1; IR (neat, cm−1): 2958,
2897, 1789; HRMS (ESI+): calcd. m/z 139.0366 [M + Na]+,
found 139.0366.
trans-2,3-Butene Carbonate (11d). Obtained as a white
solid (116 mg, 60%): mp 29−31 °C (lit.25,30,47,48 29−30 °C);
1H NMR (400 MHz, CDCl3, 298 K): δ 4.36−4.26 (m, 2H,
CH), 1.44 (d, J 6.1 Hz, 3H, CH3); 13C{1H} NMR (100 MHz,
CDCl3, 298 K) δ 154.4, 79.8, 18.2; IR (neat, cm−1) 2951, 2873,
1781; HRMS (ESI+) calcd m/z 139.0366 [M + Na]+, found
139.0371.
trans-1,2-Diphenylethylene Carbonate (11e). Obtained as
a white solid (355 mg, 89%): mp 111−112 °C (lit.25,30,47,48 mp
1
110−111 °C); H NMR (400 MHz, CDCl3, 298 K) δ 7.35−
5022
DOI: 10.1021/acscatal.6b01386
ACS Catal. 2016, 6, 5012−5025

ACS Catalysis
Research Article
Journal of CO2 Utilization 2013, 3−4, 74−92. (h) Aresta, M.;
Dibenedetto, A.; Angelini, A. Chem. Rev. 2014, 114, 1709−1742.
(i) Otto, A.; Grube, T.; Schiebahn, S.; Stolten, D. Energy Environ. Sci.
2015, 8, 3283−3297. (j) Fiorani, G.; Guo, W.; Kleij, A. W. Green
7.45 (m, 6H, ArH), 7.25−7.35 (m, 4H, ArH), 5.43 (s, 2H,
CH); 13C{1H} NMR (100 MHz, CDCl3, 298 K) δ 154.1, 134.8,
129.8, 129.2, 126.1, 85.3; IR (neat, cm−1) 3048, 2974, 1810,
1459; HRMS (ESI+) calcd m/z 263.0679 [M + Na]+, found
263.0687.
Chem. 2015, 17, 1375−1389. (k) Martín, A. J.; Larrazab
́
al, G. O.;
Perez-Ramírez, J. Green Chem. 2015, 17, 5114−5130. (l) Liu, Q.; Wu,
́
trans-1-Phenyl-2-methylethylene Carbonate (11f). Ob-
L.; Jackstell, R.; Beller, M. Nat. Commun. 2015, 6, 5933. (m) Poliakoff,
M.; Leitner, W.; Streng, E. S. Faraday Discuss. 2015, 183, 9−17.
(2) For textbooks see: (a) Carbon Dioxide as Chemical Feedstock;
Aresta, M., Ed.; Wiley-VCH: Weinheim, Germany, 2010. (b) New and
Future Developments in Catalysis: Activation of Carbon Dioxide; Suib, S.
L., Ed.; Elsevier: Oxford, U.K., 2013. (c) Carbon Dioxide Utilisation:
Closing the Carbon Cycle; Styring, P., Quadrelli, A., Armstrong, K., Eds.;
Elsevier: Oxford, U.K., 2015.
tained as a white solid (257 mg, 87%): mp 110−113 °C
(lit.25,30,47,48 mp 110−111 °C); H NMR (400 MHz, CDCl3,
1
298 K) δ 7.50−7.29 (m, 5H, ArH), 5.13 (d, J 8.0 Hz, 1H, CH),
4.60 (m, 1H, CH), 1.56 (d, J 6.2 Hz, 3H, CH3); 13C{1H} NMR
(100 MHz, CDCl3, 298 K) δ 154.4, 135.1, 129.8, 129.3, 126.1,
85.0, 80.8, 18.4; IR (neat, cm−1) 3010, 2950, 1800, 1459;
HRMS (ESI+) calcd m/z 201.0522 [M + Na]+, found
201.0527.
(3) Data obtained from the NIST database: http://webbook.nist.
gov/chemistry/name-ser.html, accessed on Dec 12, 2013.
(4) Calculated using data obtained from ref 3.
Determination of the Reaction Kinetics. Styrene oxide
8a (0.2 g, 1.66 mmol), catalyst 7l (1.0, 2.0, 3.0, 4.0, or 5.0 mol
%), and Bu4NBr (1.0, 2.0, 3.0, 4.0, or 5.0 mol %) were placed in
a sample vial fitted with a magnetic stirrer bar and placed in a
large conical flask. Cardice pellets were added to the conical
flask, which was fitted with a rubber stopper pierced by a
deflated balloon. The reaction mixture was stirred at 50 °C.
Samples were taken at convenient intervals (approximately
every 30 min) and analyzed by HPLC.
(5) For recent reviews of cyclic carbonate synthesis see: (a) Martín,
C.; Fiorani, G.; Kleij, A. W. ACS Catal. 2015, 5, 1353−1370.
(b) Kathalikkattil, A. C.; Babu, R.; Tharun, J.; Roshan, R.; Park, D.-W.
Catal. Surv. Asia 2015, 19, 223−235. (c) Yu, B.; He, L.-N.
ChemSusChem 2015, 8, 52−62. (d) Cokoja, M.; Wilhelm, M. E.;
Anthofer, M. H.; Herrmann, W. A.; Kuhn, F. E. ChemSusChem 2015,
̈
8, 2436−2454. (e) Xu, B.-H.; Wang, J.-Q.; Sun, J.; Huang, Y.; Zhang,
J.-P.; Zhang, X.-P.; Zhang, S.-J. Green Chem. 2015, 17, 108−122.
(f) D’Elia, V.; Pelletier, J. D. A.; Basset, J.-M. ChemCatChem 2015, 7,
1906−1917.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b01386.
■
(6) For recent reviews covering cyclic and polycarbonate synthesis
see: (a) Martín, R.; Kleij, A. W. ChemSusChem 2011, 4, 1259−1263.
(b) Lu, X.-B.; Darensbourg, D. J. Chem. Soc. Rev. 2012, 41, 1462−
1484. (c) Wan Isahak, W. N. R.; Che Ramli, Z. A.; Mohamed Hisham,
M. W.; Yarmo, M. A. Renewable Sustainable Energy Rev. 2015, 47, 93−
106.
S
1H and 13C NMR spectra and mass spectra for cyclic
carbonates 9a−j and 11a−f and kinetic plots for the
study of the synthesis of styrene carbonate 9a catalyzed
by catalyst 7l and tetrabutylammonium bromide (PDF)
(7) For recent reviews of polycarbonate synthesis see: (a) Klaus, S.;
Lehenmeier, M. W.; Anderson, C. E.; Rieger, B. Coord. Chem. Rev.
2011, 255, 1460−1479. (b) Kember, M. R.; Buchard, A.; Williams, C.
K. Chem. Commun. 2011, 47, 141−163. (c) Lu, X.-B.; Ren, W.-M.; Wu,
G.-P. Acc. Chem. Res. 2012, 45, 1721−1735. (d) Darensbourg, D. J.;
Wilson, S. J. Green Chem. 2012, 14, 2665−2671. (e) Ikpo, N.;
Flogeras, J. C.; Kerton, F. M. Dalton Trans. 2013, 42, 8998−9006.
(f) Darensbourg, D. J.; Yeung, A. D. Polym. Chem. 2014, 5, 3949.
(g) Taherimehr, M.; Pescarmona, P. P. J. Appl. Polym. Sci. 2014, 131,
41141. (h) Qin, Y.; Sheng, X.; Liu, S.; Ren, G.; Wang, X.; Wang, F. J.
CO2 Util. 2015, 11, 3−9. (i) Paul, S.; Zhu, Y.; Romain, C.; Brooks, R.;
Saini, P. K.; Williams, C. K. Chem. Commun. 2015, 51, 6459−6479.
(8) Peppel, W. J. Ind. Eng. Chem. 1958, 50, 767−770.
AUTHOR INFORMATION
Corresponding Authors
■
*J.A.C.-O.: e-mail, JoseAntonio.Castro@uclm.es.
*M.N.: e-mail, Michael.north@york.ac.uk; fax, +44 1904-322-
705.
Notes
The authors declare no competing financial interest.
(9) Schaffner, B.; Schaffner, F.; Verevkin, S. P.; Borner, A. Chem. Rev.
̈
̈
̈
ACKNOWLEDGMENTS
■
2010, 110, 4554−4581.
The authors gratefully acknowledge financial support from the
European Union Seventh Framework Programme FP7-NMP-
2012 under grant agreement number 309497, the EPSRC for a
studentship (to K.J.L., grant code EP/L505122), and the
(10) Beattie, C.; North, M.; Villuendas, P. Molecules 2011, 16, 3420−
3432 and references cited therein.
(11) (a) Xu, K. Chem. Rev. 2004, 104, 4303−4417. (b) Zhang, S. S. J.
Power Sources 2006, 162, 1379−1394. (c) Etacheri, V.; Marom, R.;
Elazari, R.; Salitra, G.; Aurbach, D. Energy Environ. Sci. 2011, 4, 3243−
3262. (d) Scrosati, B.; Hassoun, J.; Sun, Y.-K. Energy Environ. Sci.
2011, 4, 3287−3295.
(12) Besse, V.; Camara, F.; Voirin, C.; Auvergne, R.; Caillol, S.;
Boutevin, B. Polym. Chem. 2013, 4, 4545−4561.
(13) (a) Knifton, J. F.; Duranleau, R. G. J. Mol. Catal. 1991, 67, 389−
399. (b) Fukuoka, S.; Kawamura, M.; Komiya, K.; Tojo, M.; Hachiya,
H.; Hasegawa, K.; Aminaka, M.; Okamoto, H.; Fukawa, I.; Konno, S.
Green Chem. 2003, 5, 497−507.
(14) Tullo, A. Chem. Eng. News 2008, 86, 21.
(15) Pescarmona, P. P.; Taherimehr, M. Catal. Sci. Technol. 2012, 2,
́
Ministerio de Economıa y Competitividad (MINECO) of
Spain (Grant No. CTQ2014-51912-REDC and CTQ2014-
52899-R). J.A.C.-O. acknowledges financial support from the
Plan Propio de la Universidad de Castilla-La Mancha. Dr.
́
Helen Parker and Andrea Munoz Garcıa are thanked for their
̃
assistance in performing the ICP-MS analysis.
REFERENCES
■
(1) For recent reviews see: (a) Quadrelli, E. A.; Centi, G.; Duplan, J.-
L.; Perathoner, S. ChemSusChem 2011, 4, 1194−1215. (b) Peters, M.;
Kohler, B.; Kuckshinrichs, W.; Leitner, W.; Markewitz, P.; Muller, T.
2169−2187.
̈
̈
(16) (a) Gurtler, C.; Hofmann, J.; Muller, T. E.; Wolf, A.; Grasser, S.;
E. ChemSusChem 2011, 4, 1216−1240. (c) Styring, P.; Armstrong, K.
Chem. Today 2012, 29, 28−31. (d) Omae, I. Coord. Chem. Rev. 2012,
256, 1384−1405. (e) Hu, B.; Guild, C.; Suib, S. L. Journal of CO2
Utilization 2013, 1, 18−27. (f) Aresta, M.; Dibenedetto, A.; Angelini,
A. Journal of CO2 Utilization 2013, 3−4, 65−73. (g) Olajire, A. A.
̈
̈
Kohler, B. (Bayer Material Science) Method for producing polyether
̈
carbonate polyols; WO117332, 2011. (b) Hofmann, J.; Gurtler, C.;
̈
Nefzger, H.; Hahn, N.; Lorenz, K.; Muller, T. E. (Bayer Material
Science) Method for producing polyether carbonate polyols having
̈
5023
DOI: 10.1021/acscatal.6b01386
ACS Catal. 2016, 6, 5012−5025

ACS Catalysis
Research Article
primary hydroxyl end groups and polyurethane polymers produced
therefrom; WO080192, 2012. (c) von der Assen, N.; Sternberg, A.;
Gimenez, M.; Pelletier, J. D. A.; Kuhn, F. E.; D’Elia, V.; Basset, J.-M.
Green Chem. 2016, 18, 3116−3123.
(33) (a) Lu, X.-B.; Feng, X.-J.; He, R. Appl. Catal., A 2002, 234, 25−
34. (b) Chun, J.; Kang, S.; Kang, N.; Lee, S. M.; Kim, H. J.; Son, S. U.
J. Mater. Chem. A 2013, 1, 5517−5523. (c) Darensbourg, D. J.; Chung,
W.-C.; Wilson, S. J. ACS Catal. 2013, 3, 3050−3057.
(34) (a) Paddock, R. L.; Nguyen, S. T. J. Am. Chem. Soc. 2001, 123,
11498−11499. (b) Darensbourg, D. J.; Yarbrough, J. C.; Ortiz, C.;
Fang, C. C. J. Am. Chem. Soc. 2003, 125, 7586−7591. (c) Alvaro, M.;
Baleizao, C.; Das, D.; Carbonell, E.; García, H. J. Catal. 2004, 228,
254−258. (d) Darensbourg, D. J.; Fang, C. C.; Rodgers, J. L.
Organometallics 2004, 23, 924−927. (e) Ramin, M.; Jutz, F.;
Grunwaldt, J.-D.; Baiker, A. J. Mol. Catal. A: Chem. 2005, 242, 32−
39. (f) Darensbourg, D. J.; Bottarelli, P.; Andreatta, J. R. Macro-
molecules 2007, 40, 7727−7729. (g) Zhang, X.; Jia, Y.-B.; Lu, X.-B.; Li,
B.; Wang, H.; Sun, L.-C. Tetrahedron Lett. 2008, 49, 6589−6592.
(35) Shen, Y.-M.; Duan, W.-L.; Shi, M. J. Org. Chem. 2003, 68,
1559−1562.
̈
Katelhon, A.; Bardow, A. Faraday Discuss. 2015, 183, 291−307.
̈
̈
(17) Yoshida, M.; Ihara, M. Chem. - Eur. J. 2004, 10, 2886−2893.
(18) (a) McClellan, P. P. (Jefferson Chemical Company) Catalytic
process for producing alkylene carbonates, US2873282, 1959.
(b) McMullen, C. H.; Nelson, J. R.; Ream, B. C.; Sims, J. A., Jr.
(Union Carbide Corporation) Alkylene carbonate process,
US4314945, 1982. (c) Plotkin, J. S.; Miller, M. M.; Taylor, P. D.
(ISP Investment Inc.) Process for the preparation of alk-1-enyl ether
cyclocarbonate. US5095124, 1990. (d) Zhu, P.; Ji, Z.; Gu, Z.; Wang,
Y.; Li, B.; Xu, Y. Process for preparation of ethylene carbonate,
CN1432557, 2003. (e) Tian, H.; Zhu, Y.; Hao, Y. Process for
preparation of ethylene carbonate derivatives, CN1699359, 2005.
(f) Tian, H.; Zhu, Y.; Liu, J.; Hao, Y.; Wang, H.; Huang, H. Process for
combined preparation of dimethylcarbonate and diols, CN1733696,
2006. (g) Zhang, S.; Sun, J.; Cheng, W.; Meng, Z.; Li, Q.; Wang, L.
Method for preparing cyclic carbonate, CN1995032, 2007.
(19) (a) Harmsen, G. J.; van der Heide, E.; Vrouwenvelder, C. L.
(Shell international research) Process for the preparation of alkanediol,
WO089866, 2004. (b) Beckers, J. G. J.; van der Heide, E.; van Kessel,
G. M. M.; Lange, J.-P. (Shell International Research) Process for the
preparation of propylene carbonate, WO051939, 2005. (c) van der
Heide, E.; van Kessel, G. M. M.; Nisbet, T. M.; Vaporciyan, G. G.
(Shell Oil Company) Process for the production of alkylene carbonate
and use of alkylene carbonate thus produced in the manufacture of an
alkane diol and a dialkyl carbonate, US0197802, 2007. (d) Nisbet, T.
M.; Vaporciyan, G. G. (Shell International Research) Process for
preparing an 1,2-alkylene carbonate, WO128956, 2008. (e) Evans, W.
E.; Hess, M. L.; Matusz, M.; van Kruchten, E. M. G. A. (Shell
International Research) Process for the preparation of an alkylene
carbonate and an alkylene glycol, WO140318, 2009.
(36) (a) Lu, X.-B.; Xiu, J.-H.; He, R.; Jin, K.; Luo, L.-M.; Feng, X.-J.
Appl. Catal., A 2004, 275, 73−78. (b) Paddock, R. L.; Nguyen, S. T.
Chem. Commun. 2004, 1622−1623. (c) Lu, X.-B.; Liang, B.; Zhang, Y.-
J.; Tian, Y.-Z.; Wang, Y.-M.; Bai, C.-X.; Wang, H.; Zhang, R. J. Am.
Chem. Soc. 2004, 126, 3732−3733. (d) Lu, X.-B.; Wang, Y. Angew.
Chem., Int. Ed. 2004, 43, 3574−3577. (e) Berkessel, A.; Brandenburg,
M. Org. Lett. 2006, 8, 4401−4404. (f) Chang, T.; Jing, H.; Jin, L.; Qiu,
W. J. Mol. Catal. A: Chem. 2007, 264, 241−247. (g) Chen, S.-W.;
Kawthekar, R. B.; Kim, G.-J. Tetrahedron Lett. 2007, 48, 297−300.
(h) Miao, C.-X.; Wang, J.-Q.; Wu, Y.; Du, Y.; He, L.-N. ChemSusChem
2008, 1, 236−241. (i) Jin, L.; Huang, Y.; Jing, H.; Chang, T.; Yan, P.
Tetrahedron: Asymmetry 2008, 19, 1947−1953. (j) Yan, P.; Jing, H.
Adv. Synth. Catal. 2009, 351, 1325−1332. (k) Zhang, S.; Huang, Y.;
Jing, H.; Yao, W.; Yan, P. Green Chem. 2009, 11, 935−938. (l) Chang,
T.; Jin, L.; Jing, H. ChemCatChem 2009, 1, 379−383. (m) Zhang, S.;
Song, Y.; Jing, H.; Yan, P.; Cai, Q. Chin. J. Catal. 2009, 30, 1255−1260.
(n) Song, Y. Y.; Jin, Q. R.; Zhang, S. L.; Jing, H. W.; Zhu, Q. Q. Sci.
China: Chem. 2011, 54, 1044−1050. (o) Jang, D. Y.; Jang, H. G.; Kim,
G. R.; Kim, G.-J. Catal. Today 2012, 185, 306−312. (p) Ren, W.-M.;
Wu, G.-P.; Lin, F.; Jiang, J.-Y.; Liu, C.; Luo, Y.; Lu, X.-B. Chem. Sci.
2012, 3, 2094−2102. (q) Roy, T.; Kureshy, R. I.; Khan, N. H.; Abdi, S.
H. R.; Bajaj, H. C. Catal. Sci. Technol. 2013, 3, 2661−2667. (r) Xie, Y.;
Wang, T.-T.; Liu, X.-H.; Zou, K.; Deng, W. Q. Nat. Commun. 2013, 4,
1960.
(20) North, M. ARKIVOC 2012, 610−628.
(21) (a) North, M.; Pasquale, R. Angew. Chem., Int. Ed. 2009, 48,
2946−2948. (b) Clegg, W.; Harrington, R. W.; North, M.; Pasquale, R.
Chem. - Eur. J. 2010, 16, 6828−6843. (c) North, M.; Young, C.
ChemSusChem 2011, 4, 1685−1693.
(22) North, M.; Quek, S. C. Z.; Pridmore, N. E.; Whitwood, A. C.;
Wu, X. ACS Catal. 2015, 5, 3398−3402.
(23) (a) Castro-Osma, J. A.; North, M.; Wu, X. Chem. - Eur. J. 2014,
20, 15005−15008. (b) Castro-Osma, J. A.; North, M.; Offermans, W.
K.; Leitner, W.; Muller, T. E. ChemSusChem 2016, 9, 791−794.
̈
(24) Metcalfe, I. S.; North, M.; Pasquale, R.; Thursfield, A. Energy
Environ. Sci. 2010, 3, 212−215.
(37) Tasç i, Z.; Ulusoy, M. J. Organomet. Chem. 2012, 713, 104−111.
(38) (a) Jutz, F.; Grunwaldt, J.-D.; Baiker, A. J. Mol. Catal. A: Chem.
2008, 279, 94−103. (b) Jutz, F.; Grunwaldt, J.-D.; Baiker, A. J. Mol.
Catal. A: Chem. 2009, 297, 63−72.
(25) Beattie, C.; North, M.; Villuendas, P.; Young, C. J. Org. Chem.
2013, 78, 419−426.
(26) Melen
2577−2579.
́
dez, J.; North, M.; Villuendas, P. Chem. Commun. 2009,
(39) (a) Ren, Y.; Cheng, X.; Yang, S.; Qi, C.; Jiang, H.; Mao, Q.
Dalton Trans. 2013, 42, 9930−9937. (b) Ren, Y.; Shi, Y.; Chen, J.;
Yang, S.; Qi, C.; Jiang, H. RSC Adv. 2013, 3, 2167−2170.
(40) Jing, H.; Chang, T.; Jin, L.; Wu, M.; Qiu, W. Catal. Commun.
2007, 8, 1630−1634.
(27) North, M.; Villuendas, P. ChemCatChem 2012, 4, 789−794 and
references cited therein.
(28) North, M.; Wang, B.; Young, C. Energy Environ. Sci. 2011, 4,
4163−4170.
(41) Jing, H.; Edulji, S. K.; Gibbs, J. M.; Stern, C. L.; Zhou, H.;
Nguyen, S. T. Inorg. Chem. 2004, 43, 4315−4327.
(42) Wang, Y.; Qin, Y.; Wang, X.; Wang, F. Catal. Sci. Technol. 2014,
4, 3964−3972.
(29) North, M.; Young, C. Catal. Sci. Technol. 2011, 1, 93−99.
́
(30) (a) Castro-Osma, J. A.; Lara-Sanchez, A.; North, M.; Otero, A.;
Villuendas, P. Catal. Sci. Technol. 2012, 2, 1021−1026. (b) Castro-
Osma, J. A.; Alonso-Moreno, C.; Lara-Sanchez, A.; Martínez, J.; North,
́
́
(43) Haak, R. M.; Decortes, A.; Escudero-Adan, E. C.; Belmonte, M.
M.; Otero, A. Catal. Sci. Technol. 2014, 4, 1674−1684. (c) Martinez, J.;
M.; Martin, M.; Benet-Buchholz, J.; Kleij, A. W. Inorg. Chem. 2011, 50,
7934−7936.
Castro-Osma, J. A.; Earlam, A.; Alonso-Moreno, C.; Otero, A.; Lara-
Sanchez, A.; North, M.; Rodríguez-Dieg
21, 9850−9862.
́
uez, A. Chem. - Eur. J. 2015,
(44) (a) Decortes, A.; Martinez-Belmonte, M.; Benet-Buchholz, J.;
Kleij, A. W. Chem. Commun. 2010, 46, 4580−4582. (b) Decortes, A.;
Kleij, A. W. ChemCatChem 2011, 3, 831−834. (c) Martin, C.;
(31) (a) Rulev, Y. A.; Larionov, V. A.; Lokutova, A. V.; Moskalenko,
M. A.; Lependina, O. L.; Maleev, V. I.; North, M.; Belokon, Y. N.
ChemSusChem 2016, 9, 216−222. (b) Castro-Osma, J. A.; North, M.;
Wu, X. Chem. - Eur. J. 2016, 22, 2100−2107.
Whiteoak, C. J.; Martin, E.; Martinez-Belmonte, M.; Escudero-Adan
E. C.; Kleij, A. W. Catal. Sci. Technol. 2014, 4, 1615−1621. (d) Castro-
Gomez, F.; Salassa, G.; Kleij, W. A.; Bo, C. Chem. - Eur. J. 2013, 19,
́
,
(32) For recent examples see: (a) D’Elia, V.; Dong, H.; Rossini, A. J.;
Widdifield, C. M.; Vummaleti, S. V. C.; Minenkov, Y.; Poater, A.;
Abou-Hamad, E.; Pelletier, J. D. A.; Cavallo, L.; Emsley, L.; Basset, J.-
M. J. Am. Chem. Soc. 2015, 137, 7728−7739. (b) Barthel, A.; Saih, Y.;
́
6289−6298.
(45) Darensbourg, D. J.; Moncada, A. I.; Wei, S.-H. Macromolecules
2011, 44, 2568−2576.
5024
DOI: 10.1021/acscatal.6b01386
ACS Catal. 2016, 6, 5012−5025

ACS Catalysis
Research Article
(46) Darensbourg, D. J.; Mackiewicz, R. M.; Rodgers, J. L.; Fang, C.
C.; Billodeaux, D. R.; Reibenspies, J. H. Inorg. Chem. 2004, 43, 6024−
6034.
(47) (a) Whiteoak, C. J.; Kielland, N.; Laserna, V.; Escudero-Adan
C.; Martin, E.; Kleij, A. W. J. Am. Chem. Soc. 2013, 135, 1228−1231.
(b) Whiteoak, C. J.; Martin, E.; Escudero-Adan, E.; Kleij, A. W. Adv.
Synth. Catal. 2013, 355, 2233−2239. (c) Whiteoak, C. J.; Kielland, N.;
Laserna, V.; Castro-Gomez, F.; Martin, E.; Escudero-Adan, E. C.; Bo,
́
, E.
́
́
́
C.; Kleij, A. W. Chem. - Eur. J. 2014, 20, 2264−2275. (d) Whiteoak, C.
J.; Martin, E.; Belmonte, M. M.; Benet-Buchholz, J.; Kleij, A. W. Adv.
Synth. Catal. 2012, 354, 469−7610. (e) Vignesh Babu, H.;
Muralidharan, K. Dalton Trans. 2013, 42, 1238−1248. (f) Qin, J.;
Wang, P.; Li, Q.; Zhang, Y.; Yuan, D.; Yao, Y. Chem. Commun. 2014,
50, 10952−10955.
(48) (a) Gabriele, B.; Mancuso, R.; Salerno, G.; Veltri, L.; Costa, M.;
Dibenedetto, A. ChemSusChem 2011, 4, 1778−1786. (b) Buonerba,
A.; De Nisi, A.; Grassi, A.; Milione, S.; Capacchione, C.; Vagin, S.;
Rieger, B. Catal. Sci. Technol. 2015, 5, 118−123.
(49) Data given in the Supporting Information.
(50) Darensbourg, D. J.; Moncada, A. I. Inorg. Chem. 2008, 47,
10000−10008.
5025
DOI: 10.1021/acscatal.6b01386
ACS Catal. 2016, 6, 5012−5025