Synthesis of cyclic carbonates catalysed by aluminium heteroscorpionate complexes

Article abstract of DOI:10.1039/c3cy00810j

Parallel catalyst screening was used to develop new aluminium scorpionate based catalysts for the synthesis of cyclic carbonates from epoxides and carbon dioxide. Nineteen complexes were included in the catalyst screening, which resulted in the developmen

Full text of DOI:10.1039/c3cy00810j

Catalysis
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Synthesis of cyclic carbonates catalysed by
aluminium heteroscorpionate complexes†‡
Cite this: Catal. Sci. Technol., 2014,
4, 1674
ab
b
b
José A. Castro-Osma, Carlos Alonso-Moreno, Agustín Lara-Sánchez,
b
a
b
Javier Martínez, Michael North* and Antonio Otero*
Parallel catalyst screening was used to develop new aluminium scorpionate based catalysts for the
synthesis of cyclic carbonates from epoxides and carbon dioxide. Nineteen complexes were included in
the catalyst screening, which resulted in the development of trimetallic complex 27 which had the same
catalytic activity at one bar carbon dioxide pressure that the initial lead complex (8) had at ten bar carbon
dioxide pressure. The combination of complex 27 and tetrabutylammonium bromide could be used to
form cyclic carbonates from a range of terminal epoxides and kinetic studies showed that the reactions
were first order in epoxide, complex 27 and tetrabutylammonium bromide concentrations. Based on this
data a catalytic cycle has been proposed which accounts for all of features of the catalyst system.
Received 15th October 2013,
Accepted 19th November 2013
DOI: 10.1039/c3cy00810j
www.rsc.org/catalysis
Another exothermic reaction of carbon dioxide is its reac-
Introduction
tion with epoxides 1 and this can be controlled to form either
7
7,8
Carbon dioxide is a stable molecule with very low standard
heat of formation (ΔH = −394 kJ mol ) and standard Gibbs
energy of formation (ΔGf = −395 kJ mol ). However, there
are many reactions of carbon dioxide which are exothermic
and/or exergonic. In general, such reactions require the reac-
tion of carbon dioxide with one or more molecules with a sig-
cyclic carbonates 2 or aliphatic-polycarbonates 3 (Scheme 1).
o
−1
−1
These reactions are both highly exothermic (ΔH = −144 kJ mol
for the synthesis of ethylene carbonate 2a) being driven
f
r
o
−1
1
1
largely by the release of the strain energy in the three-membered
epoxide ring. However, neither reaction occurs spontaneously,
rather a suitable catalyst is required.
o
o
nificantly higher ΔH
also has a low ΔHf and ΔG . Well known commercially
f
and ΔG
f
to produce a product which
Aliphatic-polycarbonates 3 have the potential to provide
o
o
f
9
greener alternatives to both aromatic-polycarbonates (derived
important reactions include the reaction of carbon dioxide
from bisphenol-A and phosgene) and to the polyether-polyols
used within polyurethane foams. A number of companies
−
1 2
10
with ammonia to form urea (ΔH
reaction of carbon dioxide with sodium phenolate to form
the sodium salt of salicylic acid (ΔH = −31 kJ mol ). Thus,
r
= −101 kJ mol ) and the
are working to commercialize the production of aliphatic-
polycarbonates, and particularly effective catalysts for this
−
1 3
r
1
1
carbon dioxide is increasingly being seen as a sustainable
carbon source for a future chemicals industry which would
not be dependent on crude oil supplies. This has resulted in
a significant increase in interest in carbon dioxide chemistry
reaction have been developed by the groups of Coates,
Darensbourg,
9
,12
13
14
Lee and Williams. In contrast, the synthe-
sis of cyclic carbonates 2 has been a commercial process since
1
5
the 1950's and cyclic carbonates already have a number of
4
16
in recent years and the challenge is not so much overcoming
important applications including as electrolytes for lithium-
1
6,17
16,18
the thermodynamics of the reactions, but rather finding suit-
able catalysts to lower the often rather high activation energies
and allow reactions to be carried out at closer to ambient tem-
ion batteries
and as polar aprotic solvents.
Commer-
cially, the two most important cyclic carbonates are ethylene
carbonate 2a and propylene carbonate 2b. Current commercial
processes for the production of cyclic carbonates rely on
the use of relatively inefficient quaternary ammonium or
5
perature and pressure.
a
Green Chemistry Centre of Excellence, Department of Chemistry, The University
of York, Heslington, York, YO10 5DD, UK. E-mail: michael.north@york.ac.uk;
Fax: +44 (0)1904 322 705; Tel: +44 (0)1904 324 545
b
Departamento de Química Inorgánica, Orgánica y Bioquímica, Universidad de
Castilla-La Mancha, 13071 Ciudad Real, Spain. E-mail: Antonio.Otero@uclm.es;
Fax: +34 926 295 318; Tel: +34 926 295 300
†
This article is published in celebration of the 50th aniversary of the opening
of the chemistry department at the University of York.
Electronic supplementary information (ESI) available: C NMR
1
13
‡
H
and
spectra and mass spectra for cyclic carbonates 2a–k and all kinetic plots. See
DOI: 10.1039/c3cy00810j
Scheme 1 Synthesis of cyclic- and polycarbonates.
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19
phosphonium salts as catalysts and this necessitates the reac-
tions being carried out at high temperatures and pressures.
However, in recent years, metal(salen) or metal(salophen) com-
2
0,21
plexes have been developed by the groups of North
and
2
2
Kleij respectively which have a synergistic catalytic effect
when used with quaternary ammonium cocatalysts and allow
the synthesis of cyclic carbonates to occur at or close to room
temperature and pressure.
Fig. 2 Heteroscorpionate aluminium complex which catalyse the synthesis
2
3
Our previous work started with the synthesis of bimetallic
aluminium(salen) complex 4 (Fig. 1) and the demonstration that,
in the presence of a tetrabutylammonium bromide cocatalyst, it
would catalyse the synthesis of cyclic carbonates from terminal
epoxides and carbon dioxide at ambient temperature and pres-
of cyclic carbonates from terminal epoxides and carbon dioxide.
acen ligand and showed that it was also an effective catalyst
for the synthesis of cyclic carbonates from terminal epoxides
and carbon dioxide at room temperature and 1 bar pressure.
20
27
sure. Subsequent mechanistic studies highlighted the impor-
tance of the bimetallic nature of complex 4 as this allows the
epoxide and carbon dioxide to both be activated and to be
Recently, other workers have also reported active catalysts for
cyclic carbonate synthesis based on μ-oxo bridged bimetallic
2
4
28
III
29
pre-organised for an intramolecular reaction. Subsequently,
one-component and immobilized catalysts 5 and 6 (Fig. 1)
were prepared and shown to be active in both batch reactions
and a gas-phase flow reactor. Complexes 4 and 6 were shown
to be compatible with waste carbon dioxide present in power
aluminium or iron complexes.
In recent years, the use of heteroscorpionate ligands has
been widely explored due to their coordination abilities and
3
0
the catalytic applications of their complexes. In this context,
Otero and coworkers have reported the synthesis of acetamidate
and thioacetamidate heteroscorpionate aluminium complexes
2
5
station flue gas and complex 4 could also utilize the carbon
dioxide present in compressed air and catalyse the synthesis
of cyclic carbonates from internal epoxides at elevated temper-
3
1
based on a bis(pyrazol-1-yl)methane moiety. In these com-
plexes, the heteroscorpionate ligand exhibits high versatility
in its coordination mode due to the presence of three possible
26
atures and pressures.
3
2
Based on the importance of a bimetallic catalyst, we also
prepared aluminium complex 7 (Fig. 1) based on the simpler
tautomers in solution. These complexes act as efficient and
versatile single-site initiators in the ring-opening polymeriza-
tion and copolymerization of cyclic esters and in the ring-
3
1
expansion polymerization of ε-caprolactone. In a previous
3
3
paper, we also reported a different class of bimetallic alu-
minium complex 8 (Fig. 2), as a catalyst for the synthesis of
cyclic carbonates from terminal epoxides. Complex 8 was
active at room temperature, but did require an elevated pres-
sure (10 bar) of carbon dioxide to display good levels of cata-
lytic activity. In this paper we report the results of an extended
study on the use of nineteen complexes related to complex
8
as catalysts for cyclic carbonate synthesis under mild reac-
tion conditions.
Results and discussion
The structures of the aluminium complexes used in this
study are shown in Fig. 3. These complexes have all been pre-
3
1
viously reported and characterized and fall into five classes:
Complexes 9–12 are mononuclear alkylaluminium com-
plexes in which the metal ion is coordinated to a single bis-
pyrazole ligand.
Complexes 13–14 are mononuclear aluminium phenoxide
complexes in which the metal ion is coordinated to a single
bis-pyrazole ligand.
Complexes 15–20 are mononuclear alkylaluminium com-
plexes in which the metal ion is coordinated to two bis-
pyrazole ligands.
Complexes 21–22 are binuclear alkylaluminium complexes
in which the metal ions are coordinated to a single bis-
pyrazole ligand.
Fig. 1 Aluminium complexes which catalyse the synthesis of cyclic
carbonates from terminal epoxides and carbon dioxide.
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could be screened at once and the low volatility of styrene
oxide and styrene carbonate combined with the raised pres-
sure ensured that there was no cross-contamination of the
1
reactions. Each reaction was then analysed by H NMR spec-
troscopy without any purification to determine the conver-
sion of epoxide 1c into cyclic carbonate 2c and the results are
shown in Table 1. In all cases, the only species detected by
1
H NMR spectroscopy were unreacted epoxide and styrene
carbonate. In particular, no formation of polycarbonate was
observed in any of these reactions.
Mononuclear imidate complexes 10–12 all displayed good
levels of catalytic activity under these reaction conditions
(
Table 1, entries 2–4, compared to the previously reported
mononuclear thioamidate complex 9 (Table 1, entry 1). This
suggests that the more electron-withdrawing oxygen–nitrogen
ligand of complexes 10–12 generates a more Lewis acidic and
hence more active aluminium ion than the nitrogen–nitrogen
ligand of complex 9. Changing the alkyl groups attached to
the aluminium ion in complexes 10–12 to aryloxy groups in
complexes 13–14 however had a very detrimental effect on
the catalytic activity of the complexes (Table 1, entries 5–6).
Based on previous work with complex 8, this is likely to be
due to the rapid hydrolysis of the alkyl groups of complexes
10–12, leading to catalytically active μ-oxo bridged dimers or
oligomers, a process which will be much slower for aryloxy
complexes 13–14. This is consistent with the fact that
complexes 10–12 all had essentially identical catalytic activity,
suggesting that the alkyl groups attached to the aluminium
may be displaced during the conversion of complexes 10–12
into catalytically active species.
Table 1 Conversion of epoxide 1c into styrene carbonate 2c using
a
catalysts 9–27
b
c
−1
b
c
−1
Entry Catalyst 10 bar (%) TOF (h
)
1 bar (%) TOF (h
)
Fig. 3 Aluminium scorpionate complexes.
1
2
3
4
5
6
7
8
9
1
11
12
1
1
1
9
44
78
77
79
28
38
46
51
39
23
26
31
100
60
92
100
92
100
100
0.37
0.65
0.64
0.66
0.23
0.32
0.38
0.43
0.33
0.19
0.22
0.26
0.83
0.50
0.77
0.83
0.77
0.83
0.83
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Complexes 23–27 are trinuclear alkylaluminium complexes
in which the metal ions are coordinated to a single bis-
pyrazole ligand.
Complex 9 was included in our previous study of alumin-
3
3
ium scorpionate complexes and was included here as a
moderately active mononuclear catalyst with which com-
plexes 10–27 could be compared.
0
3
4
5
77
0.64
Each of complexes 9–27 was initially screened for the con-
version of styrene oxide 1c into styrene carbonate 2c at room
temperature and 10 bar carbon dioxide pressure. Reactions
were carried out in the absence of any solvent for 24 hours
using 5 mol% of both complex 9–27 and tetrabutylammonium
bromide, these conditions having previously been found to
be optimal for reactions catalysed by complex 8. The cata-
lyst screening was facilitated by carrying out reactions in
parallel in magnetically stirred glass tubes within a two litre
stainless steel pressure reactor. In this way, nine catalysts
73
77
52
77
100
0.61
0.64
0.43
0.64
0.83
16
1
1
1
7
8
9
a
3
3
Reactions carried out at room temperature and 1 or 10 bar CO
2
pressure for 24 hours using 5 mol% of catalyst and 5 mol% of
b
1
4
Bu NBr cocatalyst. Conversion determined by H NMR spectroscopy
c
of the crude reaction mixture. TOF = moles of product/(moles of
catalyst·time).
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Table 2 Optimization of the synthesis of styrene carbonate 2c using
Complexes 15–20 in which the aluminium ion is attached
to two bidentate bis-pyrazole ligands, as well as an alkyl
group, all displayed low to moderate catalytic activity (Table 1,
entries 7–12). In these cases, the aluminium ion is sterically
hindered by the two large ligands, so even if the alkyl group
is displaced, the resulting μ-oxo bridged aluminium com-
plexes will not be good Lewis acids and will not be able
to effectively coordinate too and hence activate epoxide 1c
towards ring-opening.
Bimetallic complex 21 was found to be a highly active cata-
lyst, giving complete conversion of epoxide 1c into styrene
carbonate 2c under the reaction conditions (Table 1, entry 13).
Comparison of the results obtained for complexes 10–12 and
a
4
catalyst 27 and Bu NBr
b
c
−1
4
Entry 27 (mol%) Bu NX (mol%) Conversion (%) TOF (h )
1
2
3
4
5
6
7
8
9
5
2.5
1
0.5
5
0
5
5
5
Br (5)
Br (2.5)
Br (1)
Br (0.5)
0
Br (5)
F (5)
Cl (5)
I (5)
100
40
31
16
0
5
0
34
69
0.83
0.33
0.26
0.13
0.00
0.04
0.00
0.28
0.58
a
2
Reactions carried out at room temperature and 1 bar CO pressure
b
1
for 24 hours. Determined by H NMR spectroscopy of the crude
reaction mixture. TOF = moles of product/(moles of catalyst·time).
c
2
1 (Table 1, entries 2–4, 13) shows the added benefit of using
a bimetallic complex. However, in this case the nature of the
group attached to the enamine nitrogen atom was critical as
changing this from a large alkyl group to a flat aryl group
(Table 2, entry 4). Control experiments showed that under
these reaction conditions, complex 27 alone had no catalytic
activity (Table 2, entry 5), and tetrabutylammonium bromide
alone gave only 5% conversion (Table 2, entry 6). The influ-
ence of the halide counterion in the tetraalkylammonium salt
was also investigated and it was found that Br > I > Cl > F
(Table 2, entries 1, 7–9). This is the same order of reactivity
found in previous work on cyclic carbonate synthesis using
complex 4 and suggests that the optimal results are obtained
when the halide is a good nucleophile (to ring-open the epoxide)
and a good leaving group (to allow the cyclic carbonate to form).
However, if the halide is too good a leaving group (iodide),
then reformation of the epoxide can compete with reaction
with carbon dioxide resulting in a slower rate of reaction.
These results indicate, as far as we are aware, that complex 27
is the third most active aluminium based catalyst for styrene
carbonate synthesis known (behind complexes 4 and 7).
Since complex 27 contains seven aluminium bound methyl
groups in three different environments and the results
discussed above had suggested that hydrolysis of these methyl
groups by adventitious moisture might be important for the
catalysis, the influence of added water on the catalytic activity
of complex 27 was investigated. As shown in Fig. 4, the addition
of up to 0.75 mol% of water had no detectable detrimental
effect on the catalytic activity when 5 mol% of complex 27 and
tetrabutylammonium bromide were used as catalysts. How-
ever, addition of 1 mol% of water (enough to hydrolyse just 1
in every 17.5 aluminium bound methyl groups in complex 27)
reduced the conversion from 100 to 73%. Further addition
of water resulted in a rapid drop in catalytic activity, giving
a conversion of just 14% when 2.5 mol% of water was added.
Thereafter, addition of up to 20 mol% of water (sufficient
to hydrolyse all of the aluminium bound methyl groups) had no
further detrimental effect on the catalytic activity of complex 27.
The above experiments resulted in complete conversion of
styrene oxide into styrene carbonate when up to 0.75 mol%
of water was added. To more fully investigate the effect of
small amounts of water (0–1.5 mol%), the study was repeated
using just 2.5 mol% of both complex 27 and tetrabutylammonium
bromide so that the reactions would not go to completion.
(complex 22) resulted in a significant decrease in catalytic
activity of the resulting complex (Table 1, entry 14), to a level
below that seen for monometallic complexes 10–12.
Finally coordination of an additional trialkylaluminium
to the oxygen or sulphur atoms of bimetallic complexes
gave trimetallic complexes 23–27. These complexes all formed
very active catalysts (Table 1, entries 15–19) with complexes
2
3
2
4, 26 and 27 giving complete conversion of styrene oxide to
styrene carbonate.
It is notable that all of the highly active catalysts were
binuclear or trinuclear complexes. Four complexes (21, 24, 26,
2
7) had given complete conversion of styrene oxide to styrene
carbonate at 10 bar pressure and two other complexes (23 and
5) had given 92% conversion. Therefore, these six complexes
2
were selected for a second round of catalyst screening in
which the pressure was reduced from 10 bar to 1 bar, the
results again being given in Table 1. All six complexes were
still catalytically active under these conditions, and trimetallic
complexes 23 and 25 which had given less than 100% conver-
sion at 10 bar pressure again gave the lowest conversions at
1
bar carbon dioxide pressure. Bimetallic complex 21 and
trimetallic complexes 24 and 26 all gave the same conversion
77%) at 1 bar pressure, a significant decrease compared to
(
the 100% conversion observed at 10 bar pressure. In contrast,
trimetallic complex 27 was now clearly seen to be the most
active catalyst as it still gave complete conversion even at
1
bar carbon dioxide pressure.
Complex 27 was therefore selected for further reaction
optimization and the effect of reducing the catalyst and
cocatalyst loading (whilst maintaining a 1 : 1 catalyst to
cocatalyst ratio) was investigated (Table 2). However, at room
temperature and 1 bar carbon dioxide pressure, reducing the
complex 27 and tetrabutylammonium bromide loadings to
2
.5 mol% each reduced the conversion of epoxide 1c to cyclic
carbonate 2c from 100 to 40% (Table 2, entries 1 and 2).
Further reduction of the catalyst loadings to 1 mol% further
reduced the conversion to 31% (Table 2, entry 3), though
interestingly even at a 0.5 mol% loading of complex 27 and
tetrabutylammonium bromide, a 16% conversion was obtained
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epoxides 1a–1k was investigated using 5 mol% of complex 27
and tetrabutylammonium bromide at room temperature
(
except for propylene oxide 1b which was used at 0 °C due to
its volatility) and 1 bar carbon dioxide pressure (except for gas-
eous ethylene oxide 1a which was used in a sealed reactor at
2.5 bar) with no added water. In each case the reaction was left
for 24 hours, then analysed to give a conversion and the cyclic
carbonate separated and purified to give an isolated yield. The
results of this study are shown in Table 3.
In general, all of the terminal epoxides studied gave
good to excellent conversions to the corresponding cyclic
carbonates under these conditions. Notably, this includes the
functionalized epoxides 1i, 1j (Table 3, entries 11 and 12).
The two exceptions were the aryl substituted epoxides 1g, 1h
which under the standard conditions gave only moderate
conversions to the corresponding cyclic carbonates 2g, 2h
Fig. 4 Influence of added water on the conversion of styrene oxide 1c
into styrene carbonate 2c after 24 hours at room temperature catalysed
by complex 27 and Bu
4
NBr (5 mol% each) with 1 bar CO
2
. The inset is
NBr.
the results obtained using 2.5 mol% of complex 27 and Bu
4
(
Table 3, entries 7 and 9). However, these reaction mixtures
The results are shown in the inset to Fig. 4 and indicate that
there is an optimal amount of water (0.75 mol% when using
were observed to solidify during the reaction, and by simply
carrying out the reactions at 50 °C this could be avoided
and the reactions then went to completion (Table 3, entries
8 and 10).
2
.5 mol% of catalysts) and that use of more or less than this
amount of water results in a lower conversion.
These results suggest that the catalytically active species
derived from complex 27 still contains most of the aluminium
bound methyl groups, but that approximately 9% of the
aluminium methyl groups will have been hydrolysed to
aluminium oxides. The same trend was previously seen for
catalysis using complex 8 where the optimal amount of water
As an alternative way of avoiding problems due to reaction
mixtures solidifying, the use of a solvent was also investigated.
Table 4 summarises the results obtained when six different
aprotic solvents were used. In all cases, the conversion was
lower than when the reaction was carried out under solvent
free conditions, but the combination of complex 27 and
tetrabutylammonium bromide was more solvent tolerant
3
3
was 0.75 mol% when using 5 mol% of complex 8 as catalyst.
6
,19
The results can be explained on the basis of the catalytically
active species being a dimer or higher oligomer with oxygen
bridges between aluminium units replacing some of the methyl
groups. However, addition of too much water will result in
over hydrolysis to either a cross-linked catalytically inactive
polymer or to aluminium oxides.
than other catalyst systems for cyclic carbonate synthesis.
Butan-2-one was included in this study due to the excellent
results that Kleij et al. have previously reported in this
3
4
solvent. The results in Table 4 show that >80% conversions
could be obtained in both toluene (Table 4, entry 1) and the
1
6,18
polar aprotic solvents propylene carbonate
and acetoni-
Having optimized the reaction conditions, the synthesis
of eleven cyclic carbonates 2a–2k derived from terminal
trile (Table 4, entries 2 and 3) whilst other functionalized sol-
vents gave lower conversions (Table 4, entries 4–6).
Table 3 Conversion of epoxides 1a–1k into cyclic carbonates 2a–2k using catalyst 27 and Bu
4
NBra
b
c
d
−1
Entry
Epoxide
Temperature (°C)
Conversion
Yield
TOF (h
)
e
e
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1g
1g
1h
1h
1i
18
0
67
85
89
85
72
92
54
84
39
86
66
82
78
7
0.56
0.71
0.77
0.74
0.71
0.60
0.45
0.70
0.33
0.72
0.55
0.68
0.65
0.06
18
18
18
18
18
50
18
50
18
18
18
18
100
100
84
100
63
100
43
100
71
10
11
12
13
14
1j
1k
28
94
90
11
f
a
2 4
Reactions carried out at room temperature and 1 bar CO pressure for 24 hours using 5 mol% of complex 27 and 5 mol% of Bu NBr
c d
b
1
cocatalyst. Determined by H NMR spectroscopy of the crude reaction mixture. Yield of pure isolated cyclic carbonate. TOF = moles of
product/(moles of catalyst·time). The volatile nature of epoxides 1a, 1b meant that conversions could not be determined. Reaction carried
out at 10 bar CO pressure for 72 hours.
e
f
2
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Table 4 Influence of solvent on the synthesis of styrene carbonate 2c
All the kinetic experiments showed a good fit to first order
kinetics,‡ which, since carbon dioxide is present in excess,
implies that the reactions are first order in styrene oxide:
rate = kobs[1c]. This was confirmed by reactions carried out
at four different initial concentrations of styrene oxide 1c
which showed that the rate of formation of styrene carbonate
a
4
using catalyst 27 and Bu NBr
b
c
−1
Entry
Solvent
Conversion
TOF (h
)
1
2
3
4
5
6
Toluene
Propylene carbonate 2b
Acetonitrile
Butan-2-one
Dichloromethane
Tetrahydrofuran
83
81
87
41
72
58
0.69
0.68
0.72
0.34
0.60
0.48
2
c increased as the initial concentration of styrene oxide 1c
increased (Fig. 5).
The concentrations of complex 27 and tetrabutylammonium
bromide do not change during the reaction, so their contri-
a
Reactions carried out at room temperature and 1 bar CO
2
pressure
NBr
cocatalyst at an epoxide concentration of 1.7 M. Determined by
for 24 hours using 5 mol% of complex 27 and 5 mol% of Bu
4
bution to the rate equation is incorporated within kobs: k
k[27] [Bu NBr] . Thus, log(kobs) = log(k) + x·log[27]
4
=
+
b
obs
x
y
1
c
H NMR spectroscopy of the crude reaction mixture. TOF = moles
of product/(moles of catalyst·time).
y·log[Bu NBr], so by carrying out reactions at various concen-
4
trations of complex 27 or tetrabutylammonium bromide whilst
keeping all other conditions constant, a plot of log(kobs
)
against [27] or [Bu NBr] allows the order with respect to these
4
In view of the high catalytic activity shown by the complex
7–tetrabutylammonium bromide catalyst system, an attempt
two reaction components to be determined.‡ Control experi-
ments showed that even at 60 °C in propylene carbonate,
complex 27 (5 mol%) or tetrabutylammonium bromide
2
was made to extend the chemistry to internal epoxides. These
are well known to be much more challenging substrates
for cyclic carbonate synthesis, but recently some significant
(
5 mol%) alone had minimal catalytic activity (after 7 hours;
2
6,35
less than 1% reaction with complex 27 and 12% reaction with
tetrabutylammonium bromide).‡ Reactions were thus carried
out (in duplicate) at four concentrations of complex 27
and the resulting log/log plot had a slope of 0.93 suggesting
that the reaction was first order in complex 27 concentration
successes have been made in this area.
conversion of cyclohexene oxide 28 into its cyclic carbonate
9 was investigated (Scheme 2). However, only an 11% con-
Therefore, the
2
version was obtained, even when the carbon dioxide pressure
was increased to 10 bar and the reaction time extended to
2 hours (Table 2, entry 14). It was however possible to isolate
(i.e. that x = 1). This was confirmed by a plot of kobs versus [27]
7
which was also found to fit to a straight line (Fig. 6).
a pure sample of compound 29, and comparison of its
2
6,36
Similarly, reactions were carried out in duplicate at four
concentrations of tetrabutylammonium bromide and the
resulting log/log plot had a slope of 1.09 suggesting that
the reaction was first order in tetrabutylammonium bromide
concentration (i.e. that y = 1). This was confirmed by a
NMR spectra with those reported in the literature
con-
firmed that the cyclic carbonate was formed as the cis-isomer,
thus indicating that cyclic carbonate synthesis catalysed by
complex 27–tetrabutylammonium bromide proceeds with
retention of the epoxide stereochemistry through a double
inversion process.
4
plot of kobs versus [Bu NBr] which was also found to fit
to a straight line (Fig. 7). Thus, the synthesis of styrene
carbonate 2c from styrene oxide 1c catalysed by complex 27
and tetrabutylammonium bromide can be represented by
To investigate the mechanism of cyclic carbonate synthesis
catalysed by complex 27 and tetrabutylammonium bromide,
a study of the reaction kinetics was carried out. This work
required that the reaction was carried out in a solvent, and
based on the results shown in Table 4, the use of acetonitrile
was first investigated. However, it was desirable that kineti-
cally monitored reactions went to high conversion in less than
eight hours, and to achieve this, the reactions were carried
out at 60 °C. Unfortunately, under these conditions, evapora-
tion of the acetonitrile was a problem, so propylene carbonate
was used as the solvent. Reactions were then carried out with
styrene carbonate 1c as substrate with an excess of carbon
dioxide. Samples were removed from the reaction every hour
the rate equation: rate = k[Bu NBr][27][1c]. This rate equation
4
has the same dependence on catalyst, tetrabutylammonium
bromide and epoxide concentrations as that previously determined
1
and analysed by H NMR spectroscopy to determine the con-
version of epoxide 1c into cyclic carbonate 2c and hence to
allow the concentrations of 1c and 2c to be calculated.
Fig. 5 Dependence of the rate of formation of 2c on the concentration
of epoxide 1c. Reactions were carried out at 60 °C in propylene
carbonate with [Bu
empty squares, [1c]
circles, [1c] = 2.51 M.
4
NBr] = [27] = 70 mM. Filled diamonds, [1c]
0
= 0.81 M;
0
= 1.58 M; filled triangles, [1c] = 2.03 M; empty
0
Scheme 2 Synthesis of cis-cyclohexyl carbonate.
0
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Fig. 6 Plot of the observed first order rate constant versus [27]. kobs(avg)
is the average of two separate kinetic experiments at each concentration.
The error bars are based on the results of the two separate kinetic
experiments. Reactions were carried out at 60 °C in propylene carbonate
Scheme 3 Catalytic cycle for cyclic carbonate synthesis catalysed by
complex 27 and Bu NBr.
4
0 4
with [1c] = 1.6 M, [Bu NBr] = 70 mM and [27] = 42–84 mM.
between aluminium ions. Such cooperative catalysis was
2
3,24
previously seen for reactions catalysed by complexes 4–7,
but not for reactions catalysed by complex 8.
3
3
Conclusions
Parallel catalyst screening has been used to enhance the activity
of an initial aluminium scorpionate complex 8, resulting in
the development of trimetallic complex 27. A combination
of complex 27 and tetrabutylammonium bromide forms an
active catalyst system for the synthesis of cyclic carbonates
from carbon dioxide and terminal epoxides at ambient tem-
−
1
perature and pressure. The turn over frequency (0.8 h )
observed with complex 27 at one bar carbon dioxide pressure
is the same as that previously found for complex 8 at ten bar
carbon dioxide pressure.
Fig. 7 Plot of the observed first order rate constant versus [Bu
4
NBr].
k
obs(avg) is the average of two separate kinetic experiments at
each concentration. The error bars are based on the results of the
two separate kinetic experiments. Reactions were carried out at
Whilst a large number of catalysts have been reported for
the synthesis of cyclic carbonates from terminal epoxides and
carbon dioxide, the vast majority of these require pressures
6
0 °C in propylene carbonate with [1c]
0
= 1.6 M, [27] = 70 mM and
[
Bu NBr] = 42–84 mM.
4
6
above 15 bar and temperatures above 90 °C. Only a small
number of catalyst systems have been reported to be active at
(or below) room temperature and at one bar carbon dioxide
pressure. Of the aluminium based systems which are active
under such mild conditions, complex 27 with a TOF of up
for reactions catalysed by complex 8 and in both cases, the
reactions showed a good fit to overall first order kinetics.
3
3
This suggests that cyclic carbonate synthesis catalysed by
complexes 8 and 27 occurs through the same mechanism
and a possible catalytic cycle is shown in Scheme 3. It is
−
1
to 0.8 h is the third most active, outperformed only by
−
1 23,37
complexes 4 and 7 (TOF 13–64 h ).
Other aluminium
6
−1
well known that the tetrabutylammonium bromide cocatalyst
based catalysts have TOFs of <0.02 to 0.5 h under similar
3
8
ring-opens the Lewis-acid coordinated epoxide to form a
halo-alkoxide. In the case of catalysis by complex 27 (or more
precisely by an in situ formed μ-oxo bridged oligomer of
complex 27), one of the three aluminium ions can act as the
Lewis acid. Carbon dioxide can then insert into the metal alkoxide
bond to give a metal carbonate which can ring-close to form
the cyclic carbonate and regenerate the tetrabutylammonium
bromide as shown in Scheme 3. The first order dependence
of the kinetics on the concentration of complex 27 indicates
that each monomer unit within the oligomer is independently
catalytically active and thus that there is no cooperative catalysis
reaction conditions.
Experimental
Commercially available chemicals (Alfa, Aldrich, Fluka) were used
as received. Complexes 9–27 were prepared as reported previ-
3
1
ously. GC–MS were recorded on a Varian CP-800-SATURN
2200 GC–MS ion-trap mass spectrometer using a FactorFour
(VF-5 ms) capillary column (30 m × 0.25 mm) with helium as
the carrier gas. The conditions used were: initial temperature
60 °C, hold at initial temperature for 3 min then ramp rate
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−
1
1
5 °C min to 270 °C; hold at final temperature for 5 min.
Styrene carbonate (2c). Obtained as
a
white solid
2
4,33
For the first 3.50 min, the eluent was routed away from the
mass detector. Subsequently, the detector was operated in
full EI scan mode. H and C NMR spectra were recorded on
a Bruker Avance 300 spectrometer at 300 MHz for H and
5 MHz for C. All spectra were recorded at ambient temper-
(242.3 mg, 89%). Mp 49–51 °C (lit.
50–51 °C). δH (CDCl3)
), 5.70
4.36 (1H, t J 8.6 Hz, OCH ), 4.82 (1H, t J 8.4 Hz, OCH
2
2
1
13
(1H, t J 8.0 Hz, PhCHO), 7.3–7.5 (5H, m, ArH); δC (CDCl3)
71.0, 77.8, 125.7, 129.2, 129.6, 136.0, 154.5.
1
1
3
7
1,2-Butylene carbonate (2d). Obtained as a colourless liquid
(163.7 mg, 85%). δH (CDCl ) 1.00 (3H, t J 7.1 Hz, CH ),
ature and were referenced to the residual solvent peak.
3
3
1
4
8
.6–1.9 (2H, m, CH ), 4.05 (1H, dd J 6.3, 5.3 Hz, OCH ),
.49 (1H, t J 8.1 Hz, OCH ), 4.5–4.7 (1H, m, OCH); δ (CDCl )
2 C 3
2
2
General procedure for catalyst screening at 10 bar pressure
.6, 27.0, 69.1, 78.1, 155.2.
,2-Hexylene carbonate (2e). Obtained as a colourless liquid
172.1 mg, 72%). δ (CDCl ) 0.91 (3H, t J 7.1 Hz, CH ), 1.2–1.6
4H, m, 2 × CH ), 1.6–1.9 (2H, m, CH ), 4.06 (1H, dd J 8.4,
Styrene oxide 1c (1.7 mmol), catalyst (83.0 μmol) and Bu
4
NBr
1
(26.7 mg, 83.0 μmol) were placed in a sample vial fitted with
(
(
H
3
3
a magnetic stirrer bar and placed in a two litre stainless steel
pressure reactor. Sufficient cardice pellets were added to the
reactor to generate a pressure of 10 bar once they evaporated.
The reaction mixture was stirred at room temperature for 24 h,
then the conversion of styrene oxide 1c to styrene carbonate 2c
2
2
7.2 Hz, OCH ), 4.52 (1H, t J 8.1 Hz, OCH ), 4.68 (1H, qd J 7.5,
2
2
5.4 Hz, OCH); δ
C
3
(CDCl ) 13.5, 22.1, 26.3, 33.4, 69.2, 77.0, 154.8.
1,2-Decylene carbonate (2f). Obtained as a colourless liquid
(
305.4 mg, 92%). δH (CDCl ) 0.86 (3H, t J 6.8 Hz, CH ), 1.1–1.6
3
3
1
was determined by analysis of a sample by H-NMR spectroscopy.
(
12H, m, 6 × CH
2
), 1.6–1.9 (2H, m, CH
2
), 4.04 (1H, dd J 8.4,
7
.2 Hz, OCH ), 4.50 (1H, dd J 8.4, 7.8 Hz, OCH ), 4.8–4.6
2
2
General procedure for catalyst screening at 1 bar pressure
(1H, m, OCH); δ
C
(CDCl
3
) 14.2, 22.7, 24.5, 29.2, 29.2, 29.4,
3
1.9, 34.0, 69.5, 77.1, 155.2.
-Chlorostyrene carbonate (2g). Obtained as a white solid
Styrene oxide 1c (1.7 mmol), catalyst (83.0 μmol) and Bu NBr
4
4
(26.7 mg, 83.0 μmol) were placed in a sample vial fitted with
2
4,33
(
276.8 mg, 84%). Mp 66–69 °C (lit.
68–69 °C). δ
), 4.82 (1H, t J 8.4 Hz, OCH
.68 (1H, t J 7.9 Hz, OCH), 7.32 (2H, d J 8.5 Hz, ArH), 7.42
(CDCl ) 70.7, 77.0, 127.1, 129.4,
H 3
(CDCl )
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 room temperature for 24 h,
then the conversion of styrene oxide 1c to styrene carbonate 2c
4
5
.31 (1H, t J 7.8 Hz, OCH
2
2
),
(
2H, d J 8.5 Hz, ArH); δ
C
3
1
34.5, 135.8, 154.1.
4-Bromostyrene carbonate (2h). Obtained as a white solid
1
was determined by analysis of a sample by H-NMR spectroscopy.
2
4,33
(
346.9 mg, 86%). Mp 72–75 °C (lit.
68–69 °C). δ
H
3
(CDCl )
4
.31 (1H, t J 8.2 Hz, OCH
2
), 4.81 (1H, t J 8.4 Hz, OCH
2
), 5.66
General procedure for synthesis of cyclic carbonates 2b–2k at
(
(
1H, t J 8.0 Hz, OCH), 7.25 (2H, dd J 8.4, 1.8 Hz, ArH), 7.58
2H, dd J 8.1 Hz, 2.0 Hz, ArH); δ (CDCl ) 70.7, 77.1, 123.8,
1
bar pressure
C
3
An epoxide 1b–1k (1.7 mmol), catalyst 27 (42.1 mg, 83.0 μmol)
127.3, 132.4, 135.0, 154.2.
Glycerol carbonate (2i). Obtained as a colourless liquid
(129.3 mg, 66%). δ (DMSO-d ) 3.50 (1H, dd J 12.6, 3.2 Hz,
CH OH), 3.66 (1H, dd J 12.6, 2.6 Hz, CH OH), 4.28 (1H, dd J 8.1,
and Bu NBr (26.7 mg, 83.0 μmol) were placed in a sample vial
4
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 for 24 h at 0 °C for epoxide
H
6
2
2
5.8 Hz, CH O), 4.48 (1H, t J 8.3 Hz, CH O), 4.7–4.8 (1H, m,
2
2
OCH), 5.26 (1H, br, OH); δ
C
(DMSO-d
6
) 60.6, 65.8, 76.9, 155.0.
1
b, 50 °C for epoxides 1g, h or 18 °C for all other substrates.
3-Chloropropylene carbonate (2j). Obtained as a colourless
liquid (185.8 mg, 82%). δH (CDCl ) 3.6–3.9 (2H, m, CH Cl),
For epoxides 1c–1k, the conversion of epoxide to cyclic
3
2
carbonate was then determined by analysis of a sample by
H-NMR spectroscopy. The remaining sample was filtered
4.41 (1H, dd J 9.0, 8.7 Hz, OCH
4.9–5.0 (1H, m, OCH); δ (CDCl
3-Phenoxypropylene carbonate (2k). Obtained as a white
2
), 4.60 (1H, t J 8.5 Hz, OCH
2
),
1
C
3
) 43.9, 67.1, 74.4, 154.4.
through a plug of silica, eluting with CH Cl to remove the
2
2
2
4,33
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 puri-
fied by flash chromatography using a solvent system of first
hexane, then hexane–EtOAc (9 : 1), then hexane–EtOAc (3 : 1),
then EtOAc to give the pure cyclic carbonate. Cyclic carbon-
ates 2a–2k are all known compounds and the spectroscopic
data for samples prepared using catalyst 27 were consistent
solid (270.5 mg, 84%). Mp 94–97 °C (lit.
(CDCl ) 4.16 (1H, dd J 10.6, 3.6 Hz, CH
94–95 °C).
OPh), 4.26
δ
H
3
2
(1H, dd J 10.6, 4.2 Hz, CH OPh), 4.5–4.7 (2H, m, OCH ),
2
2
5.0–5.1 (1H, m, OCH), 6.9–7.0 (2H, m, 2 × ArH), 7.04
(1H, t J 7.5 Hz, ArH), 7.2–7.5 (2H, m, 2 × ArH); δ (CDCl ) 66.2,
C 3
67.3, 74.0, 114.9, 122.1, 129.6, 154.3, 158.0.
2
4,33
Synthesis of ethylene carbonate 2a
with those reported in the literature.
Propylene carbonate (2b). Obtained as a colourless liquid
143.9 mg, 85%). δ (CDCl ) 1.50 (3H, d J 6.3 Hz, CH ), 4.04
‡
A sample vial was charged with Bu NBr (26.7 mg, 83.0 μmol)
4
(
H
3
3
and catalyst 27 (42.1 mg, 83.0 μmol) and then cooled in an
ice bath for 15 min, after which time ethylene oxide (75.0 mg,
1.7 mmol) was added through a syringe. The reaction vial was
(
1H, dd J 8.3, 7.4 Hz, OCH ), 4.57 (1H, t J 8.3 Hz, OCH ), 4.8–4.9
2
2
(
1H, m, OCH); δ (CDCl ) 19.2, 70.5, 73.2, 154.7.
C
3
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then placed into a stainless steel reactor vessel, previously sat-
urated with cardice pellets (approximately 2 g). The vessel was
sealed and the reaction mixture was vigorously stirred for 24 h
at room temperature, after which time the system had
reached an internal pressure of approximately 2.5 bar. Then,
4 M. Aresta and A. Dibenedetto, Dalton Trans., 2007, 2975–2992;
K. M. K. Yu, I. Curcic, J. Gabriel and S. C. E. Tsang,
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Feedstock, ed. M. Aresta, Wiley-VCH, Weinheim, 2010,
pp. 1–375P. Styring and K. Armstrong, Chem. Today, 2011,
29, 28–31.
2
excess CO and unreacted ethylene oxide were vented. The
remaining sample was filtered through a plug of silica, eluting
with CH Cl to remove the catalyst. The eluent was evaporated
5 M. North, Chem. Today, 2012, 30, 3–5.
6 M. Yoshida and M. Ihara, Chem.–Eur. J., 2004, 10, 2886–2893;
J. Sun, S.-I. Fujita and M. Arai, J. Organomet. Chem., 2005,
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2
2
in vacuo and the product was obtained as a colourless liquid
100 mg, 67%). δH (CDCl ) 4.54 (CH ); δ (CDCl ) 64.3, 155.0.
(
3
2
C
3
Synthesis of cyclic carbonate 29 at 10 bar pressure
Cyclohexene oxide 28 (156.8 mg, 1.6 mmol), catalyst 27 (42.1 mg,
4
9, 9822–9837.
R. Zevenhoven, S. Eloneva and S. Teir, Catal. Today, 2006,
15, 73–79; P. P. Pescarmona and M. Taherimehr, Catal. Sci.
8
3.0 μmol) and Bu NBr (26.7 mg, 83.0 μmol) were placed in a
4
7
sample vial fitted with a magnetic stirrer bar and placed in a
stainless steel pressure reactor. Sufficient cardice pellets were
added to the reactor to generate a pressure of 10 bar once they
evaporated. The reaction mixture was stirred at room tem-
perature for 72 h, then the conversion of epoxide 28 to cyclic
carbonate 29 was determined by analysis of a sample by
1
Technol., 2012, 2, 2169–2187; X.-B. Lu and D. J. Darensbourg,
Chem. Soc. Rev., 2012, 41, 1462–1484; N. Kielland, C. J. Whiteoak
and A. W. Kleij, Adv. Synth. Catal., 2013, 355, 2115–2138.
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2004, 42, 5561–5573; D. J. Darensbourg, R. M. Mackiewicz,
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1
H-NMR spectroscopy. The remaining sample was filtered
through a plug of silica, eluting with CH Cl to remove the
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2
8
36–844; G. W. Coates and D. R. Moore, Angew. Chem., Int.
Ed., 2004, 43, 6618–6639; D. J. Darensbourg, Inorg. Chem.,
010, 49, 10765–10780; M. R. Kember, A. Buchard and
catalyst. The eluent was evaporated in vacuo to give a mixture
of cyclic carbonate and unreacted epoxide. The mixture was
purified by flash chromatography using a solvent system of
first hexane, then hexane–EtOAc (9 : 1), then hexane–EtOAc
2
C. K. Williams, Chem. Commun., 2011, 47, 141–163; S. Klaus,
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(3 : 1), then EtOAc to give compound 29 (15.9 mg, 7%) as a
white solid with spectroscopic data consistent with those
2
6
26
reported in the literature. Mp 34–36 °C (lit. 34–35 °C).
(CDCl ) 1.3–1.4 (2H, m, CH ), 1.5–1.7 (2H, m, CH ),
.8–2.0 (4H, m, 2 × CH ), 4.6–4.7 (2H, m); δ (CDCl ) 19.2, 26.8,
δ
H
3
2
2
1
2
C
3
4
2, 8998–9006.
D. J. Darensbourg and S. J. Wilson, J. Am. Chem. Soc., 2011,
33, 18610–18613.
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9
1
1
1
Determination of the reaction kinetics
Styrene oxide 1c (0.10 g, 0.83 mmol), catalyst 27 (2.5, 5.0, 7.5
or 10 mol%) and Bu NBr (2.5, 5.0, 7.5 or 10 mol%) were
2
4
1
dissolved in propylene carbonate (0.50 mL) 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 was stirred at 60 °C. Samples were taken at con-
venient intervals (approximately every hour) and analysed by
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1
H NMR spectroscopy to determine the relative concentra-
tions of styrene oxide 1c and styrene carbonate 2c.
Notes and references
1
Data obtained and derived from the NIST database: http://
webbook.nist.gov/chemistry/name-ser.html accessed on 28th
July 2013.
2
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