Synthesis of cyclic carbonates from atmospheric pressure carbon dioxide using exceptionally active aluminium(salen) complexes as catalysts

Article abstract of DOI:10.1002/ejic.200700521

Dimetallic aluminium(salen) complexes show exceptionally high catalytic activity for the synthesis of cyclic carbonates from terminal epoxides at ambient temperature and pressure. The process has the potential to contribute towards decreasing atmospheric

Full text of DOI:10.1002/ejic.200700521

SHORT COMMUNICATION
DOI: 10.1002/ejic.200700521
Synthesis of Cyclic Carbonates from Atmospheric Pressure Carbon Dioxide
Using Exceptionally Active Aluminium(salen) Complexes as Catalysts
Jaisiel Meléndez,[a] Michael North,*[a] and Riccardo Pasquale[a]
Keywords: Carbon dioxide fixation / Aluminium / Schiff bases / Epoxides / Catalysis
Dimetallic aluminium(salen) complexes show exceptionally
high catalytic activity for the synthesis of cyclic carbonates
from terminal epoxides at ambient temperature and pres-
sure. The process has the potential to contribute towards
decreasing atmospheric carbon dioxide emissions from the
burning of fossil fuels.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
cially and it has been estimated that cyclic carbonates and
products derived from them could reduce atmospheric car-
bon dioxide emissions by up to 120 million tons per year if
the economics of the process could be improved.[3] The ep-
oxide co-reactants required for the chemistry shown in
Scheme 1 are readily available by the catalytic epoxidation
of alkenes, a process which can be achieved using hydrogen
peroxide as a green oxidant,[8] and the alkenes can be ob-
tained from waste plastics or biological sources. A large
number of homogeneous and heterogeneous catalysts have
been reported to induce the reaction between epoxides and
carbon dioxide.[7,9] However, almost all of these require ele-
vated reaction temperatures and/or high pressures of car-
bon dioxide; often the reaction is conducted in supercritical
carbon dioxide.
Introduction
One of the main scientific challenges facing the human
race in the 21st century is controlling global warming due
to increasing levels of atmospheric carbon dioxide.[1] In the
long term, it may be possible to move entirely to renewable,
or non-carbon-based fuels; however, for the foreseeable fu-
ture, annual consumption of fossil fuels is predicted to con-
tinue to increase markedly with a corresponding rise in at-
mospheric carbon dioxide levels.[2] The only solution cur-
rently being considered to this problem is “carbon capture
and storage” which involves concentrating and compressing
carbon dioxide and then storing it in disused oil/gas wells
or under the ocean.[3] However, it is far from certain that
long-term containment is feasible given the volatile nature
of carbon dioxide.
An alternative solution to the problem would be to con-
vert the carbon dioxide at source (taking advantage of the
relatively high concentration of carbon dioxide in for exam-
ple the exhaust stream of a fossil fuel power station) into a
chemical for which there is a significant commercial de-
mand.[1,3,4] In order to be feasible, any such procedure
would have to occur at atmospheric pressure and near am-
bient temperature; otherwise the energy required to operate
the process will generate more carbon dioxide than is reme-
diated. Unfortunately, the low chemical reactivity of carbon
dioxide severely restricts the range of chemical reactions
which might be feasible.
Scheme 1. Synthesis of polymeric or cyclic carbonates from CO2
and epoxides.
Notable amongst the catalysts which have been employed
to synthesise cyclic carbonates are metal(salen) com-
plexes[6,10] derived from aluminium,[11] nickel,[11a] cop-
per,[11a,12] zinc,[11a,12] magnesium,[11a] cobalt,[11a,12–14] chro-
mium,[11a,15] or tin.[16] In previous work from our group,[17]
we have demonstrated that dimetallic salen complexes dis-
play much higher catalytic activity than their monometallic
counterparts in asymmetric cyanohydrin synthesis because
the two metal ions can each activate one of the two compo-
nents of the reaction. Polycarbonate and cyclic carbonate
synthesis using chromium(salen) complexes are known to
be bimolecular,[15] therefore, we reasoned that a suitable di-
metallic salen complex might display significantly improved
catalytic activity in the reaction between carbon dioxide
and epoxides. Here we report the successful accomplish-
One unusually facile reaction of carbon dioxide is its in-
sertion into epoxides to generate cyclic carbonates or a
polycarbonate[5–7] as shown in Scheme 1. Ethylene, propyl-
ene and butylene carbonates are manufactured commer-
[a] School of Natural Sciences, Newcastle University, Bedson Building,
Newcastle upon Tyne, NE1 7RU, UK
Fax: +44-870-131-3783
E-mail: michael.north@ncl.ac.uk
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
Eur. J. Inorg. Chem. 2007, 3323–3326
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3323

J. Meléndez, M. North, R. Pasquale
SHORT COMMUNICATION
ment of this concept through the development of a dimet-
allic aluminium(salen) complex which will catalyse the in-
sertion of carbon dioxide into epoxides at atmospheric pres-
sure and ambient temperature.
Results and Discussion
We selected aluminium as the metal of choice due to its
low environmental impact and previous use in cyclic car-
bonate synthesis, albeit at elevated pressures of carbon di-
oxide (at least 6 atm).[11] Thus, treatment of a salen ligand
with aluminium triethoxide in toluene gave complexes 1a–
h as yellow or orange solids in 33–63% yields as shown in
Scheme 2. The µ-O atom in complexes 1a–h could be de-
rived from adventitious moisture, by decomposition of eth-
anol formed during the reaction, and/or from the ethoxy
group. Complexes 1a and 1c have previously been prepared
by a different route[18,19] and used to catalyse asymmetric
Michael additions.[19] The X-ray structure of complexes
1c[20] and 1d[21] have previously been reported, and the mass
spectra of compounds 1a–h confirmed their dimetallic
structures (see Supporting Information).
Complexes 1a–h were tested for their ability to catalyse
the formation of carbonate 3a from racemic styrene oxide
(2a) and carbon dioxide under solvent-free conditions as
shown in Scheme 3, and the results are summarised in
Table 1. In the absence of a co-catalyst, no reaction oc-
curred, even at 5 atm pressure and elevated temperature
(Table 1, Entries 1 and 2). This is in line with previous work
using aluminium(salen)-derived catalysts where reaction
only occurred in the presence of a tetrabutylammonium ha-
lide,[11a–11d] DMAP[11e,11f] or N-methylimidazole[11e] co-cat-
alyst. The combination of complex 1a and tetrabutylammo-
nium bromide was found to display extremely high catalytic
activity (Table 1, Entries 4–11). This combination of cata-
lysts displayed unprecedented catalytic activity at 1 atm car-
bon dioxide pressure and room temperature. Tetrabutylam-
monium chloride and iodide were also investigated as co-
catalysts, but were found to be less effective than the use of
tetrabutylammonium bromide. The only previous reports of
synthetic studies[11c,13c] on the preparation of cyclic carbon-
ates at ambient temperature required carbon dioxide pres-
sures of 6–15 atm[14,22] or reaction times of up to 7 d.[23]
That the observed catalysis was not due solely to the tetra-
butylammonium bromide co-catalyst was demonstrated by
Scheme 2. Synthesis of catalysts 1a–h.
Scheme 3. Synthesis of cyclic carbonates 3 using complexes 1a–h.
a reaction run in the absence of an aluminium(salen) com- to be related to the solubility of the catalyst in styrene oxide
plex (Table 1, Entry 3). Although use of just 1.0 mol-% of rather than to any steric or electronic factors, as the highest
catalyst 1a gave a good conversion after 24 h (Table 1, En- catalytic activity was observed with the most hydrophobic
try 10), use of 2.5 mol-% was adopted as standard in subse- complexes, and complexes 1d and 1f which gave the lowest
quent studies as this allowed good conversions (62% with conversions (Table 1, Entries 14 and 16) did not fully dis-
57% isolated yield) to be achieved after just 3 h (Table 1, solve during the reaction.
Entry 8).
Cyclic carbonates derived from other monosubstituted
Attempts to increase the reactivity of the catalyst by epoxides could also be obtained from the combination of
varying the substituents on the aromatic rings or the di- complex 1a and tetrabutylammonium bromide (Scheme 3),
amine of the salen ligand were not successful as none of as detailed in Table 2. Propylene oxide was a particularly
catalysts 1b–h were significantly more active than complex reactive substrate, and only 1 mol-% of catalyst was neces-
1a (Table 1, Entries 12–18). The trend in reactivity appears sary to convert it into propylene carbonate in good yield,
3324
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 3323–3326

Cyclic Carbonates from Atmospheric Pressure Carbon Dioxide
SHORT COMMUNICATION
Table 1. Synthesis of carbonate 3a catalysed by complexes 1a–h.
Entry
Catalyst [mol-%]
Co-catalyst [mol-%]
CO2 pressure [atm]
Temp. [°C]
Time [h]
Conversion[a] [%]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
1a (1.0)
1a (1.0)
1
5
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
25
50
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
3
24
3
3
3
3
3
3
24
24
24
3
3
3
3
3
3
3
0
0
4
5
Bu4NBr (1.0)
Bu4NBr (0.1)
Bu4NBr (1.0)
Bu4NBr (2.5)
Bu4NBr (1.0)
Bu4NBr (2.5)
Bu4NBr (0.1)
Bu4NBr (1.0)
Bu4NBr (2.5)
Bu4NBr (2.5)
Bu4NBr (2.5)
Bu4NBr (2.5)
Bu4NBr (2.5)
Bu4NBr (2.5)
Bu4NBr (2.5)
Bu4NBr (2.5)
1a (0.1)
1a (1.0)
1a (1.0)
1a (2.5)
1a (2.5)
1a (0.1)
1a (1.0)
1a (2.5)
1b (2.5)
1c (2.5)
1d (2.5)
1e (2.5)
1f (2.5)
1g (2.5)
1h (2.5)
38
56
51
62
27
86
98
50
52
33
41
28
51
64
1
[a] Conversions are based on H NMR analysis of the reaction mixture.
even at 0 °C (Table 2; Entries 1–3). The other monosubsti- minium ion, followed by intramolecular transfer of the alk-
tuted epoxides studied were converted into the correspond- oxide onto the coordinated carbon dioxide gives an alumin-
ing carbonates in good to high yield using the standard ium carbonate which can cyclize to form the cyclic carbon-
conditions developed above (Table 2, Entries 4–7), but use ate product and regenerate both catalysts. Work is currently
of lower amounts of catalyst/co-catalyst seriously reduced underway to study the kinetics of this process and to detect
the yields in these cases. Disubstituted epoxides were much reaction intermediates to confirm this mechanistic hypothe-
less reactive with trans-stilbene oxide giving just 8% of the sis.
corresponding cyclic carbonate after a reaction time of
48 h, whilst cyclohexene oxide and 2-phenylpropylene oxide
failed to give any cyclic carbonate under these conditions.
No polycarbonate was detected in any reaction catalysed by
complexes 1a–h with any of substrates 2a–e, indicating that
these complexes display a high selectivity for cyclic carbon-
ate rather than polycarbonate formation.
Table 2. Synthesis of carbonates 3b–e using catalyst 1a.
Entry[a]
Epoxide
1a/Bu4NBr [mol-%] Time [h] Yield [%][c]
1[b]
2[b]
3[b]
4
5
6
2b (R = Me)
2b (R = Me)
2b (R = Me)
2c (R = CH2Ph)
2c (R = CH2Ph)
2d (R = Bu)
2.5 + 2.5
1 + 1
1 + 1
2.5 + 2.5
2.5 + 2.5
2.5 + 2.5
2.5 + 2.5
3
3
24
3
24
3
3
77
40
63
44
99
87
64
7
2e (R = C8H17
)
[a] All reactions were conducted in the absence of solvent at 1 atm
pressure of carbon dioxide and at 25 °C, unless otherwise stated.
[b] Reaction at 0 °C. [c] Yield of isolated product obtained after
chromatographic purification. Conversions (based on 1H NMR
analysis of the reaction mixture) are also given in the Supporting
Information.
Scheme 4. Possible mechanism to explain the catalytic activity of
complexes 1a–h.
The exceptional catalytic activity of the catalysts 1a–h is
consistent with a mechanism in which both aluminium ions
of the complex play a role in activating the components of
Conclusions
The dimetallic aluminium(salen) complexes 1a–h display
the reaction as shown in Scheme 4. Thus, ring-opening of exceptionally high catalytic activity for the conversion of
the epoxide by bromide results in an aluminium-bound alk- terminal epoxides into cyclic carbonates at room tempera-
oxide. Coordination of carbon dioxide to the second alu- ture and 1 atm pressure.
Eur. J. Inorg. Chem. 2007, 3323–3326
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3325

J. Meléndez, M. North, R. Pasquale
SHORT COMMUNICATION
[9] For reviews of cyclic carbonate synthesis, see: a) M. Yoshida,
M. Ihara, Chem. Eur. J. 2004, 10, 2886–2893; b) J. Sun, S. Fu-
jita, M. Arai, J. Organomet. Chem. 2005, 690, 3490–3497.
[10] Unmetallated salen ligands will also catalyse this reaction, but
only at high pressures (Ͼ30 atm) and high temperatures
(Ͼ100 °C): Y. M. Shen, W. L. Duan, M. Shi, Eur. J. Org. Chem.
2004, 3080–3089.
Experimental Section
Experimental details for the synthesis of catalysts 1a–h and cyclic
carbonates 3a–e are given in the Supporting Information.
Supporting Information (see footnote on the first page of this arti-
cle): Full experimental details and characterising data for the syn-
thesis of catalysts 1a–h and cyclic carbonates 3a–e.
[11] a) X.-B. Lu, X.-J. Feng, R. He, Appl. Catal. A 2002, 234, 25–
34; b) X.-B. Lu, R. He, C.-X. Bai, J. Mol. Catal. A 2002, 186,
1–11; c) X.-B. Lu, Y.-J. Zhang, B. Liang, X. Li, H. Wang, J.
Mol. Catal. A 2004, 210, 31–34; d) X.-B. Lu, Y.-J. Zhang, K.
Jin, L.-M. Luo, H. Wang, J. Catal. 2004, 227, 537–541; e) M.
Alvaro, C. Baleizao, E. Carbonell, M. El Ghoul, H. García, B.
Gigante, Tetrahedron 2005, 61, 12131–12139; f) G. A. Luinstra,
G. R. Haas, F. Molnar, V. Bernhart, R. Eberhardt, B. Rieger,
Chem. Eur. J. 2005, 11, 6298–6314.
Acknowledgments
The authors thank Newcastle University for a studentship (to
R. P.), the Engineering and Physical Sciences Research Council
(EPSRC) for funding and the EPSRC national mass spectrometry
service at the University of Wales, Swansea for recording mass
spectra.
[12] Y.-M. Shen, W.-L. Duan, M. Shi, J. Org. Chem. 2003, 68, 1559–
1562.
[13] a) X.-B. Lu, J.-H. Xiu, R. He, K. Jin, L.-M. Luo, X.-J. Feng,
Appl. Catal. A 2004, 275, 73–78; b) R. L. Paddock, S. T.
Nguyen, Chem. Commun. 2004, 1622–1623; c) X.-B. Lu, B. Li-
ang, Y.-J. Zhang, Y.-Z. Tian, Y.-M. Wang, C.-X. Bai, H. Wang,
R. Zhang, J. Am. Chem. Soc. 2004, 126, 3732–3733; d) X.-B.
Lu, Y. Wang, Angew. Chem. Int. Ed. 2004, 43, 3574–3577.
[14] For the kinetic resolution of propylene oxide using Co(salen)
complexes, see: A. Berkessel, M. Brandenburg, Org. Lett. 2006,
8, 4401–4404.
[15] a) R. L. Paddock, S. T. Nguyen, J. Am. Chem. Soc. 2001, 123,
11498–11499; b) D. J. Darensbourg, J. C. Yarbrough, C. Ortiz,
C. C. Fang, J. Am. Chem. Soc. 2003, 125, 7586–7591; c) M.
Alvaro, C. Baleizao, D. Das, E. Carbonell, H. García, J. Catal.
2004, 228, 254–258.
[16] H. Jing, S. K. Edulji, J. M. Gibbs, C. L. Stern, H. Zhou, S. T.
Nguyen, Inorg. Chem. 2004, 43, 4315–4327.
[17] For a review of this work, see: T. R. J. Achard, L. A. Clut-
terbuck, M. North, Synlett 2005, 1828–1847.
[18] S. J. Dzugan, V. L. Goedken, Inorg. Chem. 1986, 25, 2858–
2864.
[19] For a leading reference, see: E. P. Balskus, E. N. Jacobsen, J.
Am. Chem. Soc. 2006, 128, 6810–6812.
[20] D. Rutherford, D. A. Atwood, Organometallics 1996, 15, 4417–
4422.
[21] Y. Wang, S. Bhandari, S. Parkin, D. A. Atwood, J. Organomet.
Chem. 2004, 689, 759–765.
[22] For mechanistic studies on the reaction between propylene ox-
ide and carbon dioxide at room temperature and 1 atm pres-
sure with long reaction times, see: a) K. Kasuga, S. Nagao,
T. Fukumoto, M. Handa, Polyhedron 1996, 15, 69–72; b) H.
Sugimoto, T. Kimura, S. Inoue, J. Am. Chem. Soc. 1999, 121,
2325–2326.
[1]
For a detailed analysis of this issue from a chemical catalysis
perspective, see: H. Arakawa, M. Aresta, J. N. Armor, M. A.
Barteau, E. J. Beckman, A. T. Bell, J. E. Bercaw, C. Creutz, E.
Dinjus, D. A. Dixon, K. Domen, D. L. DuBois, J. Eckert, E.
Fujita, D. H. Gibson, W. A. Goddard, D. W. Goodman, J. Kel-
ler, G. J. Kubas, H. H. Kung, J. E. Lyons, L. E. Manzer, T. J.
Marks, K. Morokuma, K. M. Nicholas, R. Periana, L. Que, J.
Rostrup-Nielson, W. M. H. Sachtler, L. D. Schmidt, A. Sen,
G. A. Somorjai, P. C. Stair, B. R. Stults, W. Tumas, Chem. Rev.
2001, 101, 953–996.
[2]
a) X. Xiaoding, J. A. Moulijin, Energy Fuels 1996, 10, 305–325;
b) M. Pervaiz, M. M. Sain, Resour., Conserv. Recycl. 2003, 39,
325–340 and references cited therein.
[3]
[4]
M. Aresta, A. Dibenedetto, Catal. Today 2004, 98, 455–462.
For reviews of the potential utilisation of atmospheric carbon
dioxide, see: a) A. Baiker, Appl. Organomet. Chem. 2000, 14,
751–762; b) I. Omae, Catal. Today 2006, 115, 33–52; c) R. Zev-
enhoven, S. Eloneva, S. Teir, Catal. Today 2006, 115, 73–79.
[5] For a review of polycarbonate synthesis, see: G. W. Coates,
D. R. Moore, Angew. Chem. Int. Ed. 2004, 43, 6618–6639.
[6] For a review of the use of metal(salen) complexes in the synthe-
sis of polycarbonates, see: D. J. Darensbourg, R. M. Mackiew-
icz, A. L. Phelps, D. R. Billodeaux, Acc. Chem. Res. 2004, 37,
836–884.
[7] For a review of the reaction between epoxides and CO2, see:
D. J. Darensbourg, M. W. Holtcamp, Coord. Chem. Rev. 1996,
153, 155–174.
[8] For leading references, see: a) R. I. Kureshy, S. Singh, N. H.
Khan, S. H. R. Abdi, I. Ahmed, A. Bhatt, R. V. Jasra, Catal.
Lett. 2006, 107, 127–130; b) K. Matsumoto, Y. Sawada, T. Kat-
suki, Synlett 2006, 3545–3547; c) H. Shitama, T. Katsuki, Tet-
rahedron Lett. 2006, 47, 3203–3207.
[23] a) M. Ratzenhofer, H. Kisch, Angew. Chem. Int. Ed. Engl.
1980, 19, 317–318; b) H. Kisch, R. Millini, I.-J. Wang, Chem.
Ber. 1986, 119, 1090–1094.
Received: May 16, 2007
Published Online: June 21, 2007
3326
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 3323–3326