One-component catalysts for cyclic carbonate synthesis

Article abstract of DOI:10.1039/b900180h

Homogeneous and immobilized one-component catalysts for the conversion of epoxides and carbon dioxide into cyclic carbonates at atmospheric pressure and room temperature have been developed. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b900180h

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One-component catalysts for cyclic carbonate synthesisw
´
Jaisiel Melendez, Michael North* and Pedro Villuendas
Received (in Cambridge, UK) 6th January 2009, Accepted 6th March 2009
First published as an Advance Article on the web 18th March 2009
DOI: 10.1039/b900180h
Homogeneous and immobilized one-component catalysts for the
conversion of epoxides and carbon dioxide into cyclic carbonates
at atmospheric pressure and room temperature have been
developed.
Two major challenges facing the global chemical community
over the next two decades are: stabilizing and reducing atmo-
spheric carbon dioxide levels to mitigate the effects of climate
change;1 and developing sustainable raw materials for the
chemicals industry to replace crude oil.2 One approach to
both of these problems is the development of commercially
viable routes to basic chemicals that employ carbon dioxide as
the starting material.3,4 However, if such a process is to
contribute to reducing atmospheric carbon dioxide levels then
it must be carried out at atmospheric pressure and at or near
room temperature; otherwise, more carbon dioxide will be
produced generating the required energy than is consumed by
the chemical reaction.
Scheme 1 Synthesis of cyclic carbonates from epoxides.
carbonates by a kinetic resolution of epoxides12,16 and alumi-
nium, chromium, cobalt and tin(salen) complexes have been
used to prepare polycarbonates.17 In 2007, however, we
reported the development of bimetallic aluminium(salen)
complex 1 as a uniquely active catalyst for the synthesis of
cyclic carbonates from terminal epoxides at atmospheric
pressure and room temperature.18 These reactions required
the presence of tetrabutylammonium bromide as a cocatalyst.
Recently, we reported detailed mechanistic studies on this
catalytic system19 and showed that the tetrabutylammonium
bromide plays two key roles in the catalytic cycle: providing
bromide to ring-open the epoxide and generating tributyl-
amine in situ, which reacts with the carbon dioxide to form a
carbamate salt. The bimetallic aluminium complex then brings
these two species together and allows them to combine
intramolecularly to complete the catalytic cycle.
Salicylic acid has been commercially prepared from carbon
dioxide for over a century,2,3 urea is currently prepared from
carbon dioxide on a 100 million ton per annum scale2,4 and a
number of other reactions of carbon dioxide are known to be
exothermic.3 There has also been considerable interest in the
hydrogenation of carbon dioxide5 to formic acid, formalde-
hyde, or methanol, though these processes will only be
viable if hydrogen becomes available on a large scale from
non-fossil-fuel sources.6
Another commercially important process2,7,8 that utilises
carbon dioxide is the 100% atom economical synthesis of
cyclic carbonates from epoxides9 shown in Scheme 1. This is
already a commercial process for the synthesis of both
ethylene and propylene carbonates, though currently used
catalysts require the reaction to be carried out at high
temperature and high pressure using highly purified carbon
dioxide.4,7 Under these conditions, the reaction generates
more carbon dioxide than it consumes. Metal(salen)
complexes have been widely studied as catalysts for cyclic
carbonate synthesis with aluminium,10 chromium,11 cobalt,12
ruthenium,13 tin14 and manganese15 complexes being reported
to be active in the presence of elevated pressures of carbon
dioxide. Cobalt and chromium(salen) complexes have
also been used for the enantioselective synthesis of cyclic
Whilst the complex 1–tetrabutylammonium bromide
catalyst system had exceptionally high catalytic activity, the
need for two separate catalysts was a disadvantage, especially
given the desirability of immobilising the catalyst and
employing it in a flow reactor. Herein, we report the synthesis
and catalytic activity of single-component catalysts which
build on the mechanistic understanding of the reaction and
combine the key features of complex 1 and tetrabutyl-
ammonium bromide within a single entity. The first design
for a single-component catalyst is represented by structure
7a–d. This incorporates four tetraalkylammonium bromide
units in each bimetallic complex unit since reaction kinetics
using complex 1 had shown the reaction to be second order in
tetrabutylammonium bromide.19 Catalysts 7 were prepared as
shown in Scheme 2. Thus, treatment of known20 salicyl-
aldehyde derivatives 2a,b with diethylamine gave tertiary
amines 3a,b, which could be condensed with 1,2-diamines 4a,b
to give salen ligands 5a–d. Reaction of diamines 5a–d with benzyl
bromide in acetonitrile gave bis-ammonium salts 6a–d. Treat-
ment of ligands 6a,b with triethoxyaluminium gave the desired
School of Chemistry and University Research Centre in Catalysis and
Intensified Processing, Bedson Building, University of Newcastle,
Newcastle upon Tyne, UK NE1 7RU.
E-mail: Michael.north@ncl.ac.uk; Fax: +44 870 131 3783;
Tel: +44 191 222 7128
w Electronic supplementary information (ESI) available: Experimental
details for the synthesis of catalysts and cyclic carbonates and char-
acterization data for all new compounds. See DOI: 10.1039/b900180h
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Scheme 2 Synthesis of one-component catalysts 7a–d, 9a,b and 10a,b.
Table 1 Synthesis of cyclic carbonates using catalysts 7a–da
catalysts 7a–b.w For the synthesis of catalysts 7c,d, it was
necessary to reverse the last two steps. Thus, treatment of ligands
5c,d with triethoxyaluminium gave bimetallic complexes 8c,d,
which when treated with benzyl bromide gave catalysts 7c,d.
Complex 7a was a highly effective catalyst for the conver-
sion of terminal epoxides into cyclic carbonates at room
temperature and atmospheric pressure. Representative results
are given in Table 1.1 Thus, ten racemic terminal epoxides
were converted into the corresponding cyclic carbonates with
good to excellent conversions and isolated yields after reaction
times of 3–6 hours using 2.5 mol% of catalyst 7a under
solvent-free conditions. When enantiomerically pure styrene
oxide was used as substrate, chiral HPLC analysis of samples
withdrawn after 3 and 6 hours indicated that the styrene
carbonate was also enantiomerically pure.
% Yield
(hours)
Catalyst
Epoxide R =
Conversion (hours)b
7a
7a
7a
7a
7a
7a
7a
7a
7a
7a
7b
7c
7d
Ph
71 (3), 97 (6)
30 (3)
44 (3), 81 (6)
55 (3), 79 (6)
84 (3), 100 (6)
50 (3)
59 (3), 85 (6), 95 (24)
37 (3), 50 (6), 93 (24)
30 (3), 44 (6), 98 (24)
37 (3), 49 (6), 66 (24)
69 (3), 88 (6), 99 (24)
48 (3), 60 (6), 73 (24)
54 (3), 69 (6), 89 (24)
89 (6)
63 (6)
68 (6)
65 (6)
81 (6)
62 (6)
89 (24)
85 (24)
83 (24)
51 (24)
90 (24)
55 (24)
74 (24)
CH3(CH2)3
CH3(CH2)7
CH2OH
CH2Cl
Mec
4-MeC6H4
4-BrC6H4
4-ClC6H4
4-MeSC6H4
Ph
Ph
Ph
a
Reactions carried out under solvent-free conditions using 2.5 mol%
b
of catalyst 7a–d. For full experimental details see ESIw. Determined
In addition to epoxides bearing simple alkyl or aryl sub-
stituents, catalyst 7a was also compatible with functionalized
1
c
by H NMR spectroscopy. Reaction carried out at 0 1C.
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epoxides such as glycidol and glycidyl chloride. The conver-
sion of propylene oxide into propylene carbonate is also
significant in view of the commercial importance of this
reaction. Another feature of these reactions is the complete
absence of polycarbonate formation.2,17
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Catalysts 7b–d in which the structure of the salen ligand has
been simplified by removal of the tert-butyl groups and/or
cyclohexyl ring were also effective catalysts. Compared to
complex 7a however, complexes 7b–d were all slightly less
effective in the conversion of styrene oxide into styrene
carbonate in a given period of time, which in the case of
catalysts 7c,d is related to the poor solubility of these
complexes in styrene oxide.
To further demonstrate the utility of a one-component
catalyst system, immobilized versions of catalysts 7 were pre-
pared from ligands 5a,b. Thus, treatment of ligands 5a,b with
triethoxyaluminium gave bimetallic complexes 8a,bz which
reacted with bromomethylpolystyrene to give immobilized
catalysts 9a,b containing a single tetraalkylammonium salt.
Subsequent reaction of complexes 9a,b with benzyl bromide
gave tetralkylammonium salt containing catalysts 10a,b.
Cyclic carbonate synthesis using catalysts 9 and 10 required
a solvent to swell the resin beads and propylene carbonate was
found to be a suitable solvent.19 Reactions were then carried
out with styrene oxide as substrate at 26 1C under an atmo-
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carbonate in yields of 100, 94 and 70% after reaction times of
20 hours. Although catalyst 9a contains only a single tetra-
alkylammonium bromide, its high level of catalytic activity is
consistent with the important role played by amino groups in
activating the carbon dioxide for cyclic carbonate synthesis.19
Under the same conditions, catalyst 10a gave styrene carbo-
nate yields of 79, 73, 66 and 60% over four consecutive
reactions. In contrast however, supported catalyst 9b dis-
played very low reactivity, giving just 8% conversion of
styrene oxide to styrene carbonate. The activity of this system
was however markedly improved by converting it to the
tetraammonium salt 10b, which gave styrene carbonate yields
of 79, 71, 67 and 64% in four consecutive reactions.
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Thus, eight one-component catalysts for cyclic carbonate
synthesis have been prepared and shown to exhibit high
activity in reactions carried out at atmospheric pressure and
room temperature. Polymer-supported catalysts 9 and 10 are
the first heterogeneous catalysts to display catalytic activity
under such mild reaction conditions and open the possibility
of using these and related systems in continuous flow reactors.
The authors thank the EPSRC and CarbonConnections for
financial support.
Notes and references
z Complexes 8a,b were also active catalysts for the conversion of
styrene oxide to styrene carbonate, but only in the presence of
tetrabutylammonium bromide.
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Chem. Commun., 2009, 2577–2579 | 2579