Electrosynthesis of cyclic carbonates from epoxides and atmospheric pressure carbon dioxide

Article abstract of DOI:10.1039/c1cc15467b

The use of CO₂ for the preparation of value-added compounds has dramatically increased due to increased global warming concerns. We herein report an electrochemical cell containing a copper cathode and a magnesium anode that effectively converts epoxides and carbon dioxide to cyclic carbonates under mild electrochemical conditions at atmospheric pressure.

Full text of DOI:10.1039/c1cc15467b

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COMMUNICATION
Electrosynthesis of cyclic carbonates from epoxides and atmospheric
pressure carbon dioxidew
Benjamin R. Buckley,* Anish P. Patel and K. G. Upul Wijayantha*
Received 2nd September 2011, Accepted 22nd September 2011
DOI: 10.1039/c1cc15467b
The use of CO for the preparation of value-added compounds
2
the two most effective solutions have been reported indepen-
has dramatically increased due to increased global warming
concerns. We herein report an electrochemical cell containing
a copper cathode and a magnesium anode that effectively
converts epoxides and carbon dioxide to cyclic carbonates under
mild electrochemical conditions at atmospheric pressure.
dently by Coates and Lee. Coates zinc based catalyst 1 has been
employed in the copolymerisation of cyclohexene oxide and
8
CO
2
and Lee’s salen cobalt catalyst 2 has been used in the
9
copolymerisation of propylene oxide and CO
2
(Fig. 1). Cyclic
, are
carbonates, which are also produced from epoxides and CO
2
widely used in the manufacture of products including solvents,
paint-strippers, biodegradable packaging, as well as having other
Carbon dioxide is arguably the main contributor to the
increased concentration of gases within the atmosphere, in
10
applications in the chemical industry. Again, however, these
processes do require high energy input, including high temperatures,
high pressures and the use of purified carbon dioxide.
2
fact since the dawn of the industrial revolution CO concen-
1
tration within the atmosphere has annually increased. The
main cause being identified as the combustion of fossil fuels.
There are a plethora of methods for the formation of cyclic
1
1
carbonates from epoxides and CO2 but recent efforts have
been focused on the development of catalytic systems which
are able to transform epoxides to cyclic carbonates at
temperatures below 100 1C and at atmospheric pressure.
Currently the only successful system is that reported by North
and co-workers which employs a bimetallic salen complex
such as 3 (Fig. 1) and tetrabutylammonium bromide as a
Methods that involve reducing the amount of CO released
2
into the atmosphere from industrial processes are therefore of
2
significant value. Current commercial methods involve CO
2
capture via absorption using amine based materials post
3
combustion. Pre-combustion methods also exist in which fuel
is firstly converted into H
2
and CO
2
followed by a capture
is achieved by storing it
process. Once captured storage of CO
2
1
2
co-catalyst, although later developments have incorporated
1
in unused oil/gas wells or under the ocean. This is a highly
energy intensive process and unlike other waste streams CO
has a variety of possible uses such as a C1 building block in
2c
the co-catalyst into the salen substructure.
We were interested in developing a low energy alternative in
2
13
4
,5
line with our current research portfolio aimed at electrosynthesis
organic synthesis.
coupled with semiconductor photoelectrodes to drive light
1
To chemists carbon dioxide presents itself as a cheap, non-
toxic and highly abundant carbon source, and if effectively
activated can allow for carboxylation of organic molecules.
4
assisted electrosynthetic reactions.
We were therefore
attracted to the possibility of employing an electrosynthetic
system as this type of process could be designed to be cost
neutral in terms of energy consumption if combined with a
2
Therefore fixation of CO into organic molecules is an area of
increasing interest and offers a much more environmentally
6
sound alternative to current storage solutions. Currently, the
suitable solar powered energy source. Dunach has previously
˜
reported the electrochemical carboxylation of epoxides using a
greatest obstacle for establishing industrial processes based on
1
5
nickel(II) cyclam complex, and in a related process Yuan and
co-workers have reported an electrochemical system using
CO
2
as a raw material is that in general a large energy input is
required to transform CO
2
.
1
6
high CO2 pressures. Both of these systems unfortunately
have several drawbacks, including expensive potentially toxic
catalysts, non-user friendly solvents and in the latter case high
There are several useful products which can be prepared
from CO , for example polycarbonates, which are also
2
synthesised from epoxides and CO (Scheme 1), are commercially
2
2
CO pressures.
important as they have several applications, for example as
7
eyewear lenses and exterior automotive components. Perhaps
Department of Chemistry, Loughborough University, Loughborough,
Leicestershire, LE11 3TU, UK. E-mail: b.r.buckley@lboro.ac.uk,
u.wijayantha@lboro.ac.uk; Fax: (+)+44 (0)1509 22 3925
w Electronic supplementary information (ESI) available: Experimental
procedures and characterization of compounds 5a–k. See DOI:
10.1039/c1cc15467b
Scheme 1 Current processes for CO
2
incorporation into epoxides.
1
1888 Chem. Commun., 2011, 47, 11888–11890
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a
Table 2 Optimization studies
Supporting
Entry electrolyte (equiv.) T/1C
b
t (h) Conv. (%)
2
Fig. 1 Current catalysts for CO incorporation into epoxides.
1
2
3
4
5
6
7
8
Bu
4
Bu
4
Bu
4
Bu
4
Bu
4
Bu
4
Bu
4
Bu
4
NBr (2.0)
NBr (1.0)
NBr (0.5)
NBr (1.0)
7 h rt, 12 h 50 1C 19
7 h rt, 12 h 50 1C 19
7 h rt, 12 h 50 1C 19
499
98
80
77
17
99 (96)
77
We believed that it should be possible to deliver a step
change in this technology by the development of an electro-
chemical system which is easy to set up, cheap, reliable,
25 1C
50 1C
50 1C
25 1C
50 1C
6
6
6
6
NPF
6
(1.0)
c
NBr (1.0)
NBr (1.0)
NBr (1.0)
requires no expensive catalysts, runs under atmospheric CO
2
3.5 76
pressures and at ambient temperatures. In order to achieve this
goal we set about screening a range of electrode materials for
the electrosynthetic incorporation of CO2 into our test
substrate styrene oxide 4a (Table 1). The reactions were run
in a single compartment cell, with acetonitrile as solvent,
tetrabutylammonium bromide as electrolyte and 60 mA current.
We were delighted to find that good to excellent conversions to
cyclic carbonate could be achieved under a variety of conditions
a
General conditions: Cu cathode, Mg anode, CO
b
2
(1 atm, balloon),
MeCN, single compartment cell, 60 mA. Conversion evaluated from
1
the H NMR spectrum by integration of epoxide vs. cyclic carbonate
peaks, isolated yield after column chromatography is shown in
c
parenthesis. On average 90–95% of the Bu
each reaction by precipitation with EtOAc.
4
NBr is recovered after
(
Table 1). Excellent conversion to cyclic carbonate 5a was
here that we typically recover 90–95% of this material after the
reaction by precipitation with EtOAc), however, below this
level we observed only 80% conversion (Table 2, entry 3). We
also found that the use of tetrabutylammonium bromide is
essential for the reaction to proceed, if we replaced the
bromide counter-ion with tetrafluoroborate then we observed
only 17% conversion to cyclic carbonate (Table 2, entry 5).
The reactions can be carried out at room temperature (77%
conversion after 6 h, Table 2, entry 7), however, for convenience
mild heating to 50 1C is employed (Table 2, entry 6); A
combination of heating (50 1C) and electrolysis (60 mA)
affords the cyclic carbonate 2a in 99% conversion and 96%
isolated yield. Reduced reaction times, for example, 3.5 h
results in only 76% conversion (Table 2, entry 8).
achieved using the copper cathode/magnesium anode combi-
nation (Table 1, entry 1). It is important to note that in the
absence of applied current we observed little to no cyclic
1
7
carbonate (3%) and only recovered unreacted epoxide. We
also did not observe the formation of any polymeric materials
from the possible competing polycarbonate reaction or any
other side products that have previously been observed for the
corresponding reactions carried out at high temperature and/
1
8,19
or pressure.
We next turned our attention to optimizing this mild system
Table 2), we found that high conversion to cyclic carbonate
(
could still be achieved if we reduced the amount of tetrabuty-
lammonium bromide to 1.0 equivalent (it is important to note
With our optimized conditions in hand we screened the
applicability of this system over a range of substrate types
(Table 3). We were delighted to find that excellent yields of
cyclic carbonates were observed, including electron rich and
poor aromatic systems, and a range of aliphatic epoxides were
also effectively converted to the corresponding cyclic carbonate
a
Table 1 Electrode screening
(
these yields represent some of the highest reported to date).
Sensitive functionalized substrates such as 4i–k, are also
tolerated under the reaction conditions affording excellent
yields of the corresponding cyclic carbonates 5i–k.
b,c
Entry
Cathode
Anode
Conv. (%)
1
2
3
4
5
6
Cu
Mg
Mg
Mg
Al
Sn
Zn
499 (3)
In order to explore the mild nature of these reaction
conditions we were interested in the applicability of this system
to enantiopure epoxides. Application of our optimized conditions
Steel
Graphite
Cu
Cu
Cu
75
80
75
10
5
2
to the CO incorporation of (S)-styrene oxide afforded the
cyclic carbonate in excellent yield (97%) with retention of
configuration and only a slight loss of optical purity
(99.5 : 0.5 e.r. to 98.6 : 1.4 e.r., (Scheme 2)).
a
General conditions: CO
2
(1 atm, balloon), Bu
4
NBr (2.0 eq.), MeCN,
b
single compartment cell, 60 mA, 7 h rt, 12 h 50 1C. Conversion
evaluated from the H NMR spectrum by integration of epoxide vs.
c
cyclic carbonate peaks. The number in parenthesis refers to the
1
In conclusion we have developed a powerful new tool in the
2
combat against CO emissions for the synthesis of value added
conversion from the reaction carried out in the absence of electrolysis.
compounds. This process is one of only a handful of
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11888–11890 11889

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a
Table 3 Application to a range of epoxides
Notes and references
1
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3
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7
8
V. Serini, ‘‘Polycarbonates’’, in Ullmann’s Encyclopedia of
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9
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1
0 M. North, R. Pasquale and C. Young, Green Chem., 2010, 12,
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1
1
1 For some recent reviews and articles on high pressure and/or
temperature approaches to cyclic carbonates see: (a) I. Omae,
Catal. Today, 2006, 115, 33–52; (b) J. H. Clements, Ind. Eng.
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2
26, 199–205; (e) R. Zevenhoven, S. Eloneva and S. Teir, Catal.
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a
2 4
General conditions: CO , Cu cathode, Mg anode, Bu NBr (2.0 equiv.),
MeCN, single compartment cell, 60 mA, 6 h 50 1C, isolated yields shown
are after column chromatography. Reaction ran for 8 h.
2
–12; (i) R. L. Paddock and S. T. Nguyen, J. Am. Chem. Soc.,
2001, 123, 11498–11499.
2 (a) J. Melendez, M. North and R. Pasquale, Eur. J. Inorg. Chem.,
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b
1
´
2
2
2
´
Commun., 2009, 2577–2579; (d) M. North, P. Villuendas and
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1
3 P. C. B. Page, F. Marken, C. Williamson, Y. Chan, B. R. Buckley
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1
4 (a) A. A. Tahir and K. G. U. Wijayantha, J. Photochem. Photo-
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enantiomerically pure cyclic carbonate with retention of configuration.
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˜
5 (a) P. Tascedda and E. Dunach, J. Chem. Soc., Chem. Commun.,
approaches that allow this reaction to proceed at atmospheric
1,15
1
1
pressure and at ambient-mild temperatures.
The yields
1
995, 43–44; (b) E. Dunach, P. Tascedda, M. Weidmann and
˜
obtained are comparable or better than those already reported
in the area and our approach benefits from not employing an
additional catalyst into the reaction, which in some cases can
be toxic or expensive. The equipment required to perform this
E. Dinjus, Appl. Organomet. Chem., 2001, 15, 141–144.
6 Y. Wang, G.-Q. Yuan, Y.-C. Zeng and H.-F. Jiang, Chin. J. Org.
Chem., 2007, 27, 1397–1400.
1
1
7 Epoxides dissolved in molten TBAB/TBAI have been converted
into cyclic carbonates under atmospheric pressure carbon dioxide,
but at elevated temperatures (120 1C): see V. Calo, A. Nacci,
A. Monopoli and A. Fanizzi, Org. Lett., 2002, 4, 2561–2563.
8 For example, isomerisation of the epoxide to ketone and aldehydes
2
CO incorporation reaction is cheap and should be readily
available in any undergraduate teaching facility i.e. copper
wire, magnesium ribbon and a power supply. We are currently
looking at the mechanism (early indications show that it does
1
or addition of H
X.-D. Yue, F. Cai and L.-N. He, Catal. Commun., 2007, 8,
67–172.
19 GC-MS and H, C NMR data revealed only the reaction
products and Bu NBr.
2
20 In an independent reaction addition of MgBr to the reaction
2
O to afford the ring opened diol: see J.-Q. Wang,
2
0
1
not involve MgBr
2
but may well be related to that postulated
21
1
13
12e
by North and co-workers) and scalability of this process
4
as well as applications of CO incorporation towards other
2
types of organic molecules and the development of a self-
contained solar driven process.
without electrolysis resulted in complete recovery of starting
materials.
1 The electrode materials required for large scale applications are
currently commercially available. Mg anodes are used in the
protection of ship hulls. See for example: http://www.corrpro.co.
uk/pdf/Anodes/Magnesium/Magnesium-Anodes.pdf.
2
B.R.B. and K.G.U.W. would like to thank Research Councils
UK for RCUK fellowships and Loughborough University for
funding a PhD studentship to A.P.P.
1
1890 Chem. Commun., 2011, 47, 11888–11890
This journal is c The Royal Society of Chemistry 2011