Carbon Dioxide Utilisation for the Synthesis of Unsymmetrical Dialkyl and Cyclic Carbonates Promoted by Basic Ionic Liquids

Article abstract of DOI:10.1071/CH17530

An efficient and greener synthesis of unsymmetrical organic carbonates mediated by Hünig's base-appended basic ionic liquids, via carbon dioxide conversion, is described here. These ionic liquids were found to be effective bases for the fixation of carbon dioxide by various alcohols and benzyl bromide, at room temperature. When the alcohol and the halide functionalities are present within the same substrate, the reaction cleanly produces a cyclic carbonate. These functionalised basic ionic liquids were fully recyclable with no loss product yields.

Full text of DOI:10.1071/CH17530

CSIRO PUBLISHING
Aust. J. Chem. 2018, 71, 181–185
Communication
https://doi.org/10.1071/CH17530
Carbon Dioxide Utilisation for the Synthesis of
Unsymmetrical Dialkyl and Cyclic Carbonates
Promoted by Basic Ionic Liquids
A
A D
,
B D
,
B
Peter Goodrich, H. Q. Nimal Gunaratne,
Lili Jin,
Yuntao Lei,
A C
,
and Kenneth R. Seddon
A
The QUILL Research Centre, School of Chemistry and Chemical Engineering,
The Queen’s University of Belfast, Stranmillis Road, Belfast, Northern Ireland,
BT9 5AG, UK.
B
Department of Organic Chemistry, China Pharmaceutical University,
Nanjing 210009, China.
C
Deceased.
D
Corresponding authors. Email: n.gunaratne@qub.ac.uk; jinlili@cpu.edu.cn
An efficient and greener synthesis of unsymmetrical organic carbonates mediated by Hu¨nig’s base-appended basic ionic
liquids, via carbon dioxide conversion, is described here. These ionic liquids were found to be effective bases for
the fixation of carbon dioxide by various alcohols and benzyl bromide, at room temperature. When the alcohol and the
halide functionalities are present within the same substrate, the reaction cleanly produces a cyclic carbonate. These
functionalised basic ionic liquids were fully recyclable with no loss product yields.
Manuscript received: 4 October 2017.
Manuscript accepted: 15 January 2018.
Published online: 14 February 2018.
The chemical conversion of carbon dioxide into value-added
chemicals has been paid much attention in the past decades.[1–3]
Since the greenhouse gas CO2 is an abundant, cheap, non-toxic,
undesirable, and renewable C1 feedstock,[4,5] it would be of
interest to the scientific community and industries to utilise it to
generate value-added chemicals. CO2 is most frequently treated
with alcohols[6,7] or epoxides[8–11] for the synthesis of sym-
metric, cyclic, or polycarbonates.[11] Unsymmetrical organic
carbonates are valuable chemicals in numerous applications,
such as intermediates for the synthesis of pharmaceuticals[12]
and fine chemicals,[13,14] as excellent polar aprotic solvents,[15]
and as monomers for organic glasses, synthetic lubricants or
plasticisers.[16] To produce organic carbonates, conventional
synthetic methods use phosgene,[17–19] haloformates,[20] or
carbon monoxide[21] as starting materials. However, these
chemicals are toxic and have harmful effects on environment
and health. Hence, developing an alternative methodology for
producing these compounds is highly desirable.
Dimethyl carbonate (DMC)[22–24] is an environmentally
friendly methylating agent which can be substituted for phos-
gene in some organic reactions, in particular in the preparation
of organic carbonates via transesterification of DMC or DEC
(diethyl carbonate) with various alcohols. The developed cata-
lysts include 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD),[25]
MCM-41-TBD,[26] immobilised TBD on magnetic g-Fe2O3
nanoparticles,[27] Mg-La mixed metal oxides,[28] CsF/a-
Al2O3,[29] nanocrystalline MgO,[30] organotin-oxomolybdate
coordination polymer,[31] and 1-(3-trimethoxysilylpropyl)-3-
methylimidazolium chloride.[32] However, these catalytic
systems are usually operated at high temperatures (.1008C).
Another method employed electrochemical synthesis starting
from CO2 and alcohols using ionic liquids (ILs)[33] as reaction
media. Unsymmetrical organic carbonates have been synthe-
sised, recently, via a coupling reaction of alcohol, CO2, and
haloalkanes in the presence of Cs2CO3 or K2CO3,[34–36] but
these reactions need tetrabutylammonium iodide or tetrabuty-
lammonium bromide as promoters and N,N-dimethylmethana-
mide as solvent.
ILs exhibit good to excellent performance in many reactions,
whether they act as catalysts or as reaction media. Their
desirable properties as organic reaction media are well docu-
mented.[33,37–40] In our previous work, we reported a new class
of non-nucleophilic basic ILs (Fig. 1, except 1d),[41,42] bearing
the structure of the Hu¨nig’s base,[43] ethyldi(isopropyl)amine
(NEtiPr2). This class of IL possesses a sterically hindered and
non-nucleophilic tertiary amine functionality which is linked to
a quaternary ammonium molecular segment. The basicity of the
tertiary amine is tuned by the nature and the length of the tether,
i.e. the choice of linker between the two nitrogen centres. It was
hypothesised that the ILs depicted in Fig. 1 would make ideal
recyclable materials for the fixation of CO2 to form alkyl
carbonates. The lack of nucleophilicity of the base unit
appended to the cation of these ILs would prevent any side
reactions emanating from haloalkane substrates.
Journal compilation Ó CSIRO 2018
www.publish.csiro.au/journals/ajc

182
P. Goodrich et al.
Ϫ
Ϫ
ϩ
Nϩ
N
NTf2
NTf2
N
N
O
Bu
O
1a
1c
1b
Nϩ
O
Ϫ
Ϫ
Nϩ
N
N
N
NTf2
O
N
NTf2
O
1d
Fig. 1. Set of Hu¨nig’s base-tagged basic ILs 1a–c and tetramethylguanidine-appended IL 1d used in this work,
where [NTf2]ꢀ represents bis{(trifluoromethyl)sulfonyl}amide.
O
Table 1. Optimisation of the synthesis of benzyl propargyl carbonate
IL, 1.0 MPa
R
RЈ
ROH
ϩ
RЈX
ϩ
CO2
O
O
25ЊC
O
CO2
ϩ
Ph
O
O
Scheme 1. The synthesis of unsymmetrical carbonates from CO2, alco-
Br
HC CCH2OH
Ph
IL
hols, and haloalkanes.
2a
Herein, we report a room-temperature, environmentally
benign method for the synthesis of dialkyl organic carbonates
from a three component reaction involving CO2, alcohols, and
benzyl bromide, mediated by basic ILs (Scheme 1). It’s worth
noting that we are the first group to use a non-nucleophilic basic
IL as a base catalyst to promote CO2 chemical fixation. This
method avoids the use of metal-based catalysts and co-catalysts.
The reaction was carried out in ILs 1a–d under prescribed
EntryA IL Amounts of ROH, RX, IL T [8C] Time [h] YieldB [%]
1
—
1a
1b
1c
1c
1d
1c
1c
1c
1c
1c
1c
1.0, 0.5, 0
25
25
25
25
25
25
30
40
40
25
25
25
48
48
48
48
60
48
48
24
30
48
48
48
0
47
42
74
80
0
2
1.0, 0.5, 1.2
1.0, 0.5, 1.2
1.0, 0.5, 1.2
1.0, 0.5, 1.2
1.0, 0.5, 1.2
1.0, 0.5, 1.2
1.0, 0.5, 1.2
1.0, 0.5, 1.2
1.0, 1.0, 1.2
1.0, 0.25, 1.2
1.0, 0.5, 0.6
3
4
5
6
7
79
76
82
45
74
76
reaction conditions (PCO ¼ 1.0 MPa, T ¼ 258C). The influences
2
8
of different basic ILs, temperature, CO2 pressure, and ratios of
substrates were investigated. In the absence of a base, no
reaction was observed (Table 1, entry 1). However, when the
reaction was repeated with 1a, an IL functionalised with NiPr2 in
the side chain, which gave only benzyl propargyl carbonate in
the presence of CO2 at 1.0 MPa and room temperature (entry 2,
47 % yield). Notably, no side products originating from direct
nucleophilic attack of alcohol or base on benzyl bromide were
observed. The comparative activity of the other three ILs (1b–d)
was also examined, and the results showed that 1c was the most
effective. As shown from our previous work,[41] 1c is the most
basic material among these ILs, as a consequence of having the
longest spacer between the cationic head group and the Hu¨nig’s
base unit. As expected, the higher basicity of the IL 1c, resulted
in obtaining a higher yield of carbonates. Thus, 1c improved the
yield to 74 % (Table 1, entry 4). Furthermore, prolonging the
reaction time to 60 h elevated the yield of the carbonate to 80 %
(Table 1, entry 5).
9
10
11
12
AReaction conditions: CO2 (1.0 MPa; carried out in a 16 mL stainless-steel
autoclave). As for the selectivity of the reaction, no detectable by-products
were observed in 1H NMR spectra of isolated product (see Supplementary
Material for spectra). We presumed that all the starting materials were
transformed into products.
BYield was determined by 1H NMR spectroscopy.
product was generated in 45 % yield when a stoicheiometric
ratio of alcohol and benzyl bromide was used (entry 10), and in
,74 % yield with either a 2-fold or 4-fold excess of alcohol
(entries 4, 11, 12).
In order to examine the effect of pressure on the reaction, the
experiment was carried out at different CO2 pressures (Fig. 2).
An effectively linear dependency of the yield upon pressure was
observed, but the gradient is shallow. It is clearly advantageous
to run the reaction at atmospheric pressure, rather than 10
atmospheres, as the slightly lower yield is offset against the
expense and hazard of working at elevated pressures.
Further investigation on the recyclability of the IL system for
these reactions, including linear and cyclic carbonation forma-
tion, proved very successful. The IL 1c could be recovered
quantitatively from the reaction system by extraction or distilla-
tion (see Supplementary Material), and used several times for
the reaction, with no negative impact upon the yield.
Based on the above results, a plausible reaction mechanism
(Fig. 3) was postulated. In the initial step, the basic (but non-
nucleophilic) cation of the IL deprotonates the alcohol, resulting
in the formation of a nucleophilic alkoxide. This then attacks the
The new IL 1d, which contains the 1,1,3,3-tetramethylguanidine
(TMG) group, is a strong organic base. It was designed specifi-
cally to improve the yield of the organic carbonate. Unfortu-
nately, the desired product was not detected after the reaction,
presumably due to the TMG group being insufficiently sterically
hindered to prevent its nucleophilic attack on benzyl bromide,
producing a quaternary salt.
The effect of the temperature on this reaction was also
investigated (Table 1, entries 4, 7, 8). The yield of benzyl
propargyl carbonate did not enhance significantly when the
temperature increased from room temperature to 408C, albeit
shortening the reaction time. The different ratio of substrates and
IL has some influence on the reaction (Table 1, entries 4, 10–12).
In general, the yield of benzyl propargyl carbonate increased
with increasing concentrations of propargyl alcohol. The

Ionic Liquids Mediated Carbon Dioxide Fixation
183
60
58
56
54
52
50
48
46
44
42
40
Table 2. Formation of unsymmetrical carbonates from ROH, R9Br,
and CO2 in the presence of IL 1c
EntryA
R
R0
Product
Time [h]
YieldB [%]
1
2
3
4
5
6
Et
Benzyl
Benzyl
Benzyl
Benzyl
Benzyl
Butyl
2b[27]
2c[44]
2d[46]
2e[45]
2f
60
72
72
72
72
72
82
80
83
78
67
–
Allyl
Benzyl
Phenyl
s-Butyl
Benzyl
2g[36b]
AReaction conditions: ROH (1.0 mmol), R0Br (0.5 mmol), 1c (0.6 mmol,
0.3414 g), CO2 (1.0 MPa) at 258C.
BIsolated yield.
0
0.2
0.4
0.6
0.8
1.0
CO2 pressure [MPa]
O
X
OH
1c
O
O
ϩ
CO2
Fig. 2. The influence of the pressure of CO2 on the reaction with propargyl
alcohol (1.0 mmol, 56 mg) and benzyl bromide (0.5 mmol, 85.5 mg) in the
presence of 1c (0.6 mmol, 0.341 g) in a 16 mL stainless-steel autoclave
for 24 h.
n
25ЊC
n
X ϭ Cl or Br
n ϭ 0 or 1
Scheme 2. The synthesis of cyclic carbonates from CO2 and v-haloalkanols
in the presence of a basic IL, at room temperature.
O
OH
CO2
ϩ
O
O
Ph
PhCH2Br
Ϫ
N
NiPr2
NTf2
Whereas the reactions between alcohols and CO2 in
the presence of chloroalkanes are decidedly unfavoured, an
intramolecular enhancement was found for the reaction between
v-chloroalkanols and CO2. Thus the reactions between
3-bromopropanol or 2-chloroethanol with CO2 in the presence
of 1.2 equivalents of 1c were compared. Remarkably, both
proceeded to effective completion (. 90 %) within 48 h (see
Fig. 4). Possible side products such as linear chain carbonates
were not found in the system: the products were exclusively
cyclic.
ϩ
R
PhCH2Br
O
Ϫ
OϪ
CO2
O
O
Ϫ
ϩ
N
NHiPr2
NTf2
Ϫ
ϩ
ϩ
R
N
NHiPr2
NTf2
ϩ
R
In conclusion, we have demonstrated a simple, efficient,
environmentally benign, and metal-free approach for the syn-
thesis of unsymmetrical and cyclic organic carbonates, utilising
CO2 under ambient conditions and a range of basic ILs. The use
of these ILs enabled facile product isolation, accompanied by IL
recycling. This represents a green route to both novel and known
unsymmetrical carbonates.
Fig. 3. Proposed mechanism for the formation of carbonate from CO2.
weakly electrophilic carbon of CO2 to form a carbonate anion,
which subsequently reacts with the benzyl bromide (SN2) to
produce the corresponding carbonate and the protonated IL.
Having established the optimal conditions for the synthesis of
benzyl propargyl carbonate, the generic nature of the reactionwas
investigated using a range of different alcohols and bromoalkanes
(see Table 2). The unsymmetrical dialkyl carbonates were gener-
allyproduced inexcellent yieldsundermild conditions. However,
no product was detected using 1-bromobutane instead of benzyl
bromide, emphasising the importance of the high electrophilicity
of the haloalkane for an efficient reaction (Table 2, entry 6).
Although some unsymmetrical organic carbonates are
known,[27,34–36,44,45] these have usually been prepared by trans-
esterification of an alcohol with DMC, thus constraining one of
the substituents to be methyl. Using other common reagents
such as 1,1-carbonyldiimidazole[46] or phosgene[19] produces
only symmetric carbonates with alcohols. Thus the reaction
between alcohols, haloalkanes, and CO2 in the presence of a
basic IL provides a new, green synthetic route to both known and
novel unsymmetrical dialkyl carbonates.
Experimental
Carbon dioxide (99.99 %) was obtained from BOC. All reagents
and analytical grade solvents were used as received. Propargyl
alcohol, phenol, benzyl alcohol, allyl alcohol, 2-butyl alcohol,
2-bromoethanol, 2-chloroethanol, 3-bromo-1-propanol, 1,1,3,3-
tetramethyl guanidine (TMG), ethyl bromide, 2-[2-dimethyla-
minoethoxy] ethanol, and 2-diisopropylaminoethyl chloride
hydrochloride, were obtained from Sigma–Aldrich or TCI.
1H and 13C NMR spectra were recorded on a Bruker Ultra-
shield 400 plus spectrometer at 258C using methanol (CD3OD)
or trichloromethane (CDCl3) as solvent. Electrospray (ES)-mass
spectrometry measurements were carried out on a Waters LCT
Premier instrument with an Advion TriVersa NanoMate injec-
tion system (cone voltage 50 V, source 1208C).
ILs 1a–c were prepared according to the previously pub-
lished report.[41] The procedure for the synthesis of 1d is given in
the Supplementary Material.
All the reactions were carried out in a 16 mL stainless-steel
autoclave with stirring at 600 rpm and equipped with an
The scope of this reaction can be extended by combining the
hydroxy and halo functionalities into one molecule, such as
v-haloalkanols (see Scheme 2), which proves to be a very
efficient route to cyclic carbonates.

184
P. Goodrich et al.
(a)
O
O
O
OH
Cl
4.9 4.8 4.7 4.6 4.5 4.4 4.3 4.2 4.1 4.0 3.9 3.8 3.7 3.6 3.5 3.4 3.3
f1 [ppm]
O
(b)
O
O
Br
OH
5.0 4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0
f1 [ppm]
Fig. 4. Partial 1H NMR (400 MHz) spectra obtained in the synthesis of cyclic carbonates by CO2 conversion
promoted by 1c. (a) 1H NMR spectra of a mixture of 2-chloroethanol and 1c after treating with CO2 (top) and 2-
chloroethanol in CDCl3 (bottom). (b) 1H NMR spectra of a mixture of 3-bromopropanol and 1c after treating with CO2
(top) and 3-bromopropanol in CDCl3 (bottom). The concentrations of haloalcohols and 1c are 0.5 mmol and 0.6 mmol,
respectively; the pressure of CO2 is 1.0 MPa.
automatic stirrer and temperature control system. In a typical
reaction procedure, IL (0.60 mmol, 1.2 equiv.), benzyl bromide
(0.5 mmol, 85.5 mg, 1.0 equiv.), and alcohols (1.0 mmol, 2.0
equiv.) were added into the autoclave successively. CO2 (1.0
MPa) was charged into the reactor at room temperature. The
autoclave was stirred at 258C for an appropriate time. After
reaction, the excess of CO2 was vented. Ethyl ether (2 mL) was
added to the mixture, and the crude product was extracted into
the ether layer. The pure product was obtained after evaporation
of solvent and excess starting materials.
Conflicts of Interest
The authors declare no conflicts of interest.
Acknowledgements
Lili Jin acknowledges the financial support from the China Scholarship
Council and the Fundamental Research Funds of Central Universities
(ZJ14011).
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