Electrocatalytic carboxylation of aromatic ketones with carbon dioxide in ionic liquid 1-butyl-3-methylimidazoliumtetrafluoborate to α-hydroxy- carboxylic acid methyl ester

Article abstract of DOI:10.1016/j.electacta.2011.03.061

A new electrochemical procedure for the electrocatalytic carboxylation of aromatic ketones with carbon dioxide in ionic liquid, 1-butyl-3- methylimidazolium tetrafluoborate (BMIMBF₄), to α- hydroxycarboxylic acid methyl ester was investigated for the first time. The electrochemical behavior of acetophenone in BMIMBF₄ was studied by cyclic voltammetry with a reduction peak at -1.9 V (vs. Ag). The electrolyses experiments were carried out in an undivided cell under mild conditions without any toxic solvents, catalysts and supporting electrolytes, followed by addition of an alkylating agent, affording the α-hydroxycarboxylic acid methyl ester in a moderate yield (62%). The results showed that the yields were strongly affected by various factors: temperature, current density, charge passed, electrode material and substrate concentration. Moreover, the ionic liquid was successfully recycled for this reaction.

Full text of DOI:10.1016/j.electacta.2011.03.061

Electrochimica Acta 56 (2011) 5137–5141
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrocatalytic carboxylation of aromatic ketones with carbon dioxide in ionic
liquid 1-butyl-3-methylimidazoliumtetrafluoborate to ␣-hydroxy-carboxylic
acid methyl ester
Qiuju Feng, Kelong Huang∗, Suqin Liu, Jingang Yu, Fangfei Liu
College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new electrochemical procedure for the electrocatalytic carboxylation of aromatic ketones with carbon
dioxide in ionic liquid, 1-butyl-3-methylimidazolium tetrafluoborate (BMIMBF4), to ␣-hydroxycarboxylic
acid methyl ester was investigated for the first time. The electrochemical behavior of acetophenone
in BMIMBF4 was studied by cyclic voltammetry with a reduction peak at −1.9 V (vs. Ag). The elec-
trolyses experiments were carried out in an undivided cell under mild conditions without any toxic
solvents, catalysts and supporting electrolytes, followed by addition of an alkylating agent, affording the
␣-hydroxycarboxylic acid methyl ester in a moderate yield (62%). The results showed that the yields were
strongly affected by various factors: temperature, current density, charge passed, electrode material and
substrate concentration. Moreover, the ionic liquid was successfully recycled for this reaction.
© 2011 Elsevier Ltd. All rights reserved.
Received 12 January 2011
Received in revised form 12 March 2011
Accepted 14 March 2011
Available online 22 March 2011
Keywords:
Aromatic ketones
Carbon dioxide
Ionic liquid
␣-Hydroxycarboxylic ester
Electrocarboxylation
1. Introduction
cations (N,N-dialkylimidazolium, phosphonium, pyridinium, qua-
−
ternaryammonium, etc.) with a variety of anions, such as BF4
,
PF6−, and AlCl4 [15,16]. Recently, ionic liquids have been used as
green reaction media in organic synthesis due to their wide electro-
chemical potential window, high ionic conductivity, good solvating
ability and low vapour pressure. Up to now, several research groups
have reported electrochemical fixations of CO2 with various organic
compounds to obtain valuable carboxylated products [15–19].
Conversely, to our best knowledge, investigations concerning
chemical fixation of CO2 with aromatic ketones in ionic liquids have
not been reported. Herein, we began studying the electrocarboxy-
lation of aromatic ketones in CO2-saturated ionic liquid BMIMBF4
(Scheme 1). The aim of this investigation was to set-up a simple
and efficient electrochemical route for the incorporation of CO2
into aromatic ketones in BMIMBF4 to yield ␣-hydroxycarboxylic
acid methyl esters, under mild conditions without supporting
electrolyte and catalysts, giving a safer and more environmental
friendly process for CO2 fixation.
−
Carbon dioxide (CO2) produced by combustion of fossil fuels
is regarded as the most significant greenhouse gas; the increas-
ing accumulation of CO2 in the atmosphere has caused serious
environmental problems [1]. On the other hand, CO2 is a naturally
abundant, economical, non-toxic and recyclable carbon source [2].
Thus, the conversion and utilization of CO2 have attracted consid-
erable attention in the whole world. The electrocarboxylation of
various organic compounds with CO2, which can replace phosgene
and cyanides, is an environmental friendly and attractive approach
to prepare valuable carboxylated products [3–9].
Direct electrocarboxylation of aromatic ketones with CO2
has attracted a considerable interest as a method to obtain
␣-hydroxycarboxylic ester and its derivatives widely used as anti-
inflammatory drugs [8–14]. Nevertheless, the use of toxic organic
solvents, such as CH3CN and DMF, and of large amounts of sup-
porting electrolyte makes it more complex to recover the solvents.
With the growing demand of environmental friendly technologies,
efforts should be devoted to avoid the use of volatile solvents.
Ionic liquids, molten salts with melting points close to room
temperature, are obtained by combination of large organic
2. Experimental
2.1. Chemicals
The ionic liquid BMIMBF4 (with a purity of more than 99%) was
dried under vacuum at 120 ◦C until cyclic voltammetric results
indicated that there was no detectable water, as the presence of
water would narrow the electrochemical potential window of the
∗
Corresponding author. Tel.: +86 731 8887 9850; fax: +86 731 8887 9850.
E-mail addresses: fqj245@126.com (Q. Feng), huangkelong@yahoo.com.cn
(K. Huang).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.03.061

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Q. Feng et al. / Electrochimica Acta 56 (2011) 5137–5141
Scheme 1.
ionic liquid BMIMBF4 [20,21]. The purity of CO2 and argon (Ar) was
99.99%. Unless otherwise noted, the reagents and solvents were
used as received from commercial suppliers.
2.2. Typical electroanalytical and electrolysis procedure
Fig. 1. Cyclic voltammograms recorded with glassy carbon electrode: (a) neat
BMIMBF4, (b) BMIMBF4 saturated with CO2, (c) BMIMBF4 containing 0.02 mol dm−3
acetophenone and (d) as (c) saturated with CO2.
The electroanalytical experiments were carried out using
CHI660B electrochemical station (Shanghai Chenhua Instrument
Company) in an undivided cell, with glassy carbon electrode (GC,
d = 3 mm) as the working electrode, platinum (Pt, area = 1 cm2) foil
as the counter electrode and Ag wire as the reference electrode.
Prior to the experiments, all electrodes were polished mechani-
cally (the GC electrode was polished until a mirror-like surface
was obtained) with sand paper and then were sonicated (KQ-
50DE) in ethanol for 5 min to remove any microparticulates.
Finally, all the electrodes were cleaned with diluted hydrochlo-
ric acid and double distilled water and then were dried with
Ar gas.
The electrolysis experiments were conducted in a standard
undivided glass cell equipped with a metallic cathode (area = 2 cm2)
and a magnesium rod (Mg, d = 0.5 cm) sacrificial anode. The gal-
vanostatic electrolysis was performed by using a CT2001C battery
test system (Wuhan Land Company, China) with two electrodes,
whereas the potentiostatic electrolysis was carried out using
CHI660B electrochemical station with three electrodes. Prior to
each test, BMIMBF4 (5 mL) with definite concentration of aromatic
ketone was bubbled with CO2 for 30 min to be saturated. Then a
suitable constant current (or potential) was applied and continu-
ous CO2 flow was maintained throughout the duration of the whole
electrolysis experiment. After a certain amount of charge (Q) had
been supplied to the electrode, the electrolysis was interrupted.
Then CH3I (3-fold excess) was added as an alkylation agent and
the mixture was stirred at 55 ◦C for 6 h. After that the reaction
mixture was extracted with diethyl ether (Et2O, 3× 5 mL). Then
the combined organic layers were acidified with 2 mol dm−3 aque-
ous HCl and dried over anhydrous Na2SO4. After evaporation of the
Et2O under reduced pressure, the crude product was obtained. This
product was purified by column chromatography with petroleum
ether/ethyl acetate mixtures.
77 (12), 43 (18). H NMR: ı 7.0–7.5 (4H, –ArH), 3.8 (s, 3H, ArOCH3),
3.7 (s, 1H, –OH), 3.7 (s, 3H, –COOCH3), 1.8 (s, 3H, –CH3).
3. Results and discussion
3.1. Cyclic voltammetry of acetophenone in BMIMBF4
Acetophenone was chosen as the model molecule to be inves-
tigated. Cyclic voltammograms (CV) of acetophenone in BMIMBF4
on GC electrode were recorded at the scan rate of 10 mV s−1 in an
undivided cell at 25 ◦C with Pt foil as counter electrode and an Ag
wire as the reference electrode.
As shown in Fig. 1a, after bubbling Ar gas for 20 min, there was
no reduction peak in the sweeping region from −0.9 V to −3.0 V in
neat BMIMBF4. It should be noted that the cathodic current began to
increase at around −2.1 V, due to the reduction of BMIMBF4 [17,18].
Fig. 1b illustrates that there is no obvious reduction peak in the
sweeping region and no enhancement of the current density before
−2.1 V in BMIMBF4 saturated with CO2, demonstrating that CO2
does not begin to reduce in this potential window. After addition
of acetophenone (0.02 mol dm−3) to neat BMIMBF4, the cathodic
current starts to increase at around −1.7 V (Fig. 1c), and a distinct
reduction peak appears at about −1.9 V; this peak is attributed to
the two successive one electron reduction of acetophenone to ketyl
radical anion then to a dianion [10–12,22,23]. The voltammetric
behavior of the acetophenone is strongly modified by addition of
CO2 to the solution of BMIMBF4 containing acetophenone. In fact,
after saturating the solution with CO2, a significant positive shift of
the peak potential (Ep) from −1.9 V to −1.7 V and a little increase of
the reduction current were observed (Fig. 1d), possibly because of
rapid chemical reaction between the electrogenerated anion radi-
cal and CO2 [10–12,22–24]. In addition, it is noteworthy that Ep of
acetophenone is more positive than that of CO2, which provides
a possible potential window to perform the electroreduction of
acetophenone without interference from reduction of CO2.
The effect of the scan rate on the CV behavior of acetophenone
in BMIMBF4 was examined at 25 ◦C. The results are presented in
Fig. 2. As shown, the reduction peak shifted toward negative poten-
tial, and the cathodic peak current enhanced with increasing the
scan rate from 5 to 80 mV s−1. The peak current varies linearly with
Gas chromatography–mass spectra (GC–MS) were recorded
with HP 6890/5973 GC–MS an Agilent 1100 series. HNMR spec-
tra were obtained with a Varian INOVA-300 spectrometer using
tetramethylsilane (TMS) as internal standard and deuterated chlo-
roform (CDCl3) as solvent.
2-Hydroxy-2-phenylpropionic acid methyl ester: GC–MS (m/z, %):
180 (M+, 29), 148 (9), 121(22), 104 (100), 77 (39), 51 (11), 32 (24).
H NMR: ı 7.2–7.6 (m, 5H, –ArH), 3.7 (s, 3H, –COOCH3), 3.9 (s, 1 H,
–OH), 1.8 (s, 3H, –CH3).
Methyl-2-hydroxy-2-p-tolylpropanoate: GC–MS (m/z, %): 194
(M+, 45), 135 (21), 118 (100), 91 (39), 77 (7), 65 (8), 43 (8). H NMR:
ı 7.1–7.5 (4H, –ArH), 3.8 (s, 3H, –COOCH3), 3.7 (s, 1H, –OH), 2.3 (s,
3H, –ArCH3), 1.8 (s, 3H, –CH3).
ꢀ
1/2, indicating that the electrode process is controlled by diffusion
rather than adsorption [11,25–27].
2-Hydroxy-2-(4-methoxyphenyl)propionate acid methyl ester:
The effect of acetophenone concentration on the CV behavior of
model molecule in BMIMBF4 was also studied on GC electrode at a
GC–MS (m/z, %): 210 (M+, 12), 152 (24), 133(100), 103 (15), 91 (18),

Q. Feng et al. / Electrochimica Acta 56 (2011) 5137–5141
5139
It is well known that the viscosity of BMIMBF4 displays an essen-
tial Arrhenius behavior [15]. Increasing temperature leads to an
increase of conductivity and a decrease of viscosity, which is help-
ful to this reaction. But increasing temperature is unfavorable to the
electrocarboxylation of acetophenone with CO2, since it also leads
to a decrease of solubility of CO2 in BMIMBF4 [15,17]. Moreover,
the yield of the corresponding alcohol increases with increasing
the temperature, suggesting that the high temperature is benefi-
cial for the formation of the alcohol [10,12]. Due to the above two
factors, the optimal temperature was chosen as 50 ◦C.
3.2.2. Influence of current density and charge passed
The influence of current density on the electrocarboyxlation of
acetophenone has been studied. As shown in Table 1 (entries 2,
4–6), increasing the current density from 1 to 2.25 mA cm−2 results
in an enhancement of the yield from 2% to 36%. However, the
yield of the target product declined when the current density was
Fig. 2. Cyclic voltammograms of acetophenone (0.02 mol dm−3) in BMIMBF4 on GC
at various scan rates: (a) ꢀ = 5 mV s−1, (b) 10 mV −1, (c) 20 mV −1, (d) 40 mV −1, (e)
increased to above 2.25 mA cm−2
.
60 mV −1, and (f) 80 mV −1
.
The larger the current density is, the more negative the elec-
trode potential will be. When the current density was raised from
1 to 2.25 mA cm−2, the potential difference between the anode
and the cathode arose, and more electrogenerated intermediates
were produced, which led to an increase of the yield. However,
CO2 reduction competed with the acetophenone reduction when
the current density was increased to above 2.25 mA cm−2. Under
this condition, some of the supplied energy was consumed by CO2
reduction instead of being used for the reduction of acetophenone,
resulting in a lower yield [10,11]. An optimized current density of
2.25 mA cm−2 was chosen finally.
Several electrolysis experiments were carried out using dif-
ferent amounts of charge passed. The results are presented in
Table 1(entries 2, 7–9). As shown, the yield is dramatically affected
by the charge passed. A shift of the yield of target product from
24% to 57% was achieved by changing the charge passed from 2 to
3.5 F mol−1. In general, increasing the charge passed increases the
yield of the target ester, but decreases the current efficiency. Taking
the current efficiency and the yield into account, the appropri-
ate charge passed for this electrolysis experiment was 3.5 F mol−1
[10,11].
scan rate of 10 mV s−1. Fig. 3 indicates, a monotonic increase of the
peak current is observed when the substrate concentration is grad-
ually increased from 0.02 to 0.12 mol dm−3. This result agrees with
the expectation that the peak current is proportional to substrate
concentration for a process under diffusion control [8,22,24].
3.2. Influence of reaction conditions
In order to optimize experimental conditions, the influ-
ence of temperature, current density, charge passed, electrode
material and substrate concentration on the yield of 2-hydroxy-
2-phenylpropionic acid methyl ester (HPPAME) was investigated
under galvanostatic condition. In addition, potentiostatic elec-
trolysis was also performed under optimized conditions. All
experimental results are summarized in Table 1. The yields were
calculated on the basis of the starting material.
3.2.1. Effect of temperature
The viscosity of ionic liquids is very high, affecting on the rate of
mass transport within solution, so the temperature plays an impor-
tant role in this electrochemical reduction reaction [15,17,28].
Electrolysis experiments were carried out under constant current
at 25, 50 and 75 ◦C. The results of Table 1 (entries 1–3) show that
an increase of the temperature from 25 ◦C to 50 ◦C brings about
a remarkable increase of the yields of HPPAME from 15% to 36%.
However, the yield is hardly changed from 50 ◦C to 75 ◦C.
3.2.3. Effect of electrode material
During electrolysis, reduction of acetophenone takes place on
the surface of the electrode. Thus, the choice of electrode materi-
als is of quite importance. Contrast tests were done by using Pt,
Cu, Ni and stainless steel (SS) as cathode under otherwise identical
experimental conditions. Table 1 (entries 2, 10–12) show the results
obtained with different cathodes. Under the same conditions, Pt,
SS and Cu cathodes gave HPPAME in moderate yields (entries 2,
11 and 12), whereas the Ni cathode afforded the product in a low
yield (14%, entry 10). Different cathodes resulted in different yields
of the electrocarboxylation, which might be attributed to the differ-
ent catalytic activities of the cathodes for acetophenone reduction
[10,11]. Although Cu and SS cathodes give the moderate yields of
the target ester, they could be considered for the electrochemical
carboxylation of acetophenone with CO2 because they are much
cheaper than Pt cathode.
3.2.4. Influence of substrate concentration
The effect of substrate concentration on the yield of the elec-
trocarboxylation was also investigated. The experimental results
are listed in Table 1 (entries 2, 13–15). The substrate concentra-
tion appears to be a crucial factor for the electrocarboxylation of
acetophenone with CO2. When the substrate concentration was
increased, the yield of the target product was decreased, while the
yield of the corresponding alcohol was increased [8,10–12,22].
Fig. 3. Cyclic voltammograms of acetophenone in BMIMBF4 on GC at various
concentrations: (a) c = 0.02 mol dm−3, (b) 0.04 mol dm−3, (c) 0.06 mol dm−3, (d)
0.08 mol dm−3, (e) 0.1 mol dm−3, and (f) 0.12 mol dm−3
.

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Q. Feng et al. / Electrochimica Acta 56 (2011) 5137–5141
Table 1
Electrocatalytic carboxylation of acetophenone with CO2 under various conditions.
Entry
T (◦C)
Cathode
j (mA cm−2
)
c (mol dm−3
)
Q (F mol−1
)
Yield (%)
Conv.(%)
1
2
3
1
2
3
4
5
6
7
8
25
50
75
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
Pt
Pt
Pt
Pt
Pt
Pt
Pt
Pt
Pt
Ni
SS
Cu
Pt
Pt
Pt
Pt
Pt
Pt
2.25
2.25
2.25
1
1.5
2.5
2.25
2.25
2.25
2.25
2.25
2.25
2.25
2.25
2.25
2.25
–
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.02
0.05
0.6
0.1
0.1
0.1
2.5
2.5
2.5
2.5
2.5
2.5
2
15
36
39
2
28
35
44
36
38
43
12
11
9
47
40
41
20
25
42
52
18
22
5
2
3
5
6
2
4
3
6
3
2
4
5
2
4
7
6
8
56
78
89
47
65
82
46
67
75
69
78
84
90
88
71
63
85
89
17
32
24
48
57
14
32
35
62
59
30
0
3
9
3.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
10
11
12
13
14
15
16a
17b
18c
59
58
–
a
Reacted without CO2.
b
c
Potentiostatic electrolysis at −1.9 V.
Potentiostatic electrolysis at −2.1 V.
Table 2
Electrocarboxylation of aromatic ketones with CO2.
Entry
Substrate
Product
Yield (%)
62
Yield (%) (2)
Conv. (%)
90
1
20
2
3
55
56
31
28
89
87
In conclusion, according to the experimental results, the
optimized conditions for this electrosynthesis in BMIMBF4 are
0.02 mol dm−3 acetophenone, Pt as cathode with 2.25 mA cm−2
current density, a charge consumption of 3.5 F mol−1 and a tem-
perature of 50 ◦C.
seen in Table 2, the electrolysis of substituted aromatic ketones
was similar to that of acetophenone, affording the corresponding
␣-hydroxycarboxylic acid methyl esters in moderate yields. The
substrates bearing electron-donating substituent (CH3 and OCH3
groups) give rise to a decrease in the yields in the target product
and conversely to increase in the yields in corresponding alcohols
[9,12].
3.2.5. Electrocarboxylation of acetophenone with CO2 under
potential control
3.4. Recycle of the ionic liquid
The potentiostatic electrolysis experiments were performed
under optimized conditions using Pt foil as working electrode
(area = 2 cm2), Mg rod (d = 0.5 cm) as counter electrode and Ag wire
as reference electrode. As shown in Fig. 1d, the peak potential of
acetophenone reduction peak is −1.9 V. Therefore, either −1.9 V or
−2.1 V was chosen as the operating potential for the electrolysis.
The results are summarized in Table 1 (entries 17 and 18). As shown
in Table 1, no appreciable difference in the yield of the target prod-
uct HPPAME was found at the end of the synthesis by changing the
working potential [9,12].
After electrolysis, the solution was first extracted with Et2O to
remove the reduction products. 20 mL of the CH2Cl2 was added
to the residue solvent and then the mixture was washed with
1 mL double distilled water. The obtained aqueous solution was
tested with silver nitrate until no precipitate formed, indicating
that no halide ion was present. Then the organic layer was evapo-
rated under reduced pressure to remove CH2Cl2 and the resulting
BMIMBF4 was dried under vacuum. The recycled BMIMBF4 could
be used for three cycles.
3.3. Electrocarboxylation of other substituted aromatic ketones
with CO2
4. Conclusions
Under the optimized reaction conditions, this method was
applied to other substituted aromatic ketones. As can be clearly
The
important
pharmaceutical
intermediate
␣-
hydroxycarboxylic ester and its derivatives were synthesized

Q. Feng et al. / Electrochimica Acta 56 (2011) 5137–5141
5141
by electrocarboxylation in BMIMBF4 from available aromatic
ketones and CO2 using some common electrode materials. Cyclic
voltammetry showed that BMIMBF4 could act as a medium for the
electroreduction of aromatic ketones. The electrolysis experiments
were carried out in an undivided cell under mild conditions,
without resorting to the use of toxic solvents and catalysts. The
influence of temperature, current density, charge passed, electrode
material and substrate concentration on the yields was inves-
tigated, and the target product could be obtained in moderate
to good yields under optimized condition. Efforts are underway
to explore more electrochemical carboxylation systems for the
electrochemical fixation of CO2. This research may contribute to
start a new research field in the application of ionic liquids to
organic synthesis aimed at “green chemistry”.
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