Electrocatalytic carboxylation of aliphatic halides at silver cathode in acetonitrile

Article abstract of DOI:10.1016/j.tet.2008.08.093

A simple and efficient electrocarboxylation reaction of aliphatic halides has been developed using silver as cathode, magnesium as anode and CH₃CN saturated CO₂ as solvent in an undivided cell. The influence of some key factors (such as the nature of electrode materials, supporting electrolytes and temperature) on this reaction was investigated. Under the optimized condition, the corresponding carboxylic acids were obtained in moderate to good yields (22-89%). The electrochemical behaviour was studied at different electrodes (Ag, Cu, Ni and Ti) by cyclic voltammetry, which showed significant electrocatalytic effect of the silver electrode towards the reductive carboxylation of aliphatic halides.

Full text of DOI:10.1016/j.tet.2008.08.093

Tetrahedron 64 (2008) 10517–10520
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Electrocatalytic carboxylation of aliphatic halides at silver cathode
in acetonitrile
Dong-Fang Niu, Li-Ping Xiao, Ai-Jian Zhang, Gui-Rong Zhang, Qi-Yun Tan, Jia-Xing Lu *
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 May 2008
Received in revised form 23 August 2008
Accepted 29 August 2008
Available online 4 September 2008
A simple and efficient electrocarboxylation reaction of aliphatic halides has been developed using silver
as cathode, magnesium as anode and CH CN saturated CO as solvent in an undivided cell. The influence
3 2
of some key factors (such as the nature of electrode materials, supporting electrolytes and temperature)
on this reaction was investigated. Under the optimized condition, the corresponding carboxylic acids
were obtained in moderate to good yields (22–89%). The electrochemical behaviour was studied at
different electrodes (Ag, Cu, Ni and Ti) by cyclic voltammetry, which showed significant electrocatalytic
effect of the silver electrode towards the reductive carboxylation of aliphatic halides.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Electrocatalytic
CO
2
Aliphatic halides
Silver
Cathode
1. Introduction
catalytic route involving reduction of aliphatic halides in solution
far from the electrode has been proposed to avoid the use of neg-
6
The global warming due to the increased atmospheric CO
2
ative potentials required by the direct electroreduction process. In
spite of the good yields achieved with several metal complexes, the
use of expensive catalytic materials makes the reaction system and
the separation impractical.
concentration resulting from increasing consumption of fossil fuel
1
is becoming an important environmental issue today. On the other
2
hand, CO is recognized to be a naturally abundant, cheap, re-
cyclable and non-toxic carbon source that can sometimes replace
toxic chemicals such as phosgene, isocyanates or carbon monox-
ide. Under these circumstances, conversion of carbon dioxide to
Very recently, a series of studies dealing with the reduction of
organic halides at Ag electrode have been reported. The most
7
2
important result emerging from such studies is that silver exhibits
extraordinary electrocatalytic activities towards the reduction of
organic halides. The possibility of exploiting the electrocatalytic na-
ture of silver for the electrocarboxylation of aliphatic halides has not
been established yet. In this work, we investigated in a detailed way
the influence of operative parameters on the electrocarboxylation of
aliphatic halides in CO –saturated CH CN solution at Ag cathode,
2 3
aiming to optimize the process and to define an applicable meth-
odology. The voltammetryand kinetics of aliphatic halides have been
industrially useful compounds has been a challenge for synthetic
chemists and has recently attracted much interest in view of the so-
3
4
called ‘sustainable society’ and ‘ green chemistry’ concepts.
Aliphatic halides represent one of the most challenging cate-
gories of organic pollutants (applied as pesticides), not only for
their highly toxic, even carcinogenic characters, but also for their
volatility. It is of great practical importance that aliphatic halides
2
combine with CO to prepare carboxylated products that are fine
chemicals of industrial interest. The electrocarboxylation of ali-
phatic halides has been studied in the past decades, however, the
existing methods are not very efficient. A drawback to use aliphatic
halides as starting material is that their reduction at the most
commonly used cathodes occurs at very negative potentials, where
2 3
examined in the absence and presence of CO in CH CN.
2. Results and discussion
2.1. Cyclic voltammetric behaviour of aliphatic halides
concomitant reduction of CO
products and a decrease of current efficiency. Homogeneous
2
may take place, resulting in undesired
5
Cyclic voltammograms recorded for reduction of 3-chloro-2-
methylpropene (1a) at Ag electrode in CH
tetraethylammonium chloride (Et NCl) are depicted in Figure 1. As
shown in curve A of Figure 1, reduction of 1a gives rise to a single
irreversible peak (E
) under an nitrogen atmosphere at ꢀ1.56 V that
3
CN containing 0.1 M
4
*
Corresponding author. Tel.: þ86 21 62233491; fax: þ86 21 62232414.
E-mail address: jxlu@chem.ecnu.edu.cn (J.-X. Lu).
p
0
040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.08.093

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D.-F. Niu et al. / Tetrahedron 64 (2008) 10517–10520
0
.7
2.5
2.0
1.5
0
.8
.6
.4
.2
.0
0.6
0.5
0.4
0.3
0.2
0.1
CO2
0
0
0
0
0.0
-
0.5 -1.0 -1.5 -2.0 -2.5
-1
E/V vs Ag/AgI/I
1
0
0
.0
.5
.0
B
A
Ni
Ti
Cu
-
0.4
-0.6
-0.8
-1.0 -1.2
E/V vs Ag/AgI/I
-1.4
1
-1.6
-1.8
-2.0
-0.5
-1.0
-1.5
E/V vs Ag/AgI/I
-2.0
-2.5
-3.0
-
-1
Figure 1. Cyclic voltammograms of 20 mM 3-chloro-2-methylpropene (1a) recorded
3
Figure 2. Cyclic voltammograms of 20 mM 3-chloro-2-methylpropene (1a) in CH CN–
0.1 M Et
4
NCl recorded at Cu, Ni and Ti electrodes, v¼100 mV sꢀ , T¼25 ꢂC.
1
at Ag electrode in CH
3
CN–0.1 M Et
4
NCl in the (A) absence and (B) presence of CO
2
,
v¼100 mV s , T¼25 ꢂC. Inset: cyclic voltammogram of CO
ꢀ1
2
3 4
in CH CN–0.1 M Et NCl at
Ag electrode.
2
Cu was also a good material for the reduction of CO . These two
points mentioned above are unfavourable to electrocarboxylation of
corresponds to two-electron reductive cleavage of the carbon–
1a. A broad reduction peak (Eꢁꢀ2 V) appeared at Ni or Ti electrode,
halogen bond. The voltammetry behaviour of 1a was very similar in
p
which is considerably more negative than the E value obtained at
8
appearance to those pictured in an earlier report pertaining to the
2
Ag, and very close to the reduction peak of CO .
cathodic behaviour of alkyl halides at glass carbon electrodes.
Voltammograms similar to those shown in Figure 1 were
obtained for other aliphatic halides. The data are shown in Table 1.
A study of peak current (i
concentration range 2–10 mM revealed a linear relationship be-
tween i and concentration, showing that 1a reduction proceeds
rather cleanly at Ag electrode (that is, without serious filming ef-
p
) for the main reduction process over the
p
2.2. Experimental investigations in preparative scale
electrolysis
ꢀ1
fects). Variation of scan rate (v) from 20 to 100 mV s produced
/2
a linear i
iour. The inset chart is the reduction of CO
2.13 V is a irreversible peak that can be attributed to the one-
electron of CO . It is noteworthy that E of 1a is about 0.6 V more
positive than that of CO and becomes even more positive when the
solution is saturated with CO (Fig. 1B). This provides a potential
window for electrocarboxylation of 1a to be performed without any
interference from reduction of CO . The E of 1a is positively shifted
from ꢀ1.56 to ꢀ1.24 V in the presence of CO shows a rapid
chemical reaction between CO and electrogenerated radical anion.
p
versus v1 plot, indicating diffusion-controlled behav-
3-Chloro-2-methylpropene (1a) was first chosen as a model
molecule to be investigated, and the obtained optimal reaction
conditions were then applied to other aliphatic halides. The elec-
2
at Ag electrode. At
ꢀ
2
p
trochemical carboxylation of 1a and CO
shown in Scheme 1. In our experiments, controlled-potential
electrolyses were carried out in CO –saturated CH CN containing
2
followed the principle as
2
2
2
3
0.2 M 1a in an undivided cell with Mg sacrificial anode. To optimize
the yields, we focused our attention on the influence of various
synthetic parameters on the process, such as electrode materials,
supporting electrolytes and the temperature. The results of the
electrolysis are summarized in Table 2.
Among all electrochemical reaction conditions, the choice of
electrode materials has great influence on the yields or reactivity in
2
CO fixation. It is important to find a proper electrode that has good
electrocatalytic effect to the reduction of aliphatic halides. The most
relevant outcome of the above voltammetric investigation is that
Ag shows a significant electrocatalytic effect towards the reduction
of 1a, so we first conducted the electrocarboxylation of 1a at Ag
electrode (Table 2, entry 1). Some experiments were performed at
Cu, Ni and Ti electrodes as well for comparison (Table 2, entries 2–
4). The electrocarboxylation yield decreased depending on the
2
p
2
2
In order to examine the electrocatalytic effect of Ag electrode
towards the reduction of 1a, the voltammetry behaviours of 1a were
also investigated at the most commonly used cathodes, e.g., Cu, Ni
and Ti (Fig. 2). The reduction peak of 1a at Cu electrode appeared at
ca. ꢀ1.5 V, which is similar to that required for its reduction at Ag
electrode (ꢀ1.56 V), but compared with Ag electrode, an obvious
thin film was formed at Cu electrode after three voltammetry cycles,
which is attributed to the absorption of electrogenerated in-
termediate species at Cu electrode, and will result in progressive
degradation of Cu cathode during the electrocarboxylation of ali-
9
phatic halides. In addition, in our previouslystudy, wereportedthat
Table 1
ꢀ
1
NCla
4
Voltammetric data for the reduction of aliphatic halides 1a–i at v¼100 mV s in CH
3
CNþ0.1 M Et
Substrate
CH3
CH
2
]CHCH
2
Cl
CH
2
]CHCH
2
Br
n-C
3
H
7
Br
n-C
4
H
9
Br
CH3
CH CHCH Br
BrCH
2
COOC
2
H
5
n-C
6
H13Br
6 2
C H11CH Br
CH =CCH Cl
2
2
3
2
1a
1b
1c
1d
1e
1f
1g
1h
1i
b
E
E
D
p
/V
/V
ꢀ1.56
ꢀ1.24
0.32
ꢀ1.61
ꢀ1.18
0.43
ꢀ1.58
ꢀ0.97
0.61
ꢀ1.78
ꢀ1.55
0.23
ꢀ1.87
ꢀ1.65
0.22
ꢀ1.93
ꢀ1.86
0.07
ꢀ1.29
ꢀ0.91
0.38
ꢀ1.89
ꢀ1.75
0.14
ꢀ1.71
ꢀ1.59
0.12
c
p
E
a
b
c
Ag working electrode, T¼25 ꢂC, C¼20 mM.
2
Measured in the absence of CO , applied potential.
2
Measured in the presence of CO .

D.-F. Niu et al. / Tetrahedron 64 (2008) 10517–10520
10519
+
Table 3
3 4
Electrochemical carboxylation of aliphatic halides (RX) in CH CN–Et
NCla
1
)
0.1 M TEACl-CH3CN
E= -1.6 V, rt
appb/V
RCO
H
Yield /%
c
Entry
RX
E
2
Ag
Mg
+
Cl CO2
CO H
2
+
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1g
1h
1i
ꢀ1.6
ꢀ1.7
ꢀ1.6
ꢀ1.8
ꢀ1.9
ꢀ2.0
ꢀ1.3
ꢀ1.9
ꢀ1.8
2a
2b
2b
2d
2e
2f
2g
2h
2i
89
57
68
48
40
22
38
27
24
2
)
H O
3
1
2
Scheme 1.
employed cathode materials in the following order: Ag>Cu>Ni>Ti,
which is in agreement with the reduction potential of 1a at these
electrodes: ꢀ1.56 V (Ag)zca. ꢀ1.5 V (Cu)>ca. ꢀ2 V (Ni)>ca. ꢀ2.5 V
a
General conditions: 1a–i¼0.2 M, Et
4
NCl¼0.1 M, CH CN¼20 mL, Ag cathode, Mg
3
(
Ti). Owing to the very negative potentials required for the re-
ꢂ
ꢀ1
anode, T¼0 C, electricity 2 F mol , PCO2¼1 atm.
b
duction of 1a at Ni and Ti electrodes, concomitant reduction of CO
probably takes place, which resulted in a high consumption of
2
Applied potential.
c
Isolated yield.
1
0
charge and give oxalate, CO and carbonate.
The influence of supporting electrolyte on the electrochemical
carboxylation of 1a was also investigated. The nature of anions and
cations especially inorganic halide anions of supporting electrolytes
plays important role on the synthesis. Silver has shown specific
affinity towards organic halides, especially in the case of bromides
and iodides. In fact, though negatively charged at Ag electrode,
inorganic halide anions were reported to remain specifically
absorbed at Ag electrode in acetonitrile solution in the sequence of
The temperature appeared to be a crucial factor for the elec-
trochemical carboxylation of 1a. The solubility of CO in CH CN was
increased with the decrease of temperature, indicating that low
temperature favours the reaction. Under an atmospheric pressure
2
3
2
of CO , the yield of carboxylic acid 2a increased from 69% (entry 1)
to 89% (entry 13) when the electrolysis temperature was changed
ꢂ
ꢂ
from 20 C to 0 C.
ꢀ
1
ꢀ1 ꢀ111
, which is consistent with many well known halide
Cl ꢃBr ꢃI
2.3. Electrocarboxylation of other aliphatic halides
properties, such as the solubility of the products with silver ions.
Adsorption of halide anions at Ag electrode is so strong as to hold
also at remarkably negative potentials: ꢁꢀ0.8 V (SCE) for bromides
To test the applicability of this electrochemical carboxylation,
the investigation was extended to other aliphatic halides 1b–i. The
results of these electrolyses under optimized conditions (Table 2,
entry 13) are reported in Table 3. In all cases, the corresponding
carboxylic acid was obtained. The best yield was obtained with 1a,
while the moderate yields were achieved with 1b–i. The electro-
carboxylation yields of aliphatic halides decreased with the nega-
tive shift of applied potentials, since the concomitant reduction of
12
and ꢁꢀ1.2 V (SCE) for iodides. Adsorption competition between
organic halides and inorganic halide anions hinders reduction
of organic halides taking place at Ag electrode, which is unfav-
ourable to electrochemical carboxylation reaction. As shown in
Table 2 (entries 1, 5–7), in the same tetraethylammonium cation
þ
(
TEA ) case, the yield of carboxylic acid increased in order of
ꢀ
1
ꢀ1 ꢀ1 ꢀ1
Cl >BF >Br >I under the same electrolytic conditions. The
4
2
CO may take place, which resulted in a high consumption of
nature of cations also had some influence on the yield of carboxylic
acid (Table 2, entries 5, 7, 10 and 11). The performance of tetraal-
kylammonium salts was better than that of imidazole salts. The
initial concentration of magnesium ions was too low to stabilize the
electrogenerated carboxylate anion since they were generated
gradually by electrolytic dissolution of the magnesium metal an-
ode. Hence, tetraalkylammonium cation played an important role
in stabilizing carboxylate anion at the initial electrocarboxylation
stage. When TEA salt was used as supporting electrolyte, carboxy-
charge and decrease of electrocarboxylation yield. When the elec-
trochemical carboxylation was carried out using 1g as a substrate,
the electron-withdrawing group may significantly weaken the
nucleophilicity of carbanion, the corresponding carboxylic acid 2g
was only obtained in 38% yield.
3. Conclusion
In conclusion, we have demonstrated a simple and efficient
þ
ꢀ1
2
},
late anion could be stabilized by forming an ion pair, {TEA –RCO
electrochemical route with silver cathode for electrocarboxylation
of aliphatic halides. We have shown that various conditions, such as
the nature of the electrode, the supporting electrolyte and the
temperature, can affect the yield of carboxylic acid. Under the op-
timized condition, aliphatic halides 1a–i were electrochemically
13
which was helpful to the electrosynthesis.
Table 2
Electrocarboxylation of 3-chloro-2-methylpropene under various synthetic
parametersa
2
reduced in the presence of CO to the corresponding carboxylic
appb/V
ꢂ
c
acids in moderate to good yields (22–89%).
Entry
E
Cathode
T/ C
Supporting electrolytes
Yield /%
Cyclic voltammograms and electrolysis data of 3-chloro-2-
methylpropene (1a) in CH CN at different electrodes showed sig-
3
1
2
3
4
5
6
7
8
9
ꢀ1.6
ꢀ1.5
ꢀ2
Ag
Cu
Ni
20
20
20
20
20
20
20
20
20
20
20
10
0
TEACl
TEACl
TEACl
TEACl
TEABr
TEAI
TEABF
TBABr
TBAI
69
52
45
38
60
50
62
54
47
38
34
77
89
nificant electrocatalytic effect of the silver electrode towards the
reductive carboxylation of aliphatic halides. Compared with the
ꢀ2.5
ꢀ1.6
ꢀ1.6
ꢀ1.6
ꢀ1.6
ꢀ1.6
ꢀ1.6
ꢀ1.6
ꢀ1.6
ꢀ1.6
Ti
Ag
Ag
Ag
Ag
Ag
Ag
Ag
Ag
Ag
5,6
electrochemical methods reported in the literatures, this elec-
trochemical route appears to be more simple and more efficient for
electrocarboxylation of aliphatic halides.
4
1
0
BMIMBF
BMIMBr
TEACl
4
4
4
. Experimental
1
1
1
2
.1. General
13
TEACl
a
General conditions: 3-chloro-2-methylpropene¼0.2 M, Mg as anode,
1H NMR and 13C NMR spectra were recorded on AVANCE 500
CN¼20 mL, PCO2¼1 atm, electricity 2 F molꢀ
1
CH
3
.
b
Applied potential.
(500 MHz) spectrometer in CDCl
3 4
with Me Si as an internal stan-
c
Isolated yield.
dard. Mass spectra were obtained on a 5973N spectrometer

10520
D.-F. Niu et al. / Tetrahedron 64 (2008) 10517–10520
connected with a HP 6890 gas chromatograph. Cyclic voltammo-
grams were measured with CHI660 electrochemical analyzer (CHI,
USA). Silver (Ag), copper (Cu), nickel (Ni) and titanium (Ti) elec-
trodes (d¼2 mm) were used as working electrodes, respectively.
The counter electrode and the reference electrode were a platinum
4.2.6. Ethyl malonic acid (2g)
þ
GC–MS (m/z, %) 132 (M , 1), 114 (5), 105 (60), 87 (100), 60 (24),
1
43 (35), 28 (27), 15 (3); H NMR
d
8.99 (s, 1H), 4.14 (q, J¼7.2 Hz, 2H),
13
3.54 (s, 2H), 1.30 (t, J¼7.2 Hz, 3H); C NMR (CDCl
3
) d 171.23, 166.90,
61.95, 40.83, 13.93.
wire and Ag–AgI–0.1 M n-Bu
Acetonitrile (CH CN) was kept over 4 Å molecular sieves. Tet-
raethylammonium chloride (Et NCl) and tetraethylammonium
tetrafluoroborate (Et NBF ) were prepared according to the litera-
ture. 1-Butyl-3-methyl imidazolium tetrafluoroborate (BMIMBF
4
NI in DMF, respectively.
3
4.2.7. Heptanoic acid (2h)
þ
4
GC–MS (m/z, %) 130 (M , 1), 113 (2), 101 (9), 87 (28), 73 (56), 60
1
4
4
3
(100), 43 (25), 28 (15); H NMR (CDCl ) d 10.52 (s, 1H), 2.32 (t,
14
4
)
J¼7.7 Hz, 2H), 1.62–1.56 (m, 2H), 1.33–1.23 (m, 6H), 0.86 (t, J¼7.7 Hz,
13
and 1-butyl-3-methyl imidazolium bromide were prepared accord-
ing to the literature.15 Other reagents were used as received.
3H); C NMR (CDCl
3
) d 180.54, 34.13, 31.46, 28.72, 24.74, 22.73,
14.01.
4
.2. General procedure for electrochemical carboxylation
4.2.8. Cyclohexaneacetic acid (2i)
þ
of aliphatic halides
GC–MS (m/z, %) 142 (M , 7), 124 (2), 99 (6), 83 (90), 60 (100), 53
1
(
10), 41 (32), 28 (29); H NMR (CDCl
3
)
d
11.16 (s, 1H), 2.20 (d,
Controlled-potential electrolysis was carried out at ꢀ1.6 V in
J¼6.9 Hz, 2H), 1.85–1.60 (m, 6H), 1.38–1.10 (m, 3H), 1.08–0.85 (m,
13
a mixture of 1a (0.2 M) and Et
4
NCl (0.1 M) in 20 mL dry MeCN
3
2H); C NMR (CDCl ) d 179.77, 41.93, 34.65, 32.95, 26.08, 25.98.
2
under a slow stream of CO in a one compartment electrochemical
cell equipped with a metallic ring cathode and a sacrificial Mg
ꢀ
1
anode until 2 F mol of charge was passed. After electrolysis, the
solvent was evaporated off under reduced pressure and the residual
Acknowledgements
ꢀ1
was acidified with 2 mol L
extracted with (25ꢄ4 mL) diethyl ether. The extracts were mixed
with a saturated aqueous solution of NaHCO . After separation of
ether and aqueous phases, the latter was acidified with 2 mol L
hydrochloric acid and the carboxylic acid was extracted with
25ꢄ4 mL) diethyl ether. Then, the organic layer was treated with
saturated aqueous NaCl, and dried with MgSO . After evaporation
of ether, an almost pure 3-methyl-3-butenoic acid was obtained.
aqueous hydrochloric acid and
This work was supported by the National Natural Science
Foundation of China (No. 20573037) and Shanghai Leading Aca-
demic Discipline Project (B409).
3
ꢀ1
(
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4
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.2.1. 3-Methyl-3-butenoic acid (2a)
þ
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1
(
64), 28 (100), 14 (2); H NMR (CDCl
3
)
d
9.36 (s, 1H), 4.95 (s, 1H),
1
3
4
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3
)
d
177.35,
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þ
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d
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)
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.2.3. Butanoic acid (2d)
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1
(
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11.65 (s, 1H), 2.32 (t, J¼7.2 Hz,
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180.44, 35.96, 18.12, 13.53.
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4
.2.4. Pentanoic acid (2e)
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GC–MS (m/z, %) 102 (M , 1), 87 (3), 73 (29), 60 (100), 55 (10), 41
1
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GC–MS (m/z, %) 102 (M , 1), 87 (20), 74 (2), 69 (5), 60 (100), 43
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(
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