Electrochemical carboxylation of cinnamate esters in MeCN

Article abstract of DOI:10.1016/j.tet.2007.10.104

Electrochemical carboxylation of cinnamate esters has been carried out by cathodic reduction of C{double bond, long}C bond in an undivided cell equipped with Mg sacrificial anode and using MeCN saturated with CO₂ as solvent. The yields and the

Full text of DOI:10.1016/j.tet.2007.10.104

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Tetrahedron 64 (2008) 314e318
www.elsevier.com/locate/tet
Electrochemical carboxylation of cinnamate esters in MeCN
*
Huan Wang, Kai Zhang, Ying-Zi Liu, Mei-Yu Lin, Jia-Xing Lu
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, China
Received 24 July 2007; received in revised form 28 October 2007; accepted 30 October 2007
Available online 1 November 2007
Abstract
Electrochemical carboxylation of cinnamate esters has been carried out by cathodic reduction of C]C bond in an undivided cell equipped
with Mg sacrificial anode and using MeCN saturated with CO2 as solvent. The yields and the ratio of mono- and dicarboxylic acids are strongly
affected by various factors: cathodic material, current, charge, and temperature. The highest yield (78.9%) was obtained starting from ethyl
cinnamate. Cyclic voltammograms have been measured and reaction pathways have been proposed.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Carboxylation; Electrochemical; Cinnamate esters; Carbon dioxide
1. Introduction
moderate or low yields by electroreduction at mercury cathode
in MeCN by Tyssee and Baizer.17 Saveant et al. studied the
cathodic reduction of some activated olefins (a,b-unsaturated
esters, ketones, nitriles), carried out under potentiostatic control
in CO2-saturated DMF solutions with moderate yields.18
2-Arylsuccinates have been prepared in good yields by electro-
chemcial reduction of styrene, yet it should be catalyzed by Nie
PMDTA,19 in the presence of N-carboalkoxyimidazoles,20 or
using expensive Pt as cathode.21
Herein, we wish to report electrochemical carboxylation of
cinnamate esters in MeCN. To optimize the yield, the effects
of various synthetic parameters on the electrocarboxylation
process, such as the electrode material, the current, the charge,
and the temperature, have been studied. The voltammetry and
kinetics have been examined without and with CO2 in MeCN.
The reaction pathways for the present electrochemical carboxy-
lation are discussed.
CO2 is the largest contributor to the green house effect,
which may increase the earth average temperature to such
a value that may cause catastrophic events. Therefore great
effort has been placed toward the reduction of CO2 at atmo-
spheric loading.1 On the other hand, CO2 can be proposed
as a C1 building block in organic synthesis. Its low cost and
its facile reaction with carbanions explain why many efforts
have been made in order to obtain speciality chemicals by
CO2 insertion to organic molecules.2e5
Carbanions can easily be generated by the electrochemical
reduction of organic halides,6e9 aromatic ketones,10e13 and
activated olefins.14e16 Hence, the electrocarboxylation of these
compounds is potentially an easy way to prepare carboxylated
products, some of them of industrial interest, particularly for
the production of anti-inflammatory drugs.
Electrocarboxylation of olefins has been studied in the past
decades; however, the results are not very efficient. Mono-
and dicarboxylation of simple activated olefins (CH2]CHX
with X¼CO2CH3, CN, and acetyl) have been prepared in
2. Results and discussion
2.1. Constant current electrocarboxylation of methyl
cinnamate
Since a constant current electrolysis is a much more conve-
nient procedure than a constant potential electrolysis, the
reaction conditions for the constant current electrolysis of
* Corresponding author. Tel.: þ86 21 62233491; fax: þ86 21 62232414.
E-mail address: jxlu@chem.ecnu.edu.cn (J.-X. Lu).
0040-4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.10.104

H. Wang et al. / Tetrahedron 64 (2008) 314e318
315
Table 1
Electrocarboxylation of methyl cinnamate under various synthetic parametersa
COOMe
COOMe
+ e-
COOMe
COOMe
COOMe
COOMe
Ph
Ph
+
+
Ph
+ CO2
Ph
COOMe
2a
COOMe
4a
3a
1a
Entry
Cathode
I/mA
Q/F molꢀ1
T/ꢁC
Yieldb/%
Ratio 2a:3a:4a
1
Stainless steel
100
100
100
100
60
2
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
35
10
0
64.4
41.6
53.6
75.2
60.2
66.8
68.6
64.9
65.8
67.1
44.3
62.2
67.8
66.1
59.2
66.0
76.5
77.2
77.4
21:13:66
48:13:39
50:11:39
31:19:50
40:15:45
30:15:55
29:18:53
37:17:46
55:17:28
37:16:47
80:12:8
2
Ni
Cu
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
2
3
2
4
2
5
2
6
80
2
7
110
120
140
160
100
100
100
100
100
100
100
100
100
2
8
2
9
2
10
11
12
13
14
15
16
17
18
19
a
2
1.2
1.6
2.4
2.8
3.2
2
56:12:32
49:17:34
62:15:23
63:15:22
64:15:21
47:12:41
24:13:63
17:10:73
2
2
2
ꢀ10
General conditions: methyl cinnamate¼0.1 M, Et4NBF4¼0.1 M, MeCN¼10 mL, PCO ¼1 atm.
2
b
GC yields.
methyl cinnamate (1a) were surveyed as summarized in Table 1.
electrode potential will be. So the proportion of the Faradaic
current will be declined, which will also decrease the carboxy-
lation yield. Therefore, the optimized current is 100 mA on the
electrodes of MgeTi.
The effect of the amount of electricity supplied to the elec-
trodes (Table 1, entries 4 and 11e15) is consistent with a two-
electron transfer process in which the current yield is not 100%.
Increasing the number of Faradays per mole of 1a, the yields in
carboxylated products increase linearly before 2 F molꢀ1, how-
ever, after which the yields decrease. So the best choice is
2 F molꢀ1 of 1a.
The yields of mono- and dicarboxylic acids were affected by
the choice of cathode material, current, charge, and temperature.
The nature of the cathodic material strongly determined the
yields or reactivity in CO2 fixation. Ti was the most efficient as
a cathode material, which resulted in good electrocarboxyla-
tion yield reaching the value of 75.2% (Table 1, entry 4). Other
cathode materials such as stainless steel, Ni, and Cu gave
moderate yields (Table 1, entries 1e3).
The yields are also strongly affected by the current (Table 1,
entries 4e10). The larger the current is, the more negative the
electrode potential will be. So there will be other reactions on
Although a strong decrease in the yields was observed at
higher temperature (Table 1, entry 16), only a slight increase
of the yields was observed at lower temperature (Table 1,
entries 17e19).
the electrode surface, such as the reduction of CO2 or Mg2þ
,
which will decrease the carboxylation yield. On the other
hand, the lower the current density is, the more positive the
Table 2
Electrocarboxylation of cinnamate estersa
COOMe
COOMe
+ e-
+ CO2
COOR
COOMe
COOR
Ph
COOR
COOR
Ph
+
+
Ph
Ph
COOMe
4
1
2
3
Entry
R
1
Yieldb/%
Ratio 2:3:4
1
2
3
4
a
Me
1a
1b
1c
1d
77.2c
78.9
61.9
73.1
24:13:63
33:11:56
35:11:54
30:5:65
Et
Pr
Bu
General conditions: cinnamate esters¼0.1 M, Et4NBF4¼0.1 M, MeCN¼10 mL, stainless steel cathode, Mg anode, T¼0 ꢁC, I¼100 mA, electric
charge¼2 F molꢀ1, PCO ¼1 atm.
2
b
GC yields.
This experiment (the same of entry 18, Table 1) was repeated here for clarity.
c

316
H. Wang et al. / Tetrahedron 64 (2008) 314e318
2.2. Electrochemical carboxylation of cinnamate esters
radical anion generated by the electroreduction of the activated
alkene is involved in several competitive reactions, which lead
to the formation of hydrodimers and saturated esters.22
When the cyclic voltammograms were recorded in MeCN
saturated with CO2, different behaviors were observed (Fig. 2).
The first reduction peak potential moved more positively and
current increased. The peak current ratios ip/i0p measured in the
presence of CO2 are included in the data presented in Table 3.
The current enhancements show the reactions between CO2
and electrogenerated radical anions.
To test the applicability of this electrochemical carboxy-
lation, the investigation was extended to other cinnamate esters.
The results of these electrolyses under the conditions de-
scribed above (Table 1, entry 18) are reported in Table 2. In
all cases, two monocarboxylic acids and one dicarboxylic
acid were obtained. The good yields were obtained with ethyl
ester and methyl ester (Table 2, entries 1 and 2). Moderate
yields were achieved with propyl ester and butyl ester (Table
2, entries 3 and 4). These indicate that this electrochemical
synthesis works well for cinnamate esters.
The similar behaviors have been observed to other cinna-
mate esters. The relative data were shown in Table 3.
2.3. Cyclic voltammetry of cinnamate esters
1.0
The cyclic voltammograms of 1aed (MeCNe0.1 M
Et4NBF4, GC cathode, v¼0.1 V sꢀ1) exhibit a one-electron re-
duction peak in the potential range ꢀ1.40 to ꢀ1.50 V vs Ag/
AgI/Iꢀ and a second chemically irreversible reduction at low
potentials. An example of the voltammetric behavior of the
compounds is shown in Figure 1 (curve b). When the scan
rate was increased, the first peak became partially reversible,
while the second remaining irreversible, showing for a chemical
reaction followed the first one-electron transfer reaction. The
b
0.8
0.6
0.4
0.2
a
0.0
0.25
d
-1.0
-1.2
-1.4
-1.6
-1.8
-2.0
0.20
E / V (vs Ag/AgI/I-)
Figure 2. Cyclic voltammograms of methyl cinnamate in MeCNe0.1 M
Et4NBF4 in the (a) absence and (b) presence of saturated CO2.
0.15
c
0.10
Table 3
Peak potentials and peak currents of cinnamate esters in MeCNe0.1 M
b
0.05
a
a
Et4NBF4
E0pb/V
i0pb/mA
Epc/V
ipc/mA
ip/i0p
0.00
R
1
Me
Et
Pr
Bu
a
1a
1b
1c
1d
ꢀ1.44
ꢀ1.43
ꢀ1.42
ꢀ1.42
0.0640
0.0775
0.0716
0.0736
ꢀ1.40
ꢀ1.40
ꢀ1.39
ꢀ1.39
0.1226
0.1434
0.1259
0.1446
1.92
1.85
1.76
1.96
-1.0
-1.2
-1.4
-1.6
-
-1.8
-2.0
E/ V (vs Ag/AgI/I )
GC working electrode, T¼25 ꢁC, C¼2 mM, v¼0.1 V sꢀ1
Measured in the absence of CO2.
Measured in the presence of CO2.
.
Figure 1. Cyclic voltammograms of methyl cinnamate in MeCNe0.1 M
Et4NBF4 at different scan rates: (a) 0.02 V sꢀ1, (b) 0.1 V sꢀ1, (c) 0.5 V sꢀ1
(d) 2 V sꢀ1, reference: Ag/AgI/Iꢀ; T¼25 ꢁC.
b
,
c
O
O
OOC
Ph
O
O
O
+CO2
OR
+e-
+e-
OR
OR
Ph
OR
Ph
Ph
G
+H+
E
C
O
O
+e-
Ph
OR
+CO2
Ph
Ph
OR
+H+
A
B
O
COO
H
OR
Ph
OR
O
OOC
Ph
COO
COO
+CO2
OR
D
F
COO
I
Scheme 1.

H. Wang et al. / Tetrahedron 64 (2008) 314e318
317
2.4. Reaction pathway
by the reaction of alcohols with cinnamate acid. Other
reagents were used as received.
According to the electrosynthesis results and cyclic voltam-
mograms, proposed reaction pathways for the present electro-
carboxylation are shown in Scheme 1. One-electron reduction
of cinnamate esters A would give the radical anion B. As the
solvent is dried and saturated with CO2, most of B undergoes
a nucleophilic attack on CO2 to give the radical anion D, while
only a little of B is protonated to radical C. Further one-electron
reduction of D affords the carbanion F, which also would react
with CO2 or Hþ. Carbanion F is spatially rigid and sterically
hindered, so that it is easier to react with Hþ to afford monocarb-
oxylic acid H and then CO2 to afford dicarboxylic acid I. On the
other hand, the radical C involves further reduction and
nucleophilic reaction to afford another monocarboxylic acid G.
4.2. Electrocarboxylation of cinnamate esters (typical
procedure)
The galvanostatic electrolysis was carried out in a mixture of
methyl cinnamate (0.1 M) and Et4NBF4 (0.1 M) in 10 mL dry
MeCN under a slow stream of CO2 in a one-compartment elec-
trochemical cell equipped with a magnesium rod sacrificial an-
ode and a metallic ring cathode until 2 F molꢀ1 of charge was
passed. The reaction mixture was distilled under reduced pres-
sure. The residue was esterified in DMF by adding anhydrous
K2CO3 (1 mmol) and MeI (3 mmol) and the mixture was stirred
at 50 ꢁC for 5 h. The solution was hydrolyzed and extracted
with Et2O, and the organic layers was washed with H2O, dried
over MgSO4, and evaporated. The methyl esters corresponding
to acids were isolated by column chromatography with petro-
leum ether/ethyl acetate mixtures as eluent. After isolation
and identification of the products, working curves were used
with biphenyl as internal standard for analysis of the electro-
chemical carboxylation.
3. Conclusion
In conclusion, CO2 fixation has been carried out, under
mild conditions, in good yields by electrocarboxylation of cin-
namate esters. It is shown that various conditions, such as the
nature of the electrode, the current, the passed charge, and the
temperature, could affect the yields of mono- and dicarboxylic
acids. Under the optimized condition, electrocarboxylation of
methyl cinnamate achieved 77.4% of global carboxylic acids.
Several cinnamate esters were electrochemically reduced in
the presence of CO2 to the corresponding carboxylic acids in
satisfactory yields (61.9e78.9%).
The cyclic voltammograms of cinnamate esters in MeCN
showed two successive reduction waves. Comparison of the
electrochemical behaviors in the absence and presence of
saturated of CO2 can be used as one criterion of the reaction
pathway. The radical anion generated by the electroreduction
is involved in several competitive reactions, which lead to
formation of the monocarboxylic acids and dicarboxylic acid.
4.2.1. Dimethyl benzylmalonate 2a
GCeMS (m/z, %) 222 (Mþ, 36), 131 (100), 162 (83), 91
(55), 159 (28), 103 (26), 161 (26), 163 (20), 77 (14), 133
1
(10), 93 (9); H NMR (CDCl3) d 7.30e7.17 (5H, m), 3.70
(6H, s), 3.68 (1H, d, J¼8 Hz), 3.23 (2H, d, J¼8 Hz); HRMS
m/z (Mþ) calcd for C12H14O4 222.0892, found 222.0890.
4.2.2. Ethyl methyl benzylmalonate 2b
GCeMS (m/z, %) 236 (Mþ, 32), 131 (100), 91 (59), 162
(51), 159 (34), 176 (29), 103 (24), 161 (18), 77 (14), 163
1
(13), 164 (13); H NMR (CDCl3) d 7.32e7.19 (5H, m), 4.15
(2H, q, J¼7 Hz), 3.70 (3H, s), 3.66 (1H, d, J¼5 Hz), 3.22
(2H, d, J¼5 Hz), 1.19 (3H, t, J¼7 Hz); HRMS m/z (Mþ) calcd
for C13H16O4 236.1049, found 236.1052.
4. Experimental section
4.1. General
4.2.3. Methyl propyl benzylmalonate 2c
GCeMS (m/z, %) 250 (Mþ, 34), 131 (100), 162 (83), 91
(73), 148 (54), 159 (39), 103 (26), 147 (24), 32 (23), 163
(22), 161 (21); 1H NMR (CDCl3) d 7.32e7.12 (5H, m),
4.10e4.02 (2H, m), 3.69 (3H, s), 3.67 (1H, d, J¼4 Hz), 3.24
(2H, d, J¼4 Hz), 1.64e1.57 (2H, m), 0.86 (3H, t, J¼8 Hz);
HRMS m/z (Mþ) calcd for C14H18O4 250.1205, found
250.1213.
1H NMR spectra were recorded on AVANCE 500 (500 MHz)
spectrometer in CDCl3 with Me4Si as an internal standard.
Mass spectra were obtained on a 5973N spectrometer con-
nected with a HP 6890 gas chromatograph. HRMS was mea-
sured on a CONCEPT 1H. Cyclic voltammograms were
measured with CHI650 electrochemical analyzer (CHI, USA).
Glassy carbon (GC) electrode (d¼2 mm) was used as a working
electrode. The counter electrode and the reference electrode
were a platinum wire and Ag/AgI/0.1 M n-Bu4NI in DMF,
respectively. Galvanostatic electrolysis was carried out using
a dc regulated power supply QJ 12001X (1 A, 120 V) in an
undivided cell equipped by two-electrode.
4.2.4. Butyl methyl benzylmalonate 2d
GCeMS (m/z, %) 264 (Mþ, 37), 162 (100), 131 (76), 91
(56), 148 (55), 159 (26), 147 (25), 163 (21), 161 (20), 103
(19), 204 (14); 1H NMR (CDCl3) d 7.32e7.19 (5H, m),
4.11e4.05 (2H, m), 3.69 (3H, s), 3.67 (1H, d, J¼7 Hz), 3.22
(2H, d, J¼8 Hz), 1.57e1.53 (2H, m), 1.31e1.26 (2H, m),
0.88 (3H, t, J¼7 Hz); HRMS m/z (Mþ) calcd for C15H20O4
264.1362, found 264.1370.
Acetonitrile (MeCN) and dimethylformamide (DMF) was
˚
kept over 4 A molecular sieves. Tetraethylammonium tetra-
fluoroborate (Et4NBF4) was prepared according to the litera-
ture.23 Propels cinnamate and butyl cinnamate was prepared

318
H. Wang et al. / Tetrahedron 64 (2008) 314e318
4.2.5. 2-Methoxycarbonyl-3-phenyl-succinic acid dimethyl
ester 4a
References and notes
GCeMS (m/z, %) 280 (Mþ, 1), 121 (100), 248 (92), 189
(82), 131 (46), 161 (40), 103 (37), 216 (32), 77 (28), 59
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(21), 177 (19); H NMR (CDCl3) d 7.33e7.26 (5H, m), 4.32
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(77), 121 (61), 103 (51), 203 (38), 161 (34), 77 (31), 175 (25),
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(30); H NMR (CDCl3) d 7.30e7.25 (5H, m), 4.24 (1H, d,
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1
(38), 210 (38); H NMR (CDCl3) d 7.27 (5H, d, J¼6 Hz),
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(2H, m), 3.75e3.41 (6H, m), 1.61e1.35 (2H, m), 1.28e1.08
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This work was supported by the National Natural Science
Foundation of China (no. 20573037), the Natural Science
Foundation of Shanghai (no. 05JC1470) and Ph.D. Program
Scholarship Fund of ECNU 2007.
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