Efficient electrochemical dicarboxylations of arylacetylenes with carbon dioxide using nickel as the cathode

Article abstract of DOI:10.1016/j.tet.2008.04.053

The electrochemical dicarboxylation of arylacetylenes with carbon dioxide could be smoothly achieved in an undivided cell using Ni as the cathode and Al as the anode with n-Bu₄NBr-DMF as the supporting electrolyte, at a constant current under CO₂ pressure of 3 MPa and room temperature in the absence of additional catalysts. The corresponding aryl-maleic anhydrides and 2-arylsuccinic acids were afforded in excellent total yields (82-94%). Under anhydrous conditions, an unsaturated aryl-maleic anhydride as the main product was obtained, while the presence of H₂O would lead to the formation of saturated 2-arylsuccinic acids. The results of cyclic voltammetric experiments show that a nickel cathode itself plays the catalytic role in the reduction reaction of arylacetylenes with CO₂.

Full text of DOI:10.1016/j.tet.2008.04.053

Tetrahedron 64 (2008) 5866–5872
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Tetrahedron
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Efficient electrochemical dicarboxylations of arylacetylenes with carbon
dioxide using nickel as the cathode
*
Gao-Qing Yuan, Huan-Feng Jiang , Chang Lin
College of Chemistry, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The electrochemical dicarboxylation of arylacetylenes with carbon dioxide could be smoothly achieved
in an undivided cell using Ni as the cathode and Al as the anode with n-Bu4NBr–DMF as the supporting
electrolyte, at a constant current under CO2 pressure of 3 MPa and room temperature in the absence of
additional catalysts. The corresponding aryl-maleic anhydrides and 2-arylsuccinic acids were afforded in
excellent total yields (82–94%). Under anhydrous conditions, an unsaturated aryl-maleic anhydride as the
main product was obtained, while the presence of H2O would lead to the formation of saturated
2-arylsuccinic acids. The results of cyclic voltammetric experiments show that a nickel cathode itself
plays the catalytic role in the reduction reaction of arylacetylenes with CO2.
Received 20 January 2008
Received in revised form 9 April 2008
Accepted 15 April 2008
Available online 18 April 2008
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
has shown that zero-valence nickel complexes could effectively
activate CO2 and the unsaturated hydrocarbons toward C–C bond
In the recent decades, the continuously increasing carbon di-
oxide content in the atmosphere has aroused an extensive atten-
tion in the whole world because it might lead to global climate
warming. Hence, the utilization of CO2 is one of the important
problems to be solved for the mankind. On the other hand, carbon
dioxide is an abundant, non-toxic, and recyclable carbon source. It
should be significant to convert carbon dioxide into valuable or-
ganic chemicals. Although CO2 is very stable under normal condi-
tions, its activation could be easily achieved via an electrochemical
reaction at mild conditions. An electrochemical method has be-
come one of the efficient routes for the conversion of CO2. As
reportedin the literatures,1–5 the directelectrochemical reduction of
carbon dioxide could give some useful small molecules, such as CO,
CH4, C2H4, CH3OH, HCOOH, oxalic acid, etc. In addition, the elec-
trochemical reaction of CO2 with various organic compounds, in-
cluding benzyl chlorides,6–10 allylic halides,11 vinyl bromides,12 vinyl
triflates,13 carbonyl compounds,14 benzoyl bromides,15 ketones,16–19
amines,20 heterocyclic compounds,21,22 and epoxides,23–25 could
also afford valuable carboxylic acids, carbamates, or cyclic
carbonates.
formation.28,33 Based on the idea of Ni(0) complex catalysis, we
recently reported the electrochemical route with nickel as the
cathode as well as the catalyst for the electrocarboxylation of CO2
with styrene derivatives in an undivided cell at room temperature,
which could afford the corresponding 2-arylsuccinic acids in
moderate to high yields.35 Without additional catalysts, an elec-
trochemical reaction system becomes much simple and more effi-
cient. In this paper, our purpose is to extend this method for the
electrocarboxylation of arylacetylenes with CO2. To our knowledge,
no systematic investigation on the electrochemical dicarboxylation
of phenylacetylene derivatives with CO2, using Ni as the cathode in
the absence of additional catalysts, has been reported.
2. Results and discussion
In order to optimize the electrochemical reaction conditions, we
first chose phenylacetylene as a model compound to be in-
vestigated, and examined the effect of various parameters (such as
electrode materials, conducting salts and solvents, water, CO2
pressure and electricity, temperature, etc.) on the result of elec-
trosynthesis. Then, under the optimized conditions, we further
studied the electrocarboxylation of phenylacetylene derivatives
with CO2.
In our experiments, all the electrosyntheses were carried out in
the high-pressure stainless-steel undivided cell with a constant
current at room temperature. To ensure that the electrosyntheses
were conducted under anhydrous conditions, the chemicals used in
the electrosyntheses were dried further. DMF was distilled under
The electrochemical fixation of CO2 into unsaturated hydrocar-
bons, such as alkenes26–32 and alkynes,33,34 is also an interesting
topic. The obtained carboxylic acids are used as synthetic in-
termediates in organic synthesis and in the production of poly-
meric materials or pharmaceuticals. Dun˜ach and co-workers’ study
* Corresponding author.
E-mail address: jianghf@scut.edu.cn (H.-F. Jiang).
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.04.053

G.-Q. Yuan et al. / Tetrahedron 64 (2008) 5866–5872
5867
O
COOH
COOH
0.05 mol L-1 nBu4NBr-DMF
Ni
H3O+
1)
2)
O
constant current, rt.
Al
+
CO
+
2
2
O
1a
2a
3a
Scheme 1. Electrochemical dicarboxylation of phenylacetylene with CO2.
reduced pressure and dried by 4 Å molecular sieves. Carbon dioxide
was dried by passing through a 4 Å molecular sieves column before
charging it to the cell. The conducting salts were dried in a vacuum
oven at 50 ꢀC. The electrochemical dicarboxylation of phenyl-
acetylene with CO2 follows the principle as shown in Scheme 1.
work, it is worthy to note that the products obtained with Ni
cathode are the mixture of di-carboxylic acids instead of mono-
and di-carboxylic acids, even under high or low pressure of CO2.
These experimental results show that the electrocarboxylation of
phenylacetylene with CO2 is strongly dependent upon the nature
of cathode materials and reaction conditions. The practical re-
duction potential of phenylacetylene or CO2 is associated with
cathodic materials and reaction conditions. The excellent result of
a nickel cathode may be linked with its catalytic activation for
phenylacetylene and CO2. The difference between Ni and Pt or Ag
cathodes would further be discussed via cyclic voltammetries in
Section 2.6.
2.1. Influence of electrode materials
During electrolysis, the reduction of phenylacetylene or CO2
takes place on the surface of cathodes. Thus, the property of
cathode materials is of importance for the reduction activity of
substrates. For comparison, the electrocarboxylation of phenyl-
acetylene with CO2 was investigated by using different cathodes
(Ni, Pt, Ag, Cu, Zn, stainless steel, and Cu–Sn alloy) and anodes (Al or
Mg) without any additional catalysts. The results of the electro-
carboxylation are presented in Table 1. With Ni as the cathode and
Al or Mg as the anode, as we expected, it could be smoothly realized
that two CO2 molecules were selectively incorporated into the tri-
ple bond of phenylacetylene to afford the corresponding phenyl-
maleic anhydride (2a) in excellent isolated yields (88% and 83%,
respectively, Table 1, entries 1 and 7). Under the same experimental
conditions, the use of Cu or Cu–Sn alloy and stainless-steel cath-
odes could also give 2a in good yields (74%, 73%, and 70%, re-
spectively, entries 2–4), while the yield of 2a obtained with Pt or Ag
as the cathode is only around 45% (entries 5 and 6). Specifically, in
the case of a Zn cathode, no any mono- or di-carboxylic acid
products were formed and phenylacetylene could be almost
quantitatively recovered at the end of the electrolysis (entry 8).
Moreover, in the Pt cathode case, we observed that a small amount
of the mono-carboxylic acid (phenylpropiolic acid) was also gen-
erated and the surface of the cathode was usually covered by a thin
gray matter, which might lead to the loss of Pt electrode activity. In
addition, with a Ag cathode under 0.1 MPa of CO2, the electro-
carboxylation of phenylacetylene afforded a non-selective mixture
of mono-and di-carboxylic acids (i.e., cis- and trans-cinnamic acids
29% yield, phenylpropiolic acid 15%, and phenylsuccinic acids 8%),
which is similar to that reported in the literature.34 In the present
According to our experimental results, the electrochemical
route using Ni as the cathode and Al or Mg as the anode is feasible
for the electrodicarboxylation of phenylacetylene with CO2, in the
absence of additional catalysts even at room temperature.
2.2. Influence of H2O
From Table 1, it could be seen that a small amount of the satu-
rated phenylsuccinic acid was also produced in all cases besides an
unsaturated phenylmaleic anhydride as the main product, and
found that the formation amount of saturated phenylsuccinic acids
increased under the un-dried DMF solvent and CO2 case (Table 1,
entry 9). Thus, we deduce that the formation of saturated phenyl-
succinic acids may be associated with the presence of residual
water in the electrochemical system. In order to verify this point,
we consciously added a small amount of H2O to the electrolysis
system. The obtained results are listed in Table 2. Under dry con-
ditions, the obtained di-carboxylic acid almost is an unsaturated
phenylmaleic anhydride (89% yield, Table 2, entry 1). However, the
yield of unsaturated phenylmaleic anhydrides dramatically de-
creased from 89% to 3% and conversely the yield of saturated
phenylsuccinic acids increased from 4% to 90% (entries 1–4) when
a small amount of water (from 0.0 to 0.30 g) was added to 35 mL n-
Bu4NBr–DMF electrolyte solution. The experimental results further
verify our deduction. The presence of water in the electrolysis
system could provide protons to make phenylacetylene easily be
reduced into styrene and finally result in the formation of saturated
phenylsuccinic acids. Since it is very difficult to keep an absolute
anhydrous state in the electrolysis system, the formation of an
unsaturated phenylmaleic anhydride is always accompanied with
a certain amount of saturated phenylsuccinic acids during the
electrolysis.
Table 1
Influence of electrode materials on the electrocarboxylation of phenylacetylene with
a
CO2
Entry
Cathode
Anode
Yieldb (%)
2a
3a
1
2
3
4
Ni
Cu
Al
Al
Al
Al
Al
Al
Mg
Al
Al
88
74
73
70
40
45
83
d
5
4
7
6
5
Table 2
Cu–Sn alloy
Stainless steel
Pt
Ag
Ni
Zn
Ni
a
Influence of H2O on the electrocarboxylation of phenylacetylene with CO2
5
Entry
H2O (g)
Yieldb of 2a (%)
Yieldb of 3a (%)
6c
7
3
4
1
2
3
4
0.00
0.10
0.20
0.30
89
50
11
3
4
42
80
90
8
d
9d
60
32
a
Experimental conditions: room temperature, phenylacetylene (5 mmol), DMF
a
(35 mL), current density 10 mA cmꢁ2, CO2 3 MPa, and electricity 4 F molꢁ1
.
Experimental conditions: room temperature, phenylacetylene (4.5 mmol), DMF
(35 mL), n-Bu4NBr (0.05 mol L-1), CO2 3 MPa, current density 10 mA cm-2, Al anode,
b
Isolated yield based on the starting phenylacetylene.
A small amount of phenylpropiolic acid was also detected.
DMF and CO2 were not further dried.
Ni cathode, and electricity 4 F mol-1
.
c
b
d
Isolated yield based on the starting phenylacetylene.

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G.-Q. Yuan et al. / Tetrahedron 64 (2008) 5866–5872
Table 3
Table 4
Influence of conducting salts and solvents on the electrocarboxylation of phenyl-
acetylene with CO2
Influence of electricity and temperature as well as CO2 pressure on the electro-
carboxylation of phenylacetylenea
a
Entry
Conducting salt
Solvent
Yieldb (%)
Entry
Electricity (F mol-1
)
Temp (ꢀC)
Pressure (MPa)
Yieldb (%)
2aþ3a
2a/3a
2aþ3a
2a/3a
1
2
3
4
5
6
n-Bu4NBr
n-Bu4NCl
n-Bu4NI
Et4NBr
n-Bu4NBr
n-Bu4NBr
DMF
DMF
DMF
DMF
MeCN
CH3OH
93
89
73
70
20
d
95:5
85:15
90:10
85:15
80:20
d
1
2
3
4
5
6
7
8
9
10
2
3
4
5
4
4
4
4
4
4
25
25
25
25
35
10
25
25
25
25
3
3
3
3
3
3
0.1
1
2
70
80
92
93
78
93
50
82
90
d
90:10
85:15
95:5
95:5
88:12
95:5
95:5
90:10
92:8
d
a
Experimental conditions: room temperature, phenylacetylene (5 mmol), solvent
(35 mL), conducting salt (0.05 mol L-1), CO2 3 MPa, current density 10 mA cm-2
,
electricity 4 F mol-1, Al anode and Ni cathode.
5
b
Isolated yield and the yield ratio of 2a/3a based on the starting phenylacetylene,
a
and data of GC–MS.
Experimental conditions: phenylacetylene (4.5 mmol), n-Bu4NBr (0.05 mol L-1),
DMF (35 mL), current density 5 mA cmꢁ2, Ni cathode and Al anode.
b
Isolated yield and the yield ratio of 2a/3a based on the starting phenylacetylene,
and data of GC–MS.
2.3. Influence of solvents and conducting salts
The effect of solvents (DMF, MeCN, and MeOH) and conducting
salts (n-Bu4NBr, n-Bu4NCl, n-Bu4NI, and Et4NBr) on the electro-
carboxylation of phenylacetylene with CO2 was also carefully in-
vestigated. With DMF as the solvent, the electrolysis could
efficiently afford the corresponding phenylmaleic anhydride (2a)
and phenylsuccinic acid (3a) in good to excellent total yields (70–
93%, Table 3, entries 1–4), whereas 2a and 3a were not detected in
MeOH solvent case (entry 6). In addition, when conducting elec-
trolysis in MeCN solvent, we found that the products precipitated
on the surface of an Al anode so that the electrolysis could not be
smoothly finished, and the electrolysis gave 2a and 3a in very low
yield (20%, entry 5), together with other by-products (such as 2-
propenoic acid). Different solvents lead to different results of
electrolysis, which may be due to the difference in solubility of
products or reactants and conducting salts in solvents. For con-
ducting salts, at the same anion (Brꢁ) case, the electrolysis con-
ducted with n-Bu4NBr gave 2a and 3a in 93% total yield (entry 1),
which was better than that of Et4NBr (70%, entry 4). In the same
cation (n-Bu4Nþ) case, n-Bu4NBr or n-Bu4NCl (89%, entry 2) seemed
to be more effective than n-Bu4NI (73%, entry 3). These results show
that the electrochemical activation process of phenylacetylene or
CO2 is related with anions or cations of conducting salts. The ac-
tivity of cations and anions for the reaction is n-Bu4Nþ>Et4Nþ and
Brꢁ>Clꢁ>Iꢁ, respectively. Among these conducting salts and sol-
vents examined, the combination of n-Bu4NBr and DMF as the
supporting electrolyte appears to be the most appropriate for the
electrocarboxylation of phenylacetylene with CO2.
CO2 pressure and electricity, the yield of 2a and 3a obtained at 10 ꢀC
or 25 ꢀC is superior to that of the electrolysis at 35 ꢀC (entries 3, 5,
and 6). When carbon dioxide pressure increased from 0.1 to 3 MPa,
the yield of 2a and 3a increased from 50% to 92% (entries 7–9 and
3). On the other hand, high pressure of CO2 makes possible the
electrochemical dicarboxylation of phenylacetylene with more
concentrated solution and restricts the polymerization of phenyl-
acetylene. But, carbon dioxide pressure is too high to favor the
electrolysis. When carbon dioxide pressure increased up to 5 MPa,
the voltage between anode and cathode increased dramatically so
that the electrolysis was not successful (entry 10). Thus, carbon
dioxide pressure should be controlled within an appropriate range
(2–3 MPa).
Based on the above investigations, the optimal conditions of
the electrocarboxylation for phenylacetylene with CO2 are sum-
marized as follows: Ni as the cathode and Al (Mg) as the anode,
n-Bu4NBr–DMF as the supporting electrolyte, temperature of
the electrolysis at 25 ꢀC, CO2 pressure at 2–3 MPa, and electricity at
4 F molꢁ1
.
2.5. Electrocarboxylation of phenylacetylene derivatives
In order to test the effectiveness and scope of this electro-
chemical route, several phenylacetylene derivatives were further
electrocarboxylated under the above optimized electrolysis condi-
tions. As shown in Table 5, the electrochemical route is also effec-
tive for the phenylacetylene derivatives in the absence of additional
catalysts. The electrocarboxylation of p-tolylacetylene, 1-phenyl-1-
propyne, 1,2-diphenylacetylene, and 1-ethynyl-naphthalene could
afford the corresponding aryl-maleic anhydrides and 2-arylsuccinic
acids in excellent total yields (94%, 82%, 91%, and 89%, respectively,
Table 5, entries 2–5). In all cases, almost no any mono-carboxylic
acids were formed in the present conditions. Earlier work of
Wawzonek and Wearring has also showed that the electrolysis of
1,2-diphenylacetylene with carbon dioxide could give di-carboxylic
acids (diphenylmaleic anhydride, diphenylfumaric acid, and meso-
diphenylsuccinic acid).36 It should be noted that the products
obtained with the nickel complex-catalyzed electrocarboxylation
of arylacetylenes are mono-carboxylic acids instead of di-carbox-
ylic acids.33 This means that the electrochemical reaction mecha-
nism using Ni as the cathode is different from the one catalyzed by
the nickel complex.
2.4. Influence of electricity and temperature as well as carbon
dioxide pressure
The electricity for electrolysis, temperature, and carbon dioxide
pressure are also important factors for the electrochemical reaction
system. Table 4 summarizes the effect of these factors on the
electrocarboxylation of phenylacetylene with CO2. It could be seen
from Table 4 that the overall yield of 2a and 3a increased from 70%
to 92% as the electricity passed through the electrolysis cell in-
creased from 2 to 4 F molꢁ1 (Table 4, entries 1–3). When the
electricity continued to increase up to 5 F molꢁ1, the yield was al-
most unchanged (93%, entry 4). Considering the efficiency of cur-
rent, we chose 4 F molꢁ1 as the optimal electricity for the
electrolysis. The solubility of CO2 in DMF solvent is limited under
0.1 MPa of CO2. According to the basic principle of thermodynam-
ics, lowering temperature or increasing CO2 pressure is able to
enhance the solubility of CO2 in DMF solvent, which is beneficial for
the electrocarboxylation of phenylacetylene with CO2. Our experi-
mental results also support this point. For example, under the same
In addition, it should be pointed out that no oxalate or poly-
phenylacetylene was detected in our experiments although CO2
radical anions could dimerize to oxalates and phenylacetylene
radical anions could polymerize under certain conditions.

G.-Q. Yuan et al. / Tetrahedron 64 (2008) 5866–5872
5869
Table 5
Electrochemical dicarboxylation of phenylacetylene derivatives with CO2
a
Entry
Substrate (1)
Product (2P)
Product (3P)
Yieldb (%)
2Pþ3P
2P/3P
O
COOH
COOH
O
1
93
94
95:5
1a
O
3a
2a
O
COOH
COOH
O
2
3
90:10
85:15
O
1b
3b
COOH
2b
O
O
COOH
82
91
1c
O
3c
2c
O
COOH
COOH
O
4
95:5
O
1d
3d
COOH
COOH
2d
O
O
5
89
75:25
O
1e
3e
2e
a
Experimental conditions: arylacetylene (5 mmol), DMF (35 mL), CO2 3 MPa, room temperature, n-Bu4NBr (0.05 mol L-1), electricity 4 F mol-1, current density 10 mA cmꢁ2
,
Ni cathode and Al anode.
b
Isolated yield and the yield ratio based on the starting arylacetylenes, and data of GC–MS.
2.6. Electrochemical reaction mechanism
understand the difference between Ni and Pt or Ag cathodes as well
as the electrochemical reaction mechanism of arylacetylene with
CO2, we further checked the cyclic voltammetry of an n-Bu4NBr–
DMF solution containing phenylacetylene (model compound) in
the absence or presence of CO2 with Ni and Pt or Ag as the working
electrodes. The obtained CV results are shown in Figures 1 and 2. It
can be seen that the result of Ag electrode is obviously different
In our previous work,35 it was observed that the result of a Ni
cathode was almost the same as good as that of a Pt one for the
electrodicarboxylation of styrene derivatives with carbon dioxide.
Interestingly, in the present case, a Ni cathode appears to be more
effective than a Pt or Ag one (Table 1, entries 1, 5, and 6). To
Figure 1. Cyclic voltammograms of a 0.05 mol L-1 n-Bu4NBr–DMF solution (35 mL)
containing 5.0 mmol phenylacetylene at room temperature, at the scan rate 100 mV s-1
with different working electrodes: (a) Pt; (b) cyclic voltammogram of 0.05 mol L-1
n-Bu4NBr–DMF solution (35 mL) in the presence of CO2 with Ni as a working electrode;
(c) Ni; (d) Ag.
Figure 2. Cyclic voltammograms of a 0.05 mol L-1 n-Bu4NBr–DMF solution (35 mL)
containing 5.0 mmol phenylacetylene in the presence of CO2 (0.1 MPa) at room tem-
perature, at the scan rate 100 mV s-1 with different working electrodes: (a) Ag; (b) Ni;
(c) Pt.

5870
G.-Q. Yuan et al. / Tetrahedron 64 (2008) 5866–5872
3e
Al3+
+
At the anode :
Al
At the cathode and in the solution :
+ e
+ 2e
+ e
+ e
CO2
Ph
CO2
+ 2OH
Ph
Ph
2H2O
+ e
Ph
Ph
COO
COO
A
CO2
CO2
Ph
CO2
Ph
+ e, CO2
CO2
+ e, CO2
COO
CO2
COO
+ 2e
2H2O
B
OOC
Ph
COO
Ph
Ph
B
Al3+
COO
Al3+
OOC
COO
Al2
COO
Al2
3
3
Ph
H3O
H3O
O
COOH
Ph
HOOC
Ph
COOH
- H2O
COOH
O
O
Ph
Scheme 2. Electrochemical reaction mechanism of phenylacetylene with CO2.
from those of Ni and Pt. In the absence of CO2, two reduction peaks
of phenylacetylene were observed within ꢁ0.5 to ꢁ2.0 V versus SCE
in the case of Ag electrode (Fig. 1d), while no reduction peaks
appeared at the same reduction potentials with Pt and Ni as elec-
trodes (Fig. 1a and c). In the presence of CO2 (Fig. 2), a similar be-
havior could be observed, and in Figure 2a (Ag as the electrode),
other reduction peak and an oxidation peak are also observed. At
ꢁ2.25 V versus SCE, the reduction current of phenylacetylene also
appears with Ni as the working electrode (Figs. 1c and 2b), whereas
it is still zero in the Pt working electrode case (Figs. 1a and 2c). The
CV results mean that the activity of electrode is Ag>Ni>Pt for the
reduction of phenylacetylene, and that a Ag cathode results in more
non-selective reduction products than Ni or Pt one. The activity of
electrode is too high or too low to favor the electrodicarboxylation
of phenylacetylene with CO2. A Pt cathode gave di-carboxylic acids
in low yield (Table 1, entry 5) due to its low activity. Compared with
a Ag or Pt electrode, a Ni cathode seemed to be more appropriate
for the electrodicarboxylation of phenylacetylene with CO2. More-
over, due to the catalytic role of a Ni cathode, the tendency of re-
duction reaction of phenylacetylene approximates to that of CO2
(see Fig. 1b and c). Consequently, it was considered that a single-
electron reduction of CO2 and phenylacetylene almost could take
place at the same time during the electrolysis with nickel as the
cathode. When phenylacetylene is added to n-Bu4NBr–DMF elec-
trolyte solution in the presence of CO2, the change of reduction
current at ꢁ2.0 to ꢁ2.5 V versus SCE is attributed to the one-elec-
tron or two-electron reduction processes of CO2 and phenyl-
acetylene (Fig. 2b).
stable aluminum carboxylates. Finally, a phenylmaleic anhydride
was formed via the dehydration of phenylmaleic acids during the
acidification of the metal carboxylates with 2 mol Lꢁ1 aqueous HCl.
If there was a small amount of water in the electrolysis system, an
unsaturated dicarboxylate anion B would be further reduced to the
saturated phenylsuccinic acids. On the other hand, phenylacetylene
might first be reduced to styrene in the presence of water. Then,
further reduction of styrene with CO2 gave the saturated phenyl-
succinic acids, as previously reported.35 The presence of water leads
to the formation of saturated phenylsuccinic acids, which is con-
firmed by our experiments (Table 2, entries 2–4). The electro-
chemical reaction of phenylacetylene with CO2 becomes complex
due to the presence of water. The detailed mechanism is needed to
study further.
3. Experimental section for the electrochemical
dicarboxylation of arylacetylenes with carbon dioxide
3.1. General methods
The electrocarboxylations of arylacetylenes with CO2 were car-
ried out in the high-pressure stainless-steel undivided cell fitted
with
a
nickel (or copper, platinum, silver, etc.) sheet
(2 cmꢂ3 cmꢂ0.02 cm) as the cathode and an aluminum (or mag-
nesium) plate (2 cmꢂ3 cmꢂ0.05 cm) as the anode, being similar to
that described in our previous paper.35 Prior to the electrolysis, the
two electrodes were cleaned with detergent and diluted hydro-
chloric acid, washed with distilled water, and then dried. In a typ-
ical procedure, n-Bu4NBr (1.75 mmol), dried DMF solvent (35 mL),
and phenylacetylene (5 mmol) were first added to the cell in turn.
Carbon dioxide was then charged into the desired pressure after the
cell was sealed. The electrolysis was performed at a suitable con-
stant current until 4 F molꢁ1 of starting substrates had been passed
through the cell at room temperature. The electrolyte solution was
continuously stirred by a magnetic stirrer during the electrolysis. At
the end of the electrolysis, the solvent was distilled off at reduced
pressure, and the residue was acidified with 2 mol Lꢁ1 aqueous HCl
and extracted with diethyl ether (4ꢂ25 mL). The ether phase was
washed twice with distilled water. After evaporation of ether, the
Based on our experimental results, a possible electrochemical
reaction mechanism of phenylacetylene with CO2 is outlined in
Scheme 2. When the electrolysis was performed under anhydrous
conditions, phenylacetylene or CO2 on the surface of a nickel
cathode first got electrons to form radical anions almost at the same
time. Then, the attack of CO2 radical anions to phenylacetylene or
the reaction of phenylacetylene radical anions with CO2 gave an
intermediate A (carboxylate radical anion). Further reaction of A
with CO2 radical anions, or reaction of A with CO2 after A getting one
electron, afforded a dicarboxylate anion B, followed by capturing of
the anion by Al3þ ions generated at the anodic Al metal to form

G.-Q. Yuan et al. / Tetrahedron 64 (2008) 5866–5872
5871
obtained product was dried in a vacuum oven at 60 ꢀC for 6 h. The
crude product was further purified by re-crystallization or using
planar chromatography and column chromatography (eluent: pe-
troleum ether–EtOAc), and was characterized by FTIR, mass spectra,
and 1H and 13C NMR.
NMR (acetone-d6, 100 MHz):
d
174.8, 173.3, 137.6, 136.7, 130.1, 128.5,
47.5, 38.2, 21.0. MS (EI): m/z 208 (Mþ). White solid, mp: 198–200 ꢀC.
3.1.8. 2-Methyl-1-phenylsuccinic acid (3c)
IR (neat): 1705 cmꢁ1
.
1H NMR (CDCl3):
d
1.31 (d, J¼4 Hz, 3H),
1H and 13C NMR were measured on a DRX-400 (Bruker) spec-
trometer with acetone-d6 as solvent in the presence of SiMe4 as an
internal standard. Fourier transform infrared (FTIR) spectrum and
mass spectra or GC–MS analyses were performed on TENSOR27 and
Shimadzu QP5050A spectrometers, respectively. Elemental analy-
ses were carried out in a Heraeus CHN-O-RAPID Elemental Ana-
lyzer. Melting points were determined on X-4 Mel-Temp apparatus
and were uncorrected. Cyclic voltammetric experiments were car-
ried out in an Auto LAB (PGSTAT30) electrochemical working sta-
tion, with a nickel or a platinum sheet as a working electrode and
a saturated calomel electrode (SCE) as a reference electrode.
3.08–3.13 (m, 1H), 3.74 (t, J¼12 Hz, 1H), 7.24–7.30 (m, 1H, Ph), 7.32–
7.36 (m, 4H, Ph), 10.78 (s, 1H), 11.02 (s, 1H). 13C NMR (acetone-d6,
100 MHz):
d
174.9, 1þ73.0, 137.7, 129.2, 128.5, 127.5, 43.0, 42.0, 16.1.
MS (EI): m/z 208 (M ). White solid, mp: 216–218 ꢀC.
3.1.9. 2,3-Diphenylsuccinic acid (3d)
IR (neat): 1710 cmꢁ1. 1H NMR (acetone-d6):
d
4.24 (s, 2H),
d 7.09–
7.15 (m, 2H, Ph), 7.35–7.39 (m, 4H, Ph), 7.42–7.48 (m, 4H, Ph), 11.32
(s, 2H). 13C NMR (acetone-d6, 100 MHz):
d 174.4, 138.3, 131.5, 129.4,
128.5, 47.8. MS (EI): m/z 270 (Mþ). Pale yellow solid, mp: 220–
222 ꢀC (Ref. 222 ꢀC).36
3.1.1. Phenylmaleic anhydride (2a)
4. Conclusions
IR (neat): 1766, 1702 cmꢁ1. 1H NMR (acetone-d6):
d
7.29–7.31 (m,
1H, Ph), 7.33–7.35 (m, 2H, Ph), 7.51 (s, 1H), 7.56–7.58 (m, 2H, Ph). 13
C
In conclusion, a simple and efficient electrochemical route with
nickel instead of noble metal (Pt or Ag) as the cathode and Al as the
anode has been developed for the electrochemical dicarboxylation
of arylacetylenes with CO2 without additional catalysts at mild
conditions (room temperature). A nickel cathode itself could ef-
fectively catalyze the reduction reactions of arylacetylenes with
CO2. With nickel as the cathode as well as the catalyst, the elec-
trochemical route appears much simple and more efficient. Under
the optimized conditions (i.e., n-Bu4NBr–DMF as the supporting
electrolyte, temperature of the electrolysis at 25 ꢀC, pressure of CO2
at 2–3 MPa, and electricity at 4 F molꢁ1), the electrochemical route
could afford the corresponding aryl-maleic anhydrides and 2-
arylsuccinic acids in excellent total yields (82–94%). The present
work might be helpful to further extend the application scope of
carbon dioxide.
NMR (acetone-d6, 100 MHz):
d 167.6, 166.6, 143.8, 135.4, 129.9,
129.5, 128.5, 128.1. MS (EI): m/z 174 (Mþ). Pale yellow solid, mp:
119–120 ꢀC (Ref. 118–119 ꢀC).37
3.1.2. p-Tolylmaleic anhydride (2b)
IR (neat): 1760, 1701 cmꢁ1. 1H NMR (acetone-d6):
d
2.32 (s, 3H),
7.16–7.19 (m, 2H, Ph), 7.23–7.26 (m, 2H, Ph), 7.43 (s, 1H). 13C NMR
(acetone-d6, 100 MHz):
d 168.4, 167.3, 143.7, 138.6, 132.7, 130.6,
130.0,128.5, 21.2. MS (EI): m/z 188 (Mþ). Pale yellow solid, mp: 107–
109 ꢀC (Ref. 106–108 ꢀC).37
3.1.3. 2-Methyl-1-phenylmaleic anhydride (2c)
IR (neat): 1773, 1695 cmꢁ1. 1H NMR (acetone-d6):
d
2.19 (s, 3H),
7.28–7.31 (m, 1H, Ph), 7.34–7.38 (m, 2H, Ph), 7.55–7.57 (m, 2H, Ph).
13C NMR (acetone-d6, 100 MHz):
d 169.9, 168.6, 137.8, 137.3, 136.5,
129.2, 128.5, 127.8, 17.3. MS (EI): m/z 188 (Mþ). Pale yellow solid,
mp: 149–151 ꢀC. C11H8O3 (188.18) calcd: C 70.21, H 4.29; found: C
70.31, H 4.23.
Acknowledgements
The authors thank National Natural Science Foundation of China
(No. 20332030, 20572027, 20625205 and 20772034) and Guang-
dong Natural Science Foundation (No. 07118070) for the financial
support of this work.
3.1.4. 2,3-Diphenylmaleic anhydride (2d)
IR (neat): 1760, 1680 cmꢁ1. 1H NMR (acetone-d6):
d
7.15–7.17 (m,
2H, Ph), 7.37–7.43 (m, 4H, Ph), 7.50–7.52 (m, 4H, Ph). 13C NMR
(acetone-d6, 100 MHz):
d 169.3, 137.5, 136.8, 129.5, 128.7, 128.0. MS
(EI): m/z 250 (Mþ). Pale yellow solid, mp: 158–159 ꢀC (Ref. 157–
References and notes
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d 2.26 (s, 3H), 2.48 (dd,
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