Selective electrocarboxylation of bromostyrene at silver cathode in DMF

Article abstract of DOI:10.1016/j.tet.2015.12.066

The electroreduction behavior of bromostyrenes 1 in DMF has been detected by cyclic voltammetry (CV) on GC and Ag electrodes. Under the atmospheric pressure of CO₂, selective electrocarboxylation of 1 was carried out in an undivided cell at Ag

Full text of DOI:10.1016/j.tet.2015.12.066

Tetrahedron xxx (2015) 1e5
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Selective electrocarboxylation of bromostyrene at silver cathode in
DMF
*
*
Hui-Mei Wang, Guo-Jiao Sui, Di Wu, Qiu Feng, Huan Wang , Jia-Xing Lu
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, 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:
The electroreduction behavior of bromostyrenes 1 in DMF has been detected by cyclic voltammetry (CV)
Received 25 September 2015
Received in revised form 19 December 2015
Accepted 25 December 2015
Available online xxx
2
on GC and Ag electrodes. Under the atmospheric pressure of CO , selective electrocarboxylation of 1 was
carried out in an undivided cell at Ag cathode under potentiostatic conditions, with the corresponding
vinyl-benzoic acid methyl ester 3 as the principal product, accompanied by styrene 2. Moderate to good
electrocarboxylation yields were obtained by preparative electrolysis. Synthetic parameters such as
supporting electrolyte, applied potential, electric charge, and temperature, were found to influence the
carboxylation efficiencies. Both CV and electrolysis confirm that the positions of CeBr and C]C groups
strongly affect the electrocarboxylation. Higher yields were obtained by changing from ortho to meta to
para isomers.
Keywords:
CO
2
Electrocarboxylation
Bromostyrene
Silver electrode
Ó 2015 Elsevier Ltd. All rights reserved.
1
. Introduction
will compete with each other during the electrolysis, complicating
the reaction pathway. Consequently, selectivity is a very important
31
Although several different types of gases contribute to the
issue. In our previous study,
electrocarboxylation of hal-
greenhouse effect (caused by the accumulation of these gases in the
oacetophenones has been carried out. Thanks to the extraordinary
electrocatalytic activities of Ag towards the reduction of organic
1
upper atmosphere), CO
the sharply increasing atmospheric CO
from consumption of fossil fuel is becoming an important envi-
2
is the largest contributor to the effect. And
17,20,21
2
concentration resulting
halides,
the enhanced reactivity of CeX bond compared with
that of the C]O bond has been achieved. Even so, because the bulk
electrolysis was carried out under galvanostatic conditions, three
kinds of products were obtained, and the yields were a little low.
As extension of our studies, we have now investigated the se-
lective electrocatalytic carboxylation of three bromostyrenes, such
as 2-bromostyrene (1a), 3-bromostyrene (1b) and 4-bromostyrene
(1c). Cyclic voltammograms have been used to study the electro-
reduction behaviors, and the bulk electrolysis have been performed
under potentiostatic conditions. The influence of various synthetic
parameters, such as supporting electrolyte (SE), electric charge (Q),
and temperature (T), have also been investigated in detail.
2
2
ronmental issue today. Meanwhile, CO is recognized to be a nat-
urally abundant, cheap, recyclable and non-toxic carbon source that
can sometimes replace toxic chemicals such as phosgene, iso-
3
cyanates or carbon monoxide. However, its high chemical stability
has made its conversion and utilization ecologically and economi-
cally challenging for chemists. Therefore, much attention has been
paid to utilize CO
make use of CO , among which electrocarboxylation is one of the
most practical routes. Many substrates, such as ketones,
2
under mild conditions. There are various ways to
2
4
e8
alkenes,9
on, could be electrochemical carboxylated into their corresponding
organic carboxylic acids in the presence of CO
e12
organic halides,
13e22
imines,
23e25
amines,
26,27
and so
2
.
So far, most studies about electrocarboxylation focused on the
substrates with only one reducible functional group, while a few
2. Experimental
papers concerned with substrates bearing two reducible
2.1. General
groups.2
8e31
It could be anticipated that the two reducible groups
1H NMR spectras were recorded on a Bruker 400 MHz spec-
4
trometer, with Me Si as an internal standard. Mass spectra were
obtained on a 5973N spectrometer connected with an HP 6890 gas
chromatograph. Cyclic voltammograms were measured with
CHI600c electrochemical station (Shanghai Chenhua Instruments
*
Corresponding authors. Tel.: þ86 21 52134935 (H.W.); tel.: þ86 21 62233491
(
J.X.L.); e-mail addresses: hwang@chem.ecnu.edu.cn (H. Wang), jxlu@chem.ecnu.
edu.cn (J.-X. Lu).
http://dx.doi.org/10.1016/j.tet.2015.12.066
0
040-4020/Ó 2015 Elsevier Ltd. All rights reserved.
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H.-M. Wang et al. / Tetrahedron xxx (2015) 1e5
Company) in an undivided glass cell, with an Ag or GC (d¼2 mm) as
electroreduction of C]C bond was scarcely catalyzed. For com-
parison, cyclic voltammograms of styrene at both GC and Ag elec-
trodes were also recorded (Fig. 1 curves c and d), which display an
irreversible reduction peak at ꢀ2.25 V and ꢀ2.23 V, respectively,
similar as the second reduction peak of 1c on both electrodes. It
implies that the first reduction peak is attributed to the two-
electron reductive cleavage of the CeBr bond, and the second one
is ascribed to the reduction of the C]C bond. GCeMS analysis
confirmed the synthesis of styrene through potentiostatic elec-
the working electrode, a platinum spiral (Pt) as the counter elec-
ꢀ
1
4
trode and an Ag/AgI/n-Bu NI (0.1 mol L ) in DMF as the reference
ꢀ
electrode. N,N-Dimethylformamide (DMF) was kept over 4 A mo-
lecular sieves. Other reagents were used as received.
2
.2. Electrocarboxylation of bromostyrenes (typical
procedure)
The potentiostatic electrolysis was carried out in 10 mL of dry
DMF solution containing 0.1 mol L bromostyrene and 0.1 mol L
2
trolysis at the first reduction peak in the presence of N . The above
ꢀ
1
ꢀ1
information reveals that Ag electrode shows better catalytic effect
for the reduction of CeBr bond than GC electrode, therefore, for the
reduction of CeBr bond convenience, Ag electrode is chosen as the
proper material in our follow-up study.
TBABr as a supporting electrolyte in an undivided glass cell, which
2
was equipped with an Ag cylinder (8 cm ) as cathode, a Mg rod as
ꢀ1
anode and an Ag/AgI/n-Bu
electrode. Prior to every experiment, the solution was bubbled with
CO for 30 min to be saturated. Continuous CO flow was main-
4
NI (0.1 mol L ) in DMF as the reference
Then, the electrochemical behavior of 1c in the presence of CO
2
2
2
was investigated. Compared curve e with curve b (Fig. 1), the first
reduction peak shifts to a slightly more positive potential with in-
creasing peak current, indicating there may be a chemical reaction
tained throughout the duration of the whole electrolysis process.
After the electrolysis, the reaction mixture was esterified by
adding anhydrous K
the mixture at 55 C for 5 h. The solution was treated with aq HCl
and extracted four times with ethyl acetate, and the organic layers
2
CO
3
(2 mmol) and MeI (5 mmol) and stirring
between the electroreduced intermediate and CO
a new big irreversible reduction peak corresponding to the re-
duction of CO
(Fig. 1 curve f) appears at ꢀ2.0 V.
2
. Meanwhile,
ꢁ
2
2 4
were washed with H O, dried over MgSO , and evaporated. The
conversion of the substrate and the yields of products were de-
termined by GC using n-decane as the internal standard.
3
.2. Electrocarboxylation of 4-bromostyrene under po-
tentiostatic electrolysis
2
.2.1. 2-Vinyl-benzoic acid methyl ester 3a. GCeMS (m/z, %):162
þ
1
The more positive reduction potential of CeBr bond than that of
C]C bond, especially at Ag electrode, suggests the possibility of
selective electrocarboxylation of CeBr rather than C]C for 1c.
(
(
M , 100), 147 (33), 131 (85), 103 (72), 77 (45), 51 (18); H NMR
400 MHz, DMSO):
3.83 (s, 3H), 5.36 (d, J¼10.8 Hz, 1H), 5.77 (d,
d
J¼17.6 Hz, 1H), 7.32 (dd, J¼17.2 Hz, J¼10.8 Hz, 1H), 7.40 (t, J¼7.6 Hz,
H), 7.57 (t, J¼7.6 Hz, 1H), 7.70 (d, J¼8 Hz, 1H), 7.78 (dd, J¼7.6 Hz,
J¼0.8 Hz, 1H).
2
Potentiostatic electrolysis was carried out in CO saturated DMF
1
ꢀ1
solution containing 0.1 mol L 1c in an undivided cell with Ag
2
.2.2. 3-Vinyl-benzoic acid methyl ester 3b. GCeMS (m/z, %):162
þ
1
(
M , 71), 131 (100), 103 (36), 77 (14), 51 (5); H NMR (400 MHz,
CDCl ):
3.92 (s, 3H), 5.32 (d, J¼11.2 Hz, 1H), 5.85e5.80 (m, 1H),
.75 (dd, J¼17.6 Hz, J¼10.8 Hz, 1H), 7.41e7.39 (m, 1H), 7.59 (d,
J¼7.6 Hz, 1H), 8.08e7.91 (m, 1H), 8.08 (s, 1H).
3
d
6
2
.2.3. 4-Vinyl-benzoic acid methyl ester 3c. GCeMS (m/z, %):162
þ
1
(
M , 44), 131 (100), 103 (29), 77 (20), 51 (7); H NMR (400 MHz,
CDCl ):
3
d
3.91 (s, 3H), 5.38 (d, J¼10.8 Hz, 1H), 5.87 (dd, J¼17.6 Hz,
J¼0.4 Hz,1H), 6.75 (dd, J¼17.6 Hz, J¼10.8 Hz,1H), 7.47e7.45 (m, 2H),
8
3
3
.01e7.98 (m, 2H).
. Results and discussion
.1. Cyclic voltammetry of 4-bromostyrene
Bromostyrene contain two different reducible groups, namely
CeBr and C]C on phenyl ring. Competition between these groups
may complicate the electrocarboxylation pathway. 4-Bromostyrene
(1c) was chosen as the model substrate for the investigation of
bromostyrene electrocarboxylation. To begin with, we focused on
its electroreduction behavior. Cyclic voltammograms recorded for
reduction of 1c at GC and Ag electrodes in DMF containing
ꢀ
1
0
.1 mol L TEABF
4
are depicted in Fig. 1. As shown in curves a and
b, at both electrodes, reduction of 1c gives rise to two irreversible
ꢀ
1
peaks in the region of ꢀ1.0 to ꢀ2.5 V at the scan rate of 0.1 V s
.
Moreover, these two reduction peak currents vary linearly with n1/
which confirms diffusion control for the electroreduction pro-
2
,
cesses. In the case of GC electrode, the first reduction peak of 1c was
detected at ꢀ1.80 V, and the second one was at ꢀ2.21 V. For Ag
electrode, the first reduction peak shifted to more positive potential
at ꢀ1.40 V, while the second one remains constant. In the previous
studies, metallic silver was recognized as a powerful catalytic ma-
terial for the electroreduction of C-X bond,32e34 while the
Fig. 1. Cyclic voltammograms recorded at Ag (b, d, e, f) or GC (a, c) electrode in DMF-
ꢀ1
TEABF
4
, containing 5 mmol L 4-bromostyrene (a, b, e) and styrene (c, d) in the
¼0.1 V s , T¼25 ꢁC.
ꢀ1
presence of N
2
(a, b, c, d) and CO
2
(e, f).
n
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H.-M. Wang et al. / Tetrahedron xxx (2015) 1e5
3
cathode and Mg sacrificial anode. As expected, 4-vinyl-benzoic acid
methyl ester 3c was obtained as the principal product, accompa-
nied by styrene 2 (Scheme 1), at the applied potential (Eapp) of
were detected by GCeMS during the electrolysis, suggesting that
these salts underwent electrochemical reduction. Moreover, pre-
þ
decessors found that TEA was much easier to be reduced than
þ 35,36
þ
ꢀ
1.8 V, which is slightly more negative than peak potential of CeBr
TBA .
That is, TEA salt surely expanded much more electric
þ
reduction. To deeply understand the selective electrocarboxylation
of 1c, the influence of various synthetic parameters, such as sup-
porting electrolyte (SE), Eapp, electric charge (Q), and temperature
charge than TBA salt in the selective electrocarboxylation of 1c,
lowering the carboxylation yield to a greater extent. On the other
hands, higher 3c yield could be obtained when TBAI was added into
the electrolyte with TEAI as supporting electrolyte after electrolysis,
(
T), was investigated. The results of the electrolysis were summa-
þ
rized in Table 1.
indicating that TBA have better performance to promote the es-
terification. Therefore, TBABr is the most suitable supporting elec-
trolyte for the target electrocarboxylation.
3.2.2. Influence of applied potential. The applied potential is the key
factor for the selective electrocarboxylation. To avoid competitive
electrocarboxylation of C]C bond on phenyl ring, a series of elec-
trolysis were carried out under potentials more positive than
ꢀ
2.2 V, under which C]C bond could be electroreduced (Fig. 1). As
shown in Table 1 (entries 5e10), the conversion is insensitive to
2
Scheme 1. Electrocarboxylation of bromostyrenes under CO atmosphere.
E
app. By shifting Eapp from ꢀ1.5 V to ꢀ2.0 V, the yield of 3c first
increased and then decreased, while the yield of 2 decreased
gradually. That may be ascribed to that a negative potential is in
favor of the reduction of the CeBr bond to some extent. A reduction
process carried out at more negative potential would normally
provide higher reaction rate due to the larger overpotential. How-
3
.2.1. Influence of supporting electrolyte. For the same tetraethy-
þ
lammonium cation (TEA ), the yields of 3c were very close to each
other, under 40% (Table 1, entries 1e3), while 1c conversion and
yields of 2 were also analogous, respectively. In the case of tetra-
þ
butylammonium cation (TBA ), higher yields of 3c were obtained,
2
ever, some side reactions, such as electroreduction of CO and the
both with 49% (Table 1, entries 4e5), while 1c conversion and yields
of 2 changed little. In both cases, the differences in yield of 3c were
supporting electrolyte TBABr, may take place, if the applied po-
tential is too negative. Actually, tributylamine was detected in the
final products by GCeMS. In that case, we may see that too negative
potential would go against the reduction of the CeBr bond. In the
cases of electrolysis under ꢀ1.8 V and ꢀ1.9 V (Table 1 entries 5 and
9), the yields of 3c are almost the same. With a view to energy
saving, the optimal applied potential is ꢀ1.8 V, providing 3c in yield
of 49%.
þ
minor when different anions were used in the electrolyte. TBA
þ
gave better yields than TEA , indicating that cations had great in-
fluence on the synthesis (Table 1, entries 1e5) to some extent. Al-
though magnesium ions could stabilize the electrogenerated
carboxylate anion,1
7,31
the initial concentration of magnesium ions
was too low to make it. Since they were generated gradually by
electrolytic dissolution of the Mg anode. When tetraalkylammo-
þ
nium (TRA ) salt was used as supporting electrolyte, carboxylate
3.2.3. Influence of electric charge. The electrocarboxylation of 1c
þ
ꢀ1
anion could be stabilized by forming an ion pair {TRA eRCO
2
},
was also examined under various electric charges (Table 1, entries
17
þ
ꢀ1
which was helpful to the electrosynthesis. In the presence of TBA
and TEA , significant amounts of tributylamine and triethylamine
5,11e17). Below 2.0 F mol , the conversion of 1c and the yield of 3c
þ
increased rapidly with increasing charge. When electric charge
ꢀ1
passed 2.0 F mol , the conversion of 1c reached 100%, indicating
that 1c was completely participated. The yield of 3c and 2 remained
constant, while much more tributylamine was detected in the
electrolyte, which suggesting the excess charge consumed in the
electrolysis was used for electroreduction of supporting electrolyte
rather than 2 and 3c.
Table 1
a
Electrocarboxylation of 4-bromostyrene under various conditions
Entry SE
E
app
Q (F) T ( C) Conversion Product yields (%)b
ꢁ
b
ꢀ
(
V vs Ag/AgI/I )
(%)
2
3c
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
TEAI
ꢀ1.8
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.2
1.4
1.6
1.8
2.2
2.4
2.6
2.0
2.0
2.0
2.0
25
25
25
25
97
98
96
99
18
15
18
20
17
24
20
22
18
15
15
19
18
11
16
18
14
19
4
33
39
33
49
49
35
37
38
50
38
27
36
39
44
50
50
51
47
59
73
66
3
.2.4. Influence of temperature. It is well known that CO
2
solubility
TEABr ꢀ1.8
in DMF depends on temperature, which is larger at lower tem-
TEABF
TBAI
4
ꢀ1.8
ꢀ1.8
37
peratures, indicating that low temperature favors the carboxyla-
tion reaction to some extent. However, as temperature decreases,
mass transport decelerates in solution while activation energy in-
creases during electrolysis. To check the effect of temperature, a set
of electrolysis was performed at different temperatures (Table 1,
entries 5, 18e21). The results indicated that the optimal tempera-
TBABr ꢀ1.8
TBABr ꢀ1.5
TBABr ꢀ1.6
TBABr ꢀ1.7
TBABr ꢀ1.9
TBABr ꢀ2.0
TBABr ꢀ1.8
TBABr ꢀ1.8
TBABr ꢀ1.8
TBABr ꢀ1.8
TBABr ꢀ1.8
TBABr ꢀ1.8
TBABr ꢀ1.8
TBABr ꢀ1.8
TBABr ꢀ1.8
TBABr ꢀ1.8
TBABr ꢀ1.8
25 100
25
25
25
25
25
25
25
25
25
25 100
25 100
25 100
10
0
94
98
98
97
97
70
79
86
88
0
1
2
3
4
5
6
7
8
9
0
1
ꢁ
ture was ꢀ5 C. So far, under the optimized condition, the highest
yield of 3c was 73% (Table 1, entries 20).
3
.3. Electrocarboxylation of other bromostyrenes
To investigate the influence of the C]C position on phenyl ring,
91
98
97
95
cyclic voltammograms of bromobenzene (4), 2-bromostyrene (1a)
and 3-bromostyrene (1b) at Ag electrode were carried out (Fig. 2).
As shown in Fig. 2, one irreversible reduction peak for 4, cor-
responded to two-electron reductive cleavage of CeBr bond is
detected at ꢀ1.61 V. The same as 1c, two irreversible reduction
peaks, ascribed to electroreduction of CeBr and C]C, respectively,
are obtained for 1a and 1b. Due to the conjugative effect of C]C
ꢀ5
6
14
ꢀ10
a
ꢀ1
ꢀ
ꢀ1
,
General conditions: DMF¼10 mL, SE¼0.1 mol L , 4-bromostyrene¼0.1 mol L
P
CO ¼1 atm, Ag as cathode, Mg as anode, Ag/AgI/I as reference electrode.
2
b
Conversion of the substrate and yields of two products were determined by GC,
based on the starting material.
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H.-M. Wang et al. / Tetrahedron xxx (2015) 1e5
Fig. 3. Comparison of the yield of 3a, 3b and 3c.
As a result, the target carboxylation yield of 3a was the highest
under the same conditions (Fig. 3).
4. Conclusions
In conclusion, we report a possible way to achieve the selective
electrocarboxylation of bromostyrenes on Ag electrode. Cyclic
voltammograms of bromostyrenes explained the competitive
electrochemical reducibility between CeBr bond and C]C bond on
phenyl ring. Since CeBr bond is much easier to be reduced than C]
C bond, target electrocarboxylation product could be obtained by
means of potentiostatic electrolysis. The performances of the pro-
cess were found to be dependent on synthetic conditions. Both CV
and electrolysis confirm that 2-bromostyrene was easier to reduce
than 3-bromostyrene, which in turn than 4-bromostyrene.
Acknowledgements
Fig. 2. Cyclic voltammograms of 5 mmol Lꢀ1 aromatic bromides in DMF-TEABF
corded at Ag electrode in the presence of N , n
2
re-
4
This work was financially supported by the Project for the Na-
tional Natural Science Foundation of China (21173085, 21203066,
¼0.1 V s , T¼25 ꢁC.
ꢀ1
21373090, 21473060).
group on phenyl ring, the reduction peak potentials of CeBr for 1a,
1
b and 1c are around ꢀ1.3 to ꢀ1.4 V, which are much more positive
References and notes
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Please cite this article in press as: Wang, H.-M.; et al., Tetrahedron (2015), http://dx.doi.org/10.1016/j.tet.2015.12.066