Stereospecific electrochemical carboxylation of β-bromostyrene by use of nickel(II) catalyst

Article abstract of DOI:10.1246/cl.2005.528

Electrochemical carboxylation of (E)- and (Z)-β-bromostyrenes (1) under an atmospheric pressure of carbon dioxide with a platinum cathode and a magnesium anode in the presence of 20 mol % of NiBr₂·bpy proceeded with retention of stereochemistry to give the corresponding (E)- or (Z)-cinnamic acids (2). The stereochemical outcome of nickel(II)-catalyzed electrochemical carboxylations was discussed by comparison with predominant formation of a (Z)-isomer from either (E)- or (Z)-β-bromostyrenes. Copyright

Full text of DOI:10.1246/cl.2005.528

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28
Chemistry Letters Vol.34, No.4 (2005)
Stereospecific Electrochemical Carboxylation of ꢀ-Bromostyrene by Use of Nickel(II) Catalyst
ꢀ
y
yy
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Chunxiang Kuang, Qing Yang, Hisanori Senboku, and Masao Tokuda
Department of Materials Chemistry, Graduate School of Engineering, Osaka University, Suita 565-0871
Department of Biopharmaceutics, Graduate School of Pharmaceutical Sciences, The University of Tokyo, Tokyo 113-0033
y
yy
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Division of Molecular Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628
(Received January 12, 2005; CL-050056)
Electrochemical carboxylation of (E)- and (Z)-ꢀ-bromostyr-
enes (1) under an atmospheric pressure of carbon dioxide with a
Table 1. Electrochemical carboxylation of (E)-ꢀ-bromostyrene
((E)-1) under various conditions
a
platinum cathode and a magnesium anode in the presence of
20 mol % of NiBr2 bpy proceeded with retention of stereochem-
Current
density
Isomer
ratio
E=Zc
.
Catalyst Temperature
ꢂ
Electricity Yield
ꢁ1
Entry
istry to give the corresponding (E)- or (Z)-cinnamic acids (2).
The stereochemical outcome of nickel(II)-catalyzed electro-
chemical carboxylations was discussed by comparison with pre-
dominant formation of a (Z)-isomer from either (E)- or (Z)-ꢀ-
bromostyrenes.
NiBr2
.
bpy
/ C
/F mol
/%b
ꢁ
2
/
mA cm
1
2
3
4
5
6
7
8
—
5
ꢁ10
ꢁ10
ꢁ10
ꢁ10
ꢁ10
20
10
10
10
10
15
5
3
81
61
66
65
47
71
72
76
30/70
21/79
78/22
88/12
67/33
99/1
—
3
10 mol %
20 mol %
20 mol %
20 mol %
20 mol %
20 mol %
3
3
3
Electrochemical carboxylation is one of the most useful
methods for the fixation of carbon dioxide to organic molecules
because it is a clean and environmentally benign process. It takes
place efficiently even in an atmospheric pressure of CO2 under
neutral and mild conditions to give carboxylic acids in high
yields when a reactive-metal such as magnesium or aluminum
3
10
5
3
91/9
ꢁ10
4.5
77/23
a(E)-ꢀ-bromostyrene (3 mmol) in 0.1 M Bu4NBF4-DMF (15 mL) was elec-
.
trolyzed in the presence of 20 mol % of NiBr2 bpy under an atmospheric
carbon dioxide with a Pt cathode and a Mg anode. Isolated yields. Isomer
b
c
1
is used as a sacrificial anode in the electrolysis. We have already
reported that electrochemical carboxylation of vinyl halides and
vinyl triflates gave the corresponding carboxylic acids in high
1
ratios were determined by H NMR.
with retention of its stereochemistry in a 71% yield (Table 1,
Entry 6).
Similar nickel-catalyzed electrochemical carboxylation of
(Z)-1 under the same condition as that of (E)-1 gave the corre-
2
yields. In 1996, we found that thermodynamically less-stable
(Z)-cinnamic acids ((Z)-2) were preferentially produced from ei-
ther (E)- or (Z)-ꢀ-bromostyrene ((E)-1 or (Z)-1) in the electro-
chemical carboxylations. Electrochemical carboxylation of
3
4
sponding acid 2 (E=Z ¼ 1=99) in a 78% yield (Scheme 2).
(E)-1 gave a 30/70 mixture of (E)-2 and (Z)-2, whereas that
of (Z)-1 also gave a 24/76 mixture of (E)-2 and (Z)-2
The use of an aluminum or zinc anode, instead of magnesium
anode, in the electrochemical carboxylation of (E)-1 resulted
in a decreased yield and stereochemistry of the acid 2. Electro-
chemical carboxylation of (E)-1 in the presence of 20 mol % of
(
Scheme 1).
Br
.
Ph
NiBr2 bpy, using a platinum cathode and an aluminum anode
CO2
Pt
Mg
gave a 76/24 mixture of (E)-2 and (Z)-2 only in a 45% yield. Ad-
ditionally, electrochemical carboxylation of (E)-1 using a zinc
anode gave a mixture of acid 2 (E=Z ¼ 73=27) in only a 9%
yield. These results show that a magnesium anode plays an im-
portant role in efficient electrochemical carboxylation and its
stereochemistry. It was also found that palladium catalyst such
as Pd(PPh3)4, PdCl2(PPh3)2 were not effective in this reaction.
(
E)-1
Ph
Z)-1
CO H
Ph
2
Ph
0.1M Bu4NBF4-DMF
+
CO H
2
(E)-2
(Z)-2
Br
(
from (E)-1 81%, E/Z=30/70
from (Z)-1 74%, E/Z=24/76
Scheme 1.
As one of our continuing studies on electrochemical carbox-
ylation, we attempt to develop a novel route for the stereoselec-
tive electrochemical carboxylation of (E)-1 and (Z)-1 by use of
nickel(II) catalyst. In this paper, we report our results concerning
stereoselective electrochemical carboxylation and mechanistic
aspects of this reaction.
Br
Ph
CO2
CO H
2
Ph
+
(E)-1
Pt
Mg
Ph
0.1M Bu4NBF4-DMF
2
CO2H
(Z)-2
o
Ph
3F/mol, 10 mA/cm , -10 C
(E)-2
"
20 mol% NiBr2 bpy"
Br
from (E)-1 71%, E/Z=99/1
from (Z)-1 78%, E/Z=1/99
(
Z)-1
We first examined the electrochemical carboxylation of
E)-1 under various conditions. The results are summarized in
(
Scheme 2.
Table 1. Various conditions were examined to optimize the yield
and stereoselectivity. It was found that electrochemical carbox-
ylation of (E)-1 in the presence of 20 mol % of nickel(II)
The present electrochemical method is useful for the synthe-
sis of substituted (E)- or (Z)-cinnamic acids since the starting
(E)- or (Z)-vinyl bromides can readily be prepared in high
yields. The electrochemical carboxylation of (Z)-vinyl bro-
mides with CO2 has especially been proposed as a useful proce-
0
.
ꢂ
bromide-2,2 -bipyridine complex (NiBr2 bpy), at ꢁ10 C and
2
5
5
mA/cm of constant current with an electricity of 3 Faradays
per mol of (E)-1 gave the corresponding acid 2 (E=Z ¼ 99=1)
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.4 (2005)
529
dure for the production of (Z)-cinnamic acids which are difficult
6
sium anode in electrolysis, plays an important role in the (Z)-se-
lectivity of electrochemical carboxylation of vinyl bromides. In
order to find a further evidence of this prospect, we investigated
the electrochemical carboxylation of (Z)-1-bromo-3-cyclohexyl-
ethylene ((Z)-3) in the absence of nickel catalyst, much to our
delight, a 51/49 mixture of (E)- and (Z)-3-cyclohexylpropenoic
acid ((E)- and (Z)-4) was obtained in a 23% yield. Because of an
absence of ꢁ-electron in cyclohexyl group, stability of the two
vinyl radicals is not so different, hence electrochemical carbox-
ylation of (Z)-3 gave the (E)- and (Z)-4 in almost same ratio.
These results showed that both ꢁ-electron of phenyl group and
magnesium ion are essential in regulation of the stereoselectivity
of electrochemical carboxylation of vinyl bromides.
In summary, we have developed a novel route for the stereo-
selective electrochemical carboxylation of ꢀ-bromostyrene by
use of nickel(II) catalyst. The stereochemical mechanistic as-
pects of nickel(II)-catalyzed electrochemical carboxylations
are discussed by comparison with predominant formation of a
(Z)-isomer from either (E)- or (Z)-ꢀ-bromostyrenes.
a
to prepare by conventional method, even though the conver-
b
6
sion of (E)-cinnamic acids to its (Z)-isomer has been reported.
The probable reaction pathways of the present electrochem-
ical carboxylation are shown in Scheme 3. At the cathode, in the
.
presence of nickel catalyst, a two-electron reduction of NiBr2
bpy gives Ni(0) species, and an oxidative addition of the Ni(0)
to (E)- or (Z)-1 would produce the intermediate A or B, isomer-
ization could not occur between A and B at this stage. A two-
electron reduction of A or B gives the corresponding vinyl car-
banion (C or D), which is trapped by atmospheric carbon dioxide
to give the corresponding alkenoate (E or F). At the anode, on
the other hand, a dissolution of magnesium metal takes place
to give magnesium ion. The magnesium ion readily captures
2-alkenoates (E or F) to give the stable magnesium carboxylate
I or J. Acid treatment of I or J gives (E)-2 or (Z)-2. Cyclic vol-
.
tammetry of (E)-1 and (Z)-1 in the presence of NiBr2 bpy
showed the existence of an additional reduction peak at ca.
ꢁ
1:98 and ꢁ1:97 V vs Ag/AgCl, respectively, while the reduc-
.
tion peaks of (E)-1, (Z)-1 and NiBr2 bpy alone appeared at
7
ꢁ
2:59, ꢁ2:62, and ꢁ1:53 V, respectively. These result suggest
This work was supported by a Grant-in-Aid for Scientific
Research (A) (No. 07555580) from The Ministry of Education,
Culture, Sports, Science and Technology, Japan.
.
that in the presence of NiBr2 bpy, a direct two-electron reduc-
tion of intermediate A or B may take place preferentially over
a one-electron reduction giving an equilibration between two
References and Notes
(E)- and (Z)-vinyl radical.
1
2
a) G. Silvestri, S. Gambino, G. Filardo, and A. Gulotta, Angew.
Chem., Int. Ed. Engl., 23, 979 (1984). b) O. Sock, M. Troupel,
and J. P e´ richon, Tetrahedron Lett., 26, 1509 (1985).
a) H. Kamekawa, H. Kudoh, H. Senboku, and M. Tokuda, Chem.
Lett., 1997, 917. b) H. Kamekawa, H. Senboku, and M. Tokuda,
Tetrahedron Lett., 39, 1591 (1998). c) H. Senboku, H. Kanaya,
Y. Fujimura, and M. Tokuda, J. Electroanal. Chem., 507, 82
(2001).
At a cathode (Pt)
i) in the presence of nickel(II) complex
+2e
NiBr2 bpy
Ni(0) bpy
+
CO2
CO2
Ni(0) bpy
NiBr +2e
Ni(0)
Br
Ph
Ph
Ph
Ph
Ph
Ph
-
H
(
E)-1
A
B
C
E
F
3
4
H. Kamekawa, H. Senboku, and M. Tokuda, Electrochim. Acta,
42, 2117 (1997).
Typical procedure for the nickel(II)-catalyst electrochemical car-
H
+CO2
Ni(0) bpy
+2e
Ni(0)
Ph
Ph
-
NiBr
boxylation of ꢀ-bromostyrene is described in the followings: A
Br
Z)-1
CO2
.
mixture of (E)-ꢀ-bromostyrene ((E)-1, 3 mmol) and NiBr2 bpy
(
D
(
0.6 mmol) and Bu4NBF4 (1.5 mmol) in 15 mL of DMF was added
into a one-compartment cell equipped with a platinum plate cath-
ii) in the absence of nickel(II) complex
Br + e
2
+ e
+
CO2
CO2
ode (2 ꢃ 3 cm ) and a magnesium rod anode (3 mmꢂ). The mix-
Ph
Ph
Ph
Ph
Ph
ꢂ
2
ture was electrolyzed at ꢁ10 C at 5 mA/cm of constant current
H
H
under a slow stream of carbon dioxide gas until an electricity of
3
(E)-1
G
C
E
Faradays per mol of (E)-1 was passed. The electrolyzed solution
+
e
+ e
+CO2
was poured into 2N HCl and extracted with diethyl ether (3 ꢃ 50
mL). After the organic layer was washed with water (4 ꢃ 50 mL),
it was extracted by saturated sodium hydrogen carbonate (3 ꢃ
Ph
H
H
Ph
Ph
Br
CO2
(Z)-1
H
D
F
5
0 mL). The aqueous layer was again acidified with 2N HCl and
At an anode (Mg)
Mg
extracted with diethyl ether (3 ꢃ 50 mL). The organic layer was
washed with brine and dried over MgSO4. Filtration and evapora-
tion of the solvent afforded cinnamic acid 2 (E=Z ¼ 99=1) in 71%
Mg2+
+
2e
Mg2+
H+
ꢂ
1
Ph-CH=CH-CO2
E or F
Ph-CH=CH-CO2 2Mg
Ph-CH=CH-CO2H
yield. Mp 133–134 C; H NMR (270 MHz, CDCl3) ꢃ 6.46 (1H, d,
J ¼ 16:1 Hz), 7.39–7.44 (3H, m), 7.52–7.57 (2H, m), 7.80 (1H, d,
I or J
(E) or (Z)-2
13
J ¼ 16:1 Hz); C NMR (67.5 MHz, CDCl3) ꢃ 117.30, 128.35,
Scheme 3.
1
28.93, 130.72, 133.98, 147.09, 172.66.
5
a) C. Kuang, H. Senboku, and M. Tokuda, Synlett, 2000, 1439. b)
C. Kuang, H. Senboku, and M. Tokuda, Tetrahedron Lett., 42,
3893 (2001). c) C. Kuang, H. Senboku, and M. Tokuda, Tetrahe-
dron, 58, 1491 (2002). d) C. Kuang, H. Senboku, and M. Tokuda,
Tetrahedron, 61, 637 (2005), and references cited therein.
a) M. B. Hocking, Can. J. Chem., 47, 4567 (1969). b) V. Galamb
and H. Alper, Tetrahedron Lett., 24, 2965 (1983).
In the absence of nickel catalyst, a one-electron reduction of
E)- or (Z)-1 gives the corresponding vinyl radical G or H, re-
(
spectively (Scheme 3). In an equilibration between vinyl radi-
cals G and H, the radical H would gain an advantage over radical
G due to some interaction of a magnesium ion with a phenyl
group and a radical center. When a platinum anode, instead of
a magnesium anode, was used in the electrochemical carboxyla-
tion of (E)-1, (E)-2 was predominantly produced. This result in-
dicates that a magnesium ion, formed by dissolution of a magne-
6
7
Reduction peak potentials of (E)- and (Z)-ꢀ-bromostyrene and
.
NiBr2 bpy were determined by cyclic voltammetry of 2 mM sub-
strates in 0.1 M Bu BF -DMF with a gold disk electrode (1.6
4
4
mmꢂ) at the scan rate of 0.1 V/s.
Published on the web (Advance View) March 5, 2005; DOI 10.1246/cl.2005.528