Convenient synthesis of cyclic α-alkoxyl-α,β-unsaturated carboxylic acids by nickel-catalyzed electrochemical carboxylation of lactone enol triflates

Article abstract of DOI:10.1055/s-2002-19350

Electrochemical carboxylation of lactone enol triflates in DMF containing a catalytic amount of NiBr₂·bpy with a platinum cathode and a magnesium anode under an atmospheric pressure of CO₂ gave the corresponding cyclic α-alkoxyl-α,β-

Full text of DOI:10.1055/s-2002-19350

1
40
LETTER
Convenient Synthesis of Cyclic -Alkoxyl- , -unsaturated Carboxylic Acids
by Nickel-catalyzed Electrochemical Carboxylation of Lactone Enol Triflates
C
H
onvenientSynthe
i
sisof
C
s
yclic -A
a
lkoxyl-
,
n
-unsaturated
o
C
arboxylic
A
r
cids i Senboku,* Hirotaka Kanaya, Masao Tokuda*
Laboratory of Organic Synthesis, Division of Molecular Chemistry, Graduate School of Engineering, Hokkaido University,
Sapporo 060-8628, Japan
Fax +81(11)7066598; E-mail: tokuda@org-mc.eng.hokudai.ac.jp
Received 9 October 2001
pyran-6-carboxylic acid (3: n = 1) was obtained by hy-
Abstract: Electrochemical carboxylation of lactone enol triflates in
drolysis of 3,4-dihydro-2H-pyran-6-carbonitrile, which
DMF containing a catalytic amount of NiBr bpy with a platinum
cathode and a magnesium anode under an atmospheric pressure of
2
was prepared from 3,4-dihydro-2H-pyran by dibromina-
9
tion and cyanation followed by dehydrobromination. In
this case, preparation of starting cyclic vinyl ethers carry-
ing various substituents, especially seven-membered
ones, was not easy. On the other hand, lactone enol tri-
CO gave the corresponding cyclic -alkoxyl- , -unsaturated car-
2
boxylic acids, captodative cycloalkenes, in good yields.
Key words: electrochemical carboxylation, carbon dioxide, vinyl
triflate , -unsaturated carboxylic acid, captodative alkene
flates are readily prepared from the corresponding lac-
tones.1
2–15
The present synthesis of carboxylic acids 3
using electrochemical carboxylation could be achieved
from these readily available lactones in only two steps.
There have been no reports on electrochemical carboxyla-
tion of lactone enol triflates giving cyclic -alkoxyl- , -
O
O
O
OTf
O
COOH
Pt
Mg
(
)n
( )n
CO2, DMF
NiBr2•bpy
( )n
1
n = 1, 2
2
3
7
unsaturated carboxylic acids 3, although direct and palla-
16
dium-catalyzed electrochemical carboxylations of vinyl
triflates have been reported. In this communication, we re-
port a convenient method for synthesizing cyclic -alkox-
yl- , -unsaturated carboxylic acids 3 by nickel-catalyzed
electrochemical carboxylation of lactone enol triflates 2.
Scheme 1
Electrochemical fixation of carbon dioxide is a useful and
attractive method for efficient synthesis of various car-
boxylic acids. We previously reported that electrochemi-
cal carboxylation of allylic halides, propargylic
bromides, 1,4-dibromo-2-bromomethylbut-2-ene, vinyl
bromides, and phenyl-substituted alkenes proceeded ef-
ficiently to give the corresponding carboxylic acids in
high yields when a magnesium metal was used as a reac-
tive-metal anode. We also reported chemoselective and
divergent electrochemical carboxylation of vinyl triflates
to give aliphatic -keto carboxylic acids or phenyl-substi-
1
Lactone enol triflates 2 were prepared from the corre-
2
3
sponding lactones by a modification of the previously re-
ported procedure.1
2,17
Reaction of appropriate lactones
4
5
with LDA in THF at –78 °C followed by the addition of
2
-bis(trifluoromethanesulfonyl)aminopyridine17 gave the
6
corresponding lactone enol triflates 2 in moderate to good
yields (Scheme 1). As the first attempt, direct electro-
chemical carboxylation of lactone enol triflate 2a was car-
ried out according to our previously reported
7
tuted , -unsaturated carboxylic acids. As an extension
7
electrochemical method. However, a complex mixture of
of our studies on electrochemical fixation of carbon diox-
ide to organic molecules, we recently found that a nickel-
catalyzed electrochemical carboxylation of lactone enol
triflates gave cyclic -alkoxyl- , -unsaturated carboxylic
acids, captodative cycloalkenes, in good yields.
carboxylic acids was only obtained in a very low yield,
and the starting triflate 2a was recovered in about 60%
yield. Thus, we next attempted electrochemical carboxy-
lation of lactone enol triflates 2a in the presence of
NiBr bpy, which has been shown to be effective for elec-
2
Although esters of dihydropyrancarboxylic acids 3 (n = 1)
could be prepared by a hetero Diels–Alder reaction,8
the synthesis of seven-membered analogs of 3 (n = 2) is
not possible using this method. It has also been reported
that an intramolecular Wadsworth–Emmons reaction of
trochemical carboxylation of alkyl-substituted vinyl bro-
–10
4b
mides. Electrochemical carboxylation of 2a in the
presence of 20 mol% of NiBr bpy gave an expected cy-
2
clic -alkoxyl- , -unsatureted carboxylic acid 3a in 65%
yield (Scheme 2).
-
( -oxoalkoxyl)phosphonoacetates gave esters of car-
1
1
boxylic acids 3 in moderate yields. In this method, how-
ever, troublesome preparation of starting substrates is
required. It has also been reported that 3,4-dihydro-2H-
10 mA/cm2
3 F/mol
O
OTf
O
COOH
65%
Pt
Mg
0
.1M Bu4NBF4 - DMF
CO2, NiBr2•bpy
2a
3a
Synlett 2002, No. 1, 28 12 2001. Article Identifier:
1
437-2096,E;2002,0,01,0140,0142,ftx,en;Y19801ST.pdf.
Georg Thieme Verlag Stuttgart · New York
ISSN 0936-5214
Scheme 2
©

LETTER
Convenient Synthesis of Cyclic -Alkoxyl- , -unsaturated Carboxylic Acids
141
The results of the present electrochemical carboxylation Table Nickel-catalyzed Electrochemical Carboxylation of Lactone
Enol Triflates 2a
of lactone enol triflates 2 are summarized in the Table.
Similar electrochemical carboxylation of seven-mem-
Entry
1
Triflate
Product and Yieldb
bered enol triflates 2c–e also gave the corresponding car-
boxylic acids 3c–e in 71–79% yields (entries 3–5). This
method was also applicable to the synthesis of bicyclic
products. Thus, triflate 2f, readily prepared from 3,4-dihy-
drocoumarine, was subjected to this electrochemical car-
boxylation to give the corresponding 4H-chromene-2-
carboxylic acid (3f) in 71% yield (entry 6). In all cases, a
small amount of diene, which was derived from the
nickel-catalyzed dimerization of triflates 2, was detected
O
OTf
OTf
O
COOH
2a
3
a 65%
O
O
COOH
2
3
4
5
1
2b
3b 63%
by H NMR.
A typical procedure for the nickel-catalyzed electrochem-
ical carboxylation is as follows. Lactone enol triflate 2a
O
OTf
O
COOH
(
700 mg, 3.0 mmol), which was readily prepared from -
valerolactone in 79% yield, in 15 mL of DMF containing
.1 M Bu NBF was electrolyzed in the presence of 20
2
c
3c 74%
0
4
4
Me
O
O
OTf
OTf
Me
O
COOH
mol% of NiBr bpy at 5 °C at a constant current (current
2
2
density: 10 mA/cm ) under an atmospheric pressure of
carbon dioxide. A one-compartment cell equipped with a
2
platinum plate cathode (2 3 cm ) and a magnesium rod
2
d
3d 79%
anode (3 mm ) was used for electrolysis. The electricity
passed was 3 F/mol. After electrolysis, the electrolyzed
solution was acidified with 1 N HCl and extracted with di-
ethyl ether. The ethereal solution was washed successive-
ly with H O and saturated NaHCO . The aqueous solution
Ph
Ph
O
COOH
2
3
2e
3e 71%
was again acidified carefully with 6 N HCl, and the result-
ing carboxylic acid was extracted with diethyl ether. The
combined ethereal solutions were washed with saturated
O
OTf
O
COOH
6
brine and dried over MgSO . Evaporation of the solvent
gave an almost pure 3,4-dihydro-2H-pyran-6-carboxylic
acid (3a) (250 mg, 65%).
4
2
f
3f 71%
a
Lactone enol triflate 2 (3 mmol) in DMF containing 0.1 M Bu NBF
4
4
Cyclic voltammetry of lactone enol triflate 2a and
was electrolyzed in the presence of 20 mol% of NiBr bpy at 5 °C with
2
2
NiBr bpy was carried out. No reduction peak of 2a was a constant current (10mA/cm ) under CO . A one-compartment cell
2
2
2
+
equipped with a platinum plate cathode (2 3 cm ) and a magnesium
rod anode (3 mm ) was used. The electricity passed was 3 F/mol.
observed at > –3.1 V vs Ag/Ag and a reduction peak of
+
4b
NiBr bpy appeared at –1.25 V vs Ag/Ag . On the other
2
b
Isolated yields.
2
NiBr •bpy
+
2 e
O
OTf
O
NiOTf
O
CO2
O
COO
Ni(0)•bpy
+
2 e
-
Ni(0)
TfO
(
)
n
( )
n
( )
n
( )
n
-
2
A
B
C
At an anode
Mg2+
Mg
+
2 e
O
COO
O
COO )
2
Mg
O
COOMgOTf
O
COOH
H+
Mg2+
+
and / or
(
)
n
( )
n
( )
n
( )
n
C
D
E
3
Scheme 3
Synlett 2002, No. 1, 140–142 ISSN 0936-5214 © Thieme Stuttgart · New York

1
42
H. Senboku et al.
LETTER
hand, cyclic voltammetry of 2a in the presence of
(6) (a) Chaussard, J.; Folest, J.-C.; Nédélec, J.-Y.; Pélichon, J.;
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2
+
at –1.75 V vs Ag/Ag and showed that the reduction cur-
987.
rent of this peak increased when the amount of NiBr bpy
2
(
7) (a) Kamekawa, H.; Senboku, H.; Tokuda, M. Tetrahedron
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increased. From these results we propose the following re-
action pathways, as shown in Scheme 3. At a cathode, a
two-electron reduction of NiBr bpy occurs to give
2
Ni(0) bpy. Oxidative addition of an Ni(0) catalyst to vinyl
triflates 2 gives nickel(II) complex A. A two-electron re-
duction of complex A at a cathode generates a vinyl anion
(
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2
Synthesis 1986, 548.
boxylate ions C. On the other hand, at an anode, dissolu-
tion of magnesium metal occurs to give magnesium ions.
The magnesium ions readily capture carboxylate ions C to
form stable magnesium carboxylates D or E, which upon
acid treatment gives carboxylic acid 3.
(
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Acknowledgement
This work was financially supported by a Grant-in-Aid for Scienti-
fic Research (B) (No. 10555314) from the Ministry of Education,
Science, Sports and Culture, and by a Grant-in-Aid for Encourage-
ment of Young Scientists (No. 12750753) from the Japan Society
for the Promotion of Science.
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Synlett 2002, No. 1, 140–142 ISSN 0936-5214 © Thieme Stuttgart · New York