Silver-catalyzed decarboxylative halogenation of carboxylic acids

Article abstract of DOI:10.1016/j.tetlet.2010.10.054

Decarboxylative halogenation of carboxylic acids catalyzed by silver carbonate is reported. ortho-Nitrobenzoic acids react with copper(II) chloride or bromide in DMF/DMSO at 130-140 °C leading to the corresponding aryl halides in moderate to good yields.

Full text of DOI:10.1016/j.tetlet.2010.10.054

Tetrahedron Letters 51 (2010) 6646–6648
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Tetrahedron Letters
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Silver-catalyzed decarboxylative halogenation of carboxylic acids
a
a
a,b,
⇑
Yong Luo , Xiaolin Pan , Jie Wu
a
Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, China
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Decarboxylative halogenation of carboxylic acids catalyzed by silver carbonate is reported. ortho-Nitro-
benzoic acids react with copper(II) chloride or bromide in DMF/DMSO at 130–140 °C leading to the cor-
responding aryl halides in moderate to good yields.
Received 24 August 2010
Revised 4 October 2010
Accepted 12 October 2010
Available online 20 October 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Aryl halide
Carboxylic acid
Decarboxylative halogenation
Silver carbonate
Recently, intense interest has been directed toward the devel-
opment of new methods for the transition metal-catalyzed decarb-
B
X
COOH
[M]
[M], CuX
2
oxylative coupling reactions.1 As highlighted by Baudoin, one of
the advantages of the reactions over C–H activation is the excellent
regioselectivity. In addition, compared to the conventional metal-
catalyzed cross-coupling reactions using organometallic reagents
as substrates, the decarboxylative coupling reaction is regarded
–5
1e
R
R
R1
NCS, NBS
or NIS
this work
ref.
Scheme 1. Proposed route for decarboxylative halogenation of carboxylic acids.
2
as an environmentally benign process since only CO is released
as the byproduct during the conversion of carboxylic acid into car-
bon nucleophile in the reaction. In the last several years, remark-
able progress in the field of decarboxylative coupling reactions
has been witnessed.
process, carboxylic acid was utilized as a replacement of organo-
metallic reagent for addition reaction. Additionally, copper(II) ha-
lide has been demonstrated as a convenient halide source.
Prompted by these results and the advancement of decarboxylative
coupling reactions, we envisioned that carboxylic acid might be a
good alternative for the halogenated reaction under suitable condi-
tions (Scheme 1). Thus, investigation aimed toward the develop-
ment of decarboxylative halogenation of carboxylic acids was
explored.
An initial experiment using 4,5-dimethoxy-2-nitrobenzoic acid
1a as substrate in the reaction of copper(II) chloride (2.0 equiv)
2 3
catalyzed by 10 mol % of Ag CO in the presence of KOH (2.0 equiv)
in DMF at 130 °C under nitrogen atmosphere afforded no reaction
(Table 1, entry 1). Interestingly, the desired 1-chloro-4,5-dime-
thoxy-2-nitrobenzene 2a could be obtained in 74% yield when
the reaction occurred under oxygen atmosphere (Table 1, entry
On the other hand, it is well established that aryl halides are
6
useful synthetic intermediates and can be found as essential com-
7
ponents in many pharmaceutical compounds. Usually, access to
halogenated arenes is through classic methods via direct electro-
philic aromatic substitution. Recently, much attention has been
paid to boron-halogen exchange reaction8
–11
for generation of hal-
ogenated arenes (Scheme 1). For instance, Hynes and co-workers
described a Cu(I)-mediated method for the conversion of arylbo-
ronic acids into aryl chlorides in the presence of NCS.1 Although
in some cases, the boron–halogen exchange was efficient for the
aryl halides formation, stoichiometric amounts of costly organo-
metallic reagent could not be avoided. Recently, we described pal-
ladium-catalyzed decarboxylative 1,2-addition of carboxylic acids
to aldehydes or imines under mild conditions.4a In this reaction
1d
2). Subsequently, a variety of bases and solvents were examined.
t
No better yields were observed when K
2
CO
3
, KOAc, or NaO Bu
was utilized as the base (Table 1, entries 3–5). Only a trace amount
of product 2a was detected when DMSO was used as a solvent
(Table 1, entry 6). Similar result was generated when the solvent
⇑
Corresponding author. Tel.: +86 2155664619; fax: +86 2165102412.
E-mail address: jie_wu@fudan.edu.cn (J. Wu).
was replaced to NMP (Table 1, entry 7). Gratifyingly, aryl chloride
2a could be isolated in 85% yield when the reaction was performed
0
040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.10.054

Y. Luo et al. / Tetrahedron Letters 51 (2010) 6646–6648
6647
Table 1
Initial studies for the decarboxylative halogenation of carboxylic acids
Table 2
Silver-catalyzed decarboxylative halogenation of carboxylic acids15
MeO
NO2
COOH
MeO
MeO
NO2
Cl
X
Ag CO3 (10 mol %)
Ag CO (10 mol %)
2 3
2
+
CuCl2
R
Entry
1
+
CuX2
R
base, solvent
DMF/DMSO (v/v 1:1)
MeO
COOH
ο
X = Cl, Br
ο
1
30-140 C
1
KOH, O , 130-140 C
2
2
1
a
2a
R1, R2
X
product
Yielda (%)
Yielda (%)
Entry
Base
Solvent
Atmosphere
1
2
3
4
5
6
7
8
9
KOH
KOH
DMF
DMF
DMF
DMF
DMF
DMSO
NMP
DMF/DMSO
DMF/NMP
NMP/DMSO
DMF
N
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
—
74
31
51
MeO
COOH
NO2
MeO
Cl
K
2
CO
3
Cl
85 (2a)
MeO
MeO
NO2
Br
KOAc
t
NaO Bu
62
1a
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
Trace
Trace
85
MeO
COOH
NO2
MeO
MeO
2
3
4
Br
Cl
Br
63 (2b)
83 (2c)
25 (2d)
MeO
NO2
—
—
1
1
1
1
0
1
2
3
1
a
b
c
Trace
Trace
Trace
MeO
COOH
NO2
MeO
Cl
DMF
DMF
d
NO2
a
b
c
Isolated yield based on carboxylic acid 1.
1b
In the presence of 20 mol % of CuCl
In the presence of 20 mol % of CuCl
In the presence of 20 mol % of CuCl
2
2
2
, 2.0 equiv of LiCl.
, 2.0 equiv of NaCl.
MeO
COOH
NO2
MeO
Br
d
n
, 2.0 equiv of Bu
4
NCl.
NO
2
1
b
in a mixed solvent DMF/DMSO (v/v, 1:1) (Table 1, entry 8). No reac-
tion occurred when the mixed solvent was changed to DMF/NMP
or DMSO/NMP (Table 1, entries 9 and 10). This result was accor-
dant with the previous report,1 wherein the rate of decarboxyl-
ation was much faster in a mixture of DMF and DMSO. We also
Cl
COOH
Cl
Cl
Cl
5
6
7
8
9
Cl
Br
Cl
Br
Cl
Br
80 (2e)
65 (2f)
61 (2g)
60 (2h)
83 (2i)
35 (2j)
NO
NO
2
2
1
c
c
Cl
COOH
Br
tested the reaction in the presence of 20 mol % of copper(II)
NO2
NO2
n
chloride and 2.0 equiv of LiCl, NaCl, or Bu
4
NCl (Table 1, entries
1c
1
1–13). However, a trace amount of product was furnished. Dimin-
COOH
Cl
ished reactivity was displayed when the amount of copper(II)
chloride was reduced to 1.0 equiv. The reaction was retarded when
the reaction temperature was lower or the catalytic amount of
Br
Br
NO
Br
Br
NO
Br
2
2
1
d
COOH
NO2
2 3
Ag CO was reduced. Blank experiment indicated that no reaction
took place in the absence of silver catalyst. Inferior results were
obtained when other silver catalysts were screened (data not
shown in Table 1). From the results shown in Table 1, it seemed
that the reaction proceeded through a radical process, as men-
tioned by Kochi and others.12
We next sought to understand the scope of the reaction and
examined a number of carboxylic acids under the optimized condi-
2 3 2
tions [Ag CO (10 mol %), CuX (2.0 equiv), KOH (2.0 equiv), DMF/
DMSO (v/v, 1:1), 130–140 °C) (Table 2). This silver-catalyzed
decarboxylative halogenation of carboxylic acids was shown to
be effective for ortho-nitrobenzoic acids. In addition, not only cop-
per(II) chloride but also copper(II) bromide worked well under the
standard conditions. For instance, 4,5-dimethoxy-2-nitrobenzoic
NO2
1d
COOH
NO
Cl
NO
2
2
1
e
COOH
NO2
Br
1
0
1
NO2
1e
1f
COOH
NO2
Cl
1
Cl
Cl
56 (2k)
NO2
acid 1a reacted with CuBr
mide 2b in 63% yield (Table 2, entry 2). 5-Methoxy-2-nitrobenzoic
acid 1b was a suitable substrate as well in the reaction of CuCl
which afforded the desired product 2c in 83% yield (Table 2, entry
). However, the yield was dramatically decreased when CuBr was
2
leading to the corresponding aryl bro-
OMe
OMe
COOH
Cl
1
1
2
3
—
2
,
OMe
OMe
Cl
1
g
3
2
COOH
used in the reaction (Table 2, entry 4). Other 2-nitrobenzoic acids
with various substituents in the aromatic ring were examined
meanwhile, and the corresponding products were generated as ex-
pected in moderate to good yields (Table 2, entries 5–11). How-
ever, no reaction took place when benzoic acids 1g–1j were
employed in the reaction with copper(II) chloride (Table 2, entries
Cl
Cl
Cl
—
OMe
OMe
Cl
1
h
COOH
14
Trace
Trace
CF3
CF3
1
i
1
2–15). Since directed processes, that is, the use of neighboring
COOH
Cl
groups to control either the reactivity or regioselectivity of organic
15
transformations has been well established,1
3,14
it seems that the
CN
CN
1
j
nitro group in the ortho-position is crucial for the decarboxylative
halogenation reaction.
a
Isolated yield based on carboxylic acid 1.

6
648
Y. Luo et al. / Tetrahedron Letters 51 (2010) 6646–6648
In conclusion, we have demonstrated the concept of decarboxy-
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lative halogenation of carboxylic acids, which provides a new route
for aryl halides formation. Clearly, this reaction has scope limita-
tion currently. However, this new discovery with environmentally
benign process will prompt us for its further development.
1
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Acknowledgment
The financial support from the National Natural Science Foun-
dation of China (No. 20772018) is gratefully acknowledged.
(
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5
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acids: Carboxylic acid (0.2 mmol), copper(II) salts (0.4 mmol), KOH (0.4 mmol),
and Ag
filled with O
syringe. The mixture was heated at 130–140 °C. After completion of reaction as
indicated by TLC, the solvent was diluted by EtOAc (10 mL), washed with
2
009, 48, 9350; (b) Gooben, L. J.; Rodríguez, N.; Lange, P. P.; Linder, C. Angew.
2
CO
3
(10 mol %) were placed in the test tube. The tube was sealed and
. Then DMF (1.0 mL) and DMSO (1.0 mL) were added via the
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2
saturated brine (2 Â 10 mL), and dried by Na
2 4
SO . Evaporation the solvent
followed by purification on silica gel provided the product 2. For details, please
see the ESI.