Synthesis of 3,3-Dihalo-2-oxindoles from 2-Substituted Indoles via Halogenation–Decarboxylation/Desulfonamidation–Oxidation Process

Article abstract of DOI:10.1002/adsc.201600771

A novel one-pot reaction which combines halogenation, decarboxylation/desulfonamidation with oxidation has been developed. Diverse valuable 3,3-dihalo-2-oxindole compounds can be produced rapidly and safely with isolated yields of up to 98% under mild conditions. (Figure presented.).

Full text of DOI:10.1002/adsc.201600771

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DOI: 10.1002/adsc.201600771
Synthesis of 3,3-Dihalo-2-oxindoles from 2-Substituted Indoles via
Halogenation–Decarboxylation/Desulfonamidation–Oxidation
Process
Xiaojian Jiang,a,* Feng Zhang,a Junjie Yang,a Pei Yu,a Peng Yi,a Yewei Sun,a,*
and Yuqiang Wanga
a
Institute of New Drug Research and Guangzhou Key Laboratory of Innovative Chemical Drug Research in
Cardio-cerebrovascular Diseases, Jinan University College of Pharmacy, Guangzhou 510632, Peopleꢀs Republic of China
Fax : +(86)-(0)20-8522-5030; phone: (+86)-(0)20-8522-6152; e-mail: chemjxj2015@jnu.edu.cn or yxy0723@163.com
Received: July 19, 2016; Revised: September 23, 2016; Published online: && &&, 0000
Supporting information for this article can be found under: http://dx.doi.org/10.1002/adsc.201600771.
Abstract: A novel one-pot reaction which combines
halogenation, decarboxylation/desulfonamidation
with oxidation has been developed. Diverse valua-
ble 3,3-dihalo-2-oxindole compounds can be pro-
duced rapidly and safely with isolated yields of up
to 98% under mild conditions.
into 3-haloindole-2-aldehyde in the presence of halo-
gen sources such as NCS and NBS. As such, the halo-
genation and oxidation activities of halogen sources
can be realized in a one-pot reaction. We reasoned
that it might be possible to combine the decarboxyla-
Keywords: decarboxylation; desulfonamidation;
3,3-dihalo-2-oxindoles; halogenation; oxidation
Synthesis of the 3,3-dihalo-2-oxindole unit is of criti-
cal importance in organic, medicinal, and material
chemistry.[1] The use of hypervalent iodine reagents
has proven to be a helpful strategy for the synthesis
of highly valued 3,3-dihalo-2-oxindoles.[2] For exam-
ple, Murphy and co-workers reported a production of
3,3-dichloro-2-oxindole from the toxic and explosive
3-diazo-2-oxindole intermediate and preformed hy-
drazone using PhICl2 (Scheme 1a).[2a,b]
Strategies applying metal halides or bromine to
generate the 3,3-dihalo-2-oxindoles are useful as
well.[1g,3] As shown in Scheme 1b, 3,3-dibromo-2-oxin-
dole can be accomplished by bromination of the cor-
responding indolin-2-one. Other methods, such as de-
oxygenation of isatin using PCl5,[1e,4] ClSO3H,[5]
Scheme 1. Previous pathways for the generation of 3,3-
dihalo-2-oxindoles.
SO2Cl2,[6] and WCl6 have also been documented
[7]
(Scheme 1c).
Halogen sources (E+ sources), such as N-chlorosuc-
cinimide (NCS) and N-bromosuccinimide (NBS) have
oxidative[8,9] and decarboxylative[10] properties besides
being well-known halogenation reagents. We have
previously disclosed a halogenation–oxidation process
in a one-pot manner.[11] Reactions in Scheme 2 dem-
onstrate that 2-hydroxymethylindole can be converted Scheme 2. Halogenation–oxidation reaction.
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of this transformation. Product 2b was obtained with
an isolated yield of 50% when 2.0 equiv. of NCS were
applied (Table 1, entry 2). Since we observed a certain
amount of starting material 1b preserved in this reac-
tion, we suspected that the subsequent decarboxyla-
tion–oxidation process might occur simultaneously.
The yield increased substantially when 3.0 equiv. of
NCS were used (Table 1, entry 3), suggesting that the
Scheme 3. A single-step procedure.
tion process with the current successful halogenation– whole halogenation–decarboxylation–oxidation pro-
oxidation procedure in a single step. As depicted in cess consumes three equiv. of the halogen source. Be-
Scheme 3, treatment of indole-2-carboxylic acid 1a cause 1 equiv. of 1,3-dichloro-5,5-dimethylhydantoin
with halogen sources such as NCS or NBS has indeed (DCDMH) can provide 2 equiv. of chlorine source,
afforded the desired 3,3-dichloro/bromo-2-oxindole 1.5 equiv. of DCDMH were sufficient for this reac-
compounds in a single step procedure.
tion. A yield of up to 93% of product 2b was obtained
Interestingly, we discovered that a sulfonamide when ethyl acetate was used as solvent at room tem-
group could be removed by halogen sources as well. perature (Table 1, entry 4). The yield decreased slight-
Herein, we report a convenient method to rapidly ly when the temperature was reduced to 08C
synthesize useful 3,3-dihalo-2-oxindole compounds (Table 1, entry 5). Ethyl acetate proved to be a suita-
from various indole-2-carboxylic acids and indole-2- ble solvent for this process as other solvents, such as
sulfonamides via a halogenation–decarboxylation/de- CHCl3, DCM, DCE, PhMe, Me2CO, THF, MeCN,
sulfonamidation–oxidation process in a one-pot reac- and Et2O failed to deliver higher yields (Table 1, en-
tion.
tries 6–13). The very low yield of product 2b was
We began our study by using indole-2-carboxylic probably due to the poor solubility of carboxylic acid
acid 1b as a model and the results are shown in substrate 1b in hexane (Table 1, entry 14).
Table 1. Only a trace amount of the desired 3,3-di-
With the optimized conditions chosen (Table 2,
chloro-2-oxindole product 2b was obtained when entry 4), a variety of indole-2-carboxylic acids 1 were
1.0 equiv. of NCS was used (Table 1, entry 1). We subjected to investigation. In general, good to excel-
found that a dominant amount of 3-chloro-substituted lent reaction yields were achieved under the “opti-
indolecarboxylic acid compound was formed, indicat- mized conditions” in 1–2 hours reaction time
ing that the chlorination process is the primary step (Table 2). Up to a 98% isolated yield of product 2a
was obtained when an indolic hydrogen was present
(Table 2, entry 1). Diverse N-substituents were well
tolerated in this chlorination–decarboxylation–oxida-
Table 1. Optimization of substrate 1b.
tion process. For instance, N-substituents containing
alkyl, aryl, allyl, and cyanide groups appeared to be
able to deliver excellent yields (Table 2, entries 2–7
and 9). However, a propargyl substituent only re-
turned a moderate yield (Table 2, entry 8). We suspect
that the alkyne moiety might trigger the substitution
Entry[a] Solvent E+ sources
Temp. [oC] Yield [%][c]
side reactions, leading to a lower yield. For the sub-
strates with electron-deficient R2 and R3 substituents,
good yields could be achieved (Table 2, entries 10, 11
and 13, 14). When a substrate with R2 as the 5-OMe
group was subjected to the reaction, only a moderate
yield could be obtained (Table 2, entry 12), indicating
that an electron-rich Ar system of the indole ring
could induce the substitution side reactions. Surpris-
ingly, N-substituents containing halide moieties, such
as Br, and Cl somehow failed to return the desired
3,3-dichloro-2-oxindole products (Table 2, entries 15
and 16).
Although we had identified the optimal conditions
for the chlorination–decarboxylation–oxidation pro-
cess, the bromination–decarboxylation–oxidation pro-
cess of indole-2-carboxylic acids was less efficient. We
investigated several substrates in the bromination–de-
carboxylation–oxidation process under “optimized
1
2
3
4
5
6
7
8
EtOAc NCS (1.0 equiv.) r.t.
EtOAc NCS (2.0 equiv.) r.t.
EtOAc NCS (3.0 equiv.) r.t.
<5
50
70
93
88
60
40
46
84
46
54
40
52
27
EtOAc DCDMH[b]
EtOAc DCDMH[b]
CHCl3 DCDMH[b]
r.t.
0
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
DCM
DCE
DCDMH[b]
DCDMH[b]
9
PhMe DCDMH[b]
Me2CO DCDMH[b]
10
11
12
13
14
THF
MeCN DCDMH[b]
Et2O
DCDMH[b]
hexane DCDMH[b]
DCDMH[b]
[a]
All reactions were carried out at 0.2 mmol scale in 8 mL
of solvent.
[b]
[c]
1.5 equivalents of E+ source were used.
Isolated yields.
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Table 2. Chlorination–decarboxylation–oxidation of 1.
mide group worked effectively for both the chlorina-
tion–desulfonamidation–oxidation and bromination–
desulfonamidation–oxidation processes, affording the
desired 3,3-dichloro/bromo-2-oxindole products with
high yields.
As shown in Table 4, indole-2-toluenesulfonamide,
indole-2-p-methoxybenzenesulfonamide, and indole-
2-methanesulfonamide substrates afforded the desired
3,3-dichloro-2-oxindole products 2 with good to excel-
lent yields (Table 4, entries 1–3, 5, 6, and 8). However,
the 4-nitrobenzenesulfonamide 4d was somehow less
effective compared to the other sulfonamide sub-
strates (Table 4, entry 4). It is important to note that
the bromination–desulfonamidation–oxidation proce-
dure worked satisfactory for the indole-2-sulfonamide
substrate 4. As we can see from Table 4, excellent
yields were obtained when indole-2-toluenesulfon-
Entry[a]
R1
H
1b Me
1c Ph
1d Bn
1e NC(CH2)3
1f i-Pr
1g allyl
1h progargyl
R2
R3 Product Yield [%][b]
1
2
3
4
5
6
7
8
1a
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
2a
2b
2c
2d
2e
2f
2g
2h
98
93
92
95
83
92
90
62
9
1i
H
H
2i
82
ACHTUNGTRENaNGNU mide and indole-2-methanesulfonamide substrates
were used (Table 4, entries 9, 12–14). Exceptional
cases, including the indole-2-p-methoxybenzenesulfo-
namide and indole-2-p-nitrobenzenesulfonamide sub-
strates, retuned moderate to low yields (Table 4, en-
tries 10 and 11).
For the mechanistic study, we believed that halo-
genation of the C-3 position of the indole ring was
the primary step in this transformation because we re-
covered a dominant amount of 3-haloindole inter-
10
11
12
13
14
15
16
1j
1k
1l
1m
1n
H
H
H
H
H
F
H
H
H
H
Br 2n
H
2j
2k
2l
89[c]
85[c]
63
Cl
OMe
Br
H
H
H
2m
86[c]
78[c]
<5
<5
1o Cl(CH2)3
1p Br(CH2)2
–
–
H
[a]
All reactions were carried out at a 0.2 mmol scale of 1 in
EtOAc (8 mL) in the presence of 1.5 equivalents of
DCDMH.
Isolated yields.
Reactions were run under reflux conditions for 4 h.
[b]
[c]
Table 4. Substrate scope of 4.
conditions 2”.[12] As demonstrated in Table 3, only low
to moderate yields could be achieved in the presence
of 3.0 equiv. of NBS in CHCl3 (Table 3, entries 1–4).
The disappointing results of this bromination–de-
ACHTUNGTRENNUNGcarboxylation–oxidation procedure urged us to try dif-
ferent functional groups in place of the carboxylic
acid group. Gratifyingly, we discovered that a sulfona-
En-
R1, R2
E+ sources
Prod-
uct
Yield
[%][d]
try[a]
1
2
3
4
5
6
7
8
4a, H, 4-MeC6H4
4b, Me, 4-MeC6H4
4c, Me, Me
4d, Me, 4-NO2C6H4
4e, Me, 4-MeOC6H4
4f, Bn, 4-MeC6H4
4g, Ph, 4-MeC6H4
4h, i-Pr, 4-MeC6H4
4b
4d
4e
4c
4f
4g
DCDMH[b]
DCDMH[b]
DCDMH[b]
DCDMH[b]
DCDMH[b]
DCDMH[b]
DCDMH[b]
DCDMH[b]
NBS[c]
2a
2b
2b
2b
2b
2d
2c
2f
3a
3a
3a
3a
3b
3e
70
90
90
50
80
95
40
70
90
60
30
90
93
90
Table 3. Bromination–decarboxylation–oxidation of 1.
9
10
11
12
13
14
NBS[c]
Entry[a]
R1
Product
Yield [%][b]
NBS[c]
1
2
3
4
1b
1d
1e
1h
Me
Bn
NC(CH2)3
propargyl
3a
3b
3c
3d
59
25
35
15
NBS[c]
NBS[c]
NBS[c]
[a]
All reactions were carried out at the 0.2 mmol scale in
EtOAc (8 mL).
1.5 equivalents of DCDMH were used.
3.0 equivalents of NBS were used.
Isolated yields.
[a]
All reactions were carried out at the 0.2 mmol scale of
1 in EtOAc (8 mL) in the presence of 3.0 equivalents of
[b]
[c]
[d]
NBS.
[b]
Isolated yields.
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at room temperature and was monitored by TLC. After re-
moving the solvent under vacuum, the residue was purified
by flash column chromatography (hexanes/EtOAc=3:1) to
yield the corresponding 3,3-dichloro-2-oxindole compounds
2.
General Procedure for Bromination–
Decarboxylation–Oxidation
To a solution of indole-2-carboxylic acid 1 (0.2 mmol,
1.0 equiv.) in chloroform (8 mL) was added N-bromosuccin-
AHCTUNGTREGiNNNU mide (NBS, 106.8 mg, 0.6 mmol, 3.0 equiv.) in one portion.
The reaction mixture was stirred at room temperature and
was monitored by TLC. After removing the solvent under
vacuum, the residue was purified by flash column chroma-
tography (hexanes/EtOAc=3:1) to yield the corresponding
3,3-dibromo-2-oxindole compounds 3.
General Procedure for Chlorination–
Desulfonamidation–Oxidation
Scheme 4. Proposed mechanism for the halogenation–decar-
boxylation/desulfonamidation–oxidation process.
To
a
solution of indole-2-sulfonamide
4
(0.2 mmol,
mediate I in the presence of 1.0 equiv. of the E+
source, such as NCS as discussed before. The nitrogen
atom in substrate 1 or 4 could contribute to the halo-
genation to give an iminium ion intermediate II
(Scheme 4). The cation intermediate II initiated the
decarboxylative or desulfonamidative process to
formed a radical intermediate III. Subsequent oxida-
tion with a trace amount of water in the media fur-
nished the desired quaternary 3,3-dihalo-2-oxindole
products 2 or 3. We also suspected that the decarbox-
ylation process might be a radical process which
might couple to the process of the second halogena-
tion. This hypothesis was partially supported by sever-
al reported halodecarboxylation literature reports.[10]
In summary, we have successfully merged three
1.0 equiv.) in ethyl acetate (8 mL) was added 1,3-dichloro-
5,5-dimethylhydantoin (DCDMH, 59.1 mg, 0.3 mmol,
1.5 equiv.) in one portion. The reaction mixture was stirred
at room temperature and was monitored by TLC. After re-
moving the solvent under vacuum, the residue was purified
by flash column chromatography (hexanes/EtOAc=3:1) to
yield the corresponding 3,3-dichloro-2-oxindole compounds
2.
General Procedure for Bromination–
Desulfonamidation–Oxidation
To
1.0 equiv.) in ethyl acetate (8 mL) was added N-bromosuc-
AHCTUNGTREGcNNNU inimide (NBS, 106.8 mg, 0.6 mmol, 3.0 equiv.) in one por-
tion. The reaction mixture was stirred at room temperature
and was monitored by TLC. After removing the solvent
a
solution of indole-2-sulfonamide
4
(0.2 mmol,
procedures, including halogenation, decarboxylation/ under vacuum, the residue was purified by flash column
chromatography (hexanes/EtOAc=3:1) to yield the corre-
sponding 3,3-dibromo-2-oxindole compounds 3.
desulfonamidation, and oxidation into a single reac-
tion. A novel one-pot reaction therefore has been dis-
covered. This novel strategy allows for the rapid syn-
thesis of diverse valuable 3,3-dichloro/bromo-2-oxin-
dole compounds under mild conditions. An isolated
yield of up to 98% of the desired 3,3-dichloro/bromo-
2-oxindole products can be achieved using inexpen-
sive and safe indole-2-carboxylic acid or indole-2-sul-
fonamide substrates in the presence of inexpensive
halogen sources.
Acknowledgements
This work was supported in part by the National Natural Sci-
ence Foundation of China (Grant No. 21502068).
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6 Synthesis of 3,3-Dihalo-2-oxindoles from 2-Substituted
Indoles via Halogenation–Decarboxylation/
Desulfonamidation–Oxidation Process
Adv. Synth. Catal. 2016, 358, 1 – 6
Xiaojian Jiang,* Feng Zhang, Junjie Yang, Pei Yu, Peng Yi,
Yewei Sun,* Yuqiang Wang
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