One-pot Sonogashira coupling, hydroamination of alkyne and intramolecular C–H arylation reactions toward the synthesis of indole-fused benzosultams

Article abstract of DOI:10.1016/j.tetlet.2018.04.081

A one-pot Sonogashira coupling, hydroamination of alkyne and C–H arylation reactions for the synthesis of indole-fused benzosultams are described. This method allows access to a variety of indole-fused seven membered benzosultams in good to excellent yiel

Full text of DOI:10.1016/j.tetlet.2018.04.081

Accepted Manuscript
One-pot Sonogashira Coupling, Hydroamination of Alkyne and Intramolecular
C-H Arylation Reactions toward the Synthesis of Indole-fused Benzosultams
Sudarshan Debnath, Shovan Mondal
PII:
S0040-4039(18)30567-7
https://doi.org/10.1016/j.tetlet.2018.04.081
TETL 49944
DOI:
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
9 March 2018
25 April 2018
29 April 2018
Please cite this article as: Debnath, S., Mondal, S., One-pot Sonogashira Coupling, Hydroamination of Alkyne and
Intramolecular C-H Arylation Reactions toward the Synthesis of Indole-fused Benzosultams, Tetrahedron Letters
(2018), doi: https://doi.org/10.1016/j.tetlet.2018.04.081
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Graphical Abstract
One-pot Sonogashira Coupling,
Hydroamination of Alkyne and
Intramolecular C-H Arylation Reactions
toward the Synthesis of Indole-fused
Benzosultams
Leave this area blank for abstract info.
Sudarshan Debnath, Shovan Mondal*
C-H arylation
O
O
Sonogashira
coupling
O
R
I
R3
S
One-pot
N
S
+
O
R
R2
N
(i) Pd(PPh3)2Cl2, CuI,
DMF, Et3N, r.t., 12 h;
(ii) KOAc, 100 oC, 2 h
R3
N
R1
Br
N
R1
Hydroamination

1
Tetrahedron Letters
One-pot Sonogashira Coupling, Hydroamination of Alkyne and Intramolecular
C-H Arylation Reactions toward the Synthesis of Indole-fused Benzosultams
Sudarshan Debnath, Shovan Mondal*
Department of Chemistry, Syamsundar College, Shyamsundar 713424, India,
E-mail: shovanku@gmail.com
ARTICLE INFO
ABSTRACT
Article history:
Received
A one-pot Sonogashira coupling, hydroamination of alkyne and C-H arylation reactions for the
synthesis of indole-fused benzosultams are described. This method allows access to a variety of
Received in revised form
Accepted
Available online
indole-fused seven membered benzosultams in good to excellent yields. The free indolyl nitrogen
containing indole-fused benzosultams are also prepared by this method. The structures of the
synthesized compounds are confirmed by single crystal XRD studies.
Keywords:
2009 Elsevier Ltd. All rights reserved.
Benzosultams
Sonogashira Coupling
Hydroamination
C-H Arylation
Indole
1. Introduction
Indole derivatives are very attractive scaffolds in medicinal
chemistry research due to their natural occurrence and
pharmacological activities.1 At present, there are approximately
1500 indole alkaloids described in the literarure2 which includes
varieties of functionalized indole derivatives. Many of the
biologically active indole derivatives are fused with six-, seven-
and eight-membered ring systems. On the other hand, compounds
containing benzosultam core moiety show a wide spectrum of
bioactivities, such as antiviral, antimicrobial, antileukemic,
anticancer, enzyme inhibition, etc.3,4 Therefore it is expected that
the indole-fused benzosultam derivatives are infused with the
potentiality of becoming pharmacologically active compounds
commodious for drug developments. Although in the literature,
different biologically active heterocycles-fused sultams are
reported such as pyridine-,5 quinoline-,6 Uracil- and Coumarin-
fused7 sultams but there are few reports available on the synthesis
of indole-fused sultums. For instance, Laha and co-workers
synthesized indole-fused sultams by palladium catalyzed
intramolecular oxidative coupling (scheme 1, equ. 1).8 Zhu et al.
reported the construction of indole-fused sultams by palladium
catalyzed diamination of alkynes (scheme 1, equ. 2).9 Therefore,
in our continuous effort in the synthesis of biologically active
heterocycles,10 it is now our challenge to synthesize and optimize
highly efficient and economical synthetic route towards the
formation of novel indole-fused seven-membered benzosultams.
Herein we report our results.
Previous work
Oxidative coupling
10 mol% Pd(OAc)2,
20 mol% CsOPiv,
R
R
N
equ. 1
Laha et al.
Org. Lett. 2015
N
3 equiv AgOAc,
PivOH, 130 oC, 12 h
O
S
S
R1
O
R1
O
O
2
1
O
O
Diamination of alkynes
Pd(TFA)2 (0.1 equiv),
Cu(OTf)2 (0.35 equiv),
nBu4NI (1.0 equiv),
S
R1HN
O
S
R1
N
R2
O
equ. 2
Zhu et al.
Org. Lett. 2015
HOAc (1.0 equiv),
DMSO (0.025 M),
O2, 100 oC, 12 h
R
R2
R
CO2Me
N
R3
N
Me
3
4
This work
C-H arylation
Sonogashira
coupling
O
S
O
(i) Pd(PPh3)2Cl2, CuI,
DMF, Et3N, r.t., 12 h;
O
R
I
R3
N
S
+
O
R3
(ii) KOAc, 100 oC, 2 h
R2
N
R
N
R1
Br
N
5
6
Hydroamination
R1
7
Scheme 1. Some synthetic approaches to indole-fused
benzosultams.
2. Results and discussion
The o-iodoaniline derivatives are one of the most prominent
starting materials that are widely used for the synthesis of indole
nucleus.11 On the other hand, the 3-position of indole are very
susceptible for substitution and based on this feature several
indole derivatives including β-carboline had been prepared in
past few years.12 Based on these chemistry here we prepare a new
class of indole-fused seven membered benzosultams. To access

2
Tetrahedron Letters
the indole-fused benzosultams we followed the synthetic route
according to the retrosynthetic analysis depicted in scheme 2. For
the purpose, the required precursors o-iodoaniline derivatives 510a
and propargylsulfonamides (6 and 10)10b were prepared following
Pd(PPh3)2Cl2 by Pd(OAc)2 for the cyclization of compound 11a
gave almost same yield of compound 7a (table 2: entry 4 and 7).
Br
O2
S
Pd(PPh3)2Cl2, CuI
Et3N, DMF, r.t., 24 h
Me
Me
I
N
SO2
+
Me
Br
6a
our previously reported procedure.
92%
NMe
O2
S
NMe2
N
R3
Me
N
R
O2
S
5a
Br
R
I
R3
N
11a
Path A
N
7
R
R2
S
R3
ref. 8
Scheme 3. Formation of indole-2-methylsulfonamide 11a by
N
R1
O2
N
R1
R2
N
R1
10
domino Sonogashira coupling and hydroamination reaction.
8
5
9
Br
O2
S
Table 2. Synthetic approach towards indole-fused benzosultam
via Pd-catalyzed cyclization of compound 11a.
R3
N
Br
R
O2
S
R3
N
R
I
Path B
N
R3
7
R
S
R2
Br
N
R1
O2
Br
N
R1
R2
O
N
5
6
Pd-catalyzed
S
11
12 R1
SO2
O
cyclization
Me
Me
N
Me
NMe
Scheme 2. Retrosynthetic approach towards indole-fused
N
N
benzosultam.
Me
Me
11a
7a
We started our research for the synthesis of indole-fused
benzosultams by the Pd-catalyzed cyclization of compound 8c.
Compound 8c was prepared according to our previously reported
procedure.10a Various combinations of Pd-catalysts, bases,
additives and solvents were tested in different temperatures for
the successful cyclization of compound 8c to afford the sultam
7c. But none of these combinations gave the fruitful results (table
1).
Entry
1b
Catalysta
Pd(PPh3)4
Base
Et3N
Solvent
Time (h)
Yield (%)
NP
DMF
DMA
DMF
DMF
DMA
Toluene
DMF
DMF
DMF
DMA
DMF
2
1
2c
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(OAc)2
KOAc
Et3N
92
3c
4b
5b,f
6b,f
7b
8d,f
9b,f
10e
11c
1
trace
96
KOAc
KOAc
KOAc
KOAc
KOAc
K2CO3
KOAc
Cs2CO3
1
1
95
1
82
Pd(OAc)2
Pd(OAc)2
1
96
12
5
NP
Table 1. Synthetic approach towards indole-fused benzosultam
via Pd-catalyzed cyclization of compound 8c.
PdCl2
NP
Pd(PPh3)4
Pd(OAc)2
1
NP
1
NP
F
Pd-catalyzed
cyclization
F
Br
a5 mol% catalyst was used in every case; NP – No Product.
SO2
SO2
NMe
b

c

d
e
NMe
Reactions were carried out at 100 C, 120 C, r.t., reflux.
N
N
Me
7c
fTBAB was used as an additive.
Me
8c
Catalysta
We then performed one-pot reaction i.e., the Sonogashira
coupling, hydroamination reaction and intramolecular C-H
arylation in one-pot for the synthesis of indole-fused
benzosultams. The coupling of precursors 5a and 6a was carried
out in the presence of Pd(PPh3)2Cl2-CuI catalyst in DMF using
triethylamine as a base at room temperature for 24 h and after
this KOAc was added and the temperature was increased from
Entry
1b
Base
Solvent
DMF
Time (h)
Yield (%)
NP
Pd(PPh3)4
Et3N
KOAc
Et3N
2
1
2c
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PdCl2
DMA
DMF
trace
NP
3c
4b
5b,f
6b,f
7b
8d,f
9b,f
10e
11c
1
KOAc
KOAc
KOAc
KOAc
KOAc
K2CO3
KOAc
Cs2CO3
DMF
1
<5
DMA
Toluene
DMF
1
<5
1
NP
o
1
<5
room temperature to 100 C for 2 h. This condition gave the
indole-fused benzosultam 7a13 in 93% yield (scheme 4).
DMF
12
5
NP
DMF
NP
Intramolecular
C-H arylation
Pd(PPh3)4
Pd(OAc)2
DMA
DMF
1
NP
Sonogashira
coupling
1
NP
a5 mol% catalyst was used in every case; NP – No Product.
Br
O2
1. Pd(PPh3)2Cl2, CuI,
Et3N, DMF, r.t., 24 h
I
SO2
N
S
b

c

d
e
N
R1
R
R1
Reactions were carried out at 100 C, 120 C, r.t., reflux.
+
R
fTBAB was used as an additive.
NMe2
2. KOAc, 100 oC, 2 h
N
6a,b
5a-f
7a-h
Me
Hydroamination
reaction
After getting the unsatisfactory results following path-A, we
then attempted the synthesis of benzosultams according to path-B
of scheme 2 with compound 11a. Compound 11a was prepared
in excellent yield by the Sonogashira coupling and
hydroamination of alkyne with the compounds o-iodoaniline
derivative 5a (1 equiv.) and propargylsulfonamide 6a (2 equiv.)
in the presence of Pd(PPh3)2Cl2 (5 mol%), CuI (10 mol%), in
DMF-Et3N at room temperature for 24 h (scheme 3). The use of
propargylsulfonamide derivative 6a less than 2 equivalent leads
to the decrease of formation of compound 11a. The Pd-catalyzed
cyclization (intramolecular C-H arylation) of compound 11a to
afford indole-fused benzosultam 7a was then studied according
to the table 1 and the summarized results were depicted in table
2. Just a small change, i.e., the alternation of the position of
bromine from indole nucleus to sulfonamide aryl ring leads to the
brilliant result for the cyclization step to afford the indole-fused
benzosultam. It is worth mentioning that alteration of catalyst
R = 4-Me (5a), 4-F (5b), R1 = Me (6a), Et (6b)
5c
5d
H ( ), 4-CO2Me ( ),
5-F (5e), 3-CO2Me (5f)
Scheme 4. One-pot synthesis of indole fused benzosultam.
After getting the satisfactory result, the coupling of other
precursors i.e. 5b-f with 6a,b gave the various indole fused
benzosultams 7b-h (figure 1). The structure of compound 7a was
confirmed by single crystal XRD analysis (figure 2).14
Me
Me
F
F
SO2
SO2
NEt
SO2
SO2
NEt
NMe
NMe
N
N
N
N
Me
Me
Me
Me
7b, 96%
7a, 93%
7c, 93%
7d, 94%
MeO2C
SO2
NMe
MeO2C
SO2
SO2
NEt
SO2
NMe
F
NMe
N
N
N
N
Me
Me
Me
Me
7e, 90%
7g, 84%
7f, 91%
7h, 79%

3
Figure 1. Summarized results of indole-fused benzosultams.
obtained with excellent yields (scheme 7). For the synthesis of
compound 18, the reaction was passed through one pot
Sonogashira coupling, hydroamination of alkyne, intramolecular
C-H arylation and desulfonation reaction (scheme 8). First
intermediate 19 was formed from 17 and 6a by Sonogashira
coupling and hydroamination reaction. Then complex 20 may be
formed by the oxidative insertion of “Pd” into C-Br bond of
intermediate 19 which may be converted to complex 21 by the
nucleophilic attack of delocalized π-electron. The reductive
elimination of Pd(0) and demesylation in complex 21 may lead to
intermediate 22 which may then rearranged to free indolyl
nitrogen containing indole-fused benzosultam 18.
Figure 2. Ortep diagram of compound 7a
The plausible mechanistic pathway for the formation of
compound 7 is shown in scheme 5. First intermediate 12 was
formed by the Sonogashira coupling between 5 and 6. Then Cu(I)
may be coordinated to the acetylene and sulfone group to form
the complex 13.10a Coordination of Cu(I) with sulfone and triple
bond of acetylene may increase the electrophilicity of acetylenic
carbon to promote the nucleophilic attack of lone pair of NMe2
group which will result the formation of compound 11. The
interaction of Cu(I) with sulfone may play a crucial role for the
cyclization of 12. Here it is worth mentioning that Larock et al.
synthesized 3-iodo-indole derivatives where this type of
hydroamination step was not occurred which might be due to the
absence of sulfone moiety.11a After that, the complex 14 may be
formed by oxidative addition of “Pd” at C-Br bond in compound
11 and then intramolecular arylation may be taken place at 3-
position of indole nucleus to form the intermediate 15. The
reductive elimination of Pd(0) from intermediate 15 may lead to
the complex 16 which was then aromatized to compound 7 in
presence of KOAc.
Br
O2
S
1. Pd(PPh3)2Cl2, CuI,
Et3N, DMF, r.t., 24 h
R
R
I
+
N
SO2
Me
2. KOAc, 100 oC, 2 h
NH
NMe
N
H
18a-d
SO2Me
6a
17a-d
17a
R = Me (
17b
), F ( ),
H (17c), CO2Me (17d)
F
MeO2C
SO2
NMe
Me
SO2
SO2
SO2
NMe
NMe
NMe
N
H
N
H
N
H
N
H
18b
18d
, 88%
, 93%
18a
, 92%
18c
, 90%
Scheme 7. Synthesis of free indolyl nitrogen containing indole-
fused benzosultams
Br
Br
SO2
Pd
Pd
SO2
NMe
R
NMe
SO2
"Pd"
R
R
B
NMe
N
N
N
SO2Me
19
SO2Me
SO2
N
R1
Br
SO2
20
21
H2C
SO2
SO2
N
H
I
N
H
R1
R
R1
-H+
+
steps
R
B
R
NMe2
N
N
Me
-BH+
-SO2=CH2
Me
-Pd(0) (reductive
elimination)
5
6
16
17 6a
+
7
Sonogashira
coupling
reductive
elimination
Indole fused benzosultam
Br
O
SO2
S
R1
SO2
H
N
Pd
R
-H+
H+
O
R
NMe
SO2
NMe
N
R1
R
N
N
N
Me
H
R
22
18
NMe
Me
15
Scheme 8. The probable mechanism for the formation of free
12
Intramolecular
C-H arylation
indolyl nitrogen containing indole-fused benzosultam
Br
R1
CuI
O
S
Br
N
O
3. Conclusion
(I)Cu
Br
Pd
SO2
SO2
N
R1
In conclusion, we have developed an innovative, facile and
efficient one-pot method for the formation of indole fused
"Pd"
R
N
R
Hydroamination
reaction
R
R1
NMe
Me
N
Me
N
Me
I
11
benzosultam
derivatives
via
Sonogashira
coupling,
14
13
hydroamination reaction and intramolecular C-H arylation. Our
method allowed the synthesis of a variety of indole fused
benzosultum derivatives with excellent yields. This method also
afforded the preparation of free indolyl nitrogen containing
indole-fused benzosultams. The structures of the newly
synthesized indole fused benzosultums have been confirmed
from XRD data. The biological activities of the synthesized
indole fused benzosultums are in progress in our laboratory and
will be published in due course.
Scheme 5. The probable mechanism for the one-pot synthesis of
indole fused benzosultam.
We investigated the role of protecting group of N atom in o-
iodoaniline derivatives. The N-methyl-o-iodoaniline derivative
5d' gave the same result as N,N-dimethyl-o-iodoaniline (scheme
6), but N-unsubstituted-o-anilines did not give any cyclized
product.
Br
O2
S
SO2
1. Pd(PPh3)2Cl2, CuI,
Et3N, DMF, r.t., 24 h
MeO2C
I
Acknowledgments
MeO2C
NEt
N
+
Et
2. KOAc, 100 oC, 2 h
N
NHMe
The Science and Engineering Research Board (SERB) of the
Department of Science and Technology (DST) under the
Government of India is gratefully acknowledged for giving a
research grant to Dr. Shovan Mondal through an Early Career
Research Award (Sanction no. ECR/2017/000537). We are
highly grateful to Professor S. K. Chandra, department of
Me
6b
5d'
7f, 90%
Scheme 6. Formation of indole fused benzosultam using N-
methyl-o-iodoaniline derivative.
When N-mesylated o-iodoaniline derivatives 17 are used, the free
indolyl nitrogen containing indole-fused benzosultam 18 was

4
Tetrahedron Letters
Mondal, S.; Debnath, S. Tetrahedron Lett. 2014, 55, 1577. (e)
Debnath, S.; Mondal, S. Synthesis 2016, 48, 3544. (f) Debnath, S.;
Mondal, S. J. Org. Chem. 2015, 80, 3940.
chemistry, Visva-Bharati for help in the studies of X-ray data.
We also thank Miss. Aditi Panja (Junior Research Fellow, Indian
Association for the Cultivation of Science) and Mr. Avijit
Mondal (Syamsundar College, Department of Chemistry) for
mass Spectroscopy.
11. (a) D. Yue, R. C. Larock, Org. Lett. 2004, 6, 1037. (b) Y. Chen, C.
H. Cho, R. C. Larock, Org. Lett. 2009, 11, 173. (c) Y. Chen, N. A.
Markina, R. C. Larock, Tetrahedron, 2009, 65, 8908-8915.
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124, 1172. (b) Richter, J. M.; Whitefield, B. W.; Maimone, T. J.;
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13. To a solution of compound 11a (0.50 mmol) and KOAc (1.50 mmol)
in anhyd DMF (3 mL) N2 gas was bubbled for 10 min. Then Pd(OAc)2
(5 mol%) was added and the whole reaction mixture was stirred at
100 oC for 1 h. Water (10 ml) was then added, and the solution was
extracted with ethyl acetate (3 x 10 ml). The organic extracts were
washed with water (4 x 20 ml) and brine (10 ml), dried over Na2SO4
and the solvent was distilled off to furnish a viscous mass that was
purified by column chromatography on silica gel to yield compound
7a (96%) as white solid. m.p.: 221-223 oC. IR (KBr) 2943, 1505,
1352, 1150 cm-1; 1H NMR (CDCl3, 400 MHz) δ = 8.17-8.15 (m, 1H),
8.08 (dd, J = 8.0, 1.2 Hz, 1H), 7.74 (s, 1H), 7.69-7.65 (m, 1H), 7.35-
7.31 (m, 1H), 7.23−7.21 (m, 1H), 7.11-7.09 (m, 1H), 4.78 (s, 2H),
3.65 (s, 3H), 2.70 (s, 3H), 2.47 (s, 3H); 13C NMR (CDCl3, 100 MHz) δ
= 135.7, 135.6, 133.6, 132.7, 132.7, 130.5, 130.2, 129.8, 126.5,
124.9, 124.1, 119.8, 109.5, 108.9, 50.8, 37.5, 30.0, 21.8; DEPT-135
(100 MHz, CDCl3) δ = 132.7, 130.6, 129.8, 124.9, 124.1, 119.8,
108.9, 50.8, 37.5, 30.0, 21.8; HRMS (ES+): MH+, found 327.1163.
C18H19N2O2S+ requires 327.1162.
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Supplementary Material
Supplementary Copies of NMR spectra of all new compounds
are available. Mass spectra of target compounds are available.
CIF file of compound 7a is also available.

5
Highlights
 A methodology for the preparation of
medicinally important Benzosultams.
 Molecules are completely new and applied
Sonogashira Coupling for synthesis.
 There are no any literature procedures,
similar to present documented molecules.
 These products are important in natural
product synthesis and medicinal chemistry
development.