Micellar catalysis enabled synthesis of indolylbenzothiazoles and their functionalization via Mn(II)-catalyzed C2–H amination using pyridones

Article abstract of DOI:10.1016/j.tetlet.2020.152017

The sustainable synthesis of indolylbenzothiazloes is reported utilising TPGS-750-M enabled nanomicellar catalysis in water. The reactions do not require additional catalysts and/or oxidants and proceed at room temperature. Subsequently, the indole rings were functionalized to construct novel tris-heterocyclic scaffolds via benzothiazole directed Mn(II)-catalyzed C₂–H amination utilizing pyridones as the amine partner.

Full text of DOI:10.1016/j.tetlet.2020.152017

Tetrahedron Letters 61 (2020) 152017
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Tetrahedron Letters
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Micellar catalysis enabled synthesis of indolylbenzothiazoles and their
functionalization via Mn(II)-catalyzed C2–H amination using pyridones
Sanjeev Kumar 1, Dinesh Parshuram Satpute 1, Gargi Nikhil Vaidya 1, Mithilesh Nagpure,
⇑
Shyam Kumar Lokhande, Deepak Meena, Dinesh Kumar
Department of Medicinal Chemistry, National Institute of Pharmaceutical Education and Research (NIPER), Ahmedabad, Palaj, Gandhinagar, 382355 Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The sustainable synthesis of indolylbenzothiazloes is reported utilising TPGS-750-M enabled nanomicel-
lar catalysis in water. The reactions do not require additional catalysts and/or oxidants and proceed at
room temperature. Subsequently, the indole rings were functionalized to construct novel tris-hetero-
cyclic scaffolds via benzothiazole directed Mn(II)-catalyzed C2–H amination utilizing pyridones as the
amine partner.
Received 30 March 2020
Revised 2 May 2020
Accepted 6 May 2020
Available online 11 May 2020
Ó 2020 Elsevier Ltd. All rights reserved.
Keywords:
Micellar catalysis
Indolylbenzothiazoles
Indole C-H amination
Manganese
Green chemistry
The introduction of sustainability into chemical processes is an
ongoing demand due to the adverse effects of manufacturing pro-
cesses on the environment [1]. Consequently, the development of
new processes for the synthesis and functionalization of carbo/
heterocycles is an important area of research. In this regard, the
application of water-mediated synthesis [2] and earth-abundant
metal catalysis [3] have emerged as viable synthetic tools to fulfil
these objectives.
Indoles [4] and benzothiazoles [5] are privileged compounds
constituting the building blocks of various pharmaceuticals, agro-
chemicals, and natural products. This inspired us to merge both
of these frameworks in order to exploit their molecular properties
via a single molecular architecture [6]. Dehydrative cyclocondensa-
tion [7] employing appropriate synthons could be a general strat-
egy for the construction of such target molecules; however,
performing such reactions in water at room temperature in the
absence of additives (catalyst, oxidants, acids/bases) are often chal-
lenging, limiting their practical utility (Figure Scheme 1[8]. Simi-
larly, the direct functionalization of indoles via C-H amination [9]
using earth-abundant catalysis offers significant challenges [10].
Herein, we have described nanomicellar catalysis employing the
‘‘designer” surfactant TPGS-750-M for the synthesis of high value
indolylbenzothiazoles [11] in water at ambient temperature and
their subsequent functionalization to construct novel tris-hetero-
cyclic scaffolds via benzothiazole directed indole C2–H amination
under Mn(II)-catalysis.
Pioneering work by the Lipshutz group [12] and others [13] has
established that designer surfactants empower catalysis by the
aggregation of amphiphiles into nanomicelles leading to facile
organic transformations. Keeping this philosophy intact, our study
began by evaluating various designer surfactants (PTS, TPGS-750-
M, and SPGS-550-M) for the nanomicellar enabled synthesis of 2-
(indol-3-yl)benzothiazole 3a by the treatment of 2-aminothiophe-
nol 1a with indole-3-carboxaldehyde 2a in water at room temper-
ature in the absence of an additional oxidant or catalyst. The
formation of 3a was observed in all cases, however, TPGS-750-M
gave best result (88% isolated yield) after 12 h (Entry 3, Table 1).
Comparative analysis with other commonly utilized surfactants,
including anionic surfactants (SDS and N-Lauroylsarcosinate Na),
non-ionic surfactants (Tritox-X-100 and Transcutol P), and the
cationic surfactant (TBAB) showed that TPGS-750-M is superior
(Entries 5–9, Table 1). To examine the specific role of water, the
model reactions were performed in various organic solvents in
the presence of TPGS-750-M. Except for 1,4-dioxane, only trace
amounts of 3a formation were observed in organic solvents, imply-
ing the specific role of the water-surfactant system. It further sug-
gests the role of TPGS-750-M as nanomicelles in water [12].
The feasibility of a generalized protocol for the synthesis of dif-
ferent (indol-3-yl)benzothiazoles was examined next. As summa-
rized in Scheme 2, electronically different 2- aminothiolphenols
⇑
Corresponding author.
E-mail addresses: dkchem79@gmail.com, dineshk@niperahm.ac.in (D. Kumar).
Authors are contributed equally.
1
https://doi.org/10.1016/j.tetlet.2020.152017
0040-4039/Ó 2020 Elsevier Ltd. All rights reserved.

2
S. Kumar et al. / Tetrahedron Letters 61 (2020) 152017
Scheme 1. Micellar catalysis enabled construction of indolyl benzothiazoles and their functionalization via C2–H amination.
and indole-3-carboxaldehydes gave the desired products (3a–3k)
in moderate to high yields (69–88%) at rt. Various functional
groups were tolerated, including –Cl, –Br, –OMe, –CF3, –CN, and
alkyl. Protected indole-3-caraboxaldehydes (N-Me; 3j and
N-acetyl; 3 k) were also compatible with yields of 72% and
79%, respectively. The protocol was compatible with

S. Kumar et al. / Tetrahedron Letters 61 (2020) 152017
3
S
N
S
S
N
NH
NH
NH
N
; 85%
NH
NH
F3C
Cl
3b
3e
3c
3a
3d
; 83%
; 88%
Cl
Br
OMe
S
S
N
S
N
NH
N
3f
; 81%
; 83%
; 84%
NC
S
N
S
N
S
N
NH
N
NH
NH
N
3h
3k
; 82%
3i
; 82%
3g
; 78%
S
N
S
N
S
N
; 80%
N
H
3j
; 72%
; 69%
4a
O
SH
TPGS-750-M
NH2
S
1a
+
(Aqeous 2% w/w)
Ph
CHO
+
NH
C
N
H
100
12 h
N
H
Ph
6b
Ph
21%, 6a
< 10%,
N
H
5a
Scheme 2. Optimized reaction conditions for the synthesis of indolylbenzothiazoles: 2-aminothiphenol (0.2 mmol), aldehyde (1.2 equiv.), aq. 2% w/w TPGS-750-M, room
temperature. Yields are for isolated, chromatographically purified materials.
indole-2-caraboxaldehyde and gave 4a in 82% yield. It should be
noted that, in the case of 2-phenyl indole-3-carboxaldehdye 5a,
no product formation was observed at rt, and the starting material
was recovered. However, under heating (100 °C), an unexpected
decarbonylation occurred, resulting in the formation of 2-phenyl
indole 6a (21%) along with the desired product 6b (8%).
The scalability of the protocol was demonstrated by performing
the model reaction on a gram scale (Scheme 3 and ESI). The spent

4
S. Kumar et al. / Tetrahedron Letters 61 (2020) 152017
Table 1
Evaluation of various surfactants for the dehydrative-cyclocondensation-oxidation protocol.
Entry
Surfactant (2% w/w)
Solvent
Yield 3a (%)b
1
2
3
4
5
6
7
8
None
PTS
Water
Water
Water
Water
Water
Water
Water
Water
Water
Toluene
1,4-Dioxane
THF
Tracec
56
88
51
40
TPGS-750-M
SPGS-550-M
SDS (SLS)
N-Lauroylsarcosinate Na
Tritox-X-100
Transcutol P
TBAB
TPGS-750-M
TPGS-750-M
TPGS-750-M
TPGS-750-M
TPGS-750-M
37
38
36
37
9
10
11
12
13
14
Tracec
19
Tracec
Tracec
Tracec
DMF
DCE
a
Reagents and conditions: 1a (0.2 mmol) was treated with 2a (0.24 mmol, 1.2 equiv.) in a micellar media (2% w/w) at room
temperature for 12 h. bIsolated yield of 3a. cStarting 1a was recovered.
CHO
optimized
conditions
SH
S
+
Scale up study
Recycle studies
NH
N
NH2
N
H
3a
2a
, yield 75%
1a
(1 g)
optimized
conditions
4th
3rd
0th
1st
2nd
75%
Cycle
3a
1a
+
2a
72%
% Yield of
72%
76%
76%
Scheme 3. Gram scale reaction and recycling studies (1 g scale).
water containing TPGS-750-M was reused for four successive reac-
tions without any significant decrease in the yield (Scheme 3 and
ESI).
containing heterocycles such as piperidines, b-lactams, indolizidi-
nes, and quinolizidines [19]. Using the model reaction involving
3a and pyridone 7a, we explored the Mn-catalysed formation of
tris-heterocyclic system 8a. Optimization of the reaction parame-
ters revealed that the use of MnBr2 (10 mol%), bathophenanthro-
line (12 mol%), and Cu(OAc)2ÁH2O (1 equiv.), in DMF at 100 °C for
24 h gave 8a (NMR, HRMS, and single XRD; CCDC number:
1963300) in 66% yield (Table 2). The use of other Mn-salts resulted
in inferior yields. The utilization of phosphine, bisphosphines, and
NHC ligands instead of bathophenanthroline also decreased the
yield. Other solvents including toluene, 1,4-dioxane, THF, DMSO,
DCE, MeCN, and EtOH (Table 2) were inferior. The addition of either
an acid or base inhibited the reaction or gave lower yields.
The direct functionalization of indoles represents a step- and
atom-economical approach over classical protocols [14]. Although
notable progress has been made in the transition metal catalyzed
directed CAH functionalization of indoles, [15] the direct function-
alization of heteroaryl-substituted indoles with N-heteroarenes to
construct tris-heterocyclic system is still in its infancy [16]. Taking
this as an opportunity, we explored the benzothiazole directed
indole C2–H amination under earth-abundant metal catalysis using
pyridones, an important heteroaromatic scaffold found in natural
products and pharmaceuticals, [17] as an amine partner. This rep-
resents a cooperative functionalization of the N-heterocycles to
construct tris-heterocyclic systems, hybrid molecules containing
indolylbenzothiazole and pyridone motifs [18]. Moreover, 2-
pyridones are also useful for the construction of other nitrogen-
We next examined the scope of this reaction (Scheme 4). The
reaction conditions were compatible with substitution on the ben-
zothiazole part (–CF3, –Cl), however it failed to produce the desired
products with substrates with substitution on the indole

S. Kumar et al. / Tetrahedron Letters 61 (2020) 152017
5
Table 2
Variation from the optimized conditions.a
Entry
Variations from the optimized conditions
Yield 8a (%)b
1
2
3
4
5
6
7
8
None
66
12
28
29
42
39
38
0c
MnF2 instead of MnBr2
MnCl2 instead of MnBr2
MnSO4 instead of MnBr2
Mn(ClO4)2 instead of MnBr2
Mn2Br(CO)5 instead of MnBr2
Mn(OAc)3 instead of MnBr2
AgOAc instead of Cu(OAc)2ÁH2O
p-Benzoquinone instead of Cu(OAc)2ÁH2O
K2S2O8 instead of Cu(OAc)2ÁH2O
PPh3 instead of Bathophen
DPPP instead of Bathophen
RuPhos instead of Bathophen
XantPhos instead of Bathophen
Bipy instead of Bathophen
Ipr.HCl instead of Bathophen
Toluene instead of DMF
1,4-Dioxane instead of DMF
THF instead of DMF
9
0c
10
11
12
13
14
15
16
17
18
19
20
21
22
23
21
41
45
49
41
52
46
12
15
0c
DMSO instead of DMF
DCE instead of DMF
MeCN instead of DMF
EtOH instead of DMF
32
52
24
28
a
Reagents and conditions: 3a (0.2 mmol) was treated with 7a (0.3 mmol, 1.5 equiv.) under different reaction
conditions. bIsolated yield of 8a. cStarting 3a was recovered.
Cl
N
N
S
S
N
N
N
N
H
H
O
O
8b
; 51%
8a
; 66%
F3C
N
N
N
S
S
S
I
Br
N
O
N
N
N
N
N
H
; 54%
H
H
O
O
8d
; 62%
8c
8e
; 58%
Cl
N
N
N
S
S
S
Br
N
N
N
O
N
O
N
N
N
H
H
H
; 58%
O
OMe
8h
; 58%
8f
8g
; 55%
Scheme 4. Synthesis of tris-heterocyclic systems via indole C2-H amination. Yields are for isolated, chromatographically purified materials.

6
S. Kumar et al. / Tetrahedron Letters 61 (2020) 152017
Scheme 5. Preliminary mechanistic investigation for the heteroatom-directed indole C2-H amination.
component. Using representative examples, we showed the com-
patibility of electronically different pyridones (–Br, –I, and –
OMe), however the reaction failed with –OH and –NO2 containing
pyridones. A variety of tautomerizable N-heterocycles were exam-
ined; however promising results were only obtained in the case of
pyrazinones. Other amine partners, including quinazolone, quino-
lone, benzoxazolone, and morpholine failed to produce the desired
products (see ESI for further details).
The reaction of 3-phenyl indole 9a with 7a under the optimized
conditions did not give the desired product 10a indicating the het-
ero-atom directed C2-H amination process. Similarly, no reaction
occurred with N-methyl 3j indicating the possible role of the free

S. Kumar et al. / Tetrahedron Letters 61 (2020) 152017
7
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The authors thank the Department of Pharmaceuticals (DoP),
Ministry of Chemical and Fertilizers and NIPER-Ahmedabad for
financial support. DK acknowledges Prof. Kiran Kalia, Director
NIPER-A for her constant support, encouragement, and motivation.
DK also acknowledges the Science and Engineering Research Board,
Department of Science and Technology (DST-SERB), Government of
India for the award of Ramanujan Fellowship (File No. SB/S2/RJN-
135/2017).
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