Visible-light photoredox catalytic approach for the direct synthesis of 2-aminobenzothiazoles from anilines

Article abstract of DOI:10.1016/j.tetlet.2020.151700

A novel, highly efficient and convenient approach for the visible-light-promoted direct synthesis of 2-aminobenzothiazoles from anilines and ammonium thiocyanate is presented. The reaction involves addition/cyclization cascade of SCN radical and anilines under photoredox catalysis with Ru(bpy)₃Cl₂. The salient features of the protocol include the utilization of atmospheric oxygen and visible light as clean, inexpensive and sustainable resources at room temperature.

Full text of DOI:10.1016/j.tetlet.2020.151700

Journal Pre-proofs
Visible-light photoredox catalytic approach for the direct synthesis of 2-ami-
nobenzothiazoles from anilines
Manjula Singh, Lal Dhar S. Yadav, Rana Krishna Pal Singh
PII:
S0040-4039(20)30123-4
DOI:
https://doi.org/10.1016/j.tetlet.2020.151700
TETL 151700
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
3 December 2019
29 January 2020
31 January 2020
Please cite this article as: Singh, M., Dhar S. Yadav, L., Krishna Pal Singh, R., Visible-light photoredox catalytic
approach for the direct synthesis of 2-aminobenzothiazoles from anilines, Tetrahedron Letters (2020), doi: https://
doi.org/10.1016/j.tetlet.2020.151700
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Graphical Abstract
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Visible-light photoredox catalytic approach for the
direct synthesis of 2-aminobenzothiazoles from
anilines
Manjula Singh, Lal Dhar S. Yadav, Rana Krishna Pal Singh
NH2
Ru(bpy)3Cl2 ( 2 mol%)
7W blue LEDs, air (O2)
CH3CN, rt, 10-18 h
N
NH2
NH4SCN
S
17 examples
74-96 % yields

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Visible-light photoredox catalytic approach for the direct synthesis of 2-aminobenzothiazoles
from anilines
Manjula Singha, Lal Dhar S. Yadavb*, Rana Krishna Pal Singha*
aElectrochemical laboratory, Department of Chemistry, University of Allahabad, Allahabad 211002, India
bGreen Synthesis Lab, Department of Chemistry, University of Allahabad, Allahabad 211002, India
Corresponding author. Tel.: +91 532 2500652; fax: +91 532 2460533; E-mail address: ldsyadav@hotmail.com (L.D.S. Yadav)
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A novel, highly efficient and convenient approach for the visible-light-promoted direct synthesis
of 2-aminobenzothiazoles from anilines and ammonium thiocyanate is presented. The reaction
involves addition/cyclization cascade of SCN radical and anilines under photoredox catalysis
with Ru(bpy)3Cl2. The salient features of the protocol include the utilization of atmospheric oxygen
and visible light as clean, inexpensive and sustainable resources at room temperature.
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Visible light
Photoredox catalysis
Cyclization
2-Aminobenzothiazoles
Visible-light has great prospects for the development of
sustainable methods for organic synthesis because sunlight is
a clean, inexpensive and unending natural energy source.1–5 In
fact the success of this methodology has been basically
derived from the pioneering work of the research groups of
However, because of its highly hazardous nature, the use
of molecular bromine has been restricted in organic synthesis.
In 2017, Fan and coworkers have been reported an elegant
synthesis of 2-aminobenzothiazoles via iodine-catalyzed and
oxygen-promoted cascade reaction of isothiocyanatobenzenes
with amines.23
MacMillan,1 Yoon2 and Stephenson,3 who
efficiently
utilized Ru(bpy)3Cl2 (bpy=2,2ꞌ-ipyridine) and Ir(drbbpy)3Cl2 as
efficient photoredox catalysts. Advantageously, visible light
photoredox catalysis has enabled
the
utilization
of
Recently, Bollikolla and coworkers have reported the
atmospheric oxygen in organic synthesis. In this process
atmospheric oxygen oxidatively regenerates photocatalysts
from their radical anion to complete the catalytic cycle with
synthesis
of
2-arylaminobenzothiazoles through copper
of
catalyzed domino C–N cross-coupling reactions
arylisothioureas with aryl iodide.24 Most of the protocols
available for the synthesis of 2-aminobenzothiazoles, suffer
from one or more drawbacks, such as the use of harsh
reaction conditions, high temperature, strong acids, toxic
reagents and hazardous liquid bromine. Some of these
drawbacks were overcome by cross-coupling reactions using
transition metals such as Cu,25 Pd,26 Ru27 and Fe28 under
comparatively milder reaction conditions. Recently, Wu and
co-workers have reported Cu(OTf)2-catalyzed synthesis of
2-aminobenzothiazoles from anilines using a strong Lewis
acid BF3.OEt2 as an additive (Scheme 1b).29 However, the
•
concomitant formation of superoxide radical (O2 ‾ ), which
have also been utilized in situ for various oxidative
functionalizations.1e, 2b, 3, 6, 7
The 2-aminobenzothiazole moiety features in many natural
products and bioactive molecules possessing a
broad
spectrum of medicinal and pharmaceutical activities such as,
antitumor,8 antibacterial9a and antirotaviral,9b ubiquitin ligase
inhibitor10 nuclear hormone11 and the adenosine receptor.12
The diverse use of biologically active 2-aminobenzothiazoles
has prompted the development of their synthetic methods
with enhanced efficiency, generality, scope and cost
effectiveness. Accordingly, many efficient strategies were
established for the synthesis of 2-aminobenzothiazoles.13–
development of more convenient environmentally
and
economically more sustainable synthetic strategies for 2-
aminobenzothiazoles are still highly desirable. In view of the
above discussion and our continued research efforts focused
on the development of new synthetic methods employing
visible light photoredox catalysis,30 herein we report the
14
Tradtionally, 2-minobenzothiazoles are synthesized from
anilines using KSCN and molecular bromine as reagents
(Scheme 1a).15-22
visible-light photoredox catalytic
synthesis
of
2-

2
Tetrahedron
bIsolated yield of pure product 2a.
aminobenzothiazoles employing air as an oxidant (Scheme
1c).
cReaction was carried out without catalyst; n.d.= not detected.
dO2 balloon was used.
eReaction was performed in the dark.
f Reaction was performed under N2
Previous work
gReaction was quenched with TEMPO (2 equiv).
(a)15-22
R
NH2
N
S
It was also noted that the same result was obtained on using
O2 balloon instead of an air atmosphere (Table 1, entry 1 vs
5). The use of another photocatalysts such as Eosin Y and
Rose Bengal was not so effective in terms of the reaction
time and yield in comparison to Ru(bpy)3Cl2 (Table 1, entries
1 vs 2 and 3). The optimum amount of catalyst loading
required for the maximum yield in the shortest reaction time
was 2 mol%. On decreasing the amount of photocatalyst
KSCN/Br2 / HCl/H2O
NH2
R
NH2
Cu(OTf)2 (20 mol%)
TMEDA (20 mol%)
BF3.OEt2 (2 equiv)
(b)29
N
S
NH2
R
DMSO, 100o C,
O2, 24 h
R
Present work
from
2 mol% to 1 mol%, the yield was significantly
decreased (Table 1, entry 1 vs 6), whereas on increasing the
amount of catalyst loading from 2 mol% to 3 mol% the
yield remained unchanged (Table 1, entry 1 vs 7). Next, we
optimized the reaction with different solvents and noted
that CH3CN was the best among THF, DMF and CH3CN,
hence it was used throughout the present study (Table 1,
entry 1 vs 8 and 9). The reaction was quenched with 2, 2,
6, 6-tetramethylpiperidyl-1-oxyl (TEMPO) (2 equiv), which
indicates that a radical intermediate involved in the reaction
(Table 1, entry 12).
NH2
N
S
(c)
NH4SCN
NH2
Ru(bpy)3Cl2 ( 2 mol%)
blue LEDs, air (O2)
Scheme 1. Synthesis of 2-aminobenzothiazoles from anilines.
To optimize the reaction conditions, a model reaction of
aniline 1a with NH4SCN was performed using ruthenium as
a photoredox catalyst in a solvent under irradiation with
blue LEDs [ λ = 475.5 nm] in an air atmosphere at room
temperature (Table 1). We obtained the desired product 2a
in 90% yield (Table 1, entry 1). Now, the control experiments
were conducted, which show that a photocatalyst, air (O2) and
light all are essential for the reaction, because in the absence of
any of the reagents/reaction parameters the product was not
detected/formed in traces (Table 1, entries 4, 10 and 11).
Table 2
Substrate scope for the synthesis of 2-aminobenzothiazolesa
NH2
Ru(bpy)3Cl2 ( 2 mol%)
7W blue LEDs, air (O2)
N
NH2
NH4SCN
CH3CN, rt, 10-18 h
S
Table 1
N
N
N
Optimization of reaction conditionsa
NH2
NH2
NH2
S
S
S
2c: 92%; 10 h
N
Ru(bpy)3Cl2 (mol%)
blue LEDs, air (O2)
solvent, rt, 10-18 h
2a: 90%; 10 h
2b: 94%; 10 h
NH2
N
N
N
NH2
NH4SCN
NH2
NH2
PhO
NH2
S
S
S
S
O
1a
2a
2d: 91%; 10 h
2e: 96%; 10 h
2f: 89%; 18 h
N
N
N
Entry
Catalyst
air
Solvent
Time Yield
(h) (%)b
NH2
NH2
NH2
S
S
S
F
Br
Cl
2g: 78%; 18 h
2h: 80%; 18 h
2i: 84%; 18 h
1
Ru(bpy)2 Cl2 (2 mol%)
Eosin Y (2 mol%)
Rose Bengal (2 mol%)
-
+
+
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
THF
10
10
10
10
10
10
10
10
18
18
10
18
90
N
N
N
NH2
NH2
S
NH2
2
76
S
S
NC
NCS
F3C
3
+
65
2j: 74%; 18 h
2k: 79%; 18 h
2l: 78%; 18 h
4
+
n. d.c
90d
N
N
N
NH2
MeO2S
NH2
5
Ru (bpy)2 Cl2 (2 mol%)
Ru (bpy)2 Cl2 (1 mol%)
Ru (bpy)2 Cl2 (3 mol%)
Ru (bpy)2 Cl2 (2 mol%)
Ru (bpy)2 Cl2 (2 mol%)
Ru (bpy)2 Cl2 (2 mol%)
Ru (bpy)2 Cl2 (2 mol%)
Ru (bpy)2 Cl2 (2 mol%)
O2
NH2
Ph
EtO
S
S
S
6
+
59
O
O
2m: 77%; 18 h
2n: 82%; 18 h
2o: 81%; 18 h
7
+
90
NH2
Br
N
N
S
8
+
62
NH2
NH2
S
N
S
9
+
DMF
72
Br
10
11
12
+
CH3CN
CH3CN
CH3CN
tracese
tracesf
tracesg
2p + 2p': 87% (5:3), 18 h
2q: 83%, 10 h
N2
+
aFor experimental procedure, see ref. 32
bAll compounds are known and were characterized by comparison of their
spectral data with those reported in the literature.33
cYields of isolated pure compounds 2.
aReaction conditions: aniline 1a (1.0 mmol), NH4SCN (1.0 mmol),
Ru(bpy)3Cl2 (2 mol%), base (2 equiv), solvent (3 mL), open to air
(without bubbling air) irradiation at rt through the flask’s bottom
side using Liuxeon Rebel high power blue LEDs (4.45 W , λ
max = 447.5 nm).

3
With the optimized reaction conditions in hand, we
surveyed the scope of the present protocol across a series
of anilines 1 incorporating various substitutents, such as CH3,
i-Pr, t-Bu, OCH3, PhO, F, Cl, Br, CF3, CN, SCN, CO2Et,
MeO2S and PhCO. The reaction worked well in all the
cases, affording the corresponding 2-aminobenzothiazoles 2
in moderate to excellent yields (Table 2). Anilines 1 bearing
an electron-donating group appeared to react faster and to
afford slightly higher yields of 2-aminobenzothiazoles
compared with anilines 1 having an electron-withdrawing
group (2b-e vs 2g-o). The regioselectivity of the present
intramolecular cyclization was investigated with a aniline
bearing a m-bromo group, and the reaction afforded a mixture
of regioisomers 2p and 2p′ in a ratio of 5:3, respectively.
The reaction is also compatible with a fused ring system such as
2-naphthylamine (2q).
References and notes
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SCN
SCN NH4 ,
+
NH2
NH2
SCN
SCN
NH4SCN
Ru (I)
A
1
H
B
O
2 (air)
Blue LEDs (7 W)
CH3CN, rt
O2(air), 10 h -18 h
Ru(bpy)3Cl2
Ru (II)*
O2
O
Ru (II)
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H
N
NH2
SCN
NH2
SC
N
Cyclization
NH2
NH
-H+
S
S
N
2
E
H
C
D
Scheme 3. A plausible mechanism for the formation of 2-
2.
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aminobenzothiazoles
2
from anilines 1 and NH4SCN is
depicted in Scheme 3. On irradiation with visible light Ru(II)
undergoes excitation to Ru(II)⁄ followed by single electron
transfer (SET) with SCN‾ to afford the SCN radical A and
Ru(I). The photoredox cycle is completed by the oxidation
of Ru(I) to Ru(II) with atmospheric oxygen. The formation
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•
which is
deprotonation of C forms intermediate D.
intramolecular cyclization involving the
oxidized to give cationic intermediate C. The
Then, the
attack of
nucleophilic amino group on the thiocyanate group produces
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the desired product 2.
In summary, we have developed
a
highly efficient,
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operationally convenient strategy for the direct synthesis of 2-
aminobenzothiazoles from anilines and ammonium thiocyanate.
The tandem reactions successfully afforded the corresponding 2-
aminobenzothiazoles in moderate to excellent yields under
visible light photoredox catalysis with Ru(II) and an air
atmosphere. Advantageously, the protocol utilizes visible light
and atmospheric oxygen as green and sustainable natural
resources at room temperature.
7.
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4
Tetrahedron
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32. General procedure for the synthesis of 2-aminobenzothiazoles 2:
A mixture of aniline 1 (1 mmol), NH4SCN 2 (1.0 mmol),
ruthinium (2 mol%), and CH3CN (3 mL) was taken in a flask
open to air and stirred at rt for 10-18 h (Table 2). After
completion of the reaction (monitored by TLC), water (5 mL)
was added and the mixture was extracted with ethyl acetate
(3 × 5 mL). The combined organic phase was dried over
anhydrous Na2SO4, filtered, and evaporated under reduced
pressure. The resulting crude product was purified by silica
gel chromatography using a mixture of hexane/ethyl acetate
(4:1) as eluent to afford an analytically pure sample of product
2. All the compounds 2 are known and were characterized by
comparison of their spectral data with those reported in the
literature.33 Characterization data of representative compounds 2
are given below with relevant reference:
Compound 2a:33b 1H NMR (400 MHz, CDCl3) δ: 7.59 (d, J = 8.3
Hz, 1H), 7.55 (d, J = 8.2 Hz, 1H), 7.31(t, J = 7.6 Hz, 1H), 7.12 (t,
J = 7.5 Hz, 1H), 5.67 (bs, 2H, NH2), 13C NMR (100 MHz, CDCl3)
δ: 166.4, 152.3, 131.8, 126.3, 122.5, 121.2, 119.5. HRMS (EI)
Calcd for C7H6N2S: 150.0252, found, 150.0256.
Compound 2f: 33a 1H NMR (400 MHz, CDCl3) δ: 7.42 (d, J = 8.7
Hz, 1H), 7.33 (dt, J = 10.6, 5.1 Hz, 3H), 7.07 (t, J = 7.4Hz, 1H),
6.95 (d, J = 8.5 Hz, 3H), 6.89 (s, 2H); 13C NMR (100 MHz,
CDCl3) δ: 167.6, 159.9, 152.7, 150.8, 134.1, 131.1, 123.9, 120.4,
119.0, 118.9,113.3 HRMS (EI) Calcd for C13H1oN2OS: 242.0514,
found, 242.0511.
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Compound 2h:33a 1H NMR (400 MHz, CDCl3) δ: 8.24 (d, J = 1.8
Hz, 1H), 7.79 (m, 1H), 7.54 (d, J = 8.5 Hz, 1H), 7.33 (s, 2H), 3.10
(s, 3H); 13C NMR (100 MHz, CDCl3) δ: 171.5, 158.5,
135.1, 133.5, 126.5, 122.2, 119.5, 45.5. HRMS (EI) Calcd for
C8H8N2OS: 228.0027, found, 228.0031.
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82: 9312;
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