K2S as Sulfur Source and DMSO as Carbon Source for the Synthesis of 2-Unsubstituted Benzothiazoles

Article abstract of DOI:10.1021/acs.orglett.0c00994

We describe a three-component reaction of o-iodoanilines with K₂S and DMSO that provides 2-unsubstituted benzothiazoles in moderate to good isolated yields with good functional group tolerance. Electron-rich aromatic amines and o-phenylenediamines instead of o-iodoanilines provided 2-unsubstituted benzothiazoles and 2-unsubstituted benzimidazoles with and without K₂S under similar conditions. Notably, DMSO plays three vital roles: carbon source, solvent, and oxidant.

Full text of DOI:10.1021/acs.orglett.0c00994

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K2S as Sulfur Source and DMSO as Carbon Source for the Synthesis
of 2‑Unsubstituted Benzothiazoles
Xiaoming Zhu,* Fuxing Zhang, Daizhi Kuang, Guobo Deng, Yuan Yang, Jiangxi Yu,* and Yun Liang*
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ABSTRACT: We describe a three-component reaction of o-iodoanilines with K2S and DMSO that provides 2-unsubstituted
benzothiazoles in moderate to good isolated yields with good functional group tolerance. Electron-rich aromatic amines and o-
phenylenediamines instead of o-iodoanilines provided 2-unsubstituted benzothiazoles and 2-unsubstituted benzimidazoles with and
without K2S under similar conditions. Notably, DMSO plays three vital roles: carbon source, solvent, and oxidant.
aminobenzenethiols as well as poor availability of starting
he 2-unsubstituted benzothiazoles are vital core struc-
Ttures for functional molecules applied in biology, materials and harsh reaction conditions. Recently, an efficient
pharmacy, and material science.1 They are also very important
strategy involving inorganic sulfurating reagents was employed
to assemble sulfur-containing heterocycles, particularly 2-
substituted benzothiazoles.5,6 However, using easily available
inorganic sulfurating reagents as sulfur source for a simple and
effective method for the synthesis of 2-unsubstituted
benzothiazoles has not been described.
During this synthesis of thiobenzothiazoles, a small amount
of benzothiazole formation was observed (Scheme 1, eqs 2 and
3).7 Based on this unexpected finding and our interest in
synthesis of sulfur-containing heterocycles using inorganic
sulfurating reagents,7,8 we studied the synthesis of 2-
unsubstituted benzothiazoles from o-iodoanilines, K2S, and
DMSO (Scheme 1, eq 4).
The initial evaluation of the reaction used o-iodoaniline
(1a), K2S, and DMSO to optimize the reaction conditions, and
the results are summarized in Table 1. The desired
benzothiazole product 2a could be obtained in 83% yield
using CuI, H2O, and NH4OAc at 140 °C under nitrogen
atmosphere (entry 1). Some control experiments were carried
out to investigate the role of each component in the reaction.
The results showed that CuI, NH4OAc, and K2S were
indispensable for this reaction (entries 2−4), and a small
precursors for synthesizing 2-substituted benzothiazoles via
C2−H functionalization.2 Accordingly, considerable efforts
have been made toward the development of diverse synthetic
methods for 2-unsubstituted benzothiazoles.3,4 Traditionally,
synthetic strategies for 2-substituted benzothiazoles involved
the condensation of o-aminobenzenethiols with suitable carbon
sources such as DMF,3a−c CH(OEt)3,3d−f CO2,3g−i
CHOOH,3j,k and CH2O3l,m and the deamination reaction of
the 2-aminobenzothiazoles (Scheme 1, eq 1).4 However, their
application is limited by instability and easy oxidation of o-
Scheme 1. Synthesis of of 2-Unsubstituted Benzothiazoles
Received: March 18, 2020
https://dx.doi.org/10.1021/acs.orglett.0c00994
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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a
Table 1. Optimization of the Reaction Conditions
Scheme 2. Synthesis of 2-Unsubstituted Benzothiazoles
a
from o-Iodoanilines
yield of 2a yield of 2ae
entry variation from the standard conditions
(%)
(%)
1
2
3
4
none
without CuI
without NH4OAc
without K2S
83
trace
trace
0
0
0
29
0
5
6
7
8
without H2O
44
0
0
16
72
0
0
0
0
25
0
0
0
0
K2CO3 instead of K2S
NMP instead of DMSO
KOAc instead of NH4OAc
NH4OOCH instead of NH4OAc
NH4I instead of NH4OAc
HOAc (6.equiv) instead of NH4OAc
a
Reaction conditions: 1 (0.2 mmol), K2S (3 equiv), CuI (20 mol %),
NH4OAc (6 equiv), H2O (80 μL), and DMSO (2 mL) in a sealed
Schlenk tube at 140 °C for 10 h under N2 atmosphere.
9
10
11
12
0
12
NH3·H2O (6 equiv) instead of
NH4OAc and H2O
the electron-donating groups such as −CH3 and −OCH3 at
the 4- and 5-positions of o-iodoaniline showed the same
reactivity (2b, 2f, 2j, 2k), whereas the response efficiency of
electron-withdrawing groups such as fluorine and chlorine at 5-
position of o-iodoaniline (1l, 1m) was higher than that at the
4-position of o-iodoaniline (1g, 1h). Notably, the disubstituted
o-iodoanilines (1n, 1o) could give the corresponding products
in moderate yields (2n, 2o). Gratifyingly, 3-iodopyridin-2-
amine and 1-iodonaphthalen-2-amine could be smoothly
transformed into the desired products in 52% and 95% yields,
respectively.
13
14
15
16
17
KOH (2 equiv)
HOAc (2 equiv)
none
none
none
60
44
32
trace
82
0
0
0
0
0
b
c
d
a
Reaction conditions: 1a (0.2 mmol), K2S (3 equiv), CuI (20 mol %),
NH4OAc (6 equiv), H2O (80 μL), and DMSO (2 mL) in a sealed
Schlenk tube at 140 °C for 10 h under N2 atmosphere; isolated yields.
Air. O2. 1a (5 mmol).
b
c
d
In our preliminary work, 2-substituted benzothiazoles and 2-
substituted naphtho[2,1-d]thiazoles compounds could be
synthesized by breaking the C(sp2)−H bond of electron-rich
aromatic amines.8b An ideal strategy would prepare the 2-
unsubstituted benzothiazoles via halogen-free aromatic amines
instead of o-iodoanilines. To our delight, when the electron-
rich aromatic amines (0.3 mmol) were treated with K2S (3
equiv), NH4OAc (4 equiv), H2O (80 μL), and DMSO (2 mL)
under transition-metal-free conditions, the corresponding 2-
unsubstituted benzothiazoles and 2-unsubstituted naphtho-
[2,1-d]thiazoles products 4 were obtained in 30−92% yields
(Scheme 3). Under these conditions, the naphtho[2,1-
d]thiazole and 6-bromonaphtho[1,2-d]thiazole were afforded
in 92% and 69% yields (4a, 4b), respectively. Subsequently, the
electron-rich anilines were examined. The results showed that
amount of water could promote the reaction (entry 5).
Product 2a could not be obtained when the reaction was
conducted using K2CO3 instead of K2S (entry 6) and NMP
instead of DMSO (entry 7). These results indicated that K2S
and DMSO served as the sulfur source and carbon source,
respectively. The yield of 2a did not improve when KOAc,
NH4OOCH, NH4I, or HOAc was used in place of NH4OAc
and NH3·H2O instead of NH4OAc and H2O (entries 8−12).
These results implied that NH4OAc played an important role
in this reaction. When KOH (2 equiv) or HOAc (2 equiv) was
added, the yield of 2a was reduced to 60% and 44%,
respectively (entries 13 and 14). These results showed the
effect of acid on the reaction was greater than base.
Conducting the reaction under air atmosphere or in the
presence of oxygen did not improve the yield. However, 2a was
formed in 82% yield when done on a 5 mmol scale (entry 17,
see the Supporting Information (SI) for details).
Scheme 3. Synthesis of 2-Unsubstituted Benzothiazoles
a
from Halogen-Free Electron-Rich Aromatic Amines
With the optimal reaction conditions in hand, the substrate
scope of o-iodoanilines (1) was next investigated (Scheme 2).
The o-iodoanilines (1b−1i) with various functional groups
were successfully applied to react with K2S and DMSO, and
the corresponding products were obtained in moderate to
good yields (2b−2i). The efficiency of the three-component
reaction was not affected by the electronic properties of the
substituents. For example, the electron-rich groups such as
methyl and electron-deficient groups such as ester-, cyano-,
and nitryl-substituted benzothiazoles were obtained in
excellent yields (2b−2e). However, the electron-donating
groups such as methoxyl as well as electron-withdrawing
groups such as fluorine-, chlorine-, and bromine-substituted
benzothiazoles led to only moderate yields (2f−2i). This might
be due to the instability of these groups.
a
Reaction conditions 3 (0.3 mmol), K2S (3 equiv), NH4OAc (4
Substituents at different positions of the aromatic ring of o-
iodoaniline were also investigated. These results indicated that
equiv), H2O (80 μL), and DMSO (2 mL) in a sealed Schlenk tube at
140 °C for 10 h under N2 atmosphere.
B
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electron-donating groups such as methyl- and methoxyl-
substituted anilines could smoothly transform into the
corresponding products in medium to good yields (4c−4e).
Furthermore, when 2-aminoanthracene was employed as a
substrate, the polycyclic anthra[2,1-d]thiazole 4f was obtained
in moderate yield. Unfortunately, the other anilines without
electron-rich substituents such as aniline and p-nitroaniline
were unsuccessful (3g, 3h).
Scheme 5. Controlled Experiments
The 2-unsubstituted benzimidazoles are useful intermediates
in fine chemicals10 and privileged scaffolds in medicinal
chemistry.11 Accordingly, we were curious about the possibility
of applying this method to the synthesis of 2-unsubstituted
benzimidazoles. Our further study indicated that the
corresponding product 6 was obtained in 35−86% yields
(Scheme 4) when the o-phenylenediamines (0.2 mmol) were
Scheme 4. Synthesis of 2-Unsubstituted Benzimidazoles
a
from o-Phenylenediamines
(eq 4) and 2-mercaptobenzothiazole (J) (eq 5) are possible
intermediates, respectively. Product 2a was not obtained.
These results indicated that the reaction underwent different
mechanisms from our previous report7b perhaps because of the
acid−base effect of NH4OAC.
We subsequently performed a 13C labeling experiment with
CH13COOH, and 2a was not marked by 13C (eq 6). In
addition, the naphtho[2,1-d]thiazole 4a could be isolated in
40% yield in the absence of NH4OAc (eq 7). This result
showed that the carbon source was derived from DMSO.
Furthermore, we used methyl phenyl sulfoxide instead of
DMSO, and benzothiazole (2a) and methyl(phenyl)sulfide
(L) could be isolated in 50% yield and 62.0 mg weight along
with benzenethiol (M) and diphenyl disulfide (N) by GC−MS
(eq 8, see the SI). These results indicated that DMSO is
concurrently the carbon source and oxidant.
To our surprise, when DMSO was replaced by DMSO-d6 in
the standard reaction, only 5% of deuterium labeling product
2a-D1 was obtained (Scheme 6, eq 1). To further verify the
reaction process and the hydrogen source of 2a, we performed
several deuterium-labeling experiments (Scheme 6, eqs 2−4).
When D2O or ND4OOCCD3 was used, 74% and 19% of the
a
Reaction conditions: 5 (0.2 mmol), NH4OAc (6 equiv), H2O (80
μL), and DMSO (2 mL) in a sealed Schlenk tube at 140 °C for 10 h
under N2 atmosphere.
treated with NH4OAc (6 equiv), H2O (80 μL), and DMSO (2
mL). Fortunately, an array of substituents such as −CH3,
−OCH3, −F, −Cl, −Br, −COOCH3, −CN, −NO2, and
−COPh on the phenyl ring at o-phenylenediamine were well
tolerated, and the corresponding products were afforded in
moderate to good yields (6b−6k). In addition, the
disubstituted o-phenylenediamines could smoothly trans-
formed into the target products in moderate yields (6l and
6m). Importantly, the N-substituted-benzene-1,2-diamines,
such as N-methylbenzene-1,2-diamine, N-benzylbenzene-1,2-
diamine, and N-phenylbenzene-1,2-diamine, were also com-
petent substrates; the corresponding N-substituted-2-unsub-
stituted benzimidazoles were obtained in moderate to good
yields (6n−6p).
Scheme 6. Deuterium Labeling Experiments
Some control experiments were performed to investigate the
possible reaction mechanism (Scheme 5). The benzothiazole
(2a), o-aminobenzenethiol (B), dimethyl trisulfide (E), and a
small amount of 2-methylbenzothiazole (K) could be detected
from the reaction mixture by GC−MS analysis when o-
iodoaniline (1a) was reacted with K2S and DMSO for 1.5 h
(eq 1, see the SI). Subsequently, the benzothiazole (2a) could
be synthesized from o-aminobenzenethiol (B) in the absence
of CuI and K2S (eq 2), but the 2-(methylthio)benzo[d]-
thiazole (k) could not be reacted with K2S and DMSO under
standard conditions (eq 3). The results showed that o-
aminobenzenethiol (B) may be the intermediate of the
reaction and CuI only worked in the early stages of the
reaction. The 2-mercaptobenzothiazole (J) could be obtained
in 29% and 25% yields in the absence of NH4OAc (Table 1,
entries 3 and 8)here, 2-(methylthio)benzo[d]thiazole (I)
C
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deuterium-substituted products 2a-D2 and 2a-D3 were
obtained, respectively (eqs 2 and 3). These results showed
that the hydrogen source of the product 2a came from DMSO,
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00994.
■
sı
+
H2O, and NH4 , which reacted with water to complete
hydrogen exchange from H2O. Finally, the benzothiazole 2a,
which had not been deuterium labeled, was used as a substrate
to react with D2O under standard conditions (eq 4). The 2a-
D4 was deuterized with only 60% of hydrogen, which was
lower than the deuterium-substituted percentage ratio of 74%
in 2a-D2 (eq 2). This result indicated that both benzothiazole
2a and the reaction intermediate might lead to the exchange of
deuterium and hydrogen from D2O before the formation of 2a-
D2.
Experimental details, NMR spectra, and details of the
experiments (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Xiaoming Zhu − Key Laboratory of Functional Metal−Organic
Compounds of Hunan Province, Hunan Province Universities
Key Laboratory of Functional Organometallic Materials, College
of Chemistry and Material Science, Hengyang Normal
University, Hengyang, Hunan 421008, China; Key Laboratory
of the Assembly and Application of Organic Functional
Molecules of Hunan Province, Hunan Normal University,
Changsha, Hunan 410081, China; orcid.org/0000-0003-
4086-4674; Email: zxmhnhy@163.com
Based on the controlled experimental results and previous
reports,7a,b,8e,b,9,12 we proposed a plausible reaction mechanism
for the formation of 2a (Scheme 7). The intermediate B was
Scheme 7. Possible Reaction Mechanism
Jiangxi Yu − Key Laboratory of Functional Metal−Organic
Compounds of Hunan Province, Hunan Province Universities
Key Laboratory of Functional Organometallic Materials, College
of Chemistry and Material Science, Hengyang Normal
University, Hengyang, Hunan 421008, China;
Email: hnhyyjx@126.com
Yun Liang − Key Laboratory of the Assembly and Application of
Organic Functional Molecules of Hunan Province, Hunan
Normal University, Changsha, Hunan 410081, China;
orcid.org/0000-0002-2550-9220; Email: yliang@
hunnu.edu.cn
Authors
Fuxing Zhang − Key Laboratory of Functional Metal−Organic
Compounds of Hunan Province, Hunan Province Universities
Key Laboratory of Functional Organometallic Materials, College
of Chemistry and Material Science, Hengyang Normal
University, Hengyang, Hunan 421008, China
Daizhi Kuang − Key Laboratory of Functional Metal−Organic
Compounds of Hunan Province, Hunan Province Universities
Key Laboratory of Functional Organometallic Materials, College
of Chemistry and Material Science, Hengyang Normal
University, Hengyang, Hunan 421008, China
Guobo Deng − Key Laboratory of the Assembly and Application
of Organic Functional Molecules of Hunan Province, Hunan
Normal University, Changsha, Hunan 410081, China;
orcid.org/0000-0002-5470-5706
Yuan Yang − Key Laboratory of the Assembly and Application of
Organic Functional Molecules of Hunan Province, Hunan
Normal University, Changsha, Hunan 410081, China;
orcid.org/0000-0002-9061-1758
first formed via acidification of A produced by a copper-
catalyzed coupling reaction of o-iodoaniline.7a,8e The NH4OAc
activated DMSO to generate C via the Pummerer
reaction.12a−d The imine F was then generated via the
elimination of methane thiol from D,12b,d−f which was
obtained via intermolecular nucleophilic addition from C and
B.12b,e Alternatively, the formaldehyde was formed by
decomposition of DMSO.12h,i F was also provided by
condensation of o-aminobenzenethiol B and formaldehyde.
Meanwhile, F reacted with water to decompose into B and
formaldehyde, and this process possibly completed the
exchange of hydrogen between F and water. Finally, the target
product 2a was formed via cyclization and oxidation of F.
Another possible reaction pathway was also proposed. Here,
D was oxidized to generate imine G,7b,8b which formed H via
intramolecular nucleophilic addition.7b Under the action of
NH4OAc, H was more likely to form 2a by the elimination
reaction of demethylmercaptan7b rather than providing I by
aromatization reaction. Thus, the synthesis of J was completely
inhibited. A more comprehensive reaction mechanism requires
further study.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00994
Notes
The authors declare no competing financial interest.
In conclusion, we report an efficient and practical protocol
for 2-unsubstituted benzothiazoles from o-iodoanilines with
K2S and DMSO under the promotion of NH4OAc.
Furthermore, the controlled experimental results indicate that
the K2S provides the sulfur source and DMSO acts as the
carbon source, oxidant, and solvent for the reaction. Notably,
the advantages of this protocol are tolerance to a wide range of
functional groups and readily available starting materials.
ACKNOWLEDGMENTS
■
This work was supported by the Natural Science Foundation
of China (21572051, 21602057, 21901071, and 21971061),
Scientific Research Projects of Education Department of
Hunan Province (18A002, 18K089, 19A070, and 19B080),
Science and Technology Planning Project of Hunan Province
D
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