Double C-S bond formation: Via C-H bond functionalization: Synthesis of benzothiazoles and naphtho[2,1- d] thiazoles from N -substituted arylamines and elemental sulfur

Article abstract of DOI:10.1039/c7cc07366f

A novel, atom economic, and environmentally friendly method for the synthesis of 2-substituted benzothiazoles and 2-substituted naphtho[2,1-d]thiazoles from N-substituted arylamines and elemental sulfur has been developed under metal-free conditions. The reaction underwent the process of double C-S bond formation through C-H bond functionalization.

Full text of DOI:10.1039/c7cc07366f

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DOI: 10.1039/C7CC07366F
Double C-S Bonds Formation via C-H Bonds Functionalization:
Synthesis of Benzothiazoles and Naphtho[2,1-d]yhiazoles from N-
Substituted Arylamines and Elemental Sulfur
Received 00th January 20xx,
Accepted 00th January 20xx
Xiaoming Zhu, a,b Yuzhong Yang, a Genhua Xiao, a Jianxin Song, a Yun Liang *a and Guobo Deng *a
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel, atom economic, and environmentally friendly method for the framework focus on transition-metal catalyzed intramolecular
synthesis of 2-substituted benzothiazoles and 2-substituted naphtho[2,1- cyclization of thiobenaznilides10 and intermolecular cyclization
d]yhiazoles from N-substituted arylamines and elemental sulfur has been of 2-haloanilides with thiol surrogates.11 Although these
developed under metal-free conditions. The reaction underwent the methods are high-efficiency and diversity, most of them are
process of double C-S bonds formation through C-H bonds limited in rigorous reaction conditions and require multistep
functionalization
.
preparation of starting materials. Accordingly, development of
straightforward, atom-economic and environmentally friendly
approaches for synthesis of benzothiazoles from readily
available precursors is highly warranted. Our group found that
benzaothiazoles could be synthesized via the cleavage of
C(sp3)-H bonds from N-benzyl-2-iodoanilines and K2S (Scheme
1, a).12 Recently, Itami and Segawa reported that
thiophanthrenes could be synthesized via the cleavage of
C(sp2)-H bonds from napthalynes with elemental sulfur
(Scheme 1, b).13 If the C(sp2)-H bonds and C(sp3)-H bonds could
be transformed into C-S bonds in a step reaction, this should
be an ideal strategy for synthesis of sulfur-containing
heterocyclic compounds. Herein, we report a method for
synthesis of benzothiazoles via the cleavage of C(sp2)-H bonds
and C(sp3)-H bonds from easily available N-substituted
arylamines with elemental sulfur (Scheme 1, c).
Sulfur-containing organic compounds play
significant role in a variety of synthetic drugs, natural products,
and functional materials, and represent ubiquitous
a particularly
a
“privileged scaffold”.1 Therefore, the direct construction of C-S
bonds for synthesis of sulfur-containing organic compounds
has received considerable attention, and a lot of methods for
construction of C-S bonds have been developed.2-4 Among
them, the classic strategies involved the transition-metal
catalyzed traditional cross-coupling reaction of ArX (X = Cl, Br,
I, OTf, and B(OH)2) with sulfides3 and oxidative coupling
reaction of C-H bonds with sulfides.4 However, the practical
application of these methods are limited by the expensive
metal catalysts and metal remain. Thus, considerable efforts
have been made to develop metal-free approaches for the
formation of C-S bonds in recent years.5 Despite the overall
efficiency and versatility of this transformation, the double
thiolation reaction of inorganic sulfur source via C-H bonds
functionalization for the straightforward synthesis of sulfur-
containing compounds is less explored6 and still highly
desirable under metal-free conditions.
Benzothiazoles constitute the key moiety of many natural
products and pharmaceutical drugs, and show their good
biological activities.1a,7 Accordingly, much effort has been
made towards the development of diverse synthetic methods
for benzothiazoles.8-11 Classical methods involve the
condensation of 2-aminothiophenols with aldehydes, ketones,
nitriles, acids, and alcohols.9 However, the unstable and readily
oxidative 2-aminothiophenols limited application of these
methods. In recent years, the construction of benzothiazoles
a National & Local Joint Engineering Laboratory for New Petro-chemical Materials
and Fine Utilization of Resources, Key Laboratory of Chemical Biology and
Traditional Chinese Medicine Research, Ministry of Education, Key Laboratory of
the Assembly and Application of Organic Functional Molecules, Hunan Normal
University, Changsha, Hunan 410081, China.
Scheme 1. Methods for the formation of C-S bonds
E-mail: yliang@hunnu.edu.cn; gbdeng@hunnu.edu.cn.
b 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.
Inspired by our previous work on synthesis of sulfur-
containing cyclic compounds using K2S as sulfur source,12,14 we
started our investigation by taking N-benzyl 2-naphthylamine
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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1a and K2S as model substrate in DMSO at 140 oC under hydroxyl and electron-withdrawing groups such as nitryl and
nitrogen atmosphere (Table 1). Intriguingly, the desired fluorine atom substituted-phenyl naphtho[2,1-d]thiazoles
product 2-phenylnaphtho[2,1-d]thiazole 2a was obtained in were given only in moderate yields. To our delight, the iodine
43% yield (entry 1). Encourage by this result, sulfur sources atom was compatible under optimized reaction conditions,
such as elemental sulfur and Na2S2O3 were used to react with affording the corresponding cyclic product in 78% yield, which
1a, and the results demonstrated that elemental sulfur was could be readily used for further transformations. Fortunately,
the best sulfur source and gave 95% yield (entries 2 and 3). the polyclic naphthyl and heterocyclic groups such as 2-furyl,
Then, the solvents including DMF, MeCN, and NMP were 3-thienyl, and 2-pyridyl substituted naphtho[2,1-d]thiazoles
screened (entries 4-6), and exhibited less efficiency in were afforded in 93%, 83%, 77%, 82% yield respectively.
promoting the reaction than DMSO. When the cyclization Further, the α-keto group substituted 2-naphthylamines could
reaction performed under air or oxygen atmosphere (entries 7 react with elemental sulfur in DMSO at 140 oC, and the desired
and 8), the yield of the desired product was decreased. To our products were isolated in moderate to good yields. Notably,
delight, no improvement of the yield was obtained with the the N-cinnamylnaphthalen-2-amine could give the product 2y
reduction of elemental sulfur dosage (entry 9). The yield was in 48% yield. Finally, we found that the inter C(sp3)-H bond
decreased significantly with lowering the reaction temperature could occur sulfur functionalization, and the product of 2-(tert-
(entry 10). Therefore, the optimized yield of the reaction was butyl)naphtho[2,1-d]thiazole was obtained in 55% yield.
obtained using elemental sulfur as sulfur source in DMSO at
140 oC under nitrogen atmosphere for 22 h (entry 9).
Table 2. Synthesis of naphtho[2,1-d]thiazoles a,b
Table 1. Optimization of reaction conditions a
Entry
1
Sulfur source
Solvent
DMSO
DMSO
DMSO
DMF
Isolated yield
K2S
43
95
59
13
37
40
73
39
95
52
2
S8
3
Na2S2O3
4
S8
S8
S8
S8
S8
S8
S8
5
MeCN
NMP
6
7b
8c
9d
10e
DMSO
DMSO
DMSO
DMSO
a Reaction conditions: 1a (0.3 mmol), S8 (0.1125 mmol), DMSO (2 mL), under N2
atmosphere in sealed Schlenk tube, at 140 oC for 22 h. b Air. c O2. d S8 (0.075
mmol). e 120 oC, S8 (0.075 mmol).
With the optimized reaction conditions in hand, we explored
the substituted group scope of 2-naphthylamines (Table 2).
Various N-substituted 2-naphthylamines were successfully
applied to react with elemental sulfur and the corresponding
2-substituted naphtho[2,1-d]thiazoles were obtained in good
to perfect yields. Substituents at different positions of the
benzene ring were tested firstly, which did not affect the
efficiency obviously. For example, para-, meta-, and ortho-
methylbenzyl substituted 2-naphthylamines can give the
corresponding products in 95%, 96%, 98% yields respectively.
The efficiency of the cyclization reaction is not much affected
by the electronic property of the substituted benzyl, which are
affected by their inherent tolerance. The electron-rich groups
such as methyl, methoxyl, and trifluoromethoxyl and electron-
deficient groups such as trifluoromethyl, ester and cyano
substituted benzyl naphtho[2,1-d]thiazoles were obtained in
perfect yields. However, the electron-donating group such as
a Reaction conditions: 1 (0.3 mmol), S8 (0.075 mmol), DMSO (2 mL), N2, at 140 oC.
b Isolated yields.
To further expand the substrate scope, the arylamines such
as anilines, 1-naphthylamines, and 2-anthranylamine instead
of 2-naphthylamines were examined in this cyclization reaction
(Table 3). Under optimized reaction conditions, only electron-
2 | J. Name., 2012, 00, 1-3
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donating groups such as methyl, methoxyl, and N,N-
dimethylamino substituted anilines could smoothly
transformed into the target products, and a close examination
of the results shows that the product yield decreases in the
order: (Me)2N-> MeO- > Me-. These results indicate the
reaction underwent a process of electrophilic attack. It is
worth noting that bromo- and alkynyl-substituted N-benzyl-1-
naphthylamines were also smoothly converted into the
corresponding products in 50%, 25% yield. Furthermore, when
N-benzylanthracen-2-amine was employed as substrate,
polycyclic 2-phenylanthra[2,1-d]thiazole 4g was obtained in 98%
yield.
DMSO act as an oxidant to promote this cyclization reaction
again.
Scheme 3. Controlled experiments
Table 3. Synthesis of 2-phenylbenzothiazolesa,b
Based on the experimental results and previous reports.13,15
a proposed reaction mechanism for the formation of 2a is
demonstrated in Scheme 4. Under the action of DMSO, the 1a
can transform into imine
of elemental sulfur (Sn) to the
gives , which eliminates elemental sulfur (Sn-1) and hydrogen
B. Subsequently, electrophilic attack
α
-position of -naphthylamine
β
C
proton along with the generation of sulfurated imine
Intermolecular nucleophilic cyclization of intermediate
D.
D
would generate thiazoline
aromatization to give
phenylnaphtho[2,1-d]thiazole 2a
E
, which proceeds through oxidative
the desired product 2-
.
a Reaction conditions: 3 (0.3 mmol), S8 (0.075 mmol), DMSO (2 mL), N2, at 140 oC.
b Isolated yields.
As a further demonstration of the application of the
protocol, the benzene-bridged 1,3-bisnaphtho[2,1-d]thiazole
6a and 1-4-bisnaphtho[2,1-d]thiazole 6b with its extended π-
conjulated system were obtained in moderate yield when the
1,3-bisbenzyl- and 1,4-bisbenzyl-bridged naphthylamines was
treated with four equivalent of elemental sulfur under the
standard reaction conditions in one-step synthesis (Scheme 2).
Scheme 4. Proposed reaction mechanism for the formation of 2a
In conclusion, we have demonstrated an efficient and metal-
free method for the synthesis of 2-substituted benzothiazoles
and 2-substituted naphthothiazoles from N-substituted
arylamines and elemental sulfur. In this reaction, the double C-
S bonds are formed through the cleavage of C(sp2)-H bond and
C(sp3)-H bonds. Furthermore, the experimental results indicate
that the DMSO acts as an oxidant and solvent for the
cyclization reaction. Notably, readily available starting material
and tolerance of a wide range of functional groups are the
advantages of this protocol.
Scheme 2. Synthesis benzene-bridged bisnaphtho[2,1-d]thiazoles
To interpret the reaction pathway, several control
experiments were performed (Scheme 3). Firstly, under the
standard reaction conditions, when the reaction of N-benzyl 2-
naphthylamine with elemental sulfur performed in 4 hours,
the imine 2aa could be detected from the reaction mixture by
GC-MS analysis (see the SI for details). Subsequently, the imine
2aa was also detected in the absence of elemental sulfur (see
the SI for details). These results show that DMSO could be
performed as an oxidant in this reaction. Crucially, 2aa can be
transformed into the final product 2-phenylnaphtho[2,1-
d]thiazole 2a in 90% yield. These results indicate this reaction
This work was supported by the Natural Science Foundation
of China (21572051，21602057), Ministry of Education of the
People's Republic of China (213027A), Education Department
of Hunan Province (15A109), Aid Programs for Technology
Innovative Team and Key Discipline in Education Department
of Hunan Province for financial support.
probably involved the imine as the intermediate. Finally, Conflicts of interest
dimethyl sulfide was obviously observed via GC analysis of the
There are no conflicts to declare.
model reaction (see the SI for details). This result proved that
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DOI: 10.1039/C7CC07366F
Double C-S Bonds Formation via C-H Bonds Functionalization:
Synthesis of Benzothiazoles and Naphtho[2,1-d]yhiazoles from
N-Substituted Arylamines and Elemental Sulfur
Xiaoming Zhu, Yuzhong Yang, Genhua Xiao, Jianxin Song, Yun Liang * and Guobo Deng *
An atom economic and environmentally friendly method for the synthesis of benzothiazoles and
naphtho[2,1-d]yhiazoles has been developed under metal-free conditions.