Assembly of 2-Arylbenzothiazoles through Three-Component Oxidative Annulation under Transition-Metal-Free Conditions

Article abstract of DOI:10.1021/acs.orglett.7b02168

Highly efficient methods for the synthesis of 2-arylbenzothiazoles and 2-arylnaphtho[2,1-d]thiazoles have been developed. Readily available aromatic amines, benzaldehydes, and elemental sulfur were directly assembled through oxidative annulation and C-H f

Full text of DOI:10.1021/acs.orglett.7b02168

Letter
pubs.acs.org/OrgLett
Assembly of 2‑Arylbenzothiazoles through Three-Component
Oxidative Annulation under Transition-Metal-Free Conditions
Xingzong Che,† Jingjing Jiang,† Fuhong Xiao,† Huawen Huang,*,† and Guo-Jun Deng*,†,‡
†Key Laboratory of Environmentally Friendly Chemistry and Application of Ministry of Education, College of Chemistry, Xiangtan
University, Xiangtan 411105, China
‡Beijing National Laboratory for Molecular Sciences and CAS Key Laboratory of Molecular Recognition and Function Institute of
Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China
S
* Supporting Information
ABSTRACT: Highly efficient methods for the synthesis of 2-
arylbenzothiazoles and 2-arylnaphtho[2,1-d]thiazoles have been
developed. Readily available aromatic amines, benzaldehydes, and
elemental sulfur were directly assembled through oxidative
annulation and C−H functionalization under transition-metal-free
conditions, where DMSO or oxygen served as the oxidant. NH4I or
KI as the catalyst was found to be effective to promote the
transformations to give the annulation products in good to excellent
yields with wide functional group tolerance.
enzothiazole represents one of the privileged motifs in
2-haloaniline substrates is apparently a more straightforward
approach for the construction of benzothiazoles.13 Within our
program on C−S bond formation using elemental sulfur,14
herein, we report the three-component synthesis of 2-
arylbenzothiazoles starting from readily available aromatic
amines, aldehydes, and sulfur powder. The reaction is
successfully achieved in one-pot fashion under simple and facile
reaction conditions.
1,2
Bnatural products and pharmaceuticals.
Specifically, 2-
arylbenzothiazoles compose core structures of many bioactive
compounds such as antitumor agents, antituberculotics,
antiparasitics, and calcium channel antagonists.3 Moreover, 2-
arylbenzothiazoles have been extensively investigated for
applications in industrial dyes, organic luminescent materials,
and agrochemical compounds.4 Accordingly, the development of
efficient methods for the synthesis of 2-arylbenzothiazoles has
attracted considerable interest. The classic routes to 2-
arylbenzothiazoles mainly rely on the condensation of 2-
aminothiophenols with aldehydes and carboxylic acids5 and
oxidative intramolecular cyclization of thiobenzanilides.6 Tran-
sition-metal-catalyzed direct arylation of preexisting benzothia-
zole cores is also an attractive research topic,7 which,
significantly, does not require preactivated heteroaryl coupling
partners.
We initiated our study by examining the reaction of
naphthalen-2-amine (1a), benzaldehyde (2a), and sulfur powder
in DMSO/chlorobenzene (1:3) under ambient atmosphere
(Table 1). When the reaction was run at 140 °C in the presence
of molecular sieves without any initiator, the desired 2-
phenylnaphtho[2,1-d]thiazole 3a was observed in 59% yield
(entry 1). This result encouraged us to screen a series of iodide
catalysts15 for this transformation. When KI and molecular
iodine were added, the reaction yields were improved to 78% and
76%, respectively (entries 2,3). To our delight, 94% yield was
obtained when NH4I was used as the catalyst (entry 4), while
Me4NI also enhanced the reaction yield to 70% (entry 5). Then,
we found solvent played an important role in this three-
component annulation, and much lower yields were obtained
when the reactions were carried out in NMP, toluene, and 1,4-
dioxane (entries 6−8). The addition of DMSO was critical to get
satisfactory yield since only modest yield was observed when
chlorobenzene was used as the sole solvent (entry 9). The ratio
of chlorobenzene/DMSO slightly affected the reaction yield
(entries 10−12), and 3 equiv of DMSO was found to be superior
to give the desired product in 95% yield. The desired product was
generated in 91% yield when decreasing catalyst loading of NH4I
Among the intensive efforts on the synthesis of 2-
arylbenzothiazoles, while the classic methods often use
stoichiometric toxic oxidant or proceed through multistep
operation, the direct arylations rely on expensive metal catalysts.
Furthermore, the prefunctionalized substrates such as 2-
aminothiophenol and its analogues are expensive and not easily
available chemicals.8 From a green chemistry perspective, it is
highly desirable to develop efficient methods for the synthesis of
2-arylbenzothiazoles from readily available starting materials
under mild and facile reaction conditions.9
Elemental sulfur widely exists in nature with nontoxic and
stable properties under ambient conditions. Thus, the direct use
of elemental sulfur as the sulfur source for the construction of
sulfur-containing molecules is believed to be a direct, simple, and
atom-economical approach.10−12 Meanwhile, the use of easily
available anilines instead of more complex 2-aminothiophenol or
Received: July 18, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02168
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Letter
a
a
Table 1. Optimization of Reaction Conditions
Scheme 1. Substrate Scope with Respect to Aldehydes
b
entry
catalyst
solvent
yield
1
2
3
4
5
6
7
8
9
PhCl/DMSO (3:1)
PhCl/DMSO (3:1)
PhCl/DMSO (3:1)
PhCl/DMSO (3:1)
PhCl/DMSO (3:1)
NMP
59
78
76
94
70
49
26
48
54
95
87
84
91
88
40
12
KI
I2
NH4I
Me4NI
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
NH4I
toluene
1,4-dioxane
PhCl
c
10
PhCl/DMSO
11
12
PhCl/DMSO (1:1)
PhCl/DMSO (1:3)
PhCl/DMSO (3:1)
PhCl/DMSO (3:1)
PhCl/DMSO (3:1)
PhCl/DMSO (3:1)
d
13
e
14
f
15
g
16
a
Reaction conditions: 1a (0.22 mmol), 2a (0.2 mmol), S8 (12.8 mg,
0.4 mmol), catalyst (10 mol %), 4 Å MS (100 mg), and solvent (0.4
b
mL, 0.5 M) under air at 140 °C for 24 h. GC yield based on 2a.
c
d
e
DMSO (3 equiv). NH4I (5 mol %). S (0.3 mmol, 1.5 equiv).
f
g
Without 4 Å MS. At 120 °C.
to 5 mol % (entry 13). Reducing the amount of elemental sulfur
to 1.5 equiv also gave good yield of 3a (88%) (entry 14). Much
lower yield was observed when the reaction was carried out in the
absence of molecular sieves (entry 15), and 3a was produced in
12% yield when the reaction was run at 120 °C (entry 16).
With the optimized reaction conditions in hand, the scope and
generality of the three-component reaction was explored
(Scheme 1). In general, the reactions with benzaldehydes
bearing substituents at the para-, meta-, and ortho-positions
worked smoothly to give the corresponding 2-arylnaphthothia-
zole products in excellent yields (3a−3v), and higher product
yields were obtained when aromatic aldehydes bearing electron-
donating groups were used (3d−3g). The position of the
substituent did not affect the reaction yield in most cases. While
para-bromobenzaldehyde afforded the product in 61% (3c), 3j
was obtained in 90% yield when the bromo group was located at
the meta-position. Notably, the hydroxyl functional group was
compatible under the optimized reaction conditions (3r),
affording the corresponding annulation product in 94% yield.
Aromatic aldehydes with strongly electron-withdrawing nitro at
the ortho-position were also highly effective to this reaction,
giving the corresponding products in good to excellent yields
(3s−3v). 2-(Naphthalen-2-yl)naphtho[2,1-d]thiazole 3w was
obtained in 93% yield when 2-naphthaldehyde (2w) was used.
Multisubstituted aldehydes efficiently reacted as well to give the
products in excellent yields (3x−3z). Heteroaromatic aldehydes
were also investigated under the optimized reaction conditions.
While 3aa was obtained in moderate yield when pyridine-2-
carbaldehyde (2aa) was used, very good yield was obtained with
thiophene-2-carbaldehyde (2ab). Anthracen-2-amine was also
well compatible with the present system to generate the 2-
phenylanthra[2,1-d]thiazole 3ac in 83% yield. Remarkably, when
5 mmol scale reactions were run under the optimized conditions,
a
Reaction conditions: 1a (0.22 mmol), 2 (0.2 mmol), S8 (12.8 mg, 0.4
mmol), NH4I (10 mol %), DMSO (3 equiv), 4 Å MS (100 mg), and
PhCl (0.4 mL) under air at 140 °C for 24 h. Five mmol scale
reaction. DMSO (0.1 mL).
b
c
the corresponding products 3a and 3n were obtained in 90% and
91% yields, respectively.
To further examine the scope and limitations of the reaction,
we tested anilines instead of naphthalenamine in this reaction
(Scheme 2). However, much lower yields were obtained when
anilines were employed under the previously optimized reaction
conditions. To get satisfactory yield, we used KI (20 mol %) as
the catalyst with an increased amount of elemental sulfur and
elevated reaction temperature. Notably, the oxygen atmosphere
was better as the oxidant for anilines than DMSO, and the
addition of molecular sieves could not enhance the trans-
formation. Under the modified reaction conditions, moderate to
good yields were generally obtained when alkyl-substituted
anilines were employed (4a−4g). Product 4h was obtained in
84% yield when 4-methoxyaniline was used as the substrate.
Other anilines bearing electron-donating groups such as ethoxy,
phenoxy, and multimethoxy groups afforded the desired
products in modest yields (4i−4m). Unfortunately, aliphatic
aldehydes such as n-octanal did not work in both cases of
naphthalenamine and aniline.
A series of control experiments were performed to gain
mechanistic information on the three-component annulation.
Although 2-aminothiophenol 5 combined with benzaldehyde
and sulfur powder generated the desired benzothiazole 4h in
excellent yield (Scheme 3a), it was not considered as the
B
DOI: 10.1021/acs.orglett.7b02168
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
a
the annulation would not proceed through a radical pathway
since the addition of radical scavengers such as BHT and
TEMPO did not inhibit the desired transformation (Scheme 3f).
Based on the experimental results, a possible reaction
mechanism was proposed (Scheme 4). The condensation of
Scheme 2. Substrate Scope with Respect to Anilines
Scheme 4. Possible Reaction Mechanism
a
Reaction conditions: 1 (0.3 mmol), 2a (0.2 mmol), S8 (32 mg, 1.0
mmol), KI (20 mol %), and NMP (0.5 mL) under O2 at 150 °C.
Scheme 3. Control Experiments
amines and aldehydes is believed to be the initial step, which
generates imine A. Electrophilic attack of sulfur Sn to the ortho-
position of anilines affords intermediate B,16 which proceeds
through elimination of Sn‑1 and deprotonation to give sulfurated
imine C. The final 2-arylbenzothiazoles is formed through
cyclization of C and subsequent oxidative aromatization. The
nucleophilic attack of sulfur Sn to imine would be comple-
mentary,17 whereby intermediate E is afforded. Then, intra-
molecular electrophilic addition of sulfur moiety of E to arene
followed by deprotonation generates dihydrobenzothiazole D,
which likewise delivers final products by oxidation. While
DMSO15c,d served as oxidant in the case of naphthalen-2-amine,
2-arylbenzothiazoles 4 were produced through aerobic oxidation
by oxygen. With respect to the role of the iodide catalyst, we
suspect that it may make the imine intermediate more active and
promote the oxidative aromatization.15i−k
In summary, we have developed a novel oxidative approach for
the synthesis of 2-arylbenzothiazoles and 2-arylnaphthothiazoles
from aromatic amines, aromatic aldehydes, and elemental sulfur
under metal-free conditions. NH4I and KI as the catalyst
combined with DMSO and oxygen as the oxidant were found to
be effective to achieve satisfactory reaction yields. This protocol
provides facile access to 2-arylbenzothiazoles from three readily
available starting materials. The mechanistic studies and further
synthetic applications of this methodology are currently in
progress in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b02168.
intermediate product of the reaction because the direct treatment
of aniline 1h in the absence of aldehyde could not afford 2-
aminothiophenol product (Scheme 3b). Furthermore, the
generation of imine 6 was superior when the reaction was
performed within 1 h (Scheme 3c). Then, we found either imine
6 or thioamide 7 could transfer into the final benzothiazole under
the standard conditions (Scheme 3d,e), which indicated that this
reaction probably involves two different pathways. Additionally,
1
Experimental procedures, characterization data, and H
NMR and 13C NMR spectra for all new products (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: hwhuang@xtu.edu.cn.
C
DOI: 10.1021/acs.orglett.7b02168
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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D.; Shi, Z.-J. Synlett 2008, 2008, 949−957. (g) Lewis, J. C.; Bergman, R.
G.; Ellman, J. A. Acc. Chem. Res. 2008, 41, 1013−1025. (h) Alberico, D.;
Scott, M. E.; Lautens, M. Chem. Rev. 2007, 107, 174−238.
*E-mail: gjdeng@xtu.edu.cn.
ORCID
Huawen Huang: 0000-0001-7079-1299
Guo-Jun Deng: 0000-0003-2759-0314
Notes
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The authors declare no competing financial interest.
(9) For reviews, see: (a) Domling, A.; Wang, W.; Wang, K. Chem. Rev.
ACKNOWLEDGMENTS
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This work was supported by the National Natural Science
Foundation of China (21572194, 21372187), the Collaborative
Innovation Center of New Chemical Technologies for Environ-
mental Benignity and Efficient Resource Utilization, Hunan
Provincial Natural Science Foundation of China (2017JJ3299),
the Hunan Provincial Innovative Foundation for Postgraduate
(CX2017B301), and the Undergraduate-Investigated-Study and
Innovated-Experiment Plan of Hunan Province.
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