Hydrosilane-promoted cyclization of 2-aminothiophenols by CO2to benzothiazoles

Article abstract of DOI:10.1039/c4ra09372k

A new route is presented to synthesize benzothiazoles via cyclization of 2-aminothiophenols by CO₂ in the presence of diethylsilane catalyzed by 1,5-diazabicyclo[4.3.0]non-5-ene, and a series of benzothiazoles were obtained in good yields.

Full text of DOI:10.1039/c4ra09372k

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Hydrosilane-promoted cyclization of 2-
aminothiophenols by CO to benzothiazoles†
2
Cite this: RSC Adv., 2014, 4, 56957
Xiang Gao, Bo Yu, Yanfei Zhao, Leiduan Hao and Zhimin Liu*
Received 28th August 2014
Accepted 27th October 2014
DOI: 10.1039/c4ra09372k
www.rsc.org/advances
5
A new route is presented to synthesize benzothiazoles via cyclization or nitriles (Scheme 1a); (II) reactions of 2-haloanilides with
6
7
of 2-aminothiophenols by CO
2
in the presence of diethylsilane cata- isothiocyanates, carbon disulde and piperidine (Scheme 1b),
8
9
lyzed by 1,5-diazabicyclo[4.3.0]non-5-ene, and a series of benzo- aldehydes and sulfur, carbon disulde and thiols, acid chlo-
10
thiazoles were obtained in good yields.
rides and Lawesson's reagent; (III) cyclization of thio-
formanilides, o-haloarylthioureas or o-iodothiobenzanilides
Scheme 1c) catalyzed by transition metals. These approaches
suffer from their inherent limitations, such as low yields, poor
selectivity, requiring harsh reaction conditions and harmful
reagents, and using metal catalysts, etc. Therefore, it is highly
desirable to develop metal-free and environmentally benign
routes by using renewable and relatively green reagents for the
synthesis of benzothiazoles.
11
12
13
(
Benzothiazole and its derivatives are very important biological
and chemical intermediates due to their broad biological and
pharmaceutical properties. The traditional methods to
1
synthesize benzothiazoles are based on the following reactions
as illustrated in Scheme 1: (I) condensation of 2-amino-
thiophenol with carbonyl or cyano containing compounds like
2
3
4
aryl ketones, aromatic aldehydes, N-acylated benzotriazoles
As a safe, abundant, cheap, and renewable C1 building
14
block, CO
2
has been used in organic synthesis for the
construction of C–N, C–C, C–O, C–H chemical bonds, and value-
1
5
16
17
added chemicals such as ureas, amides, carbonates,
18
19
methanol, formic acid have been synthesized. However, due
to its inherent thermodynamic stability and kinetic inertness,
the activation of CO is difficult. Therefore, much work has been
2
focused on the activation of CO
bases with tertiary nitrogen could react with CO
carbamate species, resulting in the activation of CO
hydrosilylation of CO with hydrosilanes has made the CO
conversion proceed under mild conditions, showing that CO
2
. It was reported that organic
2
to form the
20
2
. Recently,
2
2
2
21
could be activated by hydrosilanes. The combination of
organic base and hydrosilane may provide new possibilities for
efficient CO chemical transformation.
2
As a part of our continual investigation on the chemical
conversion of CO
of C–N, C–C bonds, we herein presented the hydrosilane-
promoted cyclization of 2-aminothiophenols with CO to ben-
2
into value-added chemicals via construction
22
2
Scheme 1 Strategies for the synthesis of benzothiazoles.
zothiazoles catalyzed by organic base (Scheme 1d). In this
protocol, 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) served as a
catalyst for activating CO to react with hydrosilane to form silyl
formate, which further reacted with 2-aminothiophenols,
2
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid,
Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy
of Sciences, Beijing 100190, China. E-mail: liuzm@iccas.ac.cn; Fax: +86-10- producing benzothiazoles. The reported reactions were
6
2562821; Tel: +86-10-62562852
successfully applied to the substituted 2-aminothiophenols,
affording corresponding benzothiazoles in moderate to
†
Electronic supplementary information (ESI) available: Materials, experimental
details, and characterization. See DOI: 10.1039/c4ra09372k
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excellent yields. To the best of our knowledge, this is the rst
The inuences of the CO
2
pressure and temperature on this
work on the synthesis of benzothiazoles via the cyclization of 2- reaction were studied. As the CO pressure was increased from 5
2
aminothiophenols using CO as a C1 resource.
to 7 MPa, the conversion of 2-aminothiophenol was almost
2
2-Aminothiophenol as a model substrate was investigated to unchanged, while the benzothiazole yield decreased from 90%
2
react with CO in the presence of hydrosilane under different to 81% (Table 1, entries 4 and 7), accompanied with the
conditions, and the results are summarized in Table 1. It was detectable benzothiazolone. This indicates that higher pressure
demonstrated that the reaction did not occur in the absence of promoted the reaction of CO with 2-aminothiophenol, leading
2
any organic bases (Table 1, entry 1). To our delight, the reaction to the formation of benzothiazolone as byproduct. Temperature
proceeded to form benzothiazole catalysed by organic bases also had an impact on this reaction. As temperature was low-
ꢀ
including 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and 1,8- ered from 150 to 130 C, the yield of benzothiazole decreased to
diazabicyclo[5.4.0]undec-7-ene (DBU) and DBN (Table 1, entries 81% (Table 1, entries 4 vs. 8), and more benzothiazolone was
2
–4). Especially, benzothiazole was obtained in a yield of >90% produced. Therefore, a CO pressure of 5 MPa and temperature
2
ꢀ
catalysed by DBN in the presence of diethylsilane (Et
methyl-2-pyrrolidinone (NMP) at 150 C (Table 1, entry 4). A
2
SiH
2
) in 1- at 150 C were selected for the synthesis of benzothiazole.
ꢀ
In the above experiments, benzothiazolone was detected as a
trace amount of benzothiazolone as byproduct was detected, byproduct, which was formed by the reaction of 2-amino-
which may result from the reaction of CO with 2-amino- thiophenol and CO . To suppress the formation of this
2
2
thiophenol under the experimental conditions. Compared to byproduct, the inuence of hydrosilane amount on the forma-
DBN, TBD and DBU afforded lower yields of product, which may tion of benzothiazole was investigated. Fig. 1 shows the
be ascribed to the joint effects of the basicity (TBD > DBU > dependence of the yields of the benzothiazole and benzothia-
23
DBN) and steric hindrance (DBN < TBD < DBU), and the zolone on the molar ratio of Et SiH to 2-aminothiophenol. It
2
2
organic base with a relatively less steric hindrance was more was obvious that benzothiazole was the main product under the
active for catalysing this reaction. Hydrosilanes had consider- tested conditions, and its yield was considerably affected by the
able inuences on the cyclization reaction of 2-amino- amount of Et
thiophenol with CO catalyzed by DBN. As PMHS to 5 equiv., the yield of benzothiazole increased from 70% to
poly(methylhydrosiloxane)) was used, both the conversion of 2- 90%, and the yield of benzothiazolone decreased from 22% to
2 2
SiH . Increasing the hydrosilane amount from 1.5
2
(
aminothiophenol and the yield of benzothiazole decreased trace amount. These ndings indicated that the formation of
dramatically (Table 1, entry 5); while triethylsilane (Et SiH) the byproduct could be restrained by enough hydrosilane.
3
afforded a product yield of 29% with 2-aminothiophenol
conversion of 96% (Table 1, entry 6). These results suggested thiophenols with electron-donating or electron-withdrawing
that Et SiH was superior to other alkylsilanes for the synthesis groups were tested to react with CO in the presence of
of benzothiazole under the tested conditions. Consequently, Et SiH using DBN as the catalyst in NMP (1-methyl-2-
DBN and Et SiH were, respectively, chosen as catalyst and pyrrolidinone), and the results are listed in Table 2. To our
hydride donor to investigate the synthesis of benzothiazole delight, all the substrates could be transformed to the corre-
Encouraged by the above results, ve substituted 2-amino-
2
2
2
2
2
2
2
under various conditions.
sponding benzothiazoles. The reaction of 1a with CO provided
2
an isolated 2A yield of 82% under the selected reaction condi-
tions (Table 2, entry 1). The substrate with electron-donating
Table 1 Reaction of 2-aminothiophenol with CO
under different conditions
2
and hydrosilane group (e.g., methyl) showed poor reactivity, affording a 2B
yield of 55% even prolonging the reaction time to 72 h (Table 2,
a
ꢀ
b
c
Entry Base
Hydrosilane P/MPa T/ C Conv. /% Yield /%
1
2
3
4
5
6
7
8
—
TBD
DBU
DBN Et
DBN PMHS
DBN Et
DBN Et
DBN Et
Et
Et
Et
2
2
2
2
SiH
SiH
SiH
SiH
2
2
2
2
5
5
5
5
5
5
7
5
150
150
150
150
150
150
150
130
0
78
88
92
69
96
91
90
0
73
70
90
57
29
81
81
3
2
2
SiH
SiH
SiH
2
2
a
Reaction conditions: 2-aminothiophenol, 2 mmol; base, 1 equiv.;
b
hydrosilane (Si–H 10 equiv.); NMP, 2 mL; 24 h. The loss of starting
material as determined by H NMR (DMSO-d , 400 MHz) using 4-
nitroacetophenone as an internal standard. The conversion to
product as determined by H NMR (DMSO-d
nitroacetophenone as an internal standard.
1
6
c
2 2
Fig. 1 Influence of Et SiH amount on the reaction. Reaction condi-
1
, 400 MHz) using 4- tions: 2-aminothiophenol, 2 mmol; DBN, 1 equiv.; NMP, 2 mL; CO
2
, 5
6
ꢀ
MPa; 150 C; 24 h.
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a
Table 2 Scope of the synthesis of various benzothiazoles
b
Entry Substrate
1
Product
Yield (%)
82
c
2
3
4
5
49/55
1
Fig. 2 H NMR spectra of: (a) pure Et
2
SiH
2
; (b) pure DBN; (c) reaction
71
78
72
2
solution from Et SiH2 reacted with CO2 in the presence of DBN.
Reaction condition: DBN, 1 mmol; Et
C; 24 h. 0.5 mL of DMSO-d solution of the reaction solution was
6
2
SiH
2
, 1.5 equiv.; CO
2
, 5 MPa; 150
ꢀ
transferred into the NMR tube.
pure Et SiH and DBN. This signal was ascribed to silyl formate
2
2
produced from the reaction of Et SiH with CO , suggesting that
2
2
2
it was an intermediate for the synthesis of benzothiazole. It was
reported that formylation reaction could accessibly proceed
between amines and silyl formate due to the relatively low Gibbs
1
6a
6
35
free energy conrmed by the theoretic calculations.
Based on the experimental results and the previous
reports,
16a,24
a possible reaction pathway was proposed, as
illustrated in Scheme 2. Firstly, silyl formate was formed via the
reaction of Et SiH with CO catalyzed by DBN, which was the
key step for the synthesis of benzothiazole. Then the nucleo-
philic NH group in 2-aminothiophenol would easily attack the
a
Reaction conditions: reactant, 2 mmol; DBN, 1 equiv.; Et
2
SiH
2
ꢀ
b
(5 equiv.); NMP, 2 mL; CO
2
, 5 MPa; 150 C; 24 h. Yields of the
c
2
2
2
isolated products. Reaction time 72 h.
2
silyl formate, leading to the formation of the formamide inter-
mediate 1. Subsequently, intermediate 2 was obtained through
the intramolecular nucleophilic attack of SH group to the
carbon atom of intermediate 1. Finally, the dehydration reac-
tion of intermediate 2 took place, producing benzothiazole. It
was worth noticing that the byproduct benzothiazolone was
inevitably formed due to the competitive side reaction of 2-
entry 2). In the case of 2-amino-5-methoxybenzenethiol as the
substrate, the product yield reached 71% (Table 2, entry 3). The
reactivity of the substrates with electron withdrawing groups
was also investigated. Experimental results showed that 2-
amino-5-bromobenzenethiol
chlorothiophenol (1e) could react smoothly with CO
(1d)
and
2-amino-4-
, giving
2
isolated product yields of 78% and 72%, respectively (Table 2,
entries 4 and 5). As for 5-aminolbenzothiazole (1f) as the
substrate, the nitro group was further reduced to amino group,
producing corresponding 5-aminolbenzothiazole (Table 2, entry
2
aminothiophenol and CO under the experimental conditions.
6) with a yield of 35%. It was worth noticing that only trace
amount of substituted benzothiazolones as byproducts were
detected in these reaction processes.
In order to explore the reaction pathway and better under-
stand the reaction mechanism, a control reaction of benzo-
thiazolone with Et SiH was performed in the presence of DBN
2
2
under the same other conditions, and no benzothiazole was
detectable. This indicated that benzothiazolone was not an
1
intermediate for the formation of benzothiazole. H NMR
analysis was employed to identify the possible intermediates
formed during the reaction process. As illustrated in Fig. 2, a
new signal appeared at d ¼ 8.12 ppm when Et
2 2
SiH reacted with
CO in the presence of DBN comparing with H NMR spectra of
1
2
Scheme 2 Possible reaction pathway.
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This work was nancially supported by the National Natural
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