Metal sulfide: An efficient promoter for the synthesis of 2-mercaptobenzothiazoles from 2-haloanilines and carbon disulfide

Article abstract of DOI:10.1080/00397911.2017.1356927

A convenient method has been developed for the preparation of a variety of 2-mercaptobenzothiazoles from 2-haloanilines and CS₂ mediated by metal sulfide. In this reaction, 2-haloanilines reacted with CS₂ in the presence of Na₂S · 9H₂O to form 2-mercaptobenzothiazoles. Na₂S · 9H₂O functioned both as an activator of CS₂ and as a base. Furthermore, NMR analysis was used to identify the different reaction mechanisms of 2-haloanilines and CS₂ mediated by Na₂S or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), which demonstrated that Na₂S interacted only with CS₂, while DBU reacted with both 2-iodoaniline and CS₂.

Full text of DOI:10.1080/00397911.2017.1356927

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
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ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
Metal sulfide: An efficient promoter for the
synthesis of 2-mercaptobenzothiazoles from 2-
haloanilines and carbon disulfide
Tianmiao Zhang, Weijing Qin, Ning Zhu, Limin Han, Liubo Wang & Hailong
Hong
To cite this article: Tianmiao Zhang, Weijing Qin, Ning Zhu, Limin Han, Liubo Wang
&
Hailong Hong (2017): Metal sulfide: An efficient promoter for the synthesis of 2-
mercaptobenzothiazoles from 2-haloanilines and carbon disulfide, Synthetic Communications, DOI:
0.1080/00397911.2017.1356927
1
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Date: 30 July 2017, At: 22:59

Metal sulfide: An efficient promoter for the synthesis of 2-
mercaptobenzothiazoles from 2-haloanilines and carbon
disulfide
Tianmiao Zhang
Chemical Engineering College, Inner Mongolia University of Technology, Hohhot, P. R. China
Weijing Qin
Chemical Engineering College, Inner Mongolia University of Technology, Hohhot, P. R. China
Ning Zhu
Chemical Engineering College, Inner Mongolia University of Technology, Hohhot, P. R. China
Limin Han*
Chemical Engineering College, Inner Mongolia University of Technology, Hohhot, P. R. China
Liubo Wang
Chemical Engineering College, Inner Mongolia University of Technology, Hohhot, P. R. China
Hailong Hong
Chemical Engineering College, Inner Mongolia University of Technology, Hohhot, P. R. China
1

Address correspondence to Limin Han. E-mail: hanlimin_442@hotmail.com
ABSTRACT
A convenient method has been developed for the preparation of a variety of 2-
mercaptobenzothiazoles from 2-haloanilines and CS
-haloanilines reacted with CS inꢀ the presence of Na
Na Sꢀ·9H O functioned both as an activator of CS and as a base. Furthermore, NMR analysis was
used to identify the different reaction mechanisms of 2-haloanilines and CS mediated by Na Sꢀ or
,8-diazabicyclo[5.4.0]undec-7-ene (DBU), which demonstrated that Na Sꢀ interacted only with
CS , while DBU reacted with both 2-iodoaniline and CS
2
mediated by metal sulfide. In this reaction,
2
2
2
Sꢀ·9H O to form 2-mercaptobenzothiazoles.
2
2
2
2
2
2
1
2
2
2
.
GRAPHICAL ABSTRACT
2

KEYWORDS: 2-haloanilines, 2-mercaptobenzothiazoles, carbon disulfide, metal sulfide,
reaction mechanism
Introduction
2
-Mercaptobenzothiazole derivatives (MBTs), which were prepared for the first time in
[
1]
1
887 by Hofmann, have been widely studied for industrial use and medicinal researches (Figure
[
2]
1
). In industry, MBTs (a and b) can be used as corrosion inhibitors of coatings, a fungicide and
[
3]
[4],
bactericide in leather processing, and a vulcanization accelerator in the production of rubber
[
5]
and sulfurized carbon materials.[6] In medicinal research, MBTs have frequently been
synthesized as important intermediates for preparing bioactive compounds[7] such as dual
[
8]
[9],
antagonists for the human CCR1 and CCR3 receptors (c), heat shock protein-90 inhibitors (d),
[
10]
and protoporphyrinogen IX oxidase inhibitors (e).[11] Moreover, the tripodal benzothiazole
derivative (f) has been developed as a sensitive chemosensor for the determination of Ba2 + .[12]
Due to the significant application of 2-thio-substituted benzothiazoles, many research groups are
exploring various efficient protocols for preparing MBTs (Scheme 1).
Traditionally, MBTs are obtained directly from the reaction of aniline, carbon disulfide
and sulfur at high temperatures and pressures (route a), which is used in industry practice.[13], [14]
MBTs are also prepared by the reaction of 2-aminothiophenol with carbon disulfide in the presence
of organic/inorganic base[15], [16] or ZnO/Al
O
catalyst[17] (route b). However, 2-aminothiophenols
2
3
are readily oxidized to their corresponding disulfide and not always easily available. Thus, 2,2’-
3

disulfanediyldianiline is used as the staring material to react with carbon disulfide to prepare MBTs
in our previously reported method (route c).[18], [19] Moreover, MBTs are prepared by the reaction
of 2-chlorobenzothiazole with Na
2
S
2
O
,[20] 1,2-ethanedithiol[21] or thiourea[22–24] (route d). But the
3
raw material 2-chlorobenzothiazole was usually prepared from MBT, which suggests this
methodology could not be used in practice. MBT could also be synthesized through the one-step
reaction from 2-chloronitrobenzenes and carbon disulfide mediated by NaHS[25] or Na
Sꢀ/S[26]
2
(route e). Recently, it is reported that 2-haloanilines (2-F, Cl or Br, route f) could react with
potassium O-ethyl dithiocarbonate to produce MBTs under heat[27] or microwave irradiation.[28]
However, the reactions of 2-iodoanilines and potassium O-ethyl dithiocarbonate are not reported.
Thus, our group successfully has synthesized MBTs by the reaction of 2-iodoanilines with
potassium O-ethyl dithiocarbonate catalyzed via copper[29] or iron.[30] Furthermore MBTs could
be prepared by a three-component reaction via 2-iodoanilines, carbon disulfide and amines[31] or
2
-iodoanilines, isocyanide and potassium sulfide.[32] Another way to prepare MBTs is the reaction
of 2-haloanilines with carbon disulfide via recyclable copper ferrite (CuFe
2
O
4
) nanoparticles[33] or
sodium hydride.[34] Xi’s group successfully prepared a variety of MBTs by the addition of DBU
to promote the tandem reaction of 2-haloanilines and carbon disulfide in the absence of any
catalyst.[35] However, the underlying mechanism of DBU promoted tandem reaction of 2-
haloanilines and carbon disulfide is still unclear. Encouraged by this highly efficient synthetic
method, we investigated other low-cost and more easily available reagent to replace DBU in the
synthesis of 2-mercaptobenzothiazoles.
4

Na
2
Sꢀ·9H
2
O is a widely available inorganic base, which could react with carbon disulfide
CS Sꢀ·9H O was used as an efficient
).[36], [37] Therefore, Na
to form sodium thiocarbonate (Na
2
3
2
2
promoter to synthesize 2-mercaptobenzothiazoles from 2-haloanilines and carbon disulfide in this
work. Furthermore, the different reaction mechanisms of 2-haloanilines and carbon disulfide
promoted by Na
2
Sꢀ·9H O or DBU were also discussed.
2
Results and Discussion
Initially, we used 2-iodoaniline 1a and carbon disulfide as the starting materials to prepare
2
-mercaptobenzothiazole in DMF. However, little 2-mercaptobenzothiazole was obtained after the
mixture solution was stirred for 12hꢀ at 110 °C (Table 1, entry 1). When 0.5 equiv. sodium sulfide
was added, the yield of 2a was improved significantly (entry 2). The experimental results
demonstrated that sodium sulfide was important in the reaction of 1a and carbon disulfide. Thus,
sodium sulfide was selected as a promoter for the reaction of 1a and carbon disulfide. The yield
of 2a increased with increasing amount of Na
Na Sꢀ was used. Further increases in the amount of Na
entries 5-6). Additionally, more CS needed to be added to the reaction mixture due to the low
boiling point and high-volatility of CS . When the amount of CS was 5 equiv. to 1a, the highest
2
Sꢀ (entries 3-5) and reached 76% when 2 equiv.
2
2
Sꢀ did not affect the yield of 2a notably
(
2
2
2
yield of 2a was obtained (entries 5, 7). Therefore, the optimal feeding ratio of 1a, sodium sulfide
and carbon disulfide was 1: 2: 5. Moreover, we found that the higher or lower reaction temperature
than 110 °C considerably reduced the yield of product 2a (entries 5, 8-10). After investigating the
5

effect of solvents, DMF was found to give the highest yield compared with glycol, 1,4-dioxane or
toluene (entries 11-13). We speculated the protic solvent glycol weakened the nucleophilicity of
sulfur and 1,4-dioxane or toluene reduced the solubility of sodium sulfide, which reduced the yield
of 2a. Furthermore, to test the efficiency of other metal sulfides in this reaction process, we found
that K
entries 14-15), which demonstrated that most metal sulfide could promote this reaction smoothly.
Finally, the optimal conditions for this transformation were identified as 1a, Na Sꢀ (2 equiv.) and
CS (5 equiv.) in DMF at 110 °C.
2
Sꢀ and NaHS could also promote the reaction of 1a and CS to form 2a in moderate yields
2
(
2
2
To demonstrate the generality of this approach for the synthesis of 2-
mercaptobenzothiazoles, various 2-haloaniline derivatives were used to react with CS at 110 °C
2
in DMF, which are summarized in Table 2. The results demonstrated that 2-F, or 2-Br substituted
anilines could also produce the product 2a (entries 2-3). Unfortunately, 2-chloroaniline could not
produce the corresponding product (entry 4). Significantly, under the same reaction conditions, 2-
iodoanilines bearing a weaker electron-withdrawing (3-Cl, 4-Cl, 4-F) or weaker electron-donating
group (4-methyl) afforded better yields than strong electron-withdrawing (4-cyano, 4-
trifluoromethyl) or strong electron-donating group (4-methoxyl) (entries 5-11). It is noteworthy
that 2-bromo-4-chloroaniline reacted with CS
yield (entry 12).
2
and produced the target product 2c in an excellent
6

To verify how Na
experiments were carried out. Firstly, we knew that 2-iodoaniline could not react with CS
absence of Na Sꢀ (Table 1, entry 1; Scheme 2a), which demonstrated that Na Sꢀ was important to
promote this reaction. Secondly, our experimental result indicated that 2-iodoaniline could not
react with Na Sꢀ directly at 110 °C to form the intermediate 2-aminobenzothiol or sodium 2-
aminobenzothiolate, which indicated that Na Sꢀ reacted with CS (Scheme 2b). Therefore, the
2
Sꢀ promoted the reaction of 2-iodoanilines with CS
2
, some control
2
inꢀ the
2
2
2
2
2
mixture solution of CS
2
and Na
2
Sꢀ in DMF was analyzed by 13Cꢀ NMR, and a new peak
1
3
13
(
214.6ppmꢀ) was present in the Cꢀ NMR spectrum compared to the Cꢀ NMR spectrum of CS
2
1
3
(
Figure 2a,b). The Cꢀ NMR experiment showed that CS
2
might react with Na
2
Sꢀ to form sodium
thiocarbonate, which was accordant with the corresponding report (Scheme 2c).[36]
In order to map out the different reaction mechanisms of 2-iodoaniline 1a and CS
2
mediated by Na
2
Sꢀ or DBU,[35] NMR was employed to analyze the interactions between the raw
1
materials and DBU or Na
2
Sꢀ. The H NMR spectrum of 1a showed the NH proton signal was
2
moved from δ = 5.28 to 5.32ppmꢀ after one equivalent (relative to 1a) of DBU was added into the
1
deuterated DMF solution of 2-iodoaniline (Figure 3a,b). Moreover, the H NMR spectrum of
DBU in the mixture solution of DBU and 1a was also changed compared with the pure DBU
(Figure S1 inꢀ SI), which further indicated some interaction existed between DBU and 2-
1
iodoaniline. In contrast, no interaction between Na
2
Sꢀ and 2-iodoaniline was observed in the H
1
NMR spectrum of the mixture of Na
2
Sꢀ and 2-iodoaniline (Figure 3a,c). Thus, the H NMR spectra
7

demonstrated that DBU could react with 2-iodoaniline, while Na
2
Sꢀ did not react with 2-
iodoaniline directly. However, there was no new peak in the Cꢀ NMR spectrum of the mixture
solution of CS and DBU (Scheme 2c). It is well known that CS is closely related to CO molecule
and exhibits similar reactivity.[38], [39] Considering bicyclic amidines (DBU, TBD, and DBN) are
able to activate the CO molecule and have been used as catalysts in reactions involving the use of
CO . A literature report also did not find a
,[38], [40] we speculated that DBU could still activate CS
new carbon peak in the Cꢀ NMR spectrum of the TBD-CS
mixture solution.[38] Therefore, these
results indicated that DBU showed interactions with both 2-iodoaniline and CS , while Na Sꢀ only
interacted with CS
1
3
2
2
2
2
2
2
1
3
2
2
2
2
.
To investigate the mechanism of the formation of 2-mercaptobenzothiazole after
Sꢀ·9H O reacted with CS to form sodium thiocarbonate, a series of experiments were carried
out. The three component reaction of Na Sꢀ·9H O, CS and iodobenzene or aniline were carried
Na
2
2
2
2
2
2
out to verify the site-selective reaction of 2-iodoaniline (Scheme 2d,e). However, no new benzo-
containing product was detected by TLC or LCMS. Hence, the reaction mechanism was proposed
as following (Scheme 3). Na
2
Sꢀ·9H
2
O reacts with CS to form sodium thiocarbonate 3, and 3 reacts
2
with 1 via nucleophilic substitution and simultaneous nucleophilic addtion to form the cyclic
intermediate 4. Furthermore, the reduced electron density of 2-haloanilines would promote the
nucleophilic substitution reaction of halo group with sodium thiocarbonate 3 while prevent the
nucleophilic addtion reaction of the amino group with sodium thiocarbonate 3. The increased
8

electron density of 2-haloanilines would show the opposite effects. Therefore, the simultaneous
reactions of amino and halo group with sodium thiocarbonate 3 would take place smoothly when
the electron density of benzene ring is in a suitable range. Additionally, these reactions of 2-
haloanilines would be inhibited when the electron density is shifted out of the suitable range by
the strong electron-withdrawing or electron-donating group in 2-iodoanilines (Table 2) and the
yields of target products decreased. Finally, the intermediate 4 is converted into product 2 after
-
HS is eliminated. Based on the proposed reaction mechanism, Na
2
Sꢀ is converted into NaHS and
Na
2
Sꢀ acts not only as an activator of CS but also as a base.
2
Conclusions
In conclusion, we have developed an efficient reaction of 2-haloanilines, Na
at 110 °C for the synthesis of 2-mercaptobenzothiazoles in one-pot procedure. In this system,
Sꢀ·9H O acted not only as the CS activator but also as a base. The protocol used simple and
2
Sꢀ·9H O, and
2
CS
2
Na
2
2
2
readily available starting materials, eliminated the need for any metal catalysts, and afforded the
corresponding 2-mercaptobenzothiazoles under mild conditions in moderate to good yields.
Experimental
General procedure for synthesis of benzothiazole derivatives (2a-2i)
A sealed tube (50mLꢀ) was charged with 2-haloaniline 1a (2mmol), CS
2
(10mmol), Na Sꢀ
2
(4mmol) and DMF (2mLꢀ) at room temperature under an argon gas atmosphere and the tube was
9

flushed with argon for three times and sealed. Then the mixture was stirred electromagnetically at
10 °C for 12hoursꢀ. The reaction process was monitored by TLC on silica gel. After the reaction
1
was completed, the reaction mixture was cooled to room temperature, 2mLꢀ HCl (3molꢀ/L) was
added and stirred for 30minutesꢀ. Then the reaction mixture solution was extracted by
dichloromethane (3*20mLꢀ). Subsequently, the combined organic solution were dried by
anhydrous magnesium sulfate and concentrated. The residue was purified by silica gel colum
chromatography (eluent: petroleum ether / ethyl acetate) give the corresponding pure product 2a.
Procedure for the reaction of CS2 with Na2Sꢀ or DBU in a NMR tube
The CS
2
(10 μL) and Na
2
Sꢀ (30mgꢀ) or DBU(18 μL) dissolved in 0.6mLꢀ DMF in a NMR
and DBU were also detected
tube were detected by 13Cꢀ NMR at 25 °C, and the pure CS
2
1
3
13
respectively by Cꢀ NMR in 0.6mLꢀ DMF. The Cꢀ NMR results are shown in Figure 2.
Procedure for the reaction of 2-iodoaniline with DBU or Na2Sꢀ in a NMR tube
The 2-iodoaniline (10.0mgꢀ) was dissolved with 0.5mLꢀ deuterated DMF in a NMR tube,
1
which was detected by H NMR at 25 °C. Then the DBU (18 μL) or Na
2
Sꢀ (30mgꢀ) was added
1
1
into the NMR tube and then detected by H NMR at 25 ºC. The H NMR results are shown in
Figure 3 and Figure S1.
Acknowledgements
1
0

This work was supported by the National Natural Science Foundation of China
21362019), Natural Science Foundation of Inner Mongolia Autonomous Region of China
2016MS0207) and CAS “Light of West China” Program.
(
(
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011.
1
2

Table 1. Optimization of the reaction conditionsa.
Entry
Temp. (°C) CS
2
(equiv.) Na
2
Sꢀ·9H
2
O
Solvent
Yield of 2a
(equiv.)
(%)b
1
2
3
4
5
6
7
8
9
110
110
110
110
110
110
110
120
100
90
5
5
5
5
5
5
4
5
5
5
5
–
DMF
5
0.5
1.0
1.5
2.0
2.5
2.0
2.0
2.0
2.0
2.0
DMF
53
62
65
76
78
55
58
67
43
NRc
DMF
DMF
DMF
DMF
DMF
DMF
DMF
1
1
0
1
DMF
110
1,4-dioxane
1
3

1
1
1
1
2
110
110
110
110
5
5
5
5
2.0
2.0
2.0
2.0
toluene
glycol
DMF
NRc
3
17
4d
5e
58
DMF
55
aThe reactions were performed in a sealed tube with 1a (0.5 mmol), carbon disulfide, and Na
2
2
Sꢀ·9H O in solvent (2 mLꢀ) for 12 h.
bIsolated yields.
cNR stand for no reaction.
dK
Sꢀ was used to replace Na Sꢀ.
2 2
eNaHS was used to replace Na
Sꢀ.
2
1
4

Table 2. Synthesis of various 2-mercaptobenzothiazoles.<TS: Please take care of images in
table.>
Entry
Substrate 1
Time(h)
Product
Yield(%)a
1
1a
1b
1c
1d
1e
12
2a
2a
2a
2a
2b
76
2
3
4
5
12
12
12
12
51
37
-
70
6
7
8
1f
12
12
7
2c
2d
2e
82
72
94
1gꢀ
1hꢀ
9
1
1
1i
12
15
12
2f
45
27
40
0
1
1j
1k
2gꢀ
2hꢀ
1
5

1
2
1l
15
2c
96
aIsolated yields.
1
6

Figure 1. Structures of representative 2-mercaptobenzothiazole derivatives.
1
7

1
3
Figure 2. Cꢀ NMR spectra of (a) CS
2
inꢀ DMF; (b) CS
2
and Na
2
S in DMF; (c) CS and DBU in
2
DMF.
1
8

1
Figure 3. H NMR spectra of (a) 2-iodoaniline; (b) 2-iodoaniline and DBU; (c) 2-iodoaniline
and Na S in deuterated DMF.
2
1
9

Scheme 1. Representative synthetic methods for preparing 2-mercaptobenzothiazoles.
2
0

Scheme 2. Control experiment.
2
1

Scheme 3. The mechanism for the reaction of 1, CS
2
and Na Sꢀ.
2
Supplementary Materials
Experimental procedures, characterization data and NMR spectra of all compounds could be
obtained in Supplementary Materials.
2
2