Convenient synthesis of benzo[4,5]thiazolo[2,3-c][1,2,4]triazoles with 1 mol% CuCl2·2H2O as catalyst in water

Article abstract of DOI:10.1039/c4gc02156h

A convenient and efficient procedure for the synthesis of benzo[4,5]thiazolo [2,3-c][1,2,4]triazoles has been developed via a tandem intermolecular C-N bond and intramolecular C-S bond formation sequence from o-bromo-arylisothiocyanates and aroylhydrazides. A series of benzo[4,5]thiazolo[2,3-c][1,2,4]triazoles were provided in excellent overall yields with 1 mol% CuCl₂·2H₂O/1 mol% 1,10-phenanthroline as the catalyst and water as the solvent. This journal is

Full text of DOI:10.1039/c4gc02156h

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GRAPHIC ABSTRACT
A
synthesis of benzo[4,5]thiazolo[2,3-c
][1,2,4]triazoles has been developed from
hydrazides and o-bromo-arylisothiocyanates with 1 mol % CuCl2·2H2O as catalyst in water.
One-pot, two-step; H2O as solvent
Low catalyst loading; High yields
R2
R1
1. Na2CO3 (2 equiv), H2O, 90 oC
NCS
Br
O
R1
R2
N
NHNH2
N
2. CuCl2 2H2O (1 mol%) / L (1 mol%)
N
S
26 examples
L =
up to 95% yield
N
N

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Convenient Synthesis of Benzo[4,5]thiazolo[2,3ꢀc][1,2,4]triazoles
with 1 mol% CuCl2•2H2O as Catalyst in Water
Li-Rong Wen,* Shou-Lei Li, Jian Zhang, and Ming Li*
State Key Laboratory Base of EcoꢀChemical Engineering, College of Chemistry and Molecular
Engineering, Qingdao University of Science and Technology, Qingdao 266042, P. R. China
A convenient and efficient procedure for the synthesis of benzo[4,5]thiazolo [2,3ꢀc][1,2,4]triazoles
has been developed via a tandem intermolecular C−N bond and intramolecular C−S bond
formation sequence from oꢀbromoꢀarylisothiocyanates and aroylhydrazides. A series of
benzo[4,5]thiazolo[2,3ꢀc][1,2,4]triazoles were provided in excellent overall yields with 1 mol %
CuCl2·2H2O/1 mol % 1,10ꢀphenanthroline as the catalyst and water as the solvent.
Keywords: oꢀbromoꢀarylisothiocyanates, benzo[4,5]thiazolo[2,3ꢀc][1,2,4]triazoles, copper(II)
catalyst, C−S bond formation
Introduction
In recent years, the research for the formation of carbonꢀsulfur bond in the
transitionꢀmetalꢀcatalyzed system has been the focus in organic chemistry.1 For
example, nickel,2 palladium,3 iron,4 rhodium5 and copper6 catalysts have emerged as
appealing catalysts for these reactions to overcome the drawbacks that the traditional
methods suffered from, such as long reaction time and low yields. Among the various
deployed metals, copper has attracted increasing attention because it is inexpensive,
environmentally benign and abundant.7 Reactions with copper catalysts usually could
carry out under mild conditions and employ simple ligands at the same time.
Benzo[4,5]thiazolo[2,3ꢀc][1,2,4]triazole derivatives have been discovered to
display promising activities. For example, 5ꢀmethylbenzo[4,5]thiazolo[2,3ꢀc][1,2,4]
triazole as a commercial compound (Figure 1, compound I), patented by Eli Lilly,
1

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could control some phytopathogenic fungi such as Magnaporthe grisea,
Colletotrichum lagenarium, and Colletotrichum lindemuthianum, as well as parasites
on rice, cucumber and bean.8 For another example, 7ꢀfluoroꢀNꢀphenylbenzo[4,5]
thiazolo[2,3ꢀc][1,2,4]triazolꢀ8ꢀamine (Figure 1, compound II) was reported to show
the antimicrobial activity.9 Also, they are active on dematiaceous human pathogenic
fungi10 and on a number of other imperfect and ascomycetous fungi.11
Fig. 1 Selected benzo[4,5]thiazolo[2,3ꢀc][1,2,4]triazoles with bioactivities.
From
benzo[4,5]thiazolo[2,3ꢀc][1,2,4]triazole derivatives are still limited.9,12 Ramakrishnan
and proposed general photochemical synthesis of
coꢀworkers12
a
synthetic point of view, to date, methods for obtaining
a
benzo[4,5]thiazolo[2,3ꢀc][1,2,4]triazoles from 1,2,4ꢀtriazoleꢀ3ꢀthiones by 254 nm
irradiation. However, this protocol was restricted to long reaction time (15−25 h), low
yields (only 34−56 %), and narrow substrate scope (7 examples). So the pursuit of
inexpensive and more environmentally benign methodologies for construction of
benzo[4,5]thiazolo[2,3ꢀc][1,2,4]triazole motif still remains a challenge.
As we all know, water is the most economical and ecoꢀfriendly media in the world,
and using water as solvent could also simplify the workup procedures, and enable the
recycle of the catalyst as well as allow the mild conditions.13 Moreover, taking into
account the economic and environmental protection, decreasing the amount of the
metal catalyst and the ligand is also of great significance.14 In view of the points
above and our ongoing research interest for the synthesis of heterocycles 15
herein, we
,
report an novel copper(II)ꢀcatalyzed carbonꢀsulfur bond formation reaction for the
synthesis of benzo[4,5]thiazolo[2,3ꢀc][1,2,4]triazoles in excellent yield in water with
oꢀbromoarylisothiocyanates 1 and aroylhydrazides 2 as raw materials in a oneꢀpot
twoꢀstep process. So far, to the best of our knowledge, it is the first report on 1 mol%
loading of CuCl2·2H2O and 1 mol% 1,10ꢀphen as the catalyst for synthesis of
benzo[4,5]thiazolo[2,3ꢀc][1,2,4]triazoles in water.
2

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Results and Discussion
In recent years we have witnessed great progress in the development of mild
Cuꢀcatalyzed Ullmannꢀtype reactions.16 We envisioned that the benzothiazole ring
could be constructed by a Cuꢀcatalyzed intramolecular C−S bondꢀforming reaction
from 4ꢀ(2ꢀbromoaryl)ꢀ5ꢀarylꢀ4Hꢀ1,2,4ꢀtriazoleꢀ3ꢀthiol compounds (3).
Compound 3a, used as a test substrate for the synthesis of the target product 4a,
was
synthesized
in
yield
of
80%
through
the
reaction
of
oꢀbromoꢀbenzylisothiocyanate (1a) with benzohydrazide (2a) in the presence of
NaOH in water at 90 °C.
Table 1 Reaction condition optimization of intramolecular C−S bond formationa
Entry
Catalyst
CuI
Ligand
Lꢀproline
Lꢀproline
Lꢀproline
1,10ꢀphen
1,10ꢀphen
—
Base
Cs2CO3
K2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
—
Solvent
DMSO
DMSO
DMSO
DMSO
H2O
Isolated Yield (%)
1
2
3
4
5
6
7
8
97
95
CuI
CuI
96
CuCl2·2H2O
CuCl2·2H2O
CuCl2·2H2O
—
96
96
H2O
Trace
NR
NR
1,10ꢀphen
1,10ꢀphen
H2O
CuCl2·2H2O
H2O
a.Reaction conditions: 3a (1.0 mmol), copper salt (0.1 mmol), ligand (0.1 mmol), base (2.0 mmol), solvent (2 mL).
Initially, we employed the classic CuI/Lꢀproline/Cs2CO3 catalyst system in DMSO
at 90 °C to prepare the ringꢀclose product 4a, and the reaction gave an excellent yield
of 97% within 30 min (Table 1, entry 1). Next, other inexpensive bases such as K2CO3
and Na2CO3 were tested and also gave similar yields as Cs2CO3 (Table 1, entries 2 and
3). These results promoted us to use cheaper CuCl2·2H2O/1,10ꢀphen/Na2CO3 catalyst
system to conduct this reaction. To our delight, the reaction proceeded well affording
the same yield (Table 1, entry 4). Considering the requirement of green chemistry,
water also was attempted as the reaction medium. More gratifyingly, this reaction
system favored water (Table 1, entry 5). The control experiments confirmed that a
3

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copper catalyst, a ligand, and a base were necessary for this coupling reaction (Table 1,
entries 6ꢀ8).
With this promising result above in hand, we focused on whether through a tandem
process from the substrates oꢀbromoarylisothiocyanates 1 and aroylhydrazides 2 as
starting materials in oneꢀpot operation to prepare the products 4. Based on the above
successful experimental results of copper(II)ꢀcatalyzed intramolecular ring closure
from 3 to 4, we performed directly the reaction of 1a and 2a with
CuCl2·2H2O/1,10ꢀphen/Na2CO3 as the catalyst in H2O at 90 °C to prepare the target
compound 4a. Disappointingly, however, 4a was not observed after 8 h, and 3a was
the only product as indicated by TLC. Next, we changed the strategy, the above model
reaction was first carried out in the presence of Na2CO3 in water at 90 °C for 2 h
without adding any metal catalyst. Consequently, the precursor 3a was provided
directly and smoothly. Then without isolating 3a, CuCl2·2H2O and 1,10ꢀphen were
directly added to the reaction system and continued to stir for 2 h. Gratifyingly, in this
case, the target product 4a was obtained in an excellent total yield of 95% after two
steps (Scheme 1).
Scheme 1 Oneꢀpot reaction design for the synthesis of 4a.
The other copper(II) salts such as CuSO4·5H2O, Cu(OAc)2·H2O also were tested
and gave similar results as CuCl2·2H2O (Table 2, entries 1 and 2). Whereas other
more inexpensive transition metal salt such as FeCl3ꢁ6H2O was less effective for this
coupling reaction (Table 2, entry 3). Finally, the catalyst load was investigated. To our
delight, when 1 mol% CuCl2·2H2O and 1,10ꢀphen were used, the same excellent yield
was obtained (Table 2, entries 4−7).
Table 2 Reaction condition optimization for the tandem reactiona
4

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Entry
Cat. (equiv)
1,10ꢀPhen (equiv)
Base
Time (h)
2+2
Yield (%)b
1
2
3
4
5
6
7
CuSO4·5H2O (0.01)
Cu(OAc)2·H2O (0.01)
FeCl3ꢁ6H2O (0.01)
CuCl2·2H2O (0.05)
CuCl2·2H2O (0.02)
CuCl2·2H2O (0.01)
CuCl2·2H2O (0.005)
0.01
0.01
0.01
0.05
0.02
0.01
0.005
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
90
91
30
94
93
94
25c
2+2
2+2
2+2
2+2
2+2
2+2
a.Reaction conditions: 1a (1.0 mmol), 2a (1.0 mmol), base (2.0 mmol), water (2 mL), 90 °C. Yields of the pure
b
product after washing with water. c 4 mmol scale.
With this promising result in hand, the scope of this reaction was investigated under
the optimized conditions (1 mol% CuCl2·2H2O and 1,10ꢀphen, Na2CO3 (2.0 equiv) in
H2O at 90 °C). Various isothiocyanates 1 and hydrazides 2 were examined, and the
results were summarized in Table 3.
Table 3 Investigation on the substrate scope of oneꢀpot copperꢀcatalyzed synthesis of 4a.
Entry
1
X
H
R
Time (h)
2+2
Yield (%) b
4
C6H5
94 (lit.[12] 41)
4a
4b
4c
2
3
H
H
4ꢀClC6H4
4ꢀBrC6H4
2+2
2+2
95
95
5

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4
5
H
H
4ꢀFC6H4
2+2
2+2
94
95
4d
4e
4ꢀOHC6H4
6
H
4ꢀCH3C6H4
2+2
94 (lit.[12] 40)
4f
7
8
9
H
H
H
4ꢀOCH3C6H4
3ꢀClC6H4
2+2
2+2
2+2
93 (lit.[12] 56)
4g
4h
95
94
3ꢀCF3C6H4
4i
10
11
H
H
3ꢀNO2C6H4
2ꢀBrC6H4
2+2
2+2
95
90
4j
4k
12
13
H
H
3,5ꢀ(CH3)2C6H3
4ꢀpyridinyl
2+2
2+2
95
92
4l
6

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4m
14
15
4ꢀF
4ꢀF
C6H5
2+2
2+2
91
92
4n
4o
4p
4ꢀClC6H4
16
17
4ꢀF
4ꢀCH3C6H4
2+2
2+2
90
91
4ꢀCl
C6H5
4q
4r
4s
18
19
20
4ꢀCl
4ꢀCl
4ꢀBr
4ꢀClC6H4
4ꢀCH3C6H4
C6H5
4+2
4+2
2+2
85
86
94
4t
21
22
4ꢀBr
4ꢀBr
4ꢀClC6H4
2+2
2+2
89
90
4u
4ꢀCH3C6H4
7

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4v
23
24
4ꢀCH3
4ꢀCH3
C6H5
2+2
2+2
95
93
4w
4ꢀClC6H4
4x
4y
25
26
27
4ꢀCH3
4ꢀOCH3C6H4
2+2
2+2
2+2
94
H
Bn
95
4z
H
CH3
Trace
4zz
a. Reaction conditions: 1 (1.0 mmol), 2 (1.0 mmol), CuCl2·2H2O (0.01 mmol), 1,10ꢀphen (0.01 mmol), Na2CO3
(2.0 mmol). b Yields of the pure products after washing with water.
As can be seen from Table 3, various isothiocyanates 1 and hydrazides 2 were
smoothly transformed to the corresponding target products 4 in excellent yields. For
the substrates 2, the substituted hydrazides both bearing electronꢀdonating groups and
electronꢀwithdrawing groups at 2ꢀ, 3ꢀ, or 4ꢀ as well as 3,5ꢀpositions all gave the
satisfactory yields (Table 3, entries 1−12). However, compared to 3ꢀ and 4ꢀpositions,
the substituent at 2ꢀposition seemed to have slight effect on the results, which might
be subject to steric hindrance. For example, 2ꢀbromobenzohydrazide could afford
only 90% yield (Table 3, entry 11). A heretocyclic hydrazide such as
isonicotinohydrazide was also investigated, which could provide 92% yield (Table 3,
entry 13). For the substrates 1, the various isothiocyanates bearing different
substituents did not display obvious difference in reactivity (Table 3, entries 14−25).
However, when benzyl hydrazide or an aliphatic hydrazide such as acetohydrazide
was employed in this tandem reaction, they performed quite differently. Benzyl
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hydrazide provided very high yield of 95%, whereas acetohydrazide only gave the
trace product (Table 3, entries 26 and 27).
The structures of benzo[4,5]thiazolo[2,3ꢀc][1,2,4]triazoles 4 were identified by their
1
IR, H NMR, 13C NMR, HRMS spectra, and ascertained by the singleꢀcrystal Xꢀray
diffraction analysis of 4h (Figure 2).
Fig. 2 Xꢀray structure of 4h.
It is noteworthy that all of the products only need washing with water rather than
column chromatography or recrystallization. This easy purification makes this
methodology facile, practical, and rapid to execute.
In order to justify the mechanism, CuCl/1,10ꢀphen catalyst was employed in this
tandem reaction. First the reaction of 1a and 2a was conducted in the presence of
Na2CO3 in H2O at 90 °C for 2 h, then CuCl/1,10ꢀphen was added in the reaction
system under N2 atmosphere. The product 4a was obtained within 1 h in 95% yield,
which suggested the key catalytic species is Cu(I).
Based on the above experimental results, a possible mechanism for the tandem
reaction for the synthesis of benzo[4,5]thiazolo[2,3ꢀc][1,2,4]triazoles 4 is depicted in
Scheme 2. First, isothiocyanates 1 and hydrazides 2 take place intermolecular
nucleophilic addition reaction to provide an thiourea intermediate A in presence of a
base, followed by an intramolecular nucleophilic addition reaction to give B, which
eliminates a molecule of water to afford cyclization product 3. Next, 3 undergoes an
oxidative addition reaction with the active copper(І) species C, which is generated by
the reduction process of the copper(II) salt,17 to yield copper(III) intermediate D. Then
D undergoes an intramolecular cyclization in the presence of a base via E to give
crossꢀcoupling products 4, completing the catalytic cycle by the reductive elimination.
9

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Scheme 2 Proposed reaction mechanism.
Conclusion
In conclusion, we have developed a general, convenient, and environmentally
benign method for the preparation of benzo[4,5]thiazolo[2,3ꢀc][1,2,4]triazoles in
excellent yields from oꢀbromoarylisothiocyanates and aroylhydrazides by a oneꢀpot
twoꢀstep procedure with cheap and air stable CuCl2·2H2O/1,10ꢀphen as the catalyst in
water. The low loading of the catalyst (1 mol %) and easy purification of products (by
simple washing with water) make this methodology facile, practical, and rapid to
execute. For these reasons, this novel process should show environmental and
economic advantages over previously reported protocols.
Experimental
General information
Unless otherwise specified, all reagents and solvents were obtained from commercial
suppliers and used without further purification. 1,4ꢀdioxane was dried over CaH2 and
distilled prior to use. All reagents were weighed and handled in air at room
temperature. Melting points were recorded on a RYꢀ1 microscopic melting apparatus
1
and uncorrected. H NMR spectra were recorded on 500 MHz and 13C NMR spectra
were recorded on 125 MHz by using a Bruker Avance 500 spectrometer. Chemical
shifts were reported in parts per million (δ) relative to tetramethylsilane (TMS). IR
spectra were recorded on a Nicolet iS10 FTꢀIR spectrometer and only major peaks are
reported in cmꢀ1. Mass spectra were performed on an Ultima Global spectrometer with
10

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an ESI source. The Xꢀray singleꢀcrystal diffraction was performed on Saturn 724+
instrument.
General procedure for 2ꢀbromoꢀ1ꢀisothiocyanatobenzenes 1
A 100 mL roundꢀbottom flask was charged with a mixture of oꢀbromoꢀaniline (5
mmol) and 10 mL of 1,4ꢀdioxane in an ice bath. Then NaH (15 mmol, 60% dispersion
in mineral oil) was added to the flask, and CS2 (20 mmol) was added dropwise to the
reactor slowly. When CS2 addition was complete, the reaction was stirred at room
temperature for 5 hours. Then the reaction system was refluxed. After completion of
the reaction (nearly 24 hours), the reaction solution was cooled to room temperature,
filtered and the solid was dissolved in 30 mL water, and extracted with ethyl acetate
(3×30 mL). The combined filtrate and the organic layer was dried over anhydrous
magnesium sulfate, rotary evaporated under reduced pressure and purified by silica
gel column chromatography to obtain products 1.
General procedure for the thiazolo[2,3ꢀc][1,2,4]triazoles 4
A 25 mL roundꢀbottom flask was charged with a mixture of hydrazides 2 (1.0 mmol),
oꢀbromoꢀ2ꢀisothiocyanatobenzenes 1 (1.0 mmol), sodium carbonate (2 mmol), and the
mixture was stirred in 2 mL of water at 90 ºC for 2 h. Then cupric chloride dihydrate
(0.01 mmol) and 1,10ꢀphen (0.01mmol) were added when the starting materials were
completely consumed. After completion of the reaction as indicated by TLC, the
reaction mixture was cooled to room temperature and filtered. The solid was washed
with water (3×10 mL) to give the pure products 4.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of
China (21372137 and 21072110) and the Natural Science Foundation of Shandong
Province (ZR2012BM003).
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