Ligand- and solvent-free synthesis of 2-aminobenzothiazoles by copper-catalyzed tandem reaction of 2-haloanilines with isothiocyanates

Article abstract of DOI:10.1002/aoc.2949

We report an environmentally friendly, efficient and practical method for the synthesis of 2-aminobenzothiazoles by a copper(II)-catalyzed tandem reaction of 2-haloanilines with isothiocyanates under ligand- and solvent-free conditions in air. The developed methodology conforms to the principles of 'green chemistry' and addresses the shortage of such methods for the synthesis of 2-aminobenzothiazoles. The reaction is quite general and generates a variety of 2-aminobenzothiazoles in good to excellent yields. Copyright

Full text of DOI:10.1002/aoc.2949

Full Paper
Received: 22 September 2012
Revised: 24 October 2012
Accepted: 26 October 2012
Published online in Wiley Online Library: 19 December 2012
(wileyonlinelibrary.com) DOI 10.1002/aoc.2949
Ligand- and solvent-free synthesis of
2-aminobenzothiazoles by copper-catalyzed
tandem reaction of 2-haloanilines with
isothiocyanates
Ruiya Yao, Haiyi Liu, Yichao Wu and Mingzhong Cai*
We report an environmentally friendly, efficient and practical method for the synthesis of 2-aminobenzothiazoles by a copper
(II)-catalyzed tandem reaction of 2-haloanilines with isothiocyanates under ligand- and solvent-free conditions in air. The
developed methodology conforms to the principles of ’green chemistry’ and addresses the shortage of such methods for
the synthesis of 2-aminobenzothiazoles. The reaction is quite general and generates a variety of 2-aminobenzothiazoles in
good to excellent yields. Copyright © 2012 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: copper catalysis; tandem reaction; 2-aminobenzothiazole; solvent-free; 2-haloaniline
and co-workers described a copper-catalyzed tandem reaction
between 2-halobenzenamines and isothiocyanates using the
Introduction
Development of new organic transformations that are not only
efficient, selective and high yielding but that are also environ-
mentally benign is one of the challenges facing chemists in the
21st century.[1] During the last decade, the topic of ’green’
chemistry has attracted much attention.[1,2] ’Green’ chemistry
aims at the total elimination (or at least the minimization) of
waste, and the implementation of sustainable processes.[1a] The
use of nontoxic chemicals, renewable materials and solvent-free
conditions are the key issues in a green synthetic strategy. In
the present work, we describe a green approach towards the
synthesis of 2-aminobenzothiazoles.
2-Aminobenzothiazoles, an important topical benzothiazole
scaffold, are broadly found in biological chemistry and medicinal
areas.[3] A large number of 2-anilinobenzothiazole derivatives are
found to be anticancer active and the 2-aminobenzothiazole
moieties are privileged pharmacophores as well as valuable reac-
tive intermediates.[4] There are several methods for the synthesis
of 2-aminobenzothiazoles, including: (i) cyclization of arylthioureas
with liquid bromine and benzyltrimethylammonium tribromide;[3a,5]
(ii) coupling of arylamines with 2-halobenzothiazole;[6] (iii) oxidative
cyclizaton of the intermediates generated by 2-aminothiophenol
with isothiocyanates;[7] and (iv) intramolecular C—S bond
formation catalyzed by palladium.[4e,8] Although these methods
are efficient for the synthesis of 2-aminobenzothiazoles, they
usually suffer from the use of highly toxic and corrosive reagents,
expensive palladium catalysts, specific ligands and not readily
available substrates. These drawbacks make the methodologies
unsuitable for large- or industrial-scale applications. Recently,
the copper- or iron-catalyzed one-pot tandem reactions of
2-halobenzenamines with isothiocyanates for the synthesis
of 2-aminobenzothiazoles have received much attention
because of their efficiency and low costs. For example, Wu
CuI (10 mol%)/1,10-phenanthroline (20 mol%) catalytic system to
prepare 2-aminobenzothiazoles;[4g] Li and Ding’s group reported
iron-catalyzed tandem reactions of 2-halobenzenamines and
isothiocyanates leading to 2-aminobenzothiazoles.[9] Meanwhile,
the ligand-free copper-catalyzed one-pot tandem reactions of
2-halobenzenamines and isothiocyanates were also reported.[10]
However, it should be pointed out that the copper or iron-
catalyzed one-pot tandem reactions of 2-halobenzenamines
with isothiocyanates generally involve organic solvents such as
DMSO, DMF and toluene. Use of organic solvents is environmen-
tally unfriendly, and so it is highly desirable to develop more
environmentally friendly and economic methodologies for
synthesizing 2-aminobenzothiazoles. To achieve this, we report
an environmentally friendly, economic, efficient and simple
ligand- and solvent-free system that allows the one-pot tandem
reactions of 2-halobenzenamines and isothiocyanates based on
a catalytic amount of CuSO4 using Bu3N as a base at 80 ꢀC in
air. The results are summarized below.
Results and Discussion
In order to identify the optimal reaction conditions, the reaction
of 2-iodoaniline with phenyl isothiocyanate was chosen as a
model reaction. In this preliminary experiment, the reaction of
*
Correspondence to: Mingzhong Cai, Department of Chemistry, Jiangxi
Normal University, Nanchang 330022, People’s Republic of China. E-mail:
caimzhong@163.com
Department of Chemistry, Jiangxi Normal University, Nanchang 330022,
People’s Republic of China
Appl. Organometal. Chem. 2013, 27, 109–113
Copyright © 2012 John Wiley & Sons, Ltd.

R. Yao et al.
2-iodoaniline with phenyl isothiocyanate (1.1 equiv.) was carried
out in various solvents, with CuSO4 (1 mol%) as a catalyst
and Bu3N (1.1 equiv.) as a base without a ligand, at 80 ꢀC in air
(Table 1). It was found that the reaction proceeded perfectly
in DMSO (Table 1, entry 1), but that the yields decreased when
the reaction was carried out in other solvents (Table 1, entries
2–10). However, the reaction surprisingly proceeded very
smoothly under solvent-free conditions to afford the desired
product in excellent yield within a shorter time (Table 1, entry
11). Considering our goal of an economic and environmentally
friendly reaction, these ligand- and solvent-free conditions are
clearly the most favorable.
Our next studies focused on the effect of various copper
catalysts on the yields under ligand- and solvent-free conditions.
The reaction did not occur in the absence of a catalyst (Table 2,
entry 1). Among the copper catalysts examined, CuSO4, CuI, Cu
(OAc)2, CuCl2 and CuCl were efficient catalysts and afforded the
product in high yields (Table 2, entries 2–4, 6 and 8), and CuSO4
was the best choice (entry 2), whereas other copper catalysts
such as Cu(OTf)2, CuBr, Cu2O and CuBr2 were substantially less
a
Table 1. Preliminary survey of solvents for the reaction of 2-iodoaniline with phenyl isothiocyanate catalyzed by CuSO4
Entry
Solvent
Time (h)
Yield (%)b
1
DMSO
DMF
12
14
24
16
15
24
24
24
24
24
2
96
81
73
80
76
60
14
75
65
54
95
2
3
EtOH
4
Toluene
NMP
5
6
Dioxane
H2O
7
8
Benzene
DCE
9
10
11
MeCN
None
aThe reaction was carried out using 2-iodoaniline (1.0 mmol), phenyl isothiocyanate (1.1 mmol), CuSO4 (1.0 mol%) and Bu3N (1.1 mmol) in solvent
(2.0 ml) at 80 ꢀC under an air atmosphere.
bIsolated yield.
Table 2. Screening of copper catalysts for the reaction of 2-iodoaniline with phenyl isothiocyanatea
Entry
Catalyst
Time (h)
Yield (%)b
1
None
CuSO4
Cu(OAc)2
CuCl2
Cu(OTf)2
CuI
24
2
2
3
3
2
3
3
4
4
5
1
0
95
90
89
82
93
84
89
85
83
93
94
2
3
4
5
6
7
CuBr
8
CuCl
9
Cu2O
10
11c
12d
CuBr2
CuSO4
CuSO4
aThe reaction was carried out using 2-iodoaniline (1.0 mmol), phenyl isothiocyanate (1.1 mmol) and Bu3N (1.1 mmol) in the presence of catalyst
(0.01 mmol) at 80 ꢀC under an air atmosphere.
bIsolated yield.
c0.5 mol% CuSO4 was used.
d2 mol% CuSO4 was used.
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Copyright © 2012 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2013, 27, 109–113

Solvent-free synthesis of 2-aminobenzothiazoles catalyzed by copper
effective (Table 2, entries 5, 7, 9 and 10). The amount of CuSO4
was optimized further. The low copper concentration usually
led to a long period of reaction, which was consistent with our
experimental result (Table 2, entry 11). Increasing the amount of
copper catalyst could shorten the reaction time, but did not increase
the yield of N-phenylbenzo[d]thiazol-2-amine (Table 2, entry 12).
In order to examine the effect of base on the model reaction,
we performed the reaction with various bases under ligand-
and solvent-free conditions in air (Table 3). As can be seen, the
reaction could not be performed without a base (Table 3, entry 1).
Among the bases examined, Bu3N was found to be the most
effective (Table 3, entry 2); Et3N, 1,8-diazabicyclo[5.4.0]undec-7-
ene (DBU), N,N,N0,N0-tetramethylethylenediamine (TMEDA) and
1,4-diazabicyclo[2.2.2]octane (DABCO) also afforded good yields
(Table 3, entries 3, 7–9), whereas other bases such as pyridine,
i-Pr2NH, piperidine, Cs2CO3 and K2CO3 were substantially less
effective (Table 3, entries 4–6, 10, 11). Therefore, the optimal
ligand- and solvent-free system for this reaction involves
CuSO4 (1 mol%) and Bu3N (1.1 equiv.) at 80 ꢀC in air.
reactions under the standard conditions. Cyclohexyl isothiocyanate,
for instance, was treated with 2-iodoaniline to afford the desired
product 3g in 87% yield (entry 7).
Substituted 2-iodoanilines were subsequently investigated
to react with a variety of isothiocyanates under the optimized
conditions, and the results are also summarized in Table 4.
We were pleased to find that the optimized conditions were
compatible with various 2-iodoanilines bearing trifluoromethyl,
chloro and methyl groups. It was observed that the tandem
reactions of electron-withdrawing 2-iodo-4-(trifluoromethyl)aniline
with various isothiocyanates were successful in affording the
desired products 3 h–l in good to excellent yields under ligand-
and solvent-free conditions in air (entries 8–12). The tandem
reactions of 4-chloro-2-iodoaniline with phenyl isothiocyanate
or substituted phenyl isothiocyanates proceeded very smoothly
to give the corresponding 2-aminobenzothiazoles 3 m–q in excel-
lent yields (entries 13–17). The reaction of 2-iodo-4-methylaniline
with a variety of isothiocyanates also proceeded efficiently under
the same conditions to afford the desired products 3r–u in good
to excellent yields (entries 18–21). Interestingly, a less active
substrate, 2-bromoaniline, also displayed high activity for the
tandem reaction with various aryl isothiocyanates in excellent
yields under ligand- and solvent-free conditions (entries 22–24).
Encouraged by the efficiency of the reaction protocol described
above, we investigated the scope of both 2-halobenzenamines
and isothiocyanates under the optimized conditions, and the
results are summarized in Table 4. As shown in the table, a
variety of isothiocyanates (2) were investigated by reacting
with 2-iodoaniline in the presence of CuSO4 (1 mol%) and Bu3N
(1.1 equiv.) at 80 ꢀC under ligand- and solvent-free conditions in
air. The results demonstrated that several functional groups, such Conclusions
as methyl, methoxy, fluoro, chloro and nitro groups, on the aryl
We have developed an environmentally friendly, efficient and
moiety were tolerated well, and both electron-rich and electron-
poor aryl isothiocyanates underwent the tandem reaction success-
fully in good to excellent yields (entries 2–6). 4-Methoxyphenyl
isothiocyanate, with a strong electron-donating group, for
example, was treated with 2-iodoaniline to give the desired
product 3c in 91% yield after 3 h (entry 3), and the reaction
of 4-nitrophenyl isothiocyanate, having a strong electron-
with-drawing group with 2-iodoaniline, afforded the target
product 3f in 88% yield within 1.5 h (entry 6). To our delight,
alkyl isothiocyanates were also suitable substrates for the tandem
practical method for the synthesis of 2-aminobenzothiazoles
by a copper-catalyzed tandem reaction of 2-haloanilines with
isothiocyanates using CuSO4 (1 mol%) as catalyst and Bu3N
(1.1 equiv.) as a base under ligand- and solvent-free conditions
in air. The procedure is suitable for various 2-haloanilines
and isothiocyanates, and a variety of 2-aminobenzothiazoles
can be produced conveniently with good to excellent yields.
The developed methodology has attractive advantages of
solvent-free, relatively low catalyst loading and shorter reaction
Table 3. Screening of bases for the reaction of 2-iodoaniline with phenyl isothiocyanatea
Entry
Base
Time (h)
Yield (%)b
1
None
24
2
0
95
77
34
24
63
76
78
74
14
11
2
Bu3N
3
Et3N
5
4
i-Pr2NH
Piperidine
Pyridine
DABCO
TMEDA
DBU
24
24
5
5
6
7
6
8
5
9
6
10
11
Cs2CO3
K2CO3
24
24
aThe reaction was carried out using 2-iodoaniline (1.0 mmol), phenyl isothiocyanate (1.1 mmol) and base (1.1 mmol) in the presence of CuSO4
(0.01 mmol) at 80 ꢀC under an air atmosphere.
bIsolated yield.
Appl. Organometal. Chem. 2013, 27, 109–113
Copyright © 2012 John Wiley & Sons, Ltd.
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R. Yao et al.
Table 4. Ligand- and solvent-free synthesis of a variety of 2-aminobenzothiazoles by CuSO4-catalyzed tandem reaction of 2-haloanilines and
isothiocyanatesa
Entry
R/X
H/I
R1
Time (h)
Product
Yield (%)b
1
C6H5
2
2
3a[10a]
3b[10a]
3c[9a]
3d[6b]
3e[9a]
3f[9a]
3g[11]
3h[11]
3i[11]
3j
3 k[9a]
3 l[9b]
3 m[9a]
3n[9a]
3o
95
93
91
93
92
88
87
93
92
94
91
85
92
93
90
91
92
94
90
92
85
92
90
93
2
H/I
4-CH3C6H4
4-CH3OC6H4
4-FC6H4
3
H/I
3
4
H/I
2
5
H/I
4-ClC6H4
4-NO2C6H4
Cyclohexyl
C6H5
2
6
H/I
1.5
4
7
H/I
8
CF3/I
CF3/I
CF3/I
CF3/I
CF3/I
Cl/I
2
9
4-CH3OC6H4
4-FC6H4
3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
2
4-ClC6H4
4-NO2C6H4
C6H5
2
1.5
2
Cl/I
4-CH3C6H4
4-CH3OC6H4
4-FC6H4
2
Cl/I
3
Cl/I
2
3p
Cl/I
4-ClC6H4
C6H5
2
3q
CH3/I
CH3/I
CH3/I
CH3/I
H/Br
H/Br
H/Br
2
3r[11]
3s[11]
3t[10a]
3u[12]
3a
4-CH3OC6H4
4-ClC6H4
Cyclohexyl
C6H5
3
2
4
3
4-CH3C6H4
4-ClC6H4
3
3b
3
3e
aReaction conditions: 1 (1.0 mmol); 2 (1.1 mmol); CuSO4 (0.01 mmol); Bu3N (1.1 mmol) at 80 ꢀC under an air atmosphere.
bIsolated yield.
times as compared with the ligand-free copper-catalyzed tandem
reactions reported by other groups.
under an air atmosphere at 80 ꢀC for 1.5–4 h. After cooling to
room temperature, the mixture was diluted with ethyl acetate
(5 ml) and filtered. The filtrate was removed under reduced pres-
sure to give the crude product, which was further purified by
silica gel chromatography (petroleum ether–ethyl acetate as
eluent) to yield the corresponding 2-aminobenzothiazoles.
Compounds 3a–3i, 3 k–3n and 3r–3u are known compounds
and were characterized by comparing their melting point, 1 H
NMR, 13C NMR and IR spectra with those found in the literature.
Experimental
All chemicals were reagent grade and used as purchased.
All solvents were dried and distilled before use. The products
were purified by flash chromatography on silica gel and a mixture
of EtOAc and petroleum ether was generally used as eluent.
IR spectra were determined on a PerkinElmer 683 instrument.
UV-visible spectra at room temperature were measured in
DMSO solution using a TU-1901 double-beam UV-visible spec-
trophotometer. 1 H NMR spectra (400 MHz) were recorded on a
Bruker Avance 400 MHz spectrometer with tetramethylsilane as
an internal standard in DMSO-d6 as solvent. 13C NMR spectra
(100 MHz) were recorded on a Bruker Avance 400MHz spectrome-
ter in DMSO-d6 as solvent. Melting points are uncorrected.
N-(4-Fluorophenyl)-6-(trifluoromethyl)benzo[d]thiazol-2-amine (3j)
White solid, m.p. 200–201 ꢀC. IR (KBr) n (cmꢁ1) 2957, 1624, 1576,
1468, 1459, 1328, 1307, 1147, 1113. UV (DMSO, nm): 306, 241. 1 H
NMR (400MHz, DMSO-d6) d 10.83 (s, 1 H, NH), 8.28 (s, 1 H, 7-CH),
7.90–7.86 (m, 2 H, 20-CH), 7.75 (d, J = 8.0 Hz, 1 H, 4-CH), 7.63 (d,
J = 8.0 Hz, 1 H, 5-CH), 7.26 (t, J = 8.8 Hz, 2 H, 30-CH). 13C NMR
1
(100 MHz, DMSO-d6) d 164.9 (C¼N), 158.2 (d, JCF = 238.0 Hz)
(C—Farom), 155.4 (C—Narom), 137.1 (C—NHarom), 131.2 (C—Sarom),
125.1 (q, 1JCF = 270.0Hz) (CF3), 123.2 (q, 3JCF = 4.0 Hz) (Carom), 122.9
General Procedure for the Copper-Catalyzed Tandem Reaction
of 2-Iodoanilines with Isothiocyanates under Ligand- and
Solvent-Free Conditions
2
3
(q, JCF = 32.0 Hz) (F3C—Carom), 120.4 (d, JCF = 8.0 Hz) (2Carom),
3
2
119.5 (Carom), 119.2 (q, JCF = 4.0Hz) (Carom), 116.0 (d, JCF = 23.0)
(2Carom). Anal. calcd for C14H8N2F4S: C, 53.84; H, 2.58; found: C,
53.58; H, 2.74.
A mixture of 2-iodobenzenamine (1.0 mmol), isothiocyanate
(1.1 mmol), Bu3N (1.1 mmol) and CuSO4 (0.01 mmol) was stirred
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Copyright © 2012 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2013, 27, 109–113

Solvent-free synthesis of 2-aminobenzothiazoles catalyzed by copper
N-(4-Methoxyphenyl)-6-chlorobenzo[d]thiazol-2-amine (3o)
Acknowledgments
White solid, m.p. 185–187 ꢀC. IR (KBr) n (cmꢁ1) 3202, 2958, 1628,
1578, 1475, 1452, 1321, 1301, 1141, 1120. UV (DMSO, nm): 300,
216. 1 H NMR (400 MHz, DMSO-d6) d 10.28 (s, 1 H, NH), 7.80
(s, 1 H, 7-CH), 7.57 (d, J = 8.8 Hz, 2 H, 20-CH), 7.43 (d, J = 8.4 Hz,
1 H, 4-CH), 7.22 (d, J = 8.4 Hz, 1 H, 5-CH), 6.87 (d, J = 8.8 Hz,
2 H, 30-CH), 3.66 (s, 3 H, OCH3). 13C NMR (100 MHz, DMSO-d6)
d 162.8 (C¼N), 154.9 (CH3O—Carom), 151.2 (C—Narom), 133.7
(C—NHarom), 131.6 (C—Sarom), 125.9 (C—Clarom), 125.7 (Carom),
120.6 (Carom), 119.9 (Carom), 119.7 (2Carom), 114.2 (2Carom), 55.2
(OCH3). Anal. calcd for C14H11N2SOCl: C, 57.82; H, 3.81; found: C,
57.59; H, 3.67.
We thank the National Natural Science Foundation of China
(Project No. 20862008) and the Natural Science Foundation of
Jiangxi Province in China (2010GZH0062) for financial support.
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N-(4-Fluorophenyl)-6-chlorobenzo[d]thiazol-2-amine (3p)
White solid, m.p. 213–214 ꢀC. IR (KBr) n (cmꢁ1) 3164, 2954, 1617,
1579, 1556, 1453, 1321, 1264, 1184, 1145, 1114. UV (DMSO, nm):
297, 223. 1 H NMR (400 MHz, DMSO-d6) d 10.63 (s, 1 H, NH), 7.99
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4-CH), 7.39 (d, J = 8.0 Hz, 1 H, 5-CH), 7.27 (t, J = 8.8 Hz, 2 H, 30-CH).
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(C—Sarom), 126.1 (C—Clarom), 121.4 (Carom), 120.8 (Carom), 120.1
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2
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N-(4-Chlorophenyl)-6-chlorobenzo[d]thiazol-2-amine (3q)
White solid, m.p. 206–207 ꢀC. IR (KBr) n (cmꢁ1) 3165, 2944,
1617, 1579, 1522, 1450, 1364, 1263, 1243, 1186, 1143, 1115.
UV (DMSO, nm): 304, 234. 1 H NMR (400 MHz, DMSO-d6) d
10.68 (s, 1 H, NH), 7.91 (s, 1 H, 7-CH), 7.81 (d, J = 7.6 Hz, 2 H,
20-CH), 7.58 (d, J = 8.0 Hz, 1 H, 4-CH), 7.39 (d, J = 7.6 Hz, 2 H,
30-CH), 7.32 (d, J = 8.0 Hz, 1 H, 5-CH). 13C NMR (100MHz, DMSO-
d6) d 162.4 (C = N), 151.3 (C—Narom), 139.7 (C—NHarom), 132.2
(C—Sarom), 129.3 (2Carom), 126.8 (C—Clarom), 126.6 (C—Clarom),
126.2 (Carom), 121.2 (Carom), 120.7 (Carom), 119.8 (2Carom). Anal.
calcd. for C13H8N2SCl2: C, 52.88; H, 2.73; found: C, 52.67; H, 2.91.
[12] H. Saad, Phosphorus Sulfur 2006, 181, 1557.
Appl. Organometal. Chem. 2013, 27, 109–113
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