Synthesis of 2-aminobenzothiazole via FeCl3-catalyzed tandem reaction of 2-iodoaniline with isothiocyanate in water

Article abstract of DOI:10.1039/c0gc00123f

An FeCl₃-catalyzed tandem reaction of 2-iodoaniline with isothiocyanate in water is described, which provides an environmentally benign, efficient, and practical route for the generation of 2-aminobenzothiazole. This present tandem process shows broad substrate scope in the presence of octadecyltrimethylammonium chloride as a phase-transfer catalyst. In addition, the reaction media can be recovered and recycled without loss of efficiency.

Full text of DOI:10.1039/c0gc00123f

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Synthesis of 2-aminobenzothiazole via FeCl3-catalyzed tandem reaction of
2-iodoaniline with isothiocyanate in water†
Qiuping Ding,*a,b Banpeng Cao,a Xianjin Liu,a Zhenzhen Zonga and Yi-Yuan Peng*b
Received 18th May 2010, Accepted 9th July 2010
DOI: 10.1039/c0gc00123f
An FeCl3-catalyzed tandem reaction of 2-iodoaniline with isothiocyanate in water is described,
which provides an environmentally benign, efficient, and practical route for the generation of
2-aminobenzothiazole. This present tandem process shows broad substrate scope in the presence
of octadecyltrimethylammonium chloride as a phase-transfer catalyst. In addition, the reaction
media can be recovered and recycled without loss of efficiency.
Introduction
The 2-aminobenzothiazole core, as a privileged scaffold, is
found in many natural products and pharmaceuticals that
exhibit remarkable biological activities.1 In addition, some
compounds with the skeleton, which have application in
drugs for the treatment of various diseases, are found, such
as tuberculosis,2 epilepsy,3 diabetes,4 glutamate (e.g. Fig. 1,
Riluzole),5 and tumors (e.g. Fig. 1, R116010).6 Therefore, many
efforts continue to be given to the development of efficient
strategies for their construction.7 Usually, the classical method
for the preparation of 2-aminobenzothiazoles is transition
metal-catalyzed (Pd or Cu was used) intramolecular cyclization
of 2-halobenzothioureas [(Scheme 1, eqn (1)].7a–7d
Scheme 1 Synthesis of 2-aminobenzothiazoles.
derivatives via copper(I)-catalyzed tandem addition–cyclization
reactions of 2-iodoanilines with isothiocyanates in toluene
[(Scheme 1, eqn (2)].7e Subsequently, Li and co-workers reported
the same transformation using FeF3 or CuBr as catalyst
[(Scheme 1, eqn (3)].7f,7g Although good yields can often be
obtained in these methods, the use of organic solvent is always
necessary but is harmful to human health. Organic reactions
without the use of conventional organic solvents have attracted
the attention of synthetic organic chemists, and many novel
and environmentally benign modern solvents such as fluorous
media,10 scCO2,11 ionic liquid12 and water13 have been exten-
sively studied recently. Organic reactions in aqueous media are
Fig. 1 Riluzole and R116010.
definitely the best option, which have recently been found many
applications in organic synthesis, due to the simple operation
relevant heterocyclic compounds are important goals in combi-
and avoiding the use of dry organic solvent. As part of our
natorial chemistry from the viewpoints of operational simplic-
Tandem reactions for the efficient construction of biologically
ity and assembly efficiency.8,9 Tandem addition/C–S coupling
reaction is another powerful strategy for construction of the
2-aminobenzothiazole core.7e–7g Recently, we described a novel
and efficient method for the synthesis of 2-aminobenzothiazole
continuing efforts for the expeditious synthesis of biologically
relevant heterocyclic compounds in a green process,14 herein we
would like to report our recent efforts towards the synthesis
of diverse 2-aminobenzothiazoles via FeCl3-catalyzed tandem
reactions of 2-iodoanilines with isothiocyanates in water. The
transformation proceeded smoothly under mild conditions in
the presence of phase-transfer catalysis (PTC) and the corre-
sponding products were generated in good to excellent yields.
aCollege of Chemistry and Chemical Engineering, Jiangxi Normal
University, 99 Ziyang Road, Nanchang, 330022, China.
E-mail: dqpjxnu@gmail.com
bKey Laboratory of Green Chemistry of Jiangxi Province, 99 Ziyang
Road, Nanchang, 330022, China
Results and discussion
† Electronic supplementary information (ESI) available: Experimental
procedures, characterization data, 1H and 13C NMR spectra of com-
pound 3. See DOI: 10.1039/c0gc00123f
Our studies commenced with the reaction of 2-iodoaniline
1a and phenyl isothiocyanate 2a to optimize the reaction
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Table 1 Condition screening for [Fe]-catalyzed tandem reaction of 2-
iodoaniline 1a with phenyl isothiocyanate 2a in watera
in organic reactions can make reactions faster, obtain higher
conversion or yield, and fewer byproducts, eliminate the need
for expensive or dangerous organic solvents and minimize waste
problems. On the basis of this consideration, we chose PTC-
1 (SDBS) as an additive. To our surprise, the product 3a was
generated in 88% yield (Table 1, entry 21). To examine the
effects of different PTCs on the reaction, several other PTCs were
applied under comparable conditions. The results showed that
PTC-2 (hexadecyldimethylbenzylammonium chloride) could
not improve the yield substantially (76% yield, Table 1, entry
22), PTC-3 (TBAB) did not efficiently promote the reaction
(Table 1, entry 23), and PTC-4 (octadecyltrimethylammonium
chloride) was unexpectedly the most efficient PTC giving an
almost quantitative yield of 3a (Table 1, entry 24). The yield
(97%) was substantially higher than that of the corresponding
reaction in organic solvents (88%7e or 86%7f).
With this promising result in hand, we started to investigate
the scope of this reaction under the optimized conditions
[FeCl3 (5 mol%), 1,10-phenanthroline (L-1, 5 mol%), DABCO
(2.0 equiv.), octadecyltrimethylammonium chloride (PTC-4, 10
mol%), H2O, 80 ◦C]. Initially, we investigated the scope of
isothiocyanates and the results are summarized in Table 2. We
found that the functional groups had little effect on the phenyl
ring for the tandem coupling reactions. Several substituted
groups, such as methoxy, methyl, chloro, fluoro and nitro groups,
onthe phenylring of isothiocyanates were tolerated well (Table2,
entries 1–6). It was found that both electron-rich and electron-
poor aryl isothiocyanates underwent the tandem coupling
reaction efficiently in good to excellent yields. For instance, 2-
iodoaniline 1a reacted with 4-methoxyphenyl isothiocyanate 2b
leading to the desired product 3b in 91% yield (Table 2, entry 2),
and 98% yield of product 3d was afforded when 4-nitrophenyl
isothiocyanate 2d was employed in the reaction (Table 2, entry 3).
To our delight, alkyl isothiocyanate was also a suitable substrate
in this process in moderate yield (Table 2, entries 7).
Encouraged by the above results, we further investigated
the scope and the generality of the method by varying the 2-
iodobenzenamine 2b–2e, which could be facilely derived from
the 4-substituted benzenamines (Table 3, entries 1–14). As
showed in Table 3, generally the reactions proceeded successfully
to afford the corresponding 2-aminobenzothiazole3 in moderate
to excellent yields. The results demonstrated that several func-
tional groups, such as trifluoromethyl, fluoro, chloro, and methyl
on the phenyl ring of 2-iodobenzenamine were tolerated well.
For example, reaction of 2-iodo-4-trifluoromethylbenzenamine
1b with phenyl isothiocyanate 2a afforded the desired product
3h in 92% yield (Table 3, entry 1). A similar result (93% yield)
was obtained for the reaction of 2-iodo-4-methylbenzenamine 1e
with phenyl isothiocyanate 2a (Table 3, entry 11). In comparison
with aryl isothiocyanates, alkyl isothiocyanates seemed to have
similar reactivity (Table 3, entries 8 and 14). Finally, two
less active substrates, 5-bromo-2-bromobenzenamine 1f, and 4-
iodo-2-chlorobenzenamine 1g were also tested. Unfortunately,
only trace desired product was obtained when 5-bromo-2-
bromobenzenamine 1f reacted with phenyl isothiocyanate 2a
(Table 3, entry 15), and no desired product was detected with
4-iodo-2-chlorobenzenamine 1g (Table 3, entry 16).
Entry
Catalyst
L
Base
PTC
Yield [%]b
1
2
3
4
5
6
7
8
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
L-1
L-1
L-1
L-1
L-1
L-1
L-1
L-1
L-1
L-1
L-1
L-1
L-1
L-1
—
L-2
L-3
L-4
L-5
L-6
L-1
L-1
L-1
L-1
Na2CO3
NaHCO3
Cs2CO3
K2CO3
K3PO4
NaOH
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
53
60
trace
trace
—
trace
trace
trace
75
71
70
44
trace
trace
34
trace
trace
trace
65
44
88
76
trace
97
DBU
Et3N
9
FeCl3
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
K3Fe(CN)6
Fe(NO3)3
Fe2O3
Fe2(SO4)3
FeSO4
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
—
—
—
—
PTC-1c
PTC-2c
PTC-3c
PTC-4c
FeCl3
a Reaction conditions: 2-iodoaniline 1a (0.3 mmol), phenyl isothio-
cyanate 2a (1.5 equiv), cat. (5 mol%), ligand (5 mol%), base (2.0
equiv), PTC (phase-transfer catalyst, 10 mol%), H2O (3 mL), 80 ◦C,
overnight. b Isolated yield based on 2-iodoaniline 1a. c PTC-1: SDBS
(sodium dodecylbenzenesulfonate), PTC-2: hexadecyldimethylbenzy-
lammonium chloride, PTC-3: TBAB (tetrabutylammonium bromide),
PTC-4: octadecyltrimethylammonium chloride.
conditions, and the results are summarized in Table 1. The
reaction was catalyzed by FeCl3 (5 mol%) in the presence of 1,10-
phenanthroline (L-1) and base (Na2CO3) in water at 80 ◦C. To
our delight, the desired 2-aminobenzothiazole 3a was generated
in 53% yield (Table 1, entry 1). Encouraged by the result, we
subsequently examined the effects of different bases (Table 1,
entries 2–9). The results showed that DABCO was the best choice
(Table 1, entry 9, 75% yield). Increasing the catalyst (FeCl3)
amount to 10 mol% gave a similar result (77% yield). Then
a series of other Fe catalysts, including K3Fe(CN)6, Fe(NO3)3,
Fe2(SO4)3, FeSO4, and Fe2O3, were evaluated (Table 1, entries
10–14), but the results were all inferior to FeCl3. Subsequently,
we examined the effect of ligands (Table 1, entries 16–20). The
results showed that both b-CD and PPh3 reduced the yield to
some extent (entries 19, 20), and three others (TMEDA, L-
proline, pentane-2,4-dione) showed no activity (Table 1, entries
16–18). A blank experiment showed that ligand L-1 was essential
to obtain a good result (Table 1, entry 15). It is well-known that
PTC is widely exploited in the industry.15 PTC as a promoter
The efficiency of the recovered reaction media was verified
with the reaction of 2-iodoaniline 1a and phenyl isothiocyanate
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Table 2 FeCl3-catalyzed tandem reaction of 2-iodoaniline 1a with
isothiocyanate 2a
Table 3 FeCl3-catalyzed tandem reaction of 2-iodobenzenamine 1 with
isothiocyanate 2a
Entry
1/R1/X
2/R2
Product 3
Yield [%]b
Entry
1
2/R2
Product 3
Yield [%]b
96
1
2
3
4
5
6
7
8
1b/4-CF3/I
1b/4-CF3/I
1b/4-CF3/I
1b/4-CF3/I
1c/4-F/I
2a/C6H5
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
3s
3t
92
65
58
86
91
73
73
77
91
78
93
97
78
73
trace
—
2a/C6H5
2b/4-MeOC6H4
2d/4-NO2C6H4
2e/4-ClC6H4
2a/C6H5
2
3
2b/4-MeOC6H4
2c/4-MeC6H4
91
90
1c/4-F/I
2c/4-MeC6H4
2d/4-NO2C6H4
2g/Et
2a/C6H5
2c/4-MeC6H4
2a/C6H5
2b/4-MeOC6H4
2d/4-NO2C6H4
2g/Et
2a/C6H5
2a/C6H5
1c/4-F/I
1c/4-F/I
9
1d/4-Cl/I
1d/4-Cl/I
1e/4-CH3/I
1e/4-CH3/I
1e/4-CH3/I
1e/4-CH3/I
1f/5-Br/Br
1g/4-I/Cl
10
11
12
13
14
15
16
3u
3v
3w
4
2d/4-NO2C6H4
98
a Reaction conditions: 2-iodoaniline 1 (0.3 mmol), isothiocyanate 2
(1.5 equiv), FeCl3 (5 mol%), 1,10-phenanthroline (5 mol%), PTC-4
(octadecyltrimet◦hylammonium chloride, 10 mol%), DABCO (2.0 equiv),
H2O (3 mL), 80 C. b Isolated yield based on 2-iodoaniline 1.
5
6
7
2e/4-ClC6H4
2f/4-FC6H4
2g/Et
90
82
65
Table 4 Efficiency of the recovered reaction media
no. of cycles
yield [%]
cycle I
97
cycle II
96
cycle III
80
isothiocyanates. The results showed that phase-transfer catalyst
PTC-4 (octadecyltrimethylammonium chloride) could promote
the reaction efficiently. The product was easily separated from
the reaction mixture by filtration, and the reaction media can be
recovered and recycled with good catalytic activity.
a Reaction conditions: 2-iodoaniline 1a (0.3 mmol), isothiocyanate 2
(1.5 equiv), FeCl3 (5 mol%), 1,10-phenanthroline (5 mol%), PTC-4
(octadecyltrimet◦hylammonium chloride, 10 mol%), DABCO (2.0 equiv),
H2O (3 mL), 80 C. b Isolated yield based on 2-iodoaniline 1.
Experimental section
General experimental
All reactions were performed in test tubes under air. Flash
column chromatography was performed using silica gel (200–
300 mesh). Analytical thin-layer chromatography was performed
using glass plates pre-coated with 0.25 mm 230–400 mesh silica
gel impregnated with a fluorescent indicator (254 nm). Thin layer
chromatography plates were visualized by exposure to ultraviolet
light. Organic solutions were concentrated on rotary evaporators
at 25–35 ◦C. Commercial reagents and solvents were used as
received.
Proton (1H NMR) and carbon (13C NMR) nuclear magnetic
resonance spectra were recorded on a Bruker AV 400 spectrome-
ter at 400 MHz and 100 MHz respectively at 293 K. The chemical
shifts are given in parts per million (ppm) on the delta scale (d)
and referenced to tetramethylsilane (0 ppm).
2a (Table 4). Using the fresh reaction media the yield of the
desired product N-phenylbenzo[d]thiazol-2-amine 3a was 97%,
while with the recovered media the yields were 96, and 80% in
the next two recyclizations.
Conclusions
We have established a highly efficient FeCl3-catalyzed tandem
reaction of 2-iodoanilines 1 with isothiocyanates 2 for the
synthesis of 2-aminobenzothiazoles 3. The advantages of this
method include simple, mild and environmentally benign reac-
tion conditions, experimental ease, good substrate generality,
and the toleration of a wide range of aromatic and aliphatic
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Synthesis of 2-aminobenzothiazoles
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A mixture of 2-iodoaniline 1 (0.3 mmol), isothiocyanate 2
(0.45 mmol, 1.5 equiv), DABCO (0.6 mmol, 2 equiv), FeCl3
(0.015 mmol, 5 mol%), 1,10-phenanthroline L-1 (0.015 mmol,
5 mol%), and octadecyltrimethylammonium chloride PTC-4
(0.03 mmol, 10 mol%) was stirred in water (3 mL) at 80
◦C. After completion of the reaction as indicated by TLC,
the mixture was cooled to room temperature. The mixture was
washed with saturated brine, and extracted with ethyl acetate.
The organic layer was dried with anhydrous MgSO4 and the
solvent was evaporated under vacuum, and then the residue
was purified by flash column chromatography on silica gel to
provide the corresponding pure product 3. Selected example: N-
phenylbenzo[d]thiazol-2◦-amine (3a):16 white solid, mp 158–160
◦C (lit. mp 157.2–159.4 C); IR (prism, KBr, cm-1) 3468, 1627;
1H NMR (400 MHz, CDCl3) d 7.13–7.19 (m, 2H), 7.32 (t, J =
7.6 Hz, 1H), 7.41 (t, J = 8.4 Hz, 2H), 7.50 (d, J = 8.0 Hz, 2H),
7.56 (d, J = 8.0 Hz, 1H), 7.63 (d, J = 8.0 Hz, 1H); 13C NMR
(100 MHz, CDCl3) d 118.7, 119.9, 120.4, 121.9, 123.9, 125.6,
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Acknowledgements
Financial support from the Natural Science Foundation of
Jiangxi Province of China (2009GQH0054), National Natural
Science Foundation of China (20962010), Jiangxi Educational
Committee (GJJ10387), and Startup Foundation for Doctors of
Jiangxi Normal University (200900266) is gratefully acknowl-
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