Synthesis of N-substituted-2-Aminobenzothiazoles by ligand-Free copper(I)-Catalyzed cross-Coupling reaction of 2-haloanilines with isothiocyanates www.eurjoc.org

Article abstract of DOI:10.1002/ejoc.200900953

A novel and efficient formation of N-substituted-2-aminobenzothiazoles by a ligand-free copper(I)-catalyzed one-pot cascade process was developed. A variety of isothiocyanates coupled with 2-iodoanilines to give N-substituted-2-amino-benzothiazoles in mod

Full text of DOI:10.1002/ejoc.200900953

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200900953
Synthesis of N-Substituted-2-Aminobenzothiazoles by Ligand-Free Copper(I)-
Catalyzed Cross-Coupling Reaction of 2-Haloanilines with Isothiocyanates
Guodong Shen,[a] Xin Lv,[a] and Weiliang Bao*[a]
Keywords: Copper / Fused-ring systems / Domino reactions / Cross-coupling / Heterocycles
A novel and efficient formation of N-substituted-2-amino- benzothiazoles in moderate to excellent yields under mild
benzothiazoles by a ligand-free copper(I)-catalyzed one-pot
cascade process was developed. A variety of isothiocyanates
conditions.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
coupled with 2-iodoanilines to give N-substituted-2-amino- Germany, 2009)
tribromide,[6,1b] (ii) coupling of arylamines with 2-halo-
Introduction
benzothiazole,[7] (iii) coupling of 2-aminobenzothiazoles
with aryl halides,[8] (iv) oxidative cyclization of the interme-
diates generated by 2-aminothiophenol with isothio-
cyanates,[9] and (v) intramolecular C–S bond formation for
the synthesis of 2-aminobenzothiazoles catalyzed by CuI/
Pd[10] and some multistep methods.[11] Although these reac-
tions might proceed efficiently, they usually suffer from the
use of highly toxic and corrosive reagents, high-costing
metal catalysts, and specific ligands. These drawbacks make
the methodologies difficult to be a choice in large- and in-
dustrial-scale applications. However, in recent years copper-
catalyzed reactions, especially ligand-free reactions, have re-
ceived considerable attention because of their efficiency and
low costs.[12] Very recently, these copper-catalyzed strategies
have been successfully applied to the synthesis of various
heterocyclic compounds through one-pot strategies.[13] Our
group is also interested in this copper-catalyzed one-pot
strategy and developed a copper-catalyzed cascade ad-
dition/cyclization methodology for the synthesis of several
types of heterocyclic compounds.[14] For example, orto-
iodophenol underwent the intermolecular addition/intra-
molecular C–S coupling reaction with isothiocyanate under
catalysis of CuI–ligand–base to give 2-iminobenzo-1,3-oxa-
thioles.
2-Aminobenzothiazoles such as 2-anilinobenzothiazoles,
2-(N-alkylamino)benzothiazoles, and 2-(N-acylamino)benzo-
thiazoles are important intermediates and broadly found in
biological chemistry and medicinal areas.[1] A great amount
of 2-anilinobenzothiazole derivatives are found to be anti-
cancer active. For example, 2-anilinobenzothiazole (A;
R116010) is a potential inhibitor of retinoic acid meta-
bolism;[2] 2-anilinobenzothiazole (B) is claimed to possess
activity for enhancing the atRA-induced expression of
CYP26B1.[3] Many 2-(N-alkylamino)benzothiazoles (C)[1b,4]
and 2-(N-acylamino)benzothiazole derivatives (D)[5] might
also be biologically active and of pharmaceutical value
(Figure 1).
Herein, we wish to report a new example of the copper-
Figure 1. Several N-substituted-2-aminobenzothiazole derivatives
reported as biologically active compounds and pharmaceutical catalyzed cascade addition/cyclization reaction to synthe-
products.
size N-substituted-2-aminobenzothiazoles catalyzed by a
copper(I) salt under ligand-free conditions. Our designed
and proposed approach is similar to our previous reactions
and is summarized in Scheme 1. The nucleophilic N atom
of 2-iodoanilines a would attack the carbon atom of NCS
There are several methods to synthesize N-substituted-2-
aminobenzothiazoles, including: (i) cyclization of arylthio-
ureas with liquid bromine and benzyltrimethylammonium
on isothiocyanatobenzene b; intermediate d could then be
formed in the presence of a proper base (step 1).[14c] With
[a] Department of Chemistry, Xixi Campus, Zhejiang University,
a proper copper(I) catalyst, the sulfur atom would take pri-
ority over the nitrogen atom in assaulting cyclization action
and d might be converted into product c through intra-
molecular C–S coupling (step 2).[15]
Hangzhou, Zhejiang 310028, P. R. China
Fax: +86-571-88273814
E-mail: wlbao@css.zju.edu.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.200900953.
Eur. J. Org. Chem. 2009, 5897–5901
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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G. Shen, X. Lv, W. Bao
SHORT COMMUNICATION
and anticipated product 1c was obtained in 45% yield after
flash chromatography (Table 1, Entry 1). To increase the
yield, the reaction temperature was set to 125 °C and 65%
yield was obtained (Table 1, Entry 2). Besides DMSO, a
number of other solvents such as NMP and nPrCN were
also surveyed under these conditions, but the yield did not
improve greatly (Table 1, Entries 3 and 4). Different ligands
such as ethyl 2-oxocyclohexanecarboxylate (β-keto ester)
and -proline were also tested, and the yield changed a little
(Table 1, Entries 5 and 6). We envisioned that the reaction
should be efficient. As changes in the temperature, ligand,
and solvent did not have much of an effect, we thought that
maybe the base played an important role. The amount of
the base was reduced to one equivalent. To our delight, the
starting materials disappeared and product 1c was formed
in excellent yield after 24 h (Table 1, Entry 7). When a much
cheaper base K2CO3 was used, an even higher yield was
obtained (97%; Table 1, Entry 8). Considering that β-diaza
compounds could serve as a ligand in the CuI-catalyzed
coupling reactions,[16] we tried a “ligand-free” reaction, be-
cause the intermediate and the product in our reaction were
β-diaza compounds. Only CuI and K2CO3 (1 equiv.) were
added. The reaction was conducted in DMSO at 125 °C
and the product was obtained in 97% yield (Table 1, En-
try 9). By lowering the reaction temperature to 95 °C, the
Scheme 1. Designed one-pot synthesis of N-substituted-2-amino-
benzothiazoles through intermolecular addition/intramolecular C–
S coupling.
Results and Discussion
Initially, 2-iodo-4-methylaniline (1a) and isothiocyanato-
benzene (1b) were selected as the model substrates (Table 1).
Catalyzed by CuI, different ligands, temperatures, bases,
and solvents were examined to set up standard reaction
conditions. The reaction was firstly carried out in DMSO
with CuI (15 mol-%), 1,10-phenanthroline (30 mol-%) as
the catalyst, Cs2CO3 (3.0 equiv.) as the base at 115 °C under
a nitrogen atmosphere. The reaction was stopped 24 h later,
Table 1. Optimization of the reaction conditions.
Entry
Catalyst/ligand
Base (equiv.)
Solvent
T
[°C]
Yield
[%][a]
1
2
3
4
5
6
7
8
CuI/1,10-phenanthroline
CuI/1,10-phenanthroline
CuI/1,10-phenanthroline
CuI/1,10-phenanthroline
CuI/-proline
CuI/β-keto ester
CuI/1,10-phenanthroline
CuI/1,10-phenanthroline
Cs2CO3 (3.0)
Cs2CO3 (3.0)
Cs2CO3 (3.0)
Cs2CO3 (3.0)
Cs2CO3 (3.0)
Cs2CO3 (3.0)
Cs2CO3 (1.0)
K2CO3 (1.0)
K2CO3 (1.0)
K2CO3 (1.0)
K2CO3 (1.0)
K2CO3 (1.0)
K2CO3 (1.0)
K2CO3 (1.0)
Et3N (1.0)
DMSO
DMSO
nPrCN
NMP
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
1,4-dioxane
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
115
125
125
125
125
125
125
125
125
95
85
65
95
95
95
95
95
95
95
95
45[b]
65[b]
68[b]
30[b]
30[b]
64[b]
96[c]
97[c]
97[d]
96[d]
90[d]
40[d]
94[d]
40[d]
35[d]
18
9
CuI
CuI
CuI
CuI
CuI
CuI
CuI
–
10
11
12
13
14
15
16
17
18
19
20
K2CO3 (1.0)
K2CO3 (1.0)
K2CO3 (1.0)
K2CO3 (1.0)
K2CO3 (1.0)
Cu2O
CuBr
Cu(OAc)2
CuI
80[d]
92[d]
94[d]
98[e]
[a] Isolated yield after flash chromatography and based on 2-iodo-4-methylaniline.[b] Reaction conditions: CuI (0.06 mmol), ligand
(0.12 mmol), 2-iodo-4-methylaniline (0.4 mmol), isothiocyanatobenzene (0.7 mmol), and base (1.2 mmol) in solvent (2.0 mL) under a
nitrogen atmosphere. [c] Reaction conditions: CuI (0.06 mmol), ligand (0.12 mmol), 2-iodo-4-methylaniline (0.4 mmol), isothiocyanatob-
enzene (0.7 mmol), and base (0.4 mmol) in solvent (2.0 mL) under a nitrogen atmosphere. [d] Reaction conditions: copper salt
(0.06 mmol), 2-iodo-4-methylaniline (0.4 mmol), isothiocyanatobenzene (0.7 mmol), and base (0.4 mmol) in solvent (2.0 mL) under a
nitrogen atmosphere. [e] Reaction conditions: CuI (0.75 mmol), 2-iodo-4-methylaniline (5 mmol), isothiocyanatobenzene (8.75 mmol), and
base (5 mmol) in solvent (10 mL) under a nitrogen atmosphere.
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Copper(I)-Catalyzed Cross-Coupling of 2-Haloanilines with Isothiocyanates
yield decreased only a little (Table 1, Entry 10). Obviously was obtained in 18% yield without the catalyst (Table 1,
these are the best conditions. Et3N was also tried but it was Entry 16). Some other catalysts such as Cu2O, CuBr, and
ineffective (Table 1, Entry 15). Interestingly, the product Cu(OAc)2 were also tested (Table 1, Entries 17–19), but CuI
provided the best yield. When the amount of 2-iodo-4-
Table 2. CuI-catalyzed one-pot synthesis of N-substituted-2-amino-
methylaniline was increased to 5 mmol (1.17 g), the yield
increased to 98% (Table 1, Entry 20).
benzothiazoles from 2-iodoanilines a and isothiocyanates b.[a]
With the optimal condition established, we tried to inves-
tigate the scope of this methodology. Firstly, the reactions
of 2-iodoanilines a with various substituted isothiocyanates
b were examined (Table 2, Entries 1–13). The isothiocyana-
tobenzenes bearing both electron-donating groups (o-Me,
m-Me, p-Me, and p-MeO) and electron-withdrawing groups
(p-Cl) were able to couple with 1a in moderate to excellent
yields (Table 2, Entries 1–5). The yields of the reaction were
not affected by the steric hindrance of both substrates
(Table 2, Entries 1–4), but electron-withdrawing groups on
the isothiocyanates were unfavorable to the reaction
(Table 2, Entry 5). Besides the isothiocyanatobenzenes,
some other isothiocyanates like benzoyl isothiocyanate and
(isothiocyanatomethyl)benzene could also react well with 2-
iodo-4-methylaniline (Table 2, Entries 6–8). These sub-
strates enlarged the scope of this reaction. Furthermore, 2-
iodoaniline was also investigated and good results were de-
tected (Table 2, Entries 9–11). As for 2-iodoanilines bearing
electron-withdrawing groups, the yields decreased obviously
(Table 2, Entries 12 and 13).
Encouraged by the above results, we further investigated
the scope and the generality of the method by varying the
2-iodoanilines to 2-bromoanilines and 2,6-dibromopyridin-
Table 3. CuI-catalyzed one-pot synthesis of N-substituted-2-amino-
benzothiazoles from 2-bromoaniline a and isothiocyanates b.[a]
[a] Reaction conditions: CuI (0.06 mmol), 2-iodo-4-methylaniline
(0.4 mmol), isothiocyanates (0.7 mmol), and base (0.4 mmol) in
DMSO (2.0 mL) at 95 °C under a nitrogen atmosphere. [b] Isolated
yield after flash chromatography based on 2-iodoanilines.
[a] Reaction conditions: CuI (0.06 mmol), 2-iodo-4-methylaniline
(0.4 mmol), isothiocyanates (0.7 mmol), and base (0.4 mmol) in
DMSO (2.0 mL) at 115 °C under a nitrogen atmosphere. [b] Iso-
lated yield after flash chromatography based on 2-bromoanilines.
Eur. J. Org. Chem. 2009, 5897–5901
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G. Shen, X. Lv, W. Bao
SHORT COMMUNICATION
N2, DMSO (2.0 mL) was added by syringe. Then, isothiocyanates
(0.7 mmol) were added by syringe under a counter flow of N2. The
reaction mixture was stirred for 24 h at 95(115) °C. The reaction
was monitored by TLC. After the starting material was consumed
completely, the reaction was stopped and cooled to room tempera-
ture. H2O (20 mL) was added to the solvent, and the mixture was
extracted with EtOAc (3ϫ15 mL). The extracts were combined
and washed with water (2ϫ10 mL) and brine (15 mL) and dried
(MgSO4). After evaporating the solvent under reduced pressure,
the residue was purified by column chromatography on silica gel
to give the pure product.
3-amine (Table 3, Entries 1–8). To our delight, generally the
reactions proceeded successfully in moderate to excellent
yields, although higher temperatures were required. Several
2-bromoanilines were coupled with various isothiocyanates
from moderate to excellent yields (Table 3, Entries 1–5). Fi-
nally, 2,6-dibromopyridin-3-amine was also tested. The
yield was relatively lower than that of the 2-bromoanilines,
probably because the electronic effect of 2,6-dibromopyr-
idin-3-amine (6-Br) influenced the intramolecular coupling
process (Table 3, Entries 6 and 7).
A plausible mechanism, which accounts for the forma-
tion of the N-substituted-2-aminobenzothiazoles from the
2-iodoanilines and isothiocyanates, is shown in Scheme 2.
The nucleophilic N atom of 2-iodoanilines a would attack
the carbon atom on NCS and intermediate d could be
formed in the presence of a proper base. Coordination of
copper to d gave e. Reductive elimination released product
c with concomitant regeneration of the catalyst.
Supporting Information (see footnote on the first page of this arti-
1
cle): Spectroscopic data and copies of the H and 13C NMR spec-
tra.
Acknowledgments
This work was financially supported by the Specialized Research
Fund for the Doctoral Program Foundation of Higher Education
of China (No. 2060335036).
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Conclusions
In summary, we have developed a novel, efficient, and
concise method to synthesize N-substituted-2-aminobenzo-
thiazoles under ligand-free copper(I)-catalyzed conditions.
A simple “one-pot” operation was conducted, readily avail-
able starting materials were employed, and relatively mild
conditions were applied. Various N-substituted-2-amino-
benzothiazoles, which might be potentially applicable in the
pharmaceutical and biochemical areas, were conveniently
synthesized in moderate to excellent yields. The reaction is
also a good example of a ligand-free copper(I)-catalyzed
one-pot cascade process.
Experimental Section
General Procedure: An oven-dried Schlenk tube equipped with a
Teflon valve was charged with a magnetic stir bar, K2CO3
(0.4 mmol, 100 mol-%), CuI (0.06 mmol, 15 mol-%), and 2-halo-
anilines (0.4 mmol). The tube was evacuated and backfilled with
N2 (this procedure was repeated 3 times). Under a counter flow of
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Received: August 22, 2009
Published Online: October 21, 2009
Eur. J. Org. Chem. 2009, 5897–5901
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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