A simple synthesis of 2-arylbenzothiazoles and its application to palladium-catalyzed Mizoroki-Heck reaction

Article abstract of DOI:10.1016/j.tetlet.2006.04.019

A variety of 2-arylbenzothiazoles were prepared by the direct reaction of 2-aminobenzenethiol and aryl aldehydes by the aid of activated carbon (Shirasagi KL or Darco KB) under oxygen atmosphere. 2-Pyridylbenzothiazole, thus obtained, was proved to work as an efficient ligand in palladium-catalyzed Mizoroki-Heck reaction.

Full text of DOI:10.1016/j.tetlet.2006.04.019

Tetrahedron Letters 47 (2006) 4231–4233
A simple synthesis of 2-arylbenzothiazoles and its application
to palladium-catalyzed Mizoroki–Heck reaction
Yuka Kawashita, Chigusa Ueba and Masahiko Hayashi*
Department of Chemistry, Faculty of Science, Kobe University, Nada, Kobe 657-8501, Japan
Received 14 March 2006; revised 6 April 2006; accepted 7 April 2006
Available online 5 May 2006
Abstract—A variety of 2-arylbenzothiazoles were prepared by the direct reaction of 2-aminobenzenethiol and aryl aldehydes by the
Ò
aid of activated carbon (Shirasagi KL or Darco KB) under oxygen atmosphere. 2-Pyridylbenzothiazole, thus obtained, was proved
to work as an efficient ligand in palladium-catalyzed Mizoroki–Heck reaction.
Ó 2006 Elsevier Ltd. All rights reserved.
2
-Arylbenzoxazoles, -benzimidazoles and -benzothi-
2-arylbenzothiazoles in the presence of activated carbon
(Shirasagi KL; Japan EnviroChemicals, Ltd or Darco
Ò
azoles are a remarkably important class of biologically
active compounds.1
–5
Among these, 2-arylbenzothi-
KB; Aldrich, Inc.) under oxygen atmosphere in high
yield (Table 1). The reactions took place at 50 °C, under
milder conditions compared with the case of the synthe-
azoles and their derivatives have attracted much atten-
tion not only in pharmaceutical but also in material
fields. Therefore, there have been some reports for the
synthetic procedures of these compounds. Oxidative
cyclization of the corresponding Schiff base provides a
general method. Recently, Ohsawa and his co-workers
reported one-step synthesis of 2-arylbenzothiazoles and
imidazoles using scandium triflate (Sc(OTf) ) as a cata-
lyst. They described that Sc(OTf) worked as a catalyst
8
sis of 2-arylbenzoxazoles and 2-arylbenzimidazoles.
The reaction also proceeded under air atmosphere
instead of oxygen, though it took longer time, for exam-
ple, in the reaction of 2-aminobenzenethiol and benz-
aldehyde (50 °C, 13 h, 82%), 4-methylbenzaldehyde
(50 °C, 13 h, 82%) and 4-chlorobenzaldehyde (50 °C,
13 h, 76%).
6
3
7
3
for both ring-closing and oxidation steps.
It should be mentioned that the reaction of 2-aminobenz-
enethiol with formaldehyde (37 wt % solution in water
containing 7–8% methanol) also took place to afford
the benzothiazole in moderate yield (52% isolated yield)
(Scheme 1). Benzothiazole is known as a very important
intermediate compound, because many functional
groups will be introduced to these as the second
On the other hand, we recently reported that direct syn-
thesis of 2-arylbenzoxazoles and 2-arylbenzimidazoles
(
or 1,2-phenylenediamines) from substituted 2-amino-
phenols and aldehydes in the presence of activated
8
carbon in xylene under an oxygen atmosphere. In this
letter, we report an ultimately simple and efficient
synthesis of 2-arylbenzothiazoles by the reaction of
9
position.
2
-aminobenzenethiol and aryl aldehydes in the presence
of activated carbon under oxygen or air atmosphere.
Also, we will reveal that 2-pyridylbenzothiazole
obtained by the reaction of 2-aminobenzenethiol and
The Mizoroki–Heck reaction has been widely used for
the construction of carbon–carbon bonds since 1971.
As ligands, phosphine and related compounds are often
used. Recently, electron-rich and bulky alkyl phos-
1
0
2
-pyridylaldehyde proved to serve as an efficient ligand
in palladium-catalyzed Mizoroki–Heck reaction.
phines, such as P(t-Bu) , were developed as highly effec-
3
1
1
tive ligands. Especially, the advancement in the field
1
2
A variety of 4-substituted benzaldehydes reacted with
of palladacycles, N-heterocyclic carbene-complexes,
1
3
2
-aminobenzenethiol to produce the corresponding
should be noteworthy. We also have interest in the
development of an active and simple catalyst system in
Mizoroki–Heck reactions, and especially, we have
focused on imidazole derivatives, because their struc-
tures are simple, stable and inexpensive. The reaction
*
Corresponding author. Tel.: +81 78 803 5687; fax: +81 78 803
5688; e-mail: mhayashi@kobe-u.ac.jp
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.04.019

4232
Y. Kawashita et al. / Tetrahedron Letters 47 (2006) 4231–4233
a
Table 1. Direct Synthesis of 2-Arylbenzthiazole
O2
SH
S
N
Shirasagi KL
+
OHC
R
R
xylene, 50 °C
NH2
b
Entry
R
Time/h
% Yield
79
1
2
3
4
5
6
H
CH
OCH
Cl
CN
3
3
3
4
4
4
c
3
82
81
c
3
72
82
86
NO
2
a
b
c
The ratio of 2-aminobenzenethiol and aldehydes was 1:1. Shirasagi KL (625 mg) was used per 5 mmol (626 mg) of 2-aminobenzenethiol.
Isolated yield by recrystallization unless otherwise noted.
Isolated yield by silica-gel column chromatography.
O2
As shown in Table 2, the combination of PdCl and the
2
SH
O
S
N
Shirasagi KL
2-pyridylbenzothiazole (entry 6) and 2-pyridylbenzimid-
azole system (entry 8) exhibited higher reactivity in
Mizoroki–Heck reaction compared with Jefferry’s
+
H
H
NH2
xylene, 50 °C
5
2%
1
3
conditions.
Scheme 1.
In conclusion, we have disclosed a simple and direct syn-
thesis of 2-arylbenzothiazole by the aid of activated car-
bon and the utility of 2-pyridylbenzothiazole as a ligand
in palladium-catalyzed Mizoroki–Heck reaction. We are
now investigating simple imidazole-containing nitrogen
coordinated palladium complexes in coupling reactions
encouraged by the result of entry 8, which will be pub-
of 2-aminobenzenethiol with 2-pyridylaldehyde, by the
aid of an activated carbon–oxygen system, produced 2-
pyridylbenzothiazole in 82% yield (Scheme 2). We then
examined the reaction of 4-bromotoluene with methyl
acrylate using 1 mol % of a palladium catalyst and
1
4–16
1
mol % of ligands under various conditions (Table 2).
lished in the near future.
O2
SH
S
N
Shirasagi KL
+
OHC
xylene, 50 °C
N
N
NH2
1
Scheme 2.
Table 2. Mizoroki–Heck reaction using 2-pyridylbenzothiazole, -benzoxazole and -benzimidazole ligands
O
1
1
mol% Pd catalyst
mol% ligand
Br
O
OCH3
+
OCH3
2 eq. K
2
CO
2 3
eq. additive
Me
Me
DMF, 120 ˚C, 24 h
a
Entry
Pd catalyst
Ligand
Additive
% Yield
1
2
3
4
5
6
7
8
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
2
None
1
n-Bu
n-Bu
n-Bu
n-Bu
None
None
None
None
4
4
4
4
NBr
NBr
NBr
NBr
34
24
26
64
9
79
42
84
2
3
PdCl
PdCl
PdCl
PdCl
2
2
2
2
None
1
2
3
a
Isolated yield.
H
N
O
N
N
N
N
2
3

Y. Kawashita et al. / Tetrahedron Letters 47 (2006) 4231–4233
4233
Acknowledgements
J. Org. Chem. 1972, 37, 2320–2322; (c) Tsuji, J. Palladium
Reagents and Catalysts: New Perspectives for the 21st
Century; John Wiley & Sons, 2004; Chapter 3.
This research was supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education,
Culture, Sports, Science and Technology, Japan (No.
B17340020), and Hyogo COE program. We also thank
Dr. Hideki Amii for helpful discussion.
¨
1
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5. General procedure for the synthesis of 2-arylbenzothi-
azoles (Table 1);
A mixture of 2-aminothiophenol
(
5 mmol), aldehyde (5 mmol) and Shirasagi KL (625 mg)
in xylene (8 mL) was placed in a 100 mL three-necked
flask under oxygen atmosphere (using 1 L of balloon) and
stirred at 50 °C for 3–4 h. The reaction mixture was then
filtered using Celite. After the filtrate was concentrated,
the product was isolated by recrystallization or silica-gel
column chromatography.
6
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0
.01 mmol), 2-pyridylbenzothiazole (4.24 mg, 0.01 mmol)
and K CO (552.8 mg, 4 mmol) in DMF (10.0 mL) and
2
3
the mixture was stirred at 50 °C for 1 h. 4-Bromotoluene
246.1 lL, 2 mmol) and methyl acrylate (358.7 lL,
mmol) in 1 mL of DMF were added. The mixture was
(
4
7
8
9
then stirred at 120 °C for 24 h. The reaction mixture was
cooled, and then the precipitates were removed by
filtration and extracted with ethyl acetate. The combined
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organic layer was dried over anhydrous Na SO and
2
4
filtered. After evaporation, the obtained residue was
purified by silica-gel column chromatography to give
methyl trans-4-methylcinnamate in 79% yield (277.5 mg).
2
006, 16, 1537–1543.
1
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