Acceptorless dehydrogenative synthesis of benzothiazoles and benzimidazoles from alcohols or aldehydes by heterogeneous Pt catalysts under neutral conditions

Article abstract of DOI:10.1016/j.tetlet.2015.06.073

Abstract Pt/Al₂O₃ and Pt/TiO₂ were effective catalysts for the synthesis of 2-substituted benzothiazoles and benzimidazoles from 2-aminothiophenol and 1,2-phenylenediamine with alcohols or aldehydes under acceptor-free and additive-free conditions.

Full text of DOI:10.1016/j.tetlet.2015.06.073

Accepted Manuscript
Acceptorless dehydrogenative synthesis of benzothiazoles and benzimidazoles
from alcohols or aldehydes by heterogeneous Pt catalysts under neutral condi-
tions
Chandan Chaudhari, S.M.A. Hakim Siddiki, Ken-ichi Shimizu
PII:
S0040-4039(15)01092-8
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.06.073
TETL 46465
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
18 May 2015
10 June 2015
24 June 2015
Please cite this article as: Chaudhari, C., Hakim Siddiki, S.M.A., Shimizu, K-i., Acceptorless dehydrogenative
synthesis of benzothiazoles and benzimidazoles from alcohols or aldehydes by heterogeneous Pt catalysts under
neutral conditions, Tetrahedron Letters (2015), doi: http://dx.doi.org/10.1016/j.tetlet.2015.06.073
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Acceptorless dehydrogenative synthesis of benzothiazoles
and benzimidazoles from alcohols or aldehydes by heterogeneous Pt catalysts under neutral conditions
Chandan Chaudhari, S. M. A. Hakim Siddiki, Ken-ichi Shimizu*

1
Tetrahedron Letters
journal homepage: www.els evier.com
Acceptorless dehydrogenative synthesis of benzothiazoles and
benzimidazoles from alcohols or aldehydes by heterogeneous Pt catalysts
under neutral conditions
a
Chandan Chaudhari, S. M. A. Hakim Siddiki, Ken-ichi Shimizu*
b
a,b
a
Catalysis Research Center, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan
Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Katsura, Kyoto 615-8520, Japan
b
ARTICLE INFO
ABSTRACT
Article history:
Received
2 3 2
Pt/Al O and Pt/TiO were effective catalysts for the synthesis of 2-substituted benzothiazoles
and benzimidazoles from 2-aminothiophenol and 1,2-phenylenediamine with alcohols or
aldehydes under acceptor-free and additive-free conditions.
2009 Elsevier Ltd. All rights reserved.
Received in revised form
Accepted
Available online
Keywords:
Benzothiazoles
Benzimidazoles
Platinum
Heterogeneous catalysis
a
Benzazoles such as benzothiazoles and benzimidazoles
Table 1 Synthesis of the benzothiazole by various catalysts.
are considered as important class of chemicals in medicinal
chemistry. In particular, 2-substituted benzothiazoles and
benzimidazoles are of importance due to their pharmacological
1
and biological activities. The conventional synthetic method of
2
Entry
Catalysts
Pt/Al
Pd/Al
Conv.(%)
99
GC yield (%)
-substituted benzothiazoles or 2-substituted benzimidazoles
1
2
2
O
3
97
89
57
48
46
44
34
13
0
involves condensation of 2-aminothiophenols or 1,2-
phenelynediamines with carboxylic acids or their derivatives
under acidic conditions or in the presence of dehydrating reagent.
2
O
3
99
3
Ir/Al
2
O
3
99
4
Cu/Al
2
O
3
99
2
Although continuous efforts have been focused on this method,
5
Re/Al
2
O
3
99
it has serious drawbacks including low atom-efficiency and
production of toxic salt wastes. The other important method is
the condensation of aldehydes with 2-aminothiophenols or 1,2-
6
Rh/Al
Ru/Al
2
2
O
3
3
99
7
O
99
8
Ag/Al
Ni/Al
(197 mg)
Pt/Nb
Pt/CeO
Pt/SiO -Al
Pt/HBEA
Pt/La
Pt/MgO
2
O
3
99
phenelynediamines, followed by oxidation by oxidants (H
3b
2
-
3c
3a
9
2
O
3
43
acceptor) such as crotononitrile,
(NH
4
)
2
S
2
O
8
,
I /KI,
2
3
d
3e
4
10
Al
2
O
3
30
0
2 2
H O /CAN, polymer supported hypervalent iodine, or air.
1
1
2
O
5
99
64
19
12
10
5
However, most of the methods suffer from the use of the
3
stoichiometric oxidants, which results in low atom-efficiency.
12
2
99
1
1
1
1
1
3
4
5
6
7
2
2
O
3
99
Use of alcohols as starting materials is also attractive because of
wide availability of alcohols. There are reports on the synthesis
of 2-substituted benzothiazoles or 2-substituted benzimidazoles
from 2-aminothiophenols or 1,2-phenelydiamines with alcohols
5b
99
2
O
3
99
99
0
5
a ®
Pt/ZrO
2
2
99
0
using stoichiometric amount of oxidants (MnO
2
,
T3P/DMSO,
5
IBX ) or H
c
3a
-acceptor or air. Use of 2-nitroanilines as a
6
7a
18
19
Pt/TiO
99
0
2
7
b
substrate and primary amines as alkylating reagents are also
reported. The most attractive method is the direct synthesis of
benzazoles using alcohols (or aldehydes) in the absence of
Pt/SiO
2
99
0
a
Conversion and yield are based on 2-aminothiophenol.

2
Tetrahedron
8
oxidants via acceptorless dehydrogenative coupling. Recently,
than 1 wt% Pt/Al
refluxing o-xylene gave lower yield (75%) that that in refluxing
mesitylene. In summary, use of 1 wt% Pt/Al the refluxing
2 3 2 3
O . The reaction with 1 wt% Pt/Al O in
8a-c
8d
homogeneous Ru, Ir and Fe catalysts and a heterogeneous
Ru catalyst have been reported to be effective for the synthesis
of benzazoles from alcohols under acceptor-free conditions.
8e
8f
2 3
O
mesitylene is the best conditions for the synthesis of the
benzothiazole from 2-aminothiophenol and 1-octanol. This is the
first successful example of acceptorless dehydrogenative
synthesis of 2-substituted benzothiazole from 2-aminothiophenol
and less reactive aliphatic alcohol. After the reaction of entry 1 in
8a-e
However, the homogeneous systems
require more than
stoichiometric amount of basic additives or excess amount of
alcohols, which results in low atom-efficiency. As a part of our
studies on heterogeneous Pt catalysis for the acceptorless
9
dehydrogenative coupling reactions of alcohols, we report herein
Table 1, Pt/Al was separated from the mixture by
centrifugation, followed by drying at 90 ºC for 12 h and by H –
reduction at 500 ºC for 0.5 h. The second cycle by the recovered
catalyst showed lower yield (66%) than the first cycle.
Table 2 shows general applicability of the dehydrogenative
2
O
3
the Pt catalyzed dehydrogenative synthesis of 2-substituted
benzothiazoles and benzimidazoles directly from alcohols or
aldehydes under acceptor-free and additive-free conditions.
2
Table 2 Synthesis of 2-substituted benzothiazoles from 2-
synthesis
aminothiophenol and 1.2 equiv. of alcohols or aldehydes using
Pt/Al containing mol% of Pt with respect to 2-
of
2-substituted
benzothiazoles
from
2-
2 3.
aminothiophenol with alcohols or aldehydes by Pt/Al O
2
O
3
1
aminothiophenol. After 24 h of the reaction, the linear aliphatic
alcohols (entries 1 and 2) and benzyl alcohol (entry 3) were
converted to the corresponding 2-substituted benzimidazoles in
moderate to high yields. The method was also effective for the
reaction with various aldehydes. After 3 h of the reaction,
aliphatic aldehydes (entries 4,5), an aldehyde with C=C group
Alcohol
Aldehyde
a
Entry
1
Product
Yield (%)
90
(
entry 6) and benzaldehydes (entries 7,8) reacted with 2-
aminothiophenol to give the corresponding 2-substituted
benzimidazoles in moderate to high yields.
2
3
68
55
N
S
OH
a
Table 3 Synthesis of the benzimidazole by various catalysts.
CHO
N
S
b
61
71
76
4
CHO
N
S
Entry
Catalysts
Pt/TiO
Ir/TiO
Pd/TiO
Conv. (%)
GC yield (%)
b
5
1
2
3
4
5
6
7
8
9
2
99
99
87
59
39
67
70
65
57
21
99
99
99
99
99
99
99
99
99
94
80
45
34
7
2
N
b
6
2
S
Rh/TiO
2
2
CHO
N
Ru/TiO
b
7
89
55
Re/TiO
Ag/TiO
Cu/TiO
Ni/TiO
TiO (197 mg)
Pt/ZrO
Pt/La
Pt/Al
2
0
S
2
0
CHO
N
b
2
0
8
S
H3CO
OCH3
2
0
a
Isolated yield based on 2-aminothiophenol.
3
1
1
1
1
1
0
1
2
3
4
2
0
b
h.
2
60
46
30
20
0
2
O
3
Initially, we investigated acceptorless synthesis of 2-
substituted benzothiazole from 2-aminothiophenol and 1-octanol
as a model reaction for optimization of catalysts and conditions.
2
O
3
Pt/SiO2-Al
Pt/MgO
2 3
O
8b,e
Note that previous successful catalytic systems
for this
15
16
reaction were not effective for the reaction with aliphatic
alcohols probably because of low reactivity of them. Table 1
summarizes the results of the catalyst screening test under the
Pt/HBEA
0
1
1
1
7
8
9
Pt/Nb
Pt/CeO
Pt/SiO
2
O
5
0
2
0
same reaction conditions (reflux in mesitylene under N
using 1 mol% of transition metal (Pt, Pd, Ir, Cu, Re, Rh, Ru, Ag,
Ni)-loaded Al pre-reduced under H at 500 °C for 0.5 h. The
Pt-loaded Al (Pt/Al , entry 1) showed the highest yield
97%) of 2-substituted benzothiazole. Pd/Al (entry 2) gave
good yield (89%), and the Ir, Cu, Re, Rh, Ru and Ag catalysts
entries 3-7) showed low to moderate yields (13%-57%) of the
benzothiazole. Al (entry 10) and Ni/Al (entry 9) did not
give the product. Then, we studied the support effect on the
activity of Pt-loaded catalysts (entries 1, 11-19). Clearly, Al
was the most effective support for this reaction, and Nb , CeO
SiO -Al , HBEA zeolite and La gave low to moderate
yields (5-64%). The Pt catalysts loaded on MgO, ZrO , TiO and
SiO showed did not give the benzothiazole. The Pt/Al
catalyst with higher Pt loading (5 wt%) gave lower yield (40%)
2
for 24 h)
2
0
a
Conversion and yield are based on 1,2-phenylenediamine.
2
O
3
2
2
O
3
2 3
O
Next, we studied the direct synthesis of 2-substituted
(
2 3
O
benzimidazoles from 1,2-phenylenediamine and alcohols or
aldehydes. We adopted the reaction of 1,2-phenylenediamine (1
mmol) with 1-octanol (1.2 mmol) as a model reaction, because
only one catalytic method was reported to be effective for the
reaction with aliphatic alcohols. Table 3 summarizes the results
of the catalyst screening under the same conditions (reflux in
(
2
O
3
2 3
O
8d
2 3
O
2
O
5
2
,
mesitylene under N
catalysts showed that Pt/TiO
the 2-substituted benzimidazole than the other Pt-loaded catalysts
entries 11-19). Then, we tested various transition metal-loaded
TiO catalysts. Ir/TiO gave good yield of 80%, and other
2
for 24 h). The support screening tests for Pt
2
2
O
3
2 3
O
2
(entry 1) showed higher yield of
2
2
2
2 3
O
(
2
.
2

3
catalysts gave moderate to low yields (entries 3-9). TiO
was inert (entry 10). The Pt/TiO catalyst with higher Pt loading
5 wt%) gave lower yield (83%) than 1 wt% Pt/TiO . The
in refluxing o-xylene gave lower
2
itself
2
(
2
reaction with 1 wt% Pt/TiO
2
yield (65%) that that in refluxing mesitylene. In summary, use of
wt% Pt/Al the refluxing mesitylene is the best conditions,
Pt/TiO was found to be the most effective catalyst for the
1
2 3
O
Scheme 1 A plausible mechanism for the synthesis of
benzimidazoles and benzothiazoles.
2
dehydrogenative synthesis of the 2-substituted benzimidazole
from 1,2-phenylenediamine and 1-octanol. After the reaction of
Considering the previous reports on acceptorless
dehydrogenative synthesis of benzazole derivatives from
2
entry 1 in Table 3, Pt/TiO was separated from the mixture by
8
alcohols and our reports on acceptorless dehydrogenative
centrifugation, followed by drying at 90ºC for 12 h and by H
2
–
9
coupling reactions by heterogeneous Pt catalysts, the present
system can proceed via a pathway shown in Scheme 1. The
reaction begins with Pt-catalyzed dehydrogenation of alcohols to
reduction at 500 ºC for 0.5 h. The second cycle by the recovered
catalyst showed lower yield (89%) than the first cycle.
As shown in the eqn. (1), the reaction with 0.1 mol% of the
2
Pt/TiO catalyst for 52 h resulted in 97% yield, corresponding to
aldehydes,
aminothiophenol or 1,2-phenylenediamine to afford saturated
intermediates via imine 1. Finally, Pt-catalyzed
which
undergoes
condensation
with 2-
the turnover number (TON) of 970 with respect to total Pt atoms
in the catalyst. The TON of 970 is more than 2 times higher than
the state-of-the-art system using a homogeneous Ir catalyst with
2
dehydrogenation of 2 gives the unsaturated product 3. A kinetic
study under the conditions in Table 4 (entries 1, 4) showed that
t
KO Bu reported by Kempe and co-workers.
the initial formation rate of the benzothiazole by the Pt/TiO
catalyzed reaction of 1,2-phenylenediamine with n-octanal was
.7 times higher than that with 1-octanol. This result is consistent
2
–
1
with the proposed pathway assuming that the dehydrogenation of
alcohols is a relatively slow step.
In summary, we have developed two heterogeneous
catalytic systems for the acceptorless dehydrogenative synthesis
of 2-substituted benzazoles under additive-free (neutral)
conditions. Pt/Al O is effective for synthesis of 2-substituted
benzothiazoles from 2-aminothiophenol and alcohols/aldehydes,
and Pt/TiO2 is effective for synthesis of 2-substituted
2
With the most effective catalyst, Pt/TiO , we investigated
substrate scope of the dehydrogenative benzimidazoles
synthesis. Table 4 shows the yields of the 2-substituted
benzimidazoles from the reaction of 1,2-phenylenediamine with
2
3
1
2
.2 equiv. of alcohols or aldehydes using 1 mol% of Pt/TiO .
Aliphatic alcohols (entries 1-3), an aliphatic aldehyde (entry 4),
benzaldehydes (entries 4,5) and pyridine-3-carbaldehyde (entry
benzimidazoles
alcohols/aldehydes.
from
2-phenylenediamine
and
7) were converted to the desired 2-substituted benzimidazoles in
good to high yields.
Experimental
Table 4 Synthesis of 2-substituted benzimidazoles from 1,2-
Commercially available organic compounds (from Tokyo
Chemical Industry) were used without further purification. The
GC (Shimadzu GC-14B) and GCMS (Shimadzu GCMS-QP2010)
analyses were carried out with Ultra ALLOY capillary column
2
phenylenediamine and alcohols or aldehydes by Pt/TiO .
+
UA -1 (Frontier Laboratories Ltd.) using nitrogen and helium as
the carrier gas.
Alcohol
Aldehyde
a
Entry
1
Product
Yield (%)
γ-Al
O
2 3
was prepared by calcination of γ-AlOOH (Catapal B
(JRC-
TIO-4), CeO (JRC-CEO-3), MgO (JRC-MGO-3), H -type BEA
N
Alumina purchased from Sasol) at 900 °C for 3 h. TiO
2
87
+
N
n-C7H15
2
H
zeolite (HBEA, SiO
Al (JRC-SAL-2, Al
Catalysis Society of Japan. ZrO
calcination (500 °C, 3 h) of hydroxide of Zr and La, which were
prepared by hydrolysis of ZrO(NO ·2H O and La(NO ·6H
with aqueous NH OH solution (1.0 mol dm ), followed by
filtration of precipitate, washing with distilled water and drying
at 100 °C for 12 h. Nb was prepared by calcination of niobic
acid (CBMM) at 500 °C for 3 h. Precursor of 1 wt% Pt/Al or
wt% Pt/TiO was prepared by impregnation method; a mixture
of the support oxide and aqueous HNO solution of
Pt(NH (NO (Furuya Metal Co, Ltd.) was evaporated at 50 °C,
followed by drying at 90 °C for 12 h. Pre-reduced catalyst,
named Pt/Al and Pt/TiO , were prepared by reduction of the
precursors in a pyrex tube under a flow of H
2
/Al
O
2 3
2
O
3
= 25±5, JRC-Z-HB25) and SiO
= 13.75 wt%) were supplied from
and La were prepared by
2
-
2
O
3
2
3
82
60
90
78
75
2
2 3
O
OH
N
3
)
2
2
3
)
3
2
O
-3
N
H
4
N
2 5
O
b
4
N
H
n-C7H15
2 3
O
1
2
CHO
N
3
c
5
3
)
2
3 2
)
N
H
CHO
CHO
N
2
O
3
2
c
6
3
−1
2
(20 cm min ) at
N
H
500 °C for 0.5 h. By using various supports, several pre-reduced
Pt catalysts (Pt = 1 wt%) were also prepared by the same method.
Al O or TiO -supported metal catalysts, M/Al O or M/TiO (M
N
c
7
54
2
3
2
2
3
2
N
N
H
= Ni, Cu, Ru, Pd, Ag, Re, Ir) with metal loading of 1 wt% were
prepared by impregnation method in a similar manner as
Pt/Al O or Pt/TiO using aqueous solution of metal nitrates (Ni,
N
a
Isolated yield based on 2-aminothiophenol.
3
b
h.
2
3
2

4
Tetrahedron
Cu, Ag), RuCl
of Pd(NH (NO
For the synthesis of 2-substituted benzothiazoles, Pt/Al
196 mg, 0.01 mmol of Pt) was used as the standard catalyst.
3
, IrCl
3
, NH
4
ReO
4
or an aqueous HNO
3
solution
5. (a) Wilfred, C. D.; Taylor, R. J. K. Synlett. 2004, 9, 1628–1630;;
(
b) Raghavendra, G. M.; Ramesha, A. B.; Revanna, C. N.;
3
)
2
3 2
) (Kojima Chemicals Co, Ltd.).
Nandeesh, K. N.; Mantelingu, K.; Rangappa, K.S. Tetrahedron
Lett. 2011, 52, 5571–5574; (c) Zhu, Y. P.; Jia, F. C.; Liu, M. C.;
Wu, A. X. Org. Lett. 2012, 14, 4414–4417.
2 3
O
(
After the pre-reduction at 500 °C, we carried out catalytic tests
using a batch-type reactor without exposing the catalyst to air as
follows. A mixture of 2-aminothiophenol (1 mmol) and
aldehydes or alcohols (1.2 mmol) with n-tetradecane (0.2 mmol)
in mesitylene (1.2 mL) was injected to the pre-reduced catalyst
inside the reactor (cylindrical pyrex tube) through a septum inlet,
6. (a) Kim, J. W.; He, J.; Yamaguchi, K.; Mizuno, N. Chem. Lett.
2009, 38, 920–921; (b) Ruiz, V. R.; Corma, A.; Sabater, M. J.
Tetrahedron 2010, 66, 730–735; (c) Yu, J.; Xu J.; Lu, M. Appl.
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7
.
(a) De Luca, L.; Porcheddu, A. Eur. J. Org. Chem. 2011, 5791–
795; (b) Li, G.; Wang, J.; Yuan, B.; Zhang, D.; Lin, Z.; Li, P.;
5
Huang, H. Tetrahedron Lett. 2013, 54, 6934–6936; (c) Pizzetti,
M.; De Luca, E.; Petricci, E.; Porcheddu, A.; Taddei, M. Adv.
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2
followed by filling N . Then, the resulting mixture was
magnetically stirred for 24 h for alcohols under reflux condition.
The products was analyzed and confirmed by GC and GC-MS.
The crude product was isolated by column chromatography using
silica gel 60 (spherical, 63-210 µm, Kanto Chemical Co. Ltd.)
with n-hexane/ethyl acetate as the eluting solvent, followed by
8
.
(a) Kondo, T.; Yang, S.; Huh, K. T.; Kobayashi, M.; Kotachi, S.;
Watanabe, Y. Chem. Lett. 1991, 1275–1278; (b) Ramachandran,
R.; Prakash, G.; Selvamurugan,S.; Viswanathamurthi, P.; Malecki,
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(d) Hille, T.; Irrgang, T.; Kempe, R. Chem. Eur. J. 2014, 20,
1
13
analyses by GCMS and H and C NMR.
For the synthesis of 2-substituted benzimidazoles, Pt/TiO
5
569–5572; (e) Bala, M.; Verma, P. K.; Sharma, U.; Kumar, N.;
2
Singh, B. Green Chem. 2013, 15, 1687–1693; (f) Wada, K.
Abstract in the 16th International Symposium on Relations
between Homogeneous and Heterogeneous Catalysis, 2013.
(
196 mg, 0.01 mmol of Pt) was used as the standard catalyst. The
reaction procedure is the same as that for the synthesis of 2-
substituted benzothiazoles.
9. (a) Siddiki, S. M. A. H.; Kon, K.; Touchy, A. S.; Shimizu, K.
Catal. Sci. Technol. 2014, 4, 1716; (b) Moromi, S. K.; Siddiki, S.
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Acknowledgments
This work was supported by Grant-in-Aids for Scientific
Research B (26289299) from MEXT (Japan), a MEXT program
2
014, 4, 3631–3635.
“
Elements Strategy Initiative to Form Core Research Center” and
a Grant-in-Aid for Scientific Research on Innovative Areas
Nano Informatics” (25106010) from JSPS.
“
Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://
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