Palladium-Catalyzed 3-Aryl-5-acyl-1,2,4-thiadiazole Formation from Ketones, Amidines, and Sulfur Powder

Article abstract of DOI:10.1002/ejoc.201700148

An efficient strategy for the preparation of 3,5-disubstituted 1,2,4-thiadiazoles from ketones, amidines, and sulfur powder under palladium-catalyzed conditions was developed. In this transformation, aromatic ketones act as both the carbon and acyl source

Full text of DOI:10.1002/ejoc.201700148

A Journal of
Accepted Article
Title: Palladium-Catalyzed 3-Aryl-5-acyl-1,2,4-thiadiazoles Formation
from Ketones, Amidines and Sulfur Powder
Authors: Zilong Wang, Hao Xie, Fuhong Xiao, Yanjun Guo, Huawen
Huang, and Guojun Deng
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201700148
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European Journal of Organic Chemistry
10.1002/ejoc.201700148
COMMUNICATION
Palladium-Catalyzed 3-Aryl-5-acyl-1,2,4-thiadiazoles Formation
from Ketones, Amidines and Sulfur Powder
[
a]
[a]
[a]
[a]
[a]
[a][b]
Zilong Wang, Hao Xie, Fuhong Xiao, Yanjun Guo, Huawen Huang* and Guo-Jun Deng*
Abstract: An efficient strategy for 3,5-disubstituted-1,2,4-
thiadiazoles from ketones, amidines and sulfur powder under
palladium-catalyzed conditions has been developed. Aromatic
ketones acted as carbon source and acyl source in this
transformation. This reaction provided an efficient approach for 3-
aryl-5-acyl-1,2,4-thiadiazoles from readily available starting materials.
preparation of other 3,5-disubstituted 1,2,4-thiadiazoles via
palladium-catalyzed Suzuki-Miyaura process.[10]
Introduction
Thiadiazole is prevalent and important five-membered
heterocyclic motif containing a sulfur atom and two nitrogen
atoms. The thiadiazole system constitutes the key structures of
many natural products, functional materials as well as
[
1]
pharmaceutical drugs. As an isomer of thiadiazole, the 1,2,4-
thiadiazole scaffold which has an unsymmetrical five-membered
ring also showed a broad range of biological activities, including
anti-inflammatory, antifungal activity, antibiotic activity,
Figure 1. Representative 3,5-disubstituted-1,2,4-thiadiazoles in natural
products and potencial pharmaceutical drugs.
[
2]
anticonvulsant and intense muscarinic activity (Figure 1). The
cefozopran, which contains a 3,5-disubstituted 1,2,4-thiadiazole
motif, is a commercial drug with antibacterial activity.[3] Despite
their wide applications in pharmacology, only few methods have
been developed for the preparation of this kind of five-
Above all, most of the mentioned methods for the synthesis of
3,5-disubstituted 1,2,4-thiadiazoles require strong oxidative
conditions or highly functionalized starting materials. Thus it
would be highly desirable to develop synthetically diverse
methodology for 1,2,4-thiadiazoles from simple starting materials.
As well known, thioamides or α-ketothioamides could be readily
formed through the three-component coupling of methyl ketone,
[4]
membered heterocycles. The classic method for the synthesis
of 3,5-disubstituted 1,2,4-thiadiazoles relys on the oxidative
[
5]
dimerization reaction of thioamides. A large number of oxidant
such as hydrogen peroxide,[5a] nitrous acid,[5b] thionyl chloride,[5c]
HCl-DMSO,[5d]
pyridinium
salt-DMSO,[5e]
amine, and elemental sulfur under facile reaction conditions, i.e.
bis(acyloxyiodo)arenes,[5f] N,N'-dibromo phenytoin,[5g] 2,4,6-
[11]
Willgerodte Kindler reaction (Scheme 1a).
We reasoned in
trichloro-1,3,5-triazine,[5h] and polymer-supported iodobenzene
diacetate[5i] have been successfully used for the conversion of
such a system using amidines, a variant of amines instead,
would form the thioamide intermediate A, which undergo an
oxidative N-S bond formation[
12]
to afford 5-acyl-1,2,4-
thioamides and thiobenzamides to the corresponding 1,2,4-
thiadiazoles with substituents at positions 3 and 5. Few other
methods have also been developed for the synthesis of 3,5-
disubstituted 1,2,4-thiadiazoles using other starting materials
thiadiazoles (Scheme 1b). Acyl substituent at the C5 position is
of great value since the alkaloids polycarpathiamines, isolated
[13]
from the ascidina Polycarpa aurata (Figure 1),
showed
significant cytotoxic activity agaist L5178Y murine lymphoma,
while efficient method for the preparation of 5-acyl substituted
such as aryl nitriles, 3,5-dichloro-1,2,4-thiadiazole,[7] amidines,
[
6]
[
8]
and isothocyanates.[9] Direct functionalization of 3,5-dihalo-
[14]
1,2,4-thiadiazoles is still rare. As our continuing efforts using
1,2,4-thiadiazoles could provide an alternative approach for
[
a]
Z. Wang, H. Xie, F. Xiao, Y. Guo, Dr. H. Huang and Prof. G.-J. Deng
Key Laboratory of Environmentally Friendly Chemistry and
Application of Ministry of Education, College of Chemistry
Xiangtan University
Xiangtan 411105, China.
E-mail: gjdeng@xtu.edu.cn; hwhuang@xtu.edu.cn
Prof. G.-J. Deng
[
b]
Beijing National Laboratory for Molecular Sciences and CAS Key
Laboratory of Molecular Recognition and Function Institute of
Chemistry
Chinese Academy of Sciences (CAS)
Beijing 100190 (P.R. China)
Scheme 1. Strategy for 1,2,4-thiadiazole formation inspired by the synthesis of
thioamides.
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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European Journal of Organic Chemistry
10.1002/ejoc.201700148
COMMUNICATION
cheap and readily available elemental sulfur as the sulfur source
to construct sulfur-containing heterocycles,[15] herein, we report a
facile annulation for the 3-aryl-5-acyl-1,2,4-thiadiazole synthesis
from amidines, acetophenones and sulfur powder[16] under
palladium-catalyzed reaction conditions. The methyl group ortho
to the carbonyl position of acetophenones acted as one carbon
source in this transformation.
1
1
1
0
1
2
PdCl
PdCl
PdCl
PdCl
PdCl
PdCl
PdCl
PdCl
PdCl
PdCl
2
2
2
2
2
2
2
2
2
2
KHCO
3
DMSO
PhOMe
PhCl
14
K
K
K
K
K
2
2
2
2
2
HPO
4
trace
trace
trace
trace
trace
24
HPO
HPO
HPO
HPO
4
4
4
4
13
DMF
1
1
1
1
4
Toluene
5
2
H O
Results and Discussion
6
DMSO
DMSO
DMSO
DMSO
Based on our previously demonstrated thiadiazole formation
from 2-methylquinolines, amidines and elemental sulfur,[17] we
started the investigation of the reaction of acetophenone (1a),
benzamidine (2a) and elemental sulfur in a base system under
7[c]
K
K
K
2
HPO
2
HPO
2
HPO
4
4
4
59
18[d]
56
1 [
9
e]
38
o
argon atmosphere at 160 C (Table 1). No desired product was
observed when the reaction was carried out in the absence of
[
a] Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), S (1.0 mmol),
transition metal catalyst using K
2
HPO
4
as base and DMSO as
o
catalyst (10 mol%), base (1.0 mmol), solvent (0.6 mL) at 130 C under
argon for 24 h. [b] Isolated yield based on 1a. [c] Under air. [d] At 100 C. [e]
S (0.2 mmol) was used.
o
solvent (entry 1). Addition of palladium catalyst could
significantly improve the reaction yield and the desired product
phenyl(3-phenyl-1,2,4-thiadiazol-5-yl)methanone
observed in 49% yield when Pd(OAc) was employed (entry 2).
Among the various palladium catalysts investigated, PdCl
(3a)
was
2
Table 2. Reaction of 2a with various aromatic ketones.[a]
2
showed the best efficiency to give the desired product in 74%
yield (entry 5). A brief base screening showed that stronger base
is not suitable to convert the starting materials to the
corresponding product (entries 6-10). Several organic solvents
2
and H O were screened and all of them are not good reaction
media for this kind of reaction (entries 11-15). A control reaction
showed that much poor yield was obtained when the reaction
was carried out in the absence of base (entry 16). Moderate
yield could still be obtained when the reaction was carried out
under air (entry 17). Decreasing the reaction temperature
resulted in a relatively low yield (entry 18). Finally, only 38%
yield was observed when employing 1 equivalent of sulfur, which
indicated the elemental sulfur served as not only a reactant but
as an oxidant in this reaction system.
Table 1. Optimization of the reaction conditions.[a]
Entry
Catalyst
Pd(OAc)
Base
Solvent
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
Yield (%)[b]
1
2
3
4
5
6
7
8
9
K
K
K
K
K
K
K
2
2
2
2
2
3
4
HPO
HPO
HPO
HPO
HPO
4
4
4
4
4
0
2
49
67
51
74
51
56
7
PdBr
2
Pd(OH)
2
PdCl
PdCl
PdCl
PdCl
PdCl
2
2
2
2
2
PO
4
2 7
P O
[
a] Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), S (1.0 mmol),
o
catalyst (10 mol%), base (1.0 mmol), solvent (0.6 mL) at 130 C under
argon for 24 h. [b] Isolated yield based on 1a. [c] Under air. [d] At 100 C. [e]
KOH
CO
o
S (0.2 mmol) was used.
K
2
3
48
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European Journal of Organic Chemistry
10.1002/ejoc.201700148
COMMUNICATION
and similar reaction yields were obtained when methyl and
chloro groups were presented at the ortho position (entries 5
and 6). Notably, pyridyl amidines were well tolerated in this
reaction system, providing 3t and 3u in moderate to good yields
(entries 7 and 8).
With the optimized conditions in hand,
a variety of
acetophenones were examined under the optimized reaction
conditions, as summarized in Table 2. Similar yields were
obtained when electron-donating groups presented at the para
position (entries 2-4). The desired product 3d was achieved in
To understand the mechanism of the reaction, we performed
a number of control experiments (Scheme 1). First, the addition
of 2,2,4,4-tetramethyl-1-piperidinyloxy (TEMPO), butylated
hydroxytoluene (BHT), or ethene-1,1-diyldibenzene (DBE) to the
reaction system did not inhibit the reaction (Scheme 2a),
suggesting the reaction may not proceed through a radical
pathway. Second, the coupling of acetophenone 1a with aniline
4a and elemental sulfur occurred well to afford thioamide 5a
under the standard reaction conditions or in the absence of
palladium catalyst (Scheme 2b). Thereby the transition metal
catalyst probably played a role not in the thioamide formation but
in the step of N-S bond formation, in which both reductive
elimination and electrophilic amination pathway were possibly
involved. Then we found that only 10 mol % of acetophenone
were recovered when the reaction was carried out in the
absence of 2a (Scheme 2c), which means the interaction of 1a
and sulfur may be the initial step. And 2-oxo-2-
phenylacetaldehyde (1a') was not observed in this reaction,
although the desired product 3a was obtained in 20% yield when
8
0% when an iso-butyl substituent was employed (entry 4).
The reaction yield significantly decreased when a strong
electron-withdrawing substituent was presented (entry 5).
Acetophenone with a phenyl group at the para position (1f)
also reacted smoothly with 2a and elemental sulfur to provide
the corresponding product 3f in 70% yield (entry 6). The steric
effect of the substituent slightly affected the reaction yield;
when 1-(3-methoxyphenyl)ethanone (1g) was used, the
product 3g was obtained in 78% yields (entry 7).
Acetophenones
with
two
electron-donating
methyl
functionalities (1h and 1j) were also suitable substrates to give
the desired products in good yields (entries 8 and 9). To our
delight, hetero aromatic ketones bearing thiophen-2-yl, furan-
2
-yl, and pyridine-2-yl functionalities worked well under the
optimized reaction conditions to provide the corresponding
,2,4-thiadiazoles in moderate yields (entries 10-13).
1
Table 3. Reaction of 1a with various amidines.[a]
1a' was subjected to the standard reaction conditions, (Scheme
2d).
[
a] Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), S (1.0 mmol),
o
catalyst (10 mol%), base (1.0 mmol), solvent (0.6 mL) at 130 C under
argon for 24 h. [b] Isolated yield based on 1a. [c] Under air. [d] At 100 oC. [e]
S (0.2 mmol) was used.
Scheme 2. Control experiments.
After screening various acetophenones for this kind of reaction,
we next investigated various amidine hydrochlorides to further
explore the reaction scope and limitations (Table 3). Several
substituents at the para position of benzamidine were
investigated under the given conditions and good yields were
obtained when methyl and trifluoromethyl substituents were
employed (entries 1 and 2). However, the reaction is less
efficient when a strong electron-withdrawing nitro group was
presented (entry 3). When a bromo group presented at the para
position, the desired product 3q was obtained in 52% yield
Conclusions
In summary, we have developed a novel approach for 3,5-
disubstituted-1,2,4-thiadiazoles from amidines, acetophenones
and elemental sulfur under palladium-catalyzed reaction
conditions. Cheap and readily available
sulfur powder was
acted as the sulfur source to react with amidine and methyl
group ortho to the carbonyl group. All of the 21 products are new
compounds and this method affords an efficient approach for the
(
entry 4). The effect of substituent position was also investigated
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European Journal of Organic Chemistry
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COMMUNICATION
rapid synthesis of 3-aryl-5-acyl-1,2,4-thiadiazoles from readily
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3
) δ
8
7
1
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.56-7.50 (m, 3H); 13C NMR (100 MHz, CDCl
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21372187, 21502160, 21572194), the
Program for Innovative Research Cultivation Team in University
of Ministry of Education of China (1337304), the Hunan
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for
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Keywords: palladium • thiadiazoles • ketones • sulfur • amidines
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European Journal of Organic Chemistry
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Cyclization
Zilong Wang, Hao Xie, Fuhong Xiao,
Yanjun Guo, Huawen Huang*, Guo-Jun
Deng*
Page No. – Page No.
Palladium-catalyzed 1,2,4-thiadiazole formation from readily available ketones,
amidines and sulfur powder has been developed.
Palladium-Catalyzed 3-Aryl-5-acyl-
1,2,4-Thiadiazoles Formation from
Ketones, Amidines and Sulfur
Powder
*methodology
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