Visible light promoted formation of N─S bond by photocatalyst Eosin Y

Article abstract of DOI:10.1002/jhet.4069

A novel efficient protocol for the synthesis of 5-imino-1,2,4-thiadiazole motif in open air and visible light has been reported. The reaction involves Eosin Y as photocatalyst which is a cost-effective organic dye. The designed protocol represents a novel, mild, metal free, green strategy for the construction of 1,2,4-thiadiazole nucleus by intramolecular cyclization via N─S bond formation. The reaction has been carried out under visible light exposure in ethanol: water (4:1) mixture using 2-aminopyridine and isocyanate as reactants.

Full text of DOI:10.1002/jhet.4069

FULL PAPER
Visible light promoted formation of N-S bond catalysed by
photocatalyst Eosin Y
Ankit Vermaa, Arjita Srivastavab, Saurabh K Tiwaria, Neetu Yadava, Mohd Danish Ansaria, Vijay B
Yadava, Hozeyfa Sagirc, Ibadur R Siddiqui*a
[a]
Ankit Verma, Saurabh K. Tiwari, Neetu Yadav, Mohd. Danish Ansari, Vijay B. Yadav and Prof. Ibadur R Siddiqui
Laboratory of Green Synthesis
Department of Chemistry
University of Allahabad
Prayagraj
E-mail: dr.irsiddiqui@gmail.com
Dr. Arjita Srivastava
Department of Chemistry
CMP Degree College, Prayagraj
A Constituent PG College of University of Allahabad
Dr. Hozeyfa Sagir
[b]
[c]
Department of Chemistry
Paliwal PG College
Shikohabad
Abstract: A novel efficient protocol for the synthesis of 5-imino-
fore-runner core structures of significant medicinal concern, are
1,2,4-thiadiazole motif in open air and visible light has been reported. widely associated with a broad spectrum of biological activity
The reaction involves Eosin Y as photocatalyst which is a cost-
effective organic dye. The designed protocol represents a novel,
mild, metal free, green strategy for the construction of 1,2,4-
thiadiazole nucleus by intramolecular cyclization via N-S bond
formation. The reaction has been carried out under visible light
exposure in ethanol: water (4:1) mixture using 2-aminopyridine and
isocyanate as reactants.
comprising antiulcerative8, antibacterial9, antidiabetic10, anti-
inflammatory11, antirheumatic12, and antimicrobial agents13. For
instance, cefozopran, a 1,2,4-thiadiazole nucleus based drug,
are available for commercial purpose as an antibiotic drug.14 In
recent years, 1,2,4-thiadiazole derivatives are widely studied as
potential drugs for the treatment of human leukemia15. However,
other fascinating biological effects are as cathepsin B inhibitors16,
as allosteric modulators binding at secondary binding site of
biomolecules17, non-ATP competitive GSK-3 inhibitors18 and as
dual 5-lipoxygenase and cyclooxygenase inhibitors19
Introduction
1,2,4-thiadiazoles have been synthesized by various methods,
like copper-catalyzed reaction of amidine hydrochlorides with
isothiocyanates,20
from
aryl
nitriles
in
1-butyl-3-
The development of sustainable stratagems for chemical
synthesis as alternative to conventional synthetic methodologies
has acknowledged a widespread cumulative consideration from
the synthetic research community1-3. Among such pursuits, the
selective activation of chemical bonds via visible light mediated
methylimidazolium bromide,21 molecular iodine catalyzed N-S
bond
annulation,23
formation,22
and
copper-catalyzed
electrochemical
aerobic
oxidative
intermolecular
dehydrogenative S-N coupling.24 Despite these interesting
reports for synthesis of these moieties, visible light promoted
methods for synthesis of 1, 2, 4-thiadiazoles remains a
relatively unexplored area. Accordingly, in persistence of our
current research attention on the incorporating the newer
alternatives to access biologically vital heterocyclic moieties in
an eco-friendly green method,25-27 in present study, we herein
devise a novel, photocatalysed, metal-free, clean and efficient
synthesis of 1,2,4-thiadiazoles nucleus using organic dye eosin
Y as a photoredox catalyst (Scheme 1).
chemical reactions as
a synthetic tool has witnessed a
prominent emergence during the last few decades.4 The
application of this synthetic tool for the eco-compatible synthesis
of heterocyclic compounds has also risen prominently.5
Literature survey suggests that many visible-light photo-redox
catalysts, based on metal or non-metal complexes, have been
used for chemical transformations in organic synthesis by
number of research groups6. In this perspective, a well-known
organic dye eosin Y (EY), a typical photoredox catalyst, has
been used and reported as a remarkable substitute to typical
inorganic transition metal photocatalysts7. Photoredox catalysis
has proven to be an optimum strategy for fulfilling the current
mandate in finding the sustainable synthetic methodologies and
newer alternative energy sources. The prominent biological
activity of N-heterocyclic compounds has encouraged the
research community to establish convenient methods for the
synthesis of theses structure through N-C, N-N, N-S and N-O
bond formation strategies. Among these, 1,2,4-thiadiazoles, a
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In an attempt to obtain better yields and reaction
optimisation, the reaction was carried out in different
solvents like toluene, benzene, THF, DMF, MeOH, EtOH
Previous works
R2
N
S
and H2O (Table 1, Entries 1-8). It was noted that polar
N
HN
solvents furnished better yields in comparison to non-polar
ones. EtOH was found to be the most effective solvent for
the reaction (Table 1, Entry 7). The desired product was
obtained in poor yields using water as a solvent, probably
because of the low solubility of isothiocyanate in water
(Table 1, Entry 8). The increase of reaction time did not
affect the yields of the product in any case. A mixed solvent
system consisting of ethanol and water was then utilized to
minimize the amount of volatile organic solvent for the
reaction (Table 1, Entries 9-11). It was noticed that
EtOH:H2O solvent system was better for enhancing the
yield and reducing the reaction time for corresponding
product. In this way we reduced the tedious workup, waste
and cost of the overall process. Later, different ratios of
ethanol: water were evaluated and the analysis revealed
that the use of 4:1 ratio of ethanol to water proved to be the
optimum solvent system for this reaction (Table 1, Entry
R1
NH
Cu(OTf)2,CS2CO3
NH2
R2
TEMPO,THF,50o C,2h
HCl
CuI (20 mol%)
S
N
R1
2,4-dimethylpyridine (2.0 equiv.)
DCE, 50o C, O2
N
R2
N
Iodine/ MeCN, r.t.
N
R1 NCS
N
NH
R2
R3
S
R2
N
R2
NH
R3
N
NH2
R1
Pt(+) Pt(-)
H
N
N
n-Bu4NI(0.2 equiv)
MeCN/MeOH (1:1)
5 mA, rt
undivided cell
R1
N
S
N
N
Ar
N
N
11)
.
(NH4)2S,TCT-DMSO
[Bmim]Br,
2ArCN
S
Ar
Our next step was optimisation of the catalyst in the
reaction. In a control experiment performed without any
photocatalyst, no yield of the desired products was obtained
This work: Scheme 1
N
NH
2
visible light/ Eosin Y
Ethanol:Water (4:1)
S
(Table 2, Entry 1).
R
NCS
N
1
N
Table 2: Optimization of catalysta
N
R
1
NCS
N
Visible Light / Photocatalyst
NH2
S
N
N
Ethanol:Water (4:1)
air
Results and Discussion
N
(1a)
(3a)
(2a)
Initially we carried out a reaction with phenyl isothiocyanate
(1a) with 2-aminopyridine (2a), in DCM and exposed to the
light of a household 22 W fluorescent lamp for tabulated
time at room temperature. The desired product was
Entry
Photocatalyst
Air
Time (h)
Yield (%)b
1
-
+
6
--
obtained in considerable yields (Table 1, Entry 1)
.
2
3
Eosin Y (2 mol%)
Eosin Y (2 mol%)
-
1
1
Trace
95
+
Table 1: Optimization of solventa
4
EosinY (1 mol%)
EosinY (3 mol%)
+
+
+
+
+
1
1.5
1
65
95
95
24
95c
NCS
N
Visible Light / Eosin Y
NH2
S
5
N
N
Solvent
air
6
Eosin Y (5 mol%)
Rose Bengal (5 mol %)
Eosin Y (2 mol%)
N
7
1
(1a)
(3a)
(2a)
8
1
Entry
Solvent
DCM
Time (h)
Yield (%)b
50
aReaction
conditions:
Phenyl
isothiocyanate
(3
mmol),
2-
aminopyridine(3mmol), in 8 ml EtOH: H2O (4:1) solvent, under 22W CFL
irradiation at room temperature. b Isolated yields, c Gram scale synthesis--
Phenyl isothiocyanate (12 mmol, 1.62g), 2-aminopyridine(12mmol, 1.12g),
in 20 ml EtOH: H2O (4:1) solvent, under 22W CFL irradiation at room
temperature, product 3a-95%, 2.60g
1
4
4
4
4
4
4
4
4
2
Toluene
Benzene
THF
trace
10
3
4
21
5
DMSO
MeOH
EtOH
48
The reaction was then carried out in the presence of 2
mol% Eosin Y, keeping other reaction conditions the same.
Role of air was also checked during the process and it was
observed that absence of air resulted in only trace amounts
of the desired product (Table 2, Entry 2). Interestingly, on
adding Eosin Y as the photocatalyst (2 mol%) in the
presence of visible light and air, the desired 1,2,4-
thiadiazole 3a was obtained in 95% yield. Optimum catalyst
loading was found to be 2 mol% (Table 2, Entry 3). On
lowering the catalyst loading from 2 mol% to 1 mol%, there
6
40
7
80
8
H2O
20
9
EtOH: H2O (1:1)
EtOH: H2O (2:1)
EtOH: H2O (4:1)
conditions:
4
88
10
3
91
11
1
95
aReaction
Phenyl
isothiocyanate
(3
mmol),
2-
b
aminopyridine(3mmol), under 22W CFL irradiation at room temperature.
Isolated yields.
was a significant decrease in the yield (Table 2, Entry 4)
;
2
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FULL PAPER
while an increase in the catalyst loading from 2 mol% to 3
mol% and 5 mol% did not improve the yield (Table 2,
Entries 5 & 6). The reaction was also carried out using
Rose Bengal as the photocatalyst, which did not provide
N
N
N
S
S
S
N
N
N
O2N
N
N
N
satisfactory results (Table 2, Entry 7)
. In order to
(3l, 87%)
(3j, 81%)
(3k, 82%)
demonstrate the practical applicability of this protocol, a gram
scale reaction was also performed and excellent results were
obtained (Table 2, Entry 8). These results indicate that the
photocatalyst, visible light, and air all are essential for the
reaction and support the photocatalytic model for the
reaction.
In prolongation of our effort to find out adequate reaction
conditions for our model reaction, we next moved our
attention to enquire the adaptability of different substrates
for projected reaction conditions (Scheme 2). The effect of
Cl
N
Me
N
N
S
S
S
N
N
N
Br
N
N
N
(3n, 86%)
(3o, 86%)
(3m, 87%)
N
S
N
Me
N
various
substituents
at
different
positions
of
arylisothiocyanate was checked in our protocol. One
noteworthy finding was that arylisothiocyanates with strong
electron withdrawing groups as well as electron donating
groups were well endured under given reaction conditions,
and N-fused 1,2,4-thiadiazole products were obtained in
good to excellent yields (3b-3g). However, it was seen that
arylisothiocyanates containing electron donating groups
such as methoxy and methyl at different positions gave
better reactivity and provided the corresponding products in
good to excellent yields (3c) 85%, (3d) 96% and (3e) 85%
respectively. Conversely, arylisothiocyanates with electron-
withdrawing groups such as chloro at para- and meta-
positions equipped corresponding products in moderate to
good yields, i.e. (3f) 83% and (3g) 91% respectively.
(3p, 86%)
aReaction
conditions:
Phenyl
isothiocyanate
(3
mmol),
2-
aminopyridine(3mmol), in 8 ml EtOH: H2O (4:1) solvent, under 22W CFL
irradiation at room temperature. b Isolated yields.
An alicyclic isothiocyanate such as cyclopropyl
isothiocyanate was also tolerated under these reaction
conditions, and the corresponding product 3h was isolated
in 82% yield. Other aliphatic isothiocyanates including,
propyl, isopropyl and cyclohexyl methyl groups were also
used to give the corresponding products in good yields
(3i−3k). To further examine the scope and generality of the
reaction, various 2-aminopyridine derivatives were studied
in proposed protocol. When a strong electron-withdrawing
nitro group was used, the desired product (3l) was obtained
in 87% yield (3l). Electron-withdrawing groups such as
choro and bromo were compatible and gave the equivalent
products (3m) and (3n) in 87 and 86% yield. Methyl-
substituted 2-aminopyridine was well tolerated, and the
position of the methyl substituent at 4 and 5 did not bear
Substrate scope for the synthesis of 3, 4-Disubstituted 5-
Imino-1,2,4-thiadiazoles- Scheme 2a
NCS
N
Visible Light / Eosin Y
NH2
S
N
R1
N
R2
Ethanol:Water (4:1)
air
R2
N
R1
(3)
(1)
(2)
any significant effect on the reaction yield of (3o) and (3p)
respectively.
In order to further explore the generality of this protocol, the
reaction of phenyl isothiocayante was carried out with a few
amidine (4) substrates (Scheme 3). Good to excellent
N
N
S
N
S
S
N
N
N
OMe
N
N
N
NO2
(3a, 95%)
(3b, 88%)
(3c, 85%)
yields of the desired product were obtained (5a-5c)
.
Substrate scope for the synthesis of 1,2,4-thiadiazoles-
Scheme 3a
N
N
N
S
S
S
N
N
N
N
N
N
NCS
S
N
NH
NH2
OMe
N
Visible Light / Eosin Y
N
H
R3
R3
Me
Cl
(3d, 96%)
Ethanol:Water (4:1)
air, 1h
(3f, 83%)
(3e, 85%)
(1a)
(4)
N
(5)
N
N
N
S
N
S
S
N
S
S
N
N
S
N
N
N
N
N
N
N
H
H
N
N
H
N
Cl
H3C
5a, 90%
5b, 88%
5c, 94%
(3i, 80%)
(3h, 82%)
(3g, 91%)
----------------------------------------------------------------------------------------
aReaction conditions: Phenyl isothiocyanate (3 mmol), amidine substrate
(3mmol), in 8 ml EtOH: H2O (4:1) solvent, under 22W CFL irradiation at
room temperature.
3
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FULL PAPER
On the basis of the above observations and the literature
precedents,28-29 a plausible mechanism involving photo
redox catalysis for the synthesis of the conveyed
the newly formed radicals. The radical pathway is
supported by the fact that the presence of radical quencher
TEMPO suppressed the reaction.
heterocycles is described in Scheme 4
.
Conclusion
S
C
N
Eosin Y
N
In conclusion, we have developed a novel and convenient
visible light promoted method for the synthesis of N-fused
1,2,4-thiadiazole from 2-aminopyridine and phenyl
isothiocyanate in a one-pot procedure by using inexpensive
eosin Y as a powerful organophotoredox catalyst, at room
temperature. The reaction involves a mixture of ethanol and
water as the solvent system. This synthetic pathway,
because of its green credentials, is an improvement over
those synthetic methods which involve the use of corrosive
and toxic chemicals. This synthesis widens the scope of
substrates for visible light photo redox reactions. The
developed synthetic approach can be easily scaled up to
gram scale, thereby providing the possibility for the scaled
production of diverse N-fused 1,2,4-thiadiazole. The
present methodology also offers other advantages of green
chemistry such as high atom economy, reduced reaction
time, one-pot consolidated procedure and high efficiency.
NH2
S
N
+
N
Air,
EtOH / H2O
N
O2
Y
E
O2
Y
E
Br
Br
HO
Br
O
O
*
Y
E
N
Br
COOH
S
NH2
N
hv
N
N
(E)
(A)
H2O2
HO2
N
NH2
NH
N
S
(B)
N
(E)
O2
-H
H
HO2
NH
H2O2
Acknowledgements
N
Mr. Ankit Verma and Mohd Danish Ansari gratefully
acknowledge UGC for research fellowship and UP C&ST
for financial support. The authors gratefully acknowledge
SAIF Chandigarh for NMR spectra.
NH
S
(C)
N
S
C
N
N
(D)
Supporting information
Scheme 4: Postulated mechanism for the Eosin-Y catalysed N-S bond
Additional supporting information may be found online in
the Supporting Information section at the end of this article.
formation.
On absorption of visible light, the organophotoredox
catalyst eosin Y (EY) is excited to its singlet state 1EY*
which through inter system crossing (ISC) comes to its
more stable triplet state 3EY* and undergoes a single
electron transfer (SET).30 3EY* may undergo both reductive
and oxidative quenching.31 A SET from 2-aminopyridine to
Keywords:Visible Light, Eosin Y, Photocatalyst, Green Chemistry
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5
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Entry for the Table of Contents
N
NH2
visible light/ Eosin Y
Ethanol:Water (4:1)
S
R1 NCS
N
N
R2
R2
N
R1
The application of visible light in combination with Eosin Y as the organic photocatalyst is being widely employed for organic
syntheses. Excellent results have been obtained by using this combination for the synthesis of thiadiazole ring via an effective N-S
bond formation reaction.
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