Aerobic Visible-Light Induced Intermolecular S?N Bond Construction: Synthesis of 1,2,4-Thiadiazoles from Thioamides under Photosensitizer-Free Conditions

Article abstract of DOI:10.1002/ejoc.202100440

Aerobic visible-light induced intermolecular S?N bond construction has been achieved without the addition of photosensitizer, metal, or base. With this strategy, 1,2,4-thiadiazoles can be obtained from thioamides. Preliminary mechanistic investigation suggested that the excited state of thioamides undergoes a single-electron-transfer (SET) process to afford thioamidyl radicals, which can be further transformed into a 1,2,4-thiadiazole through desulfurization and oxidative cyclization. The reaction has good functional group tolerance and represents a green method for the construction of S?N bonds.

Full text of DOI:10.1002/ejoc.202100440

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Aerobic Visible-light Induced Intermolecular S-N Bond
Construction: Synthesis of 1,2,4-Thiadiazoles from Thioamides
under Photosensitizer-Free Conditions
Authors: Liang Zhuo, Shihua Xie, Hui Wang, and Hongjun Zhu
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202100440
Link to VoR: https://doi.org/10.1002/ejoc.202100440
01/2020

10.1002/ejoc.202100440
European Journal of Organic Chemistry
COMMUNICATION
Aerobic Visible-light Induced Intermolecular S-N Bond
Construction: Synthesis of 1,2,4-Thiadiazoles from Thioamides
under Photosensitizer-Free Conditions
Liang Zhuo, †[a] Shihua Xie, †[a] Hui Wang†[a] and Hongjun Zhu*[a]
[a]
L. Zhuo, S. H. Xie, H. Wang and Prof. H. J. Zhu
College of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 211816, China.
†
denotes equal contribution
Corresponding author. E-mail: zhuhj@njtech.edu.cn
IV).[10] However, these methods typically employ metal catalysts,
photosensitizing dyes, and stoichiometric amounts of oxidizing
agents and bases. Therefore, it is particularly necessary to
develop a more environmentally synthetic approach for S-N
bond construction. We have noticed that aryl thioamides are
easily activated under photochemical conditions,[11] and we were
Abstract: Aerobic visible-light induced intermolecular S-N bond
construction to synthesize 1,2,4-thiadiazoles from thioamides has
been achieved without the addition of a photosensitizer, metal, or
base. The excited state of the thioamide undergoes a single-
electron-transfer (SET) process to afford a thioamidyl radical, which
is further transformed into a 1,2,4-thiadiazole through desulfurization
and oxidative cyclization. The reaction has good functional group
tolerance, and provides a green method for the construction of S-N
bond.
able to synthesize 1,2,4-thiadiazole under O2 atmosphere and
visible light without any auxiliary photosensitizer. Herein, we
report a photoinduced intermolecular S-N bond construction
through a single-electron-transfer (SET) process under O2
atmosphere and irradiation from blue LEDs (Scheme 1-V).
Introduction
In recent decades, the construction of S-N bond has
aroused great interest among organic chemists because
frameworks containing such bond are significant building blocks
of bioactive molecules, organic functional materials, and so on.[1]
As one of the representative heterocyclic moieties containing an
S-N bond, 1,2,4-thiadiazole derivatives have been extensively
applied in the field of pharmaceutical chemistry (Figure 1). For
instance, cefozopran (SCE-2787) is
a potent commercial
antibiotic containing
a
1,2,4-thiadiazole structure.[2] 3,5-
Bis(indolyl)-1,2,4-thiadiazoles derivatives (e.g., compound II)
display in-vitro anticancer activity.[3] Compound III may be a
potential drug for Alzheimer's disease due to its good β-
secretase inhibitory activity.[4] Compound IV, incorporating a
1,2,4-thiadiazole unit, shows neuroprotective properties.[5]
Therefore, much effort has been devoted to synthetic methods
for constructing S-N bond.[6] In 2013, photoredox-catalyzed
intermolecular S-N bond formation from thioamides using eosin
Y as a photosensitizer was reported by Yadav’s group (Scheme
1-I).[7] Chang,[8a] Nakka[8b] and Xu[8c] described methods for S-N
bond construction mediated by stoichiometric amounts of I2 and
bases (Scheme 1-II). Synthetic strategies for S-N bond
formation through Cu-catalyzed dehydrogenative coupling of
thioamide analogues have been disclosed by the Jiang,[9a]
Gong[9b] and Sperry[9c] groups (Scheme 1-III). In 2018, Pan and
co-workers developed an electrochemical method for the
construction of intermolecular S-N bond from thioamides in the
absence of transition metals or inorganic oxidants (Scheme 1-
Figure 1. Representative bioactive 1,2,4-thiadiazole scaffolds.
Results and Discussion
Firstly, we chose 4-methoxy-thiobenzamide 1c as
a
template substrate, and without any photosensitizer, the desired
product was obtained in 66% yield in CH2Cl2 under O2
atmosphere and irradiation from blue LEDs (450-455 nm) (Table
1, entry 1). We then screened a series of solvents (Table 1,
entries 1-12). The yield increased to 92% in 1,2-dichloroethane
(DCE) (Table 1, entry 3), and the results showed DCE to be
superior to other solvents such as CH2Cl2, tetrahydrofuran, ethyl
acetate, and so on (Table 1, entries 1 and 2, 4-12). The reaction
1
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10.1002/ejoc.202100440
European Journal of Organic Chemistry
FULL PAPER
did not proceed under nitrogen atmosphere (Table 1, entry 13),
whereas on exposure to air, 79% of the desired product was
isolated after 24 h (Table 1, entry 14). None of the desired
product was detected when the reaction was attempted under
dark conditions (Table 1, entry 15). It is worth noting that 1c
reacted slowly to afford 2c in 41% yield after 8 h of sunlight
irradiation (Table 1, entry 16).
1v and 1w were also transformed to the corresponding 1,2,4-
thiadiazoles 2v and 2w in 62% and 64% yield, respectively.
Moreover, some substrates including heterocyclic ring were
examined. Furanyl, thienyl, or indolyl substrates also reacted
well, providing products (2x-2aa) in yields of 30-55%, however,
no products bearing pyrrolyl or pyridinyl substituents were
obtained (1ab-1ae). Finally, aliphatic thioformamides (1af-1ah)
failed to react. The reason that these substrates (1ab-1ah) did
not react was ascribed to the lack of absorption in 450-455 nm
(see SI, Figure S10-S16).
Table 1. Optimization of reaction conditions[a]
entry
solvent
oxidant
optical
sources
blue
blue
blue
blue
blue
blue
blue
blue
blue
blue
blue
blue
blue
blue
/
Yield[b] (%)
1
2
CH2Cl2
THF
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
/
66
32
92(88)[c]
3
DCE
4
EtOAc
EtOH
73
5
75
6
CHCl3
1,4-Dioxane
Acetone
CH3CN
DMF
62
7
43
8
42
9
48
10
11
12
13
14
15
16
42
CH3OH
DMSO
DCE
85
50
0[d]
Scheme 1. Strategy to 1,2,4-thiadiazole structures via intermolecular S-N
bond construction
DCE
air
O2
air
79[e]
0[f]
41[g]
DCE
DCE
sunlight
After optimizing the reaction conditions, the scope of
reaction was explored (Table 2). Electron-donating and
electron-withdrawing groups on thiobenzamides were well
tolerated to provide the corresponding products in moderate to
good yields (2b-2h, 2j-2t). For thiobenzamides bearing electron-
donating groups, yields were consistently above 80% (2b-2h),
irrespective of whether the substituents were in ortho, meta, or
para positions on the phenyl ring. For example, thiobenzamide
[a] Reaction conditions: 4-methoxy-thiobenzamide (1c) (0.2 mmol, 0.033g),
solvent (5 mL), O2 balloon, irradiated by blue LEDs (3W×2, 450-455 nm), room
temperature, 8 h. [b] Determined by 1H NMR using mesitylene as the internal
standard. [c] Yield of isolated product. [d] N2, 24 h. [e] 24 h. [f] in dark. [g]
Using sunlight as optical sources (the maximum power density = 3.2 mW cm-
2), 8h.
To explore the potential of this method for constructing S-N
bond, the gram-scale reaction (8 mmol) was performed with a
moderate yield (Scheme 2-I, 64%), the result showed that our
protocol has potential for further application. 3,5-Bis[1-[(4-
chlorophenyl)methyl]-5-methoxy-indole-3-yl]-1,2,4-thiadiazole
(Scheme 2-II, 8) exhibited potent activity against cancer cell
lines LnCaP (IC50=14.6μM)[5]. To explore the potential
application of in pharmaceutical field, we applied our
methodology in the synthesis of the precursor of LnCaP inhibitor
(Scheme 2-II, 7). The 5-methoxyindole 3 underwent cyanation
with chlorosulfonyl isocyanate 4 to give the compound 5 in 91%
yield[12]. Compound 5 further converted to thioformamide 6 with
with
a
methoxy substituent furnished the 3,5-bis(4-
methoxyphenyl)-1,2,4-thiadiazole (2c) in a high yield of 88%.
The reason that 2,6-dimethyl-thiobenzamide (1i) did not afford
the target product was attributed to the effect of steric hindrance.
Thiobenzamides bearing halide atoms (F, Cl, Br) or
trifluoromethyl groups reacted smoothly to give the desired
products (2j-2t) in 61-70% yields, the substitution positions of
these electron-withdrawing groups had little influence on the
yields. In addition, thioformamides containing naphthyl groups
2
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10.1002/ejoc.202100440
European Journal of Organic Chemistry
FULL PAPER
(NH4)2S in 76% yield[13]. The corresponding 1,2,4-thiadiazole 7
was obtained with 69% yield under standard conditions. Finally,
the precursor 7 gave the target product 8 in 90% yield.
thioamidyl radical intermediates by TEMPO (see SI, Figure S17).
The yield of the reaction also decreased in 58% yield after the
addition of galvinoxyl. These experiments revealed that the
reaction might involve a radical pathway (Scheme 3-II). Next,
under the standard conditions, the substrate was replaced with
Table 2. Substrate scope of 1, 2, 4-thiadiazoles derivatives[a]
the
putative
intermediate
N-(phenylthioxomethyl)-
benzenecarboximidamide 9 (synthetic procedure see SI), the
desired product 2a was received in 70% yield (Scheme 3-III),
which implied that the reaction process may involve the
cyclization of the intermediate 9. Finally, in the crossover
reaction of two different aromatic substrates (1c and 1i), the
crossover product 2u from the condensation of 1c and 1i was
obtained in 9% yield, which implied that the crossover reaction
can be carried out between aromatic thioamide and aromatic
thioamidyl radical (Scheme 3-IV). Furthermore, when the
crossover experiment of aromatic thioamide (1c) and aliphatic
thioamide (1ag) were performed, the crossover product 10 from
the condensation of 1c and 1ag was isolated in 3% yield, which
suggested that the aliphatic thioamide 1ag could condense with
aromatic thioamidyl radical and give the desired product 10
(Scheme 3-V).
[a] Reaction conditions: aryl thioamide (0.2 mmol), DCE (5 mL), O2 balloon,
irradiated by blue LEDs (3W×2), room temperature, 8 h, isolated yields. [b] 12
h. [c] 24 h. [d] CH3OH (5 mL).
A series of control experiments were conducted to gain
better insight into the mechanism of the reaction. Firstly, 78% of
the product was monitored after irradiation from blue LEDs for 8
h, while the yield was only 51% after irradiation from white LEDs
in 24 h. As we expected, the reaction did not proceed under dark
conditions (Scheme 3-I). The results implied that aryl thioamide
molecules might have absorption in the visible region, and
indeed the UV/Vis absorption spectra of some substrates (1a,
1c) indicated that they have absorption in the wavelength range
450-455 nm (see SI, Figure S8, S9). Subsequently, in the
presence of the typical radical scavenger 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO), the reaction was
completely inhibited, which might be attributed to the capture of
Scheme 2. Gram-scale reaction (I) and synthesis of key structure of LnCaP
inhibitor (II)
3
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10.1002/ejoc.202100440
European Journal of Organic Chemistry
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Based
on
mechanistic
research
and
previous
literatures[11f,g],[14], we propose a plausible mechanism, as shown
in scheme 4. Substrate 1a is excited to generate 2* by visible
light irradiation, and then excited 2* undergoes an SET process
to form radical cation A under blue light irradiation and O2
atmosphere. Intermediate A is deprotonated by the superoxide
anion to afford thioamidyl radical B, which can be captured by
TEMPO (see SI, Figure S17). Further, another thioamide then
attacks the carbon atom of B to afford the intermediate C
through intermolecular nucleophilic addition and desulfurization,
and a yellow solid obtained by column chromatography proved
to be elemental sulfur (see SI, Figure S18, S19). Compound C is
further oxidized by O2 to give sulfur radical cation D,
deprotonation of which generates sulfur radical E. Finally, the
target product 2a is generated through the intramolecular
oxidative cyclization[15] from intermediate E.
Scheme 4. Proposed mechanism.
Conclusion
In summary, we developed
a
green method for
intermolecular S-N bond construction under blue light irradiation
and aerobic conditions. Our approach does not need
a
photosensitizer, metal, or base, thioamides react smoothly to
produce 1,2,4-thiadiazoles in moderate to good yields. This
methodology shows potential for practical application in S-N
bond construction, and further mechanistic research is in
progress.
Experimental Section
General Information: All reagents and solvents were used as purchased
without further purification. Aryl and alkyl thioamides were prepared
according to literature procedures. Column chromatography on silica gel
(300-400 mesh) was carried out using technical grade 60-90 petroleum
ether and analytical grade ethyl acetate (without further purification). 1H
and 13C NMR spectra were measured at ambient temperature on a
Bruker Advance2B/400MHz spectrometer using CDCl3 or d6-DMSO as
solvent. Chemical shifts are referenced to CDCl3 with 7.26 ppm for 1H
spectra and 77.03 ppm for 13C spectra, and to d6-DMSO with 2.50 ppm
for 1H spectra and 39.52 ppm for 13C spectra. HRMS (ESI) was obtained
with Micromass QTOF2 Quadrupole/Time-of-Flight Tandem Mass
spectrometer using electron spray ionization. UV-visible spectra were
recorded using an HP-8453 UV/Vis/Near Infrared Spectrophotometer
(Agilent). X-Ray Diffraction spectra were received from MiniFlex600 X-
ray diffractometer.
Scheme 3. Controlled experiments.
General Procedure for the Synthesis of 1,2,4-Thiadiazoles:
Thiobenzamide (0.8 mmol) in 25 mL over-dried Schlenk tube was
4
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10.1002/ejoc.202100440
European Journal of Organic Chemistry
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dissolved with 1,2-dichloroethane (5 mL) under O2 atmosphere, irradiated
by blue LEDs (3W×2, 450-455nm) for 8 h at room temperature. The
product was purified by flash column chromatography on silica gel (300-
400 mesh) (petroleum ether/ethyl acetate = 20:1).
Pan, J. Gao, J. Liu, J. Lai, H. Jiang, G. Yuan, Green. Chem. 2015, 17,
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This research was supported by the National Key Research and
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Keywords: Oxidation • Cyclization • Photocatalysis • 1,2,4-
Thiadiazole • Thioamide
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5
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Entry for the Table of Contents
The green approach for S-N construction to synthesize 1,2,4-thiadiazoles was developed by visible-light induced oxidative cyclization
of thioamides under photosensitizer-free conditions.
6
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