Eosin-Y-Catalyzed Photoredox C?S Bond Formation: Easy Access to Thioethers

Article abstract of DOI:10.1002/asia.201901060

An operationally simple Eosin Y catalyzed sulfenylation of hydrazones has been realized to afford a range of thioethers under visible light. The methodology provides high yields of thioethers under ambient conditions employing readily available and inexpensive starting materials. The reaction has broad substrate scope and is compatible with various functional groups.

Full text of DOI:10.1002/asia.201901060

DOI: 10.1002/asia.201901060
Full Paper
Eosin-Y-Catalyzed Photoredox CÀS Bond Formation: Easy Access
to Thioethers
Shiv Chand, Anand Kumar Pandey, Rahul Singh, Saurabh Kumar, and Krishna Nand Singh*[a]
Abstract: An operationally simple Eosin Y catalyzed sulfeny-
lation of hydrazones has been realized to afford a range of
thioethers under visible light. The methodology provides
high yields of thioethers under ambient conditions employ-
ing readily available and inexpensive starting materials. The
reaction has broad substrate scope and is compatible with
various functional groups.
Introduction
The carbonÀsulfur bond formation has been realized as a vital
synthetic strategy for the modern day state of the art drug
design and discovery, due to the ubiquity of sulfenylated struc-
tures in natural products, biologically active molecules, and
materials.[1] Various organic scaffolds containing thioether func-
tionality such as Montelukast (1a) and Neflinavir-HIV protease
inhibitor (1b) constitute the potential ingredients of medicinal
therapies for the treatment of diseases like asthma and HIV
(Figure 1). Such thioether structures are also known to exhibit
enhanced antiviral activity (1c), as well as avb3 integrin inhibi-
tion (1d).[2]
Considerable synthetic strategies have evolved over the
years to deal with the challenges associated with the CÀS
bond formation.[3] Yet the growing demand for newer method-
ologies and hazards associated with the conventional methods
entails the technology upgrades to develop greener and effi-
cient protocols for the formation of CÀS bonds using readily
available starting materials. The propensity of organic sulfur
compounds to act as metal deactivators due to its tendency to
bind with the metals, pose severe limitations to the conven-
tionally used transition-metal catalyzed cross-coupling reac-
tions.[4] Therefore, the existing strategies for the CÀS bond for-
mation often employ harsh reaction conditions such as high
temperature, use of toxic and high boiling polar solvents, etc.
Several other strategies comprise of a reductive generation of
thiol alternative from sulfones, sulfoxides and hydrazides,
many of which however employ strong reducing agents like
Figure 1. Some biologically active molecules containing thioethers.
DIBAL-H or LiAlH4.[5] Various transition metals viz. Pd,[6] Cu,[7]
Co,[8] Ni,[9] Fe,[10] etc., efficiently catalyze the CÀS bond forma-
tion, but the high cost, air sensitivity and requisite for more
polar solvents restrict their acceptance for large-scale prepara-
tions. Thus, despite their utmost importance, an operational
simple, cost-effective and sustainable synthetic protocol for
the sulfenylation of the organic molecules is yet far from reali-
zation.
Some recent reports by the Peng,[11] Corma,[12] and Zeng,[13]
research groups have endowed remarkable synthetic contribu-
tions to CÀS bond formation employing reasonably practical
conditions (Scheme 1). Hydrazones have been used as excel-
lent synthon for various transformations. In particular, they are
used as precursors to the diazo compounds as well as carbene
source which allows the formation of a variety of CÀC and CÀ
heteroatom bonds with good regio- and stereo-selectivity
under mild conditions.[14]
[a] S. Chand, A. K. Pandey, Dr. R. Singh, S. Kumar, Prof. Dr. K. N. Singh
Department of Chemistry (Centre of Advanced Study)
Institute of Science
Banaras Hindu University
Varanasi 221005 (India)
E-mail: knsingh@bhu.ac.in
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/asia.201901060.
This manuscript is part of a special issue celebrating the 20th anniversary of
the Chemical Research Society of India (CRSI). A link to the Table of Con-
tents of the special issue will appear here when the complete issue is pub-
lished.
The pursuit for a potential green toolbox to tackle the haz-
ards associated with age long conventional strategies, led to
the rejuvenation of photoredox chemistry. A single electron
transfer (SET) mediated strategy for the activation of organic
substrates revolutionized the area of organic synthesis as evi-
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Results and Discussion
To validate the possibility of synthetic route, a model reaction
between acetophenone hydrazone (1a) and benzenethiol (2a)
was investigated in detail, and the results are shown in Table 1.
Table 1. Optimization of reaction conditions.[a]
Entry
Photocatalyst
Solvent
Base
Yield [%][b]
1
2
3
4
5
6
7
8
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
–
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
Toluene
Dioxane
CH3CN
DCE
EtOH
DMF
DMF
–
0
NaOAc
K2CO3
DBU
trace
36
55
25
70
85
80
trace
30
15
22
25
0
Et3N
tBuOK
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
9
10
11
12
13
14
15
16
17
Eosin B
Rose bengal
Rhodamine
60
40
0
DMF
DMF
Scheme 1. Synthesis of thioethers.
[a] Using 1a (1 mmol), 2a (1.5 mmol), photocatalyst (2 mol%), base
(3 equiv), solvent (1 mL) in open vessel at room temperature for 24 h.
[b] Isolated yield after column chromatography.
dent by numerous scientific reports over the last decade.[15]
The photoredox catalysis plays the crucial role as it promotes
electron transfer and enables the generation of radical species
either by oxidation or reduction. Such reactions operating at
room temperature have allowed various otherwise tedious re-
actions. Organic dyes present a rational and rather economic
preference over the metal-based photocatalysts for thiolation
process, considering the fact that coordination of thiols could
poison the metal catalysis.[16] Eosin Y is amongst the well ex-
plored organic dyes for photo-redox organic transformations
as the redox potential of Eosin Y suits well for both the oxida-
tion as well as reduction of organic substrates,[17] thereby ren-
dering a vast range of substrate scope for the desired transfor-
mations. In this perspective, it is envisioned that a SET strategy
could be effective by activated Eosin Y under visible light for
the CÀS bond formation employing hydrazones and thiophe-
nols.
A proper setup of the model reaction was assembled in a
10 mL vial employing Eosin Y as photocatalyst under green
LED (530 nm) irradiation. The reaction without the use of a
base did not afford the product at all (Table 1, entry 1). Several
bases were then screened to effect the desired transformation.
When the reaction was carried out in the presence of 3 equiva-
lent of NaOAc in DMF at room temperature, a trace amount of
the desired product 3a was formed (Table 1, entry 2). Further
trial of different bases afforded different yields of the produc-
t 3a (Table 1, entry 2–7), and the use of NaOH (entry 7) was
concluded to be the best keeping other parameters intact.
Subsequently, the effect of different solvents was examined
(entries 8–13), which disclosed DMF as the best choice. Obvi-
ously when the reaction was carried out in the absence of
photocatalyst, no desired product was obtained (Table 1,
entry 14). Further, some other organic dyes were also screened
for their catalytic activity. It was observed that the members of
the fluorescein family viz. Eosin B and Rose Bengal (entries 15
& 16) although worked well, but could not surpass the worth
of Eosin Y. A fluorone dye viz. Rhodamine, however, failed to
produce the product under stipulated conditions (entry 17).
Control experiments revealed that both the photocatalyst
and light irradiation were indispensable for the transformation.
Additional studies showed that decreasing the amount of base
or thiol decreased the product yield considerably. A trace
Our group has explored a regioselective hydrothiolation of
alkynes using organic Ionic base-Brønsted acid combination,[18a]
as well as the synthesis of unsymmetrical sulfides.[18b] With the
proficiency in the area and influenced by the modern day
technology advancements in the organic synthesis, we report
herein a new photoredox mediated CÀS bond formation,
which features an electron mediated generation of thiyl radical
for sustainable synthesis of a range of thioethers under mild
conditions (Scheme 1).
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amount of the undesired disulfide was also observed under
the applied reaction conditions as the thiols are prone to oxi-
dative SÀS coupling. To our cherish, a set of optimized reaction
conditions were thus unveiled (Table 1, entry 7) affording a
considerably fair product formation under visible light irradia-
tion.
particular provided somewhat higher product yields. The reac-
tion involving heteroaromatic hydrazones and thiols also suc-
ceeded well affording the products 3j, 3k, 3r, and 3u in high
yields. The reaction of a hydrazone containing polycyclic aro-
matic moiety viz. 1-(naphthalen-1-yl)ethylidene)hydrazine with
thiophenol also worked efficiently under the present set of
conditions to give the product 3e in excellent yield. To our de-
light, even the hydrazone derived from an a,b-unsaturated car-
bonyl compound viz. cinnamaldehyde also underwent cou-
pling to give the desired product 3x in good yield. Aliphatic
thiols namely 1-hexanethiol, cyclohexanethiol and ethanethiol
however, could barely afford a trace of the desired products
perhaps due to over-oxidation of the relatively less stable
moiety.[19]
To demonstrate the generality and scope of the method, the
reaction was afterward investigated using different sets of
thiols and hydrazones under established conditions
(Scheme 2). As evident from Scheme 2, the optimized condi-
tions worked proficiently for the reaction of a wide range of
thiols and hydrazones derived from ketones and aldehydes to
afford the desired products 3a–3y in good to excellent yields.
A number of substituent viz. CH3, Cl, F, OMe, CF3, and CN were
well tolerated during the course of reaction. ortho-Substituted
hydrazone afforded somewhat inferior yields (3c & 3 f) possibly
due to steric hindrance. The versatility of the reaction was also
scrutinized by varying the substitution pattern on both the re-
acting partners, which gave rise to the desired products
smoothly. Hydrazones containing electron rich substituent in
To get an insight into the reaction mechanism, a control ex-
periment using a radical trap namely TEMPO (2,2,6,6-tetrame-
thylpiperidine N-oxide) in the reaction mixture was performed
under the standard conditions (Scheme 3), which suppressed
the desired transformation. The involvement of a thiyl radical
in the mechanistic pathway was further confirmed by the for-
mation of TEMPO-SPh adduct 2,2,6,6-tetramethyl-1-(phenyl-
thiooxy) piperidine (4a, 43%) which was detected by the
HRMS analysis and then characterized by the spectral data.
Scheme 3. Radical-trapping experiment.
Since the photo-excited state of Eosin Y is capable of both
single electron oxidation as well as reduction, cyclic voltammo-
gram of thiophenol 2a (1 mm) was recorded at glassy carbon
electrode in 0.1m CH3CN/TBAP at a scan rate of 100 mVsÀ1 (cf.
Figure S1, SI). The oxidation and reduction potentials of the
thiophenol were observed at À0.37 V and À1.3 V respectively.
The redox potential of Eosin Y indicates that it could allow a
single electron oxidation of the thiophenol but not the reduc-
tion.[16] Also, thermodynamically a single electron oxidation of
the thiophenol (EPhSH/PhSH +ꢀ0.95 V vs. SCE) by the excited
state of Eosin Y (EE*/EÀ =1.18 V vs. SCE) is quite feasible.[20]
Based on isolation of products, control experiments, and ex-
isting literature,
a plausible mechanism is outlined in
Scheme 4. The redox potential of thiophenol and thermody-
namic properties suggest a reductive quenching cycle for the
photo- excited state of Eosin Y. Thus a photo-excited Eosin Y is
presumed to oxidize thiol (2) to the corresponding thiyl radical
cation.[21a] The reduced Eosin Y radical anion subsequently un-
dergoes aerial oxidation to regenerate the neutral Eosin Y for
the next catalytic cycle, along with the generation of superox-
À
ide radical anion (O2 ) which deprotonates the thiyl radical
cation to form the thiyl radical and a hydroperoxyl radical
Scheme 2. Substrate scope for the synthesis of thioethers. Conditions: 1
(1 mmol), 2 (1.5 mmol), photocatalyst (2 mol%), base (3 equiv), solvent
(1 mL) in open vessel at room temperature for 24 h. Isolated yields after
column chromatography are reported.
[21b]
C
(HO2 ).
Hydrazone (1) acts as a precursor to the diazo com-
pound, which on addition with the thiyl radical affords the in-
termediate I, followed by dediazotization to generate the inter-
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solvent (1H NMR: CDCl3 at 7.26 ppm). Chemical shifts for carbons
are reported in parts per million downfield from tetramethylsilane
and are referenced to the carbon resonances of the solvent peak
(13C NMR: CDCl3 at 77.16 ppm). NMR data are represented as fol-
lows: chemical shift, multiplicity (s=singlet, d=doublet, t=triplet,
q=quintet and m=multiplet), coupling constant (J) (Hz), and inte-
gration. HRMS was determined using SCIEX X500R Q-TOF mass
spectrometer.
General Experimental Procedure
A mixture of hydrazone (1, 1 mmol), thiol (2, 1.5 mmol), Eosin Y
(2 mol%), sodium hydroxide (3 equiv) and DMF (1.0 mL), contained
in a 25-mL borosilicate RB flask, was stirred and irradiated using
green LED (530 nm) under ambient conditions for 24 h. After com-
pletion of reaction (TLC), the mixture was worked-up using water
(15 mL) and ethyl acetate (2ꢁ15 mL). The organic phase was dried
over anhydrous Na2SO4, filtered, evaporated under reduced pres-
sure, and the crude product was purified by column chromatogra-
phy using ethyl acetate and n-hexane as eluent to afford the pure
product 3.
Physical and Spectral Data of New Products
Scheme 4. Proposed mechanism.
(1-(2-Chlorophenyl)ethyl)(phenyl)sulfane (3 f): Colorless oil; yield:
171 mg (69%).
1H NMR (500 MHz, CDCl3) d=7.45 (d, J=7.5 Hz, 1H), 7.25–7.21 (m,
3H), 7.15–7.11 (m, 4H), 7.07 (t, J=7.5 Hz, 1H), 4.85 (q, J=7.0 Hz,
1H), 1.52 ppm (d, J=7.0 Hz, 3H); 13C NMR (125 MHz, CDCl3) d=
140.6, 134.8, 133.3, 132.2, 129.6, 128.9, 128.4, 128.3, 127.3, 127.2,
43.5, 21.8 ppm. HRMS calcd for C14H14ClS+ [M+H]+: 249.0499
found: 249.0509
mediate II. Alternatively, the diazo compound formed by the
reaction of hydrazone and hydroxide ion could afford an inter-
mediate carbene, which on its subsequent coupling with the
thiyl radical generates the intermediate II, followed by H-ab-
straction from the hydrogen source present in the reaction
mixture via Hydrogen Atom Transfer (HAT)[22] to give the prod-
uct 3.
4-(1-(p-Tolylthio)ethyl)benzonitrile (3i): Pale yellow oil; yield:
172 mg (68%). 1H NMR (500 MHz, CDCl3) d=7.53 (d, J=7.5 Hz,
2H), 7.31 (d, J=7.5 Hz, 2H), 7.12 (d, J=7.5 Hz, 2H), 7.03 (d, J=
7.0 Hz, 2H), 4.26 (q, J=6.5 Hz, 1H), 2.29 (s, 3H), 1.61 ppm (d, J=
7.0 Hz, 3H); 13C NMR (125 MHz, CDCl3) d=149.2, 138.3, 136.8,
133.9, 132.3, 129.8, 128.2, 118.9, 110.8, 48.4, 21.7, 21.6 ppm. HRMS
calcd for C16H16NS+ [M+H]+: 254.0998 found: 25.0990.
Conclusion
In conclusion, a photo-induced single electron transfer strategy
has been explored for a sustainable thiolation of hydrazones
derived from aldehydes and ketones to afford the correspond-
ing thioethers. The reaction operates under ambient condi-
tions and employs an eco-friendly organic dye Eosin Y as pho-
tocatalyst. The redox potentials of the Eosin Y and thiophenols
have been found to be the driving force and rationale behind
the present transformation offering a vast range of substrate
scope and functional group tolerance.
2-(1-(Phenylthio)ethyl)thiophene (3k): Light brown oil; yield:
165 mg (75%). 1H NMR (500 MHz, CDCl3) d=7.35 (d, J=7.5 Hz,
2H), 7.25 (d, J=6.0 Hz, 3H), 7.17 (d, J=5.0 Hz, 1H), 6.86 (t, J=
3.5 Hz, 1H), 6.80 (d, J=2.5 Hz, 1H), 4.62 (q, J=7.0 Hz, 1H),
1.71 ppm (d, J=7.0 Hz, 3H); 13C NMR (125 MHz, CDCl3) d=147.7,
134.7, 133.1, 128.À8, 127.7, 126.5, 124.7, 124.4, 43.5, 23.4 ppm. HRMS
calcd for C12H11S2 [MÀH]À: 219.0307 found: 219.0304.
o-Tolyl(1-(p-tolyl)ethyl)sulfane (3m): Colorless oil; yield: 184 mg
1
(76%). H NMR (500 MHz, CDCl3) d=7.22 (d, J=7.0 Hz, 1H), 7.13 (d,
J=7.5 Hz, 2H), 7.08 (d, J=7.0 Hz, 1H), 7.04–6.99 (m, 4H), 4.22 (q,
J=7.0, 1H), 2.26 (s, 3H), 2.23 (s, 3H), 1.54 ppm (d, J=7.0 Hz, 3H);
13C NMR (125 MHz, CDCl3) d=140.3, 139.9, 136.9, 135.0, 132.4,
130.2, 129.2, 127.1, 127.0, 126.3, 47.0, 22.5, 21.2, 20.8 ppm. HRMS
calcd for C16H17SÀ [MÀH]À: 241.1056 found: 241.1047.
Experimental Section
General Information and Materials
All the reagents and solvents were purchased from Sigma–Aldrich
or Merck chemical co. and were used without further purification.
Analytical thin layer chromatography (TLC) was performed on
Merck Kieselgel 60 GF254 plates (thickness 0.25 mm). Visualization
was performed with a 254 nm UV lamp and by staining in I2 cham-
ber. Product purification was done using Merck silica gel (100–
200 mesh) column chromatography. Proton and carbon magnetic
resonance spectra were recorded on a JEOL AL-500 FTNMR spec-
trometer (1H NMR at 500 MHz, 13C NMR at 125 MHz). Chemical
shifts for protons are reported in parts per million downfield from
tetramethylsilane, and are referenced to residual deuterium in the
(4-Fluorophenyl)(1-phenylpropyl)sulfane (3o): Colorless oil; yield:
1
194 mg (79%). H NMR (500 MHz, CDCl3) d=7.18 (t, J=6.0 Hz, 2H),
7.13–7.08 (m, 5H), 6.82 (t, J=8.5 Hz, 2H), 3.87 (q, J=7.0 Hz, 1H),
1.96–1.80 (m, 2H), 0.86 ppm (t, J=7.5 Hz, 3H); 13C NMR (125 MHz,
CDCl3) d=163.5, 161.6, 141.9, 135.7, 135.7, 129.9, 128.4, 128.0,
127.2, 115.9, 115.7, 56.5, 29.1, 12.5 ppm. HRMS calcd for C15H14FSÀ
[MÀH]À: 245.0805 found: 245.0803.
(4-Methoxyphenyl)(1-(p-tolyl)ethyl)sulfane (3p): Colorless oil;
yield: 178 mg (69%). 1H NMR (500 MHz, CDCl3) d=7.17 (d, J=
9.0 Hz, 2H), 7.06 (d, J=8.0 Hz, 2H), 7.00 (d, J=8.0 Hz, 2H), 6.70 (d,
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Acknowledgements
We are thankful to the CSIR, New Delhi (Scheme No. 02(0285)/
16/ EMR-II) for financial assistance. SC is thankful to CSIR, New
Delhi for JRF and AKP is thankful to DST, New Delhi for INSPIRE
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Conflict of interest
The authors declare no conflict of interest.
Keywords: Eosin Y · hydrazones · photoredox catalysis ·
sulfenylation · thioethers
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Manuscript received: August 3, 2019
Revised manuscript received: September 3, 2019
Accepted manuscript online: September 12, 2019
Version of record online: && &&, 0000
Chem. Asian J. 2019, 00, 0 – 0
www.chemasianj.org
5
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
Shiv Chand, Anand Kumar Pandey,
Rahul Singh, Saurabh Kumar,
Krishna Nand Singh*
&& – &&
Eosin-Y-Catalyzed Photoredox CÀS
Bond Formation: Easy Access to
Thioethers
Photoredox sulfenylation: An opera-
tionally simple and efficient photoredox
strategy has been realised for the CÀS
bond formation, employing organic dye
Eosin Y as catalyst. The methodology
offers a broad substrate scope and func-
tional group tolerance.
&
&
Chem. Asian J. 2019, 00, 0 – 0
www.chemasianj.org
6
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!