Visible light photocatalysis with benzophenone for radical thiol-ene reactions

Article abstract of DOI:10.1016/j.tetlet.2017.04.060

We report herein a simple, metal- and oxidant-free visible light promoted strategy for an anti-Markovnikov hydrothiolation of unactivated olefins using benzophenone as an inexpensive photocatalyst at room temperature. Anti-Markovnikov adducts of a wide va

Full text of DOI:10.1016/j.tetlet.2017.04.060

Accepted Manuscript
Visible light photocatalysis with benzophenone for radical thiol-ene reactions
Manjula Singh, Arvind K. Yadav, Lal Dhar S. Yadav, R.K.P. Singh
PII:
S0040-4039(17)30500-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.04.060
TETL 48852
Reference:
Tetrahedron Letters
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Revised Date:
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18 April 2017
19 April 2017
Please cite this article as: Singh, M., Yadav, A.K., Yadav, L.D.S., Singh, R.K.P., Visible light photocatalysis with
benzophenone for radical thiol-ene reactions, Tetrahedron Letters (2017), doi: http://dx.doi.org/10.1016/j.tetlet.
2017.04.060
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Graphical Abstract
Visible light photocatalysis with benzophenone for
radical thiol-ene reactions
Leave this area blank for abstract info.
Manjula Singha, Arvind K. Yadavb, Lal Dhar S. Yadavb*, R. K. P. Singha*
O
(10 mol%) , CH CN
3
Ph Ph
R1S
R1SH
R2
R2
visible light, rt, N2, 12-18 h
15 examples
up to 94% yield

1
Tetrahedron Letters
journal homepage: www.els evier.com
Visible light photocatalysis with benzophenone for radical thiol-ene reactions
Manjula Singha, Arvind K. Yadavb, Lal Dhar S. Yadavb*, R. K. P. Singha*
a Electrochemical laboratory, Department of Chemistry, University of Allahabad, Allahabad 211002, India
b Green Synthesis Lab, Department of Chemistry, University of Allahabad, Allahabad 211002, India
* Corresponding author. Tel.: +91 532 2500652; fax: +91 532 2460533; E-mail address: ldsyadav@hotmail.com (L.D.S. Yadav)
ARTICLE INFO
ABSTRACT
We report herein a simple, metal- and oxidant-free visible light promoted strategy for an
anti-Markovnikov hydrothiolation of unactivated olefins using benzophenone as an inexpensive
photocatalyst at room temperature. Anti-Markovnikov adducts of a wide variety of olefins and
thiols are formed in highly regioselective manner and good to excellent yields. The present radical
thiol-ene reaction is operationally simple and well tolerates a variety of functional groups.
Article history:
Received
Received in revised form
Accepted
Available online
Keywords:
Visible light
2009 Elsevier Ltd. All rights reserved.
Photocatalysis
Thiol-ene reaction
Benzophenone
Anti-Markovnikov addition
Regioselective reaction
The hydrothiolation of alkenes is a long known synthetically
useful transformation,1 described as the thiol-ene click ( TEC)
reaction by Schlaad.2 The reaction has emerged as a powerful
tool for the construction of carbon-sulfur bonds in natural
products, pharmaceuticals,3 polymers and organic materials4 as
well as sulfur-containing ligands and chiral auxillaries.5 The
radical TEC reaction6 is one of the most common methods for
effecting the synthesis of thioethers involving the anti-
Markovnikov radical addition of thiols to alkenes (Scheme 1).7
Particularly, this reaction is of significant importance in
materials and biological sciences owing to its high efficiency
and compatibility with a variety of functional groups.8
to a heteroatom.15 Based on earlier reports15 on successful use of
benzophenone as a photocatalyst, it was chosen as a catalyst in
the present study. Movassagh and Navidi have reported water
promoted catalyst-free anti-Markovnikov addition of thiols to
styrenes but the method is not applicable to aliphatic thiols and
nonstyrenic alkenes.17
In view of the above points and our continued efforts
focused on visible light mediated synthetic routes,14b,18 we
hypothesized that photoexcited benzophenone could bring
about a metal- and oxidant-free radical thiol-ene reaction under
visible light irradiation ( Scheme 1b).
Previous work
Over the past several years, the application of visible light
photoredox catalysis for the convenient generation of free
radicals has stimulated their targeted use in organic synthesis.
Basically, the success of this strategy relies on the pioneering
work of the research groups of McMillan,9 Yoon,10 and
Stephenson,11 who have used ruthenium and iridium transition
metal complexes as efficient photoredox catalysts. Recently,
Yoon and co-workers12 have reported novel applications of
visible light photoredox catalysis in radical thiol-ene reactions
employing the ruthenium complex Ru(bpz)3(PF6) (Scheme 1a).
Stephenson and co-workers13 have also reported radical thiol-
ene reactions under visible light irradiation (blue LEDs) using
ruthenium complex Ru(bpy)3Cl2 as the photoredox catalyst
(Scheme 1a). However, the use of transition metal complexes as
photoredox catalysts is disadvantageous because of high cost,
potential toxicity and problematic removal of their undesired
traces from products. In requisite of metal-free, cost effective,
operationally simple and ecofriendly catalysts, some organic
dyes,14 diarylketones15 and N-hydroxyphthalimide16 have shown
enough promise for high photocatalytic performance. It is
known that after photoexcitation with visible light,
diarylketones selectively abstract a hydrogen atom attached
Ru(bpy)32+, blue LEDs
R1SH
R
2
1
R S
(a)12,13
2
R
or
Ru(bpy)3Cl2, BrCCl3,
blue LEDs
O
Present work
Ph
Ph
1
(b) R1SH
2
R S
R
2
R
visible light
Scheme 1. Visible light mediated radical thiol-ene reactions.
In order to realize our hypothesis and optimize the reaction
conditions, a model reaction of styrene (1a) with thiophenol
(2a) was performed using catalytic amount of benzophenone in
a solvent under nitrogen and irradiation with a household 18 W
compact fluorescent lamp (18 W CFL) (Table 1). We were
delighted to get the desired product (3a) in 87% yield (Table 1,
entry 1). Then, the control experiments were carried out, which
show that benzophenone and visible light are essential for the
reaction because in the absence of any of the reagents/reaction
parameters the product was not detected (Table 1, entry 1

2
Tetrahedron
versus 6 and 7).The optimum amount of the photocatalyst
3e, 3h, 3k, and 3o). However, in this case the yield was slightly
lower as compared to styrenes and thiolphenols.
benzophenone required for the reaction was 10 mol%. On
decreasing the amount of benzophenone from 10 mol% to 5
mol% the yield was considerably reduced (Table 1, entry 1
versus 4), whereas the yield was not enhanced even on use of
15 mol% of benzophenone (Table 1, entry 1 versus 5). The
use of other organic catalyst was not so effective as
benzophenone (Table 1, entry 1, versus 2 and 3).
Table 2
Substrate scope for the radical thiol-ene reaction
O
Ph
Ph (10 mol%), CH3CN
R1S
R1SH
R2
R2
Table1
Optimization of reaction conditionsa
visible light, rt, N2, 12-18 h
1
2
3a-3ob
( 75-94% )c
O
S
S
S
Ph
Ph
Ph
S
(10 mol% ) , CH3CN
Ph
Ph
Ph
PhSH
CH3
CH3O
visible light, rt, N2, 12-24 h
3a, 12 h , 87%
3b, 12 h , 90%
3c, 12 h , 94%
1a
2a
3a
S
Yield (%)b
S
S
Entry
Catalyst (mol %)
Solvent
Time (h)
Ph
Ph
Ph
NO2
Br
Ph
1
Benzophenone (10%)
Acetone (10%)
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
EtOH
12
12
12
12
12
24
24
24
12
12
12
87
67
3d,
3e,
3f
14 h , 84%
S
12 h , 85%
S
, 16 h , 79%
S
2
Ph
3
Acetophenone (10%)
Benzophenone (5 %)
Benzophenone (15 %)
Benzophenone (10 %)
-
62
Cl
4
58
OCH3
5
87
3g, 12 h , 92%
3h, 16 h , 78%
Ph
3i, 14 h , 81%
6
n.d.c
n.d.d
tracese
53
S
Ph
S
S
n-Hex
7
Ph
Me
8
Benzophenone (10%)
Benzophenone (10% )
Benzopnenone (10%)
Benzophenone (10%)
3j
, 18 h , 84%
3k
3l
, 14 h , 76%
, 14 h , 83%
S
9
CH3
Ph
10
11
DMF
71
S
S
Ph
DMSO
76
Br
Ph
aReaction conditions: 1a (1.0 mmol), thiophenol (1.0 mmol), catalyst
(mol%), in 3 mL solvent irradiated with a household 18 W compact
fluorescent lamp Philips (18 W CFL) under nitrogen atmosphere at rt for
12-24 h. Isolated yield of the pure product 3a. Reaction was performed in
the dark, n.d. = not detected. dReaction was carried out without catalyst.
eReaction was quenched with 2,2,6,6-tetramethylpiperidyl-1-oxyl (TEMPO)
(2 equiv).
3m , 14 h , 82%
3n, 14 h , 81%
3o, 16 h , 75%
a For experimental procedure, see ref. 19.
b
c
b All compounds are known and were characterized by comparison of their
spectral data with those reported in the literature.12b,20
c Yields of isolated pure compounds 3.
Moreover, the reaction was quenched with traditional radical
seavenger 2,2,6,6-tetramethylpiperidyl-1-oxyl (TEMPO) (2
equiv) indicating that the reaction might follow a radical
pathway (Table 1, entry 6).
On the basis of our observations and the literature
reports,12,15a
a plausible mechanistic pathway is depicted in
Scheme 2. As already reported,15a on irradiation with visible
light, benzophenone is photoexcited to A, which abstracts a
hydrogen radical from thiol 2 to generate the thiyl radical C and
the ketyl radical B in the present case. The thiyl radical C adds
to olefin 1 to form a radical intermediate D, which abstracts a
hydrogen radical from the ketyl radical B to complete the
photocatalytic cycle of benzophenone and afford the final
product 3. The excitation of benzophenone by the visible light
was ensured by using a 400 nm long-pass filter.15a
Next, the reaction was optimized for an effective solvent
system. It was found that CH3CN was the best among the tested
solvents EtOH, DMF, DMSO and CH3CN (Table 1, entry 1
versus 9-11) hence it was used throughout the present study.
Under the established reaction conditions, we surveyed the
generality and scope of the present thiol-ene reaction across a
range of olefins 1 and thiophenols 2 incorporating various
substituents like Me, MeO, NO2, Cl, and Br. The reaction
worked well in all the cases and afforded sulfides 3 in good to
excellent yields and high purity. Styrenes and thiophenols with
an electron-donating group on the aromatic ring appear to react
faster and afford marginally higher yields in comparison to
those bearing an electron-withdrawing group (Table 2, products
3b and 3c versus 3d, 3f and 3i).This is probably because an
electron-donating group stabilizes the electron-deficient
benzylic and thiyl radical intermediates C and D involved in the
reaction (Scheme 2). The protocol was also applicable to
aliphatic alkenes and thiols to give satisfactory results (Table 2,
R1SH
R1S
C
B
Ph
R2
O
OH
2
1
A
Ph
Ph
Ph
R2
photocatalytic
cycle of
benzophenone
R1S
1
R2
D
H
O
R1S
R1S
R2
R2
Ph
Ph
3
3
Scheme 2. Plausible mechanism for anti-Markovnikov thiol-ene reaction.

3
13. Keylor, M. H.; Park, J. E.; Wallentin, C.-J.; Stephenson, C. R.
J. Tetrahedron 2014, 70, 4264.
In conclusion, we have developed a convenient and highly
regioselective synthesis of sulfides from readily available and
diversified olefins and thiols. The present radical thiol-ene
reaction utilizes visible light as the greenest energy source
and benzophenone as an inexpensive organophotocatalyst to
afford sulfides in good to excellent yields and high purity at
ambient temperature. The protocol is metal-and oxidant-free,
operationally simple and compatible with a variety of functional
groups in both the reaction partners.
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Acknowledgments
We sincerely thank the SAIF, Punjab University, Chandigarh,
for providing spectra. M.S. is grateful to the UGC, New Delhi,
for a research fellowship.
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Supplementary data
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Supplementary data associated with this article can be found,
in the online version, at doi…
17. Movassagh, B.; Navidi, M. ARKIVOC 2008 (xv) 47.
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19. General procedure for the synthesis of sulfides 3: A mixture of
olefin 1 (1 mmol), benzophenone (10 mol%), thiol 2 (1 mmol)
and nitrile (3 mL) was stirred at rt under N2 atmosphere and
irradiation with visible light ( 18 W compact fluorescent lamp)
for 12-18 h ( Table 2). After completion of reaction (monitored
by TLC), water (5 mL) was added and the mixture was
extracted with ethyl acetate (3 × 5 mL). The combined organic
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phase was dried over anhydrous Na2SO4, filtered
evaporated under reduced pressure. The resulting crude
product was purified by silica gel chromatography using
and
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a
mixture of hexane/ethyl acetate (4:1) as eluent to afford an
analytically pure sample of product 3 . All the compounds 3 are
known and were characterized by comparison of their spectral
data with those reported in the literature.12b,20 Characterization
data of representative compounds 3 are given below:
Compound 3a:20a 1H NMR (300 MHz, CDCl3) δ: 7.45-7.40 (m,
2H), 7.38-7.31 (m, 4H), 7.29-7.23 (m, 4H), 3.28-3.22 (m, 2H),
3.00-2.99 (m, 2H) 13C NMR (75.4 MHz, CDCl3), δ: 140.8,
136.7, 129.7, 129.3, 128.9, 128.5, 126.8, 126.3, 36.1,35.6;
HRMS (EI): calcd for C14H14S 214.0816, found 214.0812.
Compound 3i:20a 1H NMR (300 MHz, CDCl3) δ: 7.47-7.23 (m,
7H), 7.16 (m, 2H), 3.29-3.19 (m, 2H), 2.99-2.92 (m, 2H); 13C
NMR (75.4 MHz, CDCl3) δ: 138.6, 136.3, 132.1, 131.4, 130.0,
129.5, 129.0, 128.5, 35.1, 34.8; HRMS (EI): calcd for
C14H13ClS 248.0427, found 248.0430.
Compound 3m:20a 1H NMR (300 MHz, CDCl3) δ: 7.65 (d, J
=7.3 Hz, 2H), 7.55 (d, J = 8.0 Hz, 2H), 7.48 (t, J = 7.3 Hz, 2H),
7.40-7.28 (m, 5H), 7.16 (d, J=7.9 Hz, 2H), 3.26-3.19 (m, 2H ),
3.05-2.95 (m, 2H ), 2.40 (s, 3H ); 13C NMR (75.4 MHz, CDCl3)
δ: 141.3, 139.9, 136.7,135.3,134.5, 133.4, 132.8, 130.7, 130.2,
129.5, 129.1, 127.7, 127.5, 127.3, 36.3, 35.7, 21.6; HRMS (EI):
calcd for C21H20S 304.1286, found 304.1283.
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4
Tetrahedron
Miyazaki, T.; Kasai, S.; Ogiwara, Y.; Sakai, N. Eur. J. Org.
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Highlights
•
•
•
Metal- and oxidant-free one-pot
protocol for the synthesis of thioethers.
Visible light-mediated radical thiol-ene
click (TEC) reactions.
Utilization of benzophenone as a stable
and inexpensive organophotoredox
catalyst.
•
Highly regioselective efficient anti-
Markovnikov hydrothiolation of
unactivated olefins.