Facile Thiol–Ene Click Protocol Using Benzil as Sensitizer and White LED as Light Source

Article abstract of DOI:10.1002/ejoc.202001275

The thiol–ene reaction leading to a series of thioether derivatives by simple metal and oxidant free visible light promoted photosensitized protocol, following anti-Markonikov hydrothiolation of unactivated aryl and alkyl olefins at room temperature is demonstrated. Benzil served as a green photosensitizer in this reaction and white LED lights as a light source. This radical based thiol–ene reaction is operationally simple and tolerates a wide variety of functional groups present in olefins.

Full text of DOI:10.1002/ejoc.202001275

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Facile Thiol-ene Click Protocol Using Benzil as Sensitizer and
White LED as Light Source
Authors: Anupam Das and K R Justin Thomas
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202001275
Link to VoR: https://doi.org/10.1002/ejoc.202001275
01/2020

10.1002/ejoc.202001275
European Journal of Organic Chemistry
FULL PAPER
Facile Thiol-ene Click Protocol Using Benzil as Sensitizer and
White LED as Light Source
Anupam Das,[a] and K. R. Justin Thomas* [a]
Dedication ((optional))
[a]
A. Das, Prof. Dr. K. R. J. Thomas
Organic Materials Laboratory, Department of Chemistry
Indian Institute of Technology Roorkee
Roorkee – 247667, India
E-mail: krjt@cy.iitr.ac.in
Homepage: http://faculty.iitr.ac.in/~krjt8fcy/
Supporting information for this article is given via a link at the end of the document.
Abstract: The thiol-ene reaction leading to a series of thioether
derivatives by simple metal and oxidant free visible light promoted
photosensitized protocol following anti-Markonikov hydrothiolation of
unactivated aryl and alkyl olefins at room temperature is
demonstrated. Benzil served as a green photosensitizer in this
reaction and white LED lights as a light source. This radical based
thiol-ene reaction is operationally simple and tolerates a wide variety
of functional groups present in olefins.
natural
products,[14]
drug
molecules,[14]
polymers,[15]
pharmaceutical materials,[16] and peptide chemistry.[17]
Introduction
Apart from the conventional route of organic transformations, in
the early 1900s, Alexander Eibnern[1-2] followed by V.N.
Filimonov[3] came with a new concept of using photons in
organic transformations. Because of fewer expenses with an
eco-sustainable photon-induced organic transformation, it
became popular among scientists.[4] Along with the development
of photo-redox catalysts, photosensitizers are becoming a
popular choice for organic reactions.[5] And recently, because of
advantages like less toxicity, recyclability and cheapness,
photosensitizers became popular among the scientific
community interested in environmentally benign organic
transformations.[5]
The reaction between thiol and alkene to generate
thioethers was first demonstrated by Posner in 1905.[6] The
1990s and the early 2000s were the peak time for this type of
reaction because of its feasibility and wide range of
applications[7] like benchmark polymer materials synthesis,
cross-linked polymers formation, simple material applications
such as protective coatings on films, medicinal chemistry,
molecular biology, and biotechnology.[8] Apart from conventional
methods, recently several photocatalytic thiol-ene click reaction
protocols were reported ever since Yoon and his co-workers first
successfully demonstrated visible-light-induced thiol-ene click
reaction by ruthenium-based photocatalyst in 2013.[9] The
reasons behind this interest were the applicability of
photocatalytic thio-ene click reaction as a tool for a variety of
platinum drug delivery carriers,[10] syntheses of biodegradable
antibacterial hydrogels,[11] flexible hydrophilic cellulose sponge
preparation for oil-water separation,[12] and for biosensing.[13]
This chemistry also bears a huge potential in the construction of
Scheme 1. Comparison of Thiol-Ene Click Protocols.
Visible light-induced photocatalytic thiol-ene/yne click
reactions were reported in the past years (Scheme 1) by using
ruthenium[9,18] as well as iridium[19] complexes, TiO2
[20]
and
Bi2O3.[21] Metal-based photocatalyst bears a lot of disadvantages
such as high material cost, toxicity, environmental pollution,
stability issue, etc. Not only metal-based photocatalysts but also
many organic dyes like NPPOC-hexylamine,[22] eosin-y,[23] rose
bengal,[24] acridinium salt,[25] and methylene blue[26] were also
introduced as photocatalyst in recent years but most of them are
highly environmentally toxic and expensive. In 2017, Kokotos. et.
al. as well as Singh et. al. introduced phenylglyoxylic acid[27] and
benzophenone[28] respectively as photosensitizer for thiol-ene
click reaction. Some nanostructured photocatalysts such as
[29]
Bi2O3
and graphitic carbon nitride[30] were also shown to
effectively catalyst thiol-ene click reaction. Few reports on the
UV-light-induced photoinitiator assisted thio-ene click reactions
were also reported earlier. In 2011, Garrell et. al. reported a UV-
light induced 2,2-dimethoxy-2-phenylacetophenone sensitized
hydrothiolation.[31] Later in 2017, Wiesbrock et. al. demonstrated
simple UV-light assisted hydrothiolation[32] of alkenes.
Though the photochemistry of carbonyl compounds are
older by few decades, but effective application of carbonyl
group-containing photocatalysts and sensitizers have come into
1
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10.1002/ejoc.202001275
European Journal of Organic Chemistry
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light very recently. By direct photosensitization or photoinitiation
they can involve in very important organic transformations like
polymerization, cis-trans isomerization, pinacol reaction,
intramolecular atom transfer radical addition, photoredox-
coupling and many more.[33] By inspiring on the development of
carbonyl based photosensitizers we decided to implement the
concept for thiol-ene click reactions. Benzil has lower excited
state energy (Et= 53.7 kcal mol‒1) with little high triplet state
lifetime. It's photosensitization ability is almost similar or even
better to some of the existing popular ruthenium and iridium
metal-based photosensitizers (Et= 49-63 kcal mol‒1), organic
dyes like riboflavin (Et= 50 kcal mol‒1) and even carbonyl
derivatives such as Michler’s ketone (Et= 61 kcal mol‒1),
benzophenone (Et= 69.1 kcal mol‒1), acetophenone (Et= 70 kcal
mol‒1), thioxanthons (Et= 63 kcal mol‒1) etc. Even triplet state
lifetime, as well as photosensitization ability of benzil, is better in
most of the cases with advantages like less toxicity,
Our exploration started with the hydrothiolation reaction of
styrene (1a) by thiophenol (2a) in the presence of benzil as a
photosensitizer. In this control reaction 1.0 equiv. of styrene (1a)
was irradiated with 5 W white LED light in the presence of an
equivalent amount of thiophenol (2a) and 5 mol% of benzil to
afford 42% of 3a within 6 h. (Table 1, entry 1). On increasing the
catalyst loading up to 10 mol% improved the yield to 60% (Table
1, entry 2). Further, increment in the amount of benzil up to 20
mol% did not help to increase the yield beyond 60% (Table 1,
entry 3). Similarly increasing the amount of styrene (1a) was
also detrimental to the yield (Table1, entry 4 & 5). However
changing the solvent from CH3CN to THF slightly improved the
yield (Table 1, entry 6) thought not satisfactory. On the other
hand, increasing the amount of thiophenol (2a) from 1 to 4
equivalents substantially increased the yield and reached 91%
with 4 equivalents of thiophenol (Table 1, entries 6, 7 & 8). A
higher amount of 2a did not improve the yield further (Table 1,
entry 9). The use of other solvents such as DCM, 1,4-dioxane,
toluene, and CH3CN only gave a lesser yield of 3a (Table 1,
entries 10, 11, 12 & 13). Under these conditions, the yield of 3a
remains unaltered (Table 1, entry 8 & 15) on increasing the light
irradiation time from 6 h to 8 h. Any decrement of light irradiation
time from 6 h affected the yield (Table 1, entry 14). The reaction
under dark did not lead to any product formation (Table 1, entry
16). Whereas the yield of 3d decreased to 16% when benzil was
not used (Table 1, entry 17).
environment-friendliness,
low-cost
and
commercial
availability.[34] On the other hand, metal-based and organic dye-
based complex molecules always have the demerit of
photoredox capability along with desired photosensitization. So,
there always remains a possibility of side reactions. Therefore,
we set our goal to exploit benzil as a photosensitizer for
selective thio-ene click reaction under white LED light irradiation.
Apart from available photocatalytic methods, here initiation of
the reaction happened by proton abstraction from thiol via triplet
excited state of benzil under visible light irradiation. The reaction
occurred smoothly and gave an excellent yield of the desired
thioether product. A comparative study of previously available
photocatalytic thiol-ene click reaction with our developed
protocol is listed in Table S1, ESI.
Table 2. Screening of Different Photocatalysts Under Best Conditions.
Entry
1
PS
Isolated Yield (%)[a]
91
Results and Discussion
Table 1. Optimization of Reaction Conditions.[a]
2
3
84
82
Entry
1a
(Equiv.)
2a
(Equiv.)
Catalyst
loading
Solvent
Time
Isolated
Yield
(%)[b]
42
1
2
1
1
1
1
5 mol%
10 mol%
CH3CN
CH3CN
6 h.
6 h.
60
4
5
73
3
4
5
6
7
8
9
10
11
1
1.5
3
1
1
1
1
1
1
1
1
1
1
2
4
6
4
4
20 mol%
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
CH3CN
CH3CN
CH3CN
THF
THF
THF
THF
DCM
1,4-
Dioxane
Toluene
CH3CN
THF
THF
THF
6 h.
6 h.
6 h.
6 h.
6 h.
6 h.
6 h.
6 h.
6 h.
60
60
43
48
70
91
79
88
83
59
51
6
12
13
14
15
16
1
1
1
1
1
4
4
4
4
4
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
6 h.
6 h.
4 h.
8 h.
6 h.
81
78
83
[a] Isolated yield.
91
Trace[c]
On comparison to other similar keto derivatives (Table 2),
because of the lowest first triplet excited state energy (Table S2,
ESI; ET= 53.7 kcal mol‒1) with longer lifetime (36 μSec) and
pretty high phosphorescence quantum yield (2.59-3.05), benzil
(PS1) proved to be the most suitable photosensitizer for
hydrothiolation of styrene (Table 2, entry 1). PS2 and PS3 gave
17
18
1
1
4
4
-
THF
THF
6 h.
6 h.
16[d]
79
5 mol%
[a] Reaction condition: 1a (0.96 mmol), 2a (3.84 mmol), benzil (0.096 mmol) in
3 mL solvent irradiated with 5 W White LED light strip at room temperature
and N2 atmosphere. [b] Isolated yield. [c] Reaction was performed in the dark. [d]
Reaction was carried out without photosensitizer
2
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10.1002/ejoc.202001275
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moderate yields (Table 2, entry 2 & 3) for the reaction because
of the slightly high energy of the first triplet excited state (ET=
69.07 kcal mol‒1 and 62.85 kcal mol‒1) whereas PS4, PS5, and
PS6 gave a quite low yield in thiol-ene click reaction (Table 2,
entry 4, 5 & 6) because of very short triplet excited state
lifetimes (3 and 0.34 μSec). Although PS4, PS5, and PS6
possess similar triplet excited state energies comparable to
benzil (PS1) failed to serve as triplet photosensitizer for this
reaction.
thioethers (3s-3v) in reaction with thiophenol (2a). When 2-
mercaptobenzothiazole was used then it gave a trace amount of
thioether 3w on reaction with styrene. The reaction of thiophenol with
4,7-dimethylchroman-2-one also gave only a trace amount of 3x.
Finally, one reaction of thiophenol was also conducted with
phenylacetylene which leads to 85% (E:Z = 7.3:1) thiol-yne
product 3y. But when 3y further treated with excess thiophenol
no further hydrothiolation happened indicative of high selectivity
of the protocol.
Scheme 3. Gram Scale Thiol-Ene Click Reaction.
To demonstrate the potential of our protocol, a gram scale
experiment was conducted with styrene (1a, 1.00 g), 4.0 equiv.
of thiophenol (2a, 4.23 g) and 10 mol%, benzil (0.141 g) under 5
W white LED light irradiation for 6 h (Scheme 3). The desired
thioether 3a was formed in 89% of yield. Thus, the gram scale
reactions gave satisfactory results without much depression in
yield.
Scheme 2. Substrate Scope. Reaction condition: 1a (0.96 mmol), 2a (3.84
mmol), Benzil (0.096 mmol) in 3 mL THF, irradiated with 5 W White LED light
strip at room temperature and N2 atmosphere.
After achieving the optimized conditions for hydrothiolation
of styrene by thiophenol, we explored the substrate scope for a
variety of alkenes, alkyne, and thiols under optimized conditions
(Scheme 2). In the reaction between thiophenol and aromatic
alkenes, a lesser yield (75%) was obtained for the alkene with
an electron-withdrawing group (-3-NO2) whereas neutral and
electron-donating group bearing alkenes (-H, -OMe, -NMe2)
gave comparatively high yield above 90%. For unsaturated
aliphatic alkenes like methacrylic acid and its ester, the reaction
with thiophenol gave 73-76% of yield. Divinylbenzenes also
reacted readily with thiophenol to give dithiolation products in
Scheme 4. Control Experiments.
To understand the mechanistic path, we performed several
control experiments (Scheme 4). Firstly the reaction under dark
failed to produce the desired thioether (3a) which indicated the
importance of light for this reaction (Scheme 4a). After that in a
reaction of 1a and 2a in presence of light but without the
photosensitizer resulted only 16% yield of 3a which indicated the
necessity of the photosensitizer for the reaction (Scheme 4b).
An experiment was also conducted in the presence of TEMPO,
excellent
yield.
Phenylmethanethiol
and
thiophen-2-
ylmethanethiol were also found to work under the conditions with
styrene, 4-vinyl pyridine, etc. to give a decent yield of the
desired products. Ethyl ester of mercaptosuccinic acid gave 82-
83% of yield on reaction with styrene and 4-vinyl pyridine
respectively. Aliphatic alkenes like cyclohexene, 1-octene, 1-
decene, and 2-ethyl-1-hexene also gave 65-70% yield of
3
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10.1002/ejoc.202001275
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which leads to a trace amount of 3a formation, suggesting that
the presence of a radical pathway (Scheme 4c). Decreased
reaction yield from 91 to 74% under the oxygen atmosphere
points to the formation of the triplet excited state of the
photosensitizer under visible light irradiation (Scheme 4d) in the
reaction conditions. When the reaction was carried out by using
thiophenol and 10 mol% of benzil then a 50% yield of B was
observed (Scheme 4e). Instead of thiophenol when its sodium
salt was used for the reaction with styrene in the presence of
benzil no product formation was witnessed (Scheme 4f).
In the Stern-Volmer quenching study (Figure 1), a steep
and linear quenching of fluorescent intensity was observed when
thiophenol was added into a solution of benzil with quenching
constant (Kq) 3.8 x 104 Lmol-1s-1. On the other hand, a gradual
increase of fluorescent intensity was detected when styrene was
added into a solution of benzil. This indicates the reactivity of
thiophenol in presence of benzil and no reaction between
styrene and benzil under photoirradiation.
that under photoexcitation carbonyl compounds can undergo
ISC to convert singlet PSs* to triplet PSt* species. PSt* can
capture a proton from thiophenol 2a to give thiol radical
intermediate A. In the next step, A can be scavenged by styrene
1a to generate thioether radical intermediate C. There remains a
possibility of self couping of thiol radicals to generate B but it
can revert back to A again during the reaction. Intermediate C
after pickup proton from PCt*-H will give the desired product 3a.
There also remains an alternative possibility for thioether radical
intermediate C getting converted to the desired product 3a by
abstracting proton from thiophenol (2a) directly through the
radical chain propagation pathway.[35]
Conclusion
In summary, we have developed a cheap and efficient protocol
for thiol-ene/yne click reaction using benzil as a photosensitizer.
A wide variety of olefins and thiols are used to show the
applicability of this protocol in TEC reaction. Instead of the
traditional fluorescent light source mostly used in photocatalytic
thiol-ene coupling reactions, herein we demonstrated a less
energy-consuming LED light source. This method may find
application in alkene and alkyne protection, peptide, and
glycoside modification bypassing the need for transition metal
complexes as well as thermal conditions and stoichiometric
additives. Further applications of this protocol are currently
under investigation.
Figure 1. Stern-Volmer plots for Quenching of Benzil fluorescence emission in
Experimental Section
the Presence of [A] Thiophenol and [B] Styrene.
General Procedure
In a typical reaction, an oven-dried glass vial was charged with alkene 1
(0.96 mmol), thiol 2 (3.84 mmol) and benzil (20 mg, 0.096 mmol). Then 3
mL freshly distilled THF was added to the reaction mixture and purged
with N2 gas for 20 minutes. After purging, the glass vial was closed with a
screw cap and irradiated with 12 V white LED for 6 h. The progress of the
reaction was monitored by TLC. After completion of the reaction, 6 mL
water was added to the reaction mixture and extracted three times with
ethyl acetate (5 mL). The combined organic layer was washed with brine
(10 mL), dried over anhydrous sodium sulfate, and the solvent was
evaporated under reduced pressure. The product was purified by column
chromatography on silica gel.
Acknowledgements
We gratefully acknowledge the financial support from Science
and Engineering Research Board (SERB), New Delhi
(CRG/2018/003729) and Council of Scientific and Industrial
Research (CSIR), New Delhi (02(0371)/19/EMR-II).
Keywords: alkene • alkyne • thiol • photosensitizer • white LED •
eco-friendly • green chemistry
Scheme 5. Proposed Mechanism of Hydrothiolation of Alkene Using Benzil as
Photosensitizer.
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10.1002/ejoc.202001275
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Entry for the Table of Contents
Hydrothiolation
Photo-Sensitized Eco-Friendly: Light-induced photo-sensitized reactions of alkenes and alkynes with thiols to form thioethers are
described. High selectivity with large functional group compatibility for aromatic as well as aliphatic alkenes is also demonstrated.
Institute and/or researcher Twitter usernames: iitroorkee, KRJustinThomas1
6
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