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Synthesis of (E)-β-Iodovinyl Sulfones via Photoredox
Catalyzed Difunctionalization of Terminal Alkynes
Sourav Kumar,[a, b] Jaswant Kumar,[a, b] Tahira Naqvi,[c] Shabnam Raheem,[d]
Masood Ahmad Rizvi,*[d] and Bhahwal Ali Shah*[a, b]
A photoredox mediated approach enabling the synthesis of (E)β-iodovinyl sulfones at room temperature using simple alkynes
and thiols as the coupling partners in the presence of TBAI as
iodine source is reported. The direct conversion of thiols into
sulfones following the tandem introduction of C I and C S
bonds is achieved using SeO2 as an oxidizing agent. The
method is viable with a wide range of alkynes and thiols at
room temperature, employs mild conditions, and has good
yields. Besides, we could also use β-iodovinyl sulfones as the
starting material to access different β-functionalized sulfone
derivatives.
Introduction
compounds.[7] Owing to the importance of β-iodovinyl sulfones,
many methods over the years have appeared to generate these
moieties employing a variety of synthetic protocols. Their
synthesis generally involves electrophilic halogenations of
vinylsulfones,[8] Knoevenagel condensation of aromatic aldehydes with sodium sulfinates,[9] halosulfonylation using sulfonyl
halides,[10] and oxidation of the corresponding vinyl sulfides.[11]
Among the various strategies employed, the most logical and
straightforward approach corresponds to sulfonative functionalization of alkynes, the addition of sulfonyl, and other groups
across a triple bond. In one such contribution, Nakamura and
co-workers[12] explored the reactivity of alkynes with sulfonyl
halides in the presence of ligand and metal catalyst leading to
the generation of β-iodovinyl sulfones. In another work for the
synthesis of β-iodovinyl sulfones direct difunctionalization of
alkynes with sulfinic acid in the presence of molecular iodine
was reported.[13] So far, most of the efforts developed introduce
sulfone moiety using various sulfonyl reagents primarily as
precursors. Thus, it becomes even more challenging where we
may wish to install sulfone group without using any sulfonyl
precursor, viz., utilizing thiols directly. To the best of our
knowledge, the only report for the synthesis of β-iodovinyl
sulfones using thiols has been reported via coupling with
alkynes utilizing iodine as halogenating source under reflux
conditions.[14] Intriguingly, we could not come across any report
that enables the synthesis of β-iodovinyl sulfones at room
temperatures. Thus, a mild and efficient approach employing
readily available starting materials for the synthesis of βiodovinyl sulfone is highly desirable.
Thus, in continuation of our interests,[15] herein, we disclose
a first photoredox catalyzed approach for the synthesis of (E)-βiodovinyl sulfones using readily available alkynes and thiols as
coupling partners at room temperature (Scheme 1). The
method showcases the tandem introduction of C I and C S
bond using TBAI as iodine source followed by oxidation of
sulfide to sulfone with selenium dioxide. It would be pertinent
to mention that the simultaneous introduction of a C I and
C S bond is challenging due to the instability of sulfonyl vinyl-
Unarguably, the development of new transformations employing readily available and cheaper starting materials represents
an essential aspect of synthetic organic chemistry. The advent
of photoredox catalysis has further broadened the scope of
more straightforward starting materials by using unique single
electron transfer (SET) routes to design and develop novel
chemical processes.[1] Of particular intrigue is radical-induced
vicinal difunctionalization of alkynes as they provide an
efficient platform for synthesizing substituted alkenes, interacting with both carbon and heteroatom-containing radicals with
good stereoselectivity.[2] Among the various classes of functionalized alkenes, the synthesis of β-halovinyl sulfones has drawn
considerable attention of the chemists as they find pervasive
presence in biologically important molecules and serve as
fundamental building blocks in the construction of an array of
synthetic frameworks.[3] For example, cysteine protease
inhibitor,[4] covalent protease inhibitors,[5] HIV-1 inhibitors,[6]
inhibitors of a transpeptidase, which are required in Staphylococcus aureus for cell wall protein anchoring and virulence,
have all been identified as vinyl sulfone-containing
[a] S. Kumar, J. Kumar, Dr. B. A. Shah
Natural Product & Medicinal Chemistry
Academy of Scientific and Industrial Research (AcSIR),
Ghaziabad-201002 (India)
E-mail: bashah@iiim.res.in
[b] S. Kumar, J. Kumar, Dr. B. A. Shah
CSIR-Indian Institute of Integrative Medicine,
Jammu-180001 (India)
[c] Dr. T. Naqvi
Department of Chemistry
Govt. College for Women,
MA Road, Srinagar, 190001 (India)
[d] S. Raheem, Dr. M. A. Rizvi
Department of Chemistry
University of Kashmir,
Srinagar-190006 (India)
E-mail: masoodku2@gmail.com
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cptc.202200110
ChemPhotoChem 2022, 6, e202200110 (1 of 7)
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Research Articles
doi.org/10.1002/cptc.202200110
Scheme 1. Synthesis of (E)-β-iodovinyl sulfones.
iodide. The central feature of the present method is its
selectivity/general applicability to a wide array of substituted
alkynes and thiols with good yields. Also, β-iodovinyl sulfones
were found amenable to late-stage functionalization reactions
such as copper-catalyzed functionalization of N-methylimidazole, oxidation to α-tosyl phenylacetone,[16] and arylation by Suzuki–Miyaura coupling with phenylboronic acid.[3a]
Results and Discussion
Initially, phenylacetylene 1 was chosen as a model substrate for
the reaction with thiophenol 2 to determine the optimal
reaction conditions. Pleasingly, the reaction on irradiation
under blue LEDs in the presence of TBAI and SeO2 with
Ru(bpy)3Cl2 as photocatalyst using MeCN as solvent led to the
synthesis of desired (E)-β-iodovinyl sulfones 3 in 92 % yield
(Table 1, entry 1). We next examined the effect of various
photocatalysts on the reaction yields; the use of Mes-Acr + ClO4
lowered the reaction yield to 35 % (Table 1, entry 2). A further
drop in reaction yields was observed in the presence of eosin-Y
and Rose Bengal as photocatalyst (Table 1, entries 3–4). Other
oxidants (Table 1, entry 5) examined for the reaction gave
traces of product. The change of iodine source to NIS from TBAI
was not as efficient and afforded only 17 % yields of the desired
product (Table 1, entry 6). Both photocatalyst and light were
found to be critical for our reaction as no product was obtained
in their absence (Table 1, entry 7). A detailed optimization study
ChemPhotoChem 2022, 6, e202200110 (2 of 7)
Table 1. Optimization of Reaction Conditions.[a]
Entry
Deviation from standard conditions
Yield (%)[b]
3
1
2
3
4
5
6
7
none
Mes-Acr + ClO4 as photocatalyst
eosin-Y as photocatalyst
rose Bengal as photocatalyst
H2O2/TBHP instead of SeO2
NIS instead of TBAI
no photocatalyst or light
92
35
22
19
n.d.
17
n.d.
[a] Reaction conditions: phenylacetylene (1 mmol), thiophenol (1 mmol),
photocatalyst (1 mol%), selenium dioxide (1 mmol), TBAI (1 mmol),
irradiation under air, 3 h, rt, 25 °C. [b] Isolated yield.
describing the effect of different parameters, including the
substrate equivalents, solvents, time, and temperature, on
reaction yields has been shown in the supporting information
(Table S2, supporting information).
With optimized conditions in hand, we initially explored the
scope of the sulfonylation process with different alkynes
(Scheme 2). As expected, a range of alkyl-substituted phenylacetylenes (3-methyl, 4-methyl, 4-ethyl, 4-n-propyl, 4-tert-butyl,
and 2,4,5-trimethyl) participated in the reaction with thiophenol under optimized conditions to give corresponding βiodovinyl sulfones 4–9 in 81–88 % yields. Also, a variety of halo© 2022 Wiley-VCH GmbH
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Scheme 2. Substrate scope for the synthesis of β-iodovinyl sulfone.
substituted alkynes ranging from 2-fluoro, 3-fluoro, 4-fluoro, 2chloro, 3-chloro, 4-chloro, 4-bromo, 3,5-difluoro phenylacetylene furnished related products 10–17 in 73–85 % yields. The
substrates bearing electron-withdrawing and electron-donating
groups on benzene rings such as (2-trifluoromethyl, 4ChemPhotoChem 2022, 6, e202200110 (3 of 7)
trifluoromethyl, 3-methoxy, 4-methoxy, and 4-phenoxy) phenylacetylenes were found to be compatible with our reaction to
afford products 18–22 in good yields. The scope of the reaction
was also extended to 2-ethynyl-6-methoxynaphthalene and 9ethynylphenanthrene to generate the corresponding product
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23–24 in 85–81 % yields, respectively. Gratifyingly, aliphatic
alkynes such as cyclopropyl acetylene and 1-ethynyl-1-cyclohexene were also found to be feasible to participate in the
reaction to give products 25 and 26 in comparatively lower
yields.In addition, the heterocyclic 2-ethynylthiophene was also
varied to generate corresponding product 27 in excellent yield.
Next, the feasibility of the protocol was examined with a
series of thiophenols to broaden the substrate scope. The
reaction of phenylacetylene proceeded smoothly with alkylsubstituted (2-methyl, 3-methyl, 4-methyl, 2-ethyl, 4-tert-butyl
2,4-dimethyl, 2,5-dimethyl, 3,5-dimethyl) thiols to furnish the
corresponding products 28–35 in excellent yields. The 4methoxythiophenol was also found to give the product 36 in
80 % yields. The halo substituted (4-fluoro, 4-chloro, 2-bromo,
3-bromo, 4-bromo, and 2,6-dichloro) thiophenols produced βiodovinylsulfones 37–42 in up to 82 % yields. The thiophenol
bearing electron-withdrawing 4-trifluoromethyl group was also
compatible with this reaction to give product 43 in 60 % yields.
Based on single electron transfer (SET) reactivity patterns
observed by our group[15b,d] and literature precedence,[17]
control experiments to support the proposed reaction mechanism were carried out. The relative propensities of reaction
ingredients towards SET reaction with [Ru(bpy)3]Cl2 photocatalyst were explored through fluorescence quenching and
absorption kinetics experiments (Scheme 3A). The corresponding Stern Volmer plots depict reactant quenching in the order
thiophenol PSH > TBAI > PA. The highest observed quenching
rate in the case of thiophenol corroborates with the first step of
the SET reaction involving reductive quenching of blue LED
excited photocatalyst by thiophenol. To support the second
step of proposed SET based reaction mechanism involving
reaction of thiyl radical and phenylacetylene, inferences from
the experiment involving quenching studies of the ternary
system (photocatalyst, PSH, and PA) were made. The quenching
of photocatalyst by PSH had a significant synergistic effect
upon PA addition which can be explained by its effect of
Scheme 3. Photoredox studies.
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Scheme 4. Plausible mechanism.
removing thiyl radical and thereby generating positive feedback on PSH quenching of photocatalyst.
The radical nature of the reaction was evidenced by the
complete inhibition of the reaction in the presence of TEMPO
as a radical scavenger (see supporting information). The
absorption kinetic studies of reactants with [Ru(bpy)3]Cl2
catalyst indicated pseudo first-order kinetics for its absorptive
degradation under rate constants in the order PSH > TBAI > PA
(Scheme 3B). Thus, from the control experiments data, it can be
predicted that the blue LED excited [Ru(bpy)3Cl2]* undergoes
reductive quenching with thiophenol (PC1) to give thiyl radical
cation I that deprotonates in presence of a peroxy radical anion
to give thiyl radical II (Scheme 4).[18] The catalytic loop gets
completed by the regeneration of ground state photocatalyst
with molecular oxygen.[19] This SET reaction continues with the
addition of thiyl radical to the phenylacetylene to generate
vinyl sulfide radical adduct III. In another parallel catalytic cycle
(PC2), iodide anion sourced from TBAI reductively quenches the
photo excited [Ru(bpy)3Cl2]* photocatalyst to ground state Ru +,
thereby generating iodine radical.[20] The iodine radical so
formed undergoes radical-radical coupling reaction with vinyl
sulfide radical to produce β-iodovinyl sulfide intermediate IV.
The β-iodovinyl sulfide IV on oxidation in the presence of
selenium dioxide[22] affords (E)-β-iodovinyl sulfones. The selective formation of E-isomer can be attributed to the equal
abundance of the vinyl radical E-form III and Z-form IIIa.
However, to avoid steric repulsion from the sulfenyl group, the
iodine radical preferably reacts with vinyl radical IIIa, leading to
the exclusive formation of E-isomer.[11c,21]
The relative efficacy of studied photocatalyst for this
synthetic methodology (Table 1, entries 2–4) under optimized
reaction conditions can be attributed to their differing
thermodynamic feasibilities (suitability of their redox potentials
with the reaction ingredients), corroboration of reaction and
excited-state lifetime and absorption wavelengths.[23] To further
demonstrate the synthetic utility of the protocol, we undertook
a series of reactions using 3 as the starting material for further
functionalizations (Scheme 5). The copper-catalyzed substitution of iodo-group in 3 by N-methyl imidazole gave product 44
in 84 % yields. In another copper-catalyzed reaction of 3, αtosyl phenylacetone 45 was obtained in 78 % yields. Furthermore, arylation of 3 by Suzuki–Miyaura coupling with phenylboronic acid produced 46 in 80 % yields.
Scheme 5. Synthetic transformations of (E)-β-iodovinyl sulfone (3).
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Conclusion
Conflict of Interest
In conclusion, a first photoredox catalyzed approach employing
readily available alkynes and thiols as starting materials for the
synthesis of (E)-β-iodovinyl sulfones at room temperature has
been developed. The method presents a tandem introduction
of C S and C I bonds to the triple bond of alkyne followed by
the oxidation of sulfide to sulfone in the presence of selenium
dioxide. The present methodology is rather attractive due to
the mild reaction conditions, easy setup, readily available
starting materials, and broad substrate scope with good
functional group tolerance. Furthermore, the β-iodovinyl
sulfones served as good substrates for late-stage modifications
to give different 1,2-difunctionalized sulfonylated alkenes. The
further applicability and reactivity patterns of the protocol are
currently under investigation in our laboratory.
The authors declare no conflict of interest.
Experimental Section
General Information
All reactions were carried out in oven-dried glassware. The solvents
used were purified by distillation. All reactions were irradiated
under blue LEDs. 1H and 13C NMR spectra were recorded on FTNMR 500 and 400 MHz instruments. Chemical data for protons are
reported in parts per million (ppm) downfield from tetramethylsilane and are referenced to the residual proton in the NMR
solvent (CDCl3, 7.26 ppm). Carbon nuclear magnetic resonance
spectra (13C NMR) were recorded at 125 MHz or 100 MHz. Chemical
data for carbons are reported in parts per million (ppm, δ scale)
downfield from tetramethylsilane and are referenced to the carbon
resonance of the solvent. Coupling constants (J) are quoted in Hz.
Mass spectra were obtained by using Q-TOF-LC/MS spectrometer
using electron spray ionization.
General Synthetic Procedures: Synthesis of
β-iodovinylsulfones
To the oven-dried 5 mL glass vial was added phenylacetylene
(100 mg, 1 mmol) in MeCN followed by addition of Ru(bpy)3Cl2
(7.33 mg, 1 mol %), thiophenol (108 mg, 1 mmol), TBAI (362 mg,
1 mmol) and SeO2 (109 mg, 1 mmol) with continuous stirring. The
reaction mixture was then irradiated under blue LEDs. After the
completion of the reaction, as monitored by TLC, the reaction
mixture was extracted with ethyl acetate and water. The aqueous
layers were then again extracted with ethyl acetate. The combined
organic layers were dried over Na2SO4 and concentrated under
vacuum. The crude mixture was purified by silica gel column
chromatography using (hexane/ethyl acetate = 98 : 2) as a solvent
system to obtain pure product 3 as white solid (340 mg, 92 %).
Acknowledgements
BAS thank DST-SERB for Financial Assistance (000850/2021). S.K
and J.K thank CSIR for fellowship.
ChemPhotoChem 2022, 6, e202200110 (6 of 7)
Data Availability Statement
The data that support the findings of this study are available in
the supplementary material of this article.
Keywords: alkyne · thiophenol · β-iodovinyl sulfone · radical
reactions · photoredox catalysis
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Manuscript received: April 14, 2022
Revised manuscript received: June 22, 2022
Accepted manuscript online: July 8, 2022
Version of record online: August 3, 2022
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