Markovnikov-Selective Radical Addition of S-Nucleophiles to Terminal Alkynes through a Photoredox Process

Article abstract of DOI:10.1002/anie.201610000

Direct radical additions to terminal alkynes have been widely employed in organic synthesis, providing credible access to the anti-Markovnikov products. Because of the Kharasch effect, regioselective control for the formation of Markovnikov products still remains a great challenge. Herein, we develop a transition-metal-free, visible light-mediated radical addition of S-nucleophiles to terminal alkynes, furnishing a wide array of α-substituted vinyl sulfones with exclusive Markovnikov regioselectivity. Mechanistic investigations demonstrated that radical/radical cross-coupling might be the key step in this transformation. This radical Markovnikov addition protocol also provides an opportunity to facilitate the synthesis of other valuable α-substituted vinyl compounds.

Full text of DOI:10.1002/anie.201610000

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201610000
German Edition: DOI: 10.1002/ange.201610000
Photoredox Catalysis
Markovnikov-Selective Radical Addition of S-Nucleophiles to
Terminal Alkynes through a Photoredox Process
Huamin Wang, Qingquan Lu, Chien-Wei Chiang, Yi Luo, Jiufu Zhou, Guangyu Wang, and
Aiwen Lei*
Abstract: Direct radical additions to terminal alkynes have
been widely employed in organic synthesis, providing credible
access to the anti-Markovnikov products. Because of the
Kharasch effect, regioselective control for the formation of
Markovnikov products still remains a great challenge. Herein,
we develop a transition-metal-free, visible light-mediated
radical addition of S-nucleophiles to terminal alkynes, furnish-
ing a wide array of a-substituted vinyl sulfones with exclusive
Markovnikov regioselectivity. Mechanistic investigations dem-
onstrated that radical/radical cross-coupling might be the key
step in this transformation. This radical Markovnikov addition
Scheme 1. Radical addition of H-heteroatom compounds to terminal
alkynes. SET=single-electron transfer, PT=proton transfer, [pc]=pho-
toredox catalyst.
protocol also provides an opportunity to facilitate the synthesis
of other valuable a-substituted vinyl compounds.
R
adical additions to terminal alkynes have long served as
a powerful strategy to access complex molecules.[1] In this
context, anti-Markovnikov products were obtained exclu-
sively due to the Kharasch effect.[1,2] Consequently, strategies
allowing for direct efficient synthesis of a-substituted vinyl
compounds from intermolecular radical addition to terminal
alkynes have rarely been reported.[2g] In principle, the stability
of a radical intermediate generated from the radical addition
step is the key for the final selectivity control, and anti-
Markovnikov addition often gives the more stable radical
intermediate (Scheme 1a). Therefore, seeking a new channel
to synthesize valuable a-substituted vinyl compounds through
radical Markovnikov addition procedure is appealing and
extremely challenging. Inspired by the state of the art of
radical/radical cross coupling,[3] we wondered whether the
Markovnikov products, a-substituted vinyl compounds, can
be obtained through the general and useful cross-coupling of
a-vinyl carbon radical and other organic radicals (Sche-
me 1b).
cules. The development of synthetic approaches to vinyl
sulfones has received substantial attention in recent years.[5]
Up to now, the majority of methods for the synthesis of vinyl
sulfones lead to trans-configured b-substituted products.[5,6]
Nevertheless, the synthesis of a-substituted vinyl sulfones,
which have great potential application in biological sciences,[7]
still remains an outstanding challenge. Established
approaches to the synthesis of a-substituted vinyl sulfones
usually contain several steps or rely on noble transition metal
catalysts, suffering from poor regioselectivity and difficult
purification.[7,8] An alternative strategy with more simple and
sustainable perspective is therefore highly appealing.
Recently, visible light-mediated photoredox catalysis has
emerged as a powerful tool in radical reactions.[3h–j,9] Thus, we
envisioned that visible light-mediated photoredox catalysis
might enable access a-substituted vinyl sulfones through
radical processes. Herein, we describe a transition-metal-free,
visible light-mediated Markovnikov addition of sulfinic acids
to terminal alkynes via a-vinyl radical/sulfonyl radical cross-
coupling. A wide array of a-substituted vinyl sulfones can be
obtained with exclusive Markovnikov regioselectivity, some
of which are difficult to generate by existing approaches.
To test the above hypothesis, phenylacetylene (1a) and p-
toluenesulfinic acid (2a) were chosen as substrates (Support-
ing Information, Table S1). Firstly, eosin Y, widely used as
a photoredox catalyst due to its low cost and simple
purification,[10] was tested. To our delight, when the reaction
of 1a and 2a was performed, the desired product 3a was
obtained in 19% yield (Table S1, entry 1). Encouraged by the
result, various bases, reacting with sulfinic acids to form
sulfinate salts, were next investigated. Intriguingly, when the
reaction was performed with K2CO3 as base, the Markovni-
kov product was exclusively obtained in 86% yield (Table S1,
entry 5). Further experiments have shown that the solvent has
According to the aforementioned concept, a-substituted
vinyl sulfones, which not only serve as useful synthetic
intermediates but also are widely featured in biologically
active compounds,[4] were chosen as synthetic target mole-
[*] H. Wang, Dr. Q. Lu, C. Chiang, Y. Luo, J. Zhou, G. Wang, Prof. A. Lei
The Institute for Advanced Studies (IAS), College of Chemistry and
Molecular Sciences, Wuhan University
Wuhan 430072, Hubei (P.R. China)
E-mail: aiwenlei@whu.edu.cn
Prof. A. Lei
Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, Lanzhou 730000 (P.R. China)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201610000.
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a considerable influence on the efficiency of this transforma-
tion (Table S1, entries 5–10). Notably, no desired product was
obtained in the absence of photocatalyst or visible light-
irradiation (Table S1, entries 13 and 14). After extensive
screening of reaction parameters, we found that treatment of
1a with 2a in the presence of eosin Y as photoredox catalyst
and K2CO3 as the base in DMF at room temperature afforded
3a in 86% yield with high a-regioselectivity.
With the optimal reaction conditions in hand, we next
examined the radical Markovnikov addition protocol for the
synthesis of a-substituted vinyl sulfones. As shown in Table 1,
the transformation was applicable to various terminal
alkynes, giving the Markovnikov products in moderate to
good yields. Aromatic alkynes, bearing various functional
groups at the para position, reacted smoothly with p-
toluenesulfinic acid to afford the desired products (3a–3h).
Interestingly, aromatic alkynes with substituents at the ortho
position also proceeded smoothly to give the a-vinyl sulfones
in 81% yield (3i). Moreover, a range of functional groups,
including halides (F, Cl, Br), carbonyl, and even sulfonamide,
were available for this transformation. More importantly, the
reaction is compatible with 3-aminophenylacetylene to afford
the corresponding product 3k, which is difficult to synthesize
by transition metals catalysis owing to the coordination of
amino group to metal. This feature can be introduced to the
synthesis of useful compounds bearing a coordinating group.
Remarkably, fluorine and dibenzothiophene, being widely
used in organic optoelectronic materials,[11] were also efficient
coupling partners (3p and 3q). Estrone derivative 3t was also
successfully generated. It is noteworthy that heteroaromatic
alkynes were found to be suitable for the transformation,
delivering 3u and 3v in 66% and 47%, respectively. Finally,
aliphatic alkynes, such as phenylbutyne, 4-pentyn-1-ol, 1-
heptyne and so on, provided moderate yields (3w–3y).
Among them, 3x could be synthesized in 61% yield without
the use of protecting groups.
Table 1: Substrates scope for synthesis of a-vinyl sulfones.[a]
Next, the substrates scope with regard to sulfinic acids was
evaluated. As shown in Table 1, the reaction is compatible
with a series of sulfinic acids, generating Markovnikov
products with good efficiency. Notably, Br and Cl substituents
are well tolerated, thus enabling further functionalization (4d
and 4e). A good yield was also obtained with the use of
naphthalene sulfinic acid as coupling partner (4h). Intrigu-
ingly, benzenesulfinic acid bearing an acetyl amino group in
para position was easily transformed (4i); the product is
difficult to synthesize by other methods. Most importantly,
a heterocyclic substrate was a valid coupling partner, afford-
ing the desired product (4j) in 43% yield.
To elucidate our hypothesis for this visible light-mediated
Markovnikov addition of proceeding through a radical cross-
coupling process, a series of mechanistic studies were
conducted by electron paramagnetic resonance (EPR) spec-
troscopy, as shown in Scheme 2. Notably, when the reaction
mixture of 2a was irradiated with visible light, a distinct signal
of a trapped radical was observed, suggesting the involvement
of a sulfonyl radical in the transformation. These results
indicated that photocatalysis plays an essential role in the
formation of the sulfonyl radical. In further EPR experiments,
distinct mixed signals of the trapped radical were detected in
the reaction mixtures of 1a and 2a under standard conditions,
demonstrating that the transformation not only involves
[a] Isolated yields are shown. Unless otherwise noted, the standard
conditions were as follows: 1 (0.3 mmol), 2 (0.6 mmol), eosin Y
(1 mol%), K2CO3 (1.0 equiv), DMF (4.0 mL), N2, 3W green LEDs, 4 h.
[b] 6 h. [c] 4.5 h. [d] Eosin Y (2 mol%). [e] Eosin Y (2 mol%), Cs2CO3
(1.0 equiv). [f] TFA (0.6 mmol), K2CO3 (1.0 equiv), 2,5-dichlorothio-
phene-3-sulfinic acid sodium salt (0.6 mmol).
Scheme 2. EPR experiments.
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a radical pathway, but also might contain mixed radical
to the reaction mixture after 1 h in the absence of visible light
species. In addition, radical trapping experiments were also
performed (Scheme 3). The reactions were found to be
strongly inhibited by radical scavengers, 2,2,6,6-tetramethyl-
1-piperidinyloxy (TEMPO) and 2,4-di-tert-butyl-4-methyl-
phenol (BHT), supporting the proposed radical pathway of
this reaction. More importantly, a-vinyl carbon radical was
trapped by P(OEt)3 in the transformation.[12] Combining the
above information, we hypothesized that a-vinyl carbon
radical/sulfonyl radical cross-coupling might be the key step
in the transformation.
irradiation (Scheme 4c). In contrast, in the presence of visible
light rapid consumption of both 1a and “component” was
detected along with the formation of desired product 3a
(Scheme 4d). These results indicated that visible light is
indispensable for the formation of 3a (see the Supporting
Information for details).
Additionally, intermediate experiments were carried out
(see SI for further details). First, the reaction of b-substituted
vinyl sulfone 4 under standard conditions was investigated.
However, no further conversion from 4 to the desired product
3a was detected. Furthermore, 4 and 2a did not react to afford
the desired product 3a. Similarly, 1a and thiosulfonate 5 did
not react to generate 3a. These results rule out the possibility
of 4 and 5 as intermediates of this transformation en route to
a-substituted vinyl sulfones.
On the basis of these results, a plausible mechanism of
visible light-mediated Markovnikov addition of sulfonyl
radical to terminal alkynes is illustrated in Scheme 5. First,
Scheme 3. Radical trapping experiments.
To further investigate the mechanism, operando IR
experiments were performed to monitor the reaction, as
shown in Scheme 4. No transformation proceeded when 1a
was subjected to the standard reaction condition alone
(Scheme 4a). In contrast, when 2a was subjected to the
same reaction conditions a new component was formed,
presumably a sulfinate salt based on comparison with sodium
p-tolylsulfinate (Scheme 4b). To gain deeper insight into the
transformation, control experiments in the presence or
absence of the visible light were also monitored by operando
IR. No further conversion was observed when 1a was added
Scheme 5. Tentative mechanism.
sulfinate I is generated from acid–base neutralization reaction
between sulfinic acid with base. Subsequently, the excited
state of eosin Y reacts with I to afford intermediate II along
with the formation of eosin YCÀ, and intermediate II resonates
to intermediate III. Then, eosin YCÀ reacts with alkyne to
induce the formation of an a-vinyl carbon radical intermedi-
ate. Finally, the Markovnikov product is formed via a-vinyl
carbon radical/sulfonyl radical cross-coupling and proton
transfer (PT).
To verify the usefulness of this Markovnikov addition
protocol, the phenylacetylene reaction was scaled up to
8 mmol. The procedure proceed smoothly, giving the desired
product in 73% yield (Scheme 6a), which demonstrated that
the protocol opens up an opportunity to generate biologically
active compounds having a-substituted vinyl sulfone struc-
tural scaffolds on a larger scale. Pleasingly, this addition
protocol was also applicable to the construction of a-
substituted vinyl sulfides which are widely used in the
synthesis of natural products and pharmaceuticals (Sche-
me 6b).[13,14]
Scheme 4. Operando IR experiments. Kinetic profiles for eosin Y
(1 mol%), K2CO3 (0.3 mmol) in DMF (4.0 mL) under different con-
ditions: a) 1a (0.3 mmol), under visible light; b) 2a (0.6 mmol), under
visible light; c) 2a (0.6 mmol), addition of 1a (0.3 mmol) after 1 h
without visible light; d) 2a (0.6 mmol), addition of 1a (0.3 mmol) after
1 h under visible light.
In summary, we have developed a transition-metal-free,
visible light-mediated radical Markovnikov addition for the
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Scheme 6. a) Reaction scale-up. b) Expansion of the reaction scope to
vinyl sulfides.
highly regioselective synthesis of a-substituted vinyl sulfones
via a-vinyl carbon radical/sulfonyl radical cross-coupling. The
method exhibits broad functional-groups tolerance and can
be extended to larger scale. Pleasingly, the novel strategy is
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sulfides. This protocol provides not only a practical method-
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simple starting materials, but also allows to control regiose-
lectivity in radical additions to terminal alkynes. Ongoing
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Experimental Section
General procedure for the synthesis of a-substituted vinyl
sulfones: A mixture of phenylacetylene (1a, 0.3 mmol), p-toluene-
sulfinic acid (2a, 0.6 mmol), K2CO3 (0.3 mmol), eosin Y (1.0 mol%)
in degased dry DMF (4.0 mL) was stirred under an argon atmosphere
and irradiated with commercially available 3 W green LED for 4 h.
Then, water was added and the mixture was extracted with ethyl
acetate (ꢀ 4). The combined organic layers were dried over Na2SO4
and concentrated under reduced pressure. The resulting crude
product was separated on a silica gel column with hexane and ethyl
acetate as eluent to afford the desired product.
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This work was supported by the National Natural Science
Foundation of China (21390400, 21520102003, 21272180,
21302148), the Hubei Province Natural Science Foundation
of China (2013CFA081), the Research Fund for the Doctoral
Program of Higher Education of China (20120141130002),
and the Ministry of Science and Technology of China
(2012YQ120060). The Program of Introducing Talents of
Discipline to Universities of China (111 Program) is also
acknowledged.
Keywords: Markovnikov selectivity · photoredox catalysis ·
radical addition · radical cross coupling ·
transition-metal-free synthesis
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Manuscript received: October 12, 2016
Final Article published: && &&, &&&&
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Communications
Photoredox Catalysis
H. Wang, Q. Lu, C. Chiang, Y. Luo,
J. Zhou, G. Wang, A. Lei*
&&&& — &&&&
Markovnikov-Selective Radical Addition
of S-Nucleophiles to Terminal Alkynes
through a Photoredox Process
A transition-metal-free, visible light-
mediated radical Markovnikov addition
was developed enabling the highly regio-
selective synthesis of a-substituted vinyl
sulfones through a radical–radical cross-
coupling process. The reaction shows
good functional-groups tolerance and can
be easily scaled-up.
6
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Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!