Heterogeneous Carbon Nitrides Photocatalysis Multicomponent Hydrosulfonylation of Alkynes to Access β-Keto Sulfones with the Insertion of Sulfur Dioxide in Aerobic Aqueous Medium

Article abstract of DOI:10.1021/acs.orglett.9b04454

Although hydrosulfonylation of alkynes is an ideal process to generate β-keto sulfones, such an approach is rarely implemented. Here we reported a facile and efficient graphitic carbon nitride (p-g-C₃N₄) photocatalyzed hydrosulfonylation of alkynes with the insertion of sulfur dioxide in aerobic conditions. Controlled experiments and ESR results indicated both the superoxide radicals and valence band holes played an important role in the reaction. Further isotope experiments confirmed the oxygen atom of the products comes from H₂O, while the O₂ plays an important role in the reaction by quenching the DABCO radical cation. The metal-free heterogeneous semiconductor is fully recyclable at least 6 times without significant reducing activity. Furthermore, this reaction could be carried out under solar light irradiation and was applicable for a large-scale reaction with conserved results.

Full text of DOI:10.1021/acs.orglett.9b04454

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Letter
Heterogeneous Carbon Nitrides Photocatalysis Multicomponent
Hydrosulfonylation of Alkynes To Access β‑Keto Sulfones with the
Insertion of Sulfur Dioxide in Aerobic Aqueous Medium
Bangqing Ni,* Binbin Zhang, Jine Han, Bohan Peng, Yuling Shan, and Tengfei Niu*
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ABSTRACT: Although hydrosulfonylation of alkynes is an ideal process to
generate β-keto sulfones, such an approach is rarely implemented. Here we
reported a facile and efficient graphitic carbon nitride (p-g-C3N4) photocatalyzed
hydrosulfonylation of alkynes with the insertion of sulfur dioxide in aerobic
conditions. Controlled experiments and ESR results indicated both the superoxide
radicals and valence band holes played an important role in the reaction. Further
isotope experiments confirmed the oxygen atom of the products comes from H2O,
while the O2 plays an important role in the reaction by quenching the DABCO
radical cation. The metal-free heterogeneous semiconductor is fully recyclable at
least 6 times without significant reducing activity. Furthermore, this reaction could
be carried out under solar light irradiation and was applicable for a large-scale
reaction with conserved results.
Scheme 1. Methods for β-Keto Sulfones with the Insertion
of Sulfur Dioxide
n recent decades, designing feasible and environmentally
Ifriendly methods to sulfonyl unit has become crucial to
future advances not only in chemistry but also in biology and
pharmaceuticals.1 β-Keto sulfones represent important sulfonyl
compounds and are valuable building blocks for the
construction of biologically active heterocycles.2 Moreover,
they have shown a wide range of biological properties
including antifungal, antibacterial, and nonnucleoside inhib-
itors properties.3 Over the past decades, numerous methods
have been developed to build these structures.
The traditional methods for the synthesis of β-keto sulfones
involve the direct alkylation of sodium sulfinates with phenacyl
halides suffer from the drawbacks of the unavailability of
starting materials, narrow substrate scopes, low yields, or
relatively harsh reaction conditions.4 Recently, visible light
promoted oxidative difunctional sulfonylation of an unsatu-
rated bond has been used as an efficient tool, since two
functional groups could be formed in one step from simple
starting materials under very mild conditions.5 Despite great
success, these methods usually require toxic, hazard, or
corrosive preinstalled sulfonyl compounds such as sulfonyl
halides, sulfinic acids, sulfonyl hydrazines, or sodium sulfinates.
Recently, the insertion of sulfur dioxide to generation of
sulfonyl compounds has been regarded as a safety and
environmentally friendly pathway.6 For example, Wu and co-
workers7 developed a sulfonylative coupling of aryl/alkyl
halides/aryldiazonium tetrafluoroborates with DABCO·(SO2)2
and silyl enolates to form β-keto sulfones (Scheme 1a, 1b).
Moreover, they developed another difunctinal sulfonylation of
3-arylpropiolic acids with aryl diazonium salt and water via
Received: December 12, 2019
https://dx.doi.org/10.1021/acs.orglett.9b04454
© XXXX American Chemical Society
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tandem sulfonylation and decarboxylation (Scheme 1c).8
Notwithstanding the novelty of these approaches, challenges
still remain. A simple and atom-economical difunctionl
sulfonylation of easily available alkenes or alkynes via insertion
of sulfur dioxide is still highly desirable.
a
Table 1. Optimization of Reaction Conditions
b
entry
catalyst
solvent
yield (%)
In an ideal process, β-keto sulfones could be obtained by the
Markovnikov hydration of the alkenyl cation which was
generated by the addition of a sulfonyl radical with alkynes.
However, on the basis of previous reports,8 this process was
unworkable, perhaps due to the inability to generate the
alkenyl cation (Scheme 1d). Therefore, we envisioned whether
a photocatalytic system could oxidize an alkenyl radical to
produce an alkenyl cation. If possible, β-keto sulfone ketones
could be delivered. On the other hand, it is well know that
sulfonyl radicals would be produced from radical initiators and
sulfur dioxide in the presence of a photocatalyst under visible
light irradiation.9 To date, various photoredox catalyst (PC)
systems based on homogeneous photoredox catalysts such as
ruthenium- or iridium-based transition metal complexes10 or
organic dyes11 have been developed and enabled insertion of
sulfur dioxide through a radical mechanism.12 However, these
catalysts are usually difficult to recover from the reaction
mixture. Therefore, growing interest has been focused on using
metal-free and recyclable material as the photocatalyst in the
visible light promoted reaction. In this context, polymeric
carbon nitride (C3N4) has been considered a cheap, metal-free,
nontoxic, and easily recyclable photocatalyst,13 although
initially this polymeric material had been investigated in solar
water splitting, degradation of pollutants, and CO2 reduction.
Because of its high thermal, chemical, and photostability, as
well as favorable conduction and valence band positions, very
recently, the C3N4-base photocatalyst expanded the reaction
range to organic transformations such as oxidation reaction,
C−H bond activation, and C−C bond forming reactions.14
Herein, we report a C3N4 catalyzed visible-light-promoted
radical domino process to access β-keto sulfones by using
alkyne, aryldiazonium tetrafluoroborates, DABCO·(SO2)2,
oxygen, and water as the starting materials.
c
1
p-g-C3N4
p-g-C3N4
Ru(bpy)3Cl2
Ir(ppy)3
Rhodamine B
Eosin Y
p-g-C3N4
p-g-C3N4
p-g-C3N4
p-g-C3N4
p-g-C3N4
CH3CN/H2O (2:1)
CH3CN/H2O (2:1)
CH3CN/H2O (2:1)
CH3CN/H2O (2:1)
CH3CN/H2O (2:1)
CH3CN/H2O (2:1)
CH3CN/H2O (1:1)
CH3CN
NR
74
53
38
42
58
76
NR
trace
NR
NR
2
3
4
5
6
7
8
9
H2O
d
10
11
CH3CN/H2O (1:1)
CH3CN/H2O (1:1)
e
a
Reaction conditions: substrate 1a (0.5 mmol), 2a (0.4 mmol),
DABCO·(SO2)2 (0.6 mmol), p-g-C3N4 (10 mg) under Xe lamp (250
W) irradiation or homogeneous PC (2 mol %) under blue LED
(5W), at room temperature, solvent (2 mL), O2 atmosphere, reaction
for 5 h. Isolated yield. Without O2. Without light. Without
catalyst.
b
c
d
e
electron-donating groups proceeded smoothly to afford the
desired products with moderate to good yields (3aa−3qa).
Notably, a series of functional groups including halogen, nitro,
ester, ether, and methyl groups were well compatible under the
conditions, providing a chance for further functional trans-
formations. It was found that the steric hindrance had an
obvious effect on the reaction efficiency. In comparison with
the para-, ortho-, and meta-nitro and chloro substituted aryl
diazonium salts, the ortho substituted substituents gave a
relatively lower yield (3ba, 3ja). It is worth noting that
heterocycle diazonium salts such as 2p and 2q could also be
tolerated well in this reaction to produce the corresponding
products 3pa and 3qa with moderate yields.
Initially, we began our investigation with diazonium salt 1a,
phenylacetylene 2a, and DABCO·(SO2)2 by using porous
graphite phase carbon nitride (p-g-C3N4) as the catalyst under
Xe lamp (250 W) irradiation for 5 h at room temperature;
more optimization details are provided in the Supporting
Information. However, no product was detected under such
reaction conditions (Table 1, entry 1). Surprisingly, the desired
β-ketosulfones 3a was obtained with a 74% yield when the
reaction was performed under an O2 atmosphere (Table 1,
entry 2).
Switching the catalysts to homogeneous photoredox
catalysts such as Ru(bpy)3Cl2, Ir(ppy)3, Eosin Y, and
Rhodamine B gave lower catalytic activities for this trans-
formation (Table 1, entries 3−6). As expected, the trans-
formation was hampered when anhydrous solvent was used as
a replacement (Table 1, entry 8). Only a trace amount of the
desired product was observed in water (Table 1, entry 9),
perhaps due to the solubility of the reagents. Finally, no desired
product was observed when the reaction was carried out in the
absence of a photocatalyst or visible light (Table 1, entries 10,
11).
The scope of alkynes was also investigated. Aryl alkynes with
substituents such as fluoro, chloro, methyl, and methoxy were
suitable for the present reaction transformation, affording the
desired products in moderate to good yields (3ab−3ah).
Comparatively, the electronic effect of the substituents is not
obvious, while the steric effect had a slight influence on the
reaction, and the 1-fluoro-2-vinylbenzene gave a slightly lower
yield (3ac, 59%). It was noted that heterocycle alkene
vinylpyridine 2i was suited for this protocol and successfully
converted into the corresponding products 3ai in 64% yield.
Remarkably, these results further expanded the substrates
scope of the reaction for a complex small molecule. The
reaction of vinylestrone 2j smoothly proceeded to afford the
corresponding β-keto sulfones product 3aj in 58% isolated
yield. The aliphatic alkynes such as 2k, however, were inert
toward this transformation.
Furthermore, this visible light promoted difunctinal
sulfonylation of alkynes could be driven by sunlight with
conserved results compared with the model reaction condition
of irradiation by Xe light (Figure S1). The use of solar energy
makes it suitable for large-scale performance. The reaction of
1a with 2e (10 mmol scale) proceeded smoothly under the
optimized conditions to provide the product 3ae in 78% yield
(2.65 g) using 200 mg of p-g-C3N4 within 7 h irradiation under
sunlight (Scheme 3).
Under the optimal reaction conditions, the photocatalytic
sulfonylation scope of various diazonium salts was first
explored (Scheme 2). In general, reactions of aromatic
diazonium salts bearing either electron-withdrawing or
B
https://dx.doi.org/10.1021/acs.orglett.9b04454
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a
in Table 2, the model reaction was obviously inhabited by
adding free radical scavengers such as hydroquinone or 2,2,6,6-
Scheme 2. Scope of the Diazonium Salt and the Alkynes
Table 2. Active Species Trapping Reactions for The
a
Photocatalyzed Sulfonylation System
entry
quencher
quenching group
yield (%)
1
2
3
4
5
6
Hydroquinone
TEMPO
p-Benzoquinone
HCOONH4
Tertiary butanol
CCl4
Free radicals
Free radicals
trace
trace
trace
trace
70
•O2
−
Hole
•OH
e−
trace
a
Reaction conditions: substrate 1a (0.5 mmol), 2a (0.4 mmol),
DABCO·(SO2)2 (0.6 mmol), p-g-C3N4 (10 mg) at room temperature,
CH3CN/H2O (1:1, 2 mL), O2 atmosphere, irradiation by Xe light
(250 W) for 5 h.
tetramethyl-1-piperidinyloxy (TEMPO) (Table 2, entries 1, 2),
which indicated that the reaction involved a free radical
process. When the superoxide radical quencher p-benzoqui-
none was added, a trace amount of desired product was
observed (Table 2, entry 3); further ESR results confirmed the
superoxide radical. These results reveal that the generated
superoxide radicals were the major oxidative species. Besides
the superoxide radicals, valence band holes also had an
important effect on the oxidation sulfonylation (Table 2, entry
4), while the quenching of the hydroxyl radical (Table 2, entry
5) and electrons (Table 2, entry 6) had almost no influence on
the reaction.
18
Furthermore, when O2 was replaced by O2 under the
standard conditions, product 3a was isolated and no O18
labeled product was detected (Scheme 4a). While H2O was
Scheme 4. Isotope Control Experiment
a
Reaction conditions: substrate 1 (0.5 mmol), 2 (0.4 mmol),
DABCO·(SO2)2 (0.6 mmol), p-g-C3N4 (10 mg) under Xe lamp
(250 W) irradiation at room temperature, MeCN/H2O (1:1, 2 mL),
O2 atmosphere, reaction for 5 h, isolated yield.
replaced by H2O18 under the standard conditions, O18 labeled
product was isolated (Scheme 4b). These results suggested
that the oxygen atom in the product originates from H2O and
not from O2. It was also worth noting that the reaction solution
was weakly acidic (pH = 5−6). Also, no H2O2 was detected by
potassium iodide-starch test paper.
Scheme 3. Visible-Light-Promoted Difunctinal
Sulfonylation of Alkynes by Sunlight and Scale-Up Reaction
On the basis of previous reports and experimental results, a
plausible mechanism was proposed (Scheme 5). Initially,
arydiazonium cation 1 with DABCO·(SO2)2 provides complex
A through electrostatic interactions.6i Subsequent homolytic
cleavage of a N−S bond15 and single electron transfer generate
an aryl radical, sulfur dioxide, and the radical cation
intermediate B•+. Then the aryl radical reacts with sulfur
dioxide to give sulfonyl radical C. In the meantime, electrons
and holes are produced by excitation of the p-g-C3N4 under
visible light irradiation. As a consequence, an electron reduces
molecular oxygen to produce a superoxide radical which was
quenched immediately with DABCO radical cation B•+. At the
same time, the obtained sulfonyl radical C reacted with alkynes
The subsequent active species trapping experiments were
carried out to gain some insight into the mechanism. As shown
C
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efficiency. Therefore, this protocol provides a green and
sustainable sulfonylation reaction for pharmaceutical chemistry
and synthetic organic chemistry.
Scheme 5. Possible Mechanism
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b04454.
1
Experimental procedures, characterization data, and H
and 13C NMR of products (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Bangqing Ni − Jiangnan University, Wuxi, P. R. China;
Email: byron-ni@yeah.net
Tengfei Niu − Jiangnan University, Wuxi, P. R. China;
orcid.org/0000-0002-6080-2668; Email: niutf@
jiangnan.edu.cn
2 to form intermediate D. Subsequent SET oxidation of
intermediate D forms alkenyl carbocation E, which undergoes
the hydrolysis process to produce the desired β-keto sulfones
3.
Finally, the reusability of the catalyst has been checked. The
catalyst was evaluated by the model reaction under Xe lamp
(250 W) irradiation for 5 h at room temperature. After
completion of each reaction, p-g-C3N4 was recovered using a
centrifuge, washed with water and ethanol, dried at 105 °C,
and then reused for the next reaction with a fresh batch of
solvent. As shown in Figure 1, The p-g-C3N4 catalyst could be
recycled and reused six times with satisfactory yields without a
significant loss of its activity.
Other Authors
Binbin Zhang − Jiangnan University, Wuxi, P. R. China
Jine Han − Jiangnan University, Wuxi, P. R. China
Bohan Peng − Jiangnan University, Wuxi, P. R. China
Yuling Shan − Jiangnan University, Wuxi, P. R. China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.9b04454
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the National Natural
Science Foundation of China (No. 21808085), China
Postdoctoral Science Foundation (2018M630519) and Post-
doctoral Science Foundation of Jiangsu Province, China
(1701096B).
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In conclusion, a facile and efficient visible-light-promoted
radical strategy has been developed for preparation of β-keto
sulfones via p-g-C3N4-catalyzed hydrosulfonylation of alkynes
with the insertion of sulfur dioxide. The reaction proceeded
smoothly at room temperature in an aerobic aqueous medium.
A preliminary mechanism revealed that both the superoxide
radicals and valence band holes played an important role in the
reaction process. Further isotope experimentation indicated
the oxygen atom of the products comes from H2O, while the
O2 play an important role in the reaction by quenching the
DABCO radical cation. Moreover, this reaction could be
carried out under sunlight irradiation and was applicable for a
large-scale reaction with conserved results. The present p-g-
C3N4 photocatalyst is straightforwardly recoverable by simple
centrifugation of reaction mixtures, and is reusable for at least
six catalytic transformations without significant loss of its
D
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