Synergistic cooperative effect of CF3SO2Na and bis(2-butoxyethyl)ether towards selective oxygenation of sulfides with molecular oxygen under visible-light irradiation

Article abstract of DOI:10.1039/d0gc02663h

A safe, practical and eco-friendly method for the switchable synthesis of sulfoxides and sulfones through visible-light-initiated oxygenation of sulfides at ambient temperature under transition-metal-, additives-free and minimal solvent conditions. The synergistic catalytic efforts between CF3SO2Na and 2-butoxyethyl ether represents the key promoting factor for the reaction. This journal is

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Synergistic Cooperative Effect of CF3SO2Na and
Bis(2-butoxyethyl)ether Towards Selective
Cite this: DOI: 10.1039/x0xx00000x
Oxygenation of Sulfides with Molecular Oxygen under
Visible-Light Irradiation
Received 00th January 2012,
Accepted 00th January 2012
Kai-Jian Liua, Zheng Wanga, Ling-Hui Lua, Jin-Yang Chenb, Fei Zenga, Ying-Wu Linb, Zhong
Caoc, Xianyong Yud and Wei-Min He*a
DOI: 10.1039/x0xx00000x
A safe, practical and eco-friendly method for the switchable synthesis of sulfoxides and sulfones through
visible-light-initiated oxygenation of sulfides at ambient temperature under transition-metal-,
additives-free and minimal solvent conditions. The synergistic catalytic efforts between CF3SO2Na and
2-butoxyethyl ether represents the key promoting factor for the reaction.
www.rsc.org/
increases manufacturing cost but also causes the issue of
transition-metal residue in the final products. Moreover, the
Introduction
Sulfur-containing molecules perform significant roles in fine
chemical, pharmaceutical, and material science, given their diverse
oxidation-states.1 Among these S-containing compounds, sulfoxides
and sulfones are indispensable motifs in synthetic pharmaceuticals,
natural products, and advanced organic materials.2 In the fine
chemical and pharmaceutical industry, the selectivity-switchable
synthesis of distinct high-value chemicals from the same starting
material would be ideal in terms of manufacturing cost and
operational simplicity.
During the past years, considerable efforts have been made for
developing of switchable synthesis of sulfoxides and sulfones
through the selective oxidation of abundant and easily available
sulfides. In 2012, Liang-Nian He’s group pioneered the selective
oxygenation of sulfides with stoichiometric amounts of oxone as
the oxidant under heating conditions (Scheme 1a).3 Liu and our
group independently realized the temperature-controlled selective
oxygenation of sulfides using molecular oxygen (1 atm) with ether
solvent as the promoter (Scheme 1b).4 However, the usage of O2 in
the presence of flammable organic solvent at high temperature
results in a potentially inflammable and explosive mixture, which
greatly limits their industrial application. Visible-light-induced
oxidation with O2 as the sole oxidant has become a prominent
sector of synthetic chemistry over the last years as it offers
eco-friendly alternatives to the conventional thermal heating
process.5 In 2019, Jiang and coworkers disclosed the firstly
switching selectivity in the above reactions is fully dependent on
different reaction mediums, resulting in the increasing of operating
difficulty and production cost. From an environmental and
economical point of view, a novel method for the visible-light
induced switchable oxygenation of sulfides with O2 in the presence
of low cost and readily available photocatalyst under
transition-metal- and additives-free conditions would be highly
desirable, especially in the pharmacy industry.
O
O
Oxone (0.6 equiv.)
EtOH (0.2 M), 60oC
Oxone (1.5 equiv.)
H2O (0.2 M), 60oC
S
R1
S
R2
(a)
(b)
R1
R1
R2
R2
S
R1
R1
R2
R2
O
O
O
S
O2 balloon
bis(methoxypropyl) ether (1 M)
80oC, 20 h
O2 balloon
bis(methoxypropyl) ether (1 M)
100oC, 20 h
S
R1
S
R2
O
O2 balloon, H3PO4 (1 equiv.)
UO2(OAc)2.2H2O (2 mol%)
H2O (10 equiv.)
MeCN (0.2 M), bule LEDs, rt
O2 balloon,
O
S
O
S
UO2(OAc)2.2H2O (2 mol%)
S
S
R1
R2
R1
R2
(c)
(d)
R1
R2
H2O (10 equiv.)
MeCN/o-xylene (0.2 M), bule LEDs, rt
O
O2 balloon
TPPV10 (0.35 mol%)
O2 balloon
TPPV10 (0.35 mol%)
O
S
O
S
R1
R2
R1
R2
R1
R2
methyl ethyl ketone (0.025M)
400 nm, xenon lamp, rt
methyl ethyl ketone/H2O(92/8, 0.025M)
O
> 400 nm, xenon lamp, rt
λ
>
λ
Present work
O2 balloon
CF3SO2Na (25 mol%)
2-butoxyethyl ether (4 equiv.)
385-390 nm, LED, rt, longer time
O2 balloon
CF3SO2Na (25 mol%)
O
S
O
S
R1
R2
(e)
R1
R2
S
R1
R2
2-butoxyethyl ether (4 equiv.)
385-390 nm, LED, rt, shorter time
O
Synergistic catalysis
Cheap and readily avaiable CF3SO2Na: photocatalyst
Low cost and easily avaibale 2-butoxyethyl ether: solvent and cocatalyst
Scheme 1 Selective oxidation of sulfides
visible-light-induced
oxygenation of sulfides with O2 in the presence of H2O and H3PO4 as
the additives (Scheme 1c).6 Very recently, Kosuke Suzuki and Kazuya
UO2(OAc)2.2H2O-catalyzed
selective
We were aware that the cheap and easily available sodium
trifluoromethanesulfinate was used as a photocatalyst for the
visible-light-induced aerobic oxidation of C-H bonds8 and the ether
solvent was employed as a promoter/catalyst for the thermal
oxidation with O2. However, to the best of our knowledge, there
has been no report about the visible-light-induced oxidation
synergistically catalyzed by CF3SO2Na and ether. Under the
guidance of green chemistry principles,9 we herein disclose a safe,
Yamaguchi
reported
the
visible-light
induced
tetraphenylphosphonium
decavanadate
(TPPV10)-catalyzed
switchable oxygenation of sulfides.7 However, expensive uranyl
salts or commercially unavailable TPPV10 nano-catalyst was
necessary in these photocatalytic processes, which not only
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practical and sustainable protocol for the switchable preparation of
sulfoxides and sulfones through visible-light initiated
CF3SO2Na/2-butoxyethyl ether synergistic catalyzed oxygenation of
sulfides under transition-metal-, additives-free and minimal solvent
conditions (Scheme 1e). The synergistic combination of CF3SO2Na
and 2-butoxyethyl ether in a relay aerobic oxidation mechanism
allows sulfides to undergo deep oxidation.
oxygenation reaction with bis(2-butoxyethyl)ether ether (BE)
as the solvent provided 2a in 89% GC yield and the sulfone 3a
in 6% GC yield based on 95% conversion of 1a (entry 12). To
our delight but unexpectedly, reducing the loading of solvent
obviously improved the oxidation capacity of the present
reaction system (entries 13 - 15). A quantitative yield of 3a was
obtained by decreasing the amount of BE from 1 mL (about 12
equiv.) to 4 equiv. (entry 14). However, further reducing the
loading of BE to 2 equiv. led to a lower yield of 3a (entry 15).
As expected, the reaction time exerted a crucial role on the
oxygenation outcome (entries 14 VS 16 - 18). A 92% yield of 2a
and 8% yield of 3a were observed by shorting the oxidation
time from 24 hours to 6 hours (entry 17). Both lower
conversion of 1a and lower yield of 2a were obtained in the
absence of CF3SO2Na or BE (entries 17 VS 19 - 21), revealing
that the BE has synergetic effect with CF3SO2Na and enhanced
oxidation capacity of the photocatalytic system. No oxidation
reaction occurred without visible-light irradiation, and the
substrate 1a was completely recovered (entry 22).
Results and discussion
Table 1. Optimization of conditionsa
O
S
O
S
O2 balloon
Me
S
Me
CF3SO2Na (25 mol%)
+
Me
O
10 W LED, solvent, rt.
Me
Me
Me
1a
2a
3a
Yieldb
Light
Source
Conv
.
Entry
Solvent
Time
2a
3a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19c
20
21
22d
375-380 nm MeCN (1 mL)
380-385 nm MeCN (1 mL)
385-390 nm MeCN (1 mL)
390-395 nm MeCN (1 mL)
395-400 nm MeCN (1 mL)
24 h
24 h
24 h
24 h
24 h
24 h
19%
21%
68%
62%
49%
15%
34%
47%
82%
88%
89%
95%
98%
19%
21%
68%
62%
49%
15%
34%
47%
80%
85%
84%
89%
41%
0
0
0
0
0
0
0
0
0
Table 2. Reaction Scope of Sulfoxides a
O
2 balloon
O
S
O
S
385-390 nm
DMF (1 mL)
CF3SO2Na (25 mol%)
385-390 nm, 10 W LED
bis(2-butoxyethyl) ether (4 equiv.)
S
+
Ar
R
Ar
R
385-390 nm MeOH (1 mL) 24 h
Ar
R
O
385-390 nm
385-390 nm
385-390 nm
385-390 nm
385-390 nm
385-390 nm
DCE (1 mL)
THF (1 mL)
ME(1 mL)
EE (1 mL)
BE (1 mL)
BE (8 equiv.)
24 h
24 h
24 h
24 h
24 h
24 h
1
2
3
R =
2b
,
3b
94%, 24h
,
H,
2%
3%
5%
6%
57%
100%
72%
53%
8%
1%
0
87%, 9h
4-Me,
4-OMe,
4-F,
2a,
2c,
2d,
3a, 98%, 24 h
3c, 94%, 24 h
3d, 81%, 48 h
3e
85%, 48 h
,
3f, 88%, 48 h
3g 79%, 48 h
3h, 86%, 48 h
88%, 6 h
82%, 6 h
86%, 12 h
84%, 12 h
87%, 12 h
86%, 19 h
84%, 8 h
89%, 20 h
94%, 8 h
85%, 12 h
91%, 7 h
95%, 24 h
4-Cl,
4-Br,
4-Ac,
2e
,
S
Me
2f,
2g
R
385-390 nm BE (4 equiv.)
24 h 100%
24 h 99%
12 h 100% 47%
6 h
4 h
6 h
6 h
6 h
24 h
4-CO2Me, 2h,
385-390 nm
385-390 nm
BE (2 equiv.)
BE (4 equiv.)
27%
4-CN,
3-Me,
3-Cl,
2i
,
3i 78%, 48 h
,
2j,
2k,
2l,
3j, 88%, 48 h
3k, 86%, 48 h
3l, 81%, 48 h
385-390 nm BE (4 equiv.)
100% 92%
38%
4%
12%
10%
0
2-Me,
2-Br,
385-390 nm
385-390 nm
385-390 nm
385-390 nm
---
BE (4 equiv.)
BE (4 equiv.)
MeCN (4 equiv.)
---
37%
4%
12%
10%
0
2m,
S
Me
S
S
Me
Me
Me
S
S
N
0
0
0
Me
N
2n
2q
, 81%, 24 h
, 76%, 24 h
2o
2p
3n
, 83%, 9 h
, 82%, 10 h
3m
, 92%, 48 h
, 88%, 48 h
BE (4 equiv.)
S
S
Me
S
S
Me
Me
aConditions: methyl 4-tolyl sulfide (1a) (1 mmol), CF3SO2Na (25
mol%), solvent, O2 balloon; b The yields were estimated by GC-MS; c
Without CF3SO2Na; dUnder dark conditions; BE: 2-butoxyethyl
ether; EE: 2-ethoxyethyl ether.
O
S
Me
2r
2s
2u
, 74%, 24 h
, 77%, 24 h
, 92%, 24 h
S
2t
, 81%, 9 h
3o
, 84%, 48 h
S
S
S
Cl
OH
S
We first investigated the visible-light-induced oxygenation of
methyl(p-tolyl)sulfane (1a) as the model reaction (Table 1).
Conducting the oxygenation of 1a with CF3SO2Na (25 mol%) as
a photocatalyst and molecular oxygen as an oxidant in MeCN
under the irradiation of LEDs (375-380 nm, 10 W) at ambient
temperature for 24 hours delivered the desired sulfoxide (2a)
in 19% GC yield (Table 1, entry 1). The effect of the visible light
source on the model reaction was next investigated (entries
2-5). Under the irradiation wavelength of 385 - 390 nm, a 68%
GC yield of 2a was obtained after 24 hours (entry 3). As
anticipated, the oxygenation efficiency was heavily depended
on the nature of the reaction medium (entries 6 - 12). The
ether solvents (entries 9 - 12) gave better results than
2v
2w
2x
3x
2y
, 73%, 15 h
, 72%, 18 h
SPh
, 78%, 15 h
, 72%, 48 h
, 70%, 24 h
SPh
SPh
SPh
Me
2aa
tBu
2ab
MeO
2z
, 75%, 24 h
3p
, 73%, 48 h
2ac
, 81%, 24 h
, 79%, 24 h
, 75%, 48 h
, 77%, 24 h
79%, 48 h
3q
3r
3s
,
,
80%, 48 h
SPh
SPh
S
SPh
Br
Me
Me
N
2ad
2ae
2af
2ag
, 72%, 24 h
, 83%, 24 h
, 74%, 24 h
, 72%, 24 h
3w
, 78%, 48 h
3t
3u
,
3v
, 74%, 48 h
82%, 48 h
, 73%, 48 h
aConditions: sulfides 1 (1.0 mmol), CF3SO2Na (25 mol%), BE (4.0 mmol),
O2 balloon, room temperature.
With the optimal conditions, the generality of the selective
non-ether solvents (entries 3,
6 - 8). Performing the
oxygenation of sulfides 1 to sulfoxides 2 was firstly investigated
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(Table 2). As seen clearly, phenylthiomethanes with various motifs into Fenofibrate (a widely used lipid regulator) were
synthetically important functional groups underwent the smoothly achieved to provide the desired products 4a and 4b
visible-light-induced oxygenation efficiently and delivered the in good yields. The sulfone-containing drugs, including EPAC2
desired products (2a - 2m) in excellent yields. Neither the electronic antagonist 4c, Zolimidine 4d (anti-ulcer agent), and Etoricoxib
effect nor steric hindrance effect of the substituent group on the 4e (antirheumatoid arthritis drug and COX-2 selective
phenyl ring of substrate 1 had a considerable influence on the inhibitor) could be easily constructed via the current reaction.
oxidative process. It was worth noting that sulfide (1n) is readily
oxidized into 1,4-bis(methylsulfinyl)benzene (2n) in 81% yield and
the S=O bond did not have a significant influence during the double
oxidative process. To our satisfaction, the methyl sulfide substrates
with naphthalene or heteroaromatic ring were also tolerated in the
reaction system, which significantly expanded the scope of the
oxidation (2o - 2s). The phenyl sulfide with various chain lengths,
isomeric structures, and functional groups underwent the
transformation to generate the sulfoxides in good yields (2t - 2w).
The dibenzo[b,d]thiophene bearing a sulfur atom in the cyclic
framework gave the product (2x) in 78% yield. Given that the
thianthrene bearing two sulfur atoms, we were pleased to find that
Figure 1 Time course for the oxygenation of 1a
only mono-oxygenation product (2y) was produced in good yield.
The effect of visible light irradiation time on the conversion
of sulfide 1a was explored. As shown in Figure 1, in the initial
oxygenation stage of 6 hours, both 92% GC yield of sulfoxide 2a
and 8% GC yield of sulfone 3a were obtained with full
conversion of the sulfide 1a. About at 11 hours, equal amount
of 2a and 3a were detected. As the lighting time up to 24
hours, 2a was completely converted into 3a. These results
suggested that the 1a was firstly converted into 2a and then
the generated 2a was transformed into 3a in the oxidative
process.
Moreover,
a series of diaryl sulfides with electron-rich and
electron-poor groups were well tolerated, delivering the
corresponding sulfoxide products (2z - 2ag) in good to excellent
yields.
Next, the universality of the deep oxygenation of sulfides to
sulfones 3 was explored. A wide range of phenylthiomethanes
with electron-neutral, -donating, and - withdrawing groups
were well compatible under the optimized conditions
regardless of their electronic and steric properties (3a - 3l).
Both polycyclic and quinoline substrates underwent
oxygenation reactions smoothly to provide the sulfone
products 3m and 3n in 92% and 88% yields, respectively.
Phenyl ethyl sulfide was also applicable in the current
sulfonylation and provided the corresponding product 3o in
84% yield. Moreover, good to excellent yields of diaryl sulfones
(3q - 3w) were obtained for the photocatalytic oxygenation of
various diaryl sulfides. The dibenzothiophene aslo proceeded
efficiently to give the sulfone 3x in 72% yield. No reaction
occurred when dialkyl sulfides, such as dibenzyl sulfide and
propyl sulfide, were used as substrates. GC-MS analysis of the
reaction mixture showed that a small quantity of sulfone
product was formed during the oxidation of diaryl sulfide to
diaryl sulfoxide. For example, the oxidation of diaryl sulfide 1af
for 24 hours provided 77% GC yield of sulfoxide 2af and 9% GC
yield of sulfone 3v.
Figure 2 Visible-light irradiation On/Off experiments
The light on-off experiments of the oxygenation of sulfide
1a to sulfoxide 2a and oxygenation of 2a to sulfone 3a
suggested that both the transformations were inhibited in the
absence of visible-light irradiation (Figure 2a-b). These results
demonstrated that the light-dependency of the current
oxidative process.
Table 3. Late-stage modificationa
O
O
O
S
Me
O
S
standard conditions
scavenger (2 equiv.)
Tolyl
S
Me
+
Tolyl
Me
S
O
S
N
Me Me
Tolyl
Me
O
Me
O
iPrO
Me
MeO
N
Me
Me
S
O
O
O
1a
2a
3a
O
SOMe-Fenofibrate
EPAC2 antagonist
4c
, 72%, 48h
Zolimidine,
4d
, 84%, 48h
Inhibited species
Radical
Scavenger
TEMPO
BHT
3a
0
2a
7%
5%
12%
4%
4a
, 78%, 24h
O
S
(a)
(b)
(c)
(d)
O
Me
O
Me Me
Radical
0
O
Etoricoxib
4e
iPrO
Cl
O
O
S
, 78%, 48h
Me
1,4-dimethoxybenzene
0
R2S
O2
O
N
N
SO2Me-Fenofibrate
benzoquinone
<1%
4b
Me
, 75%, 48h
O
S
aConditions: sulfides 1 (1.0 mmol), CF3SO2Na (25 mol%), BE (4.0 mmol),
O2 balloon, room temperature.
O
O
2 balloon, CF3SO2Na (25 mol%)
385-390 nm, 10 W Led, MeCN
Tolyl
3a
Me
S
(e)
Tolyl
Me
O
, 21%
To further verify the applicability of the present oxidation,
some sulfoxide- and sulfone-containing pharmaceuticals and
their derivatives were prepared through the developed
method (Table 3). Both the introduction of -SOMe and -SO2Me
2a
Scheme 2 Control Experiments
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In order to gain
a
better understanding of the
In conclusion, we have successfully demonstrated a safe,
visible-light-induced oxidation of sulfides, some control practical and eco-friendly photocatalytic method for the
experiments were carried out. The reaction was suppressed by switchable synthesis of sulfoxides and sulfones under
adding TEMPO or BHT as the radical scavenger, confirming that transition-metal-, additives-free and minimal solvent
radical species should be involved in this process (Scheme conditions. With cheap and easily available CF3SO2Na as a
2a-b). When 1,4-dimethoxybenzene, a well-known sulfide photocatalyst, 2-butoxyethyl ether (4 equiv.) as a solvent and a
cation radical scavenger, was added into the reaction, only a co-catalyst, various sulfides were efficiently and selectively
trace amount of 2a was observed (Scheme 2c), suggesting that oxidized into sulfoxides (34 examples, 70-95%) and sulfones
the sulfide cation radical might be a key intermediate for the (28 examples, 72-98%) through the adjustment of reaction
transformation of sulfides.10 Furthermore, the formation of time. In sharp contrast to the previous photocatalytic
superoxide anion radical was conformed by adding strategies, the present strategy not only avoiding toxic or
benzoquinone as a superoxide radical scavenger11 (Scheme commercially unavailable photocatalysts, but also significantly
2d). Performing the oxidation of sulfoxide 2a with CF3SO2Na as simplifying
operational
procedures
and
reducing
the photocatalyst and O2 as the oxidant in MeCN under the manufacturing cost. Mechanistic studies indicated that the
irradiation of LED (385-390 nm) for 24 hours gave the desired synergistic catalytic efforts between CF3SO2Na and
sulfone 3a in 21% GC yield (Scheme 2e). Approximately 4% of 2-butoxyethyl ether represent the key promoting factor for the
BE was lost during the oxidative process, suggesting the BE oxidation reaction. The protocol will open a new avenue for
might act an catalyst in the present raction.12 The ultraviolet– developing the practical visible-light-initiated oxidation and
visible absorption-spectra showed that only the oxidized may also find broad applicability in synthetic chemistry and
CF3SO2Na could absorb visible light and serve as
a
pharmaceutical chemistry.
photocatalyst in the present oxygenation reaction (Figure S1,
Acknowledgements
Electronic Supplementary Information).13
We are grateful for financial support from the Hunan
Provincial Natural Science Foundation of China (Nos.
2019JJ40090 and 2019JJ20008).
*
C1*
O
S
F3C
ONa
S
O2
O
O
SET
O
O
hv
S
O2
+
R
R
S
R
R
R
Me
S2
S1
O
Notes and references
SET
S
F3C
O
+ Na+
O
S
O
S
S
O2
S
aDepartment of Chemistry, Hunan University of Science and Engineering,
Yongzhou 425100, China
F3C
ONa
R
Me
O
C1
O
hv
R
R
F3C
ONa
O
O
C1
1
Bu
BE
O
R'
2
bSchool of Chemistry and Chemical Engineering, University of South
China, Hengyang, 421001, China
O
Na
OOH
R'
O
S
O
S
H
2
S
O
,
H
R
R
Bu
F3C
OH
Bu
R
R
O
R'
BE1
O
C1-H
cHunan Provincial Key Laboratory of Materials Protection for Electric
Power and Transportation, Changsha University of Science and
Technology, Changsha, 410114, China
O
O
BE
O
BE2
3
Scheme 3 Possible mechanism
dSchool of Chemistry and Chemical Engineering, Hunan University of
Science and Technology, Xiangtan 411201, China
Based on the above results and related literature,4, 6, 8 we
proposed plausible mechanism for the photocatalytic
a
E-mail: weiminhe2016@yeah.net
selective oxygenation of sulfides (Scheme 3). Initially, the
visible-light-induced oxidation of CF3SO2Na with ground-state
triplet molecular oxygen generated a penta-coordinate sulfide
(C1), which was further excited by light irradiation to produce
the excited-state C1*. The excited-state C1* then underwent a
single electron transfer (SET) with substrate 1 to generate a sulfide
cation radical (S1) along with the formation of ground-state C1.
Meanwhile, excited-state C1* also reacted with O2 to form an
active superoxide anion radical with the concomitant production
of radical C1·. The subsequent coupling of the intermediate S1 with
the superoxide anion radical gave the persulfoxide intermediate
(S2), which was subsequently caught by another molecular of
substrate 1 to form the sulfoxide 2. The radical C1· abstracted
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
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hydrogen from the 2-butoxyethyl ether (BE) to generate
a
carbon-centered radical BE1 along with an intermediate C1-H,
which was converted into intermediate C1. Simultaneously, the
radical BE1 was further oxidized to generate a peroxide BE2, which
oxygenated the sulfoxide
2 into sulfone product 3 and
regenerated the BE
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Conclusions
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DOI: 10.1039/D0GC02663H
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