Selectivity switch in the aerobic oxygenation of sulfides photocatalysed by visible-light-responsive decavanadate

Article abstract of DOI:10.1039/d0gc01500h

Nanometre-sized metal oxides are promising species for the development of visible-light-responsive photocatalysts for the selective transformation of organic functional groups. In this article, we report that decavanadate ([V10O28]6-, V10) behaved as an efficient visible-light-responsive photocatalyst in the product-selective oxygenation of sulfides achieved using O2 (1 atm) as the green oxidant. In particular, we revealed that visible-light-responsive photocatalysis of V10 showed remarkable activity for the oxygenation of structurally diverse sulfides to form the corresponding sulfones using O2 in methyl ethyl ketone (MEK). Furthermore, by simply adding water to the reaction mixture, the product selectivity of sulfide oxygenation can be significantly switched toward the production of sulfoxides, without concomitant loss of photocatalytic activity. Based on experimental evidence, we inferred the following mechanistic steps for this photocatalytic system: the aerobic oxygenation of sulfides to form the corresponding sulfoxides initiated by a visible-light-induced photoredox reaction of V10. As for the formation of sulfones, MEK-derived peroxide species as the co-catalysts are probably involved in the oxygenation of sulfoxides to sulfones. The selectivity switch of the V10-photocatalysed reaction brought about by water addition is most likely achieved by suppressing the formation of MEK-derived peroxide species. This journal is

Full text of DOI:10.1039/d0gc01500h

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Page 1 of 23
Green Chemistry
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DOI: 10.1039/D0GC01500H
Selectivity switch in the aerobic oxygenation of sulfides
photocatalysed by visible-light-responsive decavanadate
a
a
b
b
c
Chifeng Li, Noritaka Mizuno, Kei Murata, Kazuyuki Ishii, Tomoyoshi Suenobu,
a
a,d
Kazuya Yamaguchi* and Kosuke Suzuki*
a
Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan.
E-mail: kyama@appchem.t.u-tokyo.ac.jp; ksuzuki@appchem.t.u-tokyo.ac.jp.
b
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo
1
53-8505, Japan.
c
Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan.
d
Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and
Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
1

Green Chemistry
Page 2 of 23
Abstract
View Article Online
DOI: 10.1039/D0GC01500H
Nanometre-sized metal oxides are promising species for the development of visible-light-responsive
photocatalysts for the selective transformation of organic functional groups. In this article, we report
6
−
that decavanadate ([V O ] , V10) behaved as an efficient visible-light-responsive photocatalyst in
1
0
28
the product-selective oxygenation of sulfides achieved using O (1 atm) as the green oxidant. In
2
particular, we revealed that visible-light-responsive photocatalysis of V10 showed remarkable activity
for the oxygenation of structurally diverse sulfides to form the corresponding sulfones using O in
2
methyl ethyl ketone (MEK). Furthermore, by simply adding water to the reaction mixture, the product
selectivity of sulfide oxygenation can be significantly switched toward the production of sulfoxides,
without concomitant loss of V10 photocatalytic activity. Based on experimental evidence, we inferred
the following mechanistic steps for this photocatalytic system: the aerobic oxygenation of sulfides to
form the corresponding sulfoxides initiated by a visible-light-induced photoredox reaction of V10. As
for the formation of sulfones, MEK-derived peroxide species as the co-catalysts are probably involved
in the oxygenation of sulfoxides to sulfones. The selectivity switch of the V10-photocatalysed reaction
brought about by water addition is most likely achieved by suppressing the formation of MEK-derived
peroxide species.
2

Page 3 of 23
Green Chemistry
Introduction
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DOI: 10.1039/D0GC01500H
Over the past few decades, the importance of photocatalysts assisting the green transformation of
functional groups has grown exponentially.1 A wide variety of chemical, electrical, and optical
properties typical of metal oxides render these species attractive in the design of efficient
2
photocatalysts. Substantial efforts have been directed at developing nanometre-sized metal oxides as
part of studies focusing on fundamental catalytic mechanisms (e.g., the determination of the relevant
size and quantum effects) and the technological applications of these materials.2,3 Polyoxometalates
(
POMs) have recently emerged as attractive materials in the search for efficient photocatalysts of
functional group transformations.4,5 POMs are structurally well-defined nanometre-sized metal oxide
clusters, and their properties, such as redox potentials and electronic structures, can be finely controlled
at molecular level by simply modifying their structures, constituent elements, and oxidation states.
Notably, POMs are more thermally and oxidatively stable than commonly utilised molecular
photocatalysts, like, for instance, organometallic complexes and organic dyes. Given their molecularity
and robustness, POMs have been used as powerful tools in photocatalytic oxidative reactions,
5
including the oxidative degradation of organic pollutants and water oxidation. POM photocatalysts,
4
− 6
e.g., decatungstate ([W O ] ), have also proven useful in assisting the oxidative transformation of
1
0
32
functional groups as part of the synthesis of fine chemicals. However, photocatalysis with
decatungstate required the use of ultraviolet (UV) light to achieve the excitation of oxygen-to-tungsten
charge transfers; in fact, the energy gap between these molecular orbitals was quite large. Various
5
a
strategies have been developed to try to utilise visible light to achieve the mentioned goal. Thus far,
we have employed HOMO- and LUMO-engineering strategies to successfully develop
visible-light-responsive POM photocatalysts able to assist the oxidative transformation of a range of
functional groups.7
This study aims to develop a system that is able to carry out the efficient green photocatalytic
oxygenation of sulfides using O as the terminal oxidant. In particular, we focused on achieving the
2
selective generation of sulfoxides and sulfones, which are very important compounds characterised by a
8
9
wide range of applications in organic synthesis, including in the preparation of pharmaceuticals and
polymeric materials.10 Traditionally, (super)stoichiometric amounts of oxidants, such as
meta-chloroperoxybenzoic acid, dioxirane, hydrogen peroxide, NaIO , CrO , and KMnO , have been
4
3
4
1
1
frequently used to drive the selective oxygenation of sulfides. Notably, although from the standpoint
of environmental sustainability, the development of a green synthetic approach relying on O as the
2
3

Green Chemistry
Page 4 of 23
terminal oxidant (instead of the aforementioned stoichiometric oxidants) is highly desirable, only a few
View Article Online
DOI: 10.1039/D0GC01500H
systems for the selective, dioxygen-driven sulfide oxygenation into both sulfoxides and sulfones have
1
2
been developed. Furthermore, most of the reported such systems require harsh reaction conditions,
like, for instance, high O pressure, high temperature, or use of a highly toxic radioactive uranyl acetate
2
photocatalyst.12
6
−
In this study, decavanadate ([V O ] , V10) was revealed to be an efficient
1
0
28
visible-light-responsive photocatalyst for the selective oxygenation of sulfides achieved using O2
1 atm) as an oxidant. Polyoxovanadates like decavanadate are composed of earth-abundant vanadium
(
1
3
atoms with mainly +5 oxidation state. They are important compounds in biological processes and are
utilised for a wide range of applications, including in medicine,14 supramolecular assemblies,15 and
redox flow batteries.16 Although polyoxovanadates have been known to exhibit strong visible light
absorption, their application in photocatalytic transformation of organic functional groups has not been
studied. For example, Streb and coworkers have developed photocatalytic systems comprising
metal-substituted polyoxovanadates, but their systems are limited to the oxidative degradation of
organic dyes.17 In this study, we found that various kinds of structurally diverse sulfones could be
synthesised via visible-light-triggered photocatalysis by V10 under homogeneous conditions. The
choice of the solvent was very significant, with the highest synthetic efficiency observed in methyl ethyl
ketone (MEK). MEK-peroxide species were likely formed by MEK and O through photocatalysis of
2
dacavanadate and acted as the “co-catalysts” (not “stoichiometric” oxidants) for the oxidation. Notably,
water played an important role in directing the product selectivity toward sulfoxides over sulfones; in
particular, water proved to have minimal influence on the photocatalytic oxygenation of sulfides, but it
strongly inhibited the oxygenation of sulfoxides to sulfones, thereby effecting the selective production
of sulfoxides (Scheme 1).
4

Page 5 of 23
Green Chemistry
View Article Online
DOI: 10.1039/D0GC01500H
Scheme 1 Selective oxygenation of sulfides to sulfoxides and sulfones photocatalysed by
visible-light-responsive decavanadate. MEK: methyl ethyl ketone.
Results and discussion
Synthesis of decavanadate photocatalysts
We synthesised a decavanadate catalyst in an aqueous acidic medium and isolated it as a
tetraphenylphosphonium (TPP) salt (TPPV10). Single crystals of TPPV10 that were suitable for X-ray
crystallographic analysis were successfully obtained by recrystallizing the compound from a mixture of
acetonitrile and MEK or from acetonitrile and ethyl acetate. Based on the results of X-ray
crystallographic analysis, elemental analysis, cold-spray ionization (CSI) mass spectroscopy, and NMR
analysis, the formula of TPPV10 was determined to be TPP H [V O ] (Table S1, Fig. S1–S5). A
4
2
10 28
tetra-n-butyl ammonium (TBA) salt of V10 (TBAV10, TBA H [V O ]) was also synthesised by
3
3
10 28
employing a similar procedure. The UV-Vis spectra of TPPV10 and TBAV10 were characterised by a
visible light absorption that showed more intense absorbance than those of TBA H[γ-PV W O ],
4
2
10 40
VO(acac)2, and other vanadium-containing POMs, such as TBA [α-PV W O ] and
4
1
11 40
TBA H [γ-SiV W O ], still, TBA [α-PV W O ] showed stronger absorbance than V10 (Fig. 1a and
4
2
2
10 40
6
3
9
40
S6). Notably, V10 showed prominent absorbance at the longer wavelength (to ca. 580 nm) than other
vanadium-containing POMs. Furthermore, according to density functional theory (DFT) calculations,
the HOMO and LUMO of V10 consisted of the O2p and V3d orbitals, respectively, and the intense
absorption band in the visible light region was likely to be assignable to an O2p-to-V3d charge transfer
(
Fig. 1b).
5

Green Chemistry
Page 6 of 23
View Article Online
DOI: 10.1039/D0GC01500H
Fig. 1 (a) UV-Vis spectra of TPPV10 (0.05 mM), TBA H[γ-PV W O ] (0.05 mM), and VO(acac)
2
4
2
10 40
(
0.1 mM) in acetonitrile (1 cm cell). (b) Representations of the HOMO and LUMO of V10 as
determined by way of DFT calculations; the orbitals are represented by dark red and green lobes.
Optimization of the reaction conditions
The catalytic activity of TPPV10 in the oxygenation of thioanisole (1a), achieved through irradiation
with visible light (λ > 400 nm) using a xenon lamp under an O atmosphere, was examined in various
2
solvents (Table 1). TPPV10 was soluble in these solvents, and the photocatalytic reactions were carried
out under homogeneous conditions. Evidence indicated that the reaction hardly proceeded in organic
solvents like acetonitrile, ethyl acetate, methanol, ethanol, or dimethyl sulfoxide (Table 1, entries 4‒8).
By contrast, and to our surprise, use of simple ketone solvents, such as MEK and diethyl ketone, was
associated with significant improvements in the described photocatalytic activity, with the oxygenation
of 1a to form the corresponding sulfone (3a) proceeding with high efficiency and selectivity (Table 1,
entries 1 and 3). Notably, although the photocatalytic oxygenation of 1a to produce the corresponding
sulfoxide (2a) proceeded to some extent in acetone, N,N-dimethylformamide, or
N,N-dimethylacetamide, formation of 3a was hardly observed when the oxygenation reaction was
conducted in these solvents (Table 1, entries 9–11). Furthermore, the product selectivity of the
6

Page 7 of 23
Green Chemistry
oxygenation reaction could be made to switch by simply adding water to the reaction mixture; when
View Article Online
DOI: 10.1039/D0GC01500H
the reaction was carried out in a mixture of MEK and water (95/5, v/v), the corresponding sulfoxide 2a
was selectively obtained (Table 1, entry 2 vs. entry 1). Importantly, only one recent report exists
describing a photocatalytic system that enables users to selectively access both sulfides and sulfoxides;
this system, however, relies on highly toxic uranyl acetate catalyst, so that its utilization might be
limited.12a
Detailed reaction profiles revealed that the present V10-catalysed oxygenation of 1a is
characterised by significant solvent dependence (Fig. 2). In particular, when the photocatalytic
oxygenation of sulfide 1a was carried out in MEK, 1a was oxygenated to produce initially the
corresponding sulfoxide 2a and, subsequently, sulfone 3a (Fig. 2a). As a result, in this system, the
selective synthesis of sulfoxide 2a was very difficult to achieve at any reaction time. By contrast, when
the reaction was carried out in MEK/H O (95/5, v/v), the oxygenation of sulfide 1a to sulfoxide 2a was
2
accelerated, whereas the further oxygenation of this compound to produce sulfone 3a was significantly
suppressed, resulting in the selective oxygenation of sulfide 1a to sulfoxide 2a (Fig. 2b).
7

Green Chemistry
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Table 1 Photocatalytic aerobic oxygenation of 1a using TPPV10 as catalyst in various solventsa
View Article Online
DOI: 10.1039/D0GC01500H
O
S
O O
S
S
TPPV10
+
visible light
3
0°C, O (1 atm)
2
1a
2a
3a
Yield (%)
solvent
Entry
Solvent
Conv. (%)
2
a
3a
96
3
1
2b
3
4
5
6
7
8
9
Methyl ethyl ketone (MEK)
98
98
>99
2
<1
93
17
2
MEK/H O (95/5, v/v)
2
Diethyl ketone
Acetonitrile
81
<1
<1
<1
<1
<1
<1
<1
1
Ethyl acetate
<1
1
<1
<1
<1
<1
10
8
Methanol
Ethanol
<1
2
Dimethyl sulfoxide
Acetone
10
17
42
1
1
0
1
N,N-Dimethylformamide
N,N-Dimethylacetamide
31
aReaction conditions: 1a (0.2 mmol), TPPV10 (0.4 mol%), solvent (4 mL), 30°C, visible
b
light (λ > 400 nm, xenon lamp), O (1 atm), 8 h. 4 h.
2
8

Page 9 of 23
Green Chemistry
O
S
O O
S
View Article Online
DOI: 10.1039/D0GC01500H
S
TPPV10
visible light
+
3
0°C, O (1 atm)
2
1a
2a
3a
solvent
Fig. 2 Reaction profiles of the photocatalytic aerobic oxygenation of 1a using TPPV10 as a
catalyst in (a) MEK and (b) MEK/H O (95/5, v/v) under visible light (λ > 400 nm)
2
irradiation realised with a xenon lamp. Reaction conditions: 1a (0.2 mmol), TPPV10
(
0.4 mol%), solvent (4 mL), 30°C, visible light (λ > 400 nm, xenon lamp), O (1 atm).
2
9

Green Chemistry
Page 10 of 23
Several control experiments we conducted revealed that the oxygenation of 1a did not proceed in
View Article Online
DOI: 10.1039/D0GC01500H
the absence of the catalyst (TPPV10, Table S2, entry 2), visible light irradiation (Table S2, entry 3), or
O (Table S2, entry 4). Notably, the reaction proceeded efficiently even under an air atmosphere (Table
2
S2, entry 5). A blue LED light (λ = 420 nm) could also be used as a light source, instead of the xenon
lamp, to trigger the catalytic reaction (Table S2, entry 6). Use of a TBA salt of V10 (TBAV10) as a
photocatalyst afforded similar results as use of TPPV10 (Table 2, entries 1 and 2), indicating that the
identities of the counter cations have little effect on photocatalytic activity. The photocatalytic activity
of TPPV10 (or TBAV10) was much superior to those of vanadium-free POM TBA [α-PW O ] and
3
12 40
other vanadium-containing POMs, including TBA H[γ-PV W O ], TBA H [γ-SiV W O ],
4
2
10 40
4
2
2
10 40
1
8
TBA [α-PV W O ], TBA [α-PV W O ], and TPP [V O (C H O ) ] (Table 2, entries 1 and 2 vs.
4
1
11 40
6
3
9
40
2
6
13
5
9
4 2
entries 4‒8). In particular, TPPV10 showed much higher catalytic activity than TBA H[γ-PV W O ],
4
2
10 40
which, in a previous study conducted by our group, displayed the highest activity among the complexes
tested in the photocatalytic oxygenation of sulfides to sulfoxides (Table 2, entries 1 and 2 vs. entry 4).7c
Simple vanadium compounds, such as VO(acac) , V O , and NaVO , showed much lower
2
2
5
3
photocatalytic activity than TPPV10 in the oxygenation of 1a (Table S3). In addition, various
transition-metal complexes hardly showed photocatalytic activity in the described reaction (Table S3).
4
−
The catalytic activity of TPPV10 was even higher than that of the TBA salt of decatungstate [W O ]
1
0
28
(
TBAW10, Table S4, entry 2 vs. entry 1). Other commonly utilised visible-light-responsive molecular
photocatalysts, such as Ru(bpy) Cl , Eosin Y, methylene blue, and rose bengal, were also tested in the
3
2
aerobic oxygenation of sulfide 1a under the same conditions detailed above (Table S4, entries 3–6).
Use of these catalysts afforded sulfoxide 2a as the major product; however, in these cases, the 2a yields
were moderate, and the formation of the corresponding disulfide side-product 4a was also observed.
10

Page 11 of 23
Green Chemistry
Table 2 Photocatalytic aerobic oxygenation of 1a using various catalysts and methyl ethyl ketone
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DOI: 10.1039/D0GC01500H
(
MEK) as a solventa
O
S
O O
S
S
catalyst
+
visible light
3
0°C, O (1 atm)
2
1a
2a
3a
Yield (%)
MEK
Entry
Catalyst (mol%)
Conv. (%)
2
a
3a
59
52
<1
16
<1
<1
<1
<1
1
2
3
4
5
6
7
8
TPPV10 (0.4)
TBAV10 (0.4)
98
>99
5
35
48
2
TBA [α-PW O ] (2)
3
12 40
TBA H[γ-PV W O ] (2)
96
15
1
80
8
4
2
10 40
TBA H [γ-SiV W O ] (2)
4
2
2
10 40
TBA [α-PV W O ] (2)
<1
15
<1
4
1
11 40
TBA [α-PV W O ] (2)
21
5
6
3
9
40
TPP V O (C H O ) (2)
2
6
13
5
9
4 2
aReaction conditions: 1a (0.2 mmol), MEK (4 mL), 30°C, visible light (λ > 400 nm, xenon
lamp), O (1 atm), 4 h.
2
11

Green Chemistry
Page 12 of 23
Reaction mechanism
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DOI: 10.1039/D0GC01500H
In order to investigate the mechanism of the V10-catalysed photocatalytic oxygenation of sulfides, the
relevant oxygenation of 1a was carried out in the presence of radical scavengers (Table S5). When
1
9
(
2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), a widely used radical scavenger, was added to the
reaction mixture, the reaction was significantly suppressed (Table S5, entry 2). Similarly, the presence
2
0
of either 1,4-dimethoxybenzene, a scavenger of sulfide cation radical species (Table S5, entry 3), or
benzoquinone, a scavenger of superoxide anion radical species (Table S5, entry 4), also suppressed the
2
1
oxygenation reaction. However, the reaction was hardly affected by the presence of isopropanol, a
2
2
1
hydroxy radical (•OH) scavenger (Table S5, entry 5). The emission of singlet oxygen ( O , 1270 nm)
2
was not observed following the excitation of TBAV10 with a neodymium-doped yttrium aluminum
garnet (Nd:YAG) laser in MEK. These results indicated the possible involvement of a single electron
transfer (SET) mechanism in the formation of the sulfide cation radical and superoxide anion radical
2
3
1
species in the catalytic cycle ; on the other hand, the formation of the •OH radical and O was less
2
likely to occur during the catalytic cycle.
1
8
A set of O-labelling experiments were subsequently conducted to investigate the oxygen-transfer
process. Firstly, the TPPV10-catalysed oxygenation of 1a was performed using 18O as an oxidant,
2
1
8
under the reaction conditions described in Scheme 2a. After 2 h, O-labelled sulfoxide 2a was formed
as the main reaction product (39% yield, 95% selectivity), with the 18O content in this product
measured to be ~80%, which demonstrates that the main oxygen source for the oxygenation of 1a to 2a
is dioxygen. After 8 h, the corresponding sulfone was exclusively obtained (96% yield, >99%
1
8
selectivity). However, evidence indicated that the O content in the sulfone product was only 60%.
Therefore, an oxygen source other than O also appears to be involved in the oxygenation of 2a to 3a,
2
which raises the question of what this oxygen source may be. In order to clarify this issue, we
performed the several control experiments described in the rest of this section. When the
1
8
TPPV10-catalysed oxygenation of 1a was carried out in a mixture of MEK/H O (95/5, v/v) under
2
1
6
18
O atmosphere, no O was found to be present in the sulfone 3a produced (Scheme 2b). Therefore,
2
water can be ruled out as an oxygen source in the reaction. In addition, considering that the amount of
TPPV10 used for the reaction was very small (only 0.4 mol%) and that the oxygen atom in TPPV10
was not replaced by 18O in the presence of 18O (as indicated by the results of CSI mass analysis),
2
oxygen atoms derived from TPPV10 are unlikely to participate directly in sulfone formation.
12

Page 13 of 23
Green Chemistry
18
18 18
O
S
O O
View Article Online
(a)
DOI: 10.1039/D0GC01500H
TPPV10
S
S
visible light
3
0°C, 18O (1 atm)
2
1a
2a
9%
3a
MEK
3
2%
2
8
h
h
(18
O: ~80%)
<1%
96%
O: ~60%)
(18
16
16 16
O
S
O O
(b)
TPPV10
S
S
visible light
3
0°C, 16O (1 atm)
2
1a
2a
6%
3a
MEK/H218
O
8
12%
( O: <1%)
4
h
(18
O: <1%)
18
Scheme 2 Photocatalytic oxygenation of 1a using TPPV10 as a catalyst in the presence of (a) 18O2
1 atm) in MEK as a solvent and (b) 16O (1 atm) in MEK/H O (95/5, v/v) as a solvent. Reaction
18
(
2
2
conditions: 1a (0.2 mmol), TPPV10 (0.4 mol%), solvent (4 mL), 30°C, visible light (λ > 400 nm, xenon
lamp), O (1 atm).
2
Notably, the oxygenation of sulfoxide 2a to afford sulfone 3a proceeded when commercially
available MEK peroxide in place of O was used as an oxidant, even in the absence of visible light
2
irradiation (Table S6, entries 1 and 2). Therefore, the photocatalytic oxygenation of 2a to produce 3a,
using TPPV10 as the catalyst, probably involved the formation of MEK-derived peroxide species. This
conclusion is in good agreement with the results of the above-described labelling experiment involving
the use of 18O as the terminal oxidant, and we assume that a part of the oxygen atoms in the
2
oxygenation from 2a to 3a can be originated from MEK-derived peroxide species. (Scheme 2a).
Importantly, the reaction products derived from MEK were hardly observed after the photocatalytic
oxygenation of 1a using TPPV10, suggesting that MEK may have acted as a catalyst of the
oxygenation reactions in this photocatalytic system. When the photocatalytic oxygenation of 2a by
TPPV10 was carried out in MEK, the rate of the photocatalytic oxygenation of 2a was extremely
slower than that of the corresponding oxygenation of 2a taking place following the oxygenation of 1a
(
Fig. S7 vs. Fig. 2a). This observation probably descends from the fact that, in the presence of 1a and
O , the MEK-derived peroxide species get formed as a result of the TPPV10-catalysed photoredox
2
reaction (i.e., the reaction consisting in the oxidation of 1a to produce the corresponding sulfide cation
13

Green Chemistry
Page 14 of 23
radical and the reduction of O to produce the superoxide anion radical). This TPPV10-catalysed
2
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DOI: 10.1039/D0GC01500H
photoredox reaction seems to be difficult to proceed when 2a is used as a substrate, because of the high
oxidation potential of 2a (~2.5 V vs. normal hydrogen electrode).24
The proposed V10-catalysed photocatalytic oxygenation of sulfides may thus proceed via the
mechanism depicted in Fig. 3. The reaction is initiated by the formation of a sulfide cation radical
resulting from a SET from the sulfide substrate to the photo-activated V10 (V10*). The reduced V10 is
then reoxidised by O to afford a superoxide anion radical. The corresponding sulfoxide is then
2
obtained as a result of the reaction between the sulfide cation radical and the superoxide anion radical.
As to the formation of a sulfone product, the MEK-derived peroxide species are probably involved in
the oxygenation of sulfoxides to produce sulfones. Importantly, the oxygenation of sulfoxide 2a by
MEK-derived peroxide species to produce sulfone 3a was not suppressed by the addition of water
(
Table S6, entries 3 and 4). Therefore, the selectivity switch for the photocatalytic reaction relying on
V10 as catalyst realised by addition of water is most likely achieved via suppression of the formation
of MEK-derived peroxide.
(
i) oxygenation into sulfoxide
S
R
visible light
R
R'
S
S
V10*
V10
O O
O
S
SET
R
R'
R'
S
R
R'
R
R'
O2
O2
SET
MEK
MEK
MEK-derived
peroxide species
O O
S
(
ii) oxygenation into sulfone
(
supressed in the presence of H O)
2
R
R'
Fig. 3 Proposed mechanism of V10-catalysed photocatalytic oxygenation of sulfides.
Substrate scope
Finally, the substrate scope of the described photocatalytic oxygenation of sulfides was investigated
under visible light irradiation at λ > 400 nm in the presence of O (1 atm) as the oxidant and TPPV10
2
as the catalyst (Fig. 4). In MEK, in the presence of catalytic amounts of TPPV10 (0.35 mol%),
thioanisole and its derivatives having either electron-donating or electron-withdrawing substituents on
the aromatic ring were all efficiently and selectively converted to the desired sulfones (3a–3f). Sulfides
14

Page 15 of 23
Green Chemistry
possessing chloro, cyclopropyl, and pyridyl groups were all selectively oxygenated to produce the
View Article Online
DOI: 10.1039/D0GC01500H
corresponding sulfones (3e, 3h, and 3j), without any observed side reactions. Notably, this
photocatalytic system could also be applicable to the oxygenation of 4-nitrothioanisole to produce the
corresponding sulfone 3c, whereas a reactivity has not been achieved by reported photocatalytic
systems.25 Importantly, substrates with alkyl functional groups, as well as alkyl sulfides like
pentamenthylene sulfide and dibutyl sulfide were also efficiently converted into the desired sulfones
(
(
3g–3i, 3k and 3l). Most remarkably, when we changed the reaction solvent from MEK to MEK/H O
2
95/5 or 92/8, v/v), sulfoxides could be obtained with high selectivity from sulfides. Unfortunately,
diphenyl sulfide and dibenzothiophene could not be converted to the corresponding sulfoxides because
of the low stability of these substrates in the MEK/H O mixed solvent. By contrast, the other sulfides
2
tested in this study were all selectively oxygenated to produce the corresponding sulfoxides in
MEK/H O.
2
15

Green Chemistry
Page 16 of 23
(
Conditions A)
(Conditions B)
O
O
View Article Online
DOI: 10.1039/D0GC01500H
O
TPPV10
TPPV10
S
S
S
R
R'
R
R'
R
R'
visible light
visible light
2
3
30°C, O (1 atm)
30°C, O (1 atm)
2
2
MEK/H O
MEK
2
O
O
O
O O
O O
O O
S
S
S
S
S
S
O N
2
O N
2
2
a, 6 h
2b, 2 h
91% (9%)
2c, 3.5 h
75% (7%)
3a, 8 h
96% (<1%)
3b, 8 h
95% (<1%)
3c, 10 h
86% (5%)
a
9
3% (3%)
O
O
O
O O
O O
O O
S
S
S
S
S
S
NC
Cl
NC
Cl
2d, 2 h
2e, 3.5 h
O
2f, 2 h
3d, 10 h
3e, 24 h
O
3f, 8 h
43% (7%)
73% (9%)
81% (11%)
>99% (<1%)
96% (<1%)
91% (<1%)
O
O
O
O O
S
O O
S
O O
S
S
S
S
Cl
Cl
2
g, 3 h
2i, 6h
55% (10%)
3g, 6 h
95% (5%)
3h, 6 h
>99% (<1%)
3i, 24 h
63% (33%)
2
h, 3.5 h
a
8
2% (8%)
89% (8%)
O
O
O O
O O
O
S
S
O O
S
S
S
S
N
N
3
l, 8h
3
k, 6 h
2
j, 9 h
5% (7%)
2k, 4 h
88% (3%)
2l, 4h
83% (5%)
3j, 24 h
88% (<1%)
9
8% (<1%)
9
7% (<1%)
8
O
O O
S
O O
S
O O
S
S
O
O
2
m, 4 h
3m, 24 h
85% (<1%)
3n, 24 h
88% (<1%)
3o, 48 h
95% (<1%)
b
b
b
67% (5%)
Fig. 4 Substrate scope for the V10-catalysed oxygenation of sulfides to produce sulfoxides and
sulfones. Reaction conditions A: substrate (0.1 mmol), TPPV10 (0.35 mol%), 30°C, visible light
(
(
(
λ > 400 nm, xenon lamp), O (1 atm), MEK/H O (4 mL, 92/8, v/v). Reaction conditions B: substrate
2 2
0.2 mmol), TPPV10 (0.35 mol%), 30°C, visible light (λ > 400 nm, xenon lamp), O (1 atm), MEK
2
4 mL). Yields were determined by GC using dodecane as an internal standard. Isolated yields are
a
b
shown in ESI. Substrate (0.2 mmol), MEK/H O (4 mL, 95/5, v/v). Substrate (0.1 mmol), TPPV10
2
(
1.4 mol%), MEK (4 mL). The values in the parentheses are the yields of sulfoxides (for reaction
conditions A) and sulfones (for reaction conditions B).
16

Page 17 of 23
Green Chemistry
Conclusion
View Article Online
DOI: 10.1039/D0GC01500H
In this article, we reported the visible-light-induced selective oxygenation of sulfides to produce both
sulfoxides and sulfones using O (1 atm) as an oxidant. In particular, we report two important findings:
2
(
i) a well-known decavanadate (V10) showed remarkable visible-light-responsive photocatalytic
activity in the oxygenation of sulfides; (ii) the selectivity of the oxygenation products (i.e., sulfoxides
vs. sulfones) can be switched by simply changing the solvent. In the presence of catalytic amounts of
V10, various aromatic and alkyl sulfides can be transformed directly into the corresponding sulfoxides
or sulfones with high efficiency and selectivity, by irradiating the reaction mixture with visible light
(
λ > 400 nm). We believe that the photocatalytic activity of decavanadate might be exploited to effect
oxidative transformations of various organic functional groups using O as an oxidant. This new
2
strategy is not simply a means to realize the photocatalytic oxygenation of sulfides, but to highlight the
considerable potential of the development of green photocatalytic oxidation systems using O and
2
MEK solvent.
Experimental section
Reagents
Sulfides (TCI), acetonitrile (Kanto Chemical), acetone (Kanto Chemical), MEK (TCI), diethyl ketone
(
TCI), 3-hexanone (TCI), cyclohexanone (TCI), dimethyl sulfoxide (Kanto Chemical),
N,N-dimethylacetamide (TCI), N,N-dimethylformamide (Kanto Chemical), methanol (Wako), ethanol
(
(
Kanto Chemical), trifluorotoluene (TCI), benzonitrile (Kanto Chemical), dodecane (TCI), VO(acac)2
Aldrich), V O (Kanto Chemical), NaVO (Nacalai Tesque), 30% H O (Kanto Chemical), and methyl
2
5
3
2
2
ethyl ketone peroxide (~50% in dimethyl phthalate, TCI) were used as received. H PW O was
3
12 40
2
6
27
28
obtained
from
Wako.
TBA [W O ],
TBA H [γ-PW O ],
TBA [α-PV W O ],
4 1 11 40
4
10 32
3
4
10 36
2
9
30
and TBA H [V O ] (TBAV10)31 were
3 3 10 28
TBA H[γ-PV W O ],
TBA H [γ-SiV W O ],
4
2
10 40
4
2
2
10 40
synthesised according to the reported procedures. The syntheses of these compounds were confirmed
by CSI mass spectroscopy, IR spectroscopy, and/or NMR spectroscopy.
Instrumentation
IR spectra were measured on a JASCO FT/IR-4100 spectrometer using KCl disks. UV-Vis spectra
were measured on a Jasco V-570 spectrometer with a quartz cell of 1 cm path length. Gas
17

Green Chemistry
Page 18 of 23
chromatography (GC) analyses were performed on a Shimadzu GC-2014 instrument with a flame
View Article Online
DOI: 10.1039/D0GC01500H
ionization detector equipped with a TC-1 capillary column. GC mass spectra were recorded on a
Shimadzu GCMS-QP2010 instrument at an ionization voltage of 70 eV. CSI mass spectra were
recorded on a JEOL JMS-T100CS spectrometer. NMR spectra were recorded on a JEOL JNM
ECA-500 spectrometer ( P, 202.47 MHz; 51V, 130.23 MHz;) using 5 mm tubes. Chemical shifts (δ)
3
1
3
1
were reported in ppm downfield from H PO (solvent: D O) for P NMR spectra and NaVO (solvent:
3
4
2
3
5
1
D O) for V NMR spectra. Thermogravimetric and differential thermal analyses were performed using
2
a Rigaku Thermo plus TG 8120 instrument. Inductively coupled plasma atomic emission spectroscopy
analyses for V and P were performed with a Shimadzu ICPS-8100 instrument. Elemental analyses were
performed with an Elementar vario MICRO cube (for C, H, N) instrument at the Elemental Analysis
Center of School of Science of the University of Tokyo.
Quantum calculations
Quantum calculations were performed using the Gaussian 09 software.32 Decavanadate (V10) was
3
3
34
optimised at the CAM-B3LYP functional with 6-31++G** (for H and O) and LanL2DZ (for V) by
using the conductor-like polarizable continuum model with the parameters of the integral equation
formalism model for acetonitrile.
X-ray crystallography
Diffraction measurements were made using a Rigaku VariMax Saturn 724 diffractometer with graphite
monochromated Mo Kα radiation (λ = 0.71069 Å, 50 kV, 24 mA) at 123 K. The data were collected
Pro 35
using CrystalClear and processed using CrysAlis . Neutral scattering factors were obtained from the
standard source. Lorentz and polarization corrections were applied for data reduction. Structural analyses
3
6
were performed using WinGX. All structures were solved by SHELXS-97 (direct methods) and refined
3
7
by SHELXL-2018/3. Vanadium, oxygen, carbon, and phosphorous atoms were refined anisotropically.
CCDC-1974262 contains the supplementary crystallographic data for TPPV10. The data can be obtained
free
of
charge
from
The
Cambridge
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cif.
Synthesis of TPPV10
A solution of NaVO (2.00 g, 16.4 mmol) was dissolved in water (66 mL, 50°C), and the pH of the
3
18

Page 19 of 23
Green Chemistry
solution was adjusted to 4.8 adding dropwise to it HCl 3M. After filtration, the pH of the solution was
View Article Online
DOI: 10.1039/D0GC01500H
further adjusted to 4.5 by adding dropwise to it HCl 3M; a solution of TPPBr (4.2 g, 10.0 mmol) in a
mixture of water and acetonitrile (20 mL, 1/1, v/v) was then added to the NaVO solution. The solution
3
thus obtained was vigorously stirred for 1 min. The precipitate formed during stirring was filtered off
and washed with water to afford crude TPP H [V O ] (TPPV10) in the form of an orange powder.
4
2
10 28
Orange crystals of TPPV10 were subsequently obtained from a solution of crude TPPV10 in a mixture
of acetonitrile and ethyl acetate at 25°C. These crystals were then used for the photocatalytic reactions.
Single crystals suitable for X-ray crystallographic analysis were obtained over 3 days from a solution
of TPPV10 in a mixture of acetonitrile and ethyl acetate, or from a solution of TPPV10 in a mixture of
acetonitrile and MEK, at 25°C. Results from X-ray crystallographic analysis, CSI mass and NMR
spectroscopies, and elemental analysis support the synthesis of TPP H [V O ]. Positive-ion MS (CSI,
4
2
10 28
+
51
acetonitrile): m/z 2317.4 (calcd. 2317.8 for [TPP H V O ] ). V NMR (130.23 MHz, acetonitrile-d ):
4
3
10 28
3
−
1
δ = −511.80 ppm (4V), −487.92 ppm (4V), −410.93 ppm (2V). IR (KCl pellet, cm ): 390, 441, 526,
04, 688, 722, 760, 836, 965, 1107, 1166, 1189, 1317, 1335, 1437, 1481, 1583, 1630. Elemental analysis
calcd. (%) for TPP H V O (CH CN) : C 49.84, H 3.60, N 0.30, P 5.30, V 21.79; found: C 49.35, H
6
4
2
10 28
3
0.5
3
.62, N 0.25, P 5.34, V 22.90.
A typical procedure for the photocatalytic oxygenation of sulfides
Into a Pyrex Schlenk flask (volume: ~20 mL), TPPV10 (0.8 μmol, 0.4 mol% with respect to 1a), 1a
(
0.2 mmol), dodecane (0.1 mmol), MEK (4 mL), and a Teflon-coated magnetic stir bar were
successively placed under O atmosphere (1 atm). The reaction was initiated at 30°C by irradiation
2
with visible light (λ > 400 nm) from a xenon lamp equipped with a 400 nm cutoff filter. The detailed
reaction conditions are described in the footnotes of Tables and Figures. The identities of the products
were confirmed by comparison of their GC retention times and GC-MS spectra with those of authentic
samples. The GC yields reported in Fig. 4 were determined using dodecane as an internal standard. The
detailed GC conditions are summarized in ESI. Column chromatography was performed to isolate the
products with silica gel and solvents were technical standard. The detailed isolation conditions and
isolated yields are summarized in ESI.
Conflicts of interest
There are no conflicts to declare.
19

Green Chemistry
Page 20 of 23
View Article Online
DOI: 10.1039/D0GC01500H
Acknowledgements
We gratefully acknowledge financial support from JST PRESTO Grant Number JPMJPR18T7, JSPS
KAKENHI Grant Number 18H04500, 20H04659, and JSPS Core-to-Core program.
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