Synthesis of an anthraquinone-containing polymeric photosensitizer and its application in aerobic photooxidation of thioethers

Article abstract of DOI:10.1039/d0ra00880j

Work on the synthesis of a polymeric photosensitizer and its application in the photooxidation of thioethers is reported herein. Firstly, the polymeric photosensitizer was designed and synthesized by the reaction of anthraquinone-2-carbonyl chloride (AQ-2-COCl) with poly(2-hydroxyethyl methacrylate) (PHEMA). Then, the visible light-induced photooxidation of thioethers under aerobic conditions was investigated. The results revealed that the reaction yielded sulfoxides highly chemoselectively in excellent yields with good substrate tolerance. Importantly, AQ-PHEMA could be easily recovered and reused more than 20 times without significant loss of the catalytic activity.

Full text of DOI:10.1039/d0ra00880j

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Synthesis of an anthraquinone-containing
polymeric photosensitizer and its application in
Cite this: RSC Adv., 2020, 10, 10661
aerobic photooxidation of thioethers†
a
a
b
*
*
Yang Chen, Jianhua Hu and Aishun Ding
Work on the synthesis of a polymeric photosensitizer and its application in the photooxidation of thioethers
is reported herein. Firstly, the polymeric photosensitizer was designed and synthesized by the reaction of
anthraquinone-2-carbonyl chloride (AQ-2-COCl) with poly(2-hydroxyethyl methacrylate) (PHEMA).
Then, the visible light-induced photooxidation of thioethers under aerobic conditions was investigated.
The results revealed that the reaction yielded sulfoxides highly chemoselectively in excellent yields with
good substrate tolerance. Importantly, AQ-PHEMA could be easily recovered and reused more than 20
times without significant loss of the catalytic activity.
Received 29th January 2020
Accepted 6th March 2020
DOI: 10.1039/d0ra00880j
rsc.li/rsc-advances
cyclopropanation of dibromomalonate derivatives and alkenes.
This is the rst report on the generation of carbanions under
Introduction
visible light photoredox catalysis.44 Guo’s group also reported
the rst activation of alkynes using a Lewis acid catalyst
together with eosin Y as a small molecular photocatalyst.45
Although small molecule photosensitizers46–48 exhibit good
catalytic properties in some reactions, their disadvantages, such
as difficulty of separation and reuse as well as high cost, remain
unsolved. Therefore, for the purpose of recycling the catalyst,
polymeric photosensitizers which were designed and synthe-
sized by linking photosensitive groups onto polymeric chains
have drawn much attention from researchers.49–55 Compared
with small molecule photosensitizers, polymeric photosensi-
tizers can be easily recycled, which not only reduces the cost,
but also simplies the process. Thus, they have been widely
used in the elds of medicine, biology and hydrogels.56–58
Sulfoxides can be used as drugs1 and preservatives2 in daily life.
They can also serve as important fragments in organic
synthesis3–5 and bioactive molecules.6 Oxidation of thioethers
into sulfoxides is the most direct method for the preparation of
sulfoxides.7–9 Some other methods for synthesizing sulfoxides
were developed in the past few decades, such as organic cata-
lytic oxidation,10 hydrogen peroxide oxidation,11,12 metal
complex-catalyzed oxidation,13,14 photooxidation,15,16 etc.
However, these methods normally require stoichiometric
oxidants, which produce a large amount of environmentally
harmful waste. At the same time, another issue of the above
methods is controlling the selectivity for sulfoxide and over-
oxidation to the sulfone by-product.17 Although some catalytic
systems exhibited high chemoselectivity with small molecular
catalysts, from a practical application point of view, some of
these catalysts could not be easily recovered aer one catalytic
cycle, or the catalyst was too expensive.10,18 Considering the
concept of “green chemistry”, an environmentally friendly,
highly catalytically efficient and highly selective oxidation of
thioethers into sulfoxides is still urgently needed.
Visible light19–32 is an economically green energy. Visible
light-induced photocatalysis is a hot research topic33–43 in
organic synthesis. For example, Guo’s group reported the
aState Key Laboratory of Molecular Engineering of Polymers, Department of
Macromolecular Science, Fudan University, 2005 Songhu Road, Shanghai 200438,
P. R. China. E-mail: hujh@fudan.edu.cn; Fax: +86-21-31242888; Tel: +86-21-
55665280
bDepartment of Chemistry, Fudan University, 2005 Songhu Road, Shanghai 200438, P.
R. China. E-mail: shunzi0522@126.com; Fax: +86-21-31249190; Tel: +86-21-
31249190
† Electronic supplementary information (ESI) available: Experimental procedures,
and characterization data for all compounds. See DOI: 10.1039/d0ra00880j
Scheme 1 The synthetic procedure of AQ-PHEMA and its application
in the photooxidation of thioethers.
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Table 1 Optimization of the reaction conditionsa
But the catalysts could not be easily recovered and reused.
Thus, we wondered whether a small molecule photosensitizer
could be attached to a polymer, forming a novel polymeric
photocatalytic material. This catalytic material could be recy-
cled and reused aer catalyzing the photoreaction. With our
continuing interest in the synthesis and application of
PHEMA,36 AQ-2-COOH59–61 and PHEMA36 were used to synthe-
size the designed polymeric photosensitizer AQ-PHEMA
(Scheme 1) by immobilizing anthraquinone (AQ) onto PHEMA.
In the next step, AQ-PHEMA (for synthesis and characterization
of AQ-PHEMA, see ESI†) was tested as a photocatalyst in the
visible light-induced photooxidation of thioethers under
aerobic conditions. Finally, recycling experiments were carefully
conducted.
NMR yieldb (%)
Entry Solvent
Catalyst (mol%) Time (h) 1a 2a
3a
1
2
3
4
5
6
7
8
9
10
Cyclohexane
Toluene
THF
Ethyl acetate
DCM
5
5
5
5
5
5
5
5
5
5
3
1
0.5
—
1
1
1
72
57
48
49
72
39
72
36
28
16
18
24
24
24
24
24
9
29 21
2
0
0
0
0
0
0
0
0
0
0
0
30
39
50
51
66
74
95
97
16
54
19
3
7
1
5
3
1
1
Acetone
Et2O
Results and discussion
Optimization and scope investigation
CH3CN
CH3NO2
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
99
In the beginning, methyl phenyl thioether (1a) was chosen as 11
99
12
13
99 (96)c
1
the model substrate for the investigation of AQ-PHEMA-cata-
lyzed photooxidation. The initial attempt was carried out in
cyclohexane under air atmosphere at room temperature. Aer
72 hours, the desired product methyl phenyl sulfoxide (2a) was
48 44
6
14
99
99
99
0
0
0
0
99
0
0
0
0
1
15d
16d,e
formed in 21% NMR yield, with 29% of 1a remaining unreacted 17f
18g
—
16
99
0
and 2% of the over-oxidized by-product methyl phenyl sulfone
a
(3a) being observed (entry 1, Table 1). Encouraged by this result,
The reaction was carried out using 1a (1 mmol) and AQ-PHEMA in
solvents (5 mL) irradiated by a purple LED under air atmosphere at rt.
(Based on AQ anchored on PHEMA, the mass of 5 mol% AQ-PHEMA
is 17 mg; the mass of 3 mol% AQ-PHEMA is 10 mg; the mass of
1 mol% AQ-PHEMA is 3 mg; and the mass of 0.5 mol% AQ-PHEMA is
a survey of solvents was carried out (entries 2–10, Table 1).
When toluene or THF was tested, the NMR yield of 2a increased.
But 3a was also produced in a higher yield (entries 2 and 3,
Table 1). Then, ethyl acetate, dichloromethane, acetone, and
ether were tested. Although a small amount of 3a was formed,
the NMR yield of 2a was remarkably increased (entries 4–7,
Table 1). When CH3CN or CH3NO2 was chosen as the solvent, 2a
was formed in an excellent yield with less than 5% 3a generated
(entries 8 and 9, Table 1). Fortunately, the reaction in CH3OH
gave a 99% NMR yield of 2a with only 1% 3a detected (entry 10,
Table 1). Thus, CH3OH was chosen as the optimal solvent for
this reaction. Notably, AQ-PHEMA is insoluble in all the above
tested solvents and can only be dispersed in the solvent, so it
can be easily recovered by a simple ltration. Subsequently,
modications of the catalytic amount of AQ-PHEMA were con-
ducted (entries 10–13, Table 1). The results showed that the
catalytic amount of AQ-PHEMA could be reduced to 1 mol%
(entry 12, Table 1). Next, control experiments were carried out
and the results demonstrated that both the catalyst and light
were necessary for this transformation (entries 14 and 15, Table
1). Furthermore, taking into account the thermal effect of the
purple LED light, the reaction was carried out at 50 ꢀC without
light. The results conrmed that no reaction occurred at all
(entry 16, Table 1). Next, the oxidation using AQ as catalyst was
investigated. The reaction was completed in 9 hours with high
chemoselectivity and excellent yield (entry 17, Table 1).
However, AQ was difficult to recover due to its solubility in
CH3OH. Finally, to test whether the bromine atom derived from
the initiator in the synthesis of PHEMA has some effect on the
reaction, the oxidation using PHEMA as catalyst was investi-
gated. No reaction took place (entry 18, Table 1). The above two
2 mg.) Yield determined by 1H NMR analysis of the crude reaction
mixture using CH2Br2 (1 mmol) as internal standard. Isolated yield
b
c
d
e
of 2a. The reaction was carried out without light. The reaction was
f
carried out at 50 ꢀC. The reaction was carried out using 1 mol% AQ
g
as catalyst. The reaction was carried out in the presence of 4 mg of
PHEMA.
results clearly demonstrated that the bromine atom did not play
an important role in this photocatalytic reaction and AQ was the
key catalyst in this transformation. Thus, Condition A (1 mol%
of AQ-PHEMA, CH3OH, purple LED, air (1 atm), and room
temperature) was considered as the optimized conditions for
further studies.
With the optimized reaction conditions in hand, the scope of
this photoreaction was explored (Table 2). Firstly, the electronic
effect of the aryl group in methyl aryl thioether was examined.
With strong electron donating groups, like methoxy, attached to
the aryl ring, excellent yields were obtained for the ortho-, meta-
and para-methoxyphenyl thioethers (2b–d). For weak electron
donating groups, like methyl, the corresponding product (2e)
could be generated in an excellent yield. Good reactivity was
also observed with halogen-attached substrates (2f–h). Then, we
tested substrates with strong electron withdrawing groups. The
results showed that substrates with aldehyde (2i), nitrile (2j),
methoxycarbonyl (2k) or triuoromethyl (2l) worked very well.
Notably, the naphthyl group was also tolerated under Condition
A (2m). Next, the effect of different alkyl groups was examined.
Reactants with ethyl, cyclopropyl and propynyl groups yielded
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the desired products (2n–p) in 90%, 91% and 68% isolated
yield, respectively. The reactivity of diaryl thioethers was also
investigated. Excellent chemoselectivity as well as excellent
yields were observed (2q–s). Finally, reactions with dialiphatic
thioethers were studied. Di-n-butyl thioether gave an excellent
yield of 2t, while tetrahydro-2H-thiopyran and tetrahy-
drothiophene led to slightly lower yields of 2u and 2v, respec-
tively. The above results indicated that this reaction showed very
good substrate scope and functional group tolerance.
Table 2 Photooxidation of thioethers into sulfoxides under Condition
Aa
Recycling experiments
To investigate the recyclability of AQ-PHEMA in this trans-
formation, 4-methoxyphenyl methyl thioether (1d) was selected
as the model substrate for the AQ-PHEMA recycling experi-
ments under Condition A. 1d could be completely consumed
aer 24 hours and the NMR yield of the product 4-methox-
yphenyl methyl sulfoxide (2d) was 99% in the rst cycle of the
photocatalytic reaction. Aer the rst cycle, the catalyst AQ-
PHEMA could be easily separated and recovered by simple
ltration and directly used for the second cycle. The following
reaction cycles were performed using the same procedure (for
the specic plan for the recycling experiments, see ESI†). The
results showed that AQ-PHEMA could be reused more than 20
times without signicant loss of the catalytic activity (Fig. 1).
At the end of the recycling experiments, the recovered AQ-
PHEMA (aer 23 cycles) was characterized by 1H NMR and GPC.
As shown in Fig. 2, no signicant changes were observed in the
1H NMR spectrum compared to the original AQ-PHEMA. The
molecular weight Mn,GPC and molecular distribution Mw/Mn of
the recovered AQ-PHEMA (aer 23 cycles) were very close to
those of the original AQ-PHEMA (Fig. 3). These results clearly
indicated that the structure of AQ-PHEMA did not change
during the whole catalytic procedure.
Proposed mechanism
Based on the Guo group’s outstanding work,9 literature prece-
dents59–64 and the cyclic voltammogram of AQ-PHEMA under
irradiation (see Fig. S2†), the following possible reaction
a
All reactions were carried out using 1 (1 mmol) and AQ-PHEMA
(1 mol%) in CH3OH (5 mL) irradiated by a purple LED light at rt
under air atmosphere. The isolated yield is reported.
Fig. 1 Recycling experiments for the photocatalytic reaction of 1d.
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radical cation 3 and a superoxide radical anion, and then 3
further interacted with the superoxide radical anion to obtain
intermediate 4. 4 reacted with another molecule of thioether 1
to form the nal sulfoxide product 2.
Conclusions
In conclusion, we successfully synthesized the polymeric
photosensitizer AQ-PHEMA by immobilizing the AQ functional
group onto the precursor PHEMA. AQ-PHEMA could be used as
a visible light sensitizer to catalyze the oxidation of thioethers
into sulfoxides under aerobic conditions. The reaction exhibi-
ted high efficiency and chemoselectivity with extensive func-
tional group tolerance. A possible mechanism was proposed.
The AQ-PHEMA catalyst could be easily recovered and reused
more than 20 times. The excellent photocatalytic properties,
easy separation, and recyclability revealed the great potential of
this catalyst in industrial applications and environment
protection.
Fig. 2 1H NMR spectra for AQ-PHEMA and the recovered AQ-PHEMA
(after 23 cycles) in d6-DMSO.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
Fig. 3 The GPC traces of AQ-PHEMA and the recovered AQ-PHEMA
(after 23 cycles).
We greatly acknowledge the nancial support from Shanghai
Scientic and Technological Innovation Project (18YF1428800).
mechanism was proposed as shown in Fig. 4. AQ-PHEMA was
excited under visible light irradiation and subsequently sensi-
tized oxygen to singlet oxygen which was more oxidative than
normal triplet oxygen. Singlet oxygen could take an electron
from the lone pair of electrons in sulde 1 to form a sulde
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