Cercosporin-bioinspired selective photooxidation reactions under mild conditions

Article abstract of DOI:10.1039/c9gc02270h

The development of an efficient system for selective oxidation of organic compounds to generate more valuable compounds with molecular oxygen is a significant challenge in industrial chemistry. Bioinspired by the ability of naturally occurring perylenequinonoid pigments (PQPs) to generate reactive oxygen species (ROS) upon photoirradiation, here we report that cercosporin, one of the perylenequinonoid pigments, can function as a cost-effective and environmentally friendly photocatalyst for a wide range of selective oxidations, including benzylic C-H bonds to carbonyls, amines to aldehydes, and sulfides to sulfoxides. All of the representative reactions proceeded smoothly with high efficiency under mild conditions. Owing to the use of inexpensive metal-free visible light-driven photocatalyst produced from microbial fermentation with cheap glucose as the starting material and the ease of handling, we expect that this developed method will be particularly attractive for many more applications in synthetic transformation.

Full text of DOI:10.1039/c9gc02270h

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DOI: 10.1039/C9GC02270H
Green Chemistry
ARTICLE
Cercosporin-Bioinspired Selective Photooxidation Reactions under
Mild Conditions
Jia Li,a Wenhao Bao,a Zhaocheng Tang,a Baodang Guoa Shiwei Zhang,b Haili Liu,a Shuping Huang,c
Received 00th January 20xx,
Accepted 00th January 20xx
Yan Zhang, *b and Yijian Rao*a
DOI: 10.1039/x0xx00000x
The development of an efficient system for selective oxidation of organic compounds to generate more valuable compounds
with molecular oxygen is a significant challenge in industrial chemistry. Bioinspired by the ability of naturally-occurring
perylenequinonoid pigments (PQPs) to generate reactive oxygen species (ROS) upon photoirradiation, here we report that
cercosporin, one of perylenequinonoid pigments, can function as a cost-effective and environmentally friendly photocatalyst
for a wide range of selective oxidations, including benzylic C–H bonds to carbonyls, amines to aldehydes, and sulfides to
sulfoxides. All of representative reactions proceeded smoothly with high efficiency under mild conditions. Owing to the use
of inexpensive metal-free visible light-driven photocatalyst produced from microbial fermentation with cheap glucose as
starting material, and the ease of handling, we expect that this developed method will be particularly attractive for much
more applications in synthetic transformation.
www.rsc.org/
related inorganic and organometallic counterparts,8 metal-free
photocatalytic system provides an alternative environmentally
friendly synthetic route in the selective oxidation. However, the
application of metal-free photocatalysts in selective oxidation is
somewhat less familiar.9
Introduction
Selective oxidation of organic compounds is one of the most
fundamental reactions in organic synthesis, as well as one of the
most significant challenges in industrial chemistry.1 Traditionally,
toxic or hazardous oxidizing agents, such as toxic metal oxides and
peroxides, in stoichiometric amounts were usually used for these
chemical transformations.2 Due to growing attention on the
environmental concerns, extensive efforts have been devoted to
develop cleaner synthetic strategies by using molecular oxygen as it
is the most economical and green terminal oxidant.3 However, owing
to the inherent triplet ground state conformation of oxygen, it is
difficult to be activated to oxidize the substrates,4 especially for the
inert C–H bonds. To overcome the high oxidation potential of the
substrates, a porous graphene/carbon nitride composite or precious
and toxic noble-metal-based catalysts at high temperature under
high oxygen pressure have been commonly employed for the
oxidation of organic compounds.5
As most of photocatalysis reactions are performed under
aerobic environment with water vapor, oxygen as well as water are
the important and widespread reaction species.10 Upon irradiation
with a photocatalyst, oxygen is able to be converted into chemical
reactive oxygen species (ROS), including singlet oxygen (1O2), the
hydroxyl radical (HO•), superoxide (O2•−), and peroxide (O22-).10 These
ROS can be important resources for advance oxidation processes in
multiple organic transformations,11 thus searching or designing a
versatile metal-free photocatalyst to efficiently generate ROS is the
critical step for a wide range of selective oxidations of organic
compounds. In fact, nature has evolved various oxygen-utilization
systems that molecular oxygen reacts spontaneously with many
organic compounds at or below room temperature in a free radical
chain process called autoxidation.12 For instance, plant pathogenic
fungi Cercospora species can produce cercosporin (CP),13 one of
natural product perylenequinonoid pigments (PQP), and works
together with molecular oxygen as a powerful “weapon” after
absorption of sunlight to infect plants and then obtain nutrients for
their propagation through the autoxidation process (Fig. 1). So far,
cercosporin together with other PQP, such as hypocrellin A, have
been widely investigated at aspects of photophysics, photochemistry
and photobiology owing to their excellent properties of
photosensitization.14 Cercosporin can be activated to singlet excited
state by visible light absorption and then partially excited-state
molecules convert into longer-lived triplet state through intersystem
crossing.14a The excited-state of cercosporin can then prompt two
favorable reactions, i.e., Type-I single electron transfer reaction (SET)
and Type-II energy transfer reaction (EnT) (Fig. 1a).13c, 15 In Type-I
Additionally, since photocatalysis has been recognized as a
useful routine and its applications are rapidly growing,6 metal-based
photocatalysis was also developed for the oxidation reactions.7 But
the use of precious and toxic metals significantly limit their industrial
applications. In fact, as metal-free photocatalysts usually have cheap
prices, broad availability and some other properties superior to their
a. Key Laboratory of Carbohydrate Chemistry and Biotechnology, Ministry of
Education, School of Biotechnology, Jiangnan University, Wuxi 214122, P. R.
China. E-mail: raoyijian@jiangnan.edu.cn.
b. School of Pharmaceutical Science, Jiangnan University, Wuxi 214122, P. R. China.
E-mail: zhangyan@jiangnan.edu.cn.
c. College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108, P. R. China.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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reaction, the excited-state of cercosporin directly reacts with Bioinspired by the ability of cercosporin to generate VRieOwSA,rttioclgeeOtnhliener
another reductive substrate to yield semiquinone anion radical, with its photochemical properties14a and phoDtOosI:t1a0b.1il0it3y9(/FCi9gG. SC10)2,1267w0He
which can react with other compounds causing oxidative damage or envisioned that this natural product PQP can be used as an effective
free radical. This photoinduced electron-transfer process is a “metal-free” organic photocatalyst for selective oxidative reactions.
predominant pathway in the radical formation. When molecular
In this study, cercosporin was obtained via microbial liquid
oxygen (O2) exists, the semiquinone anion radical can transfer fermentation using glucose as a very cheap starting material with a
electron to ground state oxygen (O2) to generate superoxide (O2•−), maximum of production,16 which is substantially higher than that of
whereby other active oxygen species such as hydroxyl radical (HO•) the previously reported methods.17 Then, it was applied as a metal-
and hydrogen peroxide (H2O2) can also be generated. These highly free photocatalyst for three representative selective oxidation
ROS then can oxidize other compounds. In Type-II reaction, the reactions, namely: visible-light driven selective oxidation of benzylic
excited state of cercosporin directly reacts with O2 to form highly C–H bonds to carbonyls, amines to aldehydes, and sulfides to
toxic singlet oxygen (1O2). Despite these remarkable properties of sulfoxides (Fig. 1b). All of them showed great conversion efficiency
photosensitization, there is no report on the use of cercosporin as a under mild conditions, indicating that cercosporin is a versatile green
visible-light driven-photocatalyst for selective aerobic oxidations. metal-free photocatalyst.
Fig. 1 The organic photocatalyst cercosporin. (a) Cercosporin (CP) as a phytotoxin and a photoredox catalyst through forming
reactive oxygen species upon photoirradiation. (b) Representative visible-light driven-photocatalysis of aerobic selective oxidation
reactions by cercosporin.
cercosporin (1 mol%) was added in 2.0 mL methanol. Next, a balloon
was purged with oxygen and fixed on the top of the schlenk tube.
The reaction mixture was stirred and irradiated by 15 W white CFL at
Experimental
General procedure for the selective oxidation benzylic C-H bonds
with cercosporin: In a dried schlenk tube, benzylic derivatives 1 (0.25
mmol), potassium bromide (0.2 equiv) and cercosporin (2 mol%) was
added in 2.0 mL methanol. Next, a balloon was purged with oxygen
and fixed on the top of the schlenk tube. The reaction mixture was
stirred and irradiated by 23 W white CFL at room temperature under
an atmospheric pressure oxygen atmosphere. When the reaction
was finished, the reaction mixture was washed with brine. The
aqueous phase was re-extracted with ethyl acetate. The combined
organic extracts were dried over Na2SO4, concentrated in vacuum,
and the resulting residue was purified by silica gel column
chromatography to afford the desired product 2.
General procedure for the selective oxidation of amines with
cercosporin: In a dried schlenk tube, amines 3 (0.25 mmol) and
cercosporin (1 mol%) was added in 2.0 mL methanol. Next, a balloon
was purged with oxygen and fixed on the top of the schlenk tube.
The reaction mixture was stirred and irradiated by 15 W white CFL at
room temperature under an atmospheric pressure oxygen
atmosphere. The reaction progress was monitored by GC analysis.
Photooxidation yields of 4 were calculated from GC measurements
using internal standards.
room temperature under an atmospheric pressure oxygen
atmosphere. When the reaction was finished, the reaction mixture
was washed with brine. The aqueous phase was re-extracted with
ethyl acetate. The combined organic extracts were dried over Na2SO4,
concentrated in vacuum, and the resulting residue was purified by
silica gel column chromatography to afford the desired product 7.
Results and discussion
The selective oxidation of C(sp3)-H bonds in alkanes to obtain higher-
value compounds is of great importance due to its broad applications
in fine chemical industry and pharmaceutical industry.18 Among
them, C(sp3)-H oxidation process is much more elaborate and can be
only achieved to limited extent, so it’s still a grand challenge to
improve this kind of reaction.19 Since C(sp3)-H bonds are both
thermodynamically stable and kinetically inert, traditional catalytic
strategies as well as photocatalytic strategies,20 suffer from limited
substrates scope, the use of expensive and complicated catalysts,21
and stoichiometric amounts of strong oxidants.22 Here, the
convenient and sustainable oxidative strategy with ROS derived from
cercosporin was first tested for this reaction to investigate its
photocatalytic effectiveness. Initially, we used ethylbenzene 1a as
the model substrate with 2 mol % cercosporin as the photocatalyst
General procedure for the selective oxidation of sulfides with
cercosporin: In a dried schlenk tube, sulfides 6 (0.25 mmol) and
2 | J. Name., 2012, 00, 1-3
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to evaluate the reaction parameters (Table 1, see also Table S1). The
With this optimal reaction condition, we nextVieexwpAlrotirceledOntlhinee
3
DOI: 10.1039/C9GC02270H
reaction was first performed in DMSO solvent under an oxygen scope of substrates with C(sp )-H bond and tested whether they
atmosphere at room temperature upon irradiation by 23W CFL were able to be converted into the corresponding ketones (Scheme
(Table 1), but the desired product was synthesized only in negligible 1). It showed that these reactions completed within 20–30 h and
amount (entry 1). Other solvents, such as toluene, DMF, CH3CN and gave the corresponding ketones (2a–2e, 2h–2w) and ester (2f, 2g) in
DCE were also tested with the similar catalytic activity (entries 2-5). moderate to good yields. The substrate scope included non-cyclic
However, when the solvent was changed to methanol, desired alkyl- (2a–2c), cyclic- (2d–2g) and aryl-substituted benzylic
product 2a was achieved in 20% yield after 40 hours (entry 6). Since methylenes substrates (2h–2w), which demonstrated the diversity of
t
the use of an additive together with the photocatalyst can normally this protocol. This oxidative reaction tolerated with aryl-Me, Bu, F,
enhances the reaction rate and chemoselectivity.6a-f Thus, our Cl and Br groups (2b, 2i–2t, 2u–2w). The C–Cl group could be further
subsequent effort was to screen a suitable additive to improve the modified to other functional groups. Compared with electron-
selective photooxidation of benzylic C(sp3)–H with cercosporin as a deficient alkylbenenes, electron-rich ones were more competent
catalyst. After screening a wide range of inorganic salts as additives substrates in this selective oxidation. However, substrates bearing
(entries 7-14), it showed that acetophenone 2a as a single product strong electron-withdrawing groups, like −CO2Me and −NO2, were
with 78% yield was obtained when 0.2 equiv. of KBr was added (entry unable to be activated.
7, see Supporting Information). Other perylenequinonoid
O
O
O
O
O
O
derivatives, such as hypocrellins A and B, were also examined as the
photocatalysts in this reaction with the same products, but in low
yields of 30% and 15%, respectively (entries 15 and 16). No reaction
proceeded in the absence of either cercosporin (entry 17), light
(entry 18) or even O2 (entry 19).
O
O
MeO
Br
2d, 73%
2e, 81%
2b, 65%
2f, 71%
Me
2c, 75%
2g, 75%
O
O
O
Me
O
Me
Me
2h, 75%
2i, 78%
2j, 77%
2k, 80%
Table 1 Optimization of the reaction conditions for the selective
oxidation of sp3 C–H bonds.
O
O
O
Me
O
tBu
Me
Me
tBu
Me
tBu
tBu
O
Cercosporin
2n, 79%
2l, 76%
2o, 76%
2m, 80%
(2 mol%)
O
O
O
O
solvent, additive, O2
room temperature, 23W CFL
tBu
1a
2a
tBu
F
F
Me
F
Conditionsa
Yield (%)b
trace
trace
trace
trace
5
2p, 82%
2q, 62%
2r, 60%
2s, 69%
Entry
1
O
O
O
O
DMSO
2
Toluene
F
F
Cl
Cl
tBu
Cl
Cl
3
DMF
2v, 58%
2t, 48%
2w, 72%
2u, 65%
4
DCE
Scheme 1 Cercosporin-photocatalyzed selective oxidation of sp3
C–H with visible light.
5
CH3CN
6
CH3OH
20
7
0.2 equiv. of KBr, CH3OH
0.2 equiv. of LiBr, CH3OH
0.2 equiv. of NaBr, CH3OH
0.2 equiv. of CsBr, CH3OH
0.2 equiv. of KF, CH3OH
0.2 equiv. of KCl, CH3OH
0.2 equiv. of KI, CH3OH
0.2 equiv. of KOtBu, CH3OH
0.2 equiv. of KBr, CH3OH
0.2 equiv. of KBr, CH3OH
0.2 equiv. of KBr, CH3OH, no catalyst
0.2 equiv. of KBr, CH3OH, no light
0.2 equiv. of KBr, CH3OH, N2
78
Having realized the cercosporin-photocatalyzed selective
oxidation of benzylic C–H with visible light, we next investigated the
cercosporin-generated ROS oxidative system to a more challenging
reaction, namely photocatalytic selective oxidation of amines to
aldehydes. Selective oxidation of amines is a very useful reaction to
get functional group interconversions in organic transformation.
Different products, such as imines, carbonyl compounds, amides and
nitriles, can be obtained depending on the amine-catalytic reaction
systems.7c, 23 However, many of these reactions still require harsh
reaction conditions and leave metal-containing wastes and products
with low selectivity. Particularly, as carbonyl compounds are the
most useful building blocks for a wide range of fine chemicals and
pharmaceuticals, there is ongoing demand for a highly efficient
approach to synthesize these compounds through the selective
oxidation of amines, which has not been realized with ROS by
photocatalysis before.24
8
26
9
25
10
11
12
13
14
15c
16d
17
18
19
20
30
24
35
29
30
15
trace
trace
trace
aAll reactions were carried out on a scale of 0.25 mmol of 1a
in 2 mL of solvent under 23W CFL for 30h. See also Table S1.
To determine the photocatalytic activity of cercosporin in the
selective oxidation of amines to aldehydes, initial investigations
focused on the oxidation of benzyl amine 3a using 1 mol %
cercosporin as the catalyst under an oxygen atmosphere at room
bYields of isolated product. 2 mol% of Hypocrellin A as the
c
photocatalyst. d2 mol% of Hypocrellin B as the photocatalyst.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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O
O
temperature with photoirradiation by 15 W CFL (Table 2, see also
Table S2). It shows that the solvents play an important role in the
oxidation reactions. The use of solvents such as CHCl3, THF and
CH3CN afforded a mixture of imines and aldehyde (Table 2, entries 1-
3), but CH3OH delivered the highest conversion (95%) and selectivity
(100%) without any further optimization after 8h at room
temperature, with no imine product 5a found in the reaction (Table
2, entry 4). No product was observed when DMSO or DMF was used
as the solvent (Table 2, entries 5-6). The control experiments showed
that trace product was detected in the absence of either cercosporin
(Table 2, entry 7), light (Table 2, entry 8) or even O2 (Table 2, entry
9). The amine scope was then explored (Scheme 2), showing that a
lot of amines containing different substituents were efficiently
converted in high yields to aldehydes when the optimized reaction
conditions were utilized. Substrates bearing electron-donating
substituents (4b-4f) gave relatively higher yields than those with
electron-withdrawing groups on the aromatic ring (4g-4l). The
reactions tolerated ortho-, meta- and para-substitutes, providing the
products in moderate to high yields. Furthermore, functional groups,
such as aromatic halides (4g-4k) were unaffected under the reaction
conditions, showing the advantages of cercosporin-generated ROS-
driven synthetic transformation.
O
O
View Article Online
DOI: 10.1039/C9GC02270H
OMe
4a, 95%
4d, 97%
4c, 92%
4b, 96%
O
O
O
O
O
Cl
O
tBu
F
4h, 80%
4g, 87%
4f, 89%
4e, 90%
O
O
O
O
Br
Cl
F3C
Cl
4k, 86%
4l, 79%
4j, 79%
4i, 86%
Scheme 2 Cercosporin-photocatalyzed selective oxidation of amines
with visible light.
Last, since sulfoxides are one of most important and valuable
building blocks utilized in organic synthesis as well as drug
candidates,25 the synthesis of sulfoxides has aroused great interests.
Traditionally, sulfoxides are synthesized through the oxidation of
sulfides using stoichiometric amounts of peracid or inorganic
oxidative reagents.26 Nowadays, they can also be synthesized
through a photocatalytic oxidation reaction.27 Given the significance
of these moieties, ROS oxidative system produced by cercosporin
was also applied to the synthesis of substituted sulfoxides from
sulfides. Different from other photocatalysis, selective oxidations of
sulfides to sulfoxides were realized in CH3OH using 1 mol %
cercosporin as the catalyst under an oxygen atmosphere at room
temperature upon photoirradiation by 15W CFL and no sulfone was
found (Table 3, see also Table S3). Diverse alkyl (7a-7f) and aryl
sulfoxides (7g-7m) were produced within 6h, in 80–95% yields
Table 2 Optimization of the reaction conditions for the selective
oxidation of amines.
Cercosporin
(1 mol%)
NH2
O
N
solvent, O2
room temperature, 15W CFL,
5a
3a
4a
t
(Scheme 3). This process was compatible with aryl-Me, OMe, Bu
Entry
Conditionsa
Yield (%)b
groups (7h-7k). Additionally, the Cl and Br group were well tolerated
in this reaction (7d, 7e, 7i, 7l, 7m), providing the possibility for further
modification. Importantly, the electron-withdrawing groups, like NO2
(7f), can also afford the desired sulfoxide products in excellent yields.
Surprisingly, other metal-free photocatalysts, such as Acr+-Mes,
rhodamine B and eosin Y, and metal-based photocatalysts, such as
Ru(bpy)3Cl2 and Ir(ppy)2bpy, were unable to deliver the desired
products in this reaction condition (Table 3), again, showing the great
advantages of cercosporin-generated ROS-driven synthetic
transformation. The reason could be that the absorption spectrum
of cercosporin overlapped with the emission spectrum of CFL to a
large extent compared with other catalysts (Fig. S2), as the catalytic
reactivity of a photocatalyst is mainly related to the extent of
absorption spectrum overlapped with the emission spectrum of light
source.
4a
5a
1
2
3
4
CHCl3
THF
60
59
49
95
23
31
29
0
CH3CN
CH3OH
5
6
7
8
9
DMF
trace
trace
trace
DMSO
trace
CH3OH, no catalyst trace
0
0
0
CH3OH, no light
CH3OH, N2
trace
trace
aAll reactions were carried out on a scale of 0.25 mmol of
3a in 2 mL of solvent under 15W CFL for 8h. See also Table
S2. Yields were calculated from GC measurements using
Table 3 Optimization of the reaction conditions for the selective
oxidation of sulfides.
b
internal standards.
O
O
O
photocatalyst (1 mol%)
S
S
S
CH3OH, O2, rt, 15 W CFL
6a
7a
8a
Entrya
Photocatalyst
Yield (%)b
7a
95
12
8a
1
2
Cercosporin
Acr+-Mes
0
trace
4 | J. Name., 2012, 00, 1-3
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3
4
4
5
Eosin Y
10
trace
trace
trace
trace
Figs. S3-S5) revealed that the reactions were totally inhibited in the
View Article Online
Rhodamine B
Ru(bpy)3Cl2
Ir(ppy)2bpy
trace
15
absence of light, indicating that continuouDs OirIr:a1d0i.a10ti3o9n/Cw9GitCh0v2i2s7ib0lHe
light was essential to the photocatalytic transformations. The
addition of the radical inhibitors 2,2,6,6-tetramethylpiperidine-1-
oxyl (TEMPO) or 1,4-benzoquinone significantly inhibited all three
transformations (Scheme 4a). Furthermore, the effects of singlet
oxygen scavengers, such as 1,4-diazabicyclo [2.2.2]-octane (DABCO)
and sodium azide, on the photo oxidative reactions were also
studied. They revealed that the photooxidations of benzylic C–H
bonds and amines proceeded without obvious inhibition, while the
photooxidative reaction of sulfides can be significantly suppressed
(Scheme 4a). Therefore, we propose that a radical pathway with a
single-electron transfer (SET) process should be involved in the
photocatalytic selective oxidation of benzylic C–H bonds and amines,
trace
aAll reactions were carried out on a scale of 0.25 mmol of 6a in
2 mL of CH3OH under 15 W CFL for 6h. See also Table S3. bYields
of isolated product.
O
O
O
O
S
S
S
O
S
S
Br
Cl
7a, 95%
7c, 94%
7e, 87%
7b, 90%
7d, 90%
O
S
O
O
S
O
S
S
Me
Cl
O2
N
MeO
Me
•−
1
7g, 90%
7h, 87%
7f, 80%
7i, 95%
while the presence of both O2 and O2 are responsible for the
photooxidation reaction of sulfides.
O
O
O
S
O
S
S
S
Me
The reaction mechanism of the ROS-mediated selective
oxidative reactions is summarized in Scheme 5. For the selective ROS
oxidation of benzylic C–H bonds, the photocatalyst cercosporin was
excited by white light irradiation to generate the excited species CP*
and then bromo radical was formed by SET from KBr to cercosporin,
tBu
Br
Br
Br
Me
Me
7j, 90%
7k, 86%
7l, 93%
7m, 92%
Scheme 3 Cercosporin-photocatalyzed selective oxidation of sulfides
퐸 ∗
which was supported by the comparison of 푟푒푑 (CP*/CP•−) = 1.87 V
with visible light.
•
-
vs SCE16 and (Br /Br ) = 1.79 V vs SCE.28 This SET process was then
followed by C–H hydrogen atom abstraction by the bromo radical at
the benzyl position to generate the alkane radical 9, which was
detected by GC-MS (Fig. S6). Then, a hydroperoxide intermediate 10
was formed through oxidation of alkane radical 9 by either O2 or O2
radical anion. The intermediate 10 was then either directly converted
to the desired ketone product 2 after losing one water molecule,6a,
퐸
TEMPO (1.5 equiv)
(a)
without 2h
72h
Cercosporin (2 mol%)
DABCO (1.5 equiv)
72h
2h (65% yield)
2h (60% yield)
Ph
Ph
CH3OH, 0.2 equiv KBr, O2
1h
NaN3 (1.5 equiv)
72h
room temperature, 23 W CFL
TEMPO (1.5 equiv)
72h
without 4a
Cercosporin (1 mol%)
20j
or to the benzyl alcohol derivative 11, which can further be
DABCO (1.5 equiv)
72h
4a (85% yield)
4a (78% yield)
Ph
NH2
CH3OH, O2
oxidized to ketone 2 verified by control experiment (Scheme 4b).
Meanwhile, the CP•− radical anion can then be oxidized by O2 to
regenerate the catalyst. Isotope labelling studies revealed that
carbonyl oxygen of 2 was originated from molecular oxygen but not
from CH3OH (Scheme 4c). Similarly, the photocatalytic oxidation of
amine was induced by CP, generating the iminium intermediate 12,
which was then transformed to the carbonyl compound 4 through
hydrolysis.29 The obvious solvent effect of the above reactions may
be related to the stability of the oxygen radical anion. This active
intermediate may have longer lifetime in methanol compared with
in other solvents due to its slower deactivation rate constant in
methanol,30 which facilitated its reaction through electron transfer
process.
3a
room temperature, 15 W CFL
NaN3 (1.5 equiv)
72h
TEMPO (1.5 equiv)
72h
8g (35% yield)
8g (28% yield)
8g (trace)
Cercosporin (1 mol%)
DABCO (1.5 equiv)
S
Ph
Ph
72h
NaN3 (1.5 equiv)
72h
CH3OH, O2
6g
room temperature, 15 W CFL
O
OH
(b)
(c)
Cercosporin (2 mol%)
CH3OH, 0.2 equiv KBr, O2
room temperature, 23 W CFL
11
2h
O
18O2, CH3OH or O2, CH3O18
Cercosporin (2 mol%)
H
using 18O2
>99% 18O incorporation
using CH3O18
0% 18O incorporation
H
0.2 equiv KBr
Different from the above two selective oxidations, both electron
transfer (ET) and energy transfer (EnT) may exist in the
photocatalytic oxidation of sulfide to sulfoxides.31 Employing EnT
process, sulfide radical cation 13 was generated, which can be
oxidized to sulfoxide 7. While 1O2 was produced by CP when
employing energy transfer pathway. Then, 1O2 could take one
electron from the lone pair electron of sulfide 6, forming radical
cation 14, which could react with another molecule of 6 to afford 7
as the product.
room temperature, 23 W CFL
2a
1a
Scheme 4 Control experiments.
Since all above experiments showed that oxygen (O2),
photoirradiation, and cercosporin as the selective oxidation system
are all essential for these photooxidative reactions with high activity
and selectivity. It motivated us to study the mechanistic details of the
reactions. The time profiles of the photooxidations (Tables S4-S6,
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DOI: 10.1039/C9GC02270H
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ARTICLE
Scheme 5 Proposed mechanism.
Acknowledgements
Conclusions
We thank for the National Key R&D Program of China
(2018YFA0901700), National Natural Science Foundation of
China (21703036), Natural Science Foundation of Jiangsu
Province (Grants No BK20160167), the Thousand Talents Plan
(Young Professionals), the Fundamental Research Funds for the
Central Universities (JUSRP51712B), Postdoctoral Foundation in
Jiangsu Province (2018K153C) and the National First-class
Discipline Program of Light Industry Technology and
Engineering (LITE2018-14) for the funding support.
In summary, we have documented perylenequinonoid derivative
cercosporin as a versatile costeffective, and environmentally friendly
photocatalyst by activating molecular oxygen (O2) for selective
oxidation of benzylic C–H bonds to carbonyls, selective oxidation of
amines to aldehydes, and selective oxidation of sulfides to sulfoxides
with high efficiency under mild conditions. The developed methods
utilize irradiation of low-cost household light and inexpensive
photocatalyst, which can be easily produced from microbial
fermentation with very cheap glucose as the starting material and
purified at a large scale. These advantages make this method
particularly attractive for much more applications with respect to
selective oxidation in a sustainable and environmentally friendly
manner. Meanwhile, this class of photoredox catalyst may be used
for other chemical reactions, awaiting further investigation.
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