Perylenequinonoid-catalyzed photoredox activation for the direct arylation of (het)arenes with sunlight

Article abstract of DOI:10.1039/c9ob00659a

Naturally occurring perylenequinonoid pigments (PQPs) have attracted considerable attention owing to their excellent properties of photosensitization. They have been widely investigated as an aspect of photophysics and photobiology. However, their applications in photocatalysis are yet to be explored. We report here that sunlight along with 1 mol% cercosporin, which is one of the perylenequinonoid pigments, catalyzes the direct C-H bond arylation of (het)arenes by a photoredox process with good regioselectivity and broad functional group compatibility. Furthermore, a gram-scale reaction with great conversions of substrates was achieved even by a cercosporin-containing supernatant without organic solvent extraction and purification after liquid fermentation. Thus we set up a bridge between microbial fermentation and organic photocatalysis for chemical reactions in a sustainable, environmentally friendly manner.

Full text of DOI:10.1039/c9ob00659a

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
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DOI: 10.1039/C9OB00659A
Organic & Biomolecular Chemistry
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Perylenequinonoid-catalyzed Photoredox Activation for the Direct
Arylation of (Het)Arenes with Sunlight
Shiwei Zhang,a,b† Zhaocheng Tang,a† Wenhao Bao,a Jia Li,a Baodang Guo,a Shuping Huang,c Yan
Received 00th January 20xx,
Accepted 00th January 20xx
Zhangb*and Yijian Raoa*
DOI: 10.1039/x0xx00000x
Naturally occurring perylenequinonoid pigments (PQPs) have attracted considerable attention owing to their excellent
properties of photosensitization. They have been widely investigated at aspect of photophysics and photobiology. However,
their applications in photocatalysis are yet to be explored. We report here that sunlight along with 1 mol % cercosporin,
which is one of perylenequinonoid pigments, catalyzes the direct C−H bond arylation of (het)arenes by a photoredox process
with good regioselectivity and broad functional group compatibility. Furthermore, a gram-scale reaction with great
conversions of substrates was achieved even by cercosporin-containing supernatant without organic solvent extraction and
purification after liquid fermentation. Thus we set up a bridge between microbial fermentation and organic photocatalysis
for chemical reactions in a sustainable, environmentally friendly manner.
www.rsc.org/
light absorption and then undergo energy transfer (EnT) and
electron transfer (ET),4c-f,6d-f,5 implying that these natural
products PQPs may be used as “metal-free” photoredox
catalysts for organic synthesis, but have never been
investigated.
Introduction
Visible light-driven photocatalysis is the cutting-edge of organic
synthesis and has emerged into a green and mild mean to
achieve unique chemical reactivity, as sunlight provides the
most sustainable and redundant energy source.1 Transition
metal-based photocatalysis, especially driven by ruthenium and
iridium polypyridyl complexes,2 has been widely investigated in
organic synthesis. Alternatively, metal-free organocatalysis has
also been studied using organic dyes or large organic
molecules,3 owing to their ability to absorbing the visible or
near-visible light to participate in photoinduced electron
transfer process, but their applications of catalytic reactivity in
organic synthesis is somewhat less familiar.
Naturally occurring perylenequinonoid pigments (PQPs,
Figure 1), such as cercosporin,4 hypocrellins A,5 elsinochrome C6
and phleichrome7 produced by endophytic fungi have aroused
considerable attention owing to their excellent properties of
photosensitization, which function as new potential
compounds in photodynamic therapy and photophysical
diagnosis.4c-f,6d-f,5 Particularly, cercosporin (CP), a phytotoxin
produced by members of the genus Cercospora, exhibits the
most active compound of PQPs against different cancer cell
lines with its excellent properties of photosensitization.4 These
photosensitizers can be activated to the excited state by visible
O
OH
O
OH
MeO
MeO
OH
OMe
OMe
HO
H3C
O
O
O
OH
MeO
MeO
O
OH
O
OH
Hypocrellin A
Cercosporin
Figure 1. Representatives of the naturally occurring mold
perylenequinones.
Arylated (het)arenes are widely used in materials science
because of their interesting optical and electronic properties, as
well as biomedical applications including peptide mimetics or
drugs.8 A variety of cross-coupling methods for the synthesis of
arylated (het)arenes have been well established, mainly
utilizing transition metal-mediated reactions between
prefunctionalized substrates and arenes or C−H arylations with
functionalization in only one coupling partner.9 Recent progress
in arylation reactions was accomplished with photoredox
catalysis,10 with notable contributions by Sanford,10c König,10j,
10m
Xiao10h and Ackermann,10g among others. The reported
aKey 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
methodologies are most efficient with electron-rich
heteroarenes (Scheme 1a),10j-n or by merging transition-metal
bSchool of Pharmaceutical Science, Jiangnan University, Wuxi 214122, P. R. China. E- and photoredox catalysis (Scheme 1b)10c-e with low
mail: zhangyan@jiangnan.edu.cn
10h
regioselectivities (Scheme 1c).10f,
However, the
cCollege of Chemistry, Fuzhou University, Fuzhou, Fujian 350108, P. R. China
†These authors contributed equally to this work.
photoinduced C−H arylation of arenes and heteroarenes,
especially for electron-deficient N-heteroarenes, with
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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ARTICLE
regioselectivity under mild conditions only by organic
Journal Name
To test its photocatalysis activity, cercosporin was used as
View Article Online
photoredox catalysis has thus far proven elusive. Thus the a photoredox catalyst to investigate the DdOirIe: c10t.C10–3H9/aCr9yOlaBt0i0o6n59oAf
development of more general and efficient organic photoredox- arene (1a) with aryl diazonium salt (2a). The reaction condition
catalyzed direct arylations of (het)aromatic bases is still a major was conducted by irradiating a mixture of 1a, 2a, and
challenge.
cercosporin (1 mol%) with sunlight at room temperature under
nitrogen atmosphere (Table 1, entry 1). It gave the
corresponding arylated product, showing that cercosporin
indeed facilitated this direct arylation. Next, this reaction
condition was optimized with various solvents, additives and
the amount of 2a. The arylated product was obtained with a
best yield at 58% when using an excess of 10 equiv of 1a (Table
1, entries 1-4) with dimethyl sulfoxide (DMSO) as solvent
(entries 5-9), of which the yield were comparable with previous
studies. The use of Cs2CO3 or K2HPO4 as an additional base had
no effect on the yield (entries 10-11). We also tested other
a
Organic photoredox catalyst for arylation of (hetero)arenes
Organic photocatalyst
Ar N2BF4
Ar
X
hv
X
X = O, S, N-Boc
Only electron-rich heteroarenes!
b
Dual catalysis transition metal/ photoredox catalyst for arylation of benzenes
DG
DG
Dual catalysis
Ar
Ar N2BF4
hv
commercially
available
perylenequinonoid
pigments,
c
Metal-photoredox catalyst for arylation of (hetero)arenes
hypocrellin A and hypocrellin B, giving comparable catalytic
activity (entries 12-13). But the use of common transition metal
photocatalysts [Ru(bpy)3]Cl2 or another organic photocatalyst
Eosin Y with sunlight resulted in lower yields (entries 14-15).
Besides sunlight, blue LED light slightly promoted the reaction
to a 63% yield (entry 16). However, irradiation of the reaction
mixture with either green LED light or CFL (compact fluorescent
lamp) delivered slightly low yields of the desired product
(entries 17-18). Control experiments confirmed that both of
light irradiation and cercosporin were necessary for significant
conversion to the product (entries 19-20).
Ar
Ar
or
Ar
Cyclometalated catalyst
or
Ar N2BF4
hv
Het
N
Ar
Het
N
low regioslectivity
d
this work
)
Metal-free arylation of (het)arenes (
Cercosporin
(1mol%)
Ar1
H
Ar1
Ar2
Ar2
N2BF4
Table
1
Optimization of the carbamoylation reaction
perylenequinonoid as organic photoredox catalyst
biosynthesis of cercosporin
conditionsa
N2BF4
Cercosporin
sunlight-induced C-H arylation
(1 mol%)
H
broad substrate scope
good regioselectivity
sunlight, solvent
EtO2C
room temperature, N2
gram-scale synthesis with fermentation supernatant
CO2Et
3a
1a
2a
Entry
1
Conditions
Yieldb (%)
45
1a (5 equiv), DMSO
Scheme 1. Photo-catalyzed C−H arylation with aryl diazonium
salt.
2
1a (10 equiv), DMSO
58
3
1a (15 equiv), DMSO
50
4
1a (20 equiv), DMSO
51
Here we report an unprecedented photoinduced direct
C−H arylations of arenes and heteroarenes catalyzed by one of
naturally occurring PQPs, cercosporin. Notable features of our
finding include (a) natural product cercosporin as an organic
photoredox catalyst for C−H functionalization, (b) good
regioselectivity and broad substrate scope, and (c) great
conversions of substrates in the gram-scale reaction using
5
1a (10 equiv), DMF
38
6
1a (10 equiv), THF
25
7
1a (10 equiv), EtOH
41
8
1a (10 equiv), CH3CN
1a (10 equiv), EtOAc
50
9
29
10
11
12c
13d
14e
15f
16g
17h
18i
19
20
1a (10 equiv), DMSO, Cs2CO3
1a (10 equiv), DMSO, KH2PO4
1a (10 equiv), DMSO
50
48
cercosporin-containing supernatant in
environmentally friendly manner (Scheme 1d).
a
sustainable,
53
1a (10 equiv), DMSO
50
1a (10 equiv), DMSO
43
1a (10 equiv), DMSO
40
Results and discussion
1a (10 equiv), DMSO, blue LED
1a (10 equiv), DMSO, green LED
1a (10 equiv), DMSO, CFL
1a (10 equiv), DMSO, no catalyst,
1a (10 equiv), DMSO, no light
63
Cercosporin was produced by a new isolated endophytic
fungus Cercospora sp. JNU001 strain with high yield through
liquid fermentation (see ESI). The photochemical and
electrochemical properties were characterized (see ESI), which
imply that cercosporin can undergo photoinduced electron
transfer (PET) to facilitate chemical reactions through an
oxidative or reductive quenching cycle.
51
45
20
trace
a All reactions were carried out on a scale of 2 mmol of 1a and 0.2 mmol
of 2a in 2 mL of solvent with sunlight for 16h under nitrogen atmosphere.
2 | J. Name., 2012, 00, 1-3
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b Yields of isolated product. c 1 mol% of hypocrellin A as photocatalyst. d
5a
X = O
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e
DOI: 10.1039/C9OB00659A
1 mol% of hypocrellin B as photocatalyst. 1 mol% of [Ru(bpy)3] Cl2 as
f
g
photocatalyst. 1 mol% of Eosin Y as photocatalyst. 5 W blue LED was
used. h 5 W green LED was used. i 5 W CFL bulb was used.
R
R
O
Using the optimized conditions, the reaction scope of the
sunlight-enabled C−H arylation of arenes 1 was explored
(Scheme 2) with electron-acceptor- (CO2Et, CN, NO2, COCH3),
electron donor-and neutral substituted (Me, OMe) diazonium
salts. All gave the corresponding products in moderate to good
yields. Notably, it was also observed that halogen-substituted
aryl diazonium salts successfully underwent C−H bond arylation
and kept the C−halogen bond intact (3ad and 3am), which is
useful for further synthetic elaboration. Furthermore, we also
found that this kind of C−H arylation were efficiently
functionalized with electron-rich heteroarenes 4 (Schemes 3),
such as furan (5aa-5ao), thiophene (5ba-5bn) and pyrrole (5ca-
5cl), in moderate to good yields and all arylations selectively
happened in 2-position of heteroarenes.
5af,
R = Cl (
62%)
63%)
78%)
5aa,
R = CO2Et (
R = COCH3
72%)
O
5ag,
R = Br (
5ab,
(
69%)
75%)
5ah,
5ac,
(
R = H (
R = CH3
R = OCH3
O2N
R = NO2
R = CN (
5am,
R = CH3
R = NO2
(
70%)
64%)
5ad,
67%)
60%)
5ai,
(
61%)
5aj,
67%)
5an,
(
5ae,
R = F (
(
O
O
O
5ak,
78%
5al,
5ao
66%
, 73%
5b
X = S
R
S
5bf,
R = Cl (
60%)
5ba,
R = CO2Et (
R = COCH3
63%)
5bb,
5bg,
R = Br (
69%)
70%)
(
82%)
S
5bh,
R = H (
5bc,
(
5bd,
R = NO2
R = CN (
61%)
Cercosporin
(1 mol%)
5bi,
R = CH3
(
62%)
75%)
5bk,
65%
Ar
N2BF4
Ar
H
R
Ar'
Ar'
5bj,
5be,
R = OCH3
(
63%)
R = F (
O2N
57%)
sunlight, DMSO
NO2
room temperature, N2
1
2
3
S
S
S
OMe
R
5bm,
75%
5bl,
58%
5bn,
67%
MeO
3aa
3aj
R = CO2Et ( , 71%)
R = CO2Et (
, 58%)
, 55%)
3ac
5c
X = NBoc
3ab
R = CN (
3ak
(
R = NO2
, 68%)
3al
, 62%)
3ar
, 52%
R = COCH3
(
, 60%)
, 53%)
3ae
R = COCH3
(
3ad
R = Cl (
3am
R = Cl (
3an
R = CH3 (
, 60%)
R
N
R = NO2
(
, 57%)
3af
, 57%)
Boc
3ao
( , 54%)
R = OCH3
(
, 51%)
R = OCH3
R
5ca,
R = CO2Et (
R = COCH3
63%)
5cd,
5ce,
5cf,
R = F (
R = Cl (
R = Br (
67%)
60%)
78%)
5cg,
R = H (
59%)
5ch,
O2N
O2N
5cb,
R = CH3
(
61%)
5ci,
70%)
(
55%)
R = OCH3
(
5cc,
R = CN (
63%)
3as
R = CO2Et (
, 67%)
3at
R = Cl ( , 62%)
3au
, 49%)
NO2
O2N
R = OCH3
(
3ag
, 56%
3ap
, 60%
R
N
N
O2N
N
O2N
Boc
Boc
5cl
, 60%
Boc
5cj
, 67%
5ck
, 72%
Scheme 3. Cercosporin-catalyzed C−H arylation of electron-rich
3ah
(
R = NO2
, 48%)
heteroarenes with sunlight.
3ai
R = Ph ( , 50%)
3aq
, 50%
3av
, 65%
Scheme 2. Cercosporin-catalyzed C−H arylation of arenes with
sunlight.
Normally the direct derivatization of electron-deficient N-
heteroarenes, such as pyridines, pyrimidines, oxazoles,
thiazoles and quinolines, are difficult to be achieved. In
particular, it has not been reported by commercial available
organic photoredox catalysts,10j-n such as Eosin Y, Acr+-Mes and
Rhodamine B. Surprisingly, we clearly showed that cercosporin-
driven photocatalysis was proved to be compatible with this
challenging arylation approach (Scheme 4) with an array of
pyridines (7aa-7ai), pyrimidines (7ba-7bb), benzothiazoles (7ca-
7cb), benzoxazoles (7da-7db) and quinolines (7ea-7ec). They
afforded the corresponding arylated derivatives in good to
excellent yields. Additional benefits of this arylation approach
are the tolerance of halides, multiple substitution, steric bulk,
and diversity in the electronic properties of the N-heteroarenes.
Next, we also explored the scope of aryldiazonium salts,
showing that they all afforded good yields irrespective of
Cercosporin
(1 mol%)
Ar
Ar
N2BF4
H
X
X
sunlight, DMSO
room temperature, N2
2
4
5
(X = O, S, NBoc)
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ARTICLE
whether they were electron-poor or electron-rich (Scheme 4).
Journal Name
Last, to demonstrate the synthetic utiVliietwy Artoicfle Otnhlinies
DOI: 10.1039/C9OB00659A
Increasing steric bulk of the diazonium salts had negative effect regioselective cercosporin-catalyzed photoredox direct
on the yields (7ai). Compared with the reported arylation of (het)arenes, the gram-scale reaction was carried
photocatalysts,10f, 10h cercosporin showed good regioselectivity out using cercosporin-containing fermentation supernatant.
in this direct C–H arylation of pyridine derivatives. With Direct use of fermentation supernatant as a catalyst avoids the
pyridine, only 2-arylated products were isolated (7aa-7ae). solvent-involved extraction and purification process of
With 2-substituted pyridines, only 6-arylated products were cercosporin. In fact, it also gave the desired products with
obtained (7af-7ai). With 2-substituted pyrimidines, 4-arylated comparable yields as purified cercosporin did (Table 2). Notably,
products were obtained (7ba and 7bb). Benzothiazole (7ca and only a very small proportion of chemical solvents (≤10%) were
7cb) and benzoxazole (7da and 7db) were also proved to used in this direct C-H arylation, thus providing a possibility for
produce the 2-arylated products in good yields. For quinolines, large scale synthesis of arylated (het)arenes in a sustainable,
both of 2-arylated and 5-arylated products were obtained in environmentally friendly manner.
ratio 5:1 (7ec). Only 5-arylated quinolines were obtained when
Table 2 Gram-scale synthesis with fermentation supernatant a
2-substituted quinolines were used as substrates (7ea, 7eb).
The reason that the excellent photocatalysis activity of
cercosporin may correlated with its structural similarity to the
odd alternant hydrocarbon phenalenyl (PLY), which is well-
EtO2C
O2N
Cl
EtO2C
known to stabilize the radical state due to the presence of a
nonbonding molecular orbital (NBMO).11 Thus it shows the
advantage of cercosporin as an organic photocatalyst with
broad substrate scope in this C–H arylation as compared with
other commercial available Eosin Y and Rhodamine B.
3aa
0.67g, 60%
3ae
0.51g, 52%
3at
3aj
0.70g, 65%
0.89g, 75%
N
N
S
O
S
Cl
Me
O2N
EtO2C
Me
7cb
5ac
0.67g, 71%
5ba
0.66g, 58%
7af
0.62g, 62%
0.76g, 68%
Cercosporin
aAll reactions were carried out on a scale of 50 mmol of
(het)arene and 5 mmol of 2 with 5 W blue LED under
nitrogen atmosphere catalyzed by fermentation
supernatant, details see Experimental. bYields of isolated
product.
(1 mol%)
Y
X
Y
Ar
N2BF4
Ar
H
n
n
sunlight, DMSO
room temperature, N2
X
n = 1, 2
2
6
7
7a
Heteroarene = Pyridine
Cl
N
Cl
Subsequently, we conducted mechanistic studies to gain
insights into the mode of action of cercosporin as a photoredox
catalyst in the direct arylation of (het)arenes (Scheme 5 and
ESI). First, the addition of the radical inhibitors TEMPO, or
hydroquinone, or the electron-transfer scavenger 1,4-
dinitrobenzene significantly inhibits the transformation, and
the TEMPO trapped intermediates 8 and 9 were detected by MS
analysis (Scheme 5-1 and ESI). This finding implies a radical
pathway that proceeds through a single-electron transfer (SET)
process should be involved. Next, intermolecular competition
experiments indicated that electron-poor aryl diazonium salts 2
can preferentially react with benzene when compared with
electron-rich one (Scheme 5-2 and ESI), which correlated with
their reduction potential. A notable intermolecular kinetic
isotope effect (KIE, kH/kD  1.13) was not observed (Scheme 5-3
and ESI), indicating a non-kinetically relevant C−H cleavage.
Last, the interaction between aryl diazonium salt 2a and
cercosporin has also been studied by means of UV−vis and
fluorescence spectroscopy (see ESI). It showed 2a has no
obvious interaction with ground state CP according to UV-vis
titration. It was found that the fluorescence intensity
diminished upon the gradual addition of 2a and the decrease
fitted well with Stern–Volmer plots. These results indicated the
electron transfer (ET) process between cercosporin and arryl
diazonium salt 2.
N
R
H3COC
N
7af
, 69%
N
7ag
, 58%
Cl
7aa
R = F ( , 67%)
Cl
N
7ab
R = Cl (
R = Br (
, 58%)
, 57%)
, 53%)
Cl
7ac
7ad
R = H (
Ph
7ae
( , 70%)
R = CH3
7ah
, 60%
7ai
, 48%
7b
7c
Heteroarene = benzothiazole
Heteroarene = pyrimidine
O2N
N
R
N
N
N
S
N
Cl
7ca
R = Cl (
, 60%)
Cl
7ba
, 63%
7bb
, 51%
7cb
R = CH3
(
, 61%)
7d
Heteroarene = benzoxazole
N
7e
Heteroarene = quinoline
CH3
R
O
Cl
N
7da
R = CO2Et ( , 59%)
7db
R = Cl (
, 43%)
7ec
, 58%, 2-aryl/5-aryl, 5:1
Me
CH3
N
N
Cl
Cl
Cl
7ea
, 60%
7eb
, 53%
Scheme 4. Cercosporin-catalyzed C−H arylation of electron-
deficient heteroarenes with sunlight.
4 | J. Name., 2012, 00, 1-3
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ARTICLE
(1) effect of radical inhibitor
Conclusions
View Article Online
DOI: 10.1039/C9OB00659A
Cercosporin
(1 mol%)
Cl
N2BF4
Cl
In summary, we reported a new class of organic photoredox
catalyst, natural occurring perylenequinonoid pigment, for
photoinduced C−H direct arylations of (het)arenes with sunlight
under mild conditions. This C−H direct arylation is compatible
with a wide array of functional groups with excellent
regioselectivity and broad substrate scope. Mechanistic studies
provided strong support for an efficient SET process. The use of
this new class of photoredox catalyst for other chemical
reactions awaits further investigation.
H
S
S
Additive (2.0 equiv)
standard reaction conditions
2d
4b
5bf
Additive
TEMPO
5bf
Yield of
trace
Cl
TEMPO
8
trace
Hydroquinone
trace
1,4-Dinitrobenzene
TEMPO
Cl
S
9
(2) competition experiment
N2BF4
EtO2C
Cercosporin
(1 mol%)
Conflicts of interest
H
3aa
, 35%
There are no conflicts to declare.
sunlight, DMSO
room temperature, N2
CO2Et/OMe
2a/2e
(each 1.0 equiv)
1a
MeO
3aa 3ag
:
= 1.6:1
3ag
3aa
, 22%
Acknowledgements
(3) KIE study
We thank for the National Natural Science Foundation of China
(21703036), Natural Science Foundation of Jiangsu Province
(Grants No BK20160167), the Thousand Talents Plan (Young
Professionals), Jiangsu Specially-Appointed Professor Program
(1016010241160390) and the Fundamental Research Funds for
the Central Universities (JUSRP51712B), the National First-class
Discipline Program of Light Industry Technology and
Engineering (LITE2018-14) for funding support.
N2BF4
EtO2C
Cercosporin
(1 mol%)
D6/H6
D
sunlight, DMSO
room temperature, N2
D
D
D
CO2Et
1a/[D]6-1a
2a
KH/KD = 1.13 : 1
D
EtO2C
[D]5-3aa
Scheme 5. Mechanistic experiments.
Based on the above observations and literature reports,10a- Notes and references
n
a plausible mechanism for this photoreaction is proposed
1.
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intermediate 11, which was further oxidized by the cercosporin
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N2BF4
2.
3.
-
R
N2
BF4
R
10
H
2
CP*
CP
R
N2BF4
1
11
R
CP
2
R
H
R
12
10
R
3
Scheme 6. Proposed mechanism for cercosporin-catalyzed C–H
arylation of (het)arenes.
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J. Name., 2013, 00, 1-3 | 5
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