Discovery and characterization of a novel perylenephotoreductant for the activation of aryl halides

Article abstract of DOI:10.1016/j.jcat.2021.05.003

To develop a photocatalyst with catalytical activity for substrates with low reactivities is always highly desired. Herein, based on the principle of structure–property relationships, we rationally designed the natural product cercosporin, the naturally occurring perylenequinonoid pigment, to develop a novel organic perylenephotoreductant, hexacetyl reduced cercosporin (HARCP), through structural manipulation. Compared with cercosporin, HARCP shows prominent electrochemical and photophysical characteristics with greatly improved photoreductive activity, fluorescence lifetime and fluorescence quantum yield. These properties allowed HARCP as a powerful photoreductant to efficiently realize a series of benchmark reactions, including photoreduction, alkoxylation and hydroxylation to construct C–H and C–O bonds using aryl halides as substrates under mild conditions, all of which have never been achieved by the same photocatalyst. Thus, this study well supports the notion that the principle between structural manipulation and photocatalytic activity is of great significance to design customized photocatalysts for photoredox chemistry.

Full text of DOI:10.1016/j.jcat.2021.05.003

Journal of Catalysis 399 (2021) 111–120
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Discovery and characterization of a novel perylenephotoreductant
for the activation of aryl halides
Min Li a,1, Jia Li b,1, Baodang Guo a, Xuanzhong Liu a, Zhenbo Yuan a, Yawen Wu a, Huimin Yin c,
Shuping Huang c, , Yan Zhang d, , Yijian Rao a,
⇑
⇑
⇑
a Key Laboratory of Carbohydrate Chemistry and Biotechnology, Ministry of Education, School of Biotechnology, Jiangnan University, Wuxi 214122, PR China
b State Key Laboratory of Radiation Medicine and Protection, Collaborative Innovation Center of Radiological Medicine of Jiangsu Higher Education Institutions, Soochow
University, Suzhou 215123, PR China
c College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108, PR China
d School of Pharmaceutical Science, Jiangnan University, Wuxi 214122, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
To develop a photocatalyst with catalytical activity for substrates with low reactivities is always highly
desired. Herein, based on the principle of structure–property relationships, we rationally designed the
natural product cercosporin, the naturally occurring perylenequinonoid pigment, to develop a novel
organic perylenephotoreductant, hexacetyl reduced cercosporin (HARCP), through structural manipula-
tion. Compared with cercosporin, HARCP shows prominent electrochemical and photophysical character-
istics with greatly improved photoreductive activity, fluorescence lifetime and fluorescence quantum
yield. These properties allowed HARCP as a powerful photoreductant to efficiently realize a series of
benchmark reactions, including photoreduction, alkoxylation and hydroxylation to construct C–H and
C–O bonds using aryl halides as substrates under mild conditions, all of which have never been achieved
by the same photocatalyst. Thus, this study well supports the notion that the principle between structural
manipulation and photocatalytic activity is of great significance to design customized photocatalysts for
photoredox chemistry.
Received 19 March 2021
Revised 20 April 2021
Accepted 3 May 2021
Available online 12 May 2021
Keywords:
Cercosporin
Organic photocatalyst
Structure-property relationship
Photoreductant
Ó 2021 Elsevier Inc. All rights reserved.
1. Introduction
Previously, bioinspired by the great photosensitive properties of
the naturally occurring perylenequinonoid pigment cercosporin
Photocatalysis has attracted a lot of research interests and been
widely utilized in organic and pharmaceutical synthesis to realize
many challenging chemical transformations under mild conditions
[1–7], in which the targeted selection of specific photocatalyst is
undoubtedly the essential step [8–13]. Currently, metal-based
photocatalysts [14–17], particularly, ruthenium and iridium-
based photocatalysts, have extensively been investigated because
of their excellent photophysical properties and photocatalytic
activities. However, in view of the elevated costs and environmen-
tal issues [18,19], the use of precious metal phototcatalysts for
chemical transformations is not practically applicable in chemical
industry. As a consequence, more and more effort has been focused
on developing novel organic photocatalysts as a sustainable alter-
native for photocatalysis [20–23].
(CP, Fig. 1) secreted by fungi Cercospora species, with the ability
to efficiently generate reactive oxygen species through energy
transfer (EnT) or electron transfer (ET) process in the presence of
sunlight [24–27], we have developed cercosporin as an efficient
organic photocatalyst, and realized a series of photocatalytic trans-
formations including oxidation, cycloaddition and cross-coupling
reactions [28–33]. However, owing to its photosensitive properties
as the result of evolution with a specific function for fungi Cer-
cospora species, this natural product CP still might have many
restrictions as a versatile photocatalyst, including poor redox activ-
ities, especially reductive activity, short fluorescence lifetime and
low fluorescence quantum yield [28–33], which may limit its
applications in a broad range of substrates with low reactivities.
Aiming to design a novel organic photocatalyst with comparable
or even better photocatalytic activity than current well-known
transition metal-based photocatalysts, we accordingly questioned
whether CP could be structurally modified and then provided pow-
erful redox properties as a versatile organic photocatalyst. In fact,
structural modification has been proved to be a power method to
⇑
Corresponding authors.
E-mail addresses: huangshp@fzu.edu.cn (S. Huang), zhangyan@jiangnan.edu.cn
(Y. Zhang), raoyijian@jiangnan.edu.cn (Y. Rao).
1
These authors contributed equally.
https://doi.org/10.1016/j.jcat.2021.05.003
0021-9517/Ó 2021 Elsevier Inc. All rights reserved.

M. Li, J. Li, B. Guo et al.
Journal of Catalysis 399 (2021) 111–120
Fig. 1. Structural manipulation of CP and its applications on mechanistically distinct and challenging photocatalytic reactions.
red
1=2
ꢀꢁ
develop organic photocatalysts for specific catalytic purposes [34–
39].
ment of its photoreductive activity (from
E
(CP/CP
) =
red
1=2
-0.46 V vs SCE to E (HARCP/HARCP ꢀꢁ) = ꢁ1.43 V vs SCE), longer
Indeed, in view of the structure of CP, many reaction sites could
be potentially modified (Fig. 1). Various electron-donor groups (e.g.
CP-methoxy, at positions 4 and 9) [40] and electron-acceptor
groups (e.g. CP-acetate, at positions 14 and 17) [41,42] could be
introduced according to the reactivity of each site in CP. Addition-
ally, the ring-opening and subsequent modifications of the charac-
teristic seven-membered cyclic ether (pre-CP, at positions 6 and 7)
could also be another option [43]. However, these structural mod-
ifications did not show significant changes in UV–Vis absorption
and fluorescence emission spectra (Fig. S1), more importantly,
and no significant improvement of their photocatalytic activities
(see Supporting Information). To our surprise, a fascinating phe-
nomenon that the solution color was rapidly changed from red to
bright green (Fig. S2) and gave a more than 250-folds increase of
the fluorescence intensity (Fig. S1) when CP was modified at posi-
tions 3 and 10 by the reduction of magnesium powder, was
appeared. We speculated that this intense fluorescence change
was the result of the conversion of the perylenequinonoid core of
CP to perylene phenol. This notion is highly supported by the supe-
excited state life (from 1 ns to 4 ns) and higher fluorescence quan-
tum yield (from 0.33% to 58%) with excellent photostability (Fig. S3
and S4, see Supporting Information). Owing to these great
improvements of the photophysical and electrochemical proper-
ties of HARCP, it allowed us to realize a series of challenging bench-
mark reactions, including photoreduction, alkoxylation and
hydroxylation using aryl halides with relatively low reactivities
as substrates (Fig. 1). With its strong photoreductive activity, many
C–H and C–O bonds were efficiently constructed by activating the
C–X (Br or Cl) bonds under mild conditions. It is worth noting that
all above chemical reactions were achieved by the same photocat-
alyst for the first time, indicating that this novel HARCP can func-
tion as an excellent and versatile organic photocatalyst with
comparable photocatalytic activities to current known best transi-
tion metal-based photocatalysts [49–51].
2. Materials and methods
rior physical properties of perylene because of its extended
p-
2.1. Materials and instrumentations
conjugation and narrow HOMO–LUMO gap [44,45]. Surprisingly,
perylene is little studied, probably due to its poor solubility and
difficulties in functionalizing perylene core directly [46–48]. Com-
pared with perylene, the existence of natural substituents in CP
causes the helix distortion between two naphthalene rings due
to the spatial tension, indicating that the intermolecular accumula-
tion is no longer tight under the oscillation of the flexible chain,
which greatly increases the solubility [48]. Moreover, these sub-
stituents provide a large number of active sites for further modifi-
cation, thus solving the problem of structural diversity. Given the
advantages of CP and the fascinating phenomenon we observed,
it motivates us to systematically investigate its molecular mecha-
nism, and then to investigate whether this type of CP derivatives
could function as versatile photocatalysts to realize a wide range
of challenging chemical reactions.
Aryl halides and all other commercially available reagents and
solvents were used without further purification unless otherwise
stated. Cercosporin was biosynthesized by a cercosporin producing
strain in our laboratory [28–33].
Thin-layer chromatography was performed using silica gel
plates F254. Visualization was accomplished with short wave-
length UV light (254 nm) and UVA light (366 nm) sources. 1H
and 13C NMR spectra were recorded on Bruker AV400 (400 MHz)
spectrometer in CDCl3 with internal solvent signals (for 1H and
13C) as reference (7.26 and 77.2 for CDCl3 respectively). Data for
13C NMR were reported in terms of chemical shift and no special
nomenclature was used for equivalent carbons. High resolution
mass spectra (HRMS) were recorded on Waters Xevo G2 Q-TOF
instrument. UV–Vis and fluorescence measurements were per-
formed with Hitachi U-3900 spectrophotometer and F-7000 spec-
trofluorometer. The fluorescence lifetime was measured on
HORIBA Instruments FL3-111 steady state transient fluorescence
spectrometer. Cyclic voltammetry (CV) was performed using an
CHI600E electrochemical workstation: Au wire (u = 1.6 mm) sealed
in a Teflon jacket as working electrode, Pt wire as the counter elec-
trode, Ag/AgCl (KCl, 3 M) electrode as the reference electrode. Scan
Combined with photophysical and electrochemical assays,
including UV–Vis absorption spectra, fluorescence emission spec-
tra, lifetime of fluorescence, quantum yield of fluorescence, cyclic
voltammetry analysis as well as TD-DFT studies, herein, we suc-
cessfully developed
a novel organic photocatalyst, hexacetyl
reduced cercosporin (HARCP), by structural manipulation of CP
(Fig. 1). Compared with CP, HARCP shows a significant improve-
112

M. Li, J. Li, B. Guo et al.
Journal of Catalysis 399 (2021) 111–120
rate: 50 mV sꢁ1 (in the range –2 to + 2 V). Et4NBF4 (0.1 M in MeCN)
was used as the supporting electrolyte. Household CFLs (compact
fluorescent lamps) were used as the light sources. The intensity
of irradiation was measured by a FZ-A radiometer (Photoelectric
Instrument Factory of Beijing Normal University) equipped with
a 400–1000 nm sensor. The sample was placed at an approximate
distance of 2 cm to the lamp. The light intensity was measured as
follows: 20.6 mW/cm2 for 15 W CFL, 32.5 mW/cm2 for 23 W CFL,
and 33.3 mW/cm2 for 32 W CFL.
was cooled to 0 °C in an ice bath, and the mixture of iodomethane
(44.7 equiv, 278
lL, measured by an eppendorf pipet) and DMF
(1 mL) was slowly dropped into the reaction tube with a syringe,
and stirred at 50 °C for 4 h. When the reaction finished, the reac-
tion mixtures were quenched with water (10 mL) and extracted
with ethyl acetate (3x 10 mL). The organic phase was washed with
brine (3x 30 mL), dried over Na2SO4 and then concentrated in vac-
uum. The resulting sample was purified by silica gel column chro-
matography to afford TMERCP.
2.2. Synthesis of CP derivatives
2.3. Theoretical calculations of CP and HARCP
CP-methoxy was obtained by the methylation of CP with iodo-
methane. CP (0.1 mmol, 53.4 mg) and potassium carbonate (10
equiv, 138 mg) were firstly added into the reaction tube at 0 °C
in an ice bath, and then the mixture containing iodomethane
All calculations were performed by using Gaussian 09 program
[52]. B3LYP density functional and 6-31G* basis sets were used to
perform geometrical optimizations of CP and HARCP. The excited-
state properties of these molecular systems were calculated by
time-dependent density functional theory (TD-DFT) using B3LYP
functional and 6-31G* basis sets.
(44.7 equiv, 278
lL, measured by an eppendorf pipet) and DMF
(1 mL) was slowly dropped into the reaction tube with a syringe
under nitrogen atmosphere. The reaction mixtures were stirred
at 50 °C for 4 h. When the reaction finished, the reaction mixtures
were quenched with water (10 mL) and extracted with ethyl acet-
ate (3x 10 mL). The organic phase was washed with brine (3x
30 mL), dried over Na2SO4 and then concentrated in vacuum. The
resulting sample was purified by silica gel column chromatography
to afford CP-methoxy.
CP-acetate was synthesized by the reaction of CP with acetic
anhydride. CP (0.037 mmol, 20 mg) and acetic anhydride
(0.4 mL) were charged into the reaction tube and reacted at
55 °C for 24 h under nitrogen atmosphere. When the reaction fin-
ished, the reaction mixtures were quenched with water (5 mL) and
extracted with ethyl acetate (3x 10 mL). The organic phase was
washed with brine (3x 30 mL), dried over Na2SO4 and then concen-
trated in vacuum. The resulting sample was purified by silica gel
column chromatography to afford CP-acetate.
2.4. Typical experimental procedure for the photoreduction of aryl
halides catalyzed by HARCP
In a 10 mL schlenk tube with a magnetic stirring bar, aryl halide
1 (0.1 mmol), HARCP (10 mol%), DIPEA (8 equiv) were dissolved in
acetone (1 mL). The reaction mixtures were then irradiated with
23 W CFL (at approximately 2 cm away from the light source)
under nitrogen atmosphere. When the reaction finished, the sol-
vent in the reaction was removed with rotary evaporation and
yields of photoreduction products 2 were calculated by 1H NMR
with hexamethyldisiloxane as the internal standard.
Pre-cercosporin was biosynthesized from microbial fermenta-
tion with cheap glucose as starting material [43]. Briefly, pre-
cercosporin was produced by liquid fermentation (500 mL) using
2.5. Typical experimental procedure for the alkoxylation of aryl halides
with alcohols catalyzed by HARCP
the Cercospora sp. JNU001
silica gel column chromatography after extraction by ethyl acetate
(3x 500 mL). Generally, when the Cercospora sp. JNU001 CTB9
mutant strain was cultivated in continuous light with S-7 culture
medium at pH = 8.5, a maximum of pre-cercosporin production
at 30 mg L-1 was obtained as dark red solid.
HARCP and HiBRCP (hexaisobutyryl reduced cercosporin) were
obtained by modifying CP with magnesium (Mg) as the reducing
agent and then followed by acylation with acetic anhydride or
isobutyric anhydride, respectively. CP (0.094 mmol, 50 mg), DMAP
(12.2 equiv, 140 mg) and Mg (33 equiv, 75 mg) were charged into
the reaction tube, followed by adding the corresponding anhydride
DCTB9 mutant strain, and purified by
In a 10 mL schlenk tube with a magnetic stirring bar, aryl halide
1 (0.2 mmol), HARCP (1.2 mol%), NiSO4ꢂ6H2O (10 mol%), 4,40-dime
thoxy-2,20-bipyridine (10 mol%), DIPEA (1.5 equiv), alcohol 3 (30
equiv) were dissolved in CH3CN (2 mL). The reaction mixtures were
then irradiated with 23 W CFL (at approximately 2 cm away from
the light source) under nitrogen atmosphere. When the reaction
finished, the reaction mixtures were quenched with water (5 mL)
and extracted with ethyl acetate (3x 10 mL). The organic phase
was washed with brine (3x 30 mL), dried over Na2SO4 and then
concentrated in vacuum. The resulting residue was purified by sil-
ica gel column chromatography to afford the desired product 4.
D
(500
500
l
L). After stirring for 2 h at the room temperature, another
lL of anhydride was added and continued to stir for another
2 h. When the reaction finished, the residual Mg was removed with
filtration. The resulting filtrate was quenched with water (10 mL)
and extracted with ethyl acetate (3x 10 mL). The organic phase
was washed with brine (3x 30 mL), dried over Na2SO4 and then
concentrated in vacuum. The resulting sample was purified by sil-
ica gel column chromatography to afford the corresponding HARCP
or HiBRCP.
The synthesis of TMERCP (tetramethyl reduced cercosporin)
includes two steps: reduction and methylation. Different from
the synthesis of CP-methoxy, sodium hypodisulfite was chosen as
the reductant. CP (0.1 mmol, 53.4 mg), sodium hypodisulfite (5
equiv, 87 mg), potassium carbonate (20 equiv, 276 mg) were added
into the reaction tube, and the mixed solvent (ether: water = 1 mL:
0.1 mL) was added to the reaction tube, and then stirred for 48 h at
room temperature under nitrogen atmosphere. Then the reaction
2.6. Typical experimental procedure for the hydroxylation of aryl
halides with water catalyzed by HARCP
In a 10 mL schlenk tube with magnetic stirring bar, aryl halide 1
(0.2 mmol), HARCP (1.2 mol%), NiSO4ꢂ6H2O (10 mol%), 4,40-dime
thoxy-2,20-bipyridine (10 mol%), DIPEA (1.5 equiv), H2O (40 equiv)
were dissolved in the mix solvent (CH3CN: DMF = 1: 1, 2 mL). The
reaction mixtures were then irradiated with 23 W CFL (at approx-
imately 2 cm away from the light source) under nitrogen atmo-
sphere. When the reaction finished, the reaction mixtures were
quenched with water (5 mL) and extracted with ethyl acetate (3x
10 mL). The organic phase was washed with brine (3x 30 mL), dried
over Na2SO4 and then concentrated in vacuum. The resulting resi-
due was purified by silica gel column chromatography to afford the
desired product 5.
113

M. Li, J. Li, B. Guo et al.
Journal of Catalysis 399 (2021) 111–120
the probability of electron transfer between photocatalyst and sub-
strate, which potentially drives the reaction easier to occur [20].
As the redox potentials of ground state and excited state of a
photocatalyst determine whether a photocatalytic reaction is actu-
ally able to be carried out or not, and which photocatalytic route
the reaction will take place, through either the oxidative or reduc-
tive quenching cycles [20], we next did cyclic voltammetry analy-
3. Results and discussion
3.1. Structural modifications of CP
To develop a novel organic photocatalyst with great photophys-
ical properties, natural product CP was structurally modified with
diverse substitutions. CP-methoxy and CP-acetate were synthe-
sized through the etherification [40] and esterification of hydroxyls
[41,42]. The seven-membered ring opening product pre-CP was
obtained by microbial fermentation [43]. Although there were a
few changes in their photophysical properties (Fig. S1), no signifi-
cant change in the photocatalytic activity was observed (see Sup-
porting Information). To our delight, more than 250-folds
increase of the fluorescence intensity was observed along with
the structural modification from perylenequinonoid core to pery-
lene phenol (Fig. S1), which needed further acetylation with acetic
anhydride because of its instability. We reasoned that this phe-
nomenon was probably induced by changes of photophysical and
electrochemical properties of this CP derivative, thus, prompting
us to unravel its molecular mechanism and then investigate
whether this version of CP derivative can function as a novel pho-
tocatalyst with higher photocatalytic activity than CP. In this study,
other modifications in the same manner using different groups
including donor group (methoxy, TMERCP) and acceptor group
(isobutyrate ester, HiBRCP) were further synthesized. However,
we did not observe obvious increase of fluorescence intensity no
matter what substituent was modified compared with HARCP
(Fig. S2). Considering the low yields and the separation difficulties
of HiBRCP and TMERCP, therefore, we here only focus on the most
easily realized acetylation product HARCP, which was obtained in
96% yield, to systematically study its photophysical properties
and photocatalytic activities.
sis to obtain the redox potentials of HARCP (Fig. 2c and S7).
red
Compared with CP (E (CP/CP ꢀꢁ) = -0.46 V vs SCE) [33], the reduc-
1=2
tion potential of HARCP was dramatically improved through this
red
structural modification (E (HARCP/HARCP ꢀꢁ) = -1.43 V vs SCE).
1=2
The reason for the above improvement are the simultaneous
changes of HOMO energy level (from ꢁ5.77 eV to ꢁ5.49 eV) and
LUMO energy level (from ꢁ3.24 eV to ꢁ2.51 eV) based on the
DFT theoretical calculations of frontier molecular orbitals for CP
and HARCP, respectively (Fig. 2d, Table S2). Especially, the signifi-
cant change of LUMO energy level reduced the electron affinity of
the molecule, which made electrons at infinite distance difficult to
fill in the LUMO orbital, and then results in the reduction potential
of HARCP toward a more negative direction [53,54]. Based on the
calculated energy levels of the first singlet excited S1 state of CP
and HARCP by TD-DFT (see Supporting Information), their redox
potentials of the excited states could be calculated according to
‘‘Gibbs Energy of Photoinduced Electron Transfer” [55] as described
in Fig. 2d (The calculation process was described at Supporting
Information). We then speculated that the remarkable improve-
ment of the ground redox potentials of HARCP made it superior
in more challenging photoredox reactions as a novel and versatile
photocatalyst than CP. Therefore, we next selected mechanistically
distinct and challenging reactions with aryl halides as substrates to
evaluate the photocatalytic activity of HARCP.
3.3. Applications of HARCP on photoreduction of aryl halides
Previously, we have shown that unmodified CP has the photo-
catalytic ability to perform C–C bond-forming arylation through
the consecutive PET process to overcome the photocatalytic limita-
tion of CP for photoreductive activation of aryl halides (Fig. 3a)
[29], but the stability of CP radical anion was severely affected
by solvent polarity and hydrogen-bond donor, which greatly lim-
ited its wide applications of this consecutive PET in photocatalysis
(Fig. 3a). As a fact, the photoreduction of aryl halides was of low
3.2. Photophysical and electrochemical properties of HARCP
Generally, to design or customize a novel photocatalyst, a set of
photophysical properties including the lowest-energy absorption
maximum kmax, the lifetime of fluorescence sf, the fluorescence
quantum yield Uf, and electrochemical properties (redox poten-
tials) should be carefully considered as they determine its photo-
catalytic reactivity in photoinduced electron transfer (PET)
process [20]. As a visible-light photocatalyst, the prerequisite is
to have a strong absorption capacity in the UV–Vis region, thus
we first did the measurement of kmax of HARCP. It showed that kmax
of HARCP was centered at 455 nm in CH3CN (Fig. 2a, red). Accord-
ing to the UV–Vis absorption spectra, its excellent photon absorp-
tion ability of HARCP in the UV–Vis range and its photostability
(Fig. S3 and S4) provide the prerequisite as a good organic photo-
catalyst even if a certain blue shift was observed, compared with
CP (kmax = 476 nm, Fig. 2a, black).
Since we observed a more than 250-folds increase of the fluo-
rescence intensity of the emission maxima (kem) of HARCP, cen-
tered at 503 nm in CH3CN (Fig. 2b, red) with a blue shift
compared with CP (kem = 605 nm, Fig. 2b, black), it prompted us
to study its photophysical properties to reveal its molecular mech-
anism, which could give an explanation of its great improvements
in photocatalytic activity [20]. We then measured sf and Uf for CP
and HARCP, respectively. It shows that the lifetime of fluorescence
yield when CP was used as the photocatalyst (Table 1).
red
1=2
Since the reduction potential of HARCP was measured as E
(HARCP/HARCP ꢀꢁ) = ꢁ1.43 V vs SCE, which reached or exceeded
the reduction potentials of aromatic halides [56,57], it means that
HARCP could activate reductive cleavage of ArꢁX of aromatic
halides to generate the key aryl radical intermediates (Fig. 3b)
without the requirement of the consecutive PET process like CP.
Therefore, photoreduction of aryl halides was selected to test the
photocatalytic activity of HARCP with methyl 4-bromobenzoate
(1a) as a model reaction under visible light conditions (Fig. S8).
After a comprehensive optimization of all reaction parameters
including photocatalysts (Table S3), electron donors (Table S4), sol-
vents (Table S5) and light sources (Table S6), aryl bromide 1a was
almost completely converted to reductive product 2a in the pres-
ence of 10 mol% HARCP, 8 equiv DIPEA, upon irradiation with
23 W CFL for 40 h in acetone (Table 1, entry 1). HARCP shows out-
standing photocatalytic activity compared with CP which has low
photocatalytic effect (Table 1, entry 2). CP-methoxy, CP-acetate
and pre-CP were also tried, but with low photocatalytic activities
similar to CP (Table S3, entries 1–4). HiBRCP and TMERCP, prepared
through the same strategy as HARCP, had the similar photocat-
alytic effect (Table S3, entries 5–6), which confirmed that this type
of structural modification for CP did bring great improvements of
for HARCP is sf = 4 ns (Fig. S5) and the fluorescence quantum yield
Uf is up to 58% (Table S1, Fig. S6), while the corresponding sf and
Uf for CP were 1 ns [33] and 0.33% (Table S1, Fig. S6), respectively.
With the prolongation of fluorescence lifetime from 1 ns of CP to
4 ns of HARCP and the fluorescence quantum yield Uf of HACRP
with 175-folds increase, it indicates that HARCP greatly enhances
114

M. Li, J. Li, B. Guo et al.
Journal of Catalysis 399 (2021) 111–120
Fig. 2. Photophysical and electrochemical properties of CP and HARCP. (a) Absorption spectra of CP (2.75 * 10ꢁ5 mol Lꢁ1) and HARCP (2.75 * 10ꢁ5 mol Lꢁ1) in CH3CN; (b)
Emission spectra of CP (2.75 * 10ꢁ5 mol Lꢁ1) and HARCP (2.75 * 10ꢁ5 mol Lꢁ1) in CH3CN with 380 nm excitation; (c) Cyclic voltammogram of HARCP (1.2 * 10ꢁ3 mol Lꢁ1) in
CH3CN; (d) Comparison of photophysical and electrochemical properties between CP and HARCP.
Fig. 3. Mechanism of photoreductive activation of aryl halides through (a) the consecutive PET of CP or (b) PET of HARCP.
photocatalytic activities. Metal complex Ir[dF(CF3)ppy]2(dtbbpy)
PF6 was also used as the photocatalyst for this reaction under same
conditions, showing lower photocatalytic activity than that of
HARCP (Table S3, entries 7). N,N-diisopropylethylamine (DIPEA)
was identified as the best single-electron reductant for this reac-
tion (Table S4), so did acetone and 23 W CFL performed as the best
solvent (Table S5) and light source (Table S6), respectively. Control
experiments indicated that the reaction did not proceed in the
absence of HARCP, DIPEA or visible light (Table 1, entries 3–5).
Nitrogen atmosphere was essential as the presence of oxygen dra-
matically decreased the efficiency of this reaction (Table 1, entry
6).
all afforded the desired hydrodehalide products in good to excel-
lent yields (Scheme 1). Importantly, medicinally relevant pyridine
derivative (2j) was also efficiently synthesized with bromo- and
chloro- substrates in this transformation.
3.4. Applications of HARCP on alkoxylation and hydroxylation of aryl
halides
Next, we tested the photocatalytic activity of HARCP for the
alkoxylation and hydroxylation of aryl halides, which currently
could be achieved through the combination of nickel and transition
metal-based photoredox catalysis Ir[dF(CF3)ppy]2(dtbbpy)PF6 [49],
or semiconductors including C3N4 and CdS [58,59]. Additionally,
the organic photocatalyst BODIPY was also reported to realize
the hydroxylation of aryl halides [60]. However, no organic photo-
Under the optimized reaction conditions, a variety of aryl bro-
mides and aryl chlorides bearing esters (2a and 2c), cyano (2b,
2f-2i), ketones (2d and 2e) and trifluoromethyl (2j) functionalities
115

M. Li, J. Li, B. Guo et al.
Journal of Catalysis 399 (2021) 111–120
Table 1
Optimization for the photoreduction of aryl halides
.
a
Reaction conditions: 1a (0.1 mmol), HARCP (10 mol%), DIPEA (8 equiv), acetone (1 mL) upon irradiation with 23 W CFL. Yields were
calculated from 1H NMR using hexamethyldisiloxane as an internal standard.
red
1=2
catalyst has good catalytic effect on both alkoxylation and hydrox-
ylation for aryl halides.
(E
= -1.43 V vs SCE) of HARCP, it indicates that HARCP could
function as an efficient organic photocatalyst to actively promote
the cycle of Ni and then facilitate the above photocatalytic reac-
tions (Fig. 3b) [49]. Thus, we initiated our investigations into the
Given a moderate excited state reduction potential (Erꢃed
=
+0.87 V vs SCE) and the powerful ground state reduction potential
a
Scheme 1. Photoreduction of aryl halides with HARCP as photocatalyst. Yields were calculated from 1H NMR using hexamethyldisiloxane as an internal standard. CH3CN
was used as solvent.
116

M. Li, J. Li, B. Guo et al.
Journal of Catalysis 399 (2021) 111–120
Scheme 2. Alkoxylation of aryl halides photocatalyzed by HARCP and Ni. Yields were for the isolated products.
photocatalytic alkoxylation system with 4-bromobenzonitrile (1b)
and n-butanol (3a) as a model reaction under visible light condi-
tions. After a comprehensive optimization of all reaction parame-
ters including photocatalysts (Table S7), Ni salts, ligands
(Table S8), bases (Table S9) and light sources (Table S10), the opti-
mum reaction conditions were as follows: HARCP in combination
with catalytic amounts of NiSO4ꢂ6H2O and 4,40-dimethoxy-2,20-bi
pyridine resulting in the selective synthesis of the alkoxylation
product (4a) upon irradiation with 23 W CFL for 24 h in acetonitrile
under mild basic conditions. HARCP shows outstanding photocat-
alytic activity compared with CP (Table S7, entries 1–2), which
DIPEA could not only serve as a sacrificial reductant but also take
effect as
a mild organic base in current reaction system
(Table S9) [60]. 23 W CFL performed as the best light source
(Table S10). Nitrogen atmosphere was essential as no product
was formed under air or oxygen conditions (Table S11). Control
experiments indicated that the reaction did not proceed in the
absence of HARCP, Ni salt, ligand, visible light or DIPEA (Table S12).
Next, a series of aryl halides were then investigated under this
optimized reaction conditions to determine the efficiency of this
alkoxylation reaction. As shown in Scheme 2, electron-deficient
aryl halides with diverse substituents at different positions showed
good reactivities and selectivities. Different functional groups
including nitriles (4a, 4f-4m), esters (4b), and ketones (4c and
4d) in the substrates were tolerated well. Furthermore, hetero-
cyclic ether (4e), the important structural skeleton of drugs, could
also be obtained with moderate yield. The scope of another compo-
nent alcohol in the coupling reaction was also explored. We found
that a series of primary alcohols, including n-butanol (4a-4e), n-
hexanol (4f), methanol (4h), d4-methanol (4k), ethanol (4g), n-
propanol (4i) and benzyl alcohol (4j), could be used and delivered
excellent reaction efficiencies. Moreover, secondary alcohols
including cyclohexanol (4l) and isopropanol (4m) showed equally
effective. These results demonstrated the universality of the
HARCP-photocatalyzed alkoxylation.
has no photocatalytic effect because of its poor reduction potential
red
of the ground state (E (CP/CPꢀꢁ) = -0.46 V vs SCE). Although the
1=2
consecutive PET process of CP could reach the reduction potential
to enable the reduction of Ni(I) to Ni(0), CPꢀ– is unstable and con-
verted to dianion CP2– in the presence of alcohol or water (Fig. 3a),
which was observed in our previous study [29]. Other common
organic photocatalysts, such as Eosin Y, Rose Bengal were also
applied in the reaction, but no catalytic activity was detected
(Table S7, entries 3–7). Metal complex Ir[dF(CF3)ppy]2(dtbbpy)
PF6 was also tested in this reaction and showed low photocatalytic
activity under the above conditions (Table S7, entries 8). Some
other combinations of Ni catalysts with different ligands could also
delivered the desired product, but with low efficiencies (Table S8).
117

M. Li, J. Li, B. Guo et al.
Journal of Catalysis 399 (2021) 111–120
Scheme 3. Hydroxylation of aryl halides photocatalyzed by HARCP and Ni. Yields were for the isolated products.
Scheme 4. Gram-scale reaction photocatalyzed by HARCP and Ni.
After the above screening results and further optimization of
the solvent (Table S13), we also chose water as another nucle-
ophilic substrate to replace alcohols, and then reacted with aryl
halides including both bromides and chlorides for the construction
of C–O bond to synthesize a series of phenolic compounds, which
are extremely important in pharmaceutical and synthetic indus-
tries [61]. With this HARCP-photocatalytic system, both aryl bro-
mides and aryl chlorides were efficiently activated and delivered
the corresponding phenols (5a-5k) in moderate to excellent yields
under mild conditions (Scheme 3). To verify the origin of O atom,
H128O was used to replace H2O in the hydroxylation protocol (5j),
and the 18O labelled phenol was indeed obtained in excellent yield
as shown in Scheme 3, confirming that the oxygen atom comes
from water (see Supporting Information).
3.5. Gram-scale reaction photocatalyzed by HARCP
Last, to show the scalability of the protocols catalyzed by this
novel organic photocatalyst HARCP, gram-scale alkoxylation reac-
tion was performed (Scheme 4). Gratifyingly, with HARCP as the
photocatalyst, the desired product 4a was obtained with compara-
ble yield to that of the small scale reaction conditions.
4. Conclusions
In summary, we have developed a novel broadly applicable
organic perylenephotocatalyst HARCP through structural modifica-
tion of the natural product cercosporin with a rational design
strategy. It gives a great improvement of photophysical and
118

M. Li, J. Li, B. Guo et al.
Journal of Catalysis 399 (2021) 111–120
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Declaration of Competing Interest
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