Molecular Design of Donor-Acceptor-Type Organic Photocatalysts for Metal-free Aromatic C?C Bond Formations under Visible Light

Article abstract of DOI:10.1002/adsc.201800950

Metal-free and photocatalytic radical-mediated aromatic C?C bond formations offer a promising alternative pathway to the conventional transition metal-catalyzed cross-coupling reactions. However, the formation of aryl radicals from common precursors such as aryl halides is highly challenging due to their extremely high reductive potential. Here, we report a structural design strategy of donor-acceptor-type organic photocatalysts for visible light-driven C?C bond formations through the reductive dehalogenation of aryl halides. The reduction potential of the photocatalysts could be systematically aligned to be ?2.04 V vs. SCE via a simple heteroatom engineering of the donor-acceptor moieties. The high reductive potential of the molecular photocatalyst could reduce various aryl halides into aryl radicals to form the C?C bond with heteroarenes. The designability of the molecular photocatalyst further allowed the synthesis of a high LUMO (lowest unoccupied molecular orbital) polymer photocatalyst by a self-initiated free radical polymerization without compromising its LUMO level. (Figure presented.).

Full text of DOI:10.1002/adsc.201800950

Title: Molecular Design of Donor-Acceptor-Type Organic
Photocatalysts for Metal-free Aromatic C-C Bond Formations
under Visible Light
Authors: Lei Wang, Jeehye Byun, Run Li, Wei Huang, and Kai Zhang
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800950
Link to VoR: http://dx.doi.org/10.1002/adsc.201800950

10.1002/adsc.201800950
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Molecular Design of Donor-Acceptor-Type Organic
Photocatalysts for Metal-free Aromatic C-C Bond Formations
under Visible Light
Lei Wang,a,§ Jeehye Byun,a,§ Run Li,a Wei Huang,a and Kai A. I. Zhanga*
a
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128, Mainz, Germany.
Fax: (+49)-6131-379-370; e-mail: kai.zhang@mpip-mainz.mpg.de
§
Equal contribution
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
the activation of target molecules under light
aromatic C-C bond formations offer a promising alternative irradiation. Through
proper combination of
Abstract. Metal-free and photocatalytic radical-mediated
a
pathway to the conventional transition metal-catalyzed
cross-coupling reactions. However, the formation of aryl
radicals from common precursors such as aryl halides is
highly challenging due to their extremely high reductive
potential. Here, we report a structural design strategy of
donor-acceptor-type organic photocatalysts for visible
light-driven C-C bond formations through reductive
dehalogenation of aryl halides. The reduction potential of
the photocatalysts could be systematically aligned to be -
2.04 V vs. SCE via a simple heteroatom engineering of the
donor-acceptor moieties. The high reductive potential of
the molecular photocatalyst could reduce various aryl
halides into aryl radicals to form the C-C bond with
heteroarenes. The designability of the molecular
photocatalyst further allowed the synthesis of a high
LUMO (lowest unoccupied molecular orbital) polymer
photocatalyst by a self-initiated free radical polymerization
without compromising its LUMO level.
monomeric units, the molecular organic photocatalyst
can be tuned to catalyze highly challenging organic
reactions, such as aromatic C-C bond formation.
Aromatic C-C bond formation via arylation
reaction is a primary and key organic reaction for
producing natural, pharmaceutical and high value fine
chemicals.[5] Conventional way of the aromatic cross
coupling is to utilize transition metal catalyst,
typically palladium under the elevated temperature,
giving high efficiency and selectivity[6]. However, the
precious metal-based reaction has to face the issue in
cost and sustainability, therefore visible light-driven
C-C bond formation via radical-mediated arylation
has drawn attention as a viable route of synthesizing
the cross-coupled aryl products under mild
condition.[7] Recent developments in photocatalytic
aromatic C-C bond formation showed that the aryl
precursors with relatively low reductive potentials
(ERed. ≤ 1.0 V vs. saturated calomel electrode (SCE)),
for instance, aryl diazonium[8], diaryliodonium salts,[9]
and aryl sulfonyl chlorides[10] could be reduced
mainly by transition metal-based photocatalysts such
as Ru(bpy)3Cl2 and organic dyes such as eosin Y. In
case of arylazo sulfone substrate[11], it can produce
aryl radical under light irradiation even without
photocatalyst owing to the photolability. In addition,
the photo-induced Meerwein arylation reactions using
aryl diazonium precursors have been often used to
form various aromatic C-C bonds.[8b, 12] Dual catalytic
systems containing a photocatalyst and a metal
cocatalyt such as gold offered a useful strategy to
form carbon-heteroatom bonds.[13]
Keywords: C-C bond formation; Metal-free;
Photocatalysis; Organic photocatalyst; Donor-Acceptor
Direct utilization of sunlight for catalyzing
chemical reactions has been considered as an
inexpensive, non-polluting, and sustainable method
from last few decades.[1] Since the original reports on
Ruthenium bipyridyl complexes as visible light active
photocatalysts[2], tremendous efforts have been made
to develop the effective photocatalytic systems for
organic transformation reactions. Early examples of
molecular photocatalysts involve transition metal
complexes[3], and further molecular organic
photocatalysts[4] have been suggested as the metal-
free alternatives with non-toxicity and stability. As
the photocatalytic reactions mostly undergo the
photogenerated-radical synthetic pathways, the
molecular organic photocatalysts have strong
advantage owing to designable energy structures for
Nonetheless, the formation of aryl radicals from
common precursors such as aryl halides is still
challenging due to their extremely high reduction
potential (ERed. ≥ 2.0 V vs. SCE). Among the very few
examples of suitable photocatalysts reported, König
et al. demonstrated
a
perylenediimide (PDI)
photocatalyst, which could reduce various aryl
1
This article is protected by copyright. All rights reserved.

10.1002/adsc.201800950
Advanced Synthesis & Catalysis
halides via a consecutive photo-induced electron
transfer in the excited state.[14] The authors also
reported a chromoselective C-C bond formation
mediated by rhodamine 6G.[15] Hawker and de Alaniz
et al. reported a 10-phenylphenothiazine (PTH)
photocatalyst with a reduction potential of -2.10 V vs.
SCE for photocatalytic dehalogenation reactions.[16]
Effect of electron-deficient substituents on aryl
substrates was also investigated by König et al. to
lower the reduction potential of aryl halides.[17] Our
group recently reported a photocatalytic Stille-type
aromatic C-C bond formation catalyzed by azulene-
based conjugated microporous polymer, which
indicate the reduction potential and oxidation potential,
respectively.
To achieve extremely high LUMO band position in
molecular photocatalyst, we aim to fine-tune the D-A
units by substituting the type of heteroatoms. Three
molecular photocatalysts were synthesized to
demonstrate the structural design strategy, in
particular,
benzo[c][1,2,5]thiadiazole (Th-BT-Th),[22] 2-butyl-
4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3] triazole
(Th-BTz-Th), and 2-butyl-4,7-bis(1-methyl-1H-
4,7-di(thiophen-2-yl)
underwent the oxidative aryl formation mechanism.[18] pyrrol-2-yl)-2H-benzo[d][1,2,3]triazole (Py-BTz-Py).
In order to thereby achieve aromatic C-C coupling Theoretical simulations of three molecular
reaction with aryl halide substrates under visible light, photocatalysts showed a clear difference in electron
the photocatalysts should have high reductive
potential, which can cleave carbon-halogen bond and
generate aryl radical. The most promising way for
controlling energy level of molecular organic
photocatalysts is to introduce alternating composition
of donor (D) and acceptor (A) moieties into the
conjugated system.[19] The combination of electron-
rich donor and electron-deficient acceptor leads to a
compressed energy band gap by the molecular orbital
hybridization and intramolecular charge transfer.[20]
Thus, the intrinsic attributes of the donor and
acceptor units can affect the energy level of the
densities over D-A combinations, leading to the
gradual increase in energy levels from Th-BT-Th to
Py-BTz-Py (Figure S1). The structures and redox
potentials of the molecular photocatalysts are shown
in Figure 1. Their synthetic and characteristic details
are described in the Supporting Information (SI).
The reduction potential of Th-BT-Th was
measured to be -1.15 V vs. SCE via the cyclic
voltammetry (CV) measurement (Figure S2). By
replacing sulfur atom on the benzothiadiazole (BT)
unit to nitrogen atom, the reduction potential could be
elevated to -1.68 V vs. SCE, as shown in Th-BTz-Th.
The lone pair of the nitrogen atom is more basic than
that of sulfur, thereby makes benzotriazole ring more
electron-rich, leading to higher LUMO energy level
of Th-BTz-Th.[23] By further replacement of sulfur
atom on adjacent thiophene units by more electron-
rich 1-methylpyrrole, the reduction potential could be
gradually aligned to -2.04 V, as observed by Py-BTz-
Py.[24] The results correspond to the theoretical
calculation and demonstrate the precise energy level
control of the molecular organic photocatalysts via a
heteroatom engineering on donor/acceptor moieties.
Similar to the reduction potentials, tendentiously
elevating oxidation potentials of the molecular
photocatalysts were also observed, resulting into
decreasing photo-oxidation potentials for the organic
photocatalysis. The UV/vis absorption and
fluorescence emission spectra of the molecular
photocatalysts were displayed in Figure S3. Th-BT-
Th exhibited a broader UV/Vis absorption range until
ca. 550 nm with an absorption maximum at 453 nm,
while Th-BTz-Th and Py-BTz-Py rather showed a
narrower absorption band until ca. 450 nm, with
maxima at 392 and 382 nm, respectively. The
fluorescence bands of the molecular photocatalysts
ranged from 420 to 800 nm with maxima at 590, 530,
and 490 nm for Th-BT-Th, Th-BTz-Th, and Py-BTz-
Py, respectively. Despite of the high reductive
potential, Py-BTz-Py exhibited good stability, in
which the fluorescent emission spectrum of Py-BTz-
Py was unchanged even after the light treatment for 2
d under oxidative atmosphere (Figure S4).
molecular photocatalysts, allowing
a
precise
structural modulation for the targeted reactions.[21]
Herein, we report the structural design of donor-
acceptor-type organic photocatalysts for metal-free
and visible light-driven aromatic C-C bond formation
via aryl radical-mediated reaction at room
temperature. By a heteroatom engineering within D-
A units, the reductive potential of molecular
photocatalyst could be systematically elevated up to -
2.04 V vs. SCE, which was able to successfully
catalyze cross-coupling reaction between aryl halide
and heteroarene. Thanks to the designability, the
molecular organic photocatalyst was further tethered
into linear chain polymer via a self-initiated free
radical polymerization without compromising its
reductive potential (-1.89 V vs. SCE). The unfading
photoactivity in the polymeric system was showcased
by the synthesis of a drug molecule through a metal-
free two-step aromatic C-C bond formation.
Figure 1. Structures and redox potentials of the designed
molecular organic photocatalysts, oxidation potential of the
sacrificial reagent R3N (R=4-MeO-C6H4), and reduction
potential of methyl 4-iodobenzoate substrate. ERed and EOx
Based on the aligned redox potentials of designed
photocatalysts, we then conducted the photocatalytic
C-C bond formation between methyl 4-iodobenzoate
2
This article is protected by copyright. All rights reserved.

10.1002/adsc.201800950
Advanced Synthesis & Catalysis
and furan in DMSO as a model reaction. To avoid the
excess use of the electron donor and ease the
purification process, we only used stoichiometric
amounts of amines with respect to the aryl iodide.
Tris(4-methoxyphenyl)amine ((4-MeO-C6H4)3N) was
chosen due to its low oxidation potential (EOx. = 0.48
V vs. SCE), which ought to ease the electron
extraction process by the excited photocatalyst. The
screening and control experiments are listed in Table
1. As expected, the use of Th-BT-Th did not lead to
as electron donor, 5 mol% photocatalyst, 2.5 ml dry
DMSO as solvent, white LED (0.07 W/cm²), RT, 24 h.
b)Conversion determined via GC-MS (Figure S6). c)4%
methyl benzoate as a side product. d)8% methyl benzoate
as a side product. e)no side product determined. f)30%
methyl benzoate as a side product.
In order to study the role of electron donor, we
have also tested different alkylamines (Figure S7). By
any conversion after irradiation of white light for 24 h, using triethylamine (Et3N, EOx. = 0.58 V vs. SCE) and
which was obviously due to its insufficient reductive
potential (-1.15 V vs. SCE) to reduce methyl 4-
iodobenzoate (ERed. = -1.63 V vs. SCE) into the
corresponding aryl radical (entry 1) (Figure S5). The
reduction potential of Th-BTz-Th (-1.68 V vs. SCE)
was slightly higher than that of the substrate, which
resulted in 9% of conversion efficiency (entry 2). In
contrast, conversion efficiency of 54% for the C-C
bond formation was achieved using Py-BTz-Py as a
photocatalyst (entry 3), resulting from its sufficient
reduction potential of -2.04 V vs. SCE. The
continuous light irradiation for 48 h gave a higher
conversion up to 66% (entry 4). Control experiments
confirmed that light, organic photocatalyst, and
electron donor (amine) were mandatory for the
photocatalytic C-C bond formation (entry 5-7).
tributylamine (Bu3N, EOx. = 0.43 V vs. SCE) as
electron donors, only low conversions were
determined (entry 8 and 9). The reason could likely
lie at the high oxidation potentials of alkylamines and
the formation of methyl benzoate as a side product
due to abstraction of protons from alkylamines. In
comparison, (4-MeO-C6H4)3N only acted as electron
donor and not as extra proton donor during the
catalytic cycle, preventing the formation of side
product. The advantage of using (4-MeO-C6H4)3N in
stoichiometric amount is that it can be fully recycled
after the reaction, as shown in GC-MS spectrum
(Figure S8). The solvent effect showed that the use of
acetonitrile (MeCN) led to a conversion of 18%
(entry 10), while DMF as a solvent gave a conversion
of 42% (entry 11). Additionally, with DMF as a
solvent, 30% of methyl benzoate was obtained,
indicating that DMF could also act as an undesired
proton donor during the reaction, as described in
previous reports.[25] When the light wavelength varied,
the conversion efficiency was consequently changed
(entry 12 and 13). Based on the UV-vis absorption
spectrum of Py-BTz-Py with the absorbance
maximum at 382 nm (Figure S3), the conversion
efficiency was decreased with a clear wavelength
dependence, in which 23 % and 0 % of conversion
were obtained with 420 nm and 495 nm cut-off of
white light, respectively.
Table 1. Screening and control experiments of the model
reaction.
Reaction
condition
variations
Conversion
Entrya) Photocatalyst
[%]b)
Th-BT-Th
Th-BTz-Th
Py-BTz-Py
-
-
-
0
9
1
2
3
54
Double reaction
time (48 h)
Py-BTz-Py
66
4
Py-BTz-Py
Py-BTz-Py
Py-BTz-Py
No light
0
0
0
5
6
7
No photocatalyst
No amine
Et3N as electron
donor
Py-BTz-Py
Py-BTz-Py
2c)
9d)
8
9
Bu3N as electron
donor
Py-BTz-Py
Py-BTz-Py
MeCN as solvent
DMF as solvent
18e)
42f)
10
11
Figure 2. Proposed mechanism of the photocatalytic aryl
radical-mediated C-C bond formation reaction. Ar: aryl
substrates; X: halide; R3N: sacrificial reagent as an
electron donor (R=4-MeO-C6H4).
Light with 420
nm cut-off
Py-BTz-Py
23
12
Light with 495
nm cut-off
Py-BTz-Py
0
13
a)Reaction conditions: 1 equiv. (0.38 mmol) methyl 4-
iodobenzoate, 20 equiv. furan, 1 equiv. (4-MeO-C6H4)3N
3
This article is protected by copyright. All rights reserved.

10.1002/adsc.201800950
Advanced Synthesis & Catalysis
The
steady-state
fluorescence
quenching
experiments revealed that the fluorescence intensity
of Py-BTz-Py could be quenched by adding either (4-
MeO-C6H4)3N or methyl 4-iodobenzoate (Figure S9).
This indicates a possible electron transfer between the
catalyst and both active species. To confirm the aryl
radical-mediated reaction mechanism, we then
conducted a reduction reaction of methyl 4-
iodobenzoate using 2,2,6,6-Tetramethylpiperidin-1-
yl)oxyl (TEMPO) as a radical trapping agent. The
trapped adduct with TEMPO was determined,
indicating the formation of aryl radical under visible
light irradiation with Py-BTz-Py as a photocatalyst
(Figure S10). The observations suggested a reaction
mechanism similar to the literature, as illustrated in
Figure 2.[7] The initial step was the oxidative
extraction of one electron from the sacrificial reagent
(R3N) by the photogenerated hole of Py-BTz-Py,
which was followed by the reductive cleavage of the
carbon halogen (Ar1-X) bond by the photogenerated
electron from the LUMO level of the photocatalyst.
The formed aryl radical was then added on another
aryl substrate (Ar2-H) to form the aryl-aryl radical.
The aryl-aryl radical then lost one electron by the
afore-obtained amine cationic radical to complete the
catalytic cycle. After the deprotonation, the final
product was formed.
To investigate the general feasibility of Py-BTz-Py
as a photocatalyst, various aryl substrates were tested
for aromatic C-C coupling reaction under the
optimized reaction condition, as summarized in
Figure 3. Moderate to high substituents tolerance on
the aryl substrates were observed by scoping the
reactions of various aryl iodides with furan. The
electron-withdrawing substituents such as cyano- or
trifluoro- groups (b4-b9) generally led to higher
reaction yields than the electron-donating substituents
(b17-b20) (Figure 3a). However, single halogen-
containing substituents (b1-b3) did not give higher
yields than the model substrate of methyl 4-
iodobenzoate (b15). Nevertheless, aryl iodides with
multiple halogen atoms gave moderate product yields
(b11-b13). The reactions with the other carbonyl-
containing substrates showed similar product yields
to the model reaction (b14 and b16).
Figure 3. Scope of (a) aryl iodides with furan and (b)
arenes (Ar2) with different aryl iodides using Py-BTz-Py as
a photocatalyst. Reaction conditions: 1 equiv. (0.38 mmol)
aryl iodide, 20 equiv. Ar2, 1 equiv. (4-MeO-C6H4)3N, 5
mol% Py-BTz-Py in 2.5 ml dry DMSO, white LED (0.07
W/cm²), RT, 24 h. Isolated yields determined after
chromatography. Asterisk(*) in the dotted frames indicates
the product yield obtained from poly(Py-BTz-Py) as a
photocatalyst.
The scope of different arenes (Ar2) as a coupling
partner showed that the reactions with electron-rich
arenes such as pyrrole could result in very high yields
up to 95% (b21-b26) (Figure 3b). In particular, the
formation of b25 was comparable to that from state-
of-the-art
example
of
perylenediimide-based
photocatalyst (72%, 22h, at 40oC),[14] despite of using
the lesser amount of counter arene and electron donor.
The reaction of methyl 4-iodobenzoate with
thiophene led to a slightly lower yield than the model
reaction with furan (b27). The reactions with
electron-deficient arenes such as phenyl (b28) and
toluene (b29) showed lower product yields than the
ones with electron-rich arenes. When with toluene,
three different products were generated, in which
ortho-substituted product on toluene was mainly
obtained (b29).
4
This article is protected by copyright. All rights reserved.

10.1002/adsc.201800950
Advanced Synthesis & Catalysis
yield of 22% owing to the greater overpotential,
while the coupling with 4-chloro- and 4-
bromobenzonitrile resulted in insignificant yields.
The molecular Py-BTz-Py photocatalyst exhibited the
same trend in the conversion reaction with
halobenzonitriles and furan, where the increased
yields were observed with respect to decreasing
reductive potentials of halobenzonitriles (b30 in
Figure 3). When with electron-withdrawing group of
fluorine on 4-iodobenzonitrile, the product yield with
poly(Py-BTz-Py) was increased up to 39% (b10 in
Figure 3). Thanks to the long polymeric chain,
poly(Py-BTz-Py) could be easily isolated from the
reaction medium by precipitation in methanol,
providing a much simpler way of purifying the
products after the reaction. The regenerated poly(Py-
BTz-Py) exhibited an evolution of blue-shifted peak
in UV-vis emission spectrum, perhaps due to the
fragmentation of photocatalytic unit from polymer
after the coupling reaction (Figure S13). Such a
structural change of the regenerated poly(Py-BTz-Py)
led to the less conversion efficiency to produce b10,
giving 26 % after 24 h of the second cycle.
Figure 4. (a) Self-initiated free radical polymerization
using Py-BTz-Py-v as a photoinitiator and a comonomer to
make high LUMO polymer photocatalyst. (b) UV/vis
absorption and emission spectra of Py-BTz-Py molecule
and Poly(Py-BTz-Py). (c) Kubelka−Munk-transformed
reflectance spectrum and (d) redox potential of Poly(Py-
BTz-Py). ERed and EOx indicate the reduction potential and
oxidation potential, respectively.
The designable feature of molecular organic
photocatalyst has been further demonstrated by the
incorporation of Py-BTz-Py into polymeric structure.
As shown in Figure 4a, Py-BTz-Py was simply
modified to have a vinyl terminal at the end of alkyl
chain on benzotriazole, giving the Py-BTz-Py-v
molecule. The Py-BTz-Py-v could act as a visible
light active photoinitiator as well as a comonomer for
self-initiated free radical polymerization in presence
of methyl methacrylate (MMA) and amine as a
monomer and a cocatalyst, respectively[26]. The
resulting poly(Py-BTz-Py) exhibited a molecular
weight (Mn) of 21,000 g mol-1 with 3% of Py-BTz-Py
Figure 5. Metal-free synthesis of 4-([2,2'-bifuran]-5-yl)-3-
fluorobenzonitrile via two-step aromatic C-C bond
formation using (a) Py-BTz-Py and (b) poly(Py-BTz-Py)
as photocatalysts.
1
content, proven by H NMR spectrum (Figure S11).
UV/vis absorption and emission profiles of Py-BTz-
Py were retained even after the polymerization
(Figure 4b). Optical band gap of poly(Py-BTz-Py)
was found to be 2.89 eV from the Kubelka−Munk
transformed reflectance spectrum (Figure 4c),
corresponding to the cyclic voltammetry (CV)
measurement (Figure 4d and Figure S12). The
reduction potential of poly(Py-BTz-Py) was located
at -1.89 V vs. SCE, which is sufficient for aromatic
C-C bond formation reaction under visible light.
The photocatalytic activity of poly(Py-BTz-Py)
was showcased by coupling reaction between
halobenzonitrile and furan, since the bifuran coupled
product is recently found as an effective drug
molecule against methicillin-resistant Staphylococcus
aureus infection.[27] The conversion efficiency was
dependent on the reductive potentials of
halobenzonitrile, in which 4-chloro-, 4-bromo-, and
4-iodobenzonitrile have the reductive potentials at -
1.92, -1.80, and -1.62 V vs. SCE, respectively (Figure
S5). The coupling of 4-iodobenzonitrile and furan
with poly(Py-BTz-Py) showed the higher conversion
The successful formation of 3-fluoro-4-(furan-2-
yl)benzonitrile (b10) under visible light prompted us
to conduct a stepwise coupling of furan to generate 4-
([2,2'-bifuran]-5-yl)-3-fluorobenzonitrile, which is the
potential drug for anticancer treatment.[27] After the
iodination of b10 by N-iodosuccinimide (NIS), the
additional coupling of furan was conducted in the
standard reaction condition. Both Py-BTz-Py and
poly(Py-BTz-Py) showed a low conversion yield to
achieve the target molecule, leading to 16 and 10 %
after 24 h of white light irradiation, respectively
(Figure 5). Perhaps due to the electron-rich nature of
furyl group on b10, the successive coupling of furan
to form 4-([2,2'-bifuran]-5-yl)-3-fluorobenzonitrile
(c1) resulted in the less conversion than the single
step coupling reaction. Nonetheless, through the two-
step aromatic C-C bond formation under visible light
irradiation, the biologically-important molecule could
be produced without any metal catalysts.
5
This article is protected by copyright. All rights reserved.

10.1002/adsc.201800950
Advanced Synthesis & Catalysis
thaw method and irradiated under a white light (power:
0.07 W cm-2, λ > 420 nm) under argon atmosphere. The
final conversion was determined by GC-MS after 24 h
irradiation. To obtain the pure product, the mixture was
poured into 20 ml water and extracted by CH2Cl2. The
organic phase was dried over anhydrous MgSO4, and the
solvent removed under vacuum. The crude product was
then purified over a silica column chromatography to
afford the desired product using petroleum ether/ethyl
acetate (95:5 or 90:10) as eluent.
In summary, we have demonstrated a precise
energy band position tuning of molecular organic
photocatalysts for metal-free and visible light-driven
aromatic C-C formation reactions. Via a facile
heteroatom engineering on donor and acceptor
moieties, the reduction potential of the photocatalysts
could be gradually aligned to -2.04 V vs. SCE,
thereby leading to a high LUMO level. Various aryl
halides could be reduced to aryl radicals, which can
be coupled with heteroarene substrates to form the
final C-C coupled products. The designability of
molecular photocatalysts further promoted the
generation of linear polymer photocatalyst, showing
unfading activity for synthesizing a drug molecule by
two-step photocatalytic C-C coupling reaction under
visible light. We believe that this study can offer a
general design strategy to construct organic
Acknowledgements
The authors thank the Max Planck Society for financial support.
L.W., R.L. and W.H. thank the China Scholarship Council (CSC)
for scholarship. J.B. thanks the Alexander von Humboldt
foundation for postdoctoral research fellowship.
References
photocatalysts
-
of
either
molecular
or
macromolecular structure - with extreme high
reductive potentials for tackling challenging organic
transformation reactions with visible light
illumination.
[1]
a) M. Leyre, P. S. K., R. Oliver, K. Burkhard,
Angew. Chem. Int. Ed. 2018, 57, 10034-10072; b)
T. P. Yoon, M. A. Ischay, J. Du, Nat. Chem. 2010,
2, 527.
[2]
a) J. M. R. Narayanam, J. W. Tucker, C. R. J.
Stephenson, J. Am. Chem. Soc. 2009, 131, 8756-
8757; b) D. A. Nicewicz, D. W. C. MacMillan,
Science 2008, 322, 77-80.
Experimental Section
Synthesis of Py-BTz-Py
[3]
[4]
[5]
C. K. Prier, D. A. Rankic, D. W. C. MacMillan,
Chem. Rev. 2013, 113, 5322-5363.
N. A. Romero, D. A. Nicewicz, Chem. Rev. 2016,
116, 10075-10166.
L. Ackermann, Modern Arylation Methods,
Wiley-VCH Verlag GmbH & KGaA, Weinheim,
2009.
S. C. C. C. Johansson, M. O. Kitching, T. J.
Colacot, V. Snieckus, Angew. Chem. Int. Ed.
2012, 51, 5062-5085.
A 100 ml Schlenk tube equipped with a stirring bar and
stopper was heated under vacuum then cooled three times
before 4,7-dibromo-2-(n-butyl)-2H-benzo[d][1,2,3]triazole
(1 g, 3.00 mmol), 1-methyl-2-(tributylstannyl)pyrrole (2.14
ml, 6.5 mmol), and Pd(PPh3)2Cl2 (100.8 mg, 5 mol%) were
added. 15 ml dry DMF was added via syringe and the
mixture was degassed under vacuum and then backfilled
with nitrogen three times. It was heated under 90 oC
overnight, then poured into 200 ml water and extracted
with CH2Cl2 three times. The organic layer was washed
with water twice, dried by anhydrous MgSO4, and the
solvent was removed under vacuum. The crude product
was purified with a silica column chromatography using
DCM/Hexane (1/1) as eluent, giving the final product as
red brown solid (650 mg, 65%). Th-BT-Th, Th-BTz-Th,
and Py-BTz-Py-v were produced by the similar procedure
described above. The synthetic details can be found in the
supporting information.
[6]
[7]
[8]
I. Ghosh, L. Marzo, A. Das, R. Shaikh, B. König,
Acc. Chem. Res. 2016, 49, 1566-1577.
a) D. Kalyani, K. B. McMurtrey, S. R. Neufeldt,
M. S. Sanford, J. Am. Chem. Soc. 2011, 133,
18566; b) D. P. Hari, P. Schroll, B. König, J. Am.
Chem. Soc. 2012, 134, 2958-2961; c) J. Zoller, D.
C. Fabry, M. Rueping, ACS Catal. 2015, 5, 3900.
a) Y. X. Liu, D. Xue, J. D. Wang, C. J. Zhao, Q. Z.
Zou, C. Wang, J. L. Xiao, Synlett 2013, 24, 507;
b) M. Tobisu, T. Furukawa, N. Chatani, Chem.
Lett. 2013, 42, 1203.
a) P. Natarajan, A. Bala, S. K. Mehta, K. K.
Bhasin, Tetrahedron 2016, 72, 2521; b) J. D. Xia,
G. B. Deng, M. B. Zhou, W. Liu, P. Xie, J. H. Li,
Synlett 2012, 23, 2707.
Synthesis of poly(Py-BTz-Py)
[9]
The linear polymer photocatalyst was made by the
modified protocol from our previous report[28]. In a 20 ml
glass vial, methyl methacrylate (0.67 ml), Py-BTz-Py-v
(70 mg), and Et3N (28 μl) were dissolved in 5 ml dry THF.
The mixture was bubbled with nitrogen for 10 min and
shined under white LED for 24 h under vigorous stirring.
The mixture was then precipitated in methanol, filtered,
and washed with methanol. The light green powder was
dried under vacuum for further use, yield = 0.211 g. Mn =
21,000, Mw = 26,600, PDI = 1.27 (GPC in THF).
[10]
[11]
[12]
S. Crespi, S. Protti, M. Fagnoni, J. Org. Chem.
2016, 81, 9612-9619.
Standard procedure for the molecular organic
photocatalyst-mediated C-C bond formation under
visible light irradiation
a) P. Maity, D. Kundu, B. C. Ranu, Eur. J. Org.
Chem. 2015, 2015, 1727; b) J. Zhang, J. Chen, X.
Zhang, X. Lei, J. Org. Chem. 2014, 79, 10682.
a) B. Sahoo, M. N. Hopkinson, F. Glorius, J. Am.
Chem. Soc. 2013, 135, 5505; b) M. N. Hopkinson,
B. Sahoo, F. Glorius, Adv. Synth. Catal. 2014,
356, 2794; c) A. Tlahuext-Aca, M. N. Hopkinson,
B. Sahoo, F. Glorius, Chem. Sci. 2016, 7, 89; d) X.
[13]
A 25 ml Schlenk tube equipped with a stirring bar and
stopper was filled with aryl halide (0.38 mmol, 1 equiv),
furan (7.6 mmol, 20 equiv), tri(4-methoxylphenyl)amine
(0.38 mmol, 1 equiv), organic photocatalyst (5%, 0.019
mmol) in 2.5 ml dry DMSO. When poly(Py-BTz-Py) was
used as a photocatalyst, 60 mg of photocatalyst was used.
The reaction mixture was degassed via three freeze-pump-
6
This article is protected by copyright. All rights reserved.

10.1002/adsc.201800950
Advanced Synthesis & Catalysis
Z. Shu, M. Zhang, Y. He, H. Frei, F. D. Toste, J.
Am. Chem. Soc. 2014, 136, 5844.
I. Ghosh, T. Ghosh, J. I. Bardagi, B. König,
Science 2014, 346, 725-728.
I. Ghosh, B. König, Angew. Chem. Int. Ed. 2016,
55, 7676-7679.
S. O. Poelma, G. L. Burnett, E. H. Discekici, K.
M. Mattson, N. J. Treat, Y. Luo, Z. M. Hudson, S.
L. Shankel, P. G. Clark, J. W. Kramer, C. J.
Hawker, J. Read de Alaniz, J. Org. Chem. 2016,
81, 7155-7160.
[20]
[21]
B.-G. Kim, X. Ma, C. Chen, Y. Ie, E. W. Coir, H.
Hashemi, Y. Aso, P. F. Green, J. Kieffer, J. Kim,
Adv. Funct. Mater. 2013, 23, 439-445.
a) C.-H. Chen, C.-H. Hsieh, M. Dubosc, Y.-J.
Cheng, C.-S. Hsu, Macromolecules 2010, 43,
697-708; b) H. Zhou, L. Yang, S. Stoneking, W.
You, ACS Appl. Mater. Interfaces 2010, 2, 1377-
1383.
[14]
[15]
[16]
[22]
L. Wang, W. Huang, R. Li, D. Gehrig, P. W.
Blom, K. Landfester, K. A. Zhang, Angew. Chem.
Int. Ed. 2016, 55, 9783-9787.
[17]
[18]
A. U. Meyer, T. Slanina, C.-J. Yao, B. König,
ACS Catal. 2016, 6, 369-375.
S. Ghasimi, S. A. Bretschneider, W. Huang, K.
Landfester, K. A. I. Zhang, Adv. Sci. 2017, 4,
1700101.
a) Y. K. Eom, L. Nhon, G. Leem, B. D. Sherman,
D. Wang, L. Troian-Gautier, S. Kim, J. Kim, T. J.
Meyer, J. R. Reynolds, K. S. Schanze, ACS
Energy Lett. 2018, 2114-2119; b) J. Roncali,
Chem. Rev. 1997, 97, 173-206; c) L. Wang, R. Li,
K. A. I. Zhang, Macromol. Rapid Commun. 2018,
1800466; d) M. Zhu, Y. Du, P. Yang, X. Wang,
Catal. Sci. Technol. 2013, 3, 2295-2302.
[23]
[24]
[25]
[26]
S. C. Price, A. C. Stuart, L. Yang, H. Zhou, W.
You, J. Am. Chem. Soc. 2011, 133, 4625-4631.
B. H. Kim, H. S. Freeman, Dyes Pigm. 2013, 96,
313-318.
C. Dai, F. Meschini, J. M. R. Narayanam, C. R. J.
Stephenson, J. Org. Chem. 2012, 77, 4425-4431.
S. Ghasimi, S. Prescher, Z. J. Wang, K.
Landfester, J. Yuan, K. A. I. Zhang, Angew.
Chem. Int. Ed. 2015, 54, 14549-14553.
W. A. Hussin, M. A. Ismail, A. M. Alzahrani, W.
M. El-Sayed, Drug Des. Devel. Ther. 2014, 8,
963-972.
[19]
[27]
[28]
Z. J. Wang, K. Landfester, K. A. I. Zhang, Polym.
Chem. 2014, 5, 3559-3562.
7
This article is protected by copyright. All rights reserved.

10.1002/adsc.201800950
Advanced Synthesis & Catalysis
COMMUNICATION
Molecular Design of Donor-Acceptor-Type
Organic Photocatalysts for Metal-free Aromatic C-
C Bond Formations under Visible Light
Adv. Synth. Catal. Year, Volume, Page – Page
Lei Wang§, Jeehye Byun§, Run Li, Wei Huang, and
Kai A. I. Zhang*
8
This article is protected by copyright. All rights reserved.