Visible Light-Driven C-3 Functionalization of Indoles over Conjugated Microporous Polymers

Article abstract of DOI:10.1021/acscatal.8b01478

Metal-free and heterogeneous organic photocatalysts provide an environmentally friendly alternative to traditional metal-based catalysts. This paper reports a series of carbazole-based conjugated microporous polymers (CMPs) with tunable redox potentials and explores their photocatalytic performance with regard to C-3 formylation and thiocyanation of indoles. Conjugated polymers were synthesized through FeCl₃ mediated Friedel-Crafts reactions, and their redox potentials were well regulated by simply altering the nature of the core (i.e., 1,4-dibenzyl, 1,3,5-tribenzyl, or 1,3,5-triazin-2,4,6-triyl). The resulting CMPs exhibited high surface areas, visible light absorptions, and tunable semiconductor-range band gaps. With the highest oxidative capability, CMP-CSU6 derived from 1,3,5-tri(9H-carbazol-9-yl)benzene showed the highest efficiency for C-3 formylation and thiocyanation of indoles at room temperature. Notably, the as-made catalysts can be easily recovered with good retention of photocatalytic activity and reused at least five times, suggesting good recyclability. These results are significant for constructing high-performance porous polymer catalysts with tunable photoredox potentials targeting an efficient material design for catalysis.

Full text of DOI:10.1021/acscatal.8b01478

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Article
Visible Light-Driven C-3 Functionalization of
Indoles over Conjugat-ed Microporous Polymers
Weijie Zhang, Juntao Tang, Wenguang Yu, Qiao Huang,
Yu Fu, Gui-Chao Kuang, Chunyue Pan, and Guipeng Yu
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 17 Jul 2018
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Page 1 of 9
ACS Catalysis
Visible LightꢀDriven Cꢀ3 Functionalization of Indoles over Conjuꢀ
gated Microporous Polymers
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Weijie Zhang , Juntao Tang , Wenguang Yu , Qiao Huang , Yu Fu , Guichao Kuang , Chunyue
a
a,b
Pan *and Guipeng Yu *
a
College of Chemistry and Chemical Engineering, Hunan Provincial Key Laboratory of Efficient and Clean Utilization of
Manganese Resources, Central South University, Changsha 410083, China.
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State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 110762, China
Eꢀmail: panchunyue@csu.edu.cn (C. Pan); gilbertyu@csu.edu.cn (G. Yu).
ABSTRACT: Metalꢀfree and heterogeneous organic photocatalysts provide an environmentallyꢀfriendly alternative to traditional metalꢀ
based catalysts. This paper reports a series of carbazoleꢀbased conjugated microporous polymers (CMPs) with tunable redox potentials and
explores their photocatalytic performance with regard to Cꢀ3 formylation and thiocyanation of indoles. Conjugated polymers were
synthesized through FeCl mediated FriedelꢀCrafts reactions, and their redox potentials were well regulated by simply altering the nature
3
of the core (i.e. 1,4ꢀdibenzyl, 1,3,5ꢀtribenzyl or 1,3,5ꢀtriazinꢀ2,4,6ꢀtriyl). The resulting CMPs exhibited high surface areas, visible light
absorptions and tunable semiconductorꢀrange band gaps. With the highest oxidative capability, CMPꢀCSU6 derived from 1,3,5ꢀtri(9Hꢀ
carbazolꢀ9ꢀyl)benzene, showed the highest efficiency for Cꢀ3 formylation and thiocyanation of indoles at room temperature. Notably, the
asꢀmade catalysts can be easily recovered with good retention of photocatalytic activity and reused for at least 5 times, suggesting good
recyclability. These results are significant for constructing highꢀperformance porous polymer catalysts with tunable photoredox potentials
targeting an efficient material design for catalysis.
KEYWORDS: conjugated microporous polymers, visible light, photoredox catalysis, heterogeneous catalysis, Cꢀ3 functionalization,
indoles
compared with other catalysts such as small dyeꢀmolecules, enꢀ
sure high photocatalytic performance and good recyclability. In
addition, the flexible tuning of band gaps makes CMPs ideal canꢀ
didates for specific photocatalytic process. For example, strategies
like copolymerizations have been reported by doping appropriate
comonomer, and the obtained polymers have shown high photoꢀ
catalytic activity for hydrogen evolution, oxidation of benzylic
βꢀOꢀ4 alcohols, and cyclization of N, Nꢀdimethylanilines with
maleimides.
INTRODUCTION
3
ꢀSubstituted indoles, as important intermediates, have been
widely utilized in agrochemicals, alkaloids and pharmaceutical
1
drug candidates. Formylation and thiocyanation of indoles are
two important pathways for direct Cꢀ3 functionalization of inꢀ
doles. Generally, unrecyclable toxic transition metal (Cu or Ru)
catalysts under constant heating conditions (e.g. 400 K) are reꢀ
quired, leading to environment issues and excessive consumption
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2
of highꢀcost noble catalysts. Photocatalytic transformation driven
by sunlight shows promise in producing 3ꢀsubstituted indoles,
which are advantageous over transition metal catalysts. Current
photocatalysts like small dye molecules, however, suffer from
insufficient stability and difficulty in tuning optical and electronic
This study designed a series of carbazoleꢀcontaining CMPs as
efficient heterogeneous photocatalysts for Cꢀ3 functionalization of
indoles. The paper investigates the structural effect of CMPs on
their photocatalytic performance by incorporating different
electron donating or withdrawing building blocks (Figure 1a).
The asꢀmade CMPs show high porosity and their redox potentials
can be wellꢀtuned to meet the requirements of photoꢀinduced
transformation. The effects of redox potential and pore structure
on catalytic performance for the formylation and thiocyanation of
indoles are examined. The CMPs exhibit high efficiency in Cꢀ3
transformation for a broad range of indole substrates and
demonstrate high functional group tolerance. Compared to the
reported catalysts for Cꢀ3 formylation and thiocyanation of
2
properties. Therefore, it is highly desirable to develop metalꢀfree,
recyclable and heterogeneous catalyst systems for effective Cꢀ3
formylation and thiocyanation of indoles under visible light irraꢀ
diations. To the best of the authors’ knowledge, the use of organic
frameworks as photocatalysts for Cꢀ3 functionalization of indoles
has not yet been reported.
As an emerging class of organic porous materials, conjugated
microporous polymers (CMPs) combine extensive πꢀconjugated
skeletons with nanometerꢀscale pores to offer some advantages in
addressing energy and environmental issues such as gas capture
2
indoles, the designed CMP photocatalytic system works
3
4
5
efficiently at room temperature without the addition of strong
acids or bases while avoiding the use of strong oxidants. The sysꢀ
tem therefore possesses several advantages including cost
effectiveness (metalꢀfree and recyclability), simplicity (easy
operation and separation) and environmental friendliness.
and adsorption, heterogeneous catalysis, light emitting, energy
6
storage. Concerning the tunable optoelectronic properties of
CMPs at the molecular level, systematic studies have been carried
out to demonstrate their utility in various heterogeneous photoꢀ
7
catalytic reactions, including selective oxidation of thioether, oxiꢀ
4
a,8
9
dative coupling of primary amines,
radical polymerization,
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RESULTS AND DISCUSSION
hydrogen evolution and others. CMPs as catalysts possess
many noteworthy advantages. First, the robust porosityꢀrich naꢀ
ture and large surface area offer a shelter for substrates, rendering
substrate diffusion and product mass transfer more effective. Alꢀ
so, the good chemical and thermal stability of CMPs, especially
The nonꢀplanar rigid conformation, high lightꢀabsorption coeffiꢀ
cient, and electron donating ability of carbazole make it an ideal
scaffold in optoelectronic materials such as dyeꢀsensitized solar
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cells, perovskite solar cells and organic fieldꢀeffect transistor.
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Owing to these unique properties, incorporation of photosensitizꢀ
ing carbazole moiety into a polymer backbone may be conducive
to facilitating the migration of generated charges and suppressing
charge recombination. For this purpose, three carbazolic precurꢀ
sors called 1,4ꢀdi(9Hꢀcarbazolꢀ9ꢀyl)ꢀbenzene (DCB), 1,3,5ꢀtri(9Hꢀ
carbazolꢀ9ꢀyl)benzene (TCB) and 2,4,6ꢀtri(9Hꢀcarbazolꢀ9ꢀyl)ꢀ
cleophilic substitution was adopted to prepare the precursors.
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Carbazolic CMPs (hereafter denoted as CMPꢀCSU5, CMPꢀCSU6
and CMPꢀCSU7, synthesized from DCB, TCB and TCT, respecꢀ
tively) were obtained as insoluble solids via FeCl ꢀpromoted oxiꢀ
3
dative polymerizations in anhydrous CHCl under N protection
3
2
.
17
(Scheme S1) in high yields (90ꢀ95).
The porosity features of CMPs were characterized using N adꢀ
2
1
,3,5ꢀtriazine (TCT) were synthesized from 1ꢀbromoꢀ4ꢀ
iodobenzene, 1,3,5ꢀtribromobenzene and 2,4,6ꢀtrichloroꢀ1,3,5ꢀ
triazine, respectively. Either copperꢀpromoted Nꢀarylation or nuꢀ
sorptionꢀdesorption measurement. CMPꢀCSU5, CMPꢀCSU6 and
CMPꢀCSU7 revealed high BrunauerꢀEmmettꢀTeller (BET) surꢀ
2
−1
face areas of 978, 1281 and 811 m g , total pore volumes of
hv
1O2 andO2
3
−1
0.87, 0.96 and 0.58 cm g , respectively (Table 1). All CMPs
featured a hierarchical microꢀmesoporous structure; more imꢀ
portantly, over 43 % of the pore volume is contributed by mesoꢀ
pores, which appears to be promising shelters for the transforꢀ
mation of largeꢀsized substrates (Figure S1 and Table
0
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0
Donor
LUMO e
a)
O2
N
7b,18
1).
Scanning electron microscopy (SEM) images of CMPꢀ
CSU5, CMPꢀCSU6 and CMPꢀCSU7 displayed different morphoꢀ
logical structures (graphic flakeꢀlike, fiberꢀlike and plateꢀlike) at
3
+
the micrometer scale (Figure 2). Additionally, the residual Fe
Photoactive center
contents of CMPꢀCSU5, CMPꢀCSU6 and CMPꢀCSU7, which
were detected using inductively coupled plasmaꢀatomic emission
spectrometer (ICPꢀAES) measurements after fully degrading the
samples in nitrohydrochloric acid, were low at 1.1, 1.3 and 1.1‱ ,
respectively. These results indicate that the residual metal ions
were nearly completely removed.
O
S
N
N
N
N
N
N
HOMO h
Acceptor
N
N
NH4SCN
Table 1. Pore Parameters of CMPs
N
a
b
c
SABET
Vtotal
Vmeso
Vmeso/Vtotal
(%)
Sample
2
ꢀ1
3
−1
3
−1
N
(m g ) (cm g
)
(cm g
)
N
N
N
N
N
N
N
CMPꢀCSU5
CMPꢀCSU6
CMPꢀCSU7
CMPꢀCSU6@70
978
1281
811
0.87
0.96
0.58
1.36
1.32
0.49
0.63
0.25
0.69
0.74
55
44
43
51
56
N
N
N
CMP-CSU5
CMP-CSU6
CMP-CSU7
1717
b)
c)
CMPꢀCSU6@140 1451
a
Calculated using adsorption branches over the pressure range
b
0
.01ꢀ0.1 bar of N isotherm at 77 K. Calculated from the N2
2
3
−1 c
isotherm at P/P = 0.99 cm g . The volume of mesopores with
0
pore width >2 nm.
Figure 2. Scanning electron microscopy (SEM) images of CMPꢀ
CSU5 (a), CMPꢀCSU6 (b) and CMPꢀCSU7 (c)
0
day
2 days
The chemical structures of the asꢀprepared CMPs were conꢀ
firmed using Fourier transform infrared spectroscopy (FTꢀIR),
13
Figure 1. a) The structures of the photoredox catalysts CMPꢀ
CSU5, CMPꢀCSU6 and CMPꢀCSU7 and their applications in Cꢀ3
functionalization of indoles; b), c) Cꢀ3 formylation of 1ꢀmethylꢀ
solidꢀstate C nuclear magnetic resonance (NMR) and the ultraꢀ
violetꢀvisible spectroscopy (UV/Vis) diffuse reflectance spectrum.
Generally, the elimination of the absorption peak at around 720
−1
1
Hꢀindole tests were performed under solar radiation Changsha,
cm (assigned to the disubstituted phenyl ring in the carbazole
ꢀ1
China (08/04/2018ꢀ09/04/2018, 25ꢀ28 °C)
monomer) and newly generated peaks at around 800 cm (atꢀ
tributed to the vibration of CꢀH bonds on the diꢀor triꢀsubstituted
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ACS Catalysis
carbazole ring) implied successful polymerizations (Figures S2,
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a)
b)
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S3, S4 and Figure 3a) . The C CP/MAS NMR spectrum for
CMPꢀCSU6 (Figure S5) also offered valuable evidence for strucꢀ
ture identification. From the UV/Vis diffuse reflectance spectra,
the designed networks appeared to exhibit a broad absorption
band centered around 400 nm (Figure S6 and Figure 3b), and
their absorption band edges extended to the visible range (λ > 620
nm); the corresponding precursors exhibited minimal visible light
absorption. Such redꢀshift behavior could be attributed to the forꢀ
mation of extended πꢀconjugation chains, which indeed enhances
the visible range absorption. A detailed comparison between the
designed CMPs and a carbazoleꢀfree polymer such as PPꢀ
CMP@mmm indicated that the introduction of carbazolic donors
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benefited the broad absorption band.
Cyclic voltammetry measurements were conducted to investigate
the energy band structure of the polymers. The lowest unoccupied
molecular orbital (LUMO) position of CMPꢀCSU5, CMPꢀCSU6
and CMPꢀCSU7 was nearly the same and found to be lower thanꢀ
c)
hv
N
N
+ O2
O2
N
N
CMP-CSUs
0
.86 V (Figure S7), sufficiently negative to reduce oxygen into
•− 6
0–0
the active species O2 . Based on a free energy change (E
)
between the vibrational related excited state and the ground state
0
–0
7a
of 2.14, 2.29 and 2.08 eV (E = 1240/λ), the highest occupied
molecular orbital (HOMO) positions of CMPꢀCSU5, CMPꢀCSU6
and CMPꢀCSU7 were calculated to be 1.26, 1.42 and 1.19 V,
respectively (Figures S8 and 3d). Among the three samples,
CMPꢀCSU6 exhibited the highest oxidation potential; therefore, it
is expected to perform better in photocatalytic oxidation reaction
than the widely reported transition metal photocatalysts such as
(
I)
(II)
(III) (IV)
2+
21
[Ru(bpy)3] (+1.29 V vs. SCE). Furthermore, the LUMO (ꢀ
0.87 V) and HOMO (1.42 V) levels of CMPꢀCSU6 were suffiꢀ
cient for the redox process; the photoactive macromolecular sysꢀ
tems could effectively mediate the electron transfer from N, N,
8
N’N’ꢀtetramethylbenzeneꢀ1,4ꢀdiamine (+ 0.12 V vs SCE) to moꢀ
lecular oxygen, as verified by the occurrence of the colored catiꢀ
8
onic radical (Figure 3c insert). Among the three polymers,
d)
CMPꢀCSU6 demonstrated the highest intensity in UV/Vis absorpꢀ
tion spectra, and CMPꢀCSU7 ranked the lowest, which is conꢀ
sistent with the color depth of obtained solutions catalyzed by the
designed CMPs. These findings suggested the superior photooxiꢀ
dative activity of CMPꢀCSU6 over CMPꢀCSU5 and CMPꢀCSU7,
as evidenced by the highest oxidation potential (Figure 3c and
3
d). The electron paramagnetic resonance (EPR) spectra showed
an enhanced signal upon light irradiation, relative to that taken in
darkness, indicating photogenerated radicals, i.e., electron−hole
pairs in CMPꢀCSUs (Figure S9).
Given the promising photoredox properties of the designed
CMPs, their photocatalytic activities towards Cꢀ3 formylation of
indoles were investigated. 1ꢀmethylꢀ1Hꢀindole was chosen as a
model substrate, tetramethyl ethylenediamine (TMEDA) served as
the carbon source and molecular oxygen was a clean oxidant. The
reaction conditions were screened and optimized (Table 2 and
Table S1). Cꢀ3 formylation of indoles catalyzed by CSUꢀCMP6
afforded a satisfactory yield of 86%. Replacing CMPꢀCSU6 with
CMPꢀCSU5 or CMPꢀCSU7 was deleterious for the product yield
Figure 3. a) Partial FTꢀIR spectra of CMPꢀCSU5 (black), CMPꢀ
CSU6 (red) and CMPꢀCSU7 (blue); b) normalized solid state UVꢀ
Vis absorption spectra of CMPꢀCSU5 (black), CMPꢀCSU6 (red)
and CMPꢀCSU7 (blue); c) UVꢀvis absorption spectra and photoꢀ
graph of the cationic radical of TMPD generated by CMPꢀCSUs
in the presence of light and oxygen. Inserted images: (I): CMPꢀ
CSU5, (II): CMPꢀCSU6, (III):CMPꢀCSU7, (IV): No catalyst and
d) the LUMO and HOMO of CMPꢀCSU5, CMPꢀCSU6, and
CMPꢀCSU7
(
Table 2. entries 1, 2 and 5). The higher photocatalytic activity of
CMPꢀCSU6 could presumably be ascribed to its higher HOMO
level (compared to others) considering the comparatively lower
LUMO levels (ꢀ0.88 V, ꢀ0.87 V and ꢀ0.89 V) of the three CMPs.
A notably high HOMO level (1.42 V) could oxidize TMEDA
+•
(
TMEDA /TMEDA=0.65 V vs SCE) (Figure S10) more easily to
o
form radical cations. Most importantly, the designed CMPꢀCSU6
outperformed at a yield increase of 10% when compared to that
catalyzed by Rose Bengal with constant heating up to 60 C (Taꢀ
ble 2. entries 2 and 6). Also, a comparably high yield without the
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0
aid of high temperatures was obtained for the designed catalytic
system compared to Rose Bengal, CuCl , and I catalytic systems
CMPs (Figure 3c), consistent with known results. Research has
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reported that the formation of singlet oxygen ( O ) is possible
when CMPs are irradiated. To probe the feasibility of this pheꢀ
2
2
c,2f,22
7a
(
Table 2. entries 6, 7 and 8).
The gꢀC N , a stateꢀof art metꢀ
3
4
1
alꢀfree photocatalyst was also investigated under optimized condiꢀ
tion. However, only 31% yield of the product was obtained. (Taꢀ
ble 2. entry 9). Compared to metal complex Ru(bpy) Cl and the
nomenon, sodium azide (NaN ) which acts as a O scavenger was
3
2
added during photocatalytic transformation. A yield of 3a was
declined to 45% in the presence of NaN . It is clear that O also
1
3
2
3
2
dyes, the designed CMP catalysts exhibited certain advantages.
First, the robust porosityꢀrich nature and large surface area offered
a shelter for substrates, leading to more effective substrates diffuꢀ
sion and the product mass transfer. Second, the chemical and
thermal stability of CMPs remained good, even after being irradiꢀ
played an important role in the process of photocatalytic Cꢀ3
formylation of Nꢀmethylindole (Table S1. entry 6). The effect of
the water amount on yield for the formylation of Nꢀmethylindole
was investigated. At first, high yield retained with an increase in
water. (Table S1. entries 7 and 8). When the water reached 2 mL
or 3 mL, the yield decreased dramatically (Table S1. entries 9
and 10). A yield lower than 10% was observed when the water
was used as the solvent (Table S1, entry 11). These results
ated for
a long time. Comparatively, for metal complex
0
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0
1
2
3
4
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2
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0
Ru(bpy) Cl and the dye were usually unstable after being irradiꢀ
3
2
ated under visible light for a long time, the dye molecules are
usually unstable. The dye could also be subjected to “bleaching”
during the reaction. These patterns indicated that a high perforꢀ
mance catalytic system which operates under moderate reaction
conditions has been developed by simply altering the core nature
of CMPs. When referring to the methods of copolymerization to
demonstrate that the proportion of H O played a central role in the
2
formylation of Nꢀmethylindole.
Table 2. Selected Catalysts and Their Catalytic Efficiency for Cꢀ3
a
Formylation of NꢀMethylindole
8
,12,13
tune the redox potential of CMPs,
altering the core nature of
the precursors is an efficient strategy to fine tune of the redox
potential of taskꢀspecific CMPs for certain photoredox transforꢀ
mations.
Except for the redox potentials of CMPs, the mass transfer of
the substrate (related to pore features) was also thought to exert
certain effects on photocatalytic performance. To verify this asꢀ
sumption, CMPꢀCSU6 with different porosity features was preꢀ
pared by polymerization at different temperatures (denoted as
CMPꢀCSU6@T, Table 1 and 2, Figure S11) and the catalytic
performance of each was compared. Again, insoluble powders2
were obtained with surface areas, ranging from 1451 to 1717 m
b
c
entry catalyst
light temp
yield
67
ref
1
2
3
4
5
6
7
8
CMPꢀCSU5
+
+
+
+
+
+
ꢀ
RT
RT
RT
RT
RT
This work
This work
This work
This work
This work
[2c]
CMPꢀCSU6
86
CMPꢀCSU6@70
CMPꢀCSU6@140
CMPꢀCSU7
90
−1
3 ꢀ1
g
with total pore volumes of 1.36 and 1.32 cm g were obtained
(
Table 1). In general, CMPs with a higher surface area and larger
88
porosity demonstrate favorable mass transfer in chemical transꢀ
formations; results of this study are similar (Table 2. entries 2, 3
and 4). However, as the catalyst increases in surface area and pore
volume, the reaction efficiency in terms of yield did not increase
significantly. This trend suggests that the energy band structure is
more critical than porosity nature in developing CMPs for Cꢀ3
formylation of indoles when they deliver satisfactory porosity.
43
o
Rose Bengal
60 C
75
o
CuCl2
80 C
93
[2f]
o
I2
ꢀ
120 C 86
[22]
To demonstrate the product regioselectivity, 1,3ꢀdimethylindole
was subjected to the reaction system under the optimized condiꢀ
tions. No desired product was observed during this process, indiꢀ
cating that the formylation of indoles occurred at the Cꢀ3 position
9
gꢀC N4
+
RT
31
This work
3
a
Reaction conditions: Nꢀmethylindole (0.5 mmol), TMEDA (2
1
rather than others. The same conclusion can be drawn from the H
equiv), KI (4 eq), photocatalyst (20 mg), CH CN (5 mL) and 50
3
o
NMR spectrum analysis (Supporting information). To further
investigate the mechanism for Cꢀ3 formylation of indoles, several
control experiments were carried out. Only trace of or no target
product were detected by getting rid of either photocatalysts or
light irradiations (Table S1. entries 1 and 2), which verified the
necessity of a light source and photocatalyst in the reaction proꢀ
cess. Additional experiments were performed to explore the speꢀ
cific role of photogenerated electronꢀhole pairs in the photocataꢀ
lytic Cꢀ3 formylation (Table S1. entries 3 and 4). By adding
µL of H O, RT= 25 ± 2 C, O balloon (~0.1 MPa) using a 14 W
LED lamp (0.20 W/cm ) as the light source, 48 h. temp=reaction
2
2
,
2
b
c
temperature. Isolated yield (%).
A potential reaction mechanism for Cꢀ3 formylation of indoles
was also proposed (Scheme 1). Upon irradiation, the CMPꢀCSU6
catalyst was excited to CMPꢀCSU6*, and then reductively
•
−
quenched by oxygen to produce superoxide radical anion O
2
+
along with CMPꢀCSU6 via singlet electron transfer (SET). Simiꢀ
2c
TEMPO as a radical scavenger, a substantially reduced yield of
lar to the mechanism of oxidation of amines catalyzed by Ru with
9b,23
1
1% was obtained, while adding benzoquinone as a superoxide
tertꢀButyl hydroperoxide (TBHP)
, TMEDA (2) (+ 0.65 V vs
•−
+
radical anion O2 scavenger to the reaction mixture, led to an
even lower yield (9%). These results suggest that the superoxide
SCE) (Figure S10) was oxidized by CMPꢀCSU6 to form radical
cations 2A and to regenerate CMP in a photoredox cycle. Subseꢀ
quently, a hydrogen atom of a radical cation transferred to the
superoxide radical anion to form hydrogen peroxide anion and
iminium ion (2A→2B). An indole iminium ion (2B→3A) was
obtained by electrophilic addition of iminium ion to Nꢀ
radical participated in the photocatalytic reactions. When O was
2
replaced by N , the yield was quite poor, highlighting the imꢀ
2
portant role of oxygen in the reaction process (Table S1. entry 5).
O thus appeared to have been converted into superoxide radical
2
•−
anion O2 via single electron transfer from the excited state of
3
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ACS Catalysis
methylindole, which then released a hydrogen ion to provide an
obtained, suggesting that the reaction was not affected by steric
hindrance at Cꢀ2 position. For methyl substitution at different
benzene ring positions (i.e., 4ꢀ, 6ꢀ and 7ꢀposition), the methyl
group at different positions was found to exert no obvious impact
on yields (3i: 70%; 3s: 66%; 3t: 61%). The reactions between 2
and indoles functionalized by halogen (e.g., F, Cl and Br) on their
1
2
3
4
5
6
7
8
9
intermediate 3B (detected by LCꢀMS, Figure S12) and presumaꢀ
bly H O . After a second light photoredox cycle (3B→3C), an
2
2
iminium ion was formed and subjected to hydrolysis to generate
ꢀformylꢀNꢀmethylindole (3C→3a).
3
5
ꢀ or 6ꢀpositions resulted in the desired products in moderate
yields of 41ꢀ67 %. Also, the halogen groups retained well after
being subjected to optimized reaction conditions, providing a
potential site for further functionalization. The yields from indoles
with an electronꢀdonating group (OMe) and an electronꢀ
withdrawing group (F, Cl, Br) at the Cꢀ6 position were better than
those at the Cꢀ5 position (3j, 3k, 3l, 3m, 3n, 3o, 3p, 3q). Moreoꢀ
ver, 1,7ꢀdimethylꢀ1Hꢀindole and 4ꢀ(benzyloxy)ꢀ1ꢀmethylꢀ1Hꢀ
indole could also be subjected to the formylation reactions, and
satisfactory yields (61–70%) (3i, 3r, 3t) were obtained.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
To further verify the attracting photocatalytic properties of the
designed CMPꢀCSU6, efficiency of the Cꢀ3 thiocyanation of inꢀ
doles was examined. 1Hꢀindole was applied as a model substrate
and the conditions for lightꢀmediated thiocyanation were screened
and optimized (Table 4 and Table S2). A high yield of 95% could
be obtained using 2 equiv of NH SCN as an SCN source and tetꢀ
Scheme 1. Proposed Mechanism of Cꢀ3 Formylation of Indoles.
4
rahydrofuran as the reaction solvent. Notably, the reaction condiꢀ
tions for CMPꢀCSU6 catalytic system were much milder and free
of strong oxidants or metalꢀbased catalysts (Table 4) compared to
the catalysts disclosed
vealed that light, catalyst and O played critical roles in chemical
transformations (Table S2. entries 1, 2 and 3), of which the reacꢀ
tion mechanism was highly similar to the aforementioned
formylations (see Scheme 1 and Scheme 2). Upon light irradiaꢀ
tions, the CMPꢀCSU6 was excited to CMPꢀCSU6* and reductiveꢀ
ly quenched by oxygen to produce hydrogen peroxide and CMPꢀ
a,b
Table 3. Substrate Scope for Cꢀ3 Formylation of Indoles
2b, 2d, 2e, 24
. Further control experiments reꢀ
2
O
O
O
R
+
CSU6 via SET. The photoredox cycle is completed by a single
R1
N
N
N
−
+
•
electron transfer between SCN and CMPꢀCSU6 affording SCN
and CMPꢀCSU6 (Figure S10; Table S2. entry 9). Then, an elecꢀ
trophilic addition of radical attacks 1Hꢀindole afforded the interꢀ
mediate 4A. 4A was then attacked and oxidized to produce
4
R2
3
3
3
a, R2=CH3 (86%) 3d, R2=Boc (NR)
b, R2=Et (72%) 3e, R2=Ts (NR)
c, R2=Bn (64%) 3f, R2=H (67%)
3g, R1=CH3 (72%)
3h, R1=Ph (70%)
3i, R=CH3 (70%) 3l, R=Br (41%)
3j, R=F (51%)
3k, R=Cl (38%)
3m, R= OMe (55%)
R
O
O
2b,25
O
B.
Rearomatization of intermediate 4B by losing a proton
generated the final 3ꢀthiocyanoindole product 5f.
N
N
3p, R=Br (67%)
N
R
n, R=F (64%)
R
3
3
3
r, R=OBn (54%)
s, R=CH3 (66%)
3
t, R=CH3 (61%)
3o, R=Cl (44%) 3q, R= OMe (64%)
a
Reaction conditions: indole (0.5 mmol), TMEDA (2 equiv), KI
4 eq), photocatalyst (20 mg), 5 mL of CH CN and 50 µL of H O,
(
3
2
o
RT=25 ± 2 C, O (~0.1 MPa), using a 14 W LED lamp (0.20
W/cm ) as the light source, 48h. NR=No Reaction
2
2
b
Next, the general feasibility of CSUꢀCMP6 as a photocatalyst
for the Cꢀ3 formylation of indoles was elevated. A broad range of
substrates containing either electronꢀdonating or electronꢀ
withdrawing groups that connected to different indole positions
were investigated. The substrate scope of Cꢀ3 formylation of 1ꢀ
methylꢀ1H indoles was explored in Table 3. First, indoles conꢀ
taining electronꢀdonating groups (e.g., methyl, ethyl and benzyl)
on the nitrogen atom were all efficiently converted to their correꢀ
sponding products at respective in the yields of 86%, 72% and
Scheme 2. Proposed Mechanism of Cꢀ3 Thiocyanation of Indoles
Having established conditions for the standard Cꢀ3 thiocyanation
of indoles, the study next examined the generality and selectivity
of thiocyanation on various indole substrates. For Nꢀsubstituted
indoles, such as Nꢀmethylꢀ, Nꢀethylꢀ, and Nꢀbenzyl indoles, the
corresponding 3ꢀthiocyano products were afforded in high yields
of 98%, 96%, and 94%, respectively (Table 5. 5a, 5b and 5c).
However, the reaction failed when introducing a strong electronꢀ
withdrawing nitro group such as Boc and Ts group on N atom of
indole (Table 5. 5d and 5e). For an indole with a phenyl or meꢀ
6
4%. However, this investigation failed to achieve formylation of
indoles with an electronꢀwithdrawing group such as tꢀButyloxy
carbonyl (Boc) and 4ꢀToluene sulfonyl (T ) on the nitrogen atom.
s
This finding may result from the low electron density of indole
derivatives. The formylation of free (NꢀH) indoles afforded 3f in a
67% yield. For Nꢀmethyl indoles with a methyl or phenyl group
blocked at Cꢀ2 position (3g and 3h), yields of 72% and 70% were
4
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ACS Catalysis
Page 6 of 9
thyl group at the Cꢀ2 position, 5f and 5g were obtained in high
carried out using CMPꢀCSU6 as the photocatalyst and 1ꢀmethylꢀ
1Hꢀindole as the substrate. CMPꢀCSU6 could be used for another
five repeating cycles without a substantial decline in its catalytic
efficiency (Figure 4 Black), consistent with the observed trend of
almost no change in the FTꢀIR spectra for samples before and
after the photocatalytic reaction (Figure S13). High recyclability
of the catalyst was also found for Cꢀ3 thiocyanation (Figure 4
Red). These results demonstrate the apparent reusability of the
designed CMPꢀCSU networks as heterogeneous photocatalysts.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
yields, which may suggest that the steric hindrance at Cꢀ2 position
exerts no effect on the photocatalytic performance (Table 5. 5g
and 5h). Results also revealed that high yields (92ꢀ99%) (5i, 5j,
5
k, 5l, 5m, 5n, 5o, 5p, 5q, 5r, 5s, 5t) could be obtained by incorꢀ
porating either an electronꢀdonating or electronꢀwithdrawing
group on the benzene ring of indoles.
Table 4. Selected Catalysts or Oxidants and Their Catalytic Effiꢀ
ciency for Cꢀ3 Thiocyanation of 1HꢀIndolea
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
b
c
entry
oxidant
O2
catalyst
temp
yield
83
ref
o
1
2
3
4
Cu(OTf)2
80 C
[2d]
[24]
[2e]
[2b]
Oxone
Mn(OAc)3
O2
ꢀ
ꢀ
RT
98
o
120 C
83
Ru(bpy) Cl
RT
RT
87
3
2
5
6
O2
Ir(ppy)3
47
95
[2b]
Figure 4. Recyclability of CMPꢀCSU6 in Cꢀ3 formylation and
thiocyanation of indoles
O2
CMPꢀCSU6 RT
This
work
To illustrate the scalability of CMPꢀCSU6 as the photocatalyst,
the study scaled up the optimized conditions by 25 times using
1Hꢀindole as the substrate for Cꢀ3 thiocyanation. The starting
material was consumed as indicated by thin layer chromatography
(TLC) after 20 h. The residual mixtures were purified by column
chromatography to afford the desired product in 94% yield,
demonstrating the practical nature and scalability of the proposed
protocol. After purifications, CMPꢀCSU6 could be recovered
using simple filtration and before rinsing and then being reꢀused
in the transformation. The starting material was converted to the
desired product after 12 h, again reflecting the inherent robustness
of the desighed CMPꢀCSUs polymers as the photocatalysts.
a
Reaction conditions:1Hꢀ indoles (0.5 mmol), NH SCN (1.0
4
mmol, 2 equiv), catalyst (20 mg), THF (5 mL), 10 h, RT=25 ±
o
2
2
C, O (~0.1 MPa), using a 14 W LED lamp (0.20 W/cm ) as the
2
b
c
light source. temp=reaction temperature. Isolated yield (%)
a,b
Table 5. Substrate Scope for 3ꢀThiocyanation of Indole
CONCLUSION
In summary, this paper has developed a series of CMP catalysts
to effectively catalyze Cꢀ3 formylation and thiocyanation of inꢀ
doles. Altering the nature of the core (phenyl or triazine units)
was found to be an effective method for regulating the photoredox
potentials of the CMPs. Furthermore, the photocatalytic process
appeared not to introduce toxic metals or organic oxidizing agents
and the photocatalysts showed good recyclability without a subꢀ
stantial decrease in photocatalytic efficiency. This study also
highlights that this design strategy for optimizing photocatalytic
efficiency could be extended to prepare a wider range of conjuꢀ
gated microporous polymeric photocatalysts for other organic
transformations.
a
Reaction conditions: Indoles (0.5 mmol), NH SCN (1.0 mmol, 2
4
o
equiv), catalyst (20 mg), THF (5 mL), RT=25 ± 2 C,10 h, O
2
2
(
~0.1 MPa), using a 14 W LED lamp (0.20 W/cm ) as the light
ASSOCIATED CONTENT
Supporting Information
b
source. NR=No Reaction
Aside from photocatalytic activity and substrate tolerance, the
recyclability of the catalyst remained one of the most important
parameters. For Cꢀ3 formylation, recycling experiments were
Additional details such as materials and instruments, synthesis
procedures, FTꢀIR spectroscopy, NMR spectra, UV/Vis diffuse
reflectance spectrum, cyclic voltammetry, LCꢀMS data. The Supꢀ
5
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ACS Catalysis
porting Information is available free of charge on the ACS Publiꢀ
cations website.
BINOLꢀDerived Phosphoric Acid Based on a Chiral Carbaꢀ
zolic Porous Framework. Org. Lett. 2017, 19, 6072ꢀ6075. (b)
Kaur, P.; Hupp, J. T.; Nguyen, S. T. Porous organic polyꢀ
mers in catalysis: opportunities and challenges. ACS Catal.
1
2
3
4
5
6
7
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AUTHOR INFORMATION
Corresponding Authors
2
011, 1, 819ꢀ835 (c) Broicher, C.; Foit, S. R.; Rose, M.;
Hausoul, P. J. C.; Palkovits, R. A BipyridineꢀBased Conjuꢀ
gated Microporous Polymer for the IrꢀCatalyzed Dehydroꢀ
genation of Formic Acid. ACS Catal. 2017, 7, 8413ꢀ8419. (d)
Chen, Y.; Ji, G.; Guo, S.; Yu, B.; Zhao, Y.; Wu, Y.; Zhang,
H.; Liu, Z.; Han, B.; Liu, Z. Visibleꢀlightꢀdriven conversion
*C. Pan (panchunyue@csu.edu.cn)
*G. Yu (gilbertyu@csu.edu.cn).
Notes
of CO from air to CO using an ionic liquid and a conjugated
The authors declare no competing financial interests.
2
polymer. Green Chem. 2017, 19, 5777ꢀ5781. (e) Kundu, D.
S.; Schmidt, J.; Bleschke, C.; Thomas, A.; Blechert, S. A
Microporous BinolꢀDerived Phosphoric Acid. Angew.
Chem.,Int. Ed. 2012, 51, 5456ꢀ5459. (f) Wan, J. L.; Wang,
C.; deKrafft, K. E.; Lin, W. B. Crossꢀlinked Polymers with
1
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4
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5
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5
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0
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ACKNOWLEDGMENTS
We acknowledge the financially support from the National Sciꢀ
ence Foundation of China (Nos. 21674129 and 21636010), the
Hunan Provincial Science and Technology Plan Project, China
2+
Exceptionally High Ru(bipy)3 Loadings for Efficient Hetꢀ
(
(
No.2016TP1007), State Key Laboratory of Fine Chemicals
KF1604) and Innovation Mover Program of Central South Uniꢀ
erogeneous Photocatalysis. ACS Catal. 2012, 2, 417ꢀ424.
5) (a) Liras, M.; Iglesias, M.; Sánchez, F. Conjugated
microporous polymers incorporating BODIPY moieties as
lightꢀemitting materials and recyclable visibleꢀlight
photocatalysts. Macromolecules. 2016, 49, 1666ꢀ1673. (b)
Xu, Y.; Chen, L.; Guo, Z.; Nagai, A.; Jiang, D. Lightꢀ
Emitting Conjugated Polymers with Microporous Network
Architecture: Interweaving Scaffold Promotes Electronic
Conjugation, Facilitates Exciton Migration and Improves
Luminescence. J. Am. Chem. Soc. 2011, 133, 17622ꢀ17625.
(
versity (2018CX046). Q. Huang acknowledge the financially
support from Fundamental Research Funds for the Central Uniꢀ
versities of Central South University (2016zzts253). W. J. Zhang
gratefully also acknowledge the NMR measurements by The
Modern Analysis and Testing Center of CSU.
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ACS Catalysis
Table of Contents
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A series of carbazoleꢀbased conjugated microporous polymers (CMPs) with tunable redox potentials
were constructed by regulating the core structure, demonstrating high photocatalytic efficiency towards
Cꢀ3 formylation (up to 90%) and thiocyanation (exceeding 93% yield) of indoles under mild conditions
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