Visible-Light-Enhanced Suzuki–Miyaura Reactions of Aryl Chlorides in Water with Pd NPs Supported on a Conjugated Nanoporous Polycarbazole

Article abstract of DOI:10.1002/cssc.201802918

The visible-light-enhanced catalytic activation of aryl chlorides for Suzuki–Miyaura cross-coupling (SMC) reactions is highly challenging because of the strength of the C?Cl bond. In this work, palladium nanoparticles (Pd NPs) were grown on a conjugated nanoporous polycarbazole (CNP), named Pd/CNP. The hybrid material Pd/CNP could catalyze the SMC reactions of aryl chlorides with arylboronic acids in water under blue LED irradiation at room temperature with high efficiency. This protocol exhibited good functional group tolerance and the catalyst could be recycled without significant loss of its catalytic activity. CNP not only provided photogenerated electrons to enrich the electron density of the Pd NPs but also generated holes for the activation of the arylboronic acids.

Full text of DOI:10.1002/cssc.201802918

Accepted Article
Title: Visible-Light-Enhanced Suzuki-Miyaura Reactions of Aryl
Chlorides in Water Using Pd NPs Supported on a Conjugated
Nanoporous Polycarbazole
Authors: Bin Guo, Hong-Xi Li, Cheng-Hao Zha, David James Young,
Hai-Yan Li, and Jian-Ping Lang
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10.1002/cssc.201802918
ChemSusChem
FULL PAPER
Visible-Light-Enhanced Suzuki-Miyaura Reactions of Aryl
Chlorides in Water Using Pd NPs Supported on a Conjugated
Nanoporous Polycarbazole
Bin Guo,[a,b] Hong-Xi Li,*[a] Cheng-Hao Zha,[a] David James Young,[c] Hai-Yan Li,[a] and Jian-Ping
Lang*[a,b]
Abstract: The visible-light-enhanced catalytic activation of aryl chlorides for Suzuki−Miyaura cross-coupling (SMC) reactions is highly
challenging because of the strength of the C-Cl bond. In this work, we employed palladium nanoparticles (Pd NPs) to grow on a conjugated
nanoporous polycarbazole (CNP). The as-prepared hybrid material Pd/CNP could high efficiently catalyze the SMC reactions of aryl chlorides
with arylboronic acids in water under blue LED irradiation at room temperature. This protocol exhibited good functional group tolerance and
the catalyst could be recycled without significant loss of its catalytic activity. CNP not only provided photogenerated electrons to enrich the
electron density of the Pd NPs but also generated holes for the activation of the arylboronic acids.
Introduction
coupling reactions at room temperature is particularly
challenging.[15] Generally, excess strong base and/or high
temperatures (>100 ºC) are required to activate aryl chlorides
Palladium-catalysed Suzuki-Miyaura cross-coupling (SMC)
reactions have been widely used for the synthesis of bi- and
multi-aryl fine chemicals, pharmaceuticals, and materials.[1] The
traditional cross-coupling reactions are performed using
homogenous or heterogeneous Pd catalytic systems at relatively
high temperatures, which are incompatible with some functional
groups.[2] New catalytic systems based on efficient, recyclable
catalysts and green energy sources are highly desirable.[3]
Sunlight is the archetypal clean, abundant and sustainable
source of energy. In 2014, Mori, Yamashita and co-workers
demonstrated a visible-light-enhanced Suzuki-Miyaura reaction
due to the chemically inert nature of the C-Cl bond.[16]
A
heterogeneous catalyst that could activate inexpensive and less
toxic aryl chlorides by making use of photo-enhancement would
be a significant breakthrough.
Polycarbazoles have been widely employed as conjugated
nanoporous polymers for optical materials,[17] gas adsorption,
separation materials[18] and catalysts.[19a-h] Electron-hole
separation of polycarbazoles can be achieved by light irradiation,
followed by electron or hole transfer to a guest molecule.
Nanoporous polycarbazoles have very poor solubility in solvents
and possess strong visible light absorption to act as
photosensitizers, and their N sites could stabilize metal NPs or
molecular catalysts, which endows them as attractive candidates
for photocatalyst supports.[19i-k] SMC reactions involve electron
transfer between the transition metal catalyst and organic
substrates. Irradiation of a photocatalyst can produce energetic
electrons at surface catalytic sites, potentially avoiding the need
for elevated temperatures or sacrificial reagents. Herein, we
demonstrate the use of a conjugated nanoporous polycarbazole
to support Pd NPs for efficient visible light-promoted catalysis of
Suzuki-Miyaura cross-coupling of aryl chlorides with arylboronic
acids in water at room temperature. The protocol exhibited good
functional group tolerance and the catalyst could be recycled
without significant loss of its catalytic activity. Mechanism study
shows that CNP not only provided photogenerated electrons to
enrich the electron density of the Pd NPs but also generated
holes for the activation of the arylboronic acids.
using
a
Ru-Pd bimetallic complex.[4] Au-Pd nanostructure
photocatalysts have also been developed by making use of the
localized surface plasmon resonance of gold nanocrystals to
enhance SMC.[5] Other recently developed cooperative
photocatalysts include GO-Pd@Ag-AgBr (GO
= graphene
oxide),[6] 2H-WS2/Pd,[7] Pd-MoS2,[8] Pd/SiC[9] and Pd-Au/TiO2.[10]
Polydopamine nanofilms,[11] poly(benzoxadiazole)[12], thioether-
containing covalent organic frameworks[13] and metal organic
frameworks[14] with broad light absorption have also been used
as visible light-harvesting interfaces for Pd nanocrystal catalyzed
coupling reactions. However, the aforementioned photocatalytic
SMC reactions are limited to aryl iodides or bromides. Visible-
light-driven photocatalytic activation of aryl chlorides for cross-
[a]
[b]
[c]
B. Guo, Prof. Dr. H.-X. Li, C.-H. Zha, H.-Y. Li, Prof. Dr. J.-P. Lang
College of Chemistry, Chemical Engineering and Materials Science
Soochow University
Suzhou 215123 (China)
E-mail: lihx@suda.edu.cn; jplang@suda.edu.cn
B. Guo, Prof. Dr. J.-P. Lang
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences
Shanghai 200032 (China)
Prof. Dr. D. J. Young
Results and Discussion
Synthesis and characterization of Pd/CNP catalyst
Conjugated nanoporous polycarbazoles (CNP) were prepared
via the two procedures (Scheme 1, see Supporting Information).
3,5-Di(9H-carbazol-9-yl)pyridine (3,5-Cz) was obtained by CuI-
catalyzed cross-coupling of 3,5-dibromopyridine with 9H-
carbazole in the presence of CsCO3 at 110 ºC in toluene. The
FeCl3-mediated oxidation of 3,5-Cz in CHCl3 produced
CNP.[19c,i,j] The Pd NPs supported on CNP were prepared by
College of Engineering, Information Technology and Environment
Charles Darwin University
Northern Territory 0909 (Australia)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cssc.201802918
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reduction of Pd(OAc)2 with NaBH4 at room temperature in water.
The Brunauer-Emmett-Teller (BET) surface area of the resulting
Pd/CNP material was determined to be 865 m2/g (Table S1),
with a Type I nitrogen adsorption profile (Figure S1). This value
is 34 m2/g lower than that of the non-metalated CNP. Inductively
coupled plasma optical emission spectrometry (ICP-OES) of
Pd/CNP indicated 1.37 wt% Pd loading on the semiconducting
polymer CNP. The powder X-ray diffraction (PXRD) patterns of
this material showed four peaks at 2θ = 40.12º, 46.66º, 68.12º
and 82.10º, which correspond to the (111), (200), (220) and
(311) crystal planes of metallic Pd (Figure 1a). X-ray
photoelectron spectroscopy (XPS) of Pd/CNP (Figure 1b)
revealed peaks at 336.03 eV and 341.40 eV that were assigned
to Pd0 3d5/2 and Pd0 3d3/2, accompanied by two peaks at 337.63
eV and 343.20 eV belonging to PdII 3d5/2 and PdII 3d3/2. The
positive shift of 0.16 eV and 0.18 eV in the N 1s binding
energies of the pyridinic N and the pyrrolic N of Pd NPs/CNP,
compared with the corresponding values in the parent CNP,
indicated the N coordination to Pd. The TEM image showed the
homogeneous distribution of Pd NPs on the support with an
average particle size of 4.2 nm (Figure 1d). The high-resolution
TEM (HRTEM) image (Figure 1e) revealed lattice fringes of the
crystalline Pd NPs, with an interplanar spacing of 0.228 nm
which agrees well with the (111) lattice plane of metallic Pd.
Elemental mapping showed the distribution of Pd NPs (Figure
1f) to be evenly dispersed throughout the CNP supporter.
Notably, the Pd/CNP composite showed an enhancement in the
absorption of visible light relative to the polymer alone, which
was due to the interband transition and plasmonic effects of Pd
NPs.[11] The band gap energy (Eg) was determined from the
diffuse reflectance spectrum (Figure 2b) using the Kubelka-
Munk method. The energy band gaps obtained by extrapolation
of the linear portion of the absorption edge were 2.03 and 1.84
eV for CNP and Pd/CNP, respectively. Cyclic voltammetry (CV)
of CNP revealed an oxidation potential at 1.29 V, which was
assigned to the CNP+/CNP couple (Figure S2). The HOMO and
LUMO of CNP were calculated to be 1.14 V and -0.89 V vs
saturated calomel electrode (SCE) from the CV. The Eg
decrease of Pd/CNP relative to the parent polymer suggests the
photoexcited electrons were transferred from the LUMO of CNP
to the Pd NPs. These added electrons on the Pd NPs could
enhance the ability of this catalyst to activate aryl halides.[5c,15]
Upon excitation at ca. 410 nm, CNP in EtOH exhibited broad
emissions between 445 and 630 nm with a maxima at 505 nm,
which can be ascribed to the partially conjugated structure of
polycarbazoles (Figure 2c).[17a] The quantum yield of CNP was
measured to be 2.2%. As expected, the fluorescence of CNP
was quenched by Pd NPs (Figure 2c), which further confirmed
the efficient transfer of photoexcited electrons from CNP to the
Pd NPs. The electron paramagnetic resonance (EPR) signal of
Pd/CNP under visible light was more intense than that of CNP
(Figure S3), indicating a more efficient separation of the
photoexcited electron-hole pairs in Pd/CNP.
As shown in Figure 2a, CNP exhibited a broad absorption in
the UV-visible region with the absorption edge at ca. 800 nm.
Figure 1. (a) PXRD patterns of CNP and Pd/CNP. (b) Pd 3d XPS spectra of Pd NPs/CNP. (c) N 1s XPS spectra of CNP and Pd/CNP. (d) TEM image of Pd/CNP
(inset histogram illustrating the particle size distribution). (e) HRTEM image of Pd/CNP. (f) STEM-HAADF image and STEM-EDS elemental mappings of Pd/CNP.
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Table 1. Optimization of reaction conditions for photocatalysed SMC reactions.[a]
Yield (%)
Entry
Cat.
Base (eq.)
Solvent
3aa
85
3aa’
trace
<5
1
2
Pd/CNP
Pd/CNP
Pd/CNP
Pd/CNP
Pd/CNP
Pd/CNP
Pd/CNP
Pd/CNP
Pd/CNP
Pd/CNP
Pd/CNP
Pd/CNP
Pd/CNP
Pd/CNP
Pd/CNP
Pd/CNP
Pd/CNP
Pd NPs
CNP
K3PO4 (1.5)
K3PO4 (1.5)
K3PO4 (1.5)
K3PO4 (1.5)
K3PO4 (1.5)
K3PO4 (1.5)
K3PO4 (1.5)
K3PO4 (1.5)
K2CO3 (1.5)
Na2CO3 (1.5)
KOH (1.5)
NaOH (1.5)
tBuOK (1.5)
NEt3 (1.5)
H2O
CH3OH
C2H5OH
iPrOH
DMF
DMSO
CH3CN
CHCl3
H2O
82
3
79
<5
4
81
<5
5
83
10
6
56
21
7
52
18
8
35
15
9
68
<5
10
11
12
13
14
15
16
17
18
19
20
21[b]
22[c]
23
24[b,d]
H2O
71
<5
H2O
61
10
H2O
57
12
H2O
43
10
H2O
trace
trace
trace
61
trace
trace
trace
<5
iPr2NEt (1.5)
DBU (1.5
H2O
H2O
K3PO4 (1.0)
K3PO4 (1.5)
K3PO4 (1.5)
-
H2O
H2O
32
15
H2O
trace
trace
12
trace
trace
10
Pd/CNP
Pd/CNP
Pd NPs
Pd/C
H2O
K3PO4 (1.5)
K3PO4 (1.5)
K3PO4 (1.5)
K3PO4 (1.5)
H2O
H2O
58
9
H2O
<5
<5
Figure 2. The UV−vis spectra (a), solid-state optical diffuse-reflection
spectra (b) and emission spectra excited at 410 nm in EtOH (c) of
CNP and Pd NPs/CNP.
Pd/CNP
H2O
33
15
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), Cat. (8 mg, 0.5 mol% Pd), base
(eq.), solvent (5 mL), blue LED irradiation, room temperature, 48 h and HPLC yield. [b]
without blue LED irradiation. [c] Ru(2,2’-bpy)3Cl2 (5 mol%) as photocatalyst. [d] at
80 °C.
Photocatalysis
The catalytic activity of Pd/CNP was initially examined
using the SMC reaction of 1-(4-chlorophenyl)ethan-1-one
(1a) with phenylboronic acid (2a) and K3PO4 in water under a
nitrogen atmosphere (Table 1, entry 1). A standard workup
produced 1-([1,1'-biphenyl]-4-yl)ethan-1-one (3aa) in 85% yield
with a trace of 1,1'-biphenyl (3aa’) as determined by high
performance liquid chromatography (HPLC). High yields could
also be obtained in alcohols and DMF (entries 2-5). The same
reaction gave lower yields with lower selectivity in DMSO,
CH3CN and CHCl3 (entries 6-8). The screening of different
bases (entries 9-16) revealed that KOH was a little more
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10.1002/cssc.201802918
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selective. Reducing the amount of K3PO4 reduced the yield of 3a
(entry 17). Finally, control experiments indicated that CNP, Pd
NPs,[20] K3PO4 and blue light irradiation were all indispensable
(entries 19-21). Thus, the optimal reaction conditions for photo-
inducted SMC reactions were Pd/CNP (8 mg, 0.5 mol% Pd), 1.5
equiv. of K3PO4, the irradiation of blue LED for 48 hours, water
as the solvent at room temperature. When Pd NPs/CNP was
replaced by Pd NPs suspension combined with Ru(2,2’-bpy)3Cl2
as a photosensitizer, 3aa was isolated in 58% yield. Compared
with Pd/CNP, commercially available Pd/C showed much lower
catalytic activity, although it could also absorb visible light. The
lack of electron transfer between carbon and palladium could not
increase the catalytic activity of Pd NPs. The lower conversion
and lower selectivity for SMC reaction of 1a and 2a were
observed when the reaction was carried out at 80 °C in the
absence of blue LED irradiation.
reacted smoothly to produce the corresponding products 3aa-
3ad and 3ak-3al in high yields (75-86%). Substrates with an
electron-withdrawing group, including -CN (2e), -NO2 (2f), -CF3
(2g), -CHO (2h), -COCH3 (2i) and -F (2j), provided the desired
products 3ae-3aj in 70%-79% isolated yields. Heteroatom-
containing arylboronic acid benzo[b]thiophen-2-ylboronic acid
(2m)
also
gave
the
expected
product
3-phenyl-
benzo[b]thiophene (3am) in 71% yield. Ortho-substituted
arylboronic acids sterically hindered coupling as evidenced by
reduced yield (3al) compared to the corresponding reactions of
the meta (3ak) and para (3ab) substituted counterparts.
We also performed the cross-coupling reaction of 2a with a
variety of substituted aryl chlorides. Aryl chlorides (1a-1f)
substituted with electron-withdrawing groups were transformed
into the expected diaryl compounds 3aa-3fa in good yields
(73%-87%). Satisfactorily, chlorobenzene and 1-chloro-4-
methylbenzene with 2a could also be transformed into 1,1'-
biphenyl (3aa’) and 4-methyl-1,1'-biphenyl (3ga) in moderate
yields. Meta-isomeric substrates (1h-1i and 1k-1l) reacted
cleanly and gave 3ha-3ia and 3ka-3la in 78-90% yields. 1-(2-
Chlorophenyl)ethan-1-one provided 1-([1,1'-biphenyl]-2-yl)ethan-
1-one (3ja) in a reduced isolated yield (66%). Reactions of
heterocyclic aromatic chlorides 1m-1s with 2a gave 3ma-3sa in
65-80% yields. Under the visible light, Pd/CNP materials show
high photocatalytic activity for cross-couplings of arylchlorides
and arylboronic acids because (i) CNP has wide absorption
range and narrow energy gap to yield more electron-hole pair; (ii)
the introduction of Pd NPs minimizes the recombination of the
photogenerated electron–hole pairs, (iii) the photogenerated
electrons enrich the electron density of Pd NPs, increasing the
activity; (iv) the photogenerated holes activate arylboronic acids.
Pd/CNP also displayed high catalytic activity toward the visible-
light-inducted cross-coupling reactions of aryl bromides with
arylboronic acids (Scheme S2). For the coupling of iodobenzene
with 4-methylphenylboronic acid, the catalytic activity of Pd/CNP
composites (TOF = 193 h-1 in an hour with 99% yield) is in-
between that of Cu1Pd2@NH2-Uio-66(Zr)[21] (TOF = 141 h-1) and
that of Pd@NH2-Uio-66(Zr)[3b] (TOF = 422 h-1).
Scheme 1. Isolated yields of cross-coupling products from aryl chlorides and
arylboronic acids. Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), Pd/CNP (8
mg, 0.5 mol%), K3PO4 (1.5 eq.), H2O (5 mL), blue LED irradiation, at room
temperature, for 48 h.
Substrate scope was investigated with a variety of aryl
chlorides and arylboronic acids under the optimized conditions
(Scheme 1). Good and high yields of diaryl products were
obtained using various arylboronic acids as the cross-coupling
partners. The electronic nature of the substituents on the phenyl
rings had some influence on the yields. Arylboronic acids with
electron-rich or electron-neutral groups (2a-2d and 2k-2l)
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Figure 3. (a) Recycling of Pd/CNP catalyst for the synthesis of 3aa. (b) TEM
image of Pd NPs for Pd/CNP after the fourth run (Histograms illustrating the
particle size distribution are shown in the insets). (c) PXRD patterns and (d)
Pd 3d XPS spectra of Pd/CNP catalyst after the fourth run.
with those in fresh Pd/CNP (Figure 1b). The positive shifts were
caused by Pd-O species, which were probably formed from air
oxidation of Pd atoms on the surface of Pd NPs in the presence
of base.[22] As a heterogeneous catalyst, the possibility of Pd
leaching from this heterogeneous catalyst was investigated
during the reaction of 1a with 2a under standard conditions.
After 12 h, the yields of 3aa were 32% accompanied by <5%
3aa’. At this point, the mixture was filtered to remove the catalyst
and the filtrate was irradiated for another 36 hours at room
temperature. No further reaction was observed, indicating that
the as-prepared Pd/CNP was necessary for this reaction and
that the leaching of catalytically active Pd was insignificant if any.
Furthermore, a mercury poisoning experiment was performed.[23]
When one drop of mercury was added to the reaction mixture
under the standard conditions, 3aa is isolated only in 28% yield.
In this process, a Pd0 amalgamate was formed, which exhibits
low catalytic activity.
The reusability of this Pd/CNP catalyst was examined by
extracting the product with Et2O and adding fresh substrates 1a
and 2a to the lower aqueous layer for the next run. No significant
loss of catalytic activity was observed even after the catalytic
system was re-used four times. In the first four cycles, more than
80% of 1a was converted into 3aa (Figure 3a). After four cycles,
the ICP analysis indicated that 92% of Pd was remained. The
TEM image showed homogeneous distribution of Pd NPs on the
CNP with an average particle size of 5.09 nm (Figure 3b). The
PXRD patterns showed strong Pd peaks (Figure 3c). The
binding energies of Pd 3d (Figure 3d) in the reused Pd/CNP
composite slightly shifted to a higher binding energy compared
O
O
O
Conditions
Cl
B(OH)2
+
+
+
O
2a
3ai
3aa'
1a
3aa
additive
under blue LED
under blue LED
under blue LED
at 80 oC in dark
at 80 oC in dark
at 80 oC in dark
TEMPO
DIPEA
24%
11%
<5%
13%
10%
<5%
trace
10%
trace
trace
12%
trace
12%
trace
trace
25%
<5%
<5%
TEMPO + DIPEA
TEMPO
DIPEA
TEMPO + DIPEA
Scheme 2. Control experiments.
To gain more insight into these reactions, 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO) as an electron scavenger
or diisopropylethylamine (DIPEA) as a hole scavenger were
added into the photocatalytic reaction system. The addition of
TEMPO gave 3aa in 24% yield and the homocoupled byproduct
1,1'-biphenyl (3aa’) in 12% yield (Scheme 2). The catalytic
efficiency of this cross-coupling reaction decreased in the
presence of DIPEA (0.4 mmol) as only 3aa (11% yield) was
obtained along with a homocoupled side-product 1,1'-([1,1'-
biphenyl]-4,4'-diyl)bis(ethan-1-one) (3ai) in 10% yield. Upon
addition of both TEMPO and DIPEA, 3aa was isolated in <5%
yield without any homocoupled byproducts. The obvious
inhibitory effect of TEMPO and DIPEA indicated that the
photogenerated electrons and holes played major roles in these
photocatalytic coupling reactions. When the reactions were
carried out in dark at 80 °C, the addition of TEMPO (or DIPEA)
decreased the yield of the desired product 3aa but increased the
yield of the homocoupled product 3aa’ (or 3ai). The catalyst was
almost inactive for the cross-coupling in presence of TEMPO
and DIPEA. The mechanism of photocatalysis is similar to that
of the thermal reaction. But the electrons from the excited state
of CNP transfer more readily than those from the ground state of
CNP to Pd NPs.
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Scheme 3. Proposed mechanism of the photocatalytic SMC using Pd/CNP.
dropwise to anhydrous FeCl3 (0.3240 g, 2.0 mmol) in anhydrous
chloroform (15 mL). The resulting mixture was stirred under a nitrogen
atmosphere for 24 h at room temperature. Methanol (20 mL) was then
added into the mixture, which was stirred for another
1 h. The
In light of the above experimental results and previous
literature reports,[7-9,12] we proposed a reaction mechanism as
depicted in Scheme 3. Upon blue LED irradiation, CNP
generates electron-hole pairs, followed by transfer of the
photoexcited electrons to Pd0 leading to the separation of the
excited charges and holes. Electron-rich Pd0 then undergoes
oxidative addition of aryl chlorides (Ar-Cl), affording the
intermediate Ar-PdII–Cl (I). Meanwhile, the holes generated
inside the CNP can activate the C-B bond of the borate salt
precipitated polymer was filtered, and washed with hydrochloric acid for 1
h, followed by washing with water and methanol. The solid was purified
using Soxhlet extraction with methanol for one week before drying in a
vacuum oven at 80 °C for 12 h to give CNP (0.0680 g, 83%). 13C MAS
NMR (101 MHz): δ 140, 135, 125, 110.
Preparation of Pd/CNP. Pd(OAc)2 (11.2 mg) was added to a three-
necked flask with 50 ml water under a N2 atmosphere. The resulting
mixture was stirred for 30 min at 1000 rpm at room temperature. CNP
(120 mg) was added into the Pd(OAc)2 solution with stirring for 3 h.
Freshly prepared aqueous NaBH4 solution (0.05 M, 5.0 mL, 5 eq.) was
added by syringe and the mixture was stirred for a further 4 h at 1000
rpm under N2 atmosphere at room temperature. After this period, the
solid Pd NPs/CNP was separated by centrifugation, washed sequentially
with water, ethanol and ethyl ether, and then dried under vacuum.
3-
produced from the addition of PO4 to aryl boronic acid (Ar’-
B(OH)2). Transmetallation then generates
a
diaryl Pd
intermediate Ar-PdII–Ar’ (II) which experiences the reductive
elimination to give the diaryl product and regenerates the Pd0
catalyst.
Suzuki-Miyaura cross-coupling reactions. A quartz tube equipped with
a magnetic stirring bar was charged with aryl chloride (0.2 mmol),
arylboronic acid (0.3 mmol), Pd/CNP (8 mg), K3PO4 (0.3 mmol) and
water (5 mL). The reaction mixture was purged with N2 for 10 min in the
dark and then stirred continuously under blue LED irradiation for 48 h
with simultaneous cooling by a mechanical fan to keep the temperature
at approximately room temperature. After this period, the catalyst was
separated by centrifugation and washed with ethyl acetate and water.
The solution was extracted three times with ethyl acetate (3 × 5 mL) and
the combined organic layers were washed with brine, dried over
anhydrous Na2SO4 and concentrated under reduced pressure. The crude
product was purified by flash column chromatography using petroleum
ether and ethyl acetate as the eluent.
Conclusions
In the work reported here, we have demonstrated efficient
visible-light-promoted SMC reactions of aryl chlorides with aryl
boronic acids in water at room temperature catalyzed by Pd NPs
loaded on to a CNP. This catalytic system showed good
tolerance for a variety of functional groups. The composite
photocatalyst Pd/CNP could be recycled and reused without
significant decay of catalytic activity after multiple runs.
Extending the application of this photocatalytic system to other
reactions is an ongoing endeavor in our laboratory.
Acknowledgements
Experimental Section
We gratefully acknowledge financial support from the National
Natural Science Foundation of China (Grant No. 21771131,
21471108, 21531006 and 21773163), the Natural Science
Foundation of Jiangsu Province (BK20161276), the State Key
Laboratory of Organometallic Chemistry of Shanghai Institute of
Organic Chemistry (Grant No. 2018kf-05), the "Priority Academic
Program Development" of Jiangsu Higher Education Institutions,
and the Project of Scientific and Technologic Infrastructure of
Suzhou (SZS201708). We are also very grateful to the useful
comments and suggestions of the editor and reviewers.
General information. All chemicals were commercially available and
used as received without further purification. The analytical instruments
employed in this work were as described in our previous article, unless
otherwise noted.[24] The STEM-HAADF image and STEM-EDS elemental
mappings of Pd/CNP were performed on
a JEOL 2100F electron
microscope operating at 200 kV, equipped with EDS detector.
Preparation of 3,5-Cz. A mixture of 3,5-dibromopyridine (0.47 g, 2.0
mmol), 9H-carbazole (0.70 g, 4.2 mmol), CuI (0.04 g, 0.2 mmol) and
Cs2CO3 (0.97 g, 3.0 mmol) was added to a Schlenk tube equipped with a
stirring bar. Anhydrous toluene (20 mL) was added by syringe and the
solution stirred at 110 °C for 72 h under a nitrogen atmosphere. After
cooling to room temperature, the reaction was quenched with water and
extracted three times with ethyl acetate (3 × 15 mL). The combined
organic layers were washed with brine (20 mL), dried over anhydrous
Na2SO4 and concentrated under reduced pressure. The crude product
was purified by flash column chromatography using petroleum ether and
ethyl acetate as the eluent, affording the white solid of 3,5-di(9H-
carbazol-9-yl)pyridine (3,5-diCzPy, 0.55 g, 67%). 1H NMR (400 MHz,
CDCl3): δ 9.03 (s, 2H), 8.24 (s, 1H), 8.17 (d, J = 7.7 Hz, 4H), 7.59–7.43
(m, 8H), 7.36 (t, J = 7.2 Hz, 4H). 13C NMR (151 MHz, CDCl3): δ 146.0,
140.4, 136.2, 132.6, 126.8, 124.3, 121.4, 120.9, 109.5.
Conflict of interest
The authors declare no conflict of interest.
Keywords: visible light • photocatalyst • Suzuki–Miyaura cross-
coupling reaction • aryl chloride • conjugated nanoporous
polycarbazole
Preparation of CNP. CNP was obtained by modified literature
procedure.[19i] 3,5-DiCzPy (0.0818 g, 0.2 mmol) was dissolved in
anhydrous chloroform (10 mL) under a nitrogen atmosphere and added
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Induced by visible light, recyclable
conjugated nanoporous polycarbazole
supported Pd NPs catalyzed Suzuki–
Miyaura reactions of aryl chlorides in
water at room temperature.
Bin Guo, Hong-Xi Li,* Cheng-Hao Zha,
David James Young, Hai-Yan Li, and
Jian-Ping Lang*
Page No. – Page No.
Visible-Light-Enhanced Suzuki-
Miyaura Reactions of Aryl Chlorides
in Water Using Pd NPs Supported on
a Conjugated Nanoporous
Polycarbazole
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