Copper anchored on phosphorus g-C3N4as a highly efficient photocatalyst for the synthesis ofN-arylpyridin-2-amines

Article abstract of DOI:10.1039/d0gc03400b

A heterogeneous photocatalyst based on copper modified phosphorus doped g-C₃N₄(Cu/P-CN) has been prepared and characterized. This recyclable catalyst exhibited high photocatalytic activity for the synthesis ofN-arylpyridin-2-amine derivatives by the reaction of 2-aminopyridine and aryl boronic acid at room temperature under the irradiation of blue light. Importantly, the range of substrates for this coupling reaction has been expanded to include aryl boronic acids with strong electron-withdrawing groups as viable raw materials. In addition, this heterogeneous catalyst can be used at least 6 times while maintaining its catalytic activity.

Full text of DOI:10.1039/d0gc03400b

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Copper anchored on phosphorus g-C3N4 as a
highly efficient photocatalyst for the synthesis of
N-arylpyridin-2-amines†‡
Cite this: Green Chem., 2021, 23,
1041
Jia-Qi Di, Mo Zhang, Yu-Xuan Chen, Jin-Xin Wang, Shan-Shan Geng, Jia-Qi Tang
and Zhan-Hui Zhang
*
A heterogeneous photocatalyst based on copper modified phosphorus doped g-C3N4 (Cu/P-CN) has
been prepared and characterized. This recyclable catalyst exhibited high photocatalytic activity for the
synthesis of N-arylpyridin-2-amine derivatives by the reaction of 2-aminopyridine and aryl boronic acid at
room temperature under the irradiation of blue light. Importantly, the range of substrates for this coupling
reaction has been expanded to include aryl boronic acids with strong electron-withdrawing groups as
viable raw materials. In addition, this heterogeneous catalyst can be used at least 6 times while maintain-
ing its catalytic activity.
Received 9th October 2020,
Accepted 4th January 2021
DOI: 10.1039/d0gc03400b
rsc.li/greenchem
acid pinacol (BPin) esters with alkyl and arylamines.
Particularly, these conditions work effectively for the coupling
Introduction
Amines and their derivatives are core structural motifs widely of alkylamines (Scheme 1b).13 In 2018, Schaper and co-workers
found in medicinal chemistry, materials science, and catalysis.
Given the preeminence of aromatic and heteroaromatic
amines, a plethora of methods have been developed for the
synthesis of these compounds by using various named reac-
tions, primarily from aryl halides and boronic acids, for
example, Chan–Evans–Lam (CEL) reaction,1–4 Ullmann–
Goldberg coupling,5,6 and Buchwald–Hartwig amination.7–9
Among them, transition metal-catalyzed Chan–Evans–Lam oxi-
dative coupling of aryl boronic acids and nitrogen nucleo-
philes to synthesize secondary aromatic amines in the pres-
ence of various oxidants is one of the most important trans-
formations, which has attracted huge attention in the past
decade. Most of the methods require the use of transition
metal catalysts, ligands, and/or extreme conditions (high temp-
erature, high pressure, strong oxidants, etc.), which ultimately
result in reduced sustainability and a limited substrate range.
In particular, aryl boronic acids with electron-withdrawing sub-
stituted groups are found to be poor substrates
(Scheme 1a).10–12 In 2016, Watson developed effective reaction
conditions for the Chan–Evans–Lam amination of boronic
Hebei Key Laboratory of Organic Functional Molecules, National Demonstration
Center for Experimental Chemistry Education, College of Chemistry and Material
Science, Hebei Normal University, Shijiazhuang, 050024, China.
E-mail: zhanhui@mail.nankai.edu.cn, zhanhui@hebtu.edu.cn
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0gc03400b
‡Dedicated to the 100th anniversary of Chemistry at Nankai University.
Scheme 1 Previous work and present reaction design.
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developed an efficient protocol by using copper(II) pyridylimi-
Based on these backgrounds and our research group’s per-
noarylsulfonate complexes with chloride or triflate countera- sistent interest in the development of environmentally friendly
nions as catalysts in Chan–Evans–Lam couplings, which synthetic methodologies,48 herein, we report on the prepa-
avoids common side reactions such as deboration and homo- ration of new phosphorus doped g-C3N4 (Cu/P-CN) and its
coupling (Scheme 1c).14 In addition, Kobayashi and co-workers application as
a
photocatalyst for the synthesis of
established a procedure for the visible light-mediated Chan– N-arylpyridin-2-amine derivatives by the reaction of 2-amino-
Lam coupling reaction starting from aniline derivatives and pyridine and aryl boronic acid at room temperature under the
arylboronic acids (Scheme 1d).15 Through the effective combi- irradiation of blue light (Scheme 1f).
nation of copper and photoredox catalysis, the substrate range
of the oxidative coupling reaction is expanded to include elec-
tron-deficient aryl boronic acid as a viable starting material.
Pyridine-containing amines are also a basic value-added unit
Results and discussion
in organic synthesis and have widespread applications in Following a modified reported method,49 the Cu/g-C3N4 cata-
pharmaceutical science, material preparation, and anti-viral/ lyst was synthesized: typically, CuCl2·2H2O was dispersed in
anti-bacterial natural products.16–19 Chen and co-workers methanol and sonicated. Then melamine was slowly added to
developed a practical protocol for the preparation of these the above solution. The resulting mixture was then refluxed at
compounds via N-arylation of 2-aminopyridine with aryl 65 °C for 3 h. The resulting precipitate was collected by fil-
boronic acids (Scheme 1e).20 This protocol has several disad- tration and dried in an oven at 70 °C. The solid was transferred
vantages such as utilizing an excessive amount of unrecover- to a ceramic crucible and calcined in a muffle furnace at
able catalyst, running at high temperature, and very limited 520 °C under a N2 atmosphere. After cooling to room tempera-
substrate scope. From the perspective of green and sustainable ture, the final solid product (Cu-doped C3N4) was collected
chemistry, it is still greatly desirable to develop novel catalytic without further purification. Copper modified phosphorus
materials to resolve the above issues.
doped g-C3N4 (Cu/P-CN) was prepared by mixing Cu/C3N4 with
Recently, due to the inexhaustible availability of light as sodium hypophosphite (NaH2PO2·H2O).50,52 The mixture was
clean energy, visible-light-induced photo-redox catalysis has placed in a covered ceramic crucible and calcined at 300 °C for
emerged as an environmentally benign and powerful synthetic 2 h under a N2 atmosphere. The obtained solid product was
strategy for the development of new and valuable chemical collected and washed with distilled water and ethanol, then
transformations.21–23 Most photo-redox processes use precious dried in a vacuum oven at 80 °C for 24 h.
Ru and Ir-based polypyridyl photoredox catalysts.24–26 As a low-
The X-ray photoelectron spectroscopy (XPS) patterns of Cu/
cost alternative, organic dyes are also often used, but they P-CN are shown in Fig. 1. The XPS survey shows that copper,
usually have lower light stability.27–29 Copper-based photocata- carbon, nitrogen, oxygen and phosphorus are detectable on
lysts are rapidly emerging, which provide not only economic the external surface of Cu/P-CN materials. The narrow scan for
and ecological advantages, but also offer inaccessible inner- the Cu 2p (Fig. 1a) spectrum exhibited two main peaks at
sphere mechanisms that have been successfully applied to 935.5 and 955.0 eV. These peaks were attributed to the Cu
challenging transformations.30–35 Moreover, the combination 2p3/2 and Cu 2p1/2 orbitals, respectively, which indicates the
of conventional photocatalysts and copper salts has emerged presence of the Cu2+ moiety in the sample. The shift might
as an effective dual catalytic system for cross-coupling result from the strong interaction between the Cu2+ species
reactions.36–38 On the other hand, heterogeneous catalysts are and P-CN. The C 1s at 288.4 eV and N 1s at 402.2 eV are
favored by industries because of their high efficiency and easy assigned to the sp2 CvN bond in the tri-s-triazine structure.
recycling. Consequently, some heterogeneous photocatalytic The peaks at 288.4 eV and 284.5 eV in the C 1s zone are attrib-
systems based on Cu have been developed.39 Besides, poly- uted to the electrons originating from the sp2 C atom attached
meric graphitic carbon nitride (g-C3N4) has gained more atten- to an NH2 group and to an aromatic carbon atom.50 The peak
tion as an excellent support to stabilize a metal catalyst for of P 2p is located at 133.6 eV, indicating the presence of P on
heterogeneous catalysis. Moreover, low-cost g-C3N4-based cata- the Cu/P-CN material.45
lysts are easy to synthesize and separate from reaction mix-
The Cu/P-CN and the corresponding supporter g-C3N4 were
tures, thereby facilitating repeated use of the catalyst.40–44 The characterized by powder X-ray diffraction (XRD). As shown in
material is endowed with great many perfections such as high Fig. 2, the characteristic (002) interlayer stacking peak (27.6°)
thermal and physicochemical stability, appropriate band gap and the weak (100) diffraction peak (13.2°) of pure g-C3N4
energy and position (2.7 eV), and a unique electronic and (JCPDS no. 85-1326) were maintained in the Cu/P-CN material,
surface structure. In addition, it has been known that when suggesting that the structure of g-C3N4 did not change when
metallic elements were incorporated onto the g-C3N4 matrix, the copper species were doped.
delocalized electrons are generated. Particularly, phosphorus-
The morphology and microstructure of the samples were
doped g-C3N4 (P-CN) can enhance the photocatalytic perform- characterized via SEM and TEM. As depicted in Fig. 3a, it can
ance due to its relatively narrow band gap, stronger visible be seen from the SEM image that Cu/C3N4 maintains the
light response, larger specific surface area and higher charge layered structure of C3N4. The introduction of Cu and P did
carrier separation rate.45–47
not significantly change the structure of C3N4 (Fig. 3b). At the
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Fig. 3 SEM images of as-prepared samples (a) Cu/C3N4 and (b) Cu/
P-CN.
Fig. 1 XPS spectra of Cu/P-CN with (a) general scan, (b) C 1s, (c) N 1s
and (d) Cu 2p, and (e) P 2p narrow scans.
Fig. 4 EDS elemental mapping and EDX spectrum of Cu/P-CN.
of HCl to HNO3 was 3/1) to digest the material and then ana-
lyzed by atomic absorption spectrometry (AAS) analysis. The
Cu content was found to be 9.16%. The TEM image of Cu/
P-CN further illustrates that the layered structure is still main-
tained after Cu loading (Fig. 5).
Fig. 2 XRD patterns of C3N4, Cu/C3N4 and Cu/P-CN.
same time, the corresponding energy-dispersive X-ray (EDX)
spectrum of Cu/P-CN demonstrated the presence of C, N, Cu,
and P (Fig. 4). The Cu/P-CN energy dispersive spectroscopy
(EDS) elemental mapping was performed to determine the dis-
tribution pattern of the constituent elements. As shown in
Fig. 4, all four main elements (Cu, C, N, and P) are uniformly
distributed in the layer structure of the Cu/P-CN sample. Also,
in order to determine the accurate content of Cu in the cata-
Fig. 5 TEM images of as-prepared samples (a) Cu/C3N4 and (b) Cu/
lyst, it was treated with concentrated HCl and HNO3 (the ratio P-CN.
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Table 1 Reaction of pyridin-2-amine and phenylboronic acid under
different reaction conditionsa
Entry
Catalyst
Solvent
Base
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
Cu/P-CN
Cu/P-CN
Cu/P-CN
Cu/P-CN
Cu/P-CN
Cu/P-CN
Cu/P-CN
Cu/P-CN
Cu/P-CN
Cu/P-CN
No
Cu/P-CN
Cu/P-CN
Cu/P-CN
Cu/P-CN
C3N4
H2O
CHCl3
EL
DMF
EtOH
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Et3N
8
8
8
8
8
8
8
8
8
8
12
8
8
8
8
12
8
8
8
8
8
Trace
62
68
72
77
83
95
53
88
90
0
76
82
85
95
0
Fig. 6 TGA images of Cu/P-CN.
MeCN
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
Na2CO3
DBU
The thermal stability of Cu/P-CN was evaluated by thermo- 10
11
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
gravimetric analysis (TGA) under a nitrogen atmosphere at a
12c
heating rate of 10 °C min−1 (Fig. 6). It was clear from the
13d
figures that there was a little mass loss below 100 °C due to 14e
15f
the removal of adsorbed water, and further heating showed a
slight weight loss of up to 550 °C. At this temperature, the tri-
16
17
Cu/C3N4
CuCl2
Ni/P-CN
Cu/P-CN
Cu/P-CN
90
17
26
20
Trace
s-triazine structure decomposes, resulting in the total degra- 18
19
dation of the carbon nitride matrix.
20g
21h
The specific surface area of the catalyst was measured by
the nitrogen adsorption–desorption isotherm experiment
(Fig. 7). The BET specific surface area of C3N4 and Cu/P-CN
was calculated to be 29.0 m2 g−1 and 14.0 m2 g−1, respectively.
To survey the catalytic activity of the prepared catalyst, we
used simple pyridin-2-amine (1 mmol) and easily available
phenylboronic acid (1 mmol) as model reactants, with K2CO3
as a base under irradiation with a blue-light-emitting diode
(LED). As illustrated in Table 1, various solvents including
H2O, CHCl3, ethyl lactate (EL), N,N-dimethylformamide (DMF),
ethanol, acetonitrile, and methanol were screened. Only a
trace amount of the desired product was observed when the
reaction was carried out in H2O (Table 1, entry 1). Among
them, methanol gave the best result for achieving a maximum
yield of product 3a. The choice of the base also significantly
affects the success of the reaction, and to our delight, cheap
a Reaction conditions: Pyridin-2-amine (1 mmol), phenylboronic acid
(1 mmol), base (1 mmol), catalyst (0.2 mmol) in solvent (2 ml), room
temperature, air, under blue light (455–460 nm) irradiation otherwise
specified in the table. b Isolated yields. c Catalyst (0.05 mmol).
d Catalyst (0.10 mol). e Catalyst (0.15 mmol). f Catalyst (0.25 mmol). g In
the dark. h The reaction was carried out under a N2 atmosphere.
K2CO3 outperformed all other bases tested. Further control
experiments revealed that the reaction did not proceed in the
absence of the catalyst (Table 1, entry 11). Decreasing the
amount of the catalyst to 5 mol% reduced the yield to 76%
(Table 1, entry 12). In contrast, the reaction yield was not
improved when the amount of the catalyst was increased to
25 mol% (Table 1, entry 15). It is worth noting that expected
product 3a was not obtained when pure C3N4 was used as the
catalyst. The use of Cu/C3N4 afforded inferior results compared
to those using Cu/P-CN (Table 1, entry 17). Furthermore, as a
more convincing comparison, the reaction can also be per-
formed using pure CuCl2, affording 3a in 17% yield (Table 1,
entry 18). A nickel anchored on phosphorus g-C3N4 (Ni/P-CN)
was also used in place of Cu/P-C3N4 under otherwise identical
conditions, targeted 3a was formed only in 26% yield (Table 1,
entry 19). Moreover, it was found that light plays a crucial role
in the reaction, the yield of 3a dropped significantly when the
reaction was run in the absence of light (Table 1, entry 20).
Notably, air is critical in this type of reaction, since only s trace
amount of the product was obtained when the reaction was
performed under a N2 atmosphere (Table 1, entry 21).
With the establishment of the optimized reaction con-
ditions, the substrate scope was investigated to probe the gen-
erality and identify the limitations of this Cu/P-CN catalyzed
coupling reaction. As depicted briefly in Table 2, various sub-
Fig. 7 N2 adsorption–desorption isotherms of as-prepared C3N4 and
Cu/P-CN.
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Table 2 Substrate scope for the synthesis of N-arylpyridin-2-aminesa
a Reaction conditions: Pyridin-2-amine (1 mmol), aryl boronic acid (1 mmol), K2CO3 (1 mmol), Cu/P-CN (0.2 mmol) in methanol (2 ml), room
temperature, air, under blue light (455–460 nm, 10 W) irradiation, isolated yield. b The reaction was performed at 65 °C.
stituted phenylboronic acids bearing electron-rich groups suitable for heteroaryl substrates, such as thiophen-2-ylboronic
(OMe, Et, Me and CMe3), or electron-deficient groups (F, Cl, acid, dibenzo[b,d]furan-3-ylboronic acid and pyridin-3-ylboro-
Br, CN and NO2) at different positions were employed to the nic acid, affording the potentially bio-important N-arylpyridin-
reaction with pyridin-2-amine under optimal conditions. To 2-amines 3x–3z in high yields. Next, the reactions of phenyl-
our delight, most of the reactions proceeded smoothly to boronic acid with diversely substituted pyridin-2-amines were
afford the N-arylpyridin-2-amine derivatives in high yields. The also examined. Substituted pyridin-2-amines with either elec-
above results demonstrated that substituents on the aromatic tron-donating or weakly electron-withdrawing groups all gave
ring have no obvious effect on the yield. It is worth mentioning excellent yields. 2-Aminopyridine with a strong electron-with-
that the halogen group was all compatible with this reaction, drawing group such as 5-nitropyridin-2-amine decreased the
which offered opportunities for downstream structural modifi- reactivity, and the reaction was performed at 65 °C to afford
cation. Importantly, the strongly electron-withdrawing nitro the desired product 3ah in a moderate yield. To our disap-
group was tolerated and the corresponding product 3i was pointment, the reaction with secondary amines such as
obtained in 92% yield. The boronic acid with a larger aromatic N-methylpyridin-2-amine and 6-methoxy-N-methylpyridin-2-
group, such as naphthyl, also participated in the reaction, amine did not afford the expected products.
leading to the corresponding products 3v and 3w with high to
Encouraged by the good performance of this protocol for
excellent yields. The above conditions were also found to be N-arylpyridin-2-amines, coupling of conventional anilines
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Scheme 2 Coupling of anilines bearing the strong electron-withdraw-
ing group with phenylboronic acid.
bearing strong electron-withdrawing groups with aryl boronic
acid are considered to be some of the biggest challenges in C–
N bond formation via Chan–Evans–Lam.10,14,51 Gratifyingly,
3-nitroaniline or 4-nitroaniline as the substrate reacted with
phenylboronic acid to deliver the anticipated products 5a and
5b in high yields (Scheme 2).
To further showcase the synthetic utility of this method, a
gram-scale experiment was also performed on the Cu/P-CN
mediated model reaction. The reaction substrates were magni-
fied 20 times, and the results showed that the reaction pro-
ceeded smoothly to afford the desired product 3a without
obvious erosion in the yield (Scheme 3).
Scheme 4 Plausible catalytic cycle for the coupling reaction of
pyridin-2-amine and phenylboronic acid.
According to the previous literature15 and experimental
observations, a tentative mechanism for this coupling reaction
of 2-aminopyridine and aryl boronic acid is depicted in
Scheme 4. Similar to the g-C3N4 photocatalyst that promotes
organic photosynthesis,52 upon irradiation with blue LED
light, charge carriers of h+ and e− are generated on the surface
of the P-CN photocatalyst. The photogenerated e− could acti-
vate molecular O2 into its reactive superoxide form (O2•−).
Singlet oxygen (1O2) could be formed through the oxidation of
Fig. 8 EPR signals of the reaction solution in the dark (black line) and
LED light illumination (red line) in the presence of DMPO (left) and TEMP
(right) as spin-trapping reagents.
O2 by photogenerated holes (h+). The reactive oxygen species
•−
(O2•− and 1O2) are confirmed by spin-trapping by electron para-
magnetic resonance (EPR) experiments. When using 5,5-
1
dimethyl-1-pyrroline N-oxide (DMPO) as a spin-trapping agent,
In order to determine the possibility of O2 as the oxidant
•−
a
characteristic EPR signal of DMPO–O2
is detected.
responsible for the oxidation of the copper catalyst, the model
reaction was also performed in the presence of reagents
known to interact with 1O2 such as (S)-(−)-limonene, DL-
α-tocopherol, and 1,1-diphenylethylene, and the results found
that the reaction efficiency decreased dramatically. The mecha-
nism described here is speculation and the actual pathway of
this coupling reaction may be elucidated in due course.
Another basic ability to consider heterogeneous catalysts is
the recyclability of the catalyst. Due to economic and environ-
mental factors, easy separation is essential for industrial pro-
cesses that reduce waste emissions and promote recyclability.
In this study, the catalyst was separated by simple filtration,
and recycling studies were performed for the reaction between
pyridin-2-amine and phenylboronic acid under optimized con-
ditions. After the reaction is complete, the catalyst is separated
from the reaction mixture. The recovered catalyst is dried and
then directly reused in the next round under the same reaction
conditions. It can be seen from Fig. 9 that the reaction was
repeated up to six times, and no significant changes in its cata-
lytic activity were observed. The Cu leaching amount was also
measured by inductively coupled plasma mass spectrometry
(ICP-MS), and it was found that the Cu content after the sixth
Moreover, the formation of TEMP–1O2 has also been con-
firmed using 2,2,6,6-tetramethylpiperidine (TEMP) as a spin-
trapping reagent, a characteristic 1 : 1 : 1 triplet EPR signal is
•−
observed (Fig. 8). These results indicate that O2 coexists with
1O2 in this photocatalytic oxidative reaction. Then, the oxi-
dative coupling of 2-aminopyridine and aryl boronic acid
occurs through several steps. Initially, the Cu(II) catalyst could
undergo ligand exchange and transmetalation with 2-amino-
pyridine and phenylboronic acid to generate Cu(II) amide A. In
1
the presence of O2, Cu(II) is oxidized to Cu(III). Subsequently,
the reduction and elimination reaction takes place to furnish
the cross-coupling product 3a and regenerated Cu(I). Then, Cu
(I) is oxidized to Cu(II) by O2 and the next cycle occurs.
Scheme 3 A gram-scale experiment.
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10 K min−1 in a simulated air atmosphere. Inductively coupled
plasma atomic emission spectroscopy analyses were performed
with an X Series 2 spectrometer. A WP-VLH-1020 photoreactor
from Xi’an WATTECS experimental equipment Co. Ltd was
used for the irradiation experiment.
Synthesis of bulk g-C3N4. g-C3N4 is synthesized by simple
calcination from melamine. Specifically, 10 g melamine is
retained in a ceramic crucible and heated in a muffle furnace
at 520 °C for 4 h under nitrogen. The light brown aggregates
are ground into a powder using a mortar and pestle.
Fig. 9 Recycling of the catalyst.
Synthesis of bulk Cu/C3N4. CuCl2·2H2O (5.1 g, 30 mmol) was
dispersed in 200 mL of methanol and sonicated for 1 h to
obtain a clear and transparent solution. Then melamine (4 g,
32 mmol) was added slowly to the above solution under stir-
ring to prevent the formation of brown clumps. The resulting
mixture was then refluxed at 65 °C for 3 h. After cooling to
room temperature, the resulting precipitate was collected by
filtration and dried in an oven at 70 °C. The solid was trans-
ferred to a ceramic crucible and then calcined in a muffle
furnace at 520 °C for 4 h under N2 at a heating rate of 10 °C
min−1. After cooling to room temperature, the solid product
(Cu/C3N4) was collected without further purification.
Preparation of Cu modified phosphorus doped g-C3N4 (Cu/
P-CN). Cu/P-CN was prepared by mixing Cu/C3N4 with
NaH2PO2·H2O. In a typical experiment, the obtained Cu/C3N4
(300 mg) and NaH2PO2·H2O (400 mg) were ground with a
mortar and pestle. The uniform mixture was placed into a
covered ceramic crucible and calcined at 300 °C for 2 h under
a N2 atmosphere. After completion of calcination and cooling
to room temperature, the obtained solid product was collected
and washed with distilled water and ethanol, and then dried
in a vacuum oven at 80 °C for 24 h.
cycle was 0.60 ppm. These results unambiguously demonstrate
that the catalyst possesses excellent stability and robustness,
and can be used many times in catalytic conversion.
Conclusions
In summary, a new heterogeneous photocatalyst was prepared
and characterized using a variety of different techniques. Its
catalytic performance for the cross-coupling between 2-amino-
pyridine and aryl boronic acid has been investigated. The cata-
lyst showed high catalytic efficiency for the synthesis of diverse
N-arylpyridin-2-amines. The synergistic effect of the dual-reac-
tive polymer and copper nanoparticles enhances the catalytic
activity and allows the reaction to proceed under mild con-
ditions. The catalyst is separated by simple filtration and
reused six times without significant loss in its catalytic activity
and stability. The development of a more efficient and selective
heterogeneous catalytic system and extension to other related
organic synthesis are underway in our laboratory.
General procedure for the synthesis of N-arylpyridin-2-
amines. In a 15 mL quartz tube equipped with a Teflon-coated
magnetic stir bar, pyridine-2-amine (1 mmol), phenylboronic
acid (1.0 mmol), K2CO3 (1 mmol) and Cu/P-CN (20 mol% Cu,
140 mg Cu/P-CN) were added successively in methanol (2 mL).
The reaction tube was exposed to a blue LED (455–460 nm, 10
Experimental
General
All chemicals and reagents were obtained from commercial W) irradiation at a distance of approximately 3 mm from the
suppliers and used as received without further purification. bottom of the tube at room temperature in air with stirring for
The melting point was determined on an X-5 and was uncor- an appropriate time. The progress of the reaction was moni-
rected. IR spectra were measured on a Bruker Tensor 27 tored by TLC. After completion of the reaction, the catalyst was
Fourier transform infrared spectroscope using KBr disks. separated by filtration. The mixture was diluted with H2O and
Powder X-ray diffraction data were obtained from a Bruker D8 the product was extracted with EtOAc. The combined extracts
Advance X-ray diffractometer, Cu-kα ray was used as the X-ray were washed with brine and dried over Na2SO4. The solvent
source, and the scanning rate was 0.05° s−1 between 20° and was removed, and the crude mixture was purified by flash
80°. Transmission electron microscopy (TEM) was performed column chromatography and eluted by hexane/EtOAc on silica
on a Hitachi H-7650 instrument running at 80 kV. Surface mor- gel to give the pure product. The recovered catalyst was washed
phology was investigated using a Hitachi S-4800 SEM instru- with ethanol and ethyl acetate, dried in an oven and kept in a
1
ment. H NMR and 13C NMR data were acquired on Zhongke desiccator for further use.
Niujin AS 400 spectrometer (400 MHz for 1H NMR spec-
troscopy and 100 MHz for 13C NMR spectroscopy) using TMS
as the internal standard. Mass spectra were obtained on a
3200
Thermogravimetric and thermal analyses were performed
using a NETZSCH STA 449 F3 instrument at a heating rate of There are no conflicts to declare.
QTRAP
instrument
with
an
ESI
source.
Conflicts of interest
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14 V. H. Duparc, G. L. Bano and F. Schaper, ACS Catal., 2018,
8, 7308–7325.
Acknowledgements
We are grateful for financial support from the National Natural 15 W. J. Yoo, T. Tsukamoto and S. Kobayashi, Angew. Chem.,
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