The preparation of a Co@C3N4catalyst and applications in the synthesis of quinolines from 2-aminobenzyl alcohols with ketones

Article abstract of DOI:10.1039/d0nj05767c

An unsymmetrical diphenylphosphino-pyridinyl-triazole ligand was synthesized and characterized through IR, NMR and MS and the corresponding earth-abundant metal complex (cobalt) was prepared. Considering energy consumption and environmental friendliness, it is necessary to turn this diphenylphosphino-pyridinyl-triazole cobalt complex into a recyclable catalyst, which could easily be reused. Therefore, a heterogeneous catalyst was synthesized through Co-doping of C₃N₄, and the Co-nanoparticles on C₃N₄revealed high catalytic activity for the synthesis of quinolines with good recovery performance.

Full text of DOI:10.1039/d0nj05767c

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The preparation of a Co@C3N4 catalyst and
applications in the synthesis of quinolines
from 2-aminobenzyl alcohols with ketones†
Cite this: New J. Chem., 2021,
45, 6768
a
Fei Cao,a Anruo Mao,ab Bobin Yang,a Chenyang Gea and Dawei Wang
*
An unsymmetrical diphenylphosphino-pyridinyl-triazole ligand was synthesized and characterized
through IR, NMR and MS and the corresponding earth-abundant metal complex (cobalt) was prepared.
Considering energy consumption and environmental friendliness, it is necessary to turn this diphenyl-
phosphino-pyridinyl-triazole cobalt complex into a recyclable catalyst, which could easily be reused.
Therefore, a heterogeneous catalyst was synthesized through Co-doping of C3N4, and the Co-nanoparticles
on C3N4 revealed high catalytic activity for the synthesis of quinolines with good recovery performance.
Received 25th November 2020,
Accepted 12th March 2021
DOI: 10.1039/d0nj05767c
rsc.li/njc
Introduction
Synthesis of nitrogen heterocycles, such as quinolines, has
drawn considerable attention in recent years due to their
biological and pharmacological activities like anticancer, anti-
malaria, anti-inflammatory, antibacterial, anticonvulsant, anti-
tuberculosis, and antihypertensive properties (Scheme 1).1
Up to now, many methods suggested that the synthesis of
quinolines with noble metals or homogeneous catalysts or under
strong acid (base) conditions was a major strategy.2,3 Therefore,
the development of green synthetic approaches for the prepara-
tion of highly functionalized quinolines with earth-abundant
metal catalytic systems under relatively mild conditions is desirable,
for example, quinoline synthesis through borrowing hydrogen or
dehydrogenation strategies, which have been considered as efficient
tools in organic chemistry over the past several decades.4,5 In 2017,
Zhang and co-wokers developed a Co–PNP complex catalyzed
synthesis of quinolines in high yields, and this is the first
cobalt-catalyzed a-alkylation reaction of ketones with primary
alcohols.2a Although several classical borrowing hydrogen and
dehydrogenation coupling reactions, such as amines with alcohols,
and ketones with alcohols, have been well developed,6 the synthesis
of quinolines based on a dehydrogenation coupling strategy,
particularly with recyclable catalysts, remains highly challen-
ging and promising.7
Scheme 1 Quinoline-based biologically active compounds.
borrowing hydrogen and dehydrogenation coupling reactions.8
However, the development of earth-abundant metal catalysts
with high activities is highly valuable;9,10 importantly, noble
metal catalysts couldn’t be recovered in most cases.11 Herein,
we reported a new unsymmetrical diphenylphosphino-pyridinyl-
triazole ligand and the corresponding cobalt catalysts, which
could catalyze the synthesis of quinolines from 2-aminobenzyl
alcohols with ketones in high yields. Fortunately, the diphenyl-
phosphino-pyridinyl-triazole (NNP) ligand (1a) was prepared with
a three step method (Scheme 2).
Our research on triazole-based ligands and the resulting
catalysts was initiated by studies on catalytic applications of
a Key Laboratory of Synthetic and Biological Colloids, Ministry of Education,
School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122,
China. E-mail: wangdw@jiangnan.edu.cn
Preparation of Co@C3N4
b Wuxi No. 1 High School, Wuxi 214031, China
To find a compound with a lower melting temperature than
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nj05767c cobalt dichloride, a suitable Co complex might be good choice
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Scheme 2 The synthesis of diphenylphosphino-pyridinyl-triazole.
Fig. 1 ((a): 1 mm and (b): 3 mm) SEM image of Co@C3N4, (c) XRD image of
C3N4 and Co@C3N4, and (d) EDX image of Co@C3N4.
to pyrolyze it at 500 1C. Because the ligand of the Co complex
will be decomposed at a high temperature, the metal nano-
particles will be embedded in the carrier C3N4 backbone easily.
Therefore, the corresponding NNP–Co complex was synthe-
sized by mixing CoCl2 and 1a in THF at room temperature for
24 h under an N2 atmosphere. The prepared NNP–Co complex
was characterized by infrared spectroscopy IR, NMR and MS
(for details, see the ESI†). Considering the economic problem
and the trend of green chemistry, a recyclable heterogeneous
catalyst was prepared. Graphitic carbon nitride (g-C3N4) has
multiple rings, and contains a large proportion of carbon and
nitrogen.12,13 Its manufacture and application are charac-
terized by advantages such as abundant raw materials, low
economic cost, non-toxic nature, stable chemical structure and
high thermal stability. In the field of photocatalysis, applications
such as hydrogen-producing catalytic water decomposition,14
reduction of carbon dioxide to organic substances,15 pollutant
degradation16 and bacterial disinfection are adopted.17 Cobalt-
doped g-C3N4 (Co@g-C3N4) nanoparticles as a heterogenous
catalytic material were studied and used in the synthesis of
quinolines.
Initially, commercially available urea was dried at 70 1C for
12 h, and then ground with the NNP–Co complex. Next, the
mixture was then heated in an alumina crucible at 500 1C for
2 h to generate crude Co@g-C3N4. Subsequently, the crude product
was washed with water and ethanol three times respectively. After
filtration, the brown target product was obtained. Finally, after
ultrasonic washing and drying, the desired solid Co@g-C3N4 was
collected. (For the detailed preparation process, please see the
ESI.†) After the synthesis, the obtained Co@C3N4 nanoparticles
was fully characterized by energy dispersive X-ray spectroscopy
(EDX), infrared spectroscopy (IR), X-ray powder diffraction (XRD),
scanning electron microscopy (SEM), and X-ray photo-electron
spectroscopy (XPS).
absorption peaks at 2y = 27.51 and 13.11, corresponding to the
characteristic peaks of C3N4 (002) and (100), which were reported
in previous studies.18 Meanwhile, with the doping of cobalt,
the peak corresponding to the (100) crystal plane gradually
disappeared. It was observed that no diffraction peaks belong to
cobalt, cobalt oxide, cobalt nitride, and cobalt carbide, which
showed that the chemical coordination may be Co–N bonds
between Co and g-C3N4. Cobalt was uniformly incorporated into
the polymer carbonitride framework, and the successful modifi-
cation of g-C3N4 was achieved.
Energy dispersive X-ray spectroscopy (EDX) of Co@C3N4 (Fig. 1)
verified the existence of the expected elements (C, N, Co, P, Cl) in
the structure. Obviously, the main elements C, N, and Co were
scanned, and metal Co can be observed from more positions. The
peak intensity of P and Cl was relatively weak, which confirmed that
these atoms came from NNP–Co and C3N4, indicating that the
metal was doped on the carrier. Meanwhile, the cobalt loading of
the Co@C3N4 composite is 2.29 wt% by the ICP test.
The surface elemental chemical states and composition of
the Co@C3N4 catalyst were revealed by XPS. As shown in Fig. 2,
the characteristic peaks of C1s, N1s, Co2p and P2p appeared
in the sample at B288.2 eV, B398.6 eV, B781.2 eV, and B133.2 eV,
respectively, which were consistent with the chemical composi-
tion of the sample. In conclusion, the characterization results
disclosed that Co was successfully doped on the support C3N4.
Images (1a) and (1b) are scanning electron microscope
images of Co@C3N4 (Fig. 1). It could be seen that the morphology
of Co@C3N4 was thin layer, stacked in clusters. Due to Co doping,
the morphology of C3N4 slightly changed after the mixed metal
complex and urea were pyrolyzed. X-ray powder diffraction (XRD)
was conducted to verify the structure. C3N4 had characteristic
Fig. 2 XPS spectrum of Co@C3N4.
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Table 2 Co-catalyzed synthesis of quinolinesab
Catalytic activity
The catalytic activity in the synthesis of quinolines via 2-amino-
benzyl alcohols with ketones was investigated. The catalytic
conditions were screened, including the amount of catalyst,
base, solvent, reaction time and temperature (Table 1). Firstly,
the base had an important role in the reaction and the reaction
did not occur without a base, and sodium hydroxide was proven
to be the most efficient base. Then, toluene was used as a
solvent that can have a good effect in the reaction. It was also
necessary to screen the amount of catalyst. It was noted that,
with an increase of the amount of catalyst, the yield also
improved, but when the amount is increased to 50 mg the
yield no longer changed significantly. After, a series of reaction
conditions were screened with different temperature and time;
in fact, when the reaction time is 36 h, the reaction yield can
reach 86%, but compared with the yield with a reaction time of
24 h the change is not too obvious. Considering the problem
of time cost, the best reaction temperature is 135 1C and the
reaction time is 24 h. Finally, the homogeneous NNP–Co
complex was also applied to this reaction, and it was found
that a good yield was obtained.
As shown in Table 2, a wide range of substrates with
functional groups or substituents including methyl, methoxy,
chloro, bromine, and heterocycles at different positions of the
phenyl ring were tolerated, affording the target products in
moderate to good yields.
Table 1 The reaction condition screeningab
Entry Catalyst
Additive Solvent T (1C) Time (h) Yieldb [%]
a
Conditions: 2a (1.0 mmol), 3a (1.0 mmol), catalyst (50 mg), additive
1
2
3
4
5
6
7
8
Co@C3N4
Co@C3N4 NEt3
—
Toluene 135
Toluene 135
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
12
36
24
o5
o5
o5
o5
32
78
85
71
82
43
45
o5
61
72
81
36
24
85
25
46
62
86
88
b
(0.5 mmol), solvent (2.0 mL), 24 h, 135 1C. Isolated product.
Co@C3N4 Cs2CO3 Toluene 135
Co@C3N4 Na2CO3 Toluene 135
Co@C3N4 K2CO3
Co@C3N4 KOH
Co@C3N4 NaOH
Co@C3N4 KOtBu
Co@C3N4 NaOtBu Toluene 135
—
C3N4
Toluene 135
Toluene 135
Toluene 135
Toluene 135
Subsequently, under the most ideal reaction conditions, the
substrate scope was expanded. Analysis of the experimental
results from different para-substituents (4a–4f, 4t) revealed that
electronic effects of the substrates had a significant impact on
this reaction and that an electron-donating group of the
reactants might be more favourable during this transformation.
30-methyl-acetophenone (3g) participated in the oxidative annu-
lation with o-aminobenzyl alcohol affording the corresponding
products 4g in 86% yield. As for a halogen as a meta-substituent
of the substrate, 3h afforded 4h in 75%, and 3i afforded 4i in
72%. Regarding the substrate of the ortho substituent, it did not
affect the reaction yield. The reactions with 20-methylaceto-
phenone 3j and 20-chloroacetophenone 3l afforded the products 4j
and 4l in 82% and 74% yield respectively. Ditrifluoromethyl sub-
stituted substance 3p was also compatible for the synthesis, with a
51% yield. 2-Acetylpyridine 3o reacted to produce the quinoline
derivative 4o in 83% yield. 2-Amino-5-chlorobenzyl alcohol and
4-methylacetophenone afford the corresponding product 4n in 78%.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
NaOH
NaOH
Toluene 135
Toluene 135
Co@C3N4 NaOH
Co@C3N4 NaOH
Co@C3N4 NaOH
Co@C3N4c NaOH
Co@C3N4d NaOH
Co@C3N4e NaOH
Co@C3N4 NaOH
Co@C3N4 NaOH
Co@C3N4 NaOH
Co@C3N4 NaOH
Co@C3N4 NaOH
H2O
Xylene
135
135
Dioxane 135
EtOH 135
Toluene 135
Toluene 135
Toluene 135
Toluene r.t.
Toluene 90
Toluene 135
Toluene 135
Toluene 135
NNP–Cof
NaOH
a
Conditions: 2a (1.0 mmol), 3a (1.0 mmol), catalyst (50 mg) unless
b
otherwise noted, additive (0.5 mmol), solvent (2.0 mL). Isolated
c
d
e
product. Catalyst (10 mg). Catalyst (40 mg). Catalyst (60 mg).
f
NNP–Co (1.0 mol%).
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The synthesis of functionalized quinolines is very
important19,20 and Zhang’s research on the cobalt-catalyzed Fried-
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lander annulation reaction, aliphatic ketones and aromatic
ketones provided a clear proof that earth abundant metals are
efficient for borrowing hydrogen reactions.2a Although eight
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catalyst with good recovery performance.
Recyclability of the catalyst
We recovered the catalyst and repeated the experiments with
4-bromoacetophenone and 2-aminobenzyl alcohol as substrates.
The composite Co@C3N4 was washed and centrifuged with water
and ethanol, and then dried for reusing in the following reactions
(Scheme 3). As the times of recycling increase, the yield gradually
decreased.
Conclusions
In summary, we developed
a novel diphenylphosphino-
pyridinyl-triazole ligand and the corresponding heterogeneous
Co@C3N4 nanoparticles, which were carefully characterized
through energy dispersive X-ray spectroscopy, X-ray powder
diffraction, scanning electron microscopy and X-ray photo-
electron spectroscopy. The heterogeneous cobalt catalyst
revealed high catalytic activity for the synthesis of quinolines
through the reaction of 2-aminobenzyl alcohol and ketones in
high yields.
Conflicts of interest
There are no conflicts to declare.
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We gratefully acknowledge financial support of this work from
the National Natural Science Foundation of China (21776111,
21861039).
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