Immobilization of copper(II) into polyacrylonitrile fiber toward efficient and recyclable catalyst in Chan-Lam coupling reactions

Article abstract of DOI:10.1016/j.reactfunctpolym.2021.104831

A series of polyacrylonitrile fiber (PANF)-supported copper(II) catalysts were prepared through the immobilization of Cu(II) into prolinamide-modified PANF (PAN_PA-2F) and subsequently used for the synthesis of diverse N-arylimidazoles from arylboronic acids and imidazole. The prepared Cu(II)@PAN_PA-2Fs were well characterized by mechanical strength, FT-IR, XRD, XPS and SEM. Among them, CuCl₂@PAN_PA-2F exhibited excellent catalytic performance, and its activity was significantly affected by the Cu loading. This catalytic system also displayed good activity in the synthesis of N-arylsulfonamides from arylboronic acids and tosyl azide. It was highly efficient in gram-scale reactions and could be reused five times. The advantages of low cost, easy preparation, good durability and facile recovery make the fiber catalyst attractive.

Full text of DOI:10.1016/j.reactfunctpolym.2021.104831

Reactive and Functional Polymers 160 (2021) 104831
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Reactive and Functional Polymers
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Perspective Article
Immobilization of copper(II) into polyacrylonitrile fiber toward efficient
and recyclable catalyst in Chan-Lam coupling reactions
,
,
,
,c
Chenlu Zhanga 1, Hai Zhua 1, Kaiyue Gangb, Minli Taoa, Ning Maa *, Wenqin Zhanga
a Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, PR China
b School of Chemical Engineering, Tianjin University, Tianjin 300072, PR China
c Key Laboratory of Inorganic-Organic Hybrid Functional Materials Chemistry, Ministry of Education, Tianjin Key Laboratory of Structure and Performance for
Functional Molecules, Tianjin Normal University, Tianjin 300387, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
A series of polyacrylonitrile fiber (PANF)-supported copper(II) catalysts were prepared through the immobili-
zation of Cu(II) into prolinamide-modified PANF (PANPA-2F) and subsequently used for the synthesis of diverse
N-arylimidazoles from arylboronic acids and imidazole. The prepared Cu(II)@PANPA-2Fs were well characterized
by mechanical strength, FT-IR, XRD, XPS and SEM. Among them, CuCl2@PANPA-2F exhibited excellent catalytic
performance, and its activity was significantly affected by the Cu loading. This catalytic system also displayed
good activity in the synthesis of N-arylsulfonamides from arylboronic acids and tosyl azide. It was highly efficient
in gram-scale reactions and could be reused five times. The advantages of low cost, easy preparation, good
durability and facile recovery make the fiber catalyst attractive.
Polyacrylonitrile fiber
Heterogeneous copper catalysis
Chan-Lam coupling
N-Arylimidazole
N-Arylsulfonamide
1. Introduction
temperature, long reaction time and poor substrate generality reduce
their attractiveness in organic synthetic chemistry. Since the pioneering
Transition metal-catalyzed carbon‑nitrogen couplings have been
powerful for the production of amine, amide and N-heterocycle in a
wide range of pharmaceuticals, natural products, dyes and functional
materials [1–3]. Of these nitrogen-containing compounds, N-arylimi-
dazoles have attracted significant interest because of their bioactivities
such as anti-inflammatory [4], antifungal [5], and analgesic [6].
Moreover, the alkylation of imidazole moiety provides diverse ionic
liquids which were involved in organic transformations as environ-
mental friendly solvents and catalysts [7,8]. The tautomeric structure of
imidazole unit also expands its synthetic utilities as precursor to N-
heterocyclic carbenes [9,10], which acted as organocatalysts and
excellent ligands in metal catalysis [11]. More recently, the imidazole
unit was found in organic conjugated polymers for the application as
electroluminescent devices, sensors, and other semiconductor devices
[12,13].
works reported by Chan and Lam, Cu-catalyzed coupling of arylboronic
acids with imidazole (Chan-Lam coupling) has gained a lot of interest
owing to its mild reaction conditions (room temperature and weak base)
[20,21]. A large number of reports have been proposed about the
modification and extension of this reaction, but the development of
simpler and more versatile catalytic system is still highly desired
[22–27].
Despite tremendous progress in homogeneous copper-catalyzed
Chan-Lam coupling, the intrinsic disadvantages of homogeneous catal-
ysis such as the complicated separation of metal catalyst and non-
reusability remain unsolved, which inevitably cause the environ-
mental pollution and hamper its utility from laboratory scale to chem-
ical production. In this context, heterogenization of homogeneous
catalyst onto stable solid support simplifies the reclaiming of catalyst,
and ultimately leads to simple, economical, environmentally benign and
sustainable process. Recently, many solid materials such as silica [28],
graphene [29], organic polymers [30,31], metal-organic frameworks
[32,33], covalent-organic frameworks [34], and magnetic nanoparticles
[35] have been used as supports for copper catalysts in Chan-Lam
coupling. These heterogeneous catalysts were typically present as
–
Numerous protocols for C N coupling to prepare N-arylimidazole
have been proposed over past decades, including nucleophilic aromatic
substitution of imidazole [14] and Ullmann cross-couplings of aryl
halide with imidazole [15–19]. These methods usually show acceptable
efficiency, yet, some apparent drawbacks such as high reaction
* Corresponding author.
E-mail address: mntju@tju.edu.cn (N. Ma).
1
Both authors contributed equally to this work.
https://doi.org/10.1016/j.reactfunctpolym.2021.104831
Received 9 October 2020; Received in revised form 20 January 2021; Accepted 22 January 2021
Available online 29 January 2021
1381-5148/© 2021 Elsevier B.V. All rights reserved.

C. Zhang et al.
Reactive and Functional Polymers 160 (2021) 104831
small particles or powders, which could enhance their contact area be-
tween reactants and catalytic sites during heterogeneous catalysis.
However, the recovery of such heterogeneous catalysts might be tedious
and time-consuming because of the formed pressure drop in filtration.
Polyacrylonitrile fiber (PANF), an artificial synthetic polymer ma-
terial, has recently attracted much attention due to its low density,
excellent flexibility, high thermal stability and good solvent resistance.
These properties facilitate the modification of fiber through covalent
bonding into desirable polymer support. Especially, the cyano and ester
groups in PANF could be partially converted to other functional groups
via aminolysis and hydrolysis, which provide a variety of opportunities
toward the design of novel absorbents and heterogeneous catalysts.
Fiber-immobilized absorbents have been widely applied to the removal
of heavy metal ions [36] or organic pollutants [37,38], and the sepa-
ration and recovery of noble metal [39] from waste water directly and
effectively. Several groups including us have reported that fiber mate-
rials, especially PANF, could be modified into not only supported acid
[40] and base [41,42], but also immobilized metal catalysts for organic
reactions [43,44], with excellent catalytic performance and prominent
applicability. It is worth to note that, compared with other solid mate-
rials with rigid matrix, flexible structure of fiber ensures its stretching
and rotation, thus improving the mass diffusion and making the catalytic
sites accessible to the reactants adequately. In addition, the regular and
flexible structure of fiber material also facilitated its quick and conve-
nient separation by filtration from reaction system.
2. Results and discussion
2.1. Preparation of fiber immobilized copper catalysts
The preparation of fiber-immobilized copper catalyst (CuCl2@-
PANPA-2F) was shown in Scheme 1. Prolinamide-modified fiber PANPA-
2F was prepared from PANF and prolinamide derivative ((S)-N-(2-ami-
noethyl)pyrrolidine-2-carboxamide) according to our reported proced-
ure [53] with slight modification (for detailed preparation of
prolinamide derivative, see Supporting Information). The functional
degree of PANPA-2F was calculated by the equation as follows: functional
degree = ((w2-w1) /(w2 × ∆M)) × 1000, where w1 and w2 are the weights
of fiber before and after modification, respectively, and ∆M is the
changed molecular weight of fiber. Then PANPA-2F was used to immo-
bilize copper salts in water at room temperature to afford the corre-
sponding fiber-immobilized copper material (Cu(II)@PANPA-2F), in
which the Cu content (Cu loading) was determined by atomic absorption
spectrometry (AAS) (Table 1). By balancing the functional degree and
mechanical strength, the PANPA-2F with functional degree of 1.15 mmol
gꢀ 1 was generally used unless otherwise specified, which gave the Cu
loading of 0.90 mmol gꢀ 1. Mechanical strength test, FT-IR, XRD, TGA,
XPS, SEM and AAS were further employed for the characterization of Cu
(II)@PANPA-2Fs.
2.2. Characterization of fiber-immobilized copper catalyst
As a non-toxic natural amino acid, proline has been extensively
employed as organocatalyst in various organic reactions because of its
low price, stability under air and moisture, and highly catalytic activity
[45–47]. The presence of adjacent amino and carboxy groups make
proline and its derivatives good ligands in transition metal catalysis
[48,49]. For example, Alper et al. prepared MNPs-supported proline as
reusable ligand for Cu-catalyzed arylation of nitro nucleophiles [50].
You et al. reported proline ionic liquid anchored on polystyrene showed
good reactivity and recyclability for Cu-catalyzed N-arylation [51].
Recently, Iglesias et al. presented that copper and rhodium complexes
immobilized on porous aromatic frameworks with prolinamide ligand
exhibited high activity in cyclopropanation, A3 coupling and hydroge-
nation reactions [52]. In our earlier report, we described that
prolinamide-functionalized PANF was prepared from the easily avail-
able L-proline methyl ester hydrochloride and used as efficient catalyst
for Knoevenagel and the related multicomponent reactions in water
[53]. Encouraged by all the above-mentioned investigations, herein we
propose that compared with PANF, the prolinamide-modified PANF has
enhanced affinity to Cu(II) ion due to the chelation function of proli-
namide moiety, which will make Cu ions more easily accumulate and
reside in its surface microenvironment so that Cu(II) ions could be
immobilized efficiently. Thus, the novel microenvironment with active
Cu sites and the appropriate adjacent hydrophobic moieties can be
constructed as active and recyclable catalytic system for base-free Chan-
Lam coupling.
The mechanical properties of different Cu(II)@PANPA-2Fs were
measured by mechanical strength test and the results are summarized in
Table S1. The breaking strength CuCl2@PANPA-2F, Cu(OAc)2@PANPA-2
F
and CuSO4@PANPA-2F had only 0.08, 0.1 and 0.07 cN loss compared
with the parent PANPA-2F (8.52 cN), respectively, which showed their
good mechanical stability.
The FT-IR spectra of different fibers are shown in Fig. 1. For original
PANF, the strong absorption peak at 2245 cmꢀ 1 is caused from
–
ꢀ 1
–
stretching vibration of C N group, and the sharp peak at 1733 cm
–
Table 1
The copper contents of the functionalized fibers.
[a]
Entry
Fiber
Cu content (mmol gꢀ 1
)
1
PANF
–
2
PANPA-2F
–
3
CuCl2@PANPA-2
F
0.90
0.84
0.83
0.54
0.68
1.07
4
Cu(OAc)2@PANPA-2F
5
CuSO4@PANPA-2F
6[b]
7[c]
8[d]
CuCl2@PANPA-2
CuCl2@PANPA-2
CuCl2@PANPA-2
F
F
F
[a] Determined by atomic absorption spectrometry; [b,c,d] The functional de-
gree of PANPA-2F is 0.74, 0.93 and 1.39 mmol gꢀ 1, respectively.
Scheme 1. The preparation of CuCl2@PANPA-2F.
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C. Zhang et al.
Reactive and Functional Polymers 160 (2021) 104831
physically absorbed water, whereas a main weight loss in the range of
150 to 500 ◦C is related to the decomposition of organic moiety grafted
on the surface of fiber. Additionally, the DTG curves of fiber show that
CuCl2@PANPA-2F is more stable than the other two fiber-supported
copper catalysts because of its slower rate of decomposition (Fig. 2b).
Although a slight decrease of initial decomposition temperature of all
functionalized fibers was observed in comparison with original PANF, it
is sufficient for their employment in organic transformation performed
in this study.
The solvent resistance of PANPA-2F and CuCl2@PANPA-2F was
investigated by immersing the fiber in different solvents such as H2O,
MeOH, EtOH, CH3CN, EtOAc and CH2Cl2 for 24 h at room temperature.
After that, the fibers were filtered, washed completely with EtOH, and
dried overnight at 60 ◦C under vacuum. No mass change was observed
between the treated and fresh fibers, and the unchanged FT-IR spectra of
the treated fibers also confirmed their structural stability, which in-
dicates that fiber catalysts have desired solvent resistance.
The XRD patterns of PANF, PANPA-2F, CuCl2@PANPA-2F, Cu
(OAc)2@PANPA-2F and CuSO4@PANPA-2F are shown in Fig. 3a. For
original PANF, the strong reflection peaks at 17.0◦ is related to (100)
diffraction of the hexagonal lattice planes formed by the parallelly and
closely packed molecule rods. Due to the intramolecular dipole-dipole
interactions between nitrile groups, the molecular chain of PANF is of
the irregular helical conformation. This diffraction peak was well
maintained in the XRD pattern of PANPA-2F, indicating such modifica-
tion did not significant affect the basic crystal lattice. A new peak was
clearly observed in the XRD pattern of CuCl2@PANPA-2F at 15.8◦, which
corresponds to (1 1 0) lattice plane of CuCl2 [55]. This result demon-
strates the successful immobilization of Cu(II) into the fiber (Fig. 3a–b).
Moreover, the XPS spectra of different fibers are displayed in Fig. 3c–d.
The main signals of O1s, N1s, C1s, Cl 2p and Cu 2p can be clearly
observed in CuCl2@PANPA-2F at 532.6, 400.4v, 285.4, 199.0 and
953–934 ev, respectively (Fig. 3c). As shown in Fig. 3d, two signals
related to Cu 2p 1/2 and 2p 3/2 are centered at 953.7 and 934.8 ev,
respectively, along with the corresponding peaks at 938.9–945.5 ev.
These peaks are the features of divalent Cu cation [56], indicating the
oxidation state of metal does not change after immobilization. In the
spectra of Cu(OAc)2@PANPA-2F and CuSO4@PANPA-2F, these charac-
terization signals were also clearly visible (Fig. S1a-b).
Fig. 1. The FT-IR spectra of (a) PANF, (b) PANPA-2F, (c) CuCl2@PANPA-2F, (d)
Cu(OAc)2@PANPA-2F and (e) CuSO4@PANPA-2F.
–
belongs to C O stretching vibration of the methoxycarbonyl. The ab-
–
sorption peaks at 2942 cmꢀ 1 and 2848 cmꢀ 1 are ascribed to the
stretching vibrations of -CH2 group of fiber backbone. After the grafting
–
of prolinamide moiety, the relative intensity of peak related to C
O
–
group decreased considerably, which indicated the ester moiety was
consumed during the aminolysis process. And two new peaks at 1660
and 1545 cmꢀ 1 corresponding to amide I (C O stretching vibration in
–
–
–
CONHs) and amide II (N H bending vibration in CONHs) were
observed, respectively. After the immobilization of Cu(II) salts into
PANPA-2F, the characterization peaks of CuCl2@PANPA-2F, Cu(OAc)2@-
PANPA-2F and CuSO4@PANPA-2F showed no distinct change from the
parent PANPA-2F. This may be because the Cu salts were mainly adsor-
bed physically in fiber and the chelation of prolinamide moiety was
weak; also, the contents of adsorbed Cu salts were not high enough to
make the IR spectra changed obviously. This observation is consistent
with the previously reported result of amine functionalized fiber-
supported CuI [54].
The surface morphology of fiber was observed by SEM and their
images are show in Fig. 4. For original PANF, the surface of fiber was
The stability of fiber catalyst is an important factor for their practical
application. Therefore, the thermal behavior of different fiber was
investigated by TGA in nitrogen atmosphere from room temperature to
800 ◦C. The TGA curve of original PANF shows the fiber material can be
stable up to 315 ◦C (Fig. 2a). After modification, the TGA curves of
PANPA-2F, CuCl2@PANPA-2F, Cu(OAc)2@PANPA-2F and CuSO4@PANPA-
2F display the similar two-step thermal decomposition. For CuCl2@-
PANPA-2F, slight weight loss before 125 ◦C is likely due to the removal of
flat, smooth and uniform with an average diameter of about 23 μm. After
the modification by prolinamide, the shape of fiber kept well and its
morphology was nearly unchanged. Additionally, the fiber-supported
copper catalysts maintained good integrity of the fiber material and
no obvious crack or degradation was observed. EDX spectrum was also
conducted to determine the presence of Cu in the CuCl2@PANPA-2F
Fig. 2. The (a) TG and (b) DTG curves of PANF, PANPA-2F, CuCl2@PANPA-2F, Cu(OAc)2@PANPA-2F and CuSO4@PANPA-2F.
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C. Zhang et al.
Reactive and Functional Polymers 160 (2021) 104831
Fig. 3. The XRD curves of (a) PANF, PANPA-2F, CuCl2@PANPA-2F, Cu(OAc)2@PANPA-2F, CuSO4@PANPA-2F and (b) CuCl2∙2H2O; The XPS spectra of (c) CuCl2@-
PANPA-2F, (d) CuCl2@PANPA-2F in the region of Cu 2p.
Fig. 4. SEM images of the fibers (a) PANF, (b) PANPA-2F, (c) CuCl2@PANPA-2F, (d) Cu(OAc)2@PANPA-2F and (e) CuSO4@PANPA-2F.
(Fig. 5a). As shown in Fig. 5b, the nanostructured CuCl2 particles was
clearly distributed along the fiber.
catalytic site. Other fiber-supported copper catalysts were then exam-
ined and the results showed that Cu(OAc)2@PANPA-2F could afford a
moderate yield of 3a while CuSO4@PANPA-2F displayed inferior activity
with a low yield of 24% (entries 4–5). In addition to heterogeneous
supported‑copper catalyst, homogeneous copper salts, such as
CuCl2∙2H2O, Cu(OAc)2∙H2O and CuSO4∙5H2O, were employed as
catalyst, and the corresponding coupling products were isolated in 67%,
58% and 15% yields after 5 h, respectively (entries 6–8). In general,
immobilization of a homogeneous catalyst onto solid support would lead
to a decrease of activity and selectivity because of the disfavored kinetics
of biphasic catalytic system [57]. Apparently, from above results, we can
see that the immobilization of diverse copper catalysts onto fiber ma-
terial gave no detrimental effect on their catalytic performance, and the
immobilized catalysts were even slightly better than their homogeneous
2.3. Catalytic activity
After comprehensive characterization, the catalytic activity of the
fiber-supported copper catalyst was investigated in Chan-Lam coupling.
Initially, the reaction of phenylboronic acid with imidazole was chosen
as the model to optimize reaction conditions (Table 2). No target
product was detected in blank control or in the presence of PANF as the
reaction proceeded at 40 ◦C in CH3OH for 5 h (entries 1–2). When
CuCl2@PANPA-2F was used as the catalyst, the coupling reaction pro-
ceeded smoothly and the desired product 3a was obtained in a good
yield of 78% (entry 3). This result indicates the copper was the genuine
4

C. Zhang et al.
Reactive and Functional Polymers 160 (2021) 104831
Fig. 5. EDX spectrum (a) and Cu mapping (b) of CuCl2@PANPA-2F.
counterparts. Considering the convenient separation of catalyst by
simple filtration, the fiber supported-catalyst displayed remarkable ad-
vantages in the catalytic application. Further studies show that reaction
temperature played important impact on the reaction (entries 9–11).
When the reaction was performed in CH3OH at an elevated temperature
of 60 ◦C, the yield of 3a could reach up to 98% within less reaction time
in the presence of CuCl2@PANPA-2F.
Table 3
Optimization of the other reaction conditions.a
Entry
Cat. amount (mol%)
Solvent
Time (h)
Yield (%)b
1
8
5
2
10
8
8
8
8
8
8
8
8
8
CH3OH
CH3OH
CH3OH
CH3OH
EtOH
3
3
6
3
3
3
3
3
3
3
3
3
3
98
2
84
It is worth to note that the adjustment of Cu loading of fiber catalyst
can significantly influence its catalytic activity. Comparative experi-
ments suggested that lower loading resulted in distinct reduction of
catalytic performance as 0.54 mmol gꢀ 1 CuCl2@PANPA-2F only afforded
a 65% yield, while the improvement of Cu loading showed better per-
formance as 0.90 mmol gꢀ 1 CuCl2@PANPA-2F gave an excellent yield of
98% under the same conditions (Table 2, entries 12–14). The low Cu
loading indicates the sparse distribution of catalytic sites upon fiber
surface, which resulted in poor accessibility of active sites. In contrast,
the higher Cu density probably enhanced the utilization of catalytic sites
and hence increased the catalytic efficiency under the same conditions.
Next, we investigated the effect of catalyst dosage, solvent and
reactant ratio on model reaction using CuCl2@PANPA-2F as optimal
catalyst (Table 3). When we changed the catalyst amount from 8 mol%
to 5 mol%, the yield of 3a decreased to 84% (entry 2). And the use of 2
mol% of CuCl2@PANPA-2F only afforded 3a in a 45% yield, whereas
3
45
4
98
5
47
6
H2O
31
7
H2O:CH3OH (1:1)
CH3CN
EtOAc
86
8
trace
trace
trace
trace
93
9
10
11
12c
13d
CH2Cl2
Toluene
CH3OH
CH3OH
98
a
Reaction conditions: 0.6 mmol phenylboronic acid (1.2 equiv) and 0.5 mmol
imidazole (1.0 equiv), 60 ◦C, solvent (4 mL).
b
Isolated yield.
c
d
1.1 equiv. of phenylboronic acid was used.
1.3 equiv. of phenylboronic acid was used.
Table 2
Optimization of fiber catalysts.[a]
Entry
Catalyst
Temp. (◦C)
Time (h)
Yield (%)[b]
1
–
40
40
40
40
40
40
40
40
50
60
reflux
60
60
60
5
5
5
5
5
5
5
5
5
3
3
3
3
3
–
2
PANF
–
3
CuCl2@PANPA-2
F
78
72
24
67
58
15
89
98
98
65
84
98
4
Cu(OAc)2@PANPA-2F
5
CuSO4@PANPA-2
F
6
CuCl2 • 2H2O
7
Cu(OAc)2 • H2O
CuSO4 • 5H2O
8
9
CuCl2@PANPA-2
CuCl2@PANPA-2
CuCl2@PANPA-2
CuCl2@PANPA-2
CuCl2@PANPA-2
CuCl2@PANPA-2
F
10
11
12[c]
13[d]
14[e]
F
F
F
F
F
[a] Reaction conditions: phenylboronic acid (0.6 mmol) and imidazole (0.5 mmol), catalyst (8 mol%), solvent (4 mL); [b] Isolated yield; [c,d,e] The Cu loading of
CuCl2@PANPA-2F is 0.54, 0.68 and 1.07 mmol gꢀ 1, respectively.
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C. Zhang et al.
Reactive and Functional Polymers 160 (2021) 104831
Table 4
Chan-Lam coupling reaction of arylboronic acids with different 1H-imidazole nucleophiles.[a]
[a] Reaction conditions: 1 (0.6 mmol) and 2 (0.5 mmol), CuCl2@PANPA-2F (8 mol%), CH3OH (4 mL), 60 ◦C, 3 h.
additional catalyst amount gave no improvement of yield (entries 3–4).
Hence, the catalyst dosage of 8 mol% was necessary for the coupling
reaction toward completion (entry 1). Among the solvents tested, it was
found that other polar proton solvents, ethanol and water, were not
beneficial to this reaction and resulted in the formation of 3a in lower
yields of 47% and 31%, respectively (entries 5–6). The use of a mixture
solvent of methanol and water provided a good yield and was better than
that in water (entry 7). This result indicates that fiber catalyst system
could perform well in aqueous media but methanol was more preferred.
Other solvents, such as CH3CN, EtOAc, CH2Cl2, toluene, and n-hexane
gave only trace of product (entries 8–11). The phenomena were pre-
sumably due to the good solubility of reactants in these solvents failed to
make them enriched into the surface microenvironment of the fiber
catalyst and effectively contacting with the catalytic sites. Using meth-
anol as the solvent, good enrichment of the reactants into the fiber
surface was provided. In addition, it was observed that the fiber catalyst
spread better in methanol than in ethanol, which indicates the
accessibility of catalytic sites in fiber surface was more efficient in
methanol. Furthermore, when the reaction was performed with a 1:1 M
ratio of two coupling partners, desired product was obtained in 93%
yield (entry 12). Increasing the amount of phenylboronic acid could
improve the product yield and 1.2 equiv. of boronic acid was found to be
the optimal amount (entries 1 and 13).
With the optimal conditions in hand, a variety of structurally diverse
substrates were tested to explore generality of this catalytic system and
the results are shown in Table 4. We are glad to observe that the re-
actions of various substituted arylboronic acids with imidazole proceed
smoothly and the corresponding N-arylimidazoles were obtained in
good to excellent yields. Arylboronic acids with electron-donating alkyl
and alkoxy groups at para or meta position gave desired products 3b-3f
in high yields ranging from 94 to 97%, while ortho-substituted methoxy
group 3f provided relatively lower yield which was possibly attributed
to larger steric hindrance. Halogen-substituted phenylboronic acids
(fluoro, chloro and bromo) proved effective coupling partners and their
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C. Zhang et al.
Reactive and Functional Polymers 160 (2021) 104831
corresponding products were isolated in 80–88% yields, whereas strong
electron-withdrawing nitro-substituted phenylboronic acid gave a low
yield of product 3m. It was observed that phenylboronic acid, p-toly-
boronic acid, and 3,4-dimethoxyphenylboronic acid were favorably
converted to the corresponding N-arylbenzimidazoles when benzimid-
azole was used as a nucleophile. The presence of electron-withdrawing
halogen groups in the benzene ring was also well tolerated. Moreover,
the reaction between phenylboronic acid and N1-arylated 2-aminoben-
zimidazole 3u in an 89% yield. To further extend the substrate scope,
other substituted imidazoles (2-methyl-1H-imidazole, 4-methyl-1H-
imidazole and 4-bromo-1H-imidazole) have been tried in the reaction,
and the coupling products 3v-3x were generated in 93%, 89% and 85%
yields, respectively.
2.4. Recycling and leaching experiments
One of the most important features of heterogeneous catalyst is the
recyclability, namely, the supported-catalyst can be easily separated
from reaction system for multiple reuse. To verify the recyclability of
CuCl2@PANPA-2F, the model coupling reaction of phenylboronic acid
with imidazole was conducted under optimized conditions. After
completion of the reaction, the fiber catalyst was separated by filtration
and washed with ethylacetate to remove the absorbed product before
next run. The result showed that the reaction yields of the first to the
fifth runs were 98%, 96%, 91%, 84% and 78%, respectively, indicating
CuCl2@PANPA-2F could be reused five times with high activity. The
recovered fibers were subsequently examined by mechanical strength
test and SEM. The breaking strength of CuCl2-5@PANPA-2F was only
0.29 cN lower than that of fresh CuCl2@PANPA-2F, that is, 96% of
strength was maintained (Table S3, entry 2). Meanwhile, there was no
obvious collapse or fracture in the surface of CuCl2-5@PANPA-2F, albeit
wrinkled and rough (Fig. S2). These observations suggested that the
structure of fiber catalyst was not obviously destroyed after multiple
reuse.
Based on the high-efficiency of CuCl2@PANPA-2F in Chan-Lam
coupling for the synthesis of N-arylimidazoles, we further explored the
utility of the fiber catalyst in the synthesis of N-arylsulfonamides. The
coupling reaction of phenylboronic acid with tosyl azide was chosen as
the model. As show in Table S2, CuCl2@PANPA-2F was proved to be
effective in the reaction, and the desired product was obtained in a 91%
yield after 0.5 h at 60 ◦C (entry 1). Under catalyst-free conditions, only a
trace amount of product was generated. Also, PANF itself was found to
be inactive in the reaction (entries 2–3). The effect of solvent on the
reaction was studied and it was found that no case showed superiority to
CH3OH (entries 6–9). Reducing the catalyst dosage or reactant ratio of
phenylboronic acid to tosyl azide caused the obvious decline of yield
(entries 5 and 10). Regarding the reaction temperature, a lower tem-
Leaching experiment was then carried out to ascertain whether
CuCl2@PANPA-2F mediated heterogeneous catalytic process (Table S4).
When the coupling reaction of phenylboronic acid with imidazole pro-
ceeded for 2 h under the optimized conditions, fiber catalyst was taken
out quickly from the system, and the residue mixture was stirred for
another 1 h. The result showed that the reaction of remaining reaction
mixture almost gave no additional yield, which means the leaching
CuCl2 catalyst had very low activity, and the heterogeneous nature of
fiber catalytic system was confirmed. In order to evaluate the Cu
leaching, atomic absorption spectroscopy (AAS) was conducted for the
reaction mixtures after the recycled CuCl2@PANPA-2F was removed from
first to fourth run. The ratios of leaching Cu into the methanol were
found to be 72.0%, 7.5%, 3.4% and 0.5%, respectively, and AAS analysis
of the recycled catalyst after fourth run showed 12.4% of initial Cu was
kept in the catalyst. The remaining ratios of Cu (28.0%, 20.5%, 17.1%,
16.6%) did not well match the corresponding yields from second to fifth
runs in the above reuse experiments, which further supported that the
chelated Cu ions in the relatively hydrophobic microenvironment of
fiber surface were the major catalytic sites.
◦
perature of 40 C offered the product in a 45% yield (entry 11). Ac-
cording to above results, the optimal conditions were confirmed as
follows: CuCl2@PANPA-2F (5 mol%), CH3OH, 60 ◦C, 0.5 h. With the
optimized conditions in hand, the generality of fiber catalyst in the
coupling reaction of arylboronic acids with tosyl azide was evaluated
and the results are shown in Table 5. It can be seen that arylboronic acids
bearing either electron-donating or electron-withdrawing group
smoothly reacted with tosyl azide to give the corresponding products in
high yields (83–94%).
In addition, gram-scale synthesis of N-arylimidazole and N-arylsul-
fonamides was conducted. As show in Scheme 2, the reactions with 10
mmol scale underwent effectively and the desired N-arylated products
3a and 5a were obtained in stationary yields without any elevation of
reaction temperature or extension of reaction time, suggesting the po-
tential application of this fiber-mediated catalytic system in large-scale
production.
Table 5
Chan-Lam coupling reaction of arylboronic acid with tosyl azide.[a,b]
[a] Reaction conditions: 1 (0.6 mmol) and 4a (0.5 mmol), CuCl2@PANPA-2F (5 mol%), CH3OH (4 mL), 60 ◦C, 0.5 h. [b] All yields are isolated yield.
7

C. Zhang et al.
Reactive and Functional Polymers 160 (2021) 104831
Scheme 2. Gram-scale reaction.
imidazole catalyzed by CuCl2@PANPA-2F in N2 atmosphere under the
standard conditions only gave trace amount of product, Cu(III)-based
mechanism is preferred for this trnasformation. Although the chelated
CuCl2 is the minor component of the fiber catalyst, it is possibly the more
active site, whereas CuCl2 particles traped and wraped in fiber surface
are more likely the resevior of copper ions than major active sites,
because the copper complexes with ligands were commonly more active
than the corresponding copper salts in literatures, and the bulky CuCl2
particles had limited surface area to catalyze the reaction efficiently.
According to the related mechanisms proposed in literatures
[22,25,27,32] and our observations, plausible mechanism of CuCl2@-
PANPA-2F catalyzed reaction of arylboronic acid and imidazole is pro-
posed as shown in Scheme 3.
Table 6
Comparison of different catalytic system in Chan-Lam coupling reaction for the
synthesis of N-phenylimidazolea.
Entry
Catalyst
Reaction
Yield
(%)
Run
Ref
conditions
1
2
SiO2-NHC-Cu
PS-Cu
MeOH, 50 ◦C, 3 h
MeOH, 40 ◦C, 10
h
90
99
6
5
[58]
[31]
3
Cu@PI-COF
Cu-(tpa) MOF
Cu3(BTC)2
MeOH-H2O, rt., 8
h
87
99
92
92
61
88
99
98
8
5
4
7
5
6
5
5
[17]
[32]
[33]
[59]
[60]
[61]
[62]
4
EtOH, Et3N, rt.,
12 h
5
EtOH, Et3N,60 ◦C,
12 h
As the fiber surface contains excess basic prolinamide moiety, aryl-
boronic acid was more easily to access the prolinamide moiety, thus the
transmetalation of arylboronic acid with the fiber catalyst was more
accepted as the first step than that of imidazole with catalyst proposed
very recently [63]. The formed intermediate A was coordinated with
imidazole to generate intermediate B, which was oxidized by O2 to Cu
(III) species C. In this aerobic oxidation process, the generated water
would react with ClB(OH)3 to form B(OH)3 and HCl. The free prolina-
mide moiety adjacent to the chelated Cu(II) could also promote this
process cooperatively by combining with HCl. The reductive elimination
of C yielded the coupling product N-arylimidazole and Cu(I) species D.
The following aerobic oxidation step could make the Cu(II) catalyst
recovered in the aid of released HCl from prolinamide moiety.
6
Cu-doped CoFe2O4
URJC-1-MOF
CuO NPs
MeOH, Et3N, rt.,
5 h
7
DMF, K(OAc), rt.,
15 h
8
MeOH-H2O,
K2CO3, rt., 10 h
MeOH, Et3N, rt.,
5 h
9
Cu30I16(mtpmt)12
S4)
(μ10-
10
CuCl2@PANPA-2
F
MeOH, 60 ◦C, 3 h
This
work
a
Chan-Lam coupling of benzalboronic acid and imidazole as template
reaction.
2.5. Comparison of different catalytic systems in Chan-Lam coupling
toward N-phenylimidazole
3. Conclusion
In order to evaluate the efficiency of present fiber catalyst, we
compared the obtained results with those of previously reported het-
erogeneous catalysts (Table 6). Although the coupling worked well at
room temperature, additional bases and prolonged reaction time were
generally required (entries 4–9). Fiber-supported catalyst has compa-
rable catalytic activity with these commonly silica and MOF supported
catalysts without any base at an elevated temperature in short reaction
time. More importantly, the fiber catalyst exhibited the advantage of
easier separation by filtration over these particle catalysts because of the
regular and flexible structure of fiber.
In summary, we have described an efficient, stable and recyclable
fiber-immobilized copper catalyst in Chan-Lam coupling for the pro-
duction of N-arylimidazole and N-arylsulfonamides. The fiber catalysts
were readily prepared by the incorporation of prolinamide unit into
PANF and the subsequent immobilization of copper(II) in aqueous so-
lution. Cu(II) ions mainly gathered in the microenvironment of fiber
surface as particles and form an equilibrium with chelated Cu(II) ions,
although the latter was thought as the major catalytic sites. Excellent
physical and chemical stability of fiber catalysts were demonstrated by
mechanical strength test, thermal analysis and solvent resistance ex-
periments. Among the prepared catalysts, CuCl2@PANPA-2F exhibited
the best catalytic performance in the coupling of arylboronic acids with
imidazole as well as tosyl azide, giving desired products in good to
excellent yields. The catalytic activity of fiber catalyst was improved by
2.6. Reaction mechanism
Since Chan-Lam couplings of arylboronic acid with amine/imidazole
were reported, different reaction mechanisms have been proposed [27].
Recently, Cu(III) species has been increasingly favored as the inter-
midiate, which is generated through oxidation with O2 or possible dis-
proportionaliztion. Since the coupling of phenylboronic acid with
adjusting the Cu loading. Further studies show that CuCl2@PANPA-2
F
could be simply recovered by filtration, recycled for five times, and
perform well in gram-scale experiments. This fiber-mediated catalytic
8

C. Zhang et al.
Reactive and Functional Polymers 160 (2021) 104831
Scheme 3. Proposed mechanism of Chan-Lam coupling reaction between phenylboronic acid and imidazole by fiber-supported copper catalyst.
system is efficient, convenient and sustainable with possible application
in large-scale production. Moreover, the plausible mechanism for the
CuCl2@PANPA-2F-catalyzed coupling of arylboronic acid with imidazole
has been proposed.
internal standard.
4.3. Preparation of prolinamide-functionalized fiber PANPA-2
F
Dried PANF (0.47 g) was added to a solution of (S)-N-(2-aminoethyl)
pyrrolidine-2-carboxamide (5.00 g) dissolved in ethylene glycol (6.5
mL) in a round-bottom flask, followed by stirring at 140 ◦C for 3 h. After
being cooled to room temperature, the fiber was taken out using twee-
zers, washed with deionized water, and dried at 60 ◦C for 12 h to give
prolinamide-fiber PANPA-2F (1.15 mmol gꢀ 1). Other modification degree
of prolinamide-fiber was gained by reducing the reaction time (0.74
mmol gꢀ 1 for 1.5 h; 0.93 mmol gꢀ 1 for 2 h). Additionally, when above
mixture was stirred at 150 ◦C for 3 h, a higher modification degree of
PANPA-2F (1.39 mmol gꢀ 1) was obtained.
4. Experimental section
4.1. Materials and reagents
Commerically PANF (length of 10 cm and a diameter of 30 ± 0.5 μm)
with the composition of 93% acrylonitrile, 6.5% methyl acrylate, and
0.4–0.5% sodium styrene sulfonate were purchased from Fushun
Petrochemical Corporation of China. L-proline methyl ester hydrochlo-
ride, triethylamine, ethane-1,2-diamine, CuCl2∙2H2O, Cu(OAc)2∙H2O,
CuSO4∙5H2O, phenylboronic acid, imidazole, benzimidazole, tert-butyl
methyl ether, dichloromethane, methanol, petroleum ether, ethyl ace-
tate, and other reagents were analytical grade and used without further
purification.
4.4. Preparation of CuCl2@PANPA-2
F
Dried PANPA-2F (0.5 g) was added to an aqueous solution of cupric
chloride (0.5 mol Lꢀ 1, 15 mL) and the mixture was stirred at room
temperature for 24 h. After that, the fiber was collected by filtration,
washed with ethanol (200 mL) and water (200 mL), and dried at 60 ◦C
for 12 h under vacuum to give fiber-supported copper catalyst
CuCl2@PANPA-2F.
4.2. Instruments
The Fourier transform infrared spectra (FT-IR) of fiber was recorded
using AVATAR360 FT-IR spectrometer (Thermo Nicolet). The sample of
fiber was cut to pieces and compressed into KBr pellets before mea-
surement. X-ray power diffraction (XRD) of fiber was carried out with D/
MAX-2500 X-ray diffractometer (Rigaku Corporation). Thermostability
of fiber was measured using STA409PC TGA/DSC simultaneous ther-
malanalyzer (Netzsch Company, Germany). Elemental analysis was
performed on Vario Micro Cube instrument (Elementar, Germany).
Surface morphology of fiber was determined using scanning electron
microcopy (Hitachi, model S-4800). XPS was performed on Thermo
ESCALAB 250XI spectrometer (ThermoFisher Scientific, America). AAS
(atomic absorption spectrometry) was carried out using Ice3300. NMR
spectra of products (1H for 400 MHz and 13C for 101 MHz) were
recorded on BRUKER-AVANCE III instruments in CDCl3 with TMS as
4.5. Preparation of Cu(OAc)2@PANPA-2F and CuSO4@PANPA-2
F
The preparation of Cu(OAc)2@PANPA-2F and CuSO4@PANPA-2F are
similar to that of CuCl2@PANPA-2F. Typically, Cu(OAc)2@PANPA-2F was
prepared from PANPA-2F (0.536 g) with the aqueous solution of cupric
acetate (0.5 mol Lꢀ 1, 15 mL) at room temperature for 24 h, and
CuSO4@PANPA-2F was prepared from PANPA-2F with the aqueous solu-
tion of cupric sulfate (0.5 mol Lꢀ 1, 15 mL) at room temperature for 24 h.
9

C. Zhang et al.
Reactive and Functional Polymers 160 (2021) 104831
[26] X. Jia, P. Peng, N,O-Bidentate ligand-tunable copper(II) complexes as a catalyst for
Chan-Lam coupling reactions of arylboronic acids with 1H-imidazole derivatives,
Org. Biomol. Chem. 16 (2018) 8984–8988.
Declaration of Competing Interest
There are no conflicts of interest to declare.
[27] M.J. West, J.W.B. Fyfe, J.C. Vantourout, A.J.B. Watson, Mechanistic development
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Acknowledgements
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This work was financially supported by the National Natural Science
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