Recyclable polyetheretherketone fiber-supported N-heterocyclic carbene catalysts for nucleophilic acylation of fluorobenzenes

Article abstract of DOI:10.1039/d0cc04394j

We report for the first time a novel support of polyetheretherketone fiber for the synthesis of recyclable N-heterocyclic carbene (NHC) catalysts. The fiber catalysts were verified in nucleophilic acylation of fluorobenzenes with superior catalytic activities, and successfully recycled by a tiny pair of tweezers over 21 cycles with minimal loss of performance.

Full text of DOI:10.1039/d0cc04394j

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Recyclable polyetheretherketone fiber-supported
N-heterocyclic carbene catalysts for nucleophilic
acylation of fluorobenzenes†
Cite this:DOI: 10.1039/d0cc04394j
Received 24th June 2020,
Accepted 26th August 2020
a
a
a
b
c
Xian-Lei Shi, * Benyu Sun, Qianqian Hu, Kun Liu, Pengyu Li and
a
Juanjuan Wang
DOI: 10.1039/d0cc04394j
rsc.li/chemcomm
We report for the first time a novel support of polyetheretherketone applications NHCs are regenerated to their original forms on the
2
1–23
fiber for the synthesis of recyclable N-heterocyclic carbene (NHC) support so as to be easily recycled.
catalysts. The fiber catalysts were verified in nucleophilic acylation supports including polymers or silica,
In this respect, several
24–26
27,28
montmorillonite
29,30
of fluorobenzenes with superior catalytic activities, and success- and some porous materials
have been reported for this
fully recycled by a tiny pair of tweezers over 21 cycles with minimal objective. Nevertheless, some of them suffer from lower catalytic
loss of performance.
efficiency and inferior reusability, probably owing to poor stability
or degradation of the catalytic materials under a strongly alkaline
Recently, organocatalysis related to green chemistry has been environment. Therefore, exploiting more stable and efficient
stimulating much interest and has also been advanced support materials for the heterogenization of NHCs needs to be
1
–6
significantly.
are known as organocatalysts due to their lone pair of carbene
electrons with nucleophilic behavior in an extensive scope of acrylonitrile (PAN) fiber-supported catalysts for different sorts
Among them, N-heterocyclic carbenes (NHCs) further investigated.
In our previous work, we have developed a series of poly-
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–10
synthetic transformations.
However, most of the small- of reactions and displayed many superiorities, especially for
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1–35
molecule NHCs are difficult to handle on account of their high their prominent recyclability and convenient operations.
reactivity and sensitivity.
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To overcome this limitation, the However, the PAN fiber involves plenty of cyano groups on its
precursors such as imidazolium or thiazolium salts are often polymer chains, and the strong alkaline medium would inten-
employed to generate NHCs by in situ deprotonation with a sely disrupt the mechanical properties of the fiber, further
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strong base.
Although the practice has found success in restricting its application in supported-NHC catalysis. Given
this regard, the recovery of azolium salts still astricts the reuse many superiorities such as acid-proof and alkali resistant, high
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of NHCs. Some studies used poly(NHC) precursors to bypass strength and toughness, polyetheretherketone (PEEK) fiber
this restriction;
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however, tedious polymerization processes as a new rising star among synthons provides an appealing
were often required, thereby hampering their scalable applica- option to many other fibers as the support for heterogeneous
tions. Moreover, a promising method for NHC recycling is to catalysis. In addition, nucleophilic aromatic substitution is an
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incorporate the azolium salt into a material, and after catalytic important method in synthetic aromatic chemistry; however,
the nucleophilic aromatic substitutions of halogen atoms by
carbanions are infrequent due to the halonitroarenes often
a
39,40
Synerigism Innovative Center of Coal Safety Production in Henan Province,
College of Chemistry and Chemical Engineering, Henan Polytechnic University,
Jiaozuo, Henan 454003, P. R. China. E-mail: shixl@tju.edu.cn;
Tel: +86-0391-3986810
reacting with carbanions.
Based on the above considera-
tions and in continuation of our interest in novel fiber catalysts,
we here designed a new type of PEEK fiber-supported NHC
catalyst to evaluate their behaviour in nucleophilic acylation of
fluorobenzenes (Scheme 1).
The commercially available PEEK fiber was selected as a raw
material for preparing fiber-supported NHC precursor by a
post-functionalization process. And, as shown in Scheme 1,
the counterplan for the synthesis and regulation of the fiber
catalyst was performed firstly by chloromethylation to build
functional groups which would act as the active sites for
b
c
School of Environment and Materials Engineering, Yantai University, Yantai,
Shandong 264005, P. R. China
School of Materials and Chemical Engineering, Center for Advanced Materials
Research, Zhongyuan University of Technology, Zhengzhou 450007, P. R. China
Electronic supplementary information (ESI) available: Experimental details and
†
optimization results, characterizations of photographs, elemental analysis, FTIR
spectroscopy and mechanical properties of fiber materials in different stages, and
1
13
the physical data, H and C NMR data and copies of spectra of the synthesized
compounds. See DOI: 10.1039/d0cc04394j
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Scheme 1 Preparation of PEEK fiber-supported NHC catalysts and the mediated nucleophilic acylation of fluorobenzenes.
subsequent immobilization, and then conducted through imi- base activation, and not derived from the fiber backbone. More-
dazole grafting, neutralization and finally formation of onium over, the catalytic abilities of PEEK-IMMI and PEEK-IMEB were
salts to complete the precatalyst preparation process. The better than the small molecule catalyst 1,3-dimethylimidazolium
degree of functionality in each step was evaluated by weight iodide (DMII), the enhanced efficiency of PEEK-IMRX perhaps
gain (weight gain [%] = [(W À W )/W ] Â 100, where W and W are due to a synergistic effect between the active sites and the PEEK
2
1
1
1
2
the weights of the fiber sample before and after functionalization, backbone, which could stabilize the intermediate (Scheme 2) and
respectively).
prevent the dimerization or structural change in NHCs. In
In step 1, in order to retain enough mechanical properties of addition, PEEK-IMMI possessed a higher catalytic activity than
PEEK fiber for the succedent grafting, a weight gain of 27.6% PEEK-IMEB, probably because of its smaller steric hindrance
chloromethyl functionalized PEEK (PEEK-CM) was obtained for easier contact to the substrates. Furthermore, the perfor-
after optimization. Next, PEEK-CM was reacted with imidazole mance of organic base or bulk inorganic base was lower than
to immobilize imidazole groups, and the formed PEEK- NaH. Afterwards, under the optimized conditions (PEEK-IMMI
supported imidazole hydrochloride (PEEK-IMHCl) exhibited as the precatalyst, NaH as the base, with the solvent of DMF at
a weight gain of 18.5%, and the content of imidazole hydro- room temperature), we then examined the substrate scope to
À1
chloride was 2.29 mmol g calculated by mass change, which generalize the versatility of this methodology, and the nucleo-
is consistent with the elemental analysis value (Table S2, ESI†). philic acylation results are summarized in Table 1.
Then, PEEK-IMHCl was neutralized in Na
2
CO
3
aqueous
It was interesting to observe that the PEEK fiber-supported
solution to remove hydrogen chloride and the acquired imid- NHC systems have a good group tolerance; different substituted
azole functionalized PEEK (PEEK-IM) exhibited a weight gain of fluorobenzenes (Table 1, 3a, 3f and 3g) and aromatic aldehydes
À1
À9.38%, and the content of imidazole was 2.83 mmol g
.
(Table 1, 3a–e), as well as the multisubstituted substrates (Table 1,
Finally, PEEK-IM was reacted with methyl iodide and ethyl 3h) for the synthesis of a highly functionalized benzophenone in
bromide to afford two kinds of fiber-supported imidazoliums total synthesis of an anticancer natural product termicalcicola-
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1
(
PEEK-IMRX), and owing to different reactivity of the halides, none A, were all reacted efficaciously to obtain the corres-
the imidazolium contents of PEEK-supported methyl imidazo- ponding nucleophilic acylation products with isolated yields of
lonium iodide salts (PEEK-IMMI) and PEEK-supported ethyl 66% to 94%. However, lower activities of the substrates when
À1
imidazolonium bromide salts (PEEK-IMEB) were 1.37 mmol g
and 0.79 mmol g
electron-poor or sterically hindered aldehydes were used, and a
À1
calculated by mass change, respectively. higher reactivity of fluorobenzene were observed when it had a
However, the molar ratios of imidazoliums are not quite consis-
tent with the elemental analysis data, since the imidazole groups
in the fiber could not get a 100% conversion to imidazoliums.
With the PEEK-IMRX in hand, their activities for nucleophilic
acylation of fluorobenzenes were compared. Initial efforts were
focused on the optimization of the reaction conditions for the
model reaction (nucleophilic acylation of 4-fluoronitrobenzene by
benzaldehyde), and the catalyst types, solvents, base (to gen-
erate NHCs in situ), catalyst loading, reaction temperature and
time were all tested (Table S1, ESI†). On the whole, without any
catalyst or initial PEEK, PEEK-CM as well as the PEEK-IMHCl
and PEEK-IM containing the imidazole groups were all invalid
to the nucleophilic acylation, which indicated that the catalytic
Scheme 2 Possible mechanism of PEEK-IMMI mediated nucleophilic
activities came from imidazoliums (NHCs precursor) after the acylation of 4-fluoronitrobenzene by aldehyde.
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a
Table 1 PEEK-IMMI mediated nucleophilic acylation of fluorobenzenes
Fig. 1 Recyclability of the fiber-supported NHC catalysts for nucleophilic
acylation.
and 21st runs in model nucleophilic acylation all confirmed the
dependability of the preparation method for synthesizing
the PEEK fiber-supported NHC precursors and the stability of
the fiber catalyst after many cycles. In addition, the SEM images
of the fresh PEEK-IMMI and the recovered fiber precatalysts
PEEK-IMMX-1 and PEEK-IMMX-21 are shown in Fig. 2, apart
from them becoming increasingly coarser and more flecks
emerging in the surface of the recycled precatalysts (probably
because of the vigorous strong base or acid conditions, and
some nucleophilic acylation substrates or products were
grafted or adsorbed onto the fiber), there was no remarkable
change in the morphology or size of the fiber precatalyst with
repeated use. In addition, the fiber-supported NHC precatalysts
a
Reaction conditions unless otherwise specified: NaH (1.5 mmol) and
PEEK-IMMI (10 mol%, based on aldehyde), fluorobenzene (1 mmol)
and aldehyde (1 mmol) in DMF (10 mL) under N
b
2
.
Gram-scale test.
stronger electron-withdrawing group. To be more precise, the
PEEK fiber-supported NHC catalyst-mediated procedure pos-
sessed a selective applicability for nucleophilic acylation of
fluorobenzenes. On the basis of the above, an NHC-mediated
umpolung and base-promoted mechanism was proposed
13
were also determined by solid-state C NMR spectra; the peak
around 37.81 ppm could be assigned to the methyl attached to
an imidazole group, and the carbon peak of methylene which
linked the imidazole group and benzene ring of the PEEK
polymer chain is centered at 49.47 ppm, these above two
carbon signals indirectly demonstrated that the imidazolium
(Scheme 2). Beyond that, a Gram-scale test for the methodology
was also conducted, and this nucleophilic acylation proceeded
smoothly without any extension of the reaction time to obtain a
similar product yield of 86% (Table 1, 3a ).
Thereafter, the recyclability and stability of PEEK-IMMI were
also inspected. After accomplishing the reaction, a solution of
b
4.0 M HCl in dioxane was added, and the resulting mixture was
stirred for an additional 1 h to bring the NHC catalyst to its
precursor. The returned PEEK-IMMI was easily recovered with a
tiny pair of tweezers, and was used directly for the next cycle
after simple washing and drying. It could be found that the
activity of PEEK fiber-supported NHC catalysts was almost
consistent for 21 cycles (Fig. 1, from 89% to 81%, no test for
more runs). As a control, the small molecule of DMII could not
be recycled from the nucleophilic acylation using an analogous
protocol. Furthermore, a certain stability of PEEK-IMMI was
also inspected because during the research the fiber catalyst
stored without special protection (just drying before use)
remained equally active after at least up to three months.
To verify the reliability of this method, various technologies
were used to characterize the fibers resulted from different
stages. The photographs (Fig. S1, ESI†), elemental analysis
(Table S2, ESI†), FTIR spectroscopy (Fig. S2, ESI†) and mechan-
ical properties (Table S3, ESI†) of the original PEEK fiber, PEEK-
CM, PEEK-IMHCl, PEEK-IM, PEEK-IMMI, and the recovered
Fig. 2 SEM images of (I) PEEK-IMMI, (II) PEEK-IMMX-1 and (III) PEEK-
precatalysts PEEK-IMMX-1 and PEEK-IMMX-21 from the 1st IMMX-21. (Scale bars = 2 mm and 20 mm.).
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