Palygorskite-anchored Pd complexes catalyze the coupling reactions of pyrimidin-2-yl sulfonates

Article abstract of DOI:10.1039/c9ra06035a

In this work, an anchored Pd complex (PGS-APTES-Pd(OAc)₂) was prepared via simple and green steps from the natural clay mineral palygorskite and was well characterized by XPS, XRD, IR, SEM, and EDX. This complex was further utilized as a fine catalyst for the C-C/C-N coupling reactions of pyrimidin-2-yl sulfonates. Subsequently, the cyclic utilization test indicated the high stability and sustainability of this PGS-APTES-Pd(OAc)₂ catalyst, and no activation was required in the recycling process, providing an applicable and reusable catalyst in organic synthesis.

Full text of DOI:10.1039/c9ra06035a

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PAPER
Palygorskite-anchored Pd complexes catalyze the
coupling reactions of pyrimidin-2-yl sulfonates†
Cite this: RSC Adv., 2019, 9, 30526
a
a
a
a
Huiying Zhan, Rongrong Zhou, Xudong Chen, Quanlu Yang, Hongyan Jiang,b
*
*
Qiong Su,b Yanbin Wang,b Jia Li,b Lan Wub and Shang Wu
*
b
In this work, an anchored Pd complex (PGS–APTES–Pd(OAc)2) was prepared via simple and green steps
from the natural clay mineral palygorskite and was well characterized by XPS, XRD, IR, SEM, and EDX.
This complex was further utilized as a fine catalyst for the C–C/C–N coupling reactions of pyrimidin-2-yl
sulfonates. Subsequently, the cyclic utilization test indicated the high stability and sustainability of this
PGS–APTES–Pd(OAc)2 catalyst, and no activation was required in the recycling process, providing an
applicable and reusable catalyst in organic synthesis.
Received 3rd August 2019
Accepted 10th September 2019
DOI: 10.1039/c9ra06035a
rsc.li/rsc-advances
products is strictly monitored, and staining of palladium black
from the reaction would invalidate the pharmaceutical prod-
ucts.7 Above all, these deciencies limit their wide applications
in industries. In order to overcome these shortcomings of the
homogeneous Pd catalytic reactions, the development of sup-
ported palladium catalysts has become a research hotspot in
the eld of synthetic chemistry and has developed rapidly in
recent years.8 The supported heterogeneous catalyst can be
recycled via simple operations as the catalyst can be separated
and recovered aer the completion of the reaction.
Clay minerals, in view of their large specic surface area, rich
pore structure and excellent ion exchange properties, have been
adopted as excellent catalyst carriers.9 Among the previously
reported materials, palygorskite (PGS), beneting from its
crystalline morphology, depositional mode and crystal struc-
ture, preserves high internal and external surface areas. It is
a natural, non-toxic, cheap and easily available clay mineral. A
Lewis acidication center and an alkalization center can be
formed on the inner surface of palygorskite clay aer treatment,
which makes the palygorskite crystal not only meet the micro-
porous and surface characteristics required for the heteroge-
neous catalytic reaction, but also have the capabilities of the
catalytic cracking of molecular sieves.10 In 2010, Lei et al.10c re-
ported the oxidation of alcohols using palygorskite as the
catalyst and water as the solvent. Inspired by this work, we
supposed that palygorskite may not only serve as a catalyst
itself, but it might also be used as a good catalyst carrier.
Introduction
The synthesis of heterocyclic compounds has been investigated
in many studies for numerous times due to the important
structural units of many medical and other bioactive molecules.
Among them, molecules containing 3,4-dihydropyrimidin-
2(1H)-one (DHPM) scaffolds are widely found in natural prod-
ucts and compounds with unique pharmaceutical activities
(Fig. 1). For example, compounds A and B have been shown to
be potent calcium blockers,1 and compounds C and D have
been proven to possess calcium modulatory activities.2 In
addition, other DHPM derivatives can be used as mitotic kine-
sin inhibitors and hepatitis B virus replication inhibitors.3
Therefore, the investigation of the functionalization and
transformation of DHPM derivatives is of great value and
signicance.4
Palladium-catalyzed C–C coupling reactions, with the
advantages of high activity and concise reaction conditions, are
widely applied in synthetic chemistry.5 They have also been
used in the modication of DHPMs with considerable progress
reported in recent years.6 However, these methods generally
face the problem of the consumption of the precious metal
catalyst in palladium homogeneous catalysis. Also, the catalyst
is difficult to be recovered from the reaction system. This
problem may cause serious loss of palladium and metal
contamination of the products. Notably, in routine chemical
and drug syntheses, the content of residual metals in the
aCollege of Chemical Engineering, Lanzhou University of Arts and Science, Beimiantan
400, Lanzhou, Gansu 730000, People's Republic of China. E-mail: 674585005@qq.
com; yangquanlu2002@163.com
bKey Laboratory for Utility of Environment-Friendly Composite Materials and Biomass
in Universities of Gansu Province, College of Chemical Engineering, Northwest Minzu
University, Lanzhou, Gansu 730030, People's Republic of China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra06035a
Fig. 1 DHPMs with biological activities.
30526 | RSC Adv., 2019, 9, 30526–30533
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Herein, we reported the preparation of a palygorskite-anchored absorption peak at 1438 cmꢂ1 was ascribed to Lewis acid. For PGS–
Pd complex (PGS–APTES–Pd(OAc)2), which was used as the APTES, a weaker N–H bending (in-plane) vibration absorption
catalyst for DHPMs in C–C coupling with arylboronic acids and peak was found at 1562 cmꢂ1. The absorption peaks at 2933 and
terminal alkynes and C–N coupling with arylamines. In addi- 2877 cmꢂ1 corresponded to the asymmetric and symmetric
tion, the catalytic performance and recyclability of PGS–APTES– stretching vibrations of methylene. The absorption peak at
Pd(OAc)2 were investigated.
1438 cmꢂ1 disappeared, and the C–H bending (in-plane) vibration
absorption peak appeared at 1562 cmꢂ1. These changes in the
above-mentioned spectrum indicated that APTES was successfully
graed onto the surface of PGS. In Fig. 2(c), compared with the
observations for PGS–APTES, the absorption bands at 1778 cmꢂ1
and 1490 cmꢂ1 are attributed to the C]O stretching and –CH3
antisymmetric stretching vibrations of Pd(OAc)2, respectively. It
was revealed that coordination was formed between Pd(OAc)2 and
PGS–APTES.
Results and discussion
Synthesis and characterization of the catalyst
Palygorskite was pretreated with dilute HCl10a and then, the
activated PGS (1.0 g) was added to 150 mL xylene under ultra-
sonication for 1 h. A (3-aminopropyl)triethoxysilane (APTES)
ethanol aqueous solution (EtOH : H2O ¼ 9 : 1) was hydrolyzed
for 5 min; then, we added this to the PGS dispersion under
vigorous stirring at 80 ꢀC. The PGS–APTES product was ob-
tained by ltration, washed with anhydrous ethanol, and then
dried in vacuum at 50 ꢀC. Finally, the product was crushed,
sied out through 200 mesh sieves, and extracted with ethanol.
Then, 1 g PGS–APTES and 0.2 g Pd (OAc)2 were added into
20 mL deionized water, and the mixture was stirred at room
temperature for 3 h. When the reaction was complete, the liquid
was discarded by centrifugation, and the solid was washed with
deionized water (3 ꢁ 20 mL) and acetone (3 ꢁ 20 mL) and dried
in vacuum at 40 ꢀC for 4 h; the gray powder obtained was PGS–
APTES–Pd(OAc)2.
ICP-AES analysis revealed that the content of Pd in the PGS–
APTES–Pd(OAc)2 catalyst was 0.463 wt% (0.0435 mmol gꢂ1).
IR spectra were obtained for PGS, PGS–APTES and PGS–APTES–
Pd(OAc)2 (Fig. 2). In the FTIR spectrum shown in Fig. 2(a), the
absorption peaks appearing at 3614 cmꢂ1 and 3542 cmꢂ1 are
attributed to the telescopic vibration of the –OH group (Al, Fe, and
Mg) of PGS; also, the absorption peaks at 3442 cmꢂ1 and
1641 cmꢂ1 are ascribed to the telescopic vibration and the bending
vibration of the adsorbent H2O. The absorption peaks at
1031 cmꢂ1 and 779 cmꢂ1 corresponded to the Si–O bond exural
vibration and the Si–O–Si bond telescopic vibration. The
From the XRD spectrum (Fig. 3), it is revealed that PGS, PGS–
APTES and PGS–APTES–Pd(OAc)2 show the same diffraction
peaks at 2q ¼ 8.3ꢀ, 21.3ꢀ, 26.7ꢀ, 36.8ꢀ, and 60.3ꢀ, indicating that
the brous palygorskite is not likely to be layered montmoril-
lonite. This ordered structure in palygorskite is difficult to
change. In addition, the relatively low loading of Pd also caused
no obvious changes in the characteristic diffraction peaks.
The thermogravimetric analysis (Tg) of PGS, PGS–ATPES and
PGS–ATPES–Pd is shown in Fig. 4. At 100 ꢀC, there was a slight
increase in the loss of the mass of PGS–APTES and PGS–APTES–
Pd(OAc)2 compared with that for PGS, which may be because
the water molecules of PGS were substituted by APTES mole-
cules.11 In addition, at a temperature around 100 C to 600 C,
the loss of the mass of PGS–APTES and PGS–APTES–Pd(OAc)2
further increased, which was regarded as the decomposition of
the surfactant APTES molecules.11 Consequently, APTES was
successfully supported on PGS.
ꢀ
ꢀ
The SEM images of stony palygorskite, PGS–APTES and PGS–
APTES–Pd(OAc)2 are shown in Fig. 5. The structure of a paly-
gorskite-like typical rod can be clearly seen in Fig. 5(a), with
a diameter of 50 nm and a length of 500–1000 nm. When graed
onto APTES (Fig. 5(b)), the rod structure did not change
signicantly, and the arrangement became loose and had more
pores. Aer Pd(OAc)2 was loaded, no reunion of Pd particles was
Fig. 2 IR spectra of (a) PGS, (b) PGS–APTES and (c) PGS–APTES– Fig. 3 XRD spectrum of (a) PGS, (b) PGS–APTES and (c) PGS–APTES–
Pd(OAc)2.
Pd(OAc)2.
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PGS. Obviously, the interactions between Pd and N at the
surface of PGS resulted in the good dispersion of Pd particles at
a high magnication (Fig. 6(d)).
In order to further determine whether Pd(OAc)2 is loaded
onto the palygorskite surface in the form of adsorption or
coordination bonds and the valence state of Pd before and aer
the loading process, the catalysts were characterized by XPS
(Fig. 7). The N 1s binding energies of PGS–APTES and PGS–
APTES–Pd(OAc)2 were 399.18 eV and 399.88 eV, respectively,
increasing by 0.70 eV. In Fig. 7(c), the Pd 3d peaks consist of two
components with binding energies of 343.78 eV (Pd 3d3/2) and
338.48 eV (Pd 3d5/2) for Pd(OAc)2 and 343.28 eV (Pd 3d3/2) and
337.98 eV (Pd 3d5/2) for PGS–APTES–Pd(OAc)2, respectively,
which were reduced by 0.50 eV. In addition, this originated from
Pd 3d3/2 and Pd 3d5/2 of Pd(II). Therefore, a coordination bond
might be formed between Pd(OAc)2 and PGS–APTES.
Fig. 4 TG spectrum of (a) PGS, (b) PGS–APTES and (c) PGS–APTES–
Pd(OAc)2.
PGS–APTES–Pd(OAc)2–catalyzed Suzuki coupling reaction of
pyrimidin-2-yl sulfonates
found (Fig. 5(c)). The corresponding EDX and EDS spectra are
shown in Fig. 5(d–h), and it was demonstrated that N and Pd
did exist in uniform distribution though the APTES and Pd
loading was relatively less.
As the catalyst was well characterized, PGS–APTES–Pd(OAc)2
was used in the coupling reactions of pyrimidin-2-yl sulfonates.
Ethyl
4-methyl-6-phenyl-2-(tosyloxy)pyrimidine-5-carboxylate
(1a) and phenylboric acid (2a) were used as the substrates for
the investigation of the reaction conditions of this PGS–APTES–
Pd(OAc)2 catalytic system. The effects of the solvent, catalyst
dosage and additives were screened (Table S1, in ESI†).
Fig. 6 shows the TEM images of PGS, PGS–APTES and PGS–
APTES–Pd(OAc)2. The rod-like shape of palygorskite can be seen
clearly, and the Pd particles are distributed on the surface of
Fig. 5 SEM and EDX images of (a and d) PGS, (b and e) PGS–APTES, and (c and f) PGS–APTES–Pd(OAc)2; (g and h) the corresponding EDS
mapping of N and Pd.
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Fig. 6 TEM images of (a) PGS, (b) PGS–APTES, and (c) PGS–APTES–Pd(OAc)2. (d) The TEM image of the catalyst at high magnification.
Fig. 7 The XPS spectra of the catalyst: (a) full spectra, (b) N 1s, (c) Pd 3d.
First, 1,4-dioxane was used as the solvent, in which the could be well tolerated, giving the corresponding products in
ligand and base effects were studied (Table S1,† entries 1–7). As good yields. These results revealed the applicable method of
a result, DPE-Phos and X-Phos exhibited similar activities, but this PGS–APTES–Pd(OAc)2 catalytic system.
they both showed slightly lower activity than PPh3. The activity
of the base for TBAB, NaOAc, Cs2CO3, K2CO3 and K3PO4
increased in turn. Therefore, when PPh3 was used as the ligand
and K3PO4 was used as the base, the product could be obtained
in a higher yield of 88% (Table S1,† entry 3).
Subsequently, the effect of the solvent was screened. 1,4-
Dioxane can provide better conversion (Table S1,† entries 8 and
9). With regard to green chemistry, we tested other solvents, but
they did not afford higher yields. In addition, the amount of
PGS–APTES–Pd(OAc)2-catalyzed Sonogashira coupling
reaction of pyrimidin-2-yl sulfonates
Similarly, the prepared PGS–APTES–Pd(OAc)2 was used as the
catalyst in the Sonogashira coupling reactions of pyrimidin-2-yl
sulfonates with terminal alkynes. Compound 1a and phenyl-
acetylene (4a) were used as the substrates to optimize the
reaction conditions.
catalysts was also investigated. We found out that with the
increase in catalyst loading, the yield of 3a increased signi-
cantly. When the amount of the catalyst was 20 mg, the yield
reached 88% (Table S1,† entries 3, 10–12). Similarly, the activ-
ities of the homogeneous catalysts Pd(OAc)2 and PGS–APTES–
Pd(OAc)2 were compared using the same amount of Pd. The
results indicated that the catalytic efficiency of Pd in PGS–
APTES–Pd(OAc)2 was higher (Table S1,† entries 3 and 16)
(Scheme 1).
Under optimized conditions, a series of arylboronic acids
were reacted with 1a. Phenylboronic acids with ortho-, meta-,
and para-methyl groups were satisfactorily tolerated, giving the
corresponding products 3b–3d in good to moderate yields. In
addition, the steric hindrance of coupling partners had slight
inuence on this coupling reaction. Other functional groups
such as MeO- and Cl- could deliver the products 3e and 3f,
respectively, in good yields. In addition, naphthalen-1-ylboronic
acid could also afford the product 3g in 85% yield. Next, we
explored the scope of DHPM-sulfonates with phenylboronic
acid (2a). Compared with 1a, the DHPM-sulfonates with
different functional groups such as Me-, MeO-, F-, Cl-, and Br-
Scheme 1 Scope of Suzuki reaction.
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Table S2 (in ESI†) shows that the Cu(II) salt has no activity
(Table S2,† entry 4). The activity of the inorganic Cu(I) salt is very
low (Table S2,† entry 1 and 2). We therefore tested the effect of
((thiophene-2-carbonyl)oxy)copper (CuTC), and it could deliver the
corresponding product 5a in a moderate yield of 59% when X-phos
was used as the ligand (Table S2,† entry 3). We therefore screened
other ligands with CuTC as the co-catalyst. When DPE-Phos was
used as the ligand instead of X-phos, the yield of the product
increased signicantly (Table S2,† entry 6). However, when PPh3
was used as the ligand, it could only afford a yield of 22% (Table
S2,† entry 5). Further investigations indicated that 1,4-dioxane was
the best solvent in this reaction (Table S2,† entries 6–8). In addi-
tion, the amount of the catalyst was also explored. The results
showed that the reaction could be completed to the maximum
extent when the amount of the catalyst was 20 mg (Table S2,†
entries 6, 9–10). The optimized reaction time of the system showed
that the best reaction time of the system was 48 h (Table S2,†
entries 11–13). Similarly, we carried out control experiments on the
system, and the results indicated that the catalyst and the ligand
were indispensable (Table S2,† entries 14–15). From the above-
described experiments, we concluded the optimized reaction
conditions: PGS–APTES–Pd(OAc)2 (20 mg, Pd 0.463 wt%), CuTC
(10 mol%), K3PO4 (2.0 equiv.), DPE-Phos (6 mol%), 1,4-dioxane (5
mL), with the temperature of 110 ꢀC and 48 hours reaction time.
Under the optimized reaction conditions, the substituted
DHPM-sulfonates (1) and terminal alkynes (4) were used to inves-
tigate the substrate scope (Scheme 2). Both aryl and alkyl terminal
alkynes were compatible with the reaction system, and the corre-
sponding products could be produced in moderate to good yields.
For the substituted DHPM-sulfonates, the electronic effect of the
substituents was more obvious (please compare 5e, 5f, and 5g).
Scheme 3 Scope of the C–N coupling reaction.
coupling reaction of DHPM-sulfonates with phenylamine
derivatives. The results are listed in Table S3.†
Aer screening different reaction conditions, we found out
that when the amount of the catalyst was 20 mg and the solvent
was 1,4-dioxane with K3PO4 as the base and PPh3 as the ligand,
this reaction could afford the C–N coupling product in a good
yield of 76%.
Based on the optimization conditions, we explored the scope
of the DHPM-sulfonates with different substituted anilines. As
a result, when different anilines were tested, the C–N coupling
products could be obtained in a high yield regardless of an
electron-withdrawing substituent (–Cl) or an electron-donating
substituent (–Me). However, the steric hindrance effect of
anilines was more obvious as o-methyl aniline only delivered 7c
PGS–APTES–Pd(OAc)2-catalyzed C–N coupling reaction of
pyrimidin-2-yl sulfonates
Aer realizing the C–C coupling reaction catalyzed by PGS–
APTES–Pd(OAc)2, we applied the catalytic system to the C–N
Scheme 2 Scope of Sonogashira reaction.
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Fig. 8 Recycling of the catalyst in the Suzuki reaction.
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Fig. 9 (a) SEM image of the reused catalyst; (b) the corresponding EDX spectrum and (c) EDS mapping of the reused catalyst.
in 38% yield. For the substituted DHPM-sulfonates, the elec-
The content of Pd in the reaction solution was determined to
tronic effect of the substituents on the yield was negligible. be 76 ppb by ICP-MS analysis. In order to verify whether free Pd
Different functional groups could be well tolerated in good or PGS–APTES–Pd(OAc)2 in the reaction system plays a real
yields (Scheme 3).
catalytic role, we also carried out a “hot ltration test” on the
reaction system.
Under the optimum conditions, 1a and 2a were used as the
substrates and aer 3 hours of reaction, the catalyst was sepa-
rated from the reaction solution. At this time, the conversion of
substrates was determined to be only 28%. Aer that, the ltrate
was stirred for 24 hours under the standard conditions, but no
reaction was detected. This result indicated a key catalytic role
of PGS–APTES–Pd(OAc)2 against the trace amount of free Pd in
the solution.
In addition, the morphology of the catalyst aer the reaction
was characterized (Fig. 9). Compared with the catalyst before
the reaction, the palygorskite-supported catalyst was still in rod
shape (Fig. 9(a)). Aer the reaction, there was no obvious
accumulation of Pd particles on the surface, but the surface was
uneven, which may be due to the corrosion of palygorskite by
the reaction system. In addition, the EDX and EDS spectra
(Fig. 9(b and c)) show that Pd is still uniformly loaded on the
catalyst surface. Therefore, the catalyst has a good recycling
performance.
Recycling of the catalyst
The lifetime of a heterogeneous catalyst plays a key role in the
catalytic reaction for practical applications, which is superior to
a homogenous one. The reusability of the PGS–APTES–Pd(OAc)2
catalyst for the Suzuki coupling reaction of DHPM-sulfonates 1a
and phenylboronic acid 2a was also investigated. Aer each
reaction, the mixture was cooled to room temperature, and the
solid was separated by centrifugation, washed, dried, and then
used again in the next run. The Pd content of the catalyst aer
each reaction was determined.
In this test, PGS–APTES–Pd(OAc)2 and PGS–Pd (prepared by
the impregnation method) were used as the catalysts (equiva-
lent Pd). It can be seen from Fig. 8 that PGS–APTES–Pd(OAc)2
has good reusability, and the yield of the product for PGS–
APTES–Pd(OAc)2 has no obvious downward trend aer eleven
cycles of the catalyst. However, the activity of PGS–Pd in the
initial use was similar to that of PGS–APTES–Pd(OAc)2 and in
the third cycle, its activity declined signicantly. The results
further indicated that the PGS–APTES–Pd(OAc)2 catalyst was
stable, and the Pd content was almost unchanged (0.463–
0.431%) aer 11 cycles of use by ICP-AES. The cumulative TON
value of the catalyst was 2216.
In order to further testify the stability of the catalyst, the
catalyst was characterized by XPS aer the reaction. As shown in
Fig. 10, the binding energies of N 1s and Pd 3d have no
signicant change before and aer the Suzuki reaction (Fig. 10).
The XPS results indicated that the oxidation state of Pd was PdII
Fig. 10 XPS spectra of (A) fresh and (B) reused catalyst.
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Acknowledgements
We gratefully acknowledge the nancial support from the
National Natural Science Foundation of China (No. 21962017,
21464013, 51563022, 31760608 and 21467027), the Research
Funds for the Colleges and Universities of Gansu Province (No.
2014A-130), the Fundamental Research Funds for the Central
Universities (31920160010, 31920190016), and the Northwest
Minzu University’s Double First-class and Characteristic
Development Guide Special Funds-Chemistry Key Disciplines in
Gansu Province (No. 11080316).
References
Fig. 11 Proposed mechanism of the Suzuki coupling reaction.
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Based on previous literature6 and our above-mentioned studies,
a proposed mechanism for the Suzuki-type coupling reaction is
depicted in Fig. 11. This PGS–APTES–Pd(OAc)2 catalytic process
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arylboronic acid, followed by fast reductive elimination to
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Conclusions
In conclusion, a practical and easily available PGS–APTES–
Pd(OAc)2 catalyst was prepared using simple procedures. The
catalyst could be applied in the C–C and C–N coupling reactions
of pyrimidin-2-yl sulfonates with arylboronic acids, terminal
alkynes and anilines smoothly, giving the corresponding prod-
ucts in moderate to good yields. Obviously, the stability and
reusability of PGS–APTES–Pd(OAc)2 were superior to those of
the PGS–Pd catalyst (prepared by the impregnation method) in
the recycling test. This catalytic system will promote the appli-
cation of the PAL materials in catalysis with the expanding of
the palladium chemistry.
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Conflicts of interest
There are no conicts to declare.
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