Finely dispersed palladium on silk-fibroin as an efficient and ligand-free catalyst for Heck cross-coupling reaction

Article abstract of DOI:10.1002/aoc.5231

A palladium–fibroin complex (Pd/Fib.) was prepared by the addition of sonicated fibroin fiber in water to palladium acetate solution. Pd (OAc)₂ was absorbed by fibroin and reduced with NaBH₄ at room temperature to the Pd(0) nanoparticles. Powder-X-ray diffraction, scanning electron microscopy–energy-dispersive X-ray spectroscopy, Fourier transform-infrared, CHN elemental analysis and inductively coupled plasma-atomic emission spectroscopy were carried out to characterize the Pd/Fib. catalyst. Catalytic activity of this finely dispersed palladium was examined in the Heck coupling reaction. The catalytic coupling of aryl halides (-Cl, -Br, -I) and olefins led to the formation of the corresponding coupled products in moderate to high yields under air atmosphere. A variety of substrates, including electron-rich and electron-poor aryl halides, were converted smoothly to the targeted products in simple procedure. Heterogeneous supported Pd catalyst can be recycled and reused several times.

Full text of DOI:10.1002/aoc.5231

Received: 25 April 2019
DOI: 10.1002/aoc.5231
Revised: 26 July 2019
Accepted: 17 August 2019
F U L L P A P E R
Finely dispersed palladium on silk‐fibroin as an efficient
and ligand‐free catalyst for Heck cross‐coupling reaction
Hakimeh Mirzaei | Hossein Eshghi
| Seyed Mohammad Seyedi
Department of Chemistry, Faculty of
A palladium–fibroin complex (Pd/Fib.) was prepared by the addition of soni-
Science, Ferdowsi University of Mashhad,
Mashhad, Iran
cated fibroin fiber in water to palladium acetate solution. Pd (OAc) was
2
absorbed by fibroin and reduced with NaBH at room temperature to the
4
Correspondence
Hossein Eshghi, Department of
Chemistry, Faculty of Science, Ferdowsi
University of Mashhad. Mashhad, Iran.
Email: heshghi@um.ac.ir
Pd(0) nanoparticles. Powder‐X‐ray diffraction, scanning electron microscopy–
energy‐dispersive X‐ray spectroscopy, Fourier transform‐infrared, CHN ele-
mental analysis and inductively coupled plasma‐atomic emission spectroscopy
were carried out to characterize the Pd/Fib. catalyst. Catalytic activity of this
finely dispersed palladium was examined in the Heck coupling reaction.
The catalytic coupling of aryl halides (‐Cl, ‐Br, ‐I) and olefins led to the forma-
tion of the corresponding coupled products in moderate to high yields under
air atmosphere. A variety of substrates, including electron‐rich and electron‐
poor aryl halides, were converted smoothly to the targeted products in simple
procedure. Heterogeneous supported Pd catalyst can be recycled and reused
several times.
KEYWORDS
cross‐coupling, Heck reaction, ligand‐free, palladium–fibroin catalyst
1
| INTRODUCTION
whereas heterogeneous catalysts have many advantages,
such as stability of the catalyst, easy separation from the
The arylation or vinylation of alkenes with aryl or vinyl
halides, known as the Heck reaction, was discovered
reaction mixture over completion of the reaction, a good
recyclability, cost reduction and high catalytic activity.
[5]
[1]
independently by Heck in the 1970s. The palladium‐
catalyzed cross‐coupling reaction is a versatile tool for
Because the catalytic process takes place on a metal
surface, nanoparticles (NPs) are much more reactive than
the particulate metal counterpart due to their small
sizes and large surface areas. But the overall performance
of nanomaterials depends on the size, shape, textural
parameters of the particles and supporting materials
[2]
the generation of carbon–carbon bonds.
There are
many chemical and pharmaceutical intermediates with
alkene structural units being synthesized by including
[
3]
use of the Heck reaction in the presence of Pd catalyst.
[
6]
Therefore, tremendous attention has been given to
develop a new and advantageous catalytic system for
that these NPs are stabilized on. Therefore, different
types of materials, such as fibers, mesoporous materials,
metal oxide, activated carbon, polymers and dendrimers,
[4]
Pd‐catalyzed reactions.
Several catalytic systems,
[
7]
including the homogenized and the heterogeneous
palladium‐based catalyst, have been designed and made.
The problem with homogeneous catalysis is the difficulty
in separating the catalyst from the reaction mixture and
the impossibility of reusing it in consecutive reactions,
have been employed to support NPs. Most of these
supporting materials require surface activator ligands
due to their low activity in the adsorption of metal NPs.
All of these catalyst systems suffer from drawbacks of
one kind or another, such as the high ligand sensitivity
Appl Organometal Chem. 2019;e5231.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5231

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MIRZAEI ET AL.
toward air and moisture, the tedious multistep synthesis,
hence the high cost of the ligands, the use of various
additives, and the need for an inert atmosphere during
the synthesis of the catalyst as well as during the reac-
catalyst system (palladium–fibroin; Pd/Fib.), in which
fibroin is used as a stabilizer to hamper the high amount
of Pd employed for the effective ligand‐free palladium‐
catalyzed Heck reaction.
[8]
tion. On the other hand, ligand‐free catalytic systems
have many advantages in term of operational and eco-
nomic efficiency. Many researchers have focused studies
[9]
2 | EXPERIMENTAL
2.1 | General
on applying natural biopolymers, such as silk, starch,
cellulose and chitosan, as activated supports for metal
[10]
particles.
Many of the electron‐rich functional groups
in the protein bind with the metal surface easily, leading
to the bioconjugation. The main characteristic of these
biopolymers is their high amine group content, leading
All chemicals were purchased from Merck, Fluka or
Aldrich Chemical. The catalyst was synthesized and char-
acterized according to the literature. All yields refer to
isolated products. The products were characterized by
comparing their physical data with those of known sam-
ples or by their spectral data.
[
11]
to interesting chelating properties for metal cations.
Natural silk is composed of two primary proteins: silk
fibroin (SF; approximately 75%) and sericine (approxi-
[12]
mately 25%).
In raw silk, sericine is positioned across
the surface of two parallel fibroin fibers, which binds
[13]
them together.
SF is a fibrous protein with a semi‐
2
.2 | Preparation of support
crystalline structure that provides stiffness and strength.
Sericine is a glue‐like amorphous protein that acts as an
adhesive binder to keep the structural integrity of the
fibers. Sericine is soluble and can be removed by a
The raw silk fibers were processed three times (30 min
each time) in 0.5 wt.% Na CO solution at 80–90°C to
remove sericine (degumming), rinsed with deionized
water and dried at room temperature.
2
3
[
14]
thermochemical process known as degumming.
The
formation of SF heavy metals (Cu, Cr, Zn, etc.) complexes
has been studied to improve the mechanical properties
of silk fibers, removing heavy metals from water and
0
2.3 | Preparation of supported Pd
nanoparticles
[15]
to purify the organically polluted water.
In terms of
the catalyst, fibroin metal catalysts have been used
for chemoselective hydrogenation, reduction reaction of
p‐nitrophenol, hydroxylation of phenols and oxygen
reduction reactions.
Herein, we report an easy methodology for the synthe-
sis of a green and heterogeneous efficient palladium
The degummed fibroin (0.2 g) was dissolved in deionized
water (10 cc) and sonicated for 10 min. Then, a solution
of (0.1 M) palladium (П) acetate (5 cc) was added to the
mixture and was stirred for 6 hr, leading to the absorption
of Pd (П) on the fibroin surface. Afterwards, Pd (П)‐
[16]
SCHEME 1 Schematic representation
of Pd/Fib. catalyst

MIRZAEI ET AL.
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supported fibroin was removed from the solution, and
finally 0.1 M sodium borohydride solution (10 cc) was
added dropwise to this and was stirred for 24 hr to pro-
vide a Pd (0)/Fib. catalyst. After completion of the reduc-
tion procedure, the catalyst was washed four times with
water and ethanol (50:50) mixture and dried at room tem-
perature. The Pd/Fib. catalyst structure is schematically
shown in Scheme 1.
2.4 | General procedure for the Heck
coupling reaction
Aryl halide 1a–g (1 mmol), alkene 2a–d (1.2 mmol),
triethylamine (1.2 mmol) and Pd/Fib. catalyst (0.05 mol%)
were added to DMF (5 cc) and the reaction mixture was
stirred at 80°C for the appropriate time. After completion
of the reaction (monitored by thin‐layer chromatogra-
phy), the reaction mixture was cooled, diluted with chlo-
roform (20 mL) and filtered to remove the catalyst. The
organic layer was washed with water (40 mL × 2), dried
over MgSO and concentrated under reduced pressure
4
to obtain crude residue. The residue was purified by plate
chromatography (hexane:ethylacetate) to afford the cor-
responding Heck adduct 3a–l in moderate to high yields
(
Scheme 2).
All of the Heck coupling products are known, so these
products were identified by comparison of their physical
data with those of authentic samples. In spite of physical
SCHEME 2 General procedure for the synthesis of Heck adducts
FIGURE 1 Fourier transform‐infrared (FT‐IR) spectra of fibroin fibers: (a) without palladium; and (b) with palladium

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MIRZAEI ET AL.
1
comparison, spectroscopic data ( H‐NMR and MS) were
Product 3i: (E)‐1, 2‐diphenylethene: mass: m/z = 180
+
1
also studied and showed excellent accordance.
(M ); H‐NMR (300 MHz, CDCl ): δ 7.16 (s, 1H, vinyl),
3
7.28–7.30 (m, 1H, Ar), 7.40 (m, 2H, Ar), 7.56 (m, 2H, Ar).
Product 3k: (E)‐3‐(4‐methoxy‐phenyl)‐2‐methyl‐acrylic
+
1
2
.5 | Representative spectral data for the
acid methyl ester: mass: m/z = 206 (M ); H‐NMR
(300 MHz, CDCl ): δ 2.17 (d, J = 1.2 Hz, 3H, CH ), 3.84
s, 3H, OCH ), 3.87 (s, 3H, OCH ), 6.96 (d, J = 8.7 Hz,
selected products
3
3
(
3
3
Product 3d: (E)‐3‐(2‐methoxy‐phenyl)‐acrylic acid methyl
2H, Ar), 7.42 (d, J = 8.7 Hz, 2H, Ar), 7.68 (s, 1H, vinyl).
+
1
ester: mass: m/z = 192 (M ); H‐NMR (300 MHz, CDCl ):
3
δ 3.83 (s, 3H, OCH ), 3.90 (s, 3H, OCH ), 6.57 (d,
3
3
2
.6 | Recycling and reusing of the catalyst
J = 15.9 Hz, 1H, vinyl), 6.93 (d, J = 8.7 Hz, 1H, Ar),
.98 (d, J = 8.7 Hz, 1H, Ar), 7.34–7.40 (m, 1H, Ar) 7.53
dd, J = 7.8 Hz, 1.5, 1H, Ar), 8.03 (d, J = 15.9 Hz, 1H,
vinyl).
Product 3e: (E)‐3‐(4‐methoxy‐phenyl)‐acrylonitrile:
6
(
Due to the fact that the catalyst is insoluble in hot DMF,
it can be recycled by simple filtration. The separated cat-
alyst was washed with hot ethanol, dried at room temper-
ature and reused in the reaction.
+
1
mass: m/z = 159 (M ); H‐NMR (300 MHz, CDCl ): δ
3
3
.87 (s, 3H, OCH ), 5.74 (d, J = 16.5 Hz, 1H, vinyl), 6.94
3
(
7
d, J = 8.7 Hz, 2H, Ar), 7.36 (d, J = 18 Hz, 1H, vinyl),
.42 (d, J = 8.7 Hz, 2H, Ar).
3 | RESULTS AND DISCUSSION
3.1 | Characterization results of the
Pd/Fib. catalyst
3.1.1 | Fourier transform‐infrared spectra
−
1
Pure SF (Figure 1a) presented absorption bands at 1647 cm
−1
and 1545 cm that corresponded to amide ‐ I (C = O
stretching) and amide ‐ II (N‐H bending) of silk II structural
conformation (β‐sheet). Another absorption band observed
at 1247 cm corresponded to amide ‐ III (C‐N stretching),
which is typically silk I conformation (α‐helix or random
−1
[17]
−1
coil structure).
Peaks were observed at 1411 cm (due
to symmetric stretching vibrations of carboxylate group)
−
1
and 1339 cm
(indicating the presence of methylic
FIGURE 2 X‐ray diffraction (XRD) patterns of fibroin fibers: (a)
with palladium; and (b) without palladium
groups of alanine in the silk chain). The peak at
3423 cm is a symptom of the hydroxyl (‐OH) stretching
−
1
FIGURE 3 Scanning electron microscopy (SEM) images of: (a) pure fibroin; (b) Pd/Fib.; and (c) cutting section of fibroin fiber

MIRZAEI ET AL.
5 of 9
FIGURE 4 Energy‐dispersive X‐ray spectroscopy (EDS) report of fibroin fibers: (a) before; and (b) after palladium loading
vibration from the phenolic acids of Tyr molecule of SF
chain. Infrared (IR) analysis demonstrated that complex-
ation of palladium by the fibroin fiber had occurred,
with the prominent C‐N stretching vibration of the
of a fibroin fiber in Figure 3(c) proves that the sericin
was completely removed.
[
20]
−
1
imine (1247 cm ) being shifted to a lower frequency
TABLE 1 CHN component of fibroin fibers, before and after
palladium loading
−
1
−1
(
1229 cm ) by 18 cm , indicative of metal–fibroin
[17]
interaction.
Element (%w)
Pure fibroin
Palladium/fibroin
Nitrogen
Carbon
16.43
45.24
5.59
10.15
27.86
4.23
3
.1.2 | X‐ray diffraction
Hydrogen
Figure 2(a) presented an X‐ray diffraction (XRD) pattern
typical of amorphous substances with the absence of crys-
tallinity peaks, characterizing a silk I secondary struc-
TABLE 2 Synthesis of compound 3a in the presence of Pd/Fib.
NPs as catalyst under different reaction conditions
[18]
0
0
a
ture.
The evidence of formation of Pd NPs (Pd /Fib.)
into the surface of fibroin was obtained from powder
XRD analysis. Figure 2(a) shows broad and weak peaks
of 2θ values at 40.02°, 47.92° and 68.1°, which are
assigned to the corresponding (1 1 1), (2 0 0), (2 2 0) indi-
ces of FCC lattice of metallic Pd (JCPDS 87–0638). It can
be observed that the Pd diffraction peaks were clearly
weak, indicating much more homogeneous dispersion of
Catalyst
Temp. Time Yield
b
Entry (mol%Pd) Solvent Base
(°C)
(hr)
(%)
[19]
1
2
3
4
0.1
0.1
0.1
0.1
DMSO
DMSO
DMSO
Na CO 100
5
5
5
4
60
65
85
75
2
3
Pd NPs in fibrous matrix.
K
2
CO
3
100
100
100
NEt
3
3.1.3 | Scanning electron microscopy
Solvent‐ NEt
3
free
Scanning electron microscopy (SEM) images proved the
high dispersion of amorphous palladium metal particles
5
6
7
8
9
1
1
1
0.1
0.1
0.1
0.1
0.1
0.08
0.05
0.02
–
Methanol NEt
3
3
3
3
reflux
reflux
100
80
5
5
3
3
3
3
3
3
5
73
75
90
92
58
92
92
86
–
Ethanol NEt
(Figure 3b,e) on the smooth surface of the SF (Figure 3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
NEt
NEt
a). The palladium particles are completely dispersed
throughout the surface of the fibroin fibers, but in some
places their density is slightly higher, which can be seen
clearly in the SEM image as crystallized dots. This homo-
geneous dispersion was attributed to an interaction
between the Pd (OAc)2 and amino acids of the SF,
increasing the resistance to the growth of the Pd cluster.
The elemental analysis (EDS) was employed for determi-
nation of the chemical composition of Pd/Fib. catalyst
NEt3
r.t.
0
1
2
NEt
NEt
NEt
NEt
3
3
3
3
80
80
80
13
80
a
Other conditions: iodobenzene (1 mmol), methylacrylate (1.2 mmol),
(
Figure 4), and confirmed the presence of Pd in the sur-
3
triethylamine (Et N) (1.2 mmol) as base, and DMF as solvent at 80°C.
b
face of fibroin fibers. The SEM image of cutting section
Isolated yield of the pure product based on iodobenzene.

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MIRZAEI ET AL.
a
TABLE 3 Synthesis of Heck adducts 3a–3l using Pd/Fib. NPs as catalyst under the optimized conditions
b
c
Entry
Aryl halide
Alkene
Product
Time (hr)
Yield (%)
1
.
3a
3
1st use:92 2nd use;
0 3rd use:87
9
2.
3.
4.
3b
3c
3d
3.5
3
89
94
92
3
5
.
.
3c
3e
5
3
51
92
6
7.
8.
9.
3f
3g
3h
3.3
3
89
90
85
5
1
0.
1.
3i
3j
5
5
87
83
1
1
2.
3.
3k
3l
3
3
90
88
1
1
4.
5.
3k
3c
6
5
45
33
1
a
3
Reaction conditions: iodobenzene (1 mmol), methyl acrylate (1.2 mmol), triethylamine (Et N; 1.2 mmol) as base, 0.05 mol% Pd/Fib. and DMF as solvent at
80°C.
b
1
All the products were characterized by H‐NMR and MS.
c
Isolated yield.

MIRZAEI ET AL.
7 of 9
3
.1.4 | Inductively coupled plasma‐atomic
as Na
2
CO
3
and Et
3
N, were also evaluated, and DMF as
emission spectrometry
solvent and triethylamine (Et N) as base at 80°C yield
3
the highest conversion (92%) among all the solvents and
bases tested (Table 2, entry 11). To confirm that the activ-
ity of the Pd/Fib. catalyst is not due to leaching of the Pd
NPs in solution, a model Heck reaction was conducted
under optimized conditions in two reaction flasks A and
B. After 1 hr of reaction, the catalyst was filtered off from
both flasks A and B. At this point, isolation of the com-
pound 3a from flask A with column chromatography
showed 40% yield of product. Further stirring of the fil-
trate of flask B (devoid of Pd/Fib. catalyst) for 10 hr led
to no extra yield of compound 3e, which confirms that
no Pd leaching occurs in solution.
In order to evaluate the generality of this model
reaction, we extended the reaction to the use of other
aryl halides with other alkenes under the optimized
reaction conditions. As shown in Table 3, different aryl
halides (‐I, ‐Br, ‐Cl) and alkenes were reacted success-
fully, and provided the expected products 3a–3l in moder-
ate to high yields with > 99% trans‐selectivity. However,
the aryl chloride was found to be less reactive, giving
the corresponding product 3c in 33% yield (Table 3, entry
The palladium content of Pd/Fib. catalyst was deter-
mined using the inductively coupled plasma‐atomic emis-
sion spectrometry (ICP‐AES) technique. This resulted in a
value of about 15% w/w, and the palladium content after
the third reuse in reaction was about 12% w/w.
3.1.5 | CHN analysis
Due to confirmation of the palladium content resulting
from the ICP‐AES method, the CHN content was investi-
gated before and after palladium loading. The results are
shown in Table 1, which shows that the CHN content
(%w) decreased from 67.26%w to 42.24%w after palladium
loading, which is approximately in accordance with the
palladium content resulting from the ICP‐AES method.
3
.2 | Catalytic activity of the Pd/Fib.
nanocatalyst in the synthesis of Heck
adducts
15). The iodobenzenes having electron‐donating func-
The conditions for the Heck reaction were optimized by
using iodobenzene and methylacrylate as model sub-
strates to evaluate the effects of different solvents, type
of bases, amount of catalyst and required optimum tem-
perature of the reaction, and the results are summarized
in Table 2. In the initial selection, we used dimethyl
sulfoxide (DMSO) as solvent and K CO as base for the
reaction in the presence of Pd /Fib. at 100°C, whereas
trans‐methylcinnamate was obtained in 65% isolated
yield after 5 hr. Other solvents, like methanol, ethanol
and DMF, solvent‐free conditions and other bases, such
tional groups (entries 3 and 4) gave slightly higher
isolated yield than electron‐withdrawing groups (entry 2).
Therefore, our catalyst would be efficient enough to
activate the iodobenzene having electron‐withdrawing
groups to provide the desired products with excellent
yield and selectivity. The less reactive styrene (entries
9–11) also proceeded smoothly to furnish the desired
products with high yield and selectivity (i.e. > 99%
trans‐isomer). In the case of iodobenzene with electron‐
withdrawing groups (entries 2, 7) and for coupling with
styrene (entries 11), the reactions needed a comparatively
2
3
0
TABLE 4 Comparison with previously reported catalytic systems for Heck coupling of iodobenzene or 4‐iodoanisol with styrene
Entry
System, condition
Pd/Fib., Et N, DMF, 80°C, 3 hr, 0.05 mol% Pd
Product
Yield (%)
Ref.
1
3
85
This work
87
[
[
[
[
10d]
2b]
21]
2
3
4
5
CelMcPd0–1, Bu
3
N, DMF, 130°C, 0.8 mol% Pd
A
B
81
88
MCNTs@(A‐V)‐silica‐Pd, K
.5 mol% Pd
2
CO
3
, TBAB, DMF, 130°C,
A
B
91
86
1
CB[6]–Pd NPs, Na
2
CO
3
DMF, 140°C, 0.5 mol% Pd, 24 hr
A
B
85
85
22]
Pd0‐Mont., Et N, CH
3
3
CN, 82°C, 0.07 mol% Pd
A
B
87
90

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MIRZAEI ET AL.
FIGURE 5 Characterization of Pd/Fib. catalyst after recycling: (a) Fourier transform‐infrared (FT‐IR) spectrum; (b) X‐ray diffraction
XRD) pattern; (c) and (d) scanning electron microscopy (SEM) images
(
longer time, i.e. 5 hr, for completion. To show the
merit of the present work, the catalytic performance of
the Pd/Fib. catalyst was compared with other reported
4 | CONCLUSION
In conclusion, we have developed a simple new catalytic
method for the synthesis of Heck adducts from aryl
halides and alkenes in the presence of Pd/Fib. NPs as
ligand‐free, efficient, reusable and heterogeneous cata-
lysts. Some attractive features of this protocol are high
yields, simple procedure, relatively short reaction times,
easy work‐up, high catalytic activity, and recyclability
and reusability of the catalyst. The catalyst can be used
at least three times.
[2b,10d,21,22]
catalysts
used in the synthesis of Heck adducts
3
c and 3i from starting iodobenzene or 4‐iodoanisol with
styrene (Table 4). These comparative results establish
the advantage of the Pd/Fib. catalyst over the existing
methods (based on catalyst loading and reaction times).
It should be noted that our catalyst's synthesis conditions
were easier than the other catalytic systems mentioned
in Table 4.
In order to evaluate the catalyst's useful life, the reus-
ability of the catalyst was also investigated. For this pur-
pose, the same model reaction was again studied under
the optimized conditions. The catalyst was recovered
according to the procedure mentioned in the Experi-
mental section and reused for a similar reaction. As
shown in Table 3 (entry 1), the catalyst can be used at
least three times without significant loss of activity.
Powder‐XRD, SEM–energy‐dispersive X‐ray spectroscopy
ORCID
Hossein Eshghi https://orcid.org/0000-0002-8417-220X
REFERENCES
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