Palladium Nanoparticles Supported on Modified Hollow-Fe3O4@TiO2: Catalytic Activity in Heck and Sonogashira Cross Coupling Reactions

Full text of DOI:10.1080/00304948.2017.1374101

Organic Preparations and Procedures International
The New Journal for Organic Synthesis
ISSN: 0030-4948 (Print) 1945-5453 (Online) Journal homepage: http://www.tandfonline.com/loi/uopp20
Palladium Nanoparticles Supported on Modified
Hollow-Fe O @TiO : Catalytic Activity in Heck and
3
4
2
Sonogashira Cross Coupling Reactions
A. Elhampour & F. Nemati
To cite this article: A. Elhampour & F. Nemati (2017) Palladium Nanoparticles Supported
on Modified Hollow-Fe O @TiO : Catalytic Activity in Heck and Sonogashira Cross Coupling
3
4
2
Reactions, Organic Preparations and Procedures International, 49:5, 443-458, DOI:
0.1080/00304948.2017.1374101
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Date: 01 October 2017, At: 22:57

Organic Preparations and Procedures International, 49:443–458, 2017
Copyright Ó Taylor & Francis Group, LLC
ISSN: 0030-4948 print / 1945-5453 online
DOI: 10.1080/00304948.2017.1374101
Palladium Nanoparticles Supported on Modified
Hollow-Fe O @TiO : Catalytic Activity in Heck
and Sonogashira Cross Coupling Reactions
3
4
2
A. Elhampour and F. Nemati
Department of Chemistry, Semnan University, 35131-19111, Semnan, Iran
Palladium nanostructures have been extensively employed as versatile catalysts for a vari-
1
ety of transformations, such as the hydrogenation of alkenes and alkynes, carbon-fluo-
,2
3
rine bond formation and a number of C-C cross-coupling transformations including
4
–15
Suzuki, Negishi, Kumada, Hiyama and Stille reactions.
pling reactions is reflected by the fact that one of the inventors, Akira Suzuki, was
The importance of the cou-
1
awarded with the Nobel Prize in chemistry in 2010. Moreover, the resultant biaryl com-
6
1
7
pounds occur in a wide range of natural products.
The high material cost of palladium naturally restricts its applications, so many
approaches that enable one to maximize its catalytic performance and thus reduce the
required palladium loading in the catalyst have received a great deal of research attention.
Key challenges for the commercial development and practical use of any homogeneous
1
8
Pd catalyst is the recovery, recyclability and re-use of the catalyst. To address these
problems, the immobilization of Pd nano-particles on solid supports has received a lot of
1
9
interest owing to potential applications in catalysis.
Nano-particles have higher dispersion than many conventional macro-sized particles,
leading to the improved catalytic activity of the heterogeneous catalysts. Recently, a vari-
ety of heterogeneous catalysts anchoring Pd nano-particles for easy separation and recov-
2
0–29
ery have been synthesized and applied to a range of chemical reactions.
As is well known, the incorporation of magnetic nano-particles (NPs) in the support
makes separation fast and cost-effective. This makes NPs ideal candidates for practical
3
0–32
uses in catalysis processes.
area, easy recovery, self supporting capacity, cost reduction and surface permeability.
The desirable features include low density, high surface
3
3–36
Hollow-Fe O particles, may be promising candidates for many applications.
4
3
3
7–41
In continuation of our work on metal-mediated transformations,
and as part of our
we wish to report the
synthesis and full characterization of a new Pd catalyst based on hollow-Fe O @TiO par-
4
2,43
continuing interest in the synthesis of magnetic nano-catalysts,
3
4
2
ticles. The hollow-Fe O has been conveniently prepared through the solvothermal
3
4
4
method and coated with a layer of TiO in order to avoid the aggregation of nanoparticles
4
2
to a cluster and to protect the hollow magnetite from abrasion under harsh shaking condi-
4
tion After that it was functionalized with (3-aminopropyl)triethoxysilane (APTES).
5
46
Received September 16, 2016; in final form May 15, 2017.
Address correspondence to F. Nemati, Department of Chemistry, Semnan University, Semnan,
5131-19111, Iran. E-mail: fnemati@semnan.ac.ir
3
4
43

4
44
Nemati and Elhampour
Finally Pd nano-particles were immobilized on this shell system with reduction reaction
Scheme 1). We performed ligand-free cross coupling transformations, such as Heck and
(
Sonogashira reactions with this new nano-catalyst to test its activity. The ease of prepara-
tion, recovery and reusability of the catalyst, stability toward air and compatibility with a
variety of substrates make it ideal for the above mentioned coupling reactions.
Scheme 1
The hollow-Fe O @TiO -NH /Pd was fabricated via a 4-step synthetic procedure
2
3
4
2
which is given in the experimental section. The composite was characterized using X-ray
powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emis-
sion scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy
(EDX), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and
vibrating sample magnetometer (VSM) analysis and the process for the preparation of it
is graphically summarized in Scheme 1.
Having synthesized and characterized the h-Fe O @TiO -NH /Pd magnetic compos-
3
4
2
2
ite, its catalytic activity was tested in the Sonogashira coupling reaction of iodobenzene
and phenyl acetylene in an ambient atmosphere. We optimized the reaction under differ-
ent conditions and the results are summarized in Table 1.
The reaction was performed using iodobenzene (1.0 mmol) and phenylacetylene
1.2 mmol) in the presence of 2.0 mmol of base and different loadings of catalyst at
(
ꢀ
9
the optimum solvent with Et N and 20 mg composite (Table 1, entries 1–7). Other bases
0 C. Different solvents including DMF, water, THF and EtOH were tested. DMF was
3
such as K CO , NaOH or Cs CO were also examined in DMF, but Et N turned out to be
2
3
2
3
3
the best (Table 1, entries 8–10). When we changed the temperature of the reaction,
the yield was decreased (Table 1, entries 11–13). So, the best results were achieved when
ꢀ
the reaction was done in the presence of 0.02 g catalyst (0.18mol% Pd) at 80 C in DMF
using Et N as base (Table 1, entry 1).
3
In order to assess the efficiency of the novel magnetic composite further, the sub-
strate scope of the reactions for the Sonogashira coupling reaction was examined with a
variety of arylhalides and different terminal alkynes as the substrates under the optimized

Palladium Nanoparticles Supported on Modified Hollow-Fe O @TiO
2
445
3
4
Table 1
Optimization of Reaction Conditions in the Sonogashira Coupling Reaction of Iodoben-
a
zene with Phenylacetylene
b
Amount of
Catalyst (g)
Condition (solvent/
ꢀ
Time
(h)
Yield
(%)
Entry
temperature C)
Base
1
2
3
4
5
6
7
8
9
0.02
0.01
0.03
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
DMF/ 90
DMF/ 90
DMF/ 90
DMF/ 90
Et N
3
3
3
3
2
4
6
6
3
3
3
3
3
3
96
84
Et N
3
Et N
3
96
78
Et N
3
H O/ Reflux
2
Et N
3
93
55
THF/ Reflux
EtOH/ Reflux
DMF/ 90
Et N
3
Et N
3
89
86
K CO
2
3
DMF/ 90
DMF/ 90
NaOH
Cs CO
63
92
10
11
12
13
2
3
DMF/ 100
DMF/ 80
DMF/ r.t
Et N
3
95
88
Et N
3
Et N
3
Trace
a
Reactions were run in 3 mL solvent with 1 mmol iodobenzene, 1.2 mmol phenylacetylene, and
mmol base. Isolated yield.
b
2
reaction conditions, and the results are depicted in Table 2. As shown in Table 2, the h-
Fe O @TiO -NH /Pd effectively catalyzed the coupling reactions of a variety of elec-
3
4
2
2
tron-neutral, electron-rich, electron-deficient and bulky aryliodides with phenylacetylene
Table 2, entries 1–6). Substituted phenylacetylenes and aliphatic alkynes were also good
(
substrates and coupled with iodobenzene to give the corresponding products in good to
excellent yields (Table 2, entries 7–12). Moreover, the arylbromide and arylchloride also
worked well for this magnetic heterogeneous Sonogashira coupling reaction (Table 2,
entries 13–17).
To define applicability of h-Fe O @TiO -NH /Pd, we have investigated its catalyst
3
4
2
2
ability in the Heck cross-coupling reaction. The optimal conditions for the Heck reaction
were also screened. As can be seen in Table 3, DMF and Et N were found to be suitable
3
solvent and base for the model reaction of iodobenzene (1.0 mmol) and methyl acrylate
ꢀ
(1.5 mmol) in the air. The optimal reaction temperature was 120 C.
Following obtaining the optimum reaction conditions, we explored the scope of our
methodology using a variety of arylhalides with methyl and ethyl acrylates as olefins
Table 4). In all cases the desired (E)-products are obtained as confirmed from coupling val-
(
5
2
ues (J) in the NMR spectra. It was observed that a time of 2–4 h was required for complete
reaction with good yield (86–95%). However a much longer reaction time (6 h) is required
for the less reactive phenyl chloride with lower yield (62%) of the desired product.
Reusability is an important issue for catalytic reactions to make the method cheap,
industrially profitable and environmentally sustainable. So the reaction between arylio-
dide (1 mmol) and phenylacetylene (1.2 mmol) under optimal conditions was selected to
investigate the constancy of the catalyst activity. After completion of the reaction, the cat-
alyst was easily separated using an external magnet. The h-Fe O @TiO -NH /Pd could
3
4
2
2
be employed five times in the Sonogashira coupling reaction with no significant decrease
in catalytic activity. The product percent yields for the five runs were found to be 96, 96,

4
46
Nemati and Elhampour

Palladium Nanoparticles Supported on Modified Hollow-Fe O @TiO
2
447
3
4

4
48
Nemati and Elhampour

Palladium Nanoparticles Supported on Modified Hollow-Fe O @TiO
2
449
3
4

4
50
Nemati and Elhampour
Table 3
Optimization of Reaction Conditions for Heck Cross Coupling Reactions of Iodobenzene
a
and Methylacrylate
Amount of
Catalyst (g)
Condition (solvent/
ꢀ
b
Yield (%)
Entry
temperature C)
Base
Time (h)
1
2
3
4
5
6
7
8
9
0.02
0.01
0.03
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
DMF/ 120
DMF/ 120
DMF/ 120
DMF/ 120
Et N
3
2
2
95
74
Et N
3
Et N
3
2
94
80
Et N
3
1.5
3
H O/ Reflux
2
Et N
3
45
85
DMSO/ 120
EtOH/ Reflux
DMF/ 120
DMF/ 120
DMF/ 120
DMF/ 130
DMF/ 110
DMF/ r.t
Et N
3
2
Et N
3
3
60
82
K CO
2
2
3
NaOH
Cs CO
2
67
73
10
11
12
13
2
2
3
Et N
3
2
93
83
Et N
3
2
10
Et N
3
Trace
a
Reactions were run in 3 mL solvent with 1 mmol iodobenzene, 1.5 mmol methyl acrylate, and
mmol base. Isolated yield.
b
2
94, 94 and 92, respectively. It is worth mentioning that the XRD of the catalyst was not
changed after five cycles.
To verify whether the observed catalysis activity was due to the heterogeneous cata-
lyst h-Fe O @TiO -NH /Pd or to a leached Pd species in solution, we performed a hot fil-
3
4
2
2
5
6
tration test. Before the completion of the model reaction, we excluded the catalyst using
an external magnet and allowed the reaction to occur. No increase in the yield was
observed by GC analysis, showing a negligible amount of leached Pd species from the
catalyst. It was confirmed by ICP analysis that no Pd could be detected in the hot filtered
solution. The palladium is heterogeneous and remains on the support during the reaction.
Conclusion
A novel and low-density h-Fe O @TiO -NH /Pd was effectively prepared in a four step
3
4
2
2
procedure using simple starting materials. It exhibited high catalytic efficiency in the
ligand-free Heck and copper-free Sonogashira cross-coupling reactions. The magnetic
composite could be facilely recovered using an external magnet. This methodology had
the advantages of high yield and elimination of ligand or copper for cross-coupling reac-
tions. In addition the catalyst is moisture-stable, which was much superior to commercial
Pd catalysts. In this catalytic process an inert atmosphere is not necessary, thus overcom-
ing the high operating costs of the traditional method.
Experimental Section
All chemicals, including FeCl ¢6H O, NH Ac, ethylene glycol, ethanol, acetonitrile,
3
2
4
NH ¢H O, tetrabutyl orthotitanate (TBOT) and polyvinylpyrrolidone (PVP) were analyti-
3
2
cal grade reagents, purchased from Sigma-Aldrich, and used without further purification.

Palladium Nanoparticles Supported on Modified Hollow-Fe O @TiO
2
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3
4

4
52
Nemati and Elhampour

Palladium Nanoparticles Supported on Modified Hollow-Fe O @TiO
2
453
3
4

4
54
Nemati and Elhampour
The progress of the reaction was monitored by TLC on commercial aluminum-backed
plates of silica gel 60 F254, visualized using ultraviolet light. Melting points were deter-
1
mined in open capillaries using an Electro thermal 9100 without further correction. H
1
3
NMR and C NMR spectra were recorded using a Bruker DRX-400 spectrometer. Mag-
netic h-Fe O @m-TiO /Ag hollow spheres were characterized by the following methods:
FT-IR spectra were obtained with potassium bromide pellets in the range of 400–
3
4
2
¡
1
4
000 cm using a Shimadzu 8400 s spectrometer; X-ray diffraction (XRD) was detected
ꢀ
by a Philips instrument using Cu-Ka radiation of wavelength 1.54 A; scanning electron
microscopy, FE-SEM-EDX, analysis was performed using a Tescanvega II XMU Digital
Scanning Microscope. Samples were coated with gold at 10mA for 2min prior to analysis;
transmission electron microscope (TEM) images were taken with a CM30300Kv field
emission transmission electron microscope; the magnetic properties were characterized
using a vibrating sample magnetometer (VSM, Lakeshore 7407) at room temperature.
Thermo-gravimetric analyses (TGA) were conducted with a LINSEIS modele STS PT
ꢀ
¡1
1
13
1
6000 thermal analyzer in air at a heating rate of 5 C min . For oils, the H and
C
NMR spectra matched those reported in the literature references indicated in Tables 2
and 4 and were submitted for review.
Synthesis of magnetic Fe O hollow spheres
3
4
4
4
Hollow magnetic particles were synthesized through a solvothermal reaction. Briefly,
PVP (1.0 g) was dissolved in 40 mL of ethylene glycol (EG) and then 5 mmol of ferric
chloride hexahydrate (FeCl ¢6H O) and 30 mmol of sodium acetate (NaAc) were added.
3
2
The resulting mixture was stirred vigorously for 15min, and was transferred to a Teflon-
lined stainless-steel autoclave (100 mL capacity). Then, the autoclave was heated to
ꢀ
200 C for 10 h, and the autoclave was subsequently cooled to room temperature. The
resulting black solid was washed several times with ethanol and deionized water and
ꢀ
dried at 80 C in a vacuum oven overnight.
Synthesis of hollow-Fe O @TiO core shell
3
4
2
The Fe O @TiO hollow spheres were synthesized by directly coating a layer of TiO on
3
4
2
2
the Fe O by hydrolyzing tetrabutyl orthotitanate (TBOT) in ethanol and acetonitrile
4
3
3
solution. Hence, 50 mg of the hollow Fe O was dispersed in a mixed solvent contain-
8
3
4
ing 90 ml of ethanol and 30 ml of acetonitrile with the aid of ultrasound. Then, 0.5 ml of
NH /H O was added to the mixture. After that, 1 ml of TBOT was added to the above
3
2
suspension with stirring for 1.5 h. The products were collected by magnetic separation
and washed three times with ethanol.
Synthesis of Aminopropyl-modified hollow Fe O @TiO core shell
2
3
4
(h-Fe O @TiO -NH )
3 4 2 2
4
6
Hollow-Fe O @TiO -NH was synthesized by a procedure explained previously.
2
3
4
2
Hence, 500 mg of h-Fe O @TiO was dispersed in 20 mL dry toluene for 30 min using
3
4
2
ultrasound assistance. To the resulting solution, 2 mmol of 3-aminopropyltriethoxysilane
was added and the mixture was refluxed for 24 h under a nitrogen atmosphere. The (3-
aminopropyl)triethoxysilane functionalized hollow Fe O @TiO was subjected to mag-
3
4
2
netic separation and was washed sequentially with ethanol (3 £ 20 mL) and dried under
vacuum for 24 h.

Palladium Nanoparticles Supported on Modified Hollow-Fe O @TiO
2
455
3
4
Loading of nano-Pd on Aminopropyl-modified hollow-Fe O @TiO
2
3
4
(h-Fe O @TiO -NH -Pd)
3 4 2 2
Doping of Pd nanoparticles onto hollow-Fe O @TiO - NH (h-Fe O @TiO -NH ), was
3
4
2
2
3
4
2
2
4
5
performed according to the previous literature report. Therefore, 500 mg of h-Fe O @-
3
4
TiO -NH was dispersed in 20 mL ethanol solution under ultra-sonication for 0.5 h.
2
2
Then, a solution of PdCl (12.5 mg, 0.07 mmol) in 10 mL hot water was added. To the
2
resulting mixture, a solution of 1 mmol NaBH in 5 mL water was added and the mixture
4
was stirred for 12 h at room temperature. The solid catalyst was separated by external
magnet, washed sequentially with EtOH (3 £ 20mL) and dried under vacuum for 24 h.
General procedure for the Heck coupling reaction
Aryl halide (1 mmol), alkene (1.5 mmol), triethylamine (2 mmol) and h-Fe O @mTiO -
3
4
2
ꢀ
NH -Pd (20 mg) were added to 3 mL DMF and the reaction mixture was stirred at 120 C
2
for the required time. After completion of the reaction, the reaction mixture was cooled to
room temperature and diluted with hot ethanol (10 ml). Then, the catalyst was separated
by an external magnet from the cooled mixture, washed with acetone, dried in an oven
and re-used for a consecutive run under the same reaction conditions. The filtrate was
concentrated and the resulting residue was purified by short column chromatography on
silica gel to afford the desired product.
General procedure for the Sonogashira coupling reaction
Aryl halide (1 mmol), terminal alkyne (1.2 mmol), triethylamine (2 mmol) and h-
Fe O @mTiO -NH -Pd (20 mg) were added to 3 ml DMF and the reaction mixture was
3
4
2
2
ꢀ
stirred at 90 C for the appropriate time. After completion of the reaction, the reaction
mixture was cooled to room temperature and diluted with hot ethanol (10 ml). After that,
the catalyst was separated by an external magnet from the cooled mixture, washed with
acetone, dried in an oven and re-used for a consecutive run under the same reaction condi-
tions. The filtrate was concentrated and the resulting residue was purified by short column
chromatography on silica gel to afford the desired product.
Selected spectra data
1
,2-Diphenylethyne (Table 2, entry 1)
H NMR (400MHz, CDCl ) d 7.52–7.36 (m, 4H), 7.29–7.11 (m, 6H); C NMR
1
13
3
(100MHz, CDCl ) d 130.55, 128.12, 127.29, 122.22, 88.35.
3
1
-Chloro-4-(phenylethynyl)benzene (Table 2, entry 6)
1
H NMR (400MHz, CDCl ) d 7.43 (dd, J D 6.6, 3.0Hz, 2H), 7.38–7.31 (m, 3H), 7.29–
3
1
.17 (m, 4H); C NMR (100MHz, CDCl ) d 133.20, 131.76, 130.55, 127.64, 127.43,
3
7
3
1
27.35, 121.88, 120.73, 89.29, 87.22.
Methyl (E)-3-(p-tolyl)acrylate (Table 4, entry 3)
1
H NMR (400MHz, CDCl ) d 2.40 (s, 3H), 3.82 (s, 3H), 6.42 (d, J D 16.0Hz, 1H), 7.21 (d,
3
1
3
J D 8.0Hz, 2H), 7.44 (d, J D 8.0Hz, 2H), 7.70 (d, J D 16.0Hz, 1H); C NMR (100MHz,
CDCl ) d 21.4, 51.6, 116.7 128.0, 129.6, 131.6, 140.7, 144.8, 167.6.
3

4
56
Nemati and Elhampour
(
E)-Ethyl 4-nitrocinnamate(Table 4, entry 8)
HNMR (400MHz, CDCl ) d(ppm): 1.38 (t, J D 6.8, 3H),4.35–4.29 (m, 2H), 6.58 (d, J D
1
3
1
3
1
6, 1H), 7.80–7.69 (m, 3H), 8.28 (d, J D 8.4, 2H); C NMR (100MHz, CDCl ) d
3
(
ppm):166.06, 148.47, 141.63, 140.60, 128.64, 124.19, 122.60, 61.04, 14.28.
Calculation of h-Fe O density
3
4
It is important to determine the physical properties of the shell in hollow-sphere particles,
because it directly affects the functionality of the final catalyst. So, in this study the shell
density was determined using specific surface area and shell thickness as reported earlier
5
7
in the literature.
The rshell was calculated with shell thickness of “t” (nm) (obtained from XRD), inner
radius of “r” (nm) [obtained from BET (Brunauer-Emmett-Teller)] and specific surface
2
area SA (m . g ) [obtained from BJH (Barrett-Joyner-Halenda)]. Therefore, the rshell
¡1
58
¡
3
value was calculated as follows: 4.73 g.cm . The magnetite’s density is established at
¡
3 59
,
5
sphere-like Fe O .
.18g.cm
so the as-synthesized hollow spheres have a lower density than solid
3
4
Acknowledgments
The authors gratefully acknowledge Semnan University Science and Technology Park,
the Research Council and the Vice-chancellor for Education and Postgraduate Students
for financial support of this work.
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