Palladium supported on zinc ferrite: An efficient catalyst for ligand free C-C and C-O cross coupling reactions

Article abstract of DOI:10.1016/j.tetlet.2013.09.027

An efficient superparamagnetic Pd-ZnFe₂O₄ solid catalyst has been synthesized by loading Pd(0) species on zinc ferrite nanoparticles. Sonogashira cross couplings between terminal alkynes and aryl halides were achieved in the absence of any Cu co-catalyst. A Heck-Matsuda coupling reaction of structurally different aryldiazonium tetrafluoroborate substrates was preceded at 40 C in water. Cyanation of aryl halides was successfully done using K₄[Fe(CN)₆] as the cyanide source over Pd-ZnFe₂O₄. The catalyst was also employed for Ullmann type cross coupling reactions. Excellent yield of the products, reusability, and uncomplicated work-up make this catalyst efficient for C-C and C-O coupling reactions. Good yield of products, easy separation, and negligible leaching of Pd from the catalyst surface confirm the true heterogeneity in these catalytic reactions.

Full text of DOI:10.1016/j.tetlet.2013.09.027

Accepted Manuscript
Palladium supported on zinc ferrite: An efficient catalyst for ligand free C-C
and C-O cross coupling reactions
Abhilash S Singh, Suresh S Shendage, Jayashree M Nagarkar
PII:
S0040-4039(13)01581-5
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.09.027
TETL 43527
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
23 July 2013
5 September 2013
11 September 2013
Please cite this article as: Singh, A.S., Shendage, S.S., Nagarkar, J.M., Palladium supported on zinc ferrite: An
efficient catalyst for ligand free C-C and C-O cross coupling reactions, Tetrahedron Letters (2013), doi: http://
dx.doi.org/10.1016/j.tetlet.2013.09.027
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1
Tetrahedron Letters
journal homepage: www.elsevier.com
Palladium supported on zinc ferrite: An efficient catalyst for ligand free C-C and C-O
cross coupling reactions
Abhilash S Singha , Suresh S Shendagea and Jayashree M Nagarkara,
aDepartment of Chemistry, Institute of Chemical Technology (Deemed), N. M. Parekh Marg, Matunga, Mumbai – 400 019, India.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
An efficient superparamagnetic Pd–ZnFe2O4 solid catalyst has been synthesized by loading
Pd(0) species on zinc ferrite nanoparticles. Sonogashira cross couplings between terminal
alkynes and aryl halides were achieved in the absence of any Cu co-catalyst. A Heck–Matsuda
coupling reaction of structurally different aryldiazonium tetrafluoroborate substrates was
preceded at 40oC in water. Cyanation of aryl halides was successfully done using K4[Fe(CN)6]
as the cyanide source over Pd–ZnFe2O4. The catalyst was also employed for Ullmann type cross
coupling reactions. Excellent yield of the products, reusability and uncomplicated work-up
make this catalyst efficient for C-C and C-O coupling reactions. Good yield of products, easy
separation and negligible leaching of Pd from the catalyst surface confirm the true heterogeneity
in these catalytic reactions.
Available online
Keywords:
Pd–zinc ferrite
Sonogashira
Heck–Matsuda
Cyanation
2009 Elsevier Ltd. All rights reserved.
Ullmann
Nowadays, magnetic nanomaterials as catalyst support have
been efficiently employed in heterogeneous catalysis for various
organic reactions.1 In this context, Pd-catalyst supported on
various magnetic nanoparticles has been widely used for different
organic transformation reactions.2-9 Metal nanoparticles anchored
on the oxide surface work more efficiently as compared to bulk
particles.10
present
in
pharmaceuticals
and
natural
products.20
Arenediazonium salts are a prospective alternative to aryl halides
and are commonly used in Heck–Mizoroki reactions. This
reaction has been performed in the presence of N-heterocyclic
carbene ligands, palladacycle systems, ligand-free and
heterogeneous conditions.21 The palladium nanoparticles have
also been widely used for such coupling reactions.22 Herein we
report a new catalytic application of palladium supported on zinc
ferrite for the Heck–Matsuda coupling reactions using aryl
diazonium salts. The nitrile compound and their derivatives are
of considerable interest in organic chemistry as they are building
blocks in the synthesis of agrochemicals, dyes, herbicides,
pharmaceuticals and natural products.23 Several synthetic
strategies, and various ligands, have been developed to improve
the synthesis of benzonitrile.24 However, only a few catalytic
systems in this context were able to accomplish by discarding the
lethal cyanide source and minimizing production of heavy metal
waste, together with good reusability. Here we explore the
catalytic activity of Pd–ZnFe2O4 magnetic nanoparticles as
efficient heterogeneous catalyst under mild reaction conditions.
The palladium catalyzed cross-coupling reactions are well
known for generation of carbon-carbon bonds.11-13 Sonogashira
reaction, Heck reaction and cyanation reaction are excellent
methods for the synthesis of organic compounds.14-16 Sonogashira
cross-coupling usually involves the coupling between aryl halide
with aryl alkyne and many other terminal alkynes. This reaction
was performed in an inert atmosphere using either an expensive
gold(I) catalyst or phosphine containing Pd-catalyst along with
CuI as co-catalyst.17 However, in the presence of CuI as co-
catalyst the dimerization of the alkyne as a byproduct takes place.
Numerous modifications have been made using Zn, Sn, B, Al,
AgOTf, Ag2O, etc., instead of Cu, to enhance the yield of cross-
coupling products.18 Thus several attempts have been made to
minimize the homo-coupling product by using bulky ligands in
combination with palladium. The reaction proceeds well yielding
considerable amounts of desired cross-coupling products by
Apart from C-C coupling reaction, the C-O coupling reaction
between aryl halides and phenols for the generation of diaryl
ether derivatives is also very promising for the synthesis of
medicinal and important natural products. Among the diaryl
ethers the nitro substituted diphenyl ethers are important as
bioactive compounds.25 The traditional Ullmann-type coupling
procedure requires stoichiometric amount of reagent and very
using Pd–ZnFe2O4 magnetic nanoparticles (MNPs) as
a
heterogeneous catalyst. The coupling of aryl halides with olefinic
compounds known as Heck reaction, is an excellent method for
the synthesis of organic compounds.19 These products are widely
———
 Corresponding author. Tel.: +91-22-33611111/2222; fax: +91-22-33611020; e-mail: jm.nagarkar@ictmumbai.edu.in;
jayashreenagarkar@yahoo.co.in

2
Tetrahedron Letters
high temperatures.26 In order to overcome these problems
results are depicted in (Table 1). The catalyst show excellent
chemoselectivity (iodine vs. bromine) and lack of steric
hindrance (Table 1, entries 5-6).
various ligands in combination with transition metals such as
palladium and copper have been used.27 Many of these methods
suffer from the drawback of costly ligands and non recyclability
of the catalyst. Recently we have reported palladium supported
on zinc ferrite as a highly active, magnetically separable catalyst
for ligand free Suzuki and Heck coupling.28 Herein we report the
application of our catalyst for C-C and C-O bond formation
reactions as an environmentally benign, heterogeneous and
recyclable catalyst. The catalyst was synthesized by loading Pd°
species on zinc ferrite nanoparticles by ultrasound assisted co-
precipitation in the absence of surface stabilizer or capping
agents (Supplementary data).
The scope of the catalyst was further extended to Heck–
Matsuda
coupling
reaction,
using
aryl
diazonium
tetrafluoroborate and alkenes. Initially the reaction of 4-
methylphenyl diazonium tetrafluoroborate with ethyl acrylate
was chosen as a model reaction.30 The reaction conditions were
optimized for model reaction, where 0.5mmol of aryl diazonium
tetrafluoroborate was treated with 0.75mmol of styrene or
o
acrylate at 40 C using water as solvent and under base free
condition. The reaction in the absence of catalyst under
optimized condition did not proceed at all. Various substituted
aryl diazonium tetrafluoroborate were studied and good to
excellent yield of the arylated styrene or acrylate products were
obtained. The results are presented in Table 2.
We have carried several C–C and C-O cross-coupling
reactions such as Sonogashira, Heck, cyanation and Ullmann
using ZnFe2O4 magnetic nanoparticles catalyst.
Table 1
Sonogashira reaction of various aryl halides with alkynes catalyzed by Pd–ZnFe2O4 MNPa
Aryl halides
Products
Time (h)
12
Yield (%)b
Entry
1
90, 89c, 89c, 87c
2
3
4
5
6
7
8
13
13
12
12
13
14
12
82
85
93
70
84
82
88
a Reaction condition: 0.5 mmol of aryl halide, 0.75 mmol of terminal alkyne, 1.25 mmol of K2CO3, 10mg of Pd–ZnFe2O4 MNPs and 3 mL ethanol, reflux.
b Isolated yield by column chromatography.
c The reaction was accomplished with reused catalyst.
After completion of these reactions, the catalyst was separated
simply by using an external magnet and the reaction mixture was
subjected to further work-up for collection of alkynes, arylated
styrene or acrylate, benzonitriles and nitro substituted diphenyl
ethers. Under the optimized reaction conditions, various
substituted aryl iodides were subjected to couple with aromatic
terminal alkynes to obtain di-substituted alkynes. The above
catalytic system does not require any inert atmosphere and Cu(I)
salt which leads to the formation of undesirable homocoupling
products. The reaction of iodobenzene with phenylacetylene was
chosen as a model reaction for which 90% yield of cross-
coupling product was obtained.29 Good to excellent yields were
obtained for various substituted halides which are studied and the
Table 2
Reaction of diazonium tetrafluoroborates with styrene and
ethyl acrylate in the presence of the catalyst in watera

3
Yield (%)b
Table 3
Entry
R1
R2
KCN free cyanation of aryl halides with K4Fe[(CN)6] over
H
COOEt
COOEt
COOEt
Ph
81
85
87
82
86
81
85
84
85
80
Pd-ZnFe2O4 MNPa
1
4-CH3
4-NO2
4-OCH3
4-NO2
4-OH
4-OH
4-COCH3
4-CN
4-CH3
2
3
4
Entry Substrate
1
Product
Time (h)
10
Yield(%)b
Ph
5
90
81
83
85
85
91
86
75
Ph
6
COOEt
Ph
7
2
3
4
5
6
7
8
12
13
12
11
10
10
12
8
Ph
9
Ph
10
a Reaction condition: 0.5 mmol of aryldiazonium tetrafluoroborate, 0.75
mmol of alkene, 10mg of Pd–ZnFe2O4 MNPs and 3 mL water, 40oC, reaction
time 4 h.
b Isolated yield by column chromatography.
The catalytic activity of the catalyst was also explored for the
cyanation reactions of aryl halides to form corresponding
benzonitriles. The reaction proceeds very efficiently for aryl
iodides and aryl bromides in the presence of non-toxic potassium
ferrocyanide.31 The various substituted benzonitrile derivatives
were obtained in moderate to good yield. However these C-C
coupling reaction failed in the absence of catalyst indicating the
importance of palladium in the reaction. Various aryl halides
with electron donating and withdrawing groups were studied for
the reaction and the results are shown in Table 3. The catalyst
works efficiently for iodo and bromo derivatives, however it
failed to catalyze the less reactive chloro substituent (Table 3,
entry 7).
a Reaction condition: 0.5 mmol of aryl halide, 0.35 mmol of potassium
ferrocyanide, 0.6 mmol of triethylamine, 10mg of Pd–ZnFe2O4 MNPs and 3
mL DMF, 100oC
b Isolated yield by column chromatography.
Table 4
Pd–ZnFe2O4 MNP catalyzed C–O cross coupling reactiona
Entry
1
Substrate
Product
Time (h)
4
Yield(%)b
89, 88c, 86c, 84c
3.5
4
91
88
86
92
85
2
3
4
5
6
4
3.5
5

4
Tetrahedron
5
4
81
84
7
8
9
4
88
90
3.5
10
11
4
86
a Reaction condition: 0.5 mmol of aryl halide, 0.65 mmol of phenol, (1.0 mmol) of K2CO3 10mg of Pd–ZnFe2O4 MNPs and 2 mL DMSO, 100oC.
b Isolated yield by column chromatography
c The reaction was accomplished with reused catalyst.
After C-C coupling the catalyst activity was further explored
for C-O coupling reaction. Nitro substituted aryl halides act as
important precursors for amino compounds as well as
intermediates for several bioactive molecules. Hence the
development of methodology for the synthesis of these nitro
substituted compounds through coupling reaction is highly
desirable. The reaction of 4-chloronitrobenzene with phenol was
chosen as a model reaction and the reaction afforded product
with 88% yield.32 The reaction of 1 and 2-napthol with 4-
chloronitrobenzene and 2-chloronitrobenzene gave considerable
yield of the desired products (Table 4, entries 3–5). The electron
rich phenols also afforded the corresponding diarylethers in good
yields. Reasonable yields were obtained for electron deficient
phenols (Table 4, entries 6–7). The reaction of 4-substituted
chloronitro benzenes with 4-bromo and iodo substituted phenols
under similar conditions provided chemoselective cross coupled
products (Table 4, entries 6–7). The ortho substituted iodo and
bromo nitrobenzenes under identical conditions gave good yields
of the desired products (Table 4, entries 8–9). The reaction of 2-
chloronitrobenzene and 4-chloronitrobenzene with electron rich
phenols afforded the corresponding diarylethers in good yield
(Table 4, entries 10–11).
21 ppb for Ullmann reaction. This loss in Pd content explains the
decrease in yield with increase in the number of recycle runs.
In summary, we have synthesized efficient superparamagnetic
Pd–ZnFe2O4 nanoparticles which have been applied as an
efficient heterogeneous catalyst for C–C and C-O cross-coupling
reactions. The reactions are carried out in an environmentally
benign condition, giving moderate to good yields for the
respective coupling products. The reusability of the catalyst was
tested up to three recycle runs. Copper-free Sonogashira
couplings in ethanol, Heck–Matsuda reaction of diazonium salt
in water, cyanide-free cyanations of aryl halides and Ullmann
type coupling of phenols gave respective coupling products with
the Pd–ZnFe2O4 catalyst. Hence the catalyst offers a mild and
efficient protocol over existing methods under ligand free
conditions.
Acknowledgments
We gratefully acknowledge the financial assistance from the
Green Tech. U.P.E. Government of India.
References and notes
The recyclability of the catalyst was studied for the
Sonogashira and Ullmann cross coupling reactions. The reaction
mixture was cooled to room temperature after completion and the
catalyst (MNP) was separated using an external magnet. The
catalyst was then washed with ethanol, dried at 100°C for 3h and
preserved for next run. The TEM image of the catalyst was
studied for before and after three recycles runs (Supplementary
data). The image of recycled catalyst shows that there was no
significant change in the morphology and dispersion of the
catalyst particles.
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mmol) and 10 mg of the Pd–ZnFe2O4 catalyst (0.000764mmol of
Pd or 0.153 mol% of Pd). The reaction was carried out under
reflux condition for 11-14 h and was monitored by Gas
chromatography (GC). After completion, the reaction mixture was
cooled to room temperature and the catalyst (MNPs) was
separated using an external magnet. The catalyst was then washed
with ethanol, dried at 100 °C for 3h and preserved for next cycle.
The pure product was obtained using silica gel column
chromatography using n-hexane-EtOAc as mobile phase.
30. Heck–Matsuda coupling: Aryldiazonium tetrafluoroborate (0.5
mmol) and (0.75 mmol) of alkene were added to a flask containing
10mg of the Pd–ZnFe2O4 catalyst and water (3 ml) at 40 oC. The
resulting mixture was heated in air for 4 h. The reaction mixture
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bottom flask and stirred under reflux at 120°C under atmospheric
conditions. The progress of the reaction was monitored by GC.
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mmol), phenol (0.65 mmol), catalyst Pd–ZnFe2O4 (10 mg), and
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bottomed flask. The reaction mixture was then heated at 100 oC
under magnetic stirring for 4 h. The completion of reaction was
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was separated. The reaction mixture was diluted with water, and
extracted with ethyl acetate three times (3 x 10 ml). The combined
organic layer was concentrated to yield the crude product, which
was purified by column chromatography on silica gel
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