A simple and facile Heck-type arylation of alkenes with diaryliodonium salts using magnetically recoverable Pd-catalyst

Article abstract of DOI:10.1039/c2gc35673b

The Heck-type arylation of alkenes was achieved in aqueous polyethylene glycol using a magnetically recoverable heterogenized palladium catalyst employing diaryliodonium salts under ambient conditions. The benign reaction medium and the stability of the catalyst are the salient features of this simple and facile protocol.

Full text of DOI:10.1039/c2gc35673b

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COMMUNICATION
A simple and facile Heck-type arylation of alkenes with diaryliodonium salts
using magnetically recoverable Pd-catalyst†
Buchi Reddy Vaddula, Amit Saha, John Leazer and Rajender S. Varma*
Received 2nd May 2012, Accepted 14th June 2012
DOI: 10.1039/c2gc35673b
alkenes using these salts in the presence of a magnetically retrie-
vable palladium catalyst is reported.
The Heck-type arylation of alkenes was achieved in aqueous
polyethylene glycol using a magnetically recoverable hetero-
genized palladium catalyst employing diaryliodonium salts
under ambient conditions. The benign reaction medium and
the stability of the catalyst are the salient features of this
simple and facile protocol.
Results and discussion
The advantages associated with the use of magnetic nano-ferrite
[Fe3O4] as a solid support for various catalysts have been studied
by our group16 and others.17 The ease of recovery using an exter-
nal magnet makes it an attractive choice. We have immobilized
palladium on the surface of a dopamine functionalized nano-
ferrite [Fe3O4] to obtain the magnetically retrievable hetero-
genized [Fe3O4–Dopa–Pd] catalyst16e (Fig. 1). The oxidation of
primary alcohols to aldehydes and O-allylation of phenols was
demonstrated earlier using this [Fe3O4–Dopa–Pd] catalyst.16b,c
Here, it has been employed as an easily recyclable catalyst to
develop a simple arylation reaction in aqueous polyethylene
glycol-400 (PEG-400) by sonication (1–5 min) without any
special precautions.
The [Fe3O4–Dopa–Pd] catalyst has been prepared following
the earlier reported standard protocol.16b–e The magnetic nano-
ferrite was functionalized with dopamine by sonicating the
aqueous mixture of Fe3O4 and dopamine for 2 h. Dopamine is a
suitable anchor which prevents the leaching of the metal and acts
as a pseudo-ligand. Palladium was immobilized by treating a
methanolic solution of the dopamine functionalized nano-ferrite
with PdCl2 in the presence of aqueous ammonia to obtain the
Pd(II) catalyst supported on amine functionalized magnetic
Fe3O4 nanoparticles.
Introduction
The palladium catalyzed Heck-type arylation of olefins is an
important tool in organic chemistry. This carbon–carbon
bond coupling has been widely studied and usually involves the
reaction of various halides with olefins under basic conditions.1
The palladium catalyzed application of hypervalent iodine
reagents in organic synthesis has drawn considerable attention.2
Very recently, ligand-free palladium-catalyzed application of
hypervalent iodine reagents in Heck-type arylations has been
reported.3
The diaryliodonium salts are a class of hypervalent iodine
reagents. They are trivalent iodine compounds with two carbon
ligands. These salts are stable towards heat and oxygen, are non-
toxic and non-explosive. The application of diaryliodonium salts
in the areas of photochemistry, organic chemistry, and medicinal
chemistry is widely reported.4 They are used in a variety of
organic transformations.5 The preparation of these compounds
has been improved considerably through simple one-pot proto-
cols from the corresponding arenes and aryliodides.6 They have
been employed in the synthesis of diarylamines,7 aryl esters,8
aryl ethers,9 α-arylations by rearrangement,10 and enantioselec-
tive α-arylation of aldehydes.11 They are useful substrates in the
reaction with inorganic ions12 and in Cu-catalyzed C–N cross-
coupling reactions.13 Recently, the iodonium salts have been
employed in the Heck-type coupling, however, with limited sub-
strate scope and/or limited yields.2,14
The prepared catalyst was characterized and the TEM image
attributed the spherical morphology. The size of the nanocatalyst
was found to be in the range of 13–38 nm (Fig. 2). The
In view of our interest in the general applications of diarylio-
donium salts in organic synthesis,15 the arylation of terminal
Sustainable Technology Division, National Risk Management Research
Laboratory, U.S. Environmental Protection Agency, Cincinnati, USA.
E-mail: Varma.Rajender@epa.gov; Fax: +1-513-569-7677;
Tel: +1-513-487-2701
†Electronic supplementary information (ESI) available: Experimental
procedures, and 1H and 13C NMR spectra of all compounds are pro-
vided. See DOI: 10.1039/c2gc35673b
Fig. 1 Synthesis of the catalyst, [Fe3O4–Dopa–Pd].
Green Chem., 2012, 14, 2133–2136 | 2133
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Table 1 Heck-type arylation of alkenes 3a
Entry
R
Ar
Product
Yield (%)
1
2
3
4
5
6
7
8
–COCH3
–CH2OAc
–CO2nBu
–C6H5
–CH2C6F5
–CH2OH
–CH2OAc
–CH2OAc
–CO2Me
–CO2nBu
–COCH3
–CH2OAc
–CH2OAc
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
4-MeC6H4
3-NO2C6H4
3-NO2C6H4
3-NO2C6H4
2,4,6-(Me)3C6H2
4-t-BuC6H4
4-OMeC6H4
3a
3b
3c
3d
3e
3f
3g
3h
3i
90
88
85
78
69
45
82
80
75
81
72
82
84
9
Fig. 2 TEM image of [Fe3O4–Dopa–Pd] catalyst.
10
11
12
13
3j
3k
3l
3m
a Reaction conditions: 1.1 mmol of alkene, 1 mmol of diaryliodonium
salt, and 50 mg (4.8 mol%) of [Fe–Dopa–Pd] catalyst with aq. PEG-400
(1 : 1, v/v) subjected to ultrasonication (1 to 5 min).
Table 2 Investigation of catalyst role in the transformation
Fig. 3 Catalyst retrieval from the reaction mixture using an external
magnet.
inductively coupled plasma–atomic emission spectroscopy
(ICP-AES) analysis showed the weight percentage of the Pd to
be 10.23%.
Entry
Catalyst
Time (min)
Yield (%)
Initially, ultrasonication of methyl vinyl ketone (2 equiv.) was
attempted with diphenyliodonium triflate (1 equiv.) in PEG-400
in the presence of CuBr catalyst. After this unsuccessful attempt,
the same reaction was carried out in the presence of a nano-
ferrite Pd catalyst where the iodonium salt was completely con-
sumed in a minute. In view of our past success with PEG-based
solvent systems,15,18 the reaction was further studied in PEG-400
alone and its water admixture. A 1 : 1 v/v combination of water
and PEG-400 was found to be optimum, and the reaction ran to
completion (without the need for any base) within 1 min with
the formation of iodobenzene as the sole side product. To study
the influence of PEG-400, a control experiment was carried out
replacing aqueous PEG-400 with DMF and this could yield only
traces of the desired product. The iodobenzene can be recycled
and reused in the preparation of diaryliodonium salts. The cata-
lyst could be recovered using an external magnet (Fig. 3). The
reaction mixture was then diluted with water and extracted using
ethyl acetate. The crude product was dried over anhydrous
Na2SO4 and purified by passing through a silica gel column to
afford product 3a in high yield.
1
2
3
4
No catalyst
Nano Fe3O4
[Fe–Dopa]
5
5
5
1
0
0
0
90
[Fe–Dopa–Pd]
and 3m in 82% and 84% yields, respectively. Sterically hindered
bis(2,4,6-trimethylphenyl)iodonium tosylate also reacted well
with methyl vinyl ketone to produce 3k in good yield. Bis(3-
nitrophenyl)iodonium tosylate reacted in an identical fashion
with allyl acetate, methyl acrylate and n-butyl acrylate resulting
in products 3h, 3i and 3j in appreciable yields.
The active role of palladium in the catalysis of the reaction
was established by control experiments (Table 2). No product,
3a, formation occurred in the absence of catalyst. The same reac-
tion, when carried out separately in the presence of nano Fe3O4
or [Fe–Dopa] as catalysts, resulted in vain attempts. The
[Fe–Dopa–Pd] catalyst resulted in good product yields,
indicating the reaction is catalyzed only by palladium.
The protocol was extended to other alkenes (Table 1). n-Butyl
acrylate and styrene afforded products 3c and 3d in 85% and
78% yields, respectively. The reaction with allyl alcohol,
however, afforded 3f in only 45% yield with the formation of a
single side product, 3-phenylpropanal. This observation is in
agreement with earlier findings.19
It was observed that reactions under ultrasonication generated
a minor impurity. In an effort to counter this problem, the syn-
thesis of 3a was repeated by stirring the reaction mixture at room
temperature for 12 h, resulting in a cleaner reaction with margin-
ally improved yields (Table S3, ESI†). The room temperature
protocol was not applicable to all substrates.
Allyl acetate reacted with bis(4-t-butylphenyl)iodonium
triflate and bis(4-methoxyphenyl)iodonium tosylate to afford 3l
Upon completion of the reaction, the catalyst was separated
using an external magnet and washed with acetone followed by
2134 | Green Chem., 2012, 14, 2133–2136
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drying in a vacuum oven. The recovered catalyst was used for
three consecutive cycles of reactions. The amount of palladium
on the dopamine functionalized nano-ferrites was analyzed after
recovering the catalyst from the third reaction cycle. The
ICP-AES analysis showed the same palladium concentration on
the catalyst and the absence of palladium metal in the reaction
solvent, indicating no metal leaching into the reaction mixture.
The TEM analysis demonstrated that the catalyst morphology
and size remains unchanged upon reuse (Fig. S1, ESI†). The
amine binding sites installed by functionalization of nano-ferrites
with dopamine serve as pseudo-ligands which coordinate with
palladium, thus enabling efficient catalyst recycling and prevent-
ing metal leaching.
Acknowledgements
BRV and AS were supported in part by an appointment to the
Research Participation Program for the U.S. Environmental Pro-
tection Agency, Office of Research and Development, adminis-
tered by the Oak Ridge Institute for Science and Education
through an interagency agreement between the U.S. Department
of Energy and the EPA.
Notes and references
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Conclusions
A simple, facile and expeditious (reaction time: 1–5 min) pro-
cedure for arylation of terminal alkenes with diaryliodonium
salts has been developed using a magnetically retrievable and
easily recyclable heterogeneous Pd catalyst in aqueous PEG
using ultrasonication. Unactivated alkenes such as styrene, allyl
acetate, and allyl alcohol responded equally well. Easy magnetic
recovery of the catalyst is an additional facile attribute of this
reaction as it eliminates the tedious work-up procedure for cata-
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Experimental section
The diaryliodonium salt (1 mmol), alkene (1.1 mmol), 50 mg
(4.8 mol%) of [Fe–Dopa–Pd] catalyst with aqueous PEG-400
(1 : 1, v/v, 3 mL) is subjected to ultrasonication (1–5 min)
(cycle: 20 s pulse and 10 s pause) in a high intensity sonicator.
Upon completion of the reaction, as indicated by TLC, the cata-
lyst is separated using an external magnet. The reaction mixture
is diluted with water and the crude product is extracted using
ethyl acetate. The organic layer is dried over anhydrous Na2SO4
and purified by passing through a silica gel column to afford the
products (3a–m). The recovered magnetic catalyst was washed
with acetone, dried under vacuum and reused for the next batch
of reaction. All the synthesized compounds have been reported
in the literature and characterized by comparing their spectral
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Spectral data for some of the selected compounds are: Com-
pound 3a: 1H NMR (300 MHz, CDCl3): δ = 7.56–7.49 (m, 3H),
7.41–7.39 (m, 3H), 6.72 (d, J = 16.3 Hz, 1H), 2.38 (s, 3H);
13C NMR (75 MHz, CDCl3): δ = 198.3, 143.4, 134.5, 130.5,
1
129.0, 128.3, 127.2, 27.5. Compound 3c: H NMR (300 MHz,
CDCl3): δ = 7.68 (d, J = 16.0 Hz, 1H), 7.54–7.50 (m, 2H),
7.40–7.35 (m, 3H), 6.44 (d, J = 16.0 Hz, 1H), 4.21
(t, J = 6.7 Hz, 2H), 1.74–1.65 (m, 2H), 1.50–1.38 (m, 2H),
0.97 (t, J = 7.3 Hz, 3H); 13C NMR (75 MHz, CDCl3): δ =
167.1, 144.5, 134.5, 130.2, 128.9, 128.1, 118.4, 64.4, 30.8, 19.2,
13.7. Compound 3l: 1H NMR (300 MHz, CDCl3): δ =
7.35–7.32 (m, 4H), 6.66–6.61 (m, 1H), 6.29–6.19 (m, 1H), 4.71
(dd, J = 6.5 and 1.2 Hz, 2H), 2.1 (s, 3 H), 1.31 (s, 9H);
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