O-Allylation of phenols with allylic acetates in aqueous media using a magnetically separable catalytic system

Article abstract of DOI:10.1039/c1gc16174a

Allylic ethers were synthesized in water using magnetically recoverable heterogeneous Pd catalyst via O-allylation of phenols with allylic acetates under ambient conditions. The aqueous reaction medium, easy recovery of the catasyst using an external magn

Full text of DOI:10.1039/c1gc16174a

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COMMUNICATION
O-Allylation of phenols with allylic acetates in aqueous media using a
magnetically separable catalytic system†
Amit Saha, John Leazer* and Rajender S. Varma*
Received 20th September 2011, Accepted 5th October 2011
DOI: 10.1039/c1gc16174a
Allylic ethers were synthesized in water using magnetically
recoverable heterogeneous Pd catalyst via O-allylation of
phenols with allylic acetates under ambient conditions. The
aqueous reaction medium, easy recovery of the catalyst
using an external magnet, efficient recycling, and the high
stability of the catalyst renders the protocol economic and
sustainable.
Allyl ethers are important starting materials for a wide range
of organic reactions, including the 1,3-hydrogen shift, [3,3]-
sigmatropic rearrangement, and polymerization reactions,1 in
addition to being a popular protecting group for alcohols.2
The traditional method for the synthesis of allyl ethers is
the Williamson-type ether synthesis which involves the use
of strongly basic metal alkoxide anions and highly active
allyl halides or their equivalents.3 Comparatively, the addition
3
of oxygen nucleophiles to h -allylmetal complexes of various
transition metals (Pd,4 Ru,5 Ir,6 Ni7) provides an attractive
Fig. 1 [Fe3O4–dopamine–Pd] catalyst.
mild alternative approach for their synthesis using much less
reactive allyl alcohols, allyl esters or allyl carbonates. However,
the surface of dopamine-functionalized nanoparticles8a (Fig. 1),
([Fe3O4–dopamine–Pd]); their use as an easily recyclable catalyst
to develop an economic and eco-friendly green synthesis of allyl
ethers by O-allylation of phenols with allylic acetates in water
under an open air atmosphere has been explored.
Phenols undergo unprecedented allylic substitution reactions
with various allylic acetates in air under refluxing aqueous media
conditions for 3–10 h whilst in the presence of the [Fe3O4–
dopamine–Pd] catalyst; a mild base, sodium bicarbonate, is
adequate to produce the allyl ethers (Scheme 1) in good yields.
most of the existing methods for transition metal catalyzed O-
3
allylation via h -allylmetal intermediates are accomplished using
a non-recyclable homogeneous catalyst in various toxic organic
solvents such as THF, DMF, benzene, toluene, and DCM
under an inert atmosphere. The use of heterogeneous catalysts
in organic transformations has become an interesting area of
research in the field of green chemistry facilitating catalyst
recyclability as exemplified in a recent report by Kobayashi
et al.4a wherein a heterogeneous polymer incarcerated Pd catalyst
is used for O-allylation of phenols in a THF medium. Magnetic
nano-ferrite [Fe3O4] has been widely used by our group8 and
others9 as the solid support for various catalysts because of
their easy recovery using an external magnet. In continuation
of the work with magnetically separable nano-ferrite [Fe3O4] as
a heterogeneous catalyst support, Pd has been immobilized on
Scheme 1 O-Allylation of phenols with allylic acetates.
The catalyst was prepared following the optimized protocol
established earlier.8a–c The aqueous suspension of nano-ferrite
was sonicated for 2 h with dopamine, which acts as a pseudo-
ligand and as a robust anchor preventing the leaching of
immobilized Pd. Dopamine functionalized nano-ferrite was
then treated with PdCl2 in basic medium to obtain the Pd(II)
Sustainable Technology Division, National Risk Management Research
Laboratory, U. S. Environmental Protection Agency, MS 443,
Cincinnati, Ohio 45268, USA. E-mail: varma.rajender@.epa.gov;
Fax: 513-569-7677; Tel: 513-487-2701
† Electronic supplementary information (ESI) available: Experimental
procedures; 1H and 13C NMR spectra of all products. See DOI:
10.1039/c1gc16174a
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catalyst supported on amine functionalized magnetic Fe3O4
nanoparticles. The spherical morphology and the size of the
nano catalyst (13–38 nm) were confirmed by transmission
electron microscopy (TEM) (Fig. 2). The weight percentage
of palladium was found to be 10.23% by inductively coupled
plasma–atomic emission spectroscopy (ICP–AES) analysis.
(30 mg), even after prolonging the reaction time. Further, no
improvement was observed by increasing the amount of Pd
catalyst in terms of reaction time and yield of the product. The
reaction does not proceed in either the absence of base NaHCO3
(Table 1, entry 1) or the catalyst [Fe3O4–dopamine–Pd] (Table 1,
entry 2). Also, no desired allyl ether was obtained in an identical
reaction with [Fe3O4] nanoparticles (without Pd) (Table 1, entry
3). Water was found to be the best solvent for this reaction
to obtain improved yields and fast reaction times compared to
other common organic solvents such as DMF, THF, CH3CN,
toluene, PEG (Table 1, entries 4–7, 9).
In a typical representative experimental procedure, a mixture
of allyl acetate, phenol, Pd catalyst and NaHCO3 in water was
heated to reflux in an open atmosphere for a sufficient time to
complete the reaction (as determined by TLC). After completion
of the reaction, the catalyst was separated from the reaction
mixture using an external magnet. The product was extracted
by ethyl acetate and was purified by column chromatography.
The catalyst was washed with acetone, dried under vacuum, and
recycled for 5 consecutive reactions without any significant loss
in efficiency (Table 2).
Metal leaching was studied by ICP–AES analysis of the cata-
lyst before and after the reaction cycles. The Pd concentration in
the heterogeneous catalyst was found to be the same before and
after the reaction. The TEM image of the catalyst taken after the
fifth cycle of the reaction does not show any significant changes
in the morphology and the size of the catalyst nanoparticles
(15–37 nm) (Fig. 3), which indicates retention of the catalytic
activity after recycling. No Pd metal was detected in the reaction
solvent (water) after completion of the reaction. It confirms the
fact that dopamine-functionalization provides enough amine-
binding sites on the surfaces of the Fe3O4 nanoparticles, which
serve as pseudo-ligands by coordinating with Pd and thus help
Fig. 2 TEM image of [Fe3O4–dopamine–Pd] catalyst.
To optimize the reaction conditions, a series of experiments
were conducted with a representative reaction of cinnamyl
acetate and p-cresol, with variation of reaction parameters, such
as base, solvent, reaction temperature etc. (Table 1). It was found
that a combination of [Fe3O4–dopamine–Pd], 50 mg (4.8 mol%),
and NaHCO3 (2 equiv.) in water is ideal for a fast and efficient
reaction (Table 1, entry 13). The reaction was observed to be
incomplete in the presence of a lower amount of the Pd catalyst
Table 1 Standardization of the reaction conditiona
Entry
Base
Solvent
Temp (◦C)
Time (h)
Yield (%)b
1
—
Water
Water
Water
DMF
THF
CH3CN
Toulene
Water
PEG
Water
Water
Water
Water
Water
DMF
Water
100
100
100
120
66
15
15
15
12
10
10
12
10
12
10
10
10
5
0
0
0
2c
3d
4
NaHCO3 (without catalyst)
NaHCO3 (with Fe3O4)
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
77
53
55
9
80
63
65
80
83
85
18
Trace
66
5
6
7
8
82
100
100
100
100
100
100
100
100
120
100
9
10
11
12
13
14
15
16
Na2CO3
NaHCO3
NaHCO3
NaOAc
NaH
K3PO4
10
12
10
a A mixture of p-cresol (1 mmol), cinnamyl acetate (1 mmol), base (2 mmol) and Pd catalyst (50 mg, 4.8 mol%) was heated under air in the specified
solvent. b Yields refer to those of column purified isolated products. c The reaction was performed without the catalyst [Fe3O4–dopamine–Pd]. d The
reaction was performed with Fe3O4 naoparticles (without Pd).
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Table 2 Recycling of the catalyst
Conclusions
In conclusion, a simple and efficient green procedure for the
general O-allylation of phenols with allyl acetates has been
developed using a magnetically separable and easily recyclable
heterogeneous Pd catalyst in the presence of a mild base,
NaHCO3, in water under an open atmosphere. This precludes
the requirement of using an inert atmosphere and organic
solvents. Easy magnetic separation of the catalyst eliminates
the requirement of catalyst filtration after completion of the
reaction, which is an additional sustainable aspect of this
reaction. To the best of our knowledge, this is the first example
of an O-allylation reaction using a magnetic heterogeneous Pd
catalyst in water.
No. of cycles
Yield (%)
1
2
3
4
5
6
85
84
85
83
82
82
Experimental
O-Allylation of phenols by allylic acetates using
[Fe3O4–dopamine–Pd] catalyst
A mixture of phenol (1 mmol), allylic acetate (1 mmol), NaHCO3
(2 mmol) and Pd catalyst (50 mg, 4.8 mol%) was added to
2.5 mL of water. The aqueous reaction mixture was heated to
reflux for the required time as indicated in Table 3. The progress
of the reaction was monitored by TLC. Standard work-up
with ethyl acetate followed by simple column chromatography
provided the pure product. All products listed in Table 3 were
known in the literature (except entries 6 and 11) and were
1
identified by comparison of their FT-IR, H, and 13C NMR
with literature data. The products of entries 6 and 11, Table 3,
1
were characterized by FT-IR, H and 13C NMR spectroscopy
and elemental analysis.
Fig. 3 A TEM image of the recovered catalyst after the fifth cycle of
the reaction.
2,4-Dichloro-1-(cinnamyloxy)benzene (Table 3, entry 6). Pale
yellow solid, Mp. 47–49 ◦C, Yield 79%, FT-IR (cm-1): 752, 798,
872, 970, 999, 1055, 1100, 1245, 1263, 1286, 1378, 1447, 1474,
1570, 2868, 2920, 3032, 3064. 1H NMR (300 MHz; CDCl3) dH
4.77 (d, J = 5.7 Hz, 2H), 6.40–6.46 (m, 1H), 6.79 (dd, J1 = 0.9
Hz, J2= 15.9 Hz, 1H), 6.91–6.94 (m, 1H), 7.19–7.23 (m, 1H),
7.28–7.43 (m, 6H). 13C NMR (75 MHz; CDCl3) dC 70.0, 114.7,
123.4, 124.0, 126.0, 126.7 (2C), 127.6, 128.1, 128.6 (2C), 130.1,
133.5, 136.2, 153.0. Analysis calculated for C15H12Cl2O: C 64.54,
H 4.33; Found: C 64.56, H 4.31.
to minimize deterioration and metal leaching, whilst aiding in
efficient catalyst recycling.
A series of substituted cinnamyl acetates underwent coupling
with a variety of substituted phenols by this procedure to
produce the corresponding allyl aryl ethers (Table 3). In general,
the reactions are very efficient and clean. The ensuing products
are obtained in high yields and good purity. Several functional
groups such as OMe, NO2, Cl, and Br are compatible with
this reaction, which performs consistently with o-, m-, and p-
substituted phenols (Table 3, entries 2–4). A branched allylic
acetate (Table 3, entry 13) was used with equal efficiency to
provide a linear allyl aryl ether, supporting a mechanism that
(E)-1-Chloro-2-(3-(p-tolyloxy)prop-1-en-1-yl)benzene (Table
3, entry 11). Pale yellow viscous liquid, Yield 73%, FT-IR
(cm-1): 746, 816, 963, 1175, 1237, 1289, 1376, 1440, 1469, 1508,
1584, 1611, 2858, 2919, 3027. 1H NMR (300 MHz; CDCl3) dH
2.33 (s, 3H), 4.74 (dd, J1 = 5.7 Hz, J2 = 1.8 Hz, 2H), 6.38–6.48
(m, 1H), 6.89–6.92 (m, 2H), 7.12–7.15 (m, 2H), 7.19–7.28 (m,
3H), 7.37–7.40 (m, 1H), 7.58–7.61 (m, 1H). 13C NMR (75 MHz;
CDCl3) dC 20.5, 68.7, 114.7 (2C), 126.9, 127.0, 127.7, 128.8,
129.0, 129.7, 129.9 (2C), 130.2, 133.2, 134.7, 156.4. Analysis
calculated for C16H15ClO: C 74.27, H 5.84; Found: 74.23, H
5.81.
3
entails the intermediacy of an h -allyl Pd complex as the key
intermediate in the reaction (Scheme 2).
Acknowledgements
Amit Saha was supported by the Postgraduate Research Pro-
gram at the National Risk Management Research Laboratory
administered by the Oak Ridge Institute for Science and
3
Scheme 2 Formation of h -allyl Pd complex as reaction intermediate.
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Table 3 O-Allylation of phenols with allylic acetatesa
Entry
1
Phenols
Allylic acetate
Time (h)
6
Products
Yields (%)b
80
2
5
85
3
5
87
83
90
4
5
5c
10
6
7
9
79
85
10
8
9
3
8
75
87
10
11
12
8
8
8
75
73
76
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Table 3 (Contd.)
Entry
13
Phenols
Allylic acetate
Time (h)
8
Products
Yields (%)b
81
a A mixture of phenol (1 mmol), allylic acetates (1 mmol), sodium bicarbonate (2 mmol) and Pd catalyst (50 mg, 4.8 mol%) in water (2.5 mL) was
heated to reflux in air. b Yields refer to those of column purified isolated products. c The reaction mixture in DMF (2 mL) was heated at 90 ◦C.
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