Thermally decomposed Cu-Fe-hydrotalcite: A novel highly active catalyst for o-arylation of naphthol and phenols by aryl halides

Article abstract of DOI:10.1016/j.catcom.2012.09.024

A novel, efficient and environmentally benign method has been reported for the synthesis of diaryl ethers by the o-arylation of napthol or phenols with aryl halides in dimethylformamide (as a solvent) under reflux, using a novel heterogeneous catalyst (having redox properties), obtained from thermal decomposition of Cu-Fe at 600 °C in the absence of externally added base. The catalyst comprises Cu(II) and Fe(III) species (oxides and hydroxides), which are uniformly distributed during the catalyst formation. The catalyst can be easily separated from the reaction mixture, simply by filtration and reused several times without a significant loss of its activity.

Full text of DOI:10.1016/j.catcom.2012.09.024

Catalysis Communications 29 (2012) 132–136
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Thermally decomposed Cu–Fe-hydrotalcite: A novel highly active catalyst for
o-arylation of naphthol and phenols by aryl halides
b
a
b
V.R. Choudhary a, , D.K. Dumbre , P.N. Yadav , S.K. Bhargava
⁎
a
Chemical Engineering and Process Development Division, National Chemical Laboratory, Pune 411 008, India
Centre for Advanced Materials and Industrial Chemistry, School of Applied Sciences, RMIT University, Melbourne 3000, Australia
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 August 2012
Received in revised form 17 September 2012
Accepted 18 September 2012
Available online 25 September 2012
A novel, efficient and environmentally benign method has been reported for the synthesis of diaryl ethers by
the o-arylation of napthol or phenols with aryl halides in dimethylformamide (as a solvent) under reflux,
using a novel heterogeneous catalyst (having redox properties), obtained from thermal decomposition
of Cu–Fe at 600 °C in the absence of externally added base. The catalyst comprises Cu(II) and Fe(III) species
(oxides and hydroxides), which are uniformly distributed during the catalyst formation. The catalyst can be
easily separated from the reaction mixture, simply by filtration and reused several times without a significant
loss of its activity.
Keywords:
C–O cross coupling reaction
o-Arylation
© 2012 Elsevier B.V. All rights reserved.
Aryl halides
Phenols
Naphthol
Cu–Fe-hydrotalcite
1. Introduction
Recently, a number of methodologies for the o-arylation using
copper containing reagent/catalyst have been reported. Evan and co-
Diaryl ethers are valuable intermediates in organic synthesis and
its synthesis has attracted a lot of attention in recent years, especially
after the discovery of diaryl ethers containing biological active natural
products such as vancomycin aglycon and the combrestatin [1,2]. Aryl
ethers are also an important class of ‘building blocks’ that are widely
employed in organic synthesis, polymer industries and pharmaceuti-
cal and agrochemical areas. The importance of diaryl ethers can be
realized by the number of recent reviews published time-to-time
for the synthesis of diaryl ethers [3–7].
The classic Ullmann type arylation of phenols with aryl halides
using copper powder or copper salts requires harsh reaction condi-
tions, such as large excess of phenols, high temperatures (>200 °C)
and at least stoichiometric amounts of copper [8]. This causes a
serious waste disposal problem. To overcome the limitations of
the Ullmann's arylation, Buchwald and Hartwig introduced palladi-
um based approaches [7,9]. However, because of the high cost of
palladium and the use of expensive ligands, even after 100 years
of discovery of Ullmann diaryl ether synthesis, copper mediated
o-arylation of phenols by aryl halides remains the reaction of
choice for most organic chemists and large scale o-arylation in
pharmaceutical, agrochemical, fine chemical and polymer indus-
tries [10,11].
workers described the copper(II)-promoted arylation of phenols with
aryl boronic acids [12]. Buchwald and co-workers demonstrated that
the arylation of phenols can be accomplished using the copper(I)–
(CuOTf)2-benzene) [13].
More recently a few studies have also been reported for the
o-arylation of phenols, using CuI nanoparticles [14], silica based re-
agent in stoichiometric amount [15], nanoCuO with Cs2CO3 or KOH
[16], CuFe2O4 magnetic nanoparticles with Cs2CO3 and acetyl acetone
[17], nanoceria with K2CO3 [18] and KF/clinoptilolite [19]. However,
most of these catalysts are used along with a base [14,16–18], in
sub-stoichiometric amounts [15] and/or required long reaction peri-
od [15–18]. Hence it is of great practical interest to develop a better
solid catalyst which catalyzes the o-arylation of phenols with aryl
halides in the absence of any base, requires much shorter reaction
time for desirable product yield and also can be easily separated
and reused in the reaction several times.
In this communication, we report a simple, effective and envi-
ronmentally benign protocol for the o-arylation cross coupling reac-
tion between aryl halides and naphthol and various substituted
phenols with high product yield, using a thermally decomposed
Cu–Fe-hydrotalcite (which has redox properties) as a catalyst in
the absence of externally added base (Scheme 1). The catalyst
showed high activity in the reaction under mild reaction conditions.
It can be easily separated from the reaction mixture, simply by fil-
tration and reused several times without a significant loss of its
activity.
⁎
Corresponding author. Tel.: +91 20 25902318/65265508; fax: +91 20 25902612.
E-mail addresses: vr.choudhary@ncl.res.in, vrc0001@yahoo.co.in (V.R. Choudhary).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2012.09.024

V.R. Choudhary et al. / Catalysis Communications 29 (2012) 132–136
133
Thermally decomposed
Cu-Fe-hydrotalcite
(not shown) revealed that the hydrotalcite structure is totally
destroyed after decomposition of the Cu–Fe-hydrotalcite. The main
crystalline phase present in the catalyst was found to be Cu–Fe-oxide
and Fe2O3 (major) and Fe(OH)3 and Cu(OH)2 (minor). The surface
R
R
X
R1-OH
O
R1
DMF, reflux, 7-15h
X = I, Br or Cl
area of the catalyst was 62 m2·g−1
.
R = H, Me, MeO, NO2 or Cl
Thermal gravimetric analysis (TGA) of the Cu–Fe-hydrotalcite
(shown in Fig. 1) revealed that the thermal decomposition of the
hydrotalcite occurred in two steps with weight loss of 16% in the
first step due to desorption of physically adsorbed/absorbed water
up to 300 °C and that of 21.5% in the second step between 300 °C
and 700 °C due to decomposition of the hydrotalcite with fast evolu-
tion of CO2 followed by a slower dehydroxylation (above 400 °C). The
presence of different crystalline phases in the catalyst is consistent
with this.
R1= R2-Ph (where R2 = H or MeO) or naphthyl
Scheme 1. Reaction scheme for the o-arylation.
2. Experimental
2.1. Catalyst preparation and characterization
The solid catalyst used in this investigation was obtained from
the thermal decomposition of Cu–Fe-hydrotalcite (Cu/Fe mole ratio of
3.0) in a muffle furnace at 600 °C for 4 h. The resulting mass was finely
powdered. The preparation and characterization of Cu–Fe-hydrotalcite
have been given in our earlier publications [20,21]. The catalyst was
characterized for crystalline phases by XRD [using a Phillips Diffractom-
eter (1730 series) and CuK∝ radiations]. The surface area of the catalyst
was measured by single point N2 adsorption method (using a Surface
Area Analyser, Quantachrome, USA). The thermal decomposition of
the hydrotalcite was studied by its thermal analysis (using Diamond
TG/DTA) from 30 °C to 1000 °C at a linear heating rate of 20 °C/min.
The Cu–Fe-hydrotalcite with Cu/Fe mole ratio of 3 was prepared by a
known procedure [20,21], by adding drop-wise a solution containing
copper nitrate (39 g) and ferric nitrate (22 g) in deionized water
(200 ml) to a solution containing potassium carbonate (55 g) and po-
tassium hydroxide (30.4 g) in deionized water (600 ml) into a flask
under vigorous stirring at 40 °C, while maintaining a pH of 11–12. The
resulting gel like material was aged for 30 min, filtered, thoroughly
washed with deionized water and dried at 80 °C in a vacuum oven,
and then further heated in air oven at 200 °C for 2 h. The XRD spectra
of the hydrotalcite revealed the presence of hydrotalcite (major) and
hydroxides/carbonates of copper and iron (minor) phases. The surface
area of the hydrotalcite was 66 m2·g−1. The CO32− content of the
hydrotalcite (determined by treating the hydrotalcite with 4.0 M
3.2. Catalytic o-arylation reaction
In order to choose optimum conditions (viz. desirable solvent and
reaction temperature) the o-arylation of phenol with iodobenzene
was carried out using different solvents and also by varying the reac-
tion bath temperature from 1 to 140 °C.
Results showing the influence of the use of different solvents in
the o-arylation reaction of phenol with iodobenzene on the catalyst
performance are presented in Table 1. The o-arylation reaction was
carried out using different solvents, such as toluene, p-xylene, aceto-
nitrile, dioxan, NMP and DMF, under reflux. The results (Table 1)
show a strong influence of solvent on the o-arylation reaction; DMF
was found to be the best solvent for this coupling reaction. There
was almost no reaction in the presence of dioxane or acetonitrile, as
a solvent. The choice of solvent for the o-arylation is in the following
order: DMF>Toluene>NMP⋙Dioxane (or acetonitrile). The ob-
served strong solvent effect may be attributed to increasingly stron-
ger adsorption of toluene, p-xylene, dioxan, NMP and dioxane (or
acetonitrile) as compared to that of DMF on the catalyst surface. Fur-
ther studies are required for better understanding the strong solvent
effect.
Results showing the influence of reaction temperature and
amount of catalyst used on the product yield in the o-arylation of
phenol with iodobenzene are presented in Figs. 2 and 3. It may be
noted that the product yield increases almost exponentially with
increasing the reaction temperature and/or the amount of catalyst
used in the reaction (Figs. 2 and 3).
HNO3 and measuring the CO2 evolved) was 0.29 mmol·g−1
.
In order to study the effect of Cu/Fe ratio on the catalyst performance,
we tried to synthesize Cu–Fe-hydrotalcite with Cu/Fe ratio of 5 and 10
but the concentration of pure Cu–Fe-hydrotalcite in the solid product
was very low; the formation of mostly metal hydroxides was observed.
2.2. Catalytic reaction
The catalytic o-arylation reaction was carried out in a magnetically
stirred round bottom flask (capacity: 25 cm3), at the following reac-
tion conditions: reaction mixture=1 mmol aryl halides+1.2 mmol
phenol or naphthol+3 ml solvent (DMF)+catalyst (50 mg), bath
temperature=140 °C and reaction time=7–15 h. The reaction was
monitored by TLC. After completion of the reaction, the catalyst was
separated by filtration and the filtrate was treated with water,
followed by extraction with ethyl acetate to give the crude product,
which was subsequently purified by column chromatography on sili-
ca gel with petroleum ether/ethyl acetate as eluent. The catalyst was
further washed with acetone, dried and reused. The reaction product
was isolated by column chromatography and was characterized by
comparison of its NMR spectra with that reported earlier.
3. Results and discussion
3.1. Catalyst formation and characterization
The catalyst was obtained from the thermal decomposition of the
Cu–Fe-hydrotalcite (Cu/Fe=3) at 600 °C. The XRD of the catalyst
Fig. 1. Thermal gravimetric analysis of the Cu–Fe-hydrotalcite.

134
V.R. Choudhary et al. / Catalysis Communications 29 (2012) 132–136
100
Table 1
Results of o-arylation of phenol with iodobenzene over the thermally decomposed
Cu–Fe-hydrotalcite (as a catalyst) in the presence of different solvents.
Solvent used
Reaction time (h)
Isolated product yield (%)
(Conversion %)
80
60
40
20
0
DMF
NMP
Acetonitrile
p-xylene
Dioxane
Toluene
7
15
15
15
15
15
89 (93)
10 (10.5)
Nil (0.0)
14 (15)
Nil (0.0)
30 (32)
DMF = dimethylformamide, NMP = N-methyl pyrrolidone, conversion is given in
round brackets.
Reaction mixture=1 mmol aryl halides+1.2 mmol phenols+3 ml solvent+50 mg
catalyst, and bath temperature=140 °C.
Since, the best product yield in the o-arylation was found to be
accomplished using DMF as a solvent under reflux (bath temperature
of 140 °C), the further o-arylation work was carried under these reac-
tion conditions.
0
10
20
30
40
50
60
Catalyst loading (mg)
Results of the o-arylation reactions of phenol, 4-methoxy phenol
and naphthol by different halobenzenes and substituted aryl halides,
using the thermally decomposed Cu–Fe-hydrotalcite (as the catalyst)
in dimethyl formamide (as a solvent) under reflux conditions (bath
temperature of 140 °C) in the absence of any externally added base
are presented in Table 2. The results show a strong influence of the
nature of aryl halide (fluro-, chloro-, bromo- or iodobenzene) and
substituent group of substituted aryl halide and/or substituted phenol
on the product yield of the corresponding o-arylation, as follows:
Fig. 3. Influence of catalyst loading on the product yield in the o-arylation of phenol with
iodobenzene [reaction mixture=1 mmol aryl halides+1.2 mmol phenols+3 ml solvent
(DMF)+catalyst, and bath temperature=140 °C, conversion of iodobenzene=11, 23,
44, and 93% at the catalyst loading of 12, 20, 30, and 50 mg, respectively].
presence of electron donating group (viz. \OMe or CH3) in the
iodo- or bromo-benzene (Table 2 entries 5–7) but increased due
to the presence of electron withdrawing group (viz. \NO2 or
\Cl) in the aryl halides (Table 2, entries 8–10).
– The product yield in the o-arylation cross coupling between iodo-
or bromo-benzene and phenol is increased due to the presence of
electron donating group (viz. \OCH3) in phenol (Table 2, entries
12 and 14).
– For the o-arylation of phenol by the different aryl halides, the
diphenyl ether yield for the aryl halides was in the following
order:
iodobenzene>bromobenzene>chlorobenzene⋙fluorobenzene
(Table 2 entries 1 to 4). There is no o-arylation cross coupling re-
action between phenol and fluorobenzene even after 15 h. This
is expected because of the lowest reactivity of fluorobenzene
(resulting from the highest electronegativity of fluorine) among
the halobenzenes.
– A very good product yield is obtained even in the o-arylation of
α-naphthol with iodobenzene (Table 2, entry 13).
It may be noted that the present catalyst not only provide very
good product yields but also can be easily separated from the reaction
mixture, simply by filtration, and reused several times in the
o-arylation reaction without a significant loss of its catalytic activity
(Table 2, entry 15). The observed small increase in the reaction time
for completing the reaction is expected mostly because of the de-
crease in the quantity of recovered catalyst (which is expected due
to filtration loss). The product yield for the first, third and fifth
reuse of the catalyst was 89% (for 7.3 h), 88% (for 7.5 h) and 88%
(for 8 h).
The observed very high catalytic activity of the catalyst obtained
from the thermal decomposition of Cu–Fe-hydrotalcite attributed
most probably to the uniformed distribution of Cu(II) and Fe(III)
(having redox properties) in the catalyst during its formation.
It may be noted that, when the Cu–Fe-hydrotalcite was directly
used as a catalyst, the product yield in the same reaction period
was much smaller (Table 2, entry 16). Also, when the thermally
decomposed (under similar conditions) mixed Cu(II)–Fe(III)-nitrates
were used as the catalyst in the reaction, the product yield was much
smaller (Table 2, entry 17). This may be due to the formation of solid
catalyst with non-uniform distribution of Cu(II) and Fe(III). In the
absence of any catalyst, there was no product formation (Table 2,
entry 18).
– The product yield in the o-arylation of phenol by aryl halides (viz.
iodo-, chloro- and bromo-benzene) is decreased due to the
100
80
60
40
20
0
0
20
40
60
80
100
120
140
All the above observations reveal that the thermally decomposed
Cu–Fe-hydrotalcite is highly promising redox-type catalyst for the
o-arylation of phenol, naphthol and substituted phenols with
unsubstituted chloro-, bromo- or iodobenzenes, providing very good
product yields in the absence of any externally added base. Moreover,
the catalyst is environmentally benign and has excellent reusability.
Reaction temperature (°C)
Fig. 2. Influence of reaction (bath) temperature on the product yield in the o-arylation
of phenol with iodobenzene [reaction mixture=1 mmol aryl halides+1.2 mmol
phenols+3 ml solvent (DMF)+50 mg catalyst, conversion of iodobenzene=0.0, 32
and 93% at 1, 50 and 140 °C, respectively].

V.R. Choudhary et al. / Catalysis Communications 29 (2012) 132–136
135
Table 2
Performance of the Cu–Fe-hydrotalcite catalyst in the o-arylation reaction.
Entry
1
Aryl halide
Phenol
Phenol
Reaction time (h)
7
Product
Isolated product yield (%)
Iodobenzene
89 (93)
85 (88)
21 (22)
O
O
O
2
3
Bromobenzene
Chlorobenzene
Phenol
Phenol
10
15
4
5
Flurobenzene
p-Bromoanisole
Phenol
Phenol
15
10
nil
Nil (0.0)
84 (86)
O
O
MeO
MeO
6
p-Iodoanisole
Phenol
8
87 (92)
7
8
p-Bromotoluene
Phenol
Phenol
10
7
82 (85)
O
p-Nitro-iodobenzene
96 (100)
O
O2N
O2N
O2N
Cl
9
p-Nitro-bromobenzene
p-Nitro-chlorobenzene
Phenol
Phenol
10
15
90 (95)
75 (78)
O
O
10
11
12
13
p-Bromochlorobenzene
Iodobenzene
Phenol
12
7
87 (91)
91 (94)
90 (93)
O
p-methoxyphenol
α-Napthol
O
OMe
Iodobenzene
8
O
14
Bromobenzene
p-methoxyphenol
10
88 (92)
O
OMe
15
16
17
18
Iodobenzene
Iodobenzene
Iodobenzene
Iodobenzene
Phenol
Phenol
Phenol
Phenol
8
7
88a
40b
44c
nild
O
O
O
O
7
15
Aryl halide conversion is given in round brackets. Reaction mixture=1 mmol aryl halides=1.2 mmol phenols+3 ml solvent (DMF)+50 mg catalyst and bath temperature=
140 °C.
a
Fifth reuse of the catalyst.
Cu–Fe-HT without decomposition was used.
Catalyst obtained from the decomposition of a mixture of Cu(II) and Fe(III) nitrates (with Cu/Fe mole ratio of 3.0) at 600 °C for 4 h.
Without catalyst.
b
c
d

136
V.R. Choudhary et al. / Catalysis Communications 29 (2012) 132–136
As compared to the earlier reported reusable solid catalysts [16–18],
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Acknowledgments
VRC is grateful to the National Academy of Sciences, India, for the
NASI Senior Scientist Platinum Jubilee Fellowship.