Clay entrapped Cu(OH)x as an efficient heterogeneous catalyst for ipso-hydroxylation of arylboronic acids

Article abstract of DOI:10.1016/j.apcata.2013.06.051

A remarkably active, selective and stable montmorilonite-KSF entrapped Cu(OH)_x catalyst, has been prepared for the ipso-hydroxylation of arylboronic acids under ambient conditions without requirement of any ligand or base. This catalyst shows excellent reusability without leaching and any significant loss in catalytic activity. The catalyst was characterized using, XRD, SEM, TPR, IR, XPS and BET surface area measurement techniques.

Full text of DOI:10.1016/j.apcata.2013.06.051

Applied Catalysis A: General 466 (2013) 60–67
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Clay entrapped Cu(OH) as an efficient heterogeneous catalyst for
x
ipso-hydroxylation of arylboronic acids
a,b
a
c
c
a
Bashir Ahmad Dar , Prince Bhatti , A.P. Singh , Anish Lazar , Parduman R. Sharma ,
b
a,∗
Meena Sharma , Baldev Singh
Indian Institute of Integrative Medicine (CSIR), Canal Road, Jammu 180001, India
Department of Chemistry, University of Jammu, J&K 180004, India
a
b
c
Catalysis Division, National Chemical Laboratory, Pune, Maharastra 411008, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A remarkably active, selective and stable montmorilonite-KSF entrapped Cu(OH) catalyst, has been pre-
x
Received 1 February 2013
Received in revised form 15 May 2013
Accepted 29 June 2013
Available online xxx
pared for the ipso-hydroxylation of arylboronic acids under ambient conditions without requirement of
any ligand or base. This catalyst shows excellent reusability without leaching and any significant loss
in catalytic activity. The catalyst was characterized using, XRD, SEM, TPR, IR, XPS and BET surface area
measurement techniques.
©
2013 Elsevier B.V. All rights reserved.
Keywords:
Phenols
Heterogeneous catalysis
Clay
Copper
Water chemistry
1
. Introduction
Simple metal salts like potassium peroxymonosulfate [15], and
metal free catalytic systems like N,N-dimethyl-4-toluidine N-
oxide [16], NH OH [17], 3-aminothiophenol [18], I -H O [19],
Many biologically active compounds, ranging from natural
2
2
2
2
products to pharmaceuticals are reported to possess phenols in
monomeric or polymeric forms [1,2]. Some of the phenols also
serve as important synthetic intermediates for the construction of
more complex structures [3,4]. Consequently, the development of
methods establishing mild and efficient access to such a structural
motif is of immense importance especially in the presence of many
functional groups. Hydroxylation of aryl bromides and chlorides
into phenols have been reported using palladium-based catalysts in
presence of phosphine ligands [5,6]. Hydroxylation of aryl iodides
to the corresponding phenols have been achieved using copper
catalyst and non-phosphine ligands under thermal conditions
H O –poly(N-vinylpyrrolidone) [20], H O Amberlite IR-120 [21],
2 2 2 2
aqueous H O [22], etc. have also been developed.
2
2
However longer reaction times, use of homogeneous transition
metal catalyst systems, organic solvents, stoichiometric amounts
of ligand/base, hazardous oxygen sources such as H O and ozone
2
2
could not avoided in these approaches [23]. Moreover homoge-
neous transition metal catalysts despite showing high catalytic
activity and selectivity have some serious drawbacks, such as metal
contamination, which is highly undesirable from industrial and
biological point of view and homogeneous metal catalysts can-
not be reused, which results in loss of expensive metals. Increasing
demand for the development of more efficient and environmentally
acceptable processes in the chemical industry focusing largely on
chemical yield, eliminating waste at source and circumventing the
use of perilous materials has captivated the interest of researchers
in the field of heterogeneous catalysis [24].
Clays and modified clay supported catalysts have many
advantages, such as easy handling, easy separation, recycling, envi-
ronmentally safe disposal, inexpensiveness, improved efficiency
due to stable active site and better steric control of a reaction
intermediate. The clay-based metal composites have shown their
activities in various pioneering works in the field of catalysis [25]. In
addition to that clay minerals occur abundantly in nature and their
[
7,8]. However wide range availability of arylboronic acid deriva-
tives, their harmless nature, stability toward heat, air, moisture
and their facile conversion to phenols has made them attractive
and valuable precursors [9]. Transition metal catalysts such as
AuNPs/PVP [PVP = poly(N-vinyl-2-pyrrolidone)] [10], CuSO4-
phenanthroline [11], CuCl -Brij-S-100 [12], [Ru(bpy) Cl ]·6H O
2
3
2
2
[
13], Pd(COD)Cl (COD = 1,5-cyclooctadiene) [14] have been
2
employed to accomplish hydroxylation of aryl boronic acids.
∗ Corresponding author. Tel.: +91 0191 2572002; fax: +91 0191 2548607.
E-mail address: drbaldev1@gmail.com (B. Singh).
0
926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.06.051

B.A. Dar et al. / Applied Catalysis A: General 466 (2013) 60–67
61
OH
Cu(OH)
x
-Clay
2.1.3. Synthesis of Cu/acidic KSF [acidic clay]
B
OH
In typical synthesis procedure, 300 ml (1 M) aqueous solution
H2O, RT
R
OH
R
of NH Cl was added drop wise to a suspension of montmorilonite-
4
KSF (10 g) in distilled water (200 ml) with vigorous stirring. The
resulting slurry was stirred for 15 h, and then the water was evap-
Scheme 1. Ipso-hydroxylation of arylboronic acids.
◦
orated. The powder thus formed was kept overnight at 110 C in
◦
an air oven and then calcined at 550 C for 3 h and labeled as acidic
high surface area, sportive and ion-exchange properties have been
exploited for catalytic applications through decades. Aluminosili-
cate layers of Montmorillonite-Clays found in nature are negatively
charged due to isomorphous substitution and often these negative
charged layers are compensated by the cations between the lay-
ers. In aqueous state, the negatively charged aluminosilicate layers
attract water molecules to the interlayer for charge balance. This
charge balance phenomenon allows swelling and cation exchange
to take place. Such properties can be used to intercalate a desired
cation into the clay lattice [26].
Comparatively high stability of a catalyst on solid support some-
times allows the reaction to be less sensitive to normal ambient
conditions. Therefore, clay supported catalyst as a substitute of
homogeneous catalyst or an expensive heterogeneous catalyst
seems highly desirable [27]. In continuation of our research interest
for organic transformations through eco-friendly routes [28–30],
we herein report a highly efficient synthesis of phenols by ipso-
hydroxylation of arylboronic acids using inexpensive, air-stable
and efficient Clay entrapped Cu(OH)x as a recyclable heterogeneous
catalyst under base free and ligand-free conditions (Scheme 1). The
solvent used for this catalytic system is water and the reactions
proceeds smoothly at room temperature. Using water as solvent is
highly advantageous from the viewpoint of cost, safety, availability
and environment friendliness [31].
clay. The acidic clay was suspended in 200 ml distilled water, and to
this copper oligomer (base hydrolysed cupric chloride with OH/Cu
molar ratio of 2.0) was added drop wise to prepare the required
wt.% loading of copper, and the resulting slurry was stirred at room
temperature for 8 h. The solid products were filtered, washed sev-
eral times with distilled water, dried first at room temperature and
◦
◦
then at 110 C for 12 h and then calcined at 425 C.
All synthesized catalysts were characterized by powder X-ray
diffraction using a D-8 ADVANCE (BRUKER AXS, GERMANY) X-
ray diffractometer using Ni filter with Cu-K˛ radiation Hitachi
◦
(H-7500) in the 2ꢀ range 5–70 in step scan mode (step size:
◦
0.02 , scan speed: 2 s/step). The phases where identified by
search match procedure with the help of DIFFRACPLUS soft-
ware using JCPDS databank. Temperature programmed reduction
(TPR) and BET surface area were determined by CHEMBET-3000
TPR/TPD/TPO instrument. SEM of the catalyst was carried out using
JEOL.JEM100CXII ELECTRON MICROSCOPE with ASID Accelerat-
ing Voltage 40.0 kV. IR spectra were recorded on Perkin-Elmer IR
2
−1
spectrophotometer. The specific surface areas (m /g ) of the cat-
alyst was estimated with the N2 adsorption and desorption was
◦
determined at −196 C by means of an automated CHEMBET-3000
adsorption apparatus. XPS analysis was performed on a KRATOS-
AXIS 165 instrument.
2.2. Procedure for ipso-hydroxylation of arylboronic acid
2
2
2
. Experimental
To a solution of arylboronic acid (1 mmol) in deionized water
1.5 ml), clay entrapped Cu(OH)x (13 mg) was added and this het-
erogeneous mixture was vigorously stirred at ambient temperature
for 20–100 min. The reaction progress was monitored by TLC. Ethyl
acetate was added and the aqueous phase was isolated and back
extracted with ethyl acetate. The combined organic layer was dried
(
.1. Catalyst preparation
.1.1. Synthesis of clay entrapped Cu(OH)x [Cu/KSF]
Clay entrapped Cu(OH)x [Cu/KSF] was prepared by suspen-
ding montmorillonite-KSF (10 g) with cation exchange capacity of
over MgSO . The solvent was removed under reduced pressure and
4
1
20 mequiv./100 g clay in 200 ml distilled water and the suspen-
the residue was purified by SiO column chromatography to afford
◦
2
sion was vigorously stirred at 80 C for 2 h. Copper oligomer (base
hydrolysed cupric chloride with OH/Cu molar ratio of 2.0) was
added drop wise to prepare the required wt% loading of copper and
the resulting slurry was stirred at 90 C for 8 h. The solid products
were filtered, washed several times with distilled water, dried first
at room temperature and then at 110 C for 12 h. To study the effect
of calcination on the rate of reaction, the catalyst was calcined at
different temperatures (250–425 C).
the desired product. The prepared phenols were characterized by
comparing the observed spectral data and physical properties with
those of authentic samples. Most of the products formed were
primarily identified by comparing their TLC with the standard sam-
ples. NMR spectra were recorded on Brucker-Avance DPX FT-NMR
◦
◦
4
00 MHz instrument. ESI-MS and HRMS spectra were recorded on
Agilent 1100 LC and HRMS-6540-UHD machines. Melting points
were recorded on digital melting point apparatus. The catalyst was
washed several times with ethanol followed by distilled water and
used for next reaction cycle.
◦
2.1.2. Synthesis of Cu/Cs KSF [basic clay]
Cu/Cs KSF [basic clay] was synthesized by dropwise addition
of 7.5 g of CsCO3 in 200 ml water solution into a suspension of
montmorilonite-KSF (10 g) in distilled water (200 ml) with vigor-
ous stirring. The resulting slurry was stirred for 15 h, and then the
water was evaporated. The powder thus formed was kept overnight
3. Results and discussion
3.1. Characterization of the catalyst
◦
◦
at 110 C in an air oven and then calcined at 550 C for 3 h and
labeled as basic clay. The basic clay was suspended in 200 ml dis-
tilled water, and to this, copper oligomer (base hydrolysed cupric
chloride with OH/Cu molar ratio of 2.0) was added drop wise to
prepare the required wt.% loading of copper, and the resulting
slurry was stirred at room temperature for 8 h. The solid products
were filtered, washed several times with distilled water, dried first
The XRD spectra of (a) Cu/KSF [15 wt% Cu on as such clay (cal-
◦
cined at 250 C Cu/Clay-2a2)] (b) Cu/KSF after 10th cycle of reaction
(c) Cu/Cs KSF [Cu/Clay-3] (d) Cu/acidic KSF [Cu/Clay-1] are shown
◦
◦
◦
◦
in Fig. 1. Peaks at 2ꢀ = 21 , 26.7 , 50.3 and 60 are due to reflec-
◦
◦
tion of the quartz [SiO ] impurities [32–34] and at 12.5 , 19.9 and
2
◦
27.9 are due to palygorskite [35–37]. Peaks for montomorillonite
appear at 2ꢀ = 8.9 (d 0 0 1 reflection), 19.8 , 32.2 , and 62 [38,39].
◦
◦
◦
◦
◦
◦
◦
at room temperature and then at 110 C for 12 h and then calcined
at 425 C.
Presence of Kaolinite is implied to small peaks at 38.8 , and 42.5
[38,39]. The copper loaded on montmorilonite-KSF does not show
◦

6
2
B.A. Dar et al. / Applied Catalysis A: General 466 (2013) 60–67
results indicate that the Cu2+ ions are mostly entrapped in the
interlamellar space of montmorilonite-KSF.
XPS is a surface technique which provides valuable informa-
tion about the oxidation state and chemical environment of atoms
due to the shift in binding energies. The XPS spectra of (a) Cu/KSF
d
c
(
(
Cu/Clay-2a2) (b) Cu/KSF (Cu/Clay-2a2) after 10th cycle of reaction
c) Cu/Cs KSF (d) Cu/acidic KSF are represented in Fig. 5. Two main
peaks having binding energies are about 940 eV due to Cu 2p3/2
and 948 eV due to Cu 2p1/2 and four satellite peaks around 960
and 968 eV are shown in XPS spectra with a variety of intensities,
which are corresponding to Cu(II) species [46].
3.2. Ipso-hydroxylation of arylboronic acids
b
a
At the onset of this work copper supported acidic clay [Cu/acidic
KSF], as-such clay [Cu/KSF], base modified clay [Cu/Cs KSF] each
◦
with 10 wt.% Cu loading and calcined at 425 C were prepared,
and screened for Ipso-hydroxylation of aryl boronic acids. Phenyl-
boronic acid was taken as substrate for model reaction under
10
20
30
40
50
60
70
ambient conditions with H O as solvent (Table 2). Among all the
2
2
Theta/ Degree
three catalysts, Cu supported on basic clay (Cu/Clay-3) (Table 2,
entry 3) showed slightly higher conversion than Cu on as-such
clay (Cu/Clay-2) (Table 2, entry 2) and Cu on acidic clay (Cu/Clay-
Fig. 1. XRD spectra of (a) Cu/KSF, (b) Cu/KSF after 10th cycle of reaction, (c) Cu/Cs
KSF, and (d) Cu/acidic KSF.
1
) (Table 2, entry 1). But this difference persists for first catalytic
cycle only and from second cycle onwards the results were found
to be similar for all the three catalysts that may be due to the leach-
ing of basic components from Cu/Clay-3 (Table 2, entry 3). Since
this modification of clay support could not bring any improve-
ment in the catalytic activity (except a slight difference in first
reaction cycle), it was not advisable to use the modified clay sup-
ports for the Cu catalyst. Thus the unmodified montmorillonite-KSF
was selected as the catalyst support to optimize the other fac-
tors. The amount of Cu loading on unmodified montmorillonite-KSF
was found to have significant effect on the reaction efficiency and
15 wt.% of copper on montmorilonite-KSF was found to be the
optimum loading (Table 2, entry 5). Decreasing the metal loading
to 5 wt.% decreases the reaction rate (Table 2, entry 4), whereas
no improvement in the conversion or yield was perceived upon
increasing the copper loading to 20 wt.% (Table 2, entry 6). While
studying the effect of calcination on the activity of 15 wt.% (14.7 wt%
actual loading) of copper on montmorilonite-KSF, the uncalcined
(Cu/Clay-2a1) proved excellently active, but bulk leaching of cop-
per leading to the massive loss of catalytic activity were observed
(Table 2, entry 14). This catalyst with 15 wt.% copper loading on
montmorilonite-KSF was calcined at different temperatures and
investigated further. Proper calcination plays a very useful role
at minimizing the leaching rate of active metal from the support.
Calcination leads to dehydration causing shrinkage TOT layers of
smectite (montmorilonite-KSF) and metal species inside the layers
are irreversibly entrapped. Furthermore, under proper calcina-
tion temperature treatment the metal creates very strong bonds
with clay support and as a result the leaching problem is min-
imized [47,48]. Leaching of copper metal from the catalyst was
any characteristic peak of CuO, Cu O and metallic Cu but peaks
2
corresponding to mineral atacamite (Cu Cl(OH) ) and Cu(OH) are
2
3
2
◦
◦
◦
◦
observed at 2ꢀ = 16.1, 18 , 25.1 , 32.3 and 50.3 [40–42]. Thus it
can be concluded that copper hydroxide species are deposited in
microcrystalline form.
The N2 adsorption–desorption isotherm and pore size dis-
◦
tribution of (A) Cu/KSF[Cu on as such clay calcined at 250 C
(
Cu/Clay-2a2)] (B) Cu/acidic KSF (C) Cu/Cs KSF are shown in Fig. 2.
All samples displayed type-IV isotherms with H1 hysteresis related
to capillary condensation steps. Textural properties such as BET sur-
face area, pore volume (BJH) and the pore radius (BJH) of Cu/KSF,
Cu/acidic KSF and Cu/Cs KSF samples are summarized in Table 1.
2
The Cu/KSF shows BET surface area, 54.42 m /g; pore volume,
0
.156 cc/g; and pore radius, 19.75 A˚ . As shown in Table 1, BET sur-
face area, pore volume (BJH) and the pore radius (BJH) of Cu/acidic
2
KSF and Cu/Cs KSF are exhibited to 15.78 m /g, 0.062 cc/g, 19.50 A˚
2
and 08.39 m /g, 0.042 cc/g, 19.30 A˚ , respectively. The increased
sharpness in the N2 condensation step points to the uniformity
of the mesopore structure. The SEM images of (A) KSF and (B)
the Cu(OH)x-Clay(Cu/Clay-2a2) catalyst are depicted in Fig. 3 and
showed the presence of coarse surface (thus elevated surface area).
Thus, the Cu(OH)x-Clay catalyst is able to adsorb substrate and/or
reagent to a great extent.
To understand the reduction behavior and thermal stability of
the active metal species, temperature programmed (TPR) was done.
The reduction profile provides information about the dispersion of
the metal species on the support and the interaction between these
metal species and the support. The H -TPR spectra of (a) KSF (b)
2
◦
Cu/acidic KSF (c) Cu/Cs KSF (d) Cu/KSF(Cu/Clay-2a2)are depicted in
Fig. 4. The montmorilonite-KSF support (Fig. 4a) showed a broad
stopped completely after calcinations it at 425 C, but the activ-
ity of the catalyst was dropped down drastically (Table 2, entry
◦
◦
peak at a higher reduction temperature, 524 C, which can prob-
1). The catalyst calcined at 250 C (Cu/Clay-2a2) (Table 2, entry
ably be attributed to the reduction of some iron species in the
montmorilonite-KSF material [43,44]. The first peak in Fig. 4d, cen-
15) exhibits significant catalytic activity without copper leach-
ing and copper species like Cu(OH)2 and Cu Cl(OH)3 are reported
2
◦
2+
tered at 255 C, was assigned to the reduction of Cu nanoparticles
located on the surface probably in the entrance of pores of the
montmorilonite-KSF. Moreover, the reduction peaks, centered at
to remain stable at this temperature [49]. Encapsulation of these
hydroxide nanoparticles inside the clay interlayer restrains their
tendency to undergo agglomeration, which otherwise increases
their particle size hence reduce their catalytic activity [50,51].
Optimization of the amount of catalyst revealed that 13 mg is
sufficient to catalyze model reaction within the required time
(Table 2, entry 19). Bimetallic Cu-Clay catalysts like Cu-Ni/Clay,
◦ ◦
00 C and 318 C, were assigned to the reduction of clay inter-
3
calated Cu species to Cu+ and CuO cluster to Cu , respectively.
2
+
0
◦
Further, the reduction peak, centered at 440 C is very less pro-
+
0
nounced, was assigned to the reduction of Cu to Cu [44,45]. The

B.A. Dar et al. / Applied Catalysis A: General 466 (2013) 60–67
63
A
B
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
Relative Pressure (P/Po)
C
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
Fig. 2. N2 adsorption–desorption isotherm and pore size distribution curve (inset) of (A) Cu/KSF, (B) Cu/acidic KSF, and (C) Cu/Cs KSF.
Cu-Mn/Clay, Cu-Zn/Clay, Cu-Al/Clay, Cu-Fe/Clay, Cu-Sn/Clay and
Cu-Co/Clay could not enhance the product yield (Table 2, entries
copper oxides. The percentage of loading and oxidation state of Cu
and other metals (Ni, Mn, Zn, Al, Fe, Sn, Co) are included in Table 2.
Screening of various solvents for Cu/Clay-2a2 catalyzed hydrox-
ylation of phenylboronic acid reveal that the desired phenol is
formed excellently in water under atmospheric conditions, but the
reaction conducted in solvents like ethyl acetate, CH Cl , CHCl ,
7
–13) and Cu-clay catalyst proved comparatively better than other
bimetallic clay catalysts for this transformation (Table 2, entry 5).
Further we compared the catalytic activity of this catalyst with the
commercially available CuO, and Cu O for the model reaction under
2
2
2
3
the standard conditions; it was found that Cu-clay2a2 is much
more active as well as selective than those commercially available
THF, Toluene, hexane failed to produce the expected product even
after for 3 h (Table 3, entries 4, 6, 8–11). However, little conversions
Table 1
Textural properties of Cu/KSF, Cu/acidic KSF and Cu/Cs KSF.
2
Entry
Sample
Surface area (m /g)
Pore volume (cc/g)
Pore radius (Å)
1
2
3
Cu/KSF
Cu/acidic KSF
Cu/Cs KSF
54.42
15.78
08.39
0.156
0.062
0.042
19.75
19.51
19.38

6
4
B.A. Dar et al. / Applied Catalysis A: General 466 (2013) 60–67
Table 2
Effect of various catalytic conditions for ipso-hydroxylation reaction.
a
Entry
Catalyst
Metal loading (wt.%)b
Amount of catalyst (mg)
Time
Yield (%)e
Cuc
Xd
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
Cu/Clay-1
Cu/Clay-2
Cu/Clay-3
Cu/Clay-2a
Cu/Clay-2a
Cu/Clay-2b
Cu-Ni/Clay
Cu-Mn/Clay
Cu-Zn/Clay
Cu-Al/Clay
Cu-Fe/Clay
Cu-Sn/Clay
Cu-Co/Clay
Cu/Clay-2a1
Cu/Clay-2a2
Cu/Clay-2a2
Cu/Clay-2a2
Cu/Clay-2a2
Cu/Clay-2a2
CuO
10
10
10
5
15
20
8
8
8
8
8
0
0
0
0
0
0
2
2
2
2
2
2
2
0
0
0
0
0
0
–
–
–
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
15
10
8
6 h
6 h
6 h
6 h
6 h
6 h
6 h
6 h
6 h
6 h
6 h
6 h
56
61
64
47
66
66
56
53
57
48
56
55
47
98
98
98
93
84
98
31
27
86
1
1
1
1
1
1
1
1
1
1
2
2
2
8
8
6 h
15
15
15
15
15
15
10 min
20 min
20 min
20 min
20 min
20 min
6 h
13
13
13
13
Cu2O
Cu(OH)2
6 h
45 min
a
b
c
Reaction conditions: phenylboronic acid (1 mmol, 0.1220 g), H2O (1.5 ml), and reaction temperature: r.t, air.
Calculated metal loading.
Oxidation state + 2.
X = Ni, Mn, Zn, Al, Fe, Sn, Co.
Isolated yield.
d
e
Fig. 3. SEM images of (A) KSF and (B) Cu/KSF.
were obtained in solvents like dimethylsulfoxide (DMSO) and
dimethylformamide (DMF) (Table 3, entries 4, 6, 8–11), whereas
good conversions were observed in methanol and ethanol, but the
major compounds formed were homo coupled products (biphenyl)
along with traces of corresponding phenol (Table 3, entries 1 and 2).
Using acetonitrile as solvent gave very low conversion containing
a mixture phenol and biphenyl (Table 3, entry 3).
entries 2, 4, 5, 6, 8, 10, 12–14). The reaction was also compatible
with naphthylboronic acids (Table 4, entries 7 and 15) as well as
heteroarylboronic acid (Table 4, entry 16). More significantly, 4-
hydroxyphenylboronic acid (Table 4, entry 4) also proved to be
a good substrate, thus generating hydroquinone without loss in
Table 3
With optimized conditions in hand we further studied the sub-
strate scope of the present methodology. The method proved to be
compatible with a wide range of substrates and the correspond-
ing phenols were obtained in good to excellent yields (86–98%) in
very short reaction time. Phenylboronic acid (1a) was converted
into phenol (2a) in 98% yield within 20 min (Table 4, entry 1).
Electron-rich arylboronic acids were found to react slightly faster
than the electron-deficient ones, thus electron-rich phenols can be
easily generated using this heterogeneous catalytic system which
are otherwise difficult to obtain from the traditional nucleophilic
substitution of arylhalides. Arylboronic acid derivatives bear-
ing electron-neutral, electron-donating, and electron-withdrawing
substituents like F, Br, OMe, OH, NO , CF , CHO, COOH, etc. which
a
Effect of solvents for ipso-hydroxylation of arylboronic acid.
b
Entry
Solvent
Time
Yield (%)
1
2
3
4
5
6
7
8
9
Methanol
Ethanol
Acetonitrile
Ethylacetate
DMF
THF
DMSO
CH2Cl2
CHCl3
30 min
30 min
3 h
3 h
3 h
3 h
3 h
3 h
3 h
C
C
D
Traces
15
Traces
16
Traces
Traces
Traces
Traces
1
1
0
1
Hexane
Toluene
3 h
3 h
2
3
a
Reaction conditions: phenylboronic acid (1 mmol, 0.1220 g), Solvent (1.5 ml),
seem difficult to remain intact under previously reported basic
conditions [7a], were successfully converted to the corresponding
phenols and these functional groups are well tolerated (Table 4,
Reaction temperature: r.t, air.
b
Isolated yields after purification; C = biphenyl; D = the isolated product was mix-
ture of biphenyl, phenol and toluene and the reactant conversion was 31%.

B.A. Dar et al. / Applied Catalysis A: General 466 (2013) 60–67
65
Table 4
The ipso-hydroxylation reaction of various arylboronic acids over Cu(OH)x clay catalyst.
a
Yieldb (%)
98
TOF (min )c
−1
Entry
Substrate
Product
Time (min)
20
OH
OH
B
1
OH
1.84
OH
B
OH
OH
2
3
4
45
25
35
91
93
79
0.76
1.45
0.84
F
F
OH
B
OH
OH
OH
B
OH
OH
HO
O
HO
O
OH
B
OH
OH
OH
5
6
25
25
96
89
1.44
1.34
OH
B
OH
Br
Br
OH
B
OH
OH
7
30
93
1.17
OH
B
OH
OH
8
9
30
25
60
91
94
82
1.14
1.41
0.51
F3C
OH
F3C
OH
B
OH
OH
OH
B
OH
1
0
2
O N
O N
2
OH
OH
B
OH
1
1
2
30
50
88
71
1.10
0.53
OH
B
OH
OH
1
OHC
OHC

6
6
B.A. Dar et al. / Applied Catalysis A: General 466 (2013) 60–67
Table 4 (Continued)
−1
Entry
Substrate
Product
Time (min)
Yieldb (%)
76
TOF (min )c
OH
B
OH
OH
1
3
4
100
0.29
HOOC
HOOC
OH
OH
B
OH
NO2
1
100
40
72
93
0.27
0.87
O N
2
HO
OH
OH
B
1
1
5
6
HO
OH
OH
B
40
81
0.76
O
O
a
b
c
Reaction conditions: arylboronic acid (1 mmol); Cu(OH)x clay catalyst (13 mg); H2O (1.5 ml); reaction temperature: r.t; air.
Isolated yields after purification.
−1
TOF [min ] = [the number of moles of reactant converted/the number of moles of metal active sites]/time in minutes.
Table 5
Comparison of efficiency of the clay entrapped Cu(OH)x with some reported catalysts for the ipso-hydroxylation of arylboronic acids.
Entry
Catalyst
Conditions
Time
Yield (%)
Recyclability
Reference
2
3
4
5
6
7
CuSO4-phenanthroline
CuCl2 Brij S-100
[Ru(bpy)3Cl2]·6H2O
Pd(COD)Cl2
NH2OH
Clay entrapped Cu(OH)x
KOH, H2O, rt.
O2, H2O, rt.
Et3N, DMF, irradiation, air
CHCl3, Et3N, O2, rt.
Ethanol, NaOH, rt.
H2O, rt.
2 h
95
95
93
95
94
98
No
No
No
No
No
Yes
[11]
[12]
[13]
[14]
[17]
Present work
24 h
48 h
24 h
18 h
20 min
reaction efficiency. All the compounds so prepared were stable and
were characterized by NMR, and MS analysis. Further, catalytic
activity of the synthesized catalysts was measured in TOF/min,
which are included in Table 4.
Clay entrapped Cu(OH)x being a solid heterogeneous catalyst
could be easily recovered by simple filtration; thus the recyclability
of the catalyst was investigated for model reaction. The catalyst
was washed several times with ethanol followed by distilled water
before using for every next cycle. This experiment proved that the
catalyst has excellent recycling capability as no significant loss of
activity was observed even after 10 consecutive cycles (Fig. 6). The
2
p3/2
Satellite peaks
2
55
940
968
a
b
2
98
9
60
2
p1/2
3
18
9
48
5
24
4
40
d
c
c
d
b
a
935
940
945
950
955
960
965
970
100
200
300
400
500
600
Binding Energy/ eV
0
Temperature/ C
Fig. 5. XPS spectra of (a) Cu/KSF, (b) Cu/KSF after 10th cycle of reaction, (c) Cu/Cs
Fig. 4. H2-TPR spectra of (a) KSF, (b) Cu/acidic KSF, (c) Cu/Cs KSF, and (d) Cu/KSF.
KSF, and (d) Cu/acidic KSF.

B.A. Dar et al. / Applied Catalysis A: General 466 (2013) 60–67
67
References
[
[
[
1] J.H.P. Tyman, Synthetic and Natural Phenols, Elsevier, New York, 1996.
2] Z. Rappoport, The Chemistry of Phenols, Wiley-VCH, Weinheim, 2003.
3] A. Kotschy, G. Tima’Ri, Heterocycles from Transition Metal Catalysis, Springer,
Dordrecht, 2005.
[
[
[
[
4] C.A. Fyfe, in: S. Patai (Ed.), The Chemistry of the Hydroxyl Group. Part 1, vol. 1,
Wiley Interscience, New York, 1971.
5] T. Schulz, C. Torborg, B. Schaffner, J. Huang, A. Zapf, R. Kadyrov, A. Borner, M.
Beller, Angew. Chem. 48 (2009) 918–921.
6] A.G. Sergeev, T. Schulz, C. Torborg, A. Spannenberg, H. Neumann, M. Beller,
Angew. Chem. 48 (2009) 7595–7599.
7] D. Zhao, N. Wu, S. Zhang, P. Xi, X. Su, J. Lan, J. You, Angew. Chem. 48 (2009)
8729–8732.
[
[
8] A. Tlili, N. Xia, F. Monnier, M. Taillefer, Angew. Chem. 48 (2009) 8725–8728.
9] H. Yang, Y. Li, M. Jiang, J. Wang, H. Fu, Chem. Eur. J. 17 (2011) 5652–5660.
[
10] H. Tsunoyama, H. Sakurai, N. Ichikuni, Y. Negishi, T. Tsukuda, Langmuir 20
2004) 11293–11296.
(
[
[
11] J. Xu, X. Wang, C. Shao, D. Su, G. Cheng, Y. Hu, Org. Lett. 12 (2010) 1964–1967.
12] I. Kiyofumi, N. Kanako, Y. Misato, K. Yoshinori, Chem. Commun. 47 (2011)
11775–11777.
[
[
13] Y. Zou, J.R. Chen, X.P. Liu, L.Qiu Lu, R.L. Davis, K.A. Jorgensen, W.J. Xiao, Angew.
Chem. (2012) 784–788.
14] A.D. Chowdhury, S.M. Mobin, S. Mukherjee, S. Bhaduri, G.K. Lahiri, Eur. J. Inorg.
Chem. 21 (2011) 3232–3239.
Fig. 6. Recyclability of Cu(OH)x clay catalyst in ipsohydroxylation of phenylboronic
[15] K.S. Webb, D. Levy, Tetrahedron Lett. 36 (1995) 5117–5118.
[
[
[
16] C. Zhu, R. Wang, J.R. Falck, Org. Lett. 14 (2012) 3494–3496.
17] E. Kianmehr, M. Yahyaee, K. Tabatabai, Tetrahedron Lett. 48 (2007) 2713.
18] P. Kaewmati, E. Somsook, R.N. Dhital, H. Sakurai, Tetrahedron Lett. 53 (2012)
6104–6106.
acid.
ICP-AES analysis (Table 2) of the used catalyst after 10th consesec-
utive cycle showed negligible loss of active metal from the catalyst
[19] A. Gogoi, U. Bora, Syn. Lett. 23 (2012) 1079–1081.
[
[
[
[
[
[
20] G.K. Surya Prakash, S. Chacko, C. Panja, T.E. Thomas, L. Gurung, G. Rasul, T.
Mathew, G.A. Olah, Adv. Synth. Catal. 351 (2009) 1567–1574.
21] N. Mulakayala, K.M. Ismail Kumar, R.K. Rapolu, B. Kandagatla, P. Rao, S. Oruganti,
M. Pal, Tetrahedron Lett. 53 (2012) 6004–6007.
(
<0.2 wt.% from 14.07 wt.%), which attributed the stability of the
catalysts even after recycling. Further, the actual pathway of ipso-
hydroxylation reaction of aryl boronic acids may be elucidated in
due course [11]. To evaluate the heterogeneity of catalyst, we per-
formed filtration test on model reaction. After 7 min (almost 50%
conversion (GC) the catalyst was filtered off and the filtrate was
allowed to react further, but no further reaction was observed even
after 6 h (GC) suggesting that the copper catalyst remains on the
support during the reaction.
Some of the recently reported catalyst systems and their com-
parison with that clay entrapped Cu(OH)x is illustrated in Table 5. It
is clear that clay entrapped Cu(OH)x is more efficient and beneficial
catalyst than most of the reported catalysts.
22] J. Simon, S. Salzbrunn, G.K. Surya Prakash, N.A. Petasis, G.A. Olah, J. Org. Chem.
66 (2001) 633.
23] K. Hosoi, Y. Kuriyama, S. Inagi, T. Fuchigami, Chem. Commun. 46 (2010)
1284–1286.
24] Y.Q. Zou, J.R. Chen, X.P. Liu, L.Q. Lu, R.L. Davis, K.A. Jørgensen, W.J. Xiao, Angew.
Chem. 51 (2012) 784–788.
25] M.J. Climent, A. Corma, S. Iborra, RSC Adv. 2 (2012) 16–58.
[26] K. Wilson, J.H. Clark, Pure Appl. Chem. 72 (2000) 1313–1319.
[
[
27] A.C.K. Yip, F.L.Y. Lamb, X. Hu, Chem. Commun. 25 (2005) 3218–3220.
28] B.A. Dar, A.K. Sahu, P. Patidar, P.R. Sharma, D. Vyas, S. Maity, M. Sharma, B. Singh,
J. Ind. Eng. Chem. 19 (2013) 407–412.
[29] B.A. Dar, A. Singh, A.K. Sahu, P. Patidar, A. Chakraborty, M. Sharma, B. Singh,
Tetrahedron Lett. 53 (2012) 5497–5502.
[
30] B.A. Dar, M. Sharma, B. Singh, Bull. Chem. Reac. Eng. Catal. 7 (2012) 79–84.
31] D.F. Bishop, G. Stern, M. Fleischman, L.S. Marshall, Ind. Eng. Chem. Proc. Des.
Dev. 7 (1968) 110–117.
[
4
. Conclusion
[
[
32] K.V.R. Chary, G.V. Sagar, C.S. Srikanth, V.V. Rao, J. Phy. Chem. B 111 (2007)
543–550.
In conclusion, we have described an easy, efficient, green and
33] R.A. Alvarez-Puebla, C. Aisa, J. Blasco, J.C. Echeverrı’a, B. Mosquera, J. Garrido, J.
Appl. Clay Sci. 25 (2004) 103–110.
general method for the preparation of phenols by hydroxylation
of arylboronic acids using the base free heterogeneous copper-
catalyst in water medium at room temperature. The method
involves a cheap catalyst, mild conditions, short reaction time,
and high yields. The catalyst is ligand-free, leaching-free, easy to
synthesize, environmentally friendly and recyclable several times
without significant loss of activity. So this methodology is a practi-
cal process to develop phenols from arylboronic acids at low cost.
[34] C. Deng, H. Hu, X. Ge, C. Han, D. Zhao, G. Shao, Ultra. Sono. 18 (2011)
32–937.
35] N. Wongpisutpaisan, P. Charoonsuk, N. Vittayakorn, W. Pecharapa, Energy Proc.
(2011) 404–413.
9
[
9
[36] J. Barrault, C. Bouchoule, K. Echachoni, N. Frini, S.M. Trabelsi, F. Bergaya, Appl.
Catal. B: Environ. 15 (1998) 269–274.
[
[
[39] R.A. Alvarez-Puebla, C. Aisa, J. Blascob, J.C. Echeverrı’a, B. Mosquer, J.J. Garrido,
Appl. Clay Sci. 25 (2004) 103–110.
37] J.D. Jeffers, R.C. Reynolds, Clays Clay Miner. V 35 (1987) 473–476.
38] M. Eloussaief, I. Jarraya, M. Benzina, Appl. Clay Sci. 46 (2009) 409–413.
[
[
[
40] T. Theodoros, C. Jean-FranAois, M. Enrico, J. Lubos, R. Petra, G. Dimitrios, Chem.
Eur. J. 18 (2012) 9305–9311.
41] E.N. Muhamad, R. Irmawati, A.H. Abdullah1, Y.H. Taufiq-Yap1, S.B. Abdul Hamid,
Malaysian J. Anal. Sci. 11 (2007) 294–301.
Acknowledgements
We are grateful to the Director of Indian Institute of Integrated
Medicine (IIIM), Canal Road, Jammu for providing necessary facili-
ties and good environment to carry out the research work. We are
also thankful to CSIR-DELHI for the financial support.
42] R. Ben Achma, A. Ghorbel, A. Dafinov, F. Medina, Appl. Catal. A: Gen. 349 (2008)
20–28.
[43] J.U.K. Oubagaranadin, Z.V.P. Murthy, Appl. Clay Sci. 50 (2010) 409–413.
[
[
44] T. Mishra, P. Mohapatra, K.M. Parida, Appl. Catal. B: Environ. 79 (2008) 279–285.
45] R. Ben Achma, A. Ghorbel, S. Sayadi, A. Dafinov, F. Medina, J. Phys. Chem. Solids
69 (2008) 1116–1120.
Appendix A. Supplementary data
[46] M. Fujiwara, T. Matsushita, S. Ikeda, Anal. Sci. V 9 (1993) 289–291.
[
[
[
47] S. Navalon, M. Alvaro, H. Garcia, Appl. Catal. B 99 (2010) 1–26.
48] R.S. Varma, Tetrahedron 58 (2002) 1235–1255.
49] D. Amarajothi, V. Pitchai, T.P. Vairaperumal Kasi, Catal. Comm. 16 (2011) 15–19.
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.2013.
[50] C.N. Sisik, L.J. Hope-Weeks, J. Mater. Chem. 18 (2008) 2607–2610.
51] L. Troisi, C. Granito, F. Rosato, V. Videtta, Tetrahedron Lett. 51 (2010) 371–373.
[
06.051.