Clay encapsulated Cu(OH)x promoted homocoupling of arylboronic acids: An efficient and eco-friendly protocol

Article abstract of DOI:10.1016/j.apcata.2013.10.048

Cu(OH)_x has been encapsulated over montmorillonite-KSF by simple ologomeric deposition strategy. The resulting catalyst has been employed for selective homocoupling of arylboronic acids under ambient conditions without requirement of any ligand or base. This catalyst is easy to recover and shows excellent reusability without losing its activity. Techniques like XRD, SEM, TPR, IR, BET surface area measurement and XPS were used to characterize the catalyst. The present method promises for the simple and clean homocoupling of arylboronic acids.

Full text of DOI:10.1016/j.apcata.2013.10.048

Applied Catalysis A: General 470 (2014) 232–238
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Clay encapsulated Cu(OH)x promoted homocoupling of arylboronic
acids: An efficient and eco-friendly protocol
Bashir Ahmad Dara, Snehil Singha, Nalini Pandeya, A.P. Singhb, Priti Sharmab,
Anish Lazarb, Meena Sharmac, Ram A. Vishwakarmaa,∗, Baldev Singha,∗
a Indian Institute of Integrative Medicine (CSIR), Canal Road, Jammu 180001, India
b Catalysis division, National Chemical Laboratory, Pune, MH 411008, India
c Department of Chemistry, University of Jammu, Jammu, JK 180004, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 July 2013
Received in revised form 24 October 2013
Accepted 26 October 2013
Available online 8 November 2013
Cu(OH)x has been encapsulated over montmorillonite-KSF by simple ologomeric deposition strategy.
The resulting catalyst has been employed for selective homocoupling of arylboronic acids under ambient
conditions without requirement of any ligand or base. This catalyst is easy to recover and shows excellent
reusability without losing its activity. Techniques like XRD, SEM, TPR, IR, BET surface area measurement
and XPS were used to characterize the catalyst. The present method promises for the simple and clean
homocoupling of arylboronic acids.
Keywords:
Biaryls
© 2013 Elsevier B.V. All rights reserved.
Homocoupling
Heterogenous catalyst
Copper
Clay
1. Introduction
loss of expensive metals due to difficult recovery of homogeneous
metal catalysts, chances of undesired product formation due to
The immense scientific and commercial value of symmetrical
biaryl motiffs is illustrated by their prevalence as building blocks
in natural products, functional molecules, polymers and ligands
for catalysts etc. [1]. The structural simplicity of these biaryl com-
pounds contradicts their preparative complexity. Thus search for
efficient and convergent syntheses has captivated the attention of
synthetic chemists for many decades. Oxidative homocoupling of
arylboronic acids has revolutionized our aptitude to generate sym-
metrical biaryls in a straightforward manner because of their ready
availability, nontoxic nature & bench stability. Different catalyst
systems like Ag2O + CrCl2 [2], PdCl2 [3], Pd(OAc)2 [4], RhCl (PPh3)3
[5], CuCl [6], Pd(OAc)2,/AgNO3 [7], Pd(PPh3)2Cl2 [8], Cu(OAc)2 [9],
{(1,10-phenanthroline)Cu(␮-OH)}2Cl2]·3H2O [10], AuCl [11] etc.
have been reported to catalyze the homocoupling of aryl boronic
acids. Although, excellent yields are obtained from these known
catalytic systems, several disadvantages like longer reaction times,
high temperature, use of homogeneous transition metal cata-
lyst systems, stoichiometric amounts of ligand/base, could not be
avoided in these approaches [12]. Moreover metal contamination,
requirement of ligands and additional reagents for homogeneous
catalysts demands for the development of more efficient, economic
and environmentally acceptable process.
In continuation to our research work for the development of
green chemical protocols, [13–15] we herein report clay encap-
sulated Cu(OH)x as heterogeneous catalytic system for oxidative
homocoupling of arylboronic acids at room temperature and under
atmospheric condition without using base, ligands or any other
additives.
Clays are aluminosilicates possessing lamellar structure with
˚
interlayer spacing ranging from 10 to 100 A, thus enabling the inter-
calation of various ions or neutral species, thus can act as supports
for a large variety of reagents. They can be considered as microreac-
tors because of possessing the ability to concentrate high quantities
of reactive species between the aluminosilicate layers [13–16].
Clays and clay supported catalysts have many advantages, such
as easy handling, easy separation, recycling, environmentally safe
disposal, inexpensiveness, improved efficiency due to stable active
site and better steric control of a reaction intermediate. Encapsula-
tion of Cu(OH)x species inside the clay interlayer preserves them in
nanosize and restrains their tendency to undergo agglomeration,
which otherwise increases the particle size, hence reduces the cat-
alytic activity [17]. Comparatively high stability of a catalyst on
solid support sometimes allows the reaction to be less sensitive to
∗
Corresponding author. Tel.: +191 2572002; fax: +191 2548607.
E-mail addresses: ram@iiim.ac.in (R.A. Vishwakarma), drbaldev1@gmail.com
(B. Singh).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.10.048

B.A. Dar et al. / Applied Catalysis A: General 470 (2014) 232–238
233
normal ambient conditions. Therefore, clay supported catalyst as a
substitute of homogeneous catalyst or an expensive heterogeneous
catalyst seems highly desirable [18].
2. Experimental
KSF
2.1. Preparation of the catalyst
Cu/Clay 2b
Cu/Clay 2c
Clay encapsulated Cu(OH)x was prepared by suspending
montmorillonite-KSF (10 g) with cation exchange capacity of 120
meq/100 g clay in 200 ml distilled water and the suspension was
vigorously stirred at 80 ◦C for 2 h. Copper oligomer (base hydrol-
ysed cupric chloride with OH/Cu molar ratio of 2.0) was added drop
wise to acquire the required wt% loading of copper and the result-
ing slurry was stirred at 90 ◦C for 8 h. Heating causes expansion of
clay interlayer and makes the intercalation of Cu-oligomer easy.
The solid products were filtered, washed several times with dis-
tilled 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 ranging from 200 to
425 ◦C. The catalyst was characterised by powder X-ray diffraction
using a D-8 ADVANCE (Bruker AXS, Germany) X-ray diffractome-
ter 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 were identified by search match procedure
with the help of DIFFRACPLUS software using JCPDS databank. Tem-
perature 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 Accelerating Voltage 40.0 kV. IR spectra
were recorded on Perkin-Elmer IR spectrophotometer. The spe-
cific surface areas (m2/g) of the catalyst was estimated with the
N2 adsorption and desorption determined at −196 ◦C by means of
an automated CHEMBET-3000 adsorption apparatus. XPS analysis
was performed on a KRATOS-AXIS 165 instrument.
Cu/Clay 2d
5
10 15
20 25 30 35 40 45
50 55 60
2 Theta/ Degree
Fig. 1. XRD spectra of (a) Montmorillontie-KSF (b) Cu/clay-2b; (c) Cu/clay-2c; (d)
Cu/clay-2d.
26.7◦, 50.3◦ and 60◦ are due to reflection of the quartz [SiO2]
impurities [19–21] and at 12.5◦, 19.9◦ and 27.9◦ are due to paly-
gorskite [22–24]. Peaks for montmorillonite appear at 2␪ = 8.9◦ (d
0 0 1 reflection), 19.8◦, 32.2◦, and 62◦ [25,26]. Presence of kaolin-
ite is implied t◦ small peaks at 38.8◦, and 42.5◦ [25,26]. The copper
loaded on montmorillonite-KSF does not show any characteristic
peak of CuO, Cu2O and metallic Cu but peaks with very low intensity
corresponding to mineral atacamite (Cu2Cl(OH)3) and Cu(OH)2 are
observed at 2ꢀ = 16.1, 18◦, 25.1◦, 32.3◦ and 50.3◦ [27–29]. Thus, it
can be concluded from XRD analysis that copper hydroxide species
are deposited with very low crystallinity.
The N2 adsorption–desorption isotherm and pore size distri-
bution of Cu(OH)x-clay calcined at 250 ◦C (Cu/clay-2b) are shown
in Fig. 2. The sample displayed type-IV isotherms with H1 hys-
teresis related to capillary condensation steps. Textural properties
such as BET surface area, pore volume (BJH) and the pore radius
(BJH) of Cu(OH)x-clay(Cu/clay-2b) are summarized in Table 1. The
2.2. General experimental procedure for oxidative homocoupling
of arylboronic acid
To a solution of arylboronic acid (1 mmol) in methanol (0.5 ml),
clay encapsulated Cu(OH)x (6 mg) was added and this heteroge-
neous mixture was vigorously stirred at ambient temperature for
30–120 min. After completion of reaction (monitored by TLC), the
reaction mixture was filtered to separate the catalyst. Solvent of
the filtrate was removed under reduced pressure and then worked
up in hexane:water (1:1) system. The aqueous phase was iso-
lated and back extracted with Hexane. The combined organic layer
was dried over MgSO4. The solvent was removed under reduced
pressure and the residue was purified by silica gel column chro-
matography to afford the desired product. The prepared products
were characterized NMR and mass spectral analysis. The spec-
tral data and physical properties thus obtained were compared
with data reported in literature [references herein]. NMR spectra
were recorded on Brucker-Advance DPX FT-NMR 400 MHz instru-
ment. 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. IR spectra were recorded on
Perkin-Elmer IR spectrophotometer.
3. Results and discussion
3.1. Characterization of the catalyst
The XRD spectra of KSF (montmorillonite-KSF), Cu/clay-2b,
Fig. 2. N2 adsorption–desorption isotherm and pore size distribution curve (inset)
of Cu/clay-2b
Cu/clay-2c and Cu/clay-2d is shown in Fig. 1. Peaks at 2ꢀ = 21◦,

234
B.A. Dar et al. / Applied Catalysis A: General 470 (2014) 232–238
Fig. 3. SEM images of (a) Montmorillonite-KSF (b) Cu/clay-2b.
Table 1
2p3/2
940
Satellite peaks
960
Textural properties of Cu/KSF.
a
b
968
Entry Sample Surface area [m2/g] Pore volume [cm3/g] Pore radius [Å]
Cu/KSF 54.42 0.156 19.75
2p1/2
948
1
Cu(OH)x-clay shows BET surface area, 54.42 m2/g; pore volume,
3
˚
0.156 cm /g; and pore radius, 19.75 A. 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-2b) 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 interaction of these metal
species with the support. The H2-TPR spectra of (a) KSF (b) Cu/clay-
2b are depicted in Fig. 4. The Montmorillonite-KSF support (Fig. 4a)
showed a broad peak at a higher reduction temperature, 524 ◦C,
which can probably be attributed to the reduction of some iron
species in the montmorillonite-KSF material [30,31]. The first peak
in Fig. 4d, centered at 255 ◦C, was assigned to the reduction of Cu2+
c
d
935
940
945
950
955
960
965
970
Binding Energy/ eV
Fig. 5. XPS spectra of (a) Cu/clay-2b (b) Cu/clay-2b after 10th cycle of reaction.
nanoparticles located on the surface probably at the entrance of
pores of the Montmorillonite-KSF. Moreover, the reduction peaks,
centered at 300 ◦C and 318 ◦C, were assigned to the reduction of
clay intercalated Cu2+ species to Cu+ and CuO cluster to Cu0, respec-
tively. Further, the reduction peak, centered at 440 ◦C is very less
pronounced, was assigned to the reduction of Cu+ to Cu0 [31,32].
The results indicate that the Cu2+ ions are mostly encapsulated in
the interlamellar space of Montmorillonite-KSF.
255
298
318
524
XPS is a surface technique which provides valuable information
about the oxidation state and chemical environment of atoms due
to the shift in binding energies. The XPS spectra of (a) fresh (Cu/clay-
2b) (b) Cu/clay-2b after 10th cycle of reaction are represented in
Fig. 5. Two main peaks having binding energies of 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 [33].
440
d
c
b
a
3.2. Homocoupling of arylboronic acids
Previously we studied the copper catalyzed reactions of aryl-
boronic acids and during these studies it was observed that
homo-coupling of arylboronic acids occur exclusively [34]. At the
onset of this methodology phenylboronic acid was chosen as model
substrate to optimize the reaction conditions and all the reac-
tions were conducted at room temperature. Cu-clay catalysts with
100
200
300
400
500
600
Temperature/0C
Fig. 4. H2-TPR spectra of (a) Montmorillonite-KSF (b) Cu/clay-2b.

B.A. Dar et al. / Applied Catalysis A: General 470 (2014) 232–238
235
Table 2
Effect of various catalytic conditions for homocoupling of arylboronic acidsa.
Entry
Catalyst
Metal loading (wt%)b
Amount of catalyst (mg)
Time
Yield (%)c
1
2
3
4
5
6
7
8
Cu/clay-1
Cu/clay-2d
Cu/clay-3
Cu/clay-2a
Cu/clay-2a1
Cu/clay-2b
Cu/clay-2b
Cu/clay-2b
Cu/clay-2b
Cu/clay-2b
Cu/clay-2c
CuO
Cu2O
Cu(OH)2
CuSO4·5H2O
Cu(NO3)2·3H2O
Cu(Ac)2·H2O
CuCl
10
15
20
15
15
15
15
15
15
15
15
–
–
–
–
–
–
–
–
–
20
20
20
20
20
20
15
10
5
6 h
6 h
6 h
32
48
47
10 min
20 min
20 min
20 min
20 min
20 min
20 min
100 min
6 h
69d
87d
98
98
98
96
98
51
31
27
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
6
15
15
15
15
15
15
15
15
15
15
15
15
15
6 h
45 min
45 min
45 min
45 min
45 min
45 min
45 min
45 min
3 h
62d
Traces
41
34
56
Traces
12
7
CuCl2·2H2O
CuI
CuCN
–
–
–
Ni/clay
Traces
19
Cu-Ni/claye
3 h
a
Reaction conditions: phenylboronic acid (1 mmol), Methanol (0.5 ml), Reaction temperature: r.t, air.
Calculated metal loading.
Isolated yield.
b
c
d
e
conversion > 99%, mixture of ipso-hydroxylation and homocoupling products.
catalyst contains Cu = 8 wt% and Cu = 2 wt% and is calcined at 425 ◦C.
different copper loadings and each calcined at 425 ◦C for 3 h were
screened to get optimum metal loading. Cu loading was found
to have significant effect on the reaction efficiency and 15 wt% of
copper on montmorillonite-KSF (Cu/clay-2d) was found to be the
optimum loading (Table 2, entry 2). Decreasing the metal load-
ing to 10 wt% (Cu/clay-1) decreases the reaction rate (Table 2,
entry 1), whereas no improvement in the conversion or yield was
perceived upon increasing the copper loading to 20 wt% (Cu/clay-
3) (Table 2, entry 3). Thus a fresh lot of Cu/clay catalysts with
15 wt% copper loading on montmorillonite-KSF was prepared and
calcined at different temperatures to study the effect of calcina-
tions temperature on the activity of the catalyst. We observed that
the uncalcined form of the catalyst (Cu/clay-2a) provides excellent
conversion but with decreased selectivity and ipso-hydroxylation
occurs as a significant side reaction. Moreover bulk leaching of
copper and massive loss of catalytic activity renders this cata-
lyst as non-heterogeneous (Table 2, entry 4). The catalyst calcined
at 200 ◦C (Cu/clay-2a1) was found to furnish the desired product
with increased selectivity and the copper leaching in this catalyst
was stopped to a great extent (Table 2, entry 5). Using the cata-
lyst calcined at 300 ◦C (Cu/clay-2c) the leaching of copper metal
from the catalyst was almost completely stopped, but the activ-
ity of the catalyst was dropped to a great extent (Table 2, entry
11). The catalyst calcined at 250 ◦C (Cu/clay-2b) exhibits excel-
lent catalytic activity without copper leaching (Table 2, entry 6)
and copper species like Cu(OH)2 and Cu2Cl(OH)3 are reported to
remain stable at this temperature [35]. Optimization of the cat-
alyst amount revealed that 6 mg is sufficient to catalyze model
reaction expeditiously (Table 2, entry 10). When Cu/clay-2b was
replaced by commercially available CuO (Table 2, entry 12), and
Cu2O (Table 2, entry 13) for the model reaction under the standard
conditions, it was observed that Cu/clay-2b is much more active
as well as selective than these commercially available copper
oxides. Various copper salts like CuSO4·5H2O, Cu(NO3)2·3H2O,
Cu(Ac)2·H2O, CuCl, CuCl2·2H2O, CuI and CuCN were also com-
pared with Cu/clay-2b for phenylboronic acid homocoupling under
the optimized conditions but only Cu(NO3)2·3H2O (Table 2, entry
16), CuCl (Table 2, entry 18) and Cu(Ac)2·H2O were found to
Table 3
Effect of solvents for ipso-hydroxylation of arylboronic acida.
Entry
Solvent
Time
Yieldb(%)
1
2
3
4
5
6
7
8
Methanol
Ethanol
Acetonitrile
Ethylacetate
DMF
30 min
100 min
3 h
3 h
3 h
98
41
Traces
3
Traces
Traces
11
8
7
12
13
THF
3 h
DMSO
3 h
CH2Cl2
3 h
9
CHCl3
3 h
10
11
12
Hexane
Toluene
Water
3 h
3 h
30 min
Traces
Mixture of biphenyl, phenol and toluene and the reactant conversion = 31%.
a
Reaction conditions: phenylboronic acid (1 mmol), Solvent (0.5 ml), Reaction
temperature: r.t, air.
b
Isolated yields after purification.
catalyze the reaction to some extent, all other salts were very
sluggish.
Screening of various solvents for Cu/clay-2b catalyzed homo-
coupling of phenylboronic acid reveal that the desired product
is formed excellently (98% yield within 30 min) in methanol sol-
vent under atmospheric conditions, but the reaction conducted
in solvents like acetonitrile, dimethylformamide (DMF), THF and
water failed to produce the expected product under these con-
ditions and only traces were obtained even after for 6 h (Table 3,
entries 3, 5, 6, 12). Small conversions were obtained in solvents
like ethyl acetate, dimethylsulfoxide (DMSO), CH2Cl2, CHCl3, hex-
ane and Toluene (Table 3, entries 4, 7–11), when methanol was
replaced with ethanol the product yield was decreased to a great
extent and only 41% of the desired product was obtained in
a reaction time of 100 min, but the major compounds formed
in water was corresponding phenol (Table 3, entry 12). Using
acetonitrile as solvent furnished very low conversions contain-
ing a mixture phenol, biphenyl and toluene (Table 3, entry
3).

236
B.A. Dar et al. / Applied Catalysis A: General 470 (2014) 232–238
Table 4
The homocoupling of various arylboronic acids using Cu(OH)x-clay catalysta.
Entry
Substrate
Product
Time (min)
30
Yieldb (%)
98
1
2
3
30
35
98
93
4
5
35
30
92
95
6
7
30
30
90
94
8
35
90
88
61
9
30
10
100
11
60
77

B.A. Dar et al. / Applied Catalysis A: General 470 (2014) 232–238
237
Table 4 (Continued)
Entry
Substrate
Product
Time (min)
30
Yieldb (%)
12
13
14
96
81
92
40
80
15
16
100
100
63
77
17
60
41
18
19
60
67
60
60
89
20
Traces
a
Reaction conditions: arylboronic acid (1 mmol), catalyst (6 mg%) solvent (0.5 ml) at room temperature, under aerobic conditions.
Isolated yields.
b
In order to explore the generality and scope of the present
were successfully converted to the corresponding biphenyls and
these functional groups are well tolerated. To be honest, the sub-
strates containing –COOH groups did not show homocoupling
reaction properly; instead some unknown byproducts were formed
(Table 4, entry 20). The reaction was also compatible with naphthyl-
boronic acids (Table 4, entry 12) as well as heteroarylboronic acids
(Table 4, entries 3, 13–16, 19). Thus the tolerance of different func-
tional groups to these reaction conditions illustrates the flexibility
and generality of clay-encapsulated Cu(OH)x catalyst. All the com-
pounds so prepared were stable and were characterized by NMR
and MS analysis and compared with reported data.
protocol, homocoupling of various arylboronic acids was studied
in the presence of Cu(OH)x-clay catalyst. The method proved to
be compatible with a wide range of substrates and the desired
products were obtained in good to excellent yields (41–98%) in very
short reaction times. Phenylboronic acid was converted into cor-
responding biphenyl in 98% yield within 30 min. (Table 4, entry
1). Electron–rich arylboronic acids were found to react slightly
faster than the electron-deficient ones, thus electron-rich bipenyls
can be easily generated using this heterogeneous catalytic system.
Arylboronic acid derivatives bearing electron-neutral, electron-
donating, and electron-withdrawing substituents, which seem dif-
ficult to remain intact under previously reported basic conditions,
Clay encapsulated Cu(OH)x being a solid heterogeneous catalyst
could be easily recovered by simple filtration; thus the recyclability

238
B.A. Dar et al. / Applied Catalysis A: General 470 (2014) 232–238
Acknowledgment
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.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apcata.
2013.10.048.
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Fig. 6. Recyclability of Cu(OH)x clay catalyst in homocoupling of arylboronic acid.
Scheme 1. Homocoupling of arylboronic acids.
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4. Conclusions
In conclusion, we have developed a very active, selective and
stable heterogeneous and inexpensive Cu(OH)x-clay catalyst for the
homocoupling of arylboronic acids. The catalyst is easy to prepare
and provides high yields of desired products under ligand free, base
free and mild reaction conditions in very short reaction time. The
catalyst is leaching-free, environmentally friendly and recyclable
several times without significant loss of activity. This catalyst was
found to be much more efficient than other commercially avail-
able catalysts based on copper. Overall the present protocol is fast,
practical, interesting and inexpensive for arylboronic acids homo-
coupling. These features make this methodology economical and
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