In water homocoupling of arylboronic acids using nano-rod shaped and reusable copper oxide(II) catalyst at room temperature

Article abstract of DOI:10.1016/j.tetlet.2015.11.004

The work describes a simple in situ soft chemical synthesis of rod shaped nano CuO, characterization and study of its catalytic performance in the aerobic homocoupling of arylboronic acids to synthesize symmetrical biphenyls. The catalyst is simple to prepare, environmentally benign, efficient, easy recovery, reusable, stable and heterogeneous in nature.

Full text of DOI:10.1016/j.tetlet.2015.11.004

Tetrahedron Letters xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
In water homocoupling of arylboronic acids using nano-rod shaped
and reusable copper oxide(II) catalyst at room temperature
Prasanta Kumar Raul , Abhijit Mahanta , Utpal Bora , Ashim Jyoti Thakur , Vijay Veer b
b
a
a
a,
⇑
a
Department of Chemical Sciences, Tezpur University, Napaam 784028, Assam, India
Department of Chemistry, Defence Research Laboratory (DRL), Tezpur, Post Bag No. 2, 784001 Assam, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The work describes a simple in situ soft chemical synthesis of rod shaped nano CuO, characterization and
study of its catalytic performance in the aerobic homocoupling of arylboronic acids to synthesize sym-
metrical biphenyls. The catalyst is simple to prepare, environmentally benign, efficient, easy recovery,
reusable, stable and heterogeneous in nature.
Received 16 September 2015
Revised 20 October 2015
Accepted 1 November 2015
Available online xxxx
Ó 2015 Published by Elsevier Ltd.
Keywords:
Biphenyl
Homocoupling
Nano-rod CuO
Reusability
Boronic acid
Introduction
acids.10b,18a–f,19,20 However, there are some limitations in these cat-
alytic methods, for example, (1) Palladium is expensive and addi-
2
1
One of the important discoveries in the domain of synthetic
organic chemistry is the coupling reactions for the formation of
carbon–carbon bonds. Specifically, the coupling products, symmet-
rical and unsymmetrical biaryls are in huge demand in pharmaceu-
tional ligands are usually required, (2) certain oxidants are
2
2
employed to regenerate the catalyst, (3) addition of a strong base
1
0b,23a–c
is required to achieve high yields of biaryls
and (4) require-
ment of high temperature to complete the reaction.24 Therefore,
there is a need and scope for improvement. Studies have shown
1
–7
tical, natural product synthesis and fine chemical industries
as
2
5
26
27
12,28a–c
they are found to exhibit a variety of physical and chemical prop-
that metals like Au, Rh, Ni and Cu
could also catalyze
erties8 with versatile applications as dyes, agrochemicals, semi-
the homocoupling of arylboronic acids. But, these metal catalysts
have some drawbacks such as requiring longer reaction time,
expensive, lack of reusability and environmentally not friendly.
As per reports, Au and Rh are expensive and some additives are
9
10
conductor and optically active ligands and drugs.
Now-a-days, developing environmental friendly methodologies
for the preparation of biaryls is one of the most important and
interesting topics. Recently, aerobic homocoupling of phenyl-
boronic acid has attracted much attention among researchers.
Generally, synthesis of symmetrical and unsymmetrical biaryls
has been achieved by transition-metal-catalyzed couplings such
2
5,26
necessary for catalysis.
solvent in Ni catalytic system with heating required.
reported that the homocoupling of arylboronic acids could be car-
ried out in DMF in the presence of Cu(OAc) without any additive
Pungent pyridine is generally used as
1
1–14
27
It is
2
as the Suzuki reaction,1 modified Ullmann reaction, Kumada–
Corriu–Tamao reaction,17 etc. Recently, many researchers have
reported oxidative homocoupling of arylboronic acids as an excel-
lent method to obtain symmetrical biaryls as arylboronic acids pre-
pared from the corresponding aryl halides are more stable and less
toxic than many other organometallic reagents. Palladium-based
catalysts are frequently used for the homocoupling of arylboronic
5
16
such as a ligand. However, this reaction requires harsh conditions
with high temperature (100 °C). Kirai and Yamamoto reported that
the homocoupling of arylboronic acids could be catalyzed by a
copper complex containing 1,10-phenanthroline as a ligand.2
Pitchumani and co-workers have reported that copper terephthalate
metal–organic frameworks could be used to mediate the homocou-
28
8b
2
9
pling of arylboronic acids.
It has been established that nanoparticles (NPs) could be
successfully used as catalysts to enhance the speed of a reaction
in order to achieve more yields and less bi-products.30 We have
⇑
Corresponding author.
E-mail addresses: ubora@tezu.ernet.in (U. Bora), ashim@tezu.ernet.in
2 3
reported earlier that basic nano Al O catalyst could be used for
(
A.J. Thakur).
http://dx.doi.org/10.1016/j.tetlet.2015.11.004
040-4039/Ó 2015 Published by Elsevier Ltd.
0
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2
P. K. Raul et al. / Tetrahedron Letters xxx (2015) xxx–xxx
the N-formylation reaction under NOSE approach.31 These applica-
tions seem to be motivated primarily by the intention to enhance
the catalytic efficiency through large surface areas of the NPs.
However, a more intriguing aspect of the NP catalysis arises from
their size-specific electronic and geometric structures. Till now,
to the best of our knowledge, there is no report of use of CuO
nanocatalyst to enhance the synthesis of biphenyls. In this Letter,
we report an easy and efficient heterogeneous nano CuO catalyzed
homocoupling reaction of arylboronic acids for the synthesis of
symmetrical biaryls under mild conditions without the need of
any additives.
Table 2
a
Optimization of reaction conditions for the catalyst
CuO nano, base(0.5 eqv.)
H2O(2mL), rt
B(OH)2
Entry Catalyst Catalyst
mol %)
Solvent Base
(0.5 equiv)
Time
(h)
Yieldb
(%)
(
1
2
3
4
5
6
7
8
9
1
CuO
CuO
CuO
CuO
CuO
CuO
CuO
CuO
CuO
CuO
20
17
15
12
10
9
8
7
5
3
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
O
O
O
O
O
O
O
O
O
O
K
K
K
K
2
2
2
2
CO
CO
CO
CO
3
3
3
3
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
96
96
95
95
95
89
81
75
47
23
2 3
K CO
Results and discussion
K
K
K
K
K
2
2
2
2
2
CO
CO
CO
CO
CO
3
3
3
3
3
In all the cases, it is found that nano CuO is an efficient and
environmentally-friendly heterogeneous, reusable catalyst. Opti-
mization of reaction conditions was carried out taking the same
amount of the catalyst. Initially, phenylboronic acid was selected
as a model substrate with variation of solvent as well as base with
the aim of optimizing the yield and the results are summarized in
Table 1. All reactions were conducted at room temperature (rt)
under aerobic condition. Very less amount of product was found
in the absence of a base (Table 1, entries 1–3).
The reaction was performed with different alkaline bases in
methanol solvent at rt and yield of the biphenyl was found to
2 3 2 3
increase in case of K CO base. Keeping K CO fixed, different sol-
vents were screened and the best result was obtained in case of
water. Again, the reaction was subjected to time variation in pres-
0
a
Reaction conditions: phenyl boronic acid (1 mmol), solvent (2 mL), base
0.5 equiv).
Isolated yield.
(
b
shown in Figs. S2 and S3, respectively, (ESI). It has been observed
that nano CuO is well synthesized in rod shape. The efficiency of
CuO nano-rod in homocoupling of arylboronic acids might be
due to higher surface area compared to its bulk counterpart. It
was also observed that bulk CuO could transform arylboronic acid
to biphenyl with poor yields (Table 3 and Fig. 1). Even when bulk
CuO was used 10 times more than nano CuO, the biphenyl yield
was poor. Having defined the optimized reaction conditions, we
investigated the scope of the nano CuO catalyzed homocoupling
reaction with respect to different boronic acids. As evident from
Table 3, most of the phenylboronic acids with a variety of sub-
stituents afforded the products in good to excellent yields under
the optimum reaction conditions.
Phenylboronic acids bearing electron-withdrawing fluoro,
difluoro and formyl groups provided corresponding homocoupling
products in excellent yields (92–96%; Table 4, entries 8–12). We
also investigated the homocoupling reaction of aromatic boronic
acids bearing electron-donating groups. Phenylboronic acids
bearing methyl, ethyl, tert-butyl, methoxy etc. groups provided
ence of K
6% product was formed in presence of K
rt in water within 1.5 h (entry 12).
2
CO
3
and water in order to get optimization reaction time.
9
2
CO , and CuO catalyst at
3
The biphenyl yield was also found to increase rapidly with an
increase in the amount of the catalyst. We have taken CuO catalyst
in mmol % and it was found that as the amount of catalyst was
increased to 10 mmol %, biphenyl was formed in highest yield
and the further addition of catalyst had no obvious effect on the
yield of biphenyl (Table 2). Thus, homocoupling of arylboronic
acids was performed taking CuO nano-rod as the catalyst in water
under air for 1.5 h at room temperature. It is noteworthy to men-
tion that CuO NPs were synthesized in wet chemical process as
3
2
per our earlier Letter and well characterized with the help of
TEM, SEM, EDAX and XRD. From TEM analysis it is clear that they
are nanosize particles (Fig. S1, ESI). SEM and EDAX of nano CuO are
Table 3
Nano Cuo catalyzed aerobic homocoupling reaction of aryl boronic acidsa
B(OH)2
CuO nano (10 mol%),
Table 1
Optimization of the reaction conditions for solvent, time and basea
K CO (0.5 eqv.)
2 3
H O (2 mL)
2
CuO nano, base
B(OH)2
Solvent, rt
Yieldb,c (%)
Entry
R
Products
1
2
3
4
5
6
7
8
H
4-CH
3-CH
2-OCH
4-OCH
4-C
4-tert Butyl
4-F
3,4-Difluoro
4-Cl
5-(2-Methoxy
pyridyl)
Biphenyl
4,4 -Dimethylbiphenyl
95
85
89
86
90
87
91
94
93
92
Entry Catalyst
20 mol %)
Solvent
(2 mL)
Base
(0.5 equiv)
Time
(h)
Yieldb
(%)
0
3
3
(
0
0
0
0
0
0
3,3 -Dimthylbiphenyl
1
2
3
4
5
6
7
8
9
1
1
1
CuO
CuO
CuO
CuO
CuO
CuO
CuO
CuO
CuO
CuO
CuO
CuO
MeOH
i-PrOH
Water
MeOH
MeOH
MeOH
i-PrOH
Nil
Nil
Nil
36
36
36
1.5
1.5
1.5
2
4
3
2
1
20
30
34
79
84
64
91
96
96
96
86
95
3
3
2,2 -Dimethoxybiphenyl
4,4 -Dimethoxybiphenyl
2
H
5
4,4 -Diethylbiphenyl
K
Cs
Na
2
CO
CO
CO
3
4,4 -Ditertiarybutylbiphenyl
2
3
4,4 -Difluorobiphenyl
0
0
0
0
2
3
9
10
11
3,3 ,4,4 -Tetrafluorobiphenyl
K
K
K
K
K
2
CO
2
CO
2
CO
2
CO
2
CO
3
3
3
3
3
4,4 -Dichlorobiphenyl
0
d
2
H O
2
H O
2
H O
2
H O
2
H O
3,3 -Dimethoxy-5,5 -
bipyridine
86
0
1
2
a
Reaction conditions: phenylboronic acid (1 mmol), nano CuO (10 mol %), K
0.5 equiv).
2
CO
CO
3
3
(
K
2
CO
3
1.5
b
Isolated yield.
a
c
All compounds are characterized by 1H NMR and 13C NMR spectroscopy.
2-Methoxy pyridine-5-boronic acid (1 mmol), nano CuO (10 mol %), K
2 3
Reaction conditions: phenylboronic acid (1 mmol), nano CuO (20 mol %), K CO
0.5 equiv), solvent (2 mL).
Isolated yield.
d
(
2
b
(0.5 equiv), 60 °C, methanol solvent.
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P. K. Raul et al. / Tetrahedron Letters xxx (2015) xxx–xxx
3
100
100
8
0
0
0
0
8
0
0
0
0
6
4
2
6
4
2
CuO nano
With ca tt alyst
CuO bulk
after 65 mm ins wi tt hout c aa talyst
0
0
Water solvent ,, RT & 1.5 hrs
0
50
100 150 200 250 300 350 400
Time (min)
0
20
40
60
80
100
mmol% of catalyst
Figure 2. Hot filtration test. Reaction conditions: phenylboronic acid (1 mmol), CuO
10 mol %), water (2 mL), rt.
Figure 1. Comparison of catalytic performance of CuO nano versus CuO bulk
counterpart.
(
catalyst was recovered by filtration and washed with ethyl acetate
and dry ethanol. The washed catalyst was heated at 120 °C for 2 h
and activated under vacuum at rt for 2 h, which was subsequently
reused and the results are presented in Table 4 and Figure 3. From
this figure it is clear that the decrease in the efficiency of the cat-
alyst resulted in a minor loss of product yield even after being
reused five times. Comparison of the powder XRD patterns of the
fresh, reused CuO up to three and five times was also studied
(Fig. 3) which clearly showed that the reused catalyst exhibited a
similar powder XRD pattern. The powder XRD pattern results
clearly demonstrated no appreciable changes in the structural
integrity during/after the reuse. To demonstrate the potential util-
ity of this method for preparative purposes, the reaction was also
carried out under the optimized reaction conditions on a 7.95 g
scale (0.1 mol), giving 93% yield which were comparable to those
obtained for a small scale reaction (reaction conditions: phenyl-
boronic acid, 200 mL water, rt, 1.5 h).
desired products in 87–93% yields (Table 3, entries 2–7). It was
observed that boronic acid bearing bulky group methoxypyridine
underwent homocoupling reaction yielding fewer products.
Careful inspection of the results revealed that para- and meta-
substituted arylboronic acids afforded biaryls in good to excellent
yields. Diverse results were obtained for ortho-substituted
arylboronic acids which may be attributed to steric effects.
As, for example, the reaction of p-methoxyphenyl boronic acid
0
furnished the desired product 4,4 -dimethoxybiphenyl in 90%
isolated yield (Table 3, entry 5), demonstrating good selectivity.
o-Methoxyphenylboronic acid produced the corresponding
0
product 2,2 -dimethoxybiphenyl in a lower yield of 86% (Table 3,
entry 4). With 4-ethoxypyridine boronic acid, only a trace amount
of self-coupling product was formed.
The hot filtration test33 is a standard method to check hetero-
geneity of a reaction. The reaction has been carried out at different
time intervals and ꢀ71% product found after 65 min (Fig. 2). The
nano CuO catalyst was filtered off from the reaction mixture when
The mechanism of the present reaction is not yet clear at the
present stage. Hence, we have proposed a probable monometallic
mechanism (Scheme 1) based on predicted mechanism reported
ꢀ
71% of biphenyl was formed. After removal of the catalyst, the
reaction was monitored for an additional time up to 6 h and no fur-
ther biphenyl product formation was observed. Atomic absorption
spectroscopy analysis of the filtrate from the reaction mixture and
also of the filtrate from a stirred solution of CuO in water under
identical reaction conditions confirmed no copper leaching. These
results support the heterogeneous nature of the catalyst.
2
9,28b,34
by some other researchers.
The following observations
may be useful to predict the mechanism: (1) Cu(II) species is cat-
alytically active in the homocoupling reaction, (2) air is critical to
1
000
Fresh CuO
Reusability of catalyst
After third use
After fifth use
8
00
Reusability of the nano CuO was investigated since practical use
depends on this factor. After completion of the reaction, the
600
Table 4
400
a
Reusability of nano-CuO catalyst
Catalyst, nano
Cycle run
Yieldb (%)
2
00
CuO
CuO
CuO
CuO
CuO
1
2
3
4
5
94
91
88
83
80
0
0
10
20
30
40
50
60
70
80
a
2 3
Reaction conditions: phenylboronic acid (1 mmol), nano CuO (10 mol %), K CO
2Thet aa in degrees
(
0.5 equiv), solvent (2 mL).
b
Isolated yield.
Figure 3. Powder XRD patterns of fresh and reused CuO catalyst.
Please cite this article in press as: Raul, P. K.; et al. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.11.004

4
P. K. Raul et al. / Tetrahedron Letters xxx (2015) xxx–xxx
2
Ar-B(OH)2
+
O2
+
H O
2B(OH)3
Supplementary data
2
Solvent
Supplementary data (general experimental methods; TEM, SEM
and EDAX of CuO nanoparticles; physical and spectroscopic data
for all the compounds and H and C NMR spectra of selected
compounds) associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.tetlet.2015.11.004.
b
1
13
Ar
Cu(II)
Cu(II)
O2
Ar
References and notes
Ar
2
B(OH)3
Cu(III)
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b= Transmetallation
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Scheme 1. Plausible mechanism of homocoupling reaction.
4
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.
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Experimental
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A solution of nano CuO (0.010 mmol) and substituted phenyl-
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2
1
1
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separated and collected in a conical flask. After evaporation of
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1
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Conclusions
1
2
In conclusion, we have successfully developed a simple and
efficient rod shaped nano CuO catalyzed methodology for the
homocoupling of arylboronic acids to symmetrical biaryls. The
reaction proceeds in economical, cheapest and easily available
solvent water under extremely mild conditions: in air, at room
temperature in presence of a very mild base. The synthesized
CuO NPs remain in same conditions even after performing the
catalytic reaction. The catalyst is efficient, easily recoverable and
reusable up to another three to four times without losing its activ-
ity appreciably. This protocol is relatively inexpensive and environ-
mentally friendly. The good selectivity of homocoupling over the
Suzuki cross-coupling shows potential application in synthesis of
biphenyl.
2
1
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Authors thank Tezpur University, Assam, India for providing
infrastructures for this project. P.K.R. thanks Defence Research
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