Copper Oxide Nanoparticles as a Mild and Efficient Catalyst for N-Arylation of Imidazole and Aniline with Boronic Acids at Room Temperature

Article abstract of DOI:10.1055/s-0036-1588741

The present work describes the excellent catalytic activity of copper(II) oxide nanoparticles (NPs) towards N-arylation of aniline and imidazole at room temperature. The copper(II)oxide NPs were synthesized by a thermal refluxing technique and characterized by FT-IR spectroscopy; powder XRD, SEM, EDX, TEM, TGA, XPS, BET surface area analysis, and particle size analysis. The size of the NPs was found to be around 12 nm having a surface area of 164.180 m ² g ^-1.The catalytic system was also found to be recyclable and could be reused in subsequent catalytic runs without a significant loss of activity.

Full text of DOI:10.1055/s-0036-1588741

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© Georg Thieme Verlag Stuttgart · New York
2017, 28, A–F
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A
R. K. Borah et al.
Letter
Synlett
Copper Oxide Nanoparticles as a Mild and Efficient Catalyst for
N-Arylation of Imidazole and Aniline with Boronic Acids at Room
Temperature
Raju Kumar Boraha
Prasanta Kumar Raulb
Abhijit Mahantaa
Andrey Shchukarevc
Jyri-Pekka Mikkolac,d
Ashim Jyoti Thakur*a
R2
HN
N
R2
N
N
R1
CuO NPs
(15 mol%)
R2 = 4-Me, 2-Me
7 examples
up to 88% yield
OH
OH
K2CO3 (1.5 mmol)
B
MeOH–H2O (1:1)
rt, 8–13 h
R1
a Department of Chemical Sciences, Tezpur University,
Napaam, 784028, Assam, India
H2N
ashim@tezu.ernet.in
R2
R1 = F, Cl, OMe,
b Defence Research Laboratory, Post Bag no. 2, Solmara,
Tezpur 784001, Assam, India
Me, Et, t-Bu
c Department of Chemistry, Umeå University, 90187
Umeå, Sweden
CuO NPs
(30 mol%)
N
H
R2
R1
d Industrial Chemistry & Reaction Engineering, ÅboAka-
demi University, 20500 Åbo-Turku, Finland
R
2 = Br, OMe
8 examples
up to 93% yield
Received: 02.02.2017
Accepted after revision: 14.02.2017
Published online: 09.03.2017
having free N–H groups5 and the use of expensive palladi-
um sources and ligands.
Considering the above facts, in 1998 Chan, Evan, and
Lam6 developed a mild and efficient protocol for copper-
mediated oxidative amination, where arylboronic acids
were used as arylating agents instead of aryl halides. Aryl-
boronic acids are organometallic moieties possessing wide
applicability in contemporary organic synthesis owing to
their stability, structural diversity, and low toxicity.7 How-
ever, the requirement of 1–2 equivalents of Cu(OAc)2 and
large excess of arylboronic acid8 are drawbacks associated
with this cross-coupling methodology. Further work on the
Chan–Lam coupling reaction resulted in its catalytic ver-
sion9 along with its application to other nucleophiles for
cross-coupling reactions such as amides,10 oximes,11 and
sulfoximines.12 Subsequently, a large number of modifica-
tions has been made using different copper salts in the
presence of various ligands to improve the efficiency of the
reaction.10–12 However, the long reaction times and high re-
action temperatures are disadvantages associated with
these protocols and thus, a further optimization is desirable
in the case of this reaction.
Additionally, in line with recent trends, nanoparticles
(NPs) are considered as very efficient and attractive cata-
lysts compared to their macroscopic counterparts due to
their high surface-to-volume ratio as well as very active
surface atoms. Although many methods are available for the
synthesis of NPs, routes that are based on solution-based
processes are more effective leading to well-controlled
shapes, sizes, and structures. Amongst the metal NPs, cop-
per-based NPs hold a superior position due to their versatil-
DOI: 10.1055/s-0036-1588741; Art ID: st-2017-d1756-l
Abstract The present work describes the excellent catalytic activity of
copper(II) oxide nanoparticles (NPs) towards N-arylation of aniline and
imidazole at room temperature. The copper(II)oxide NPs were synthe-
sized by a thermal refluxing technique and characterized by FT-IR spec-
troscopy; powder XRD, SEM, EDX, TEM, TGA, XPS, BET surface area
analysis, and particle size analysis. The size of the NPs was found to be
around 12 nm having a surface area of 164.180 m2 g–1.The catalytic
system was also found to be recyclable and could be reused in subse-
quent catalytic runs without a significant loss of activity.
Key words nanoparticles, base, N-arylation, aniline, imidazole, reus-
ability
In the domain of organic synthesis, transition-metal-
mediated carbon–heteroatom bond-forming reactions have
made a significant contribution to the recent growth. Nitro-
gen-containing compounds can now be easily generated by
transition-metal-catalyzed protocols that find diverse ap-
plications in the field of advanced organic synthesis. Among
them diarylamines and N-arylimidazoles attract particular
interest due to their widespread presence in natural prod-
ucts, agrochemicals, materials, dyes, pharmaceuticals,1 and
biologically active inhibitors.2
Buchwald and Hartwig developed a method for palladi-
um-catalyzed cross-coupling of aryl halides and triflates
with amines.3 Though the process is credited with milder
reaction conditions than Ullmann reaction,4 there are some
limitations, such as difficulties associated with aryl halides
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–F

B
R. K. Borah et al.
Letter
Synlett
ity in various organic conversions such as C–N bond-forma-
tion reactions and C–H activation reactions13 and find wide
range of applications due to their stability and easy synthe-
sis.14 The literature reveals various techniques relating to
the synthesis of CuO NPs but these are generally costly,
time-consuming, and require large volumes of re-
agents.13b,15 Consequently, there is a need for a simple and
mild technique to synthesize CuO NPs. Focusing on the
above facts, in this article, we report an efficient technique
for the preparation CuO NPs, and the resulting catalysts
were utilized in a mild protocol for N-arylation of imidaz-
oles and anilines with arylboronic acids at room tempera-
ture (Scheme 1).
ure 1 (c and d)] the particles possess a needle-like morphol-
ogy. However, most of the NPs have irregular shapes and
sizes, which might be due to agglomeration. The TGA analy-
sis is shown in Figure 2 (a). The initial weight loss up to 100
°C is due to elimination of moisture, and further weight loss
up to 300 °C is due to decomposition of organic materials.
In order to examine the porous behavior and estimate the
surface area, N2adsorption/desorption experiments were
performed at liquid N2 temperature and are shown in Fig-
ure 2 (b).
OH
B
R1
OH
K2CO3 (1.5 mmol)
MeOH–H2O (1:1)
rt, 8–13 h
R2
H2N
HN
N
R2
CuO NPs
(30 mol%)
CuO NPs
(15 mol%)
R2
N
N
N
R1
R2
R1
H
Figure 1 (a) FT-IR spectrum, (b) PXRD pattern, (c) and (d) SEM images
of the synthesized CuO NPs.
Scheme 1 N-Arylation of imidazoles and anilines with arylboronic acid
We synthesized the CuO NPs following a simple and
cost-effective procedure presented by Goswami et al.16 with
some modifications using CuCl2 as the starting precursor in
order to obtain uniform smaller particles. The NPs were
characterized using FT-IR spectroscopy, powder XRD, SEM,
EDX, TEM, TGA, XPS, BET surface area analysis, and particle-
size analysis.
The CuO NPs were first characterized by FT-IR spectros-
copy (Figure 1, a). The bands at 2913 and 3440 cm–1 are due
to symmetric and asymmetric stretching of O–H bond, re-
spectively. The presence of bands at 509 and 598 cm–1 cor-
respond to different modes of bending vibration of the Cu–
O bond;16,17 whereas, the band at 1632 cm–1 can be as-
signed to C–C and C–O from residual carbonates and poly-
mer. Powder XRD was used to determine the crystal do-
main size and the structure of the NPs formed. Figure 1 (b)
shows the typical XRD pattern in the 2θ range of 20° to 70°,
showing the presence of peaks that can be indexed on the
basis of monoclinic CuO (JCPDS card no. 89-5895). The
broadening of the XRD peak indicates that the synthesized
particles are in the nano range. From the SEM images [Fig-
The N2 adsorption/desorption isotherm belongs to type
IV with a combination of H1 and H3 hysteresis loop, which
is a characteristic of mesoporous materials. The N2 phy-
sisorption (BET) investigation of the CuO nanostructure
provides a surface area of about 164.180 m2 g–1 which is
much larger than that found for commercial CuO. Energy-
dispersive X-ray (EDX) analysis of the CuO NPs reveals the
presence of Cu and O signals and shows the NPs are devoid
of any other metal impurities. The TEM and HRTEM images
[Figure 2 (d and e, respectively)] show that the NPs are cy-
lindrical in shape. The interplanar distance of 0.23 nm can
be ascribed to the (111) plane of the monoclinic CuO. The
corresponding selected area electron-diffraction (SAED)
pattern of the CuO nanostructure [inset, Figure 2 (e)] shows
that the particles are crystalline in nature and possess three
well-resolved rings for (110), (–111), and (111) planes. The
two poorly resolved rings are for (200) and (–202) planes.
The size distribution of the NPs [Figure 2,(f)] showed that
most of the particles fall in the range 7–17 nm, and the
mean particle diameter is about 12 ± 0.99 nm. XPS analysis
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–F

C
R. K. Borah et al.
Letter
Synlett
Table 1 Optimization of Reaction Conditionsa
OH
CuO NPs, base
+
B
H2N
solvent, rt, air
OH
N
H
Entry Solvent
Base
Catalyst Time
(mol%) (h)
Yield
(%)b
1
2
no solvent
H2O
K2CO3
15
15
15
15
15
21
30
36
30
30
30
–
10
20
68
54
58
74
82
92
93
86
88
66
0
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
Cs2CO3
KOH
10
10
10
10
10
8
3
CH2Cl2
4
i-PrOH–H2O (1:1)
MeOH–H2O (1:1)
MeOH–H2O (1:1)
MeOH–H2O (1:1)
MeOH–H2O (1:1)
MeOH–H2O (1:1)
MeOH–H2O (1:1)
MeOH–H2O (1:1)
MeOH–H2O (1:1)
MeOH–H2O (1:1)
MeOH–H2O (1:1)
MeOH–H2O (1:1)
MeOH–H2O (1:1)
MeOH–H2O (1:1)
5
6
7
8
8
9
8
10
11
12
13c
14d
15e
16f
17g
8
8
Figure 2 (a) TGA curve, (b) N2 adsorption/desorption isotherm, (c)
EDX pattern, (d) TEM, (e) HRTEM and in inset is the SAED pattern, and
(f) particle-size distribution of the CuO NPs.
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
24
8
30
30
30
30
30
78
92
90
61
0
8
10
10
10
confirmed the presence of Cu2+ (Figure S1, Supporting In-
formation). Recycled samples contain some hydroxide but,
CuO still dominates (Figure S2, Supporting Information).
With the aim to find out the effect of various solvents,
bases, and amount of catalyst, we chose phenylboronic acid
and aniline as the model substrates, and the results are
summarized in Table 1.
a Reaction conditions: phenylboronic acid (1 mmol), aniline (0.5 mmol),
K2CO3 (1.5 mmol), r.t., stirring.
b Isolated yield.
c 1 mmol of K2CO3.
d 2 mmol of K2CO3.
e 0.75 mmol phenylboronic acid used.
f 0.60 mmol phenylboronic acid was used.
g Nitrogen atmosphere was used.
An initial screening of the effect of different solvents us-
ing K2CO3 as base and 15 mol% of the catalyst showed that
MeOH–H2O (1:1, v/v) was the most efficient solvent system
(Table 1, entry 5). After optimizing the solvent system, we
studied the effect of the amount of catalyst (Table 1, entries
5–8), and it was found that 30 mol% catalytic amount of the
catalyst with respect to aniline was the optimized amount
for the coupling reaction (Table 1, entry 7). The reaction
also proceeded in the presence of other bases such as Na2CO3,
Cs2CO3, and KOH with comparable yields (Table 1, entries 9–
11). The control reaction conducted under identical condi-
tions but devoid of catalyst gave no coupled product even
with prolonged reaction time (Table 1, entry 12). We car-
ried out the reaction with 1–2 mmol of K2CO3 (Table 1, en-
tries 13 and 14) and found that 1.5 mmol was the optimal
amount. Notably, the presence of an oxidant is critical and
an important parameter for the copper-promoted N-aryl-
ation reaction.18 The reaction did not proceed under nitro-
gen atmosphere (Table 1, entry 17), which indicates the re-
quirement of air as oxidant for the reaction. Thus, optimum
conditions involved the reaction being carried out under air
at room temperature in MeOH–H2O (1:1) in the presence of
30 mol% of nanocatalyst with respect to aniline and K2CO3
(1.5 mmol) as the base (Table 1, entry 7).
We also examined the reaction with copper(II) salts
such as Cu(OAc)2·H2O, CuSO4·5H2O, CuCl2·2H2O, and CuI for
the N-arylation reaction of aniline under the optimized re-
action conditions and found only 15–30% of the product di-
phenylamine (Table 2, entries 1–3).
After optimizing the conditions for aniline, our next en-
deavor was to extend the scope of this method for N-aryl-
ation of imidazole. For this, we took imidazole (1 mmol)
and phenylboronic acid (1.2 mmol) as the model substrates.
In this case, the reaction also proceeded smoothly but re-
quired a comparatively longer time than in the case of N-
arylation of aniline.
To evaluate the scope and limitation of the current pro-
cedure, a wide range of electronically diverse anilines and
imidazoles with different phenylboronic acid substrates
was examined. The results are summarized in Table 3.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–F

D
R. K. Borah et al.
Letter
Synlett
Table 2 N-Arylation of Aniline Using Different Copper(II) Saltsa
Table 3 (continued)
Entry Arylboronic acid
Anilinea
Imidazoleb Time Yield
(h) (%)c
OH
Cu source (30 mol%)
+
B
H2N
K2CO3 (1.5 mmol)
MeOH–H2O (1:1), rt
OH
N
B(OH)2
H
1121i
1221j
1321k
–
–
–
13 81
12 86
11 86
N
HN
F
Entry
Copper source (mol%)
Time (h)
Yield (%)b
B(OH)2
1
2
3
4
5
6
Cu(OAc)2·H2O (30)
CuSO4·5H2O (30)
CuCl2·2H2O (30)
CuCl (30)
24
24
24
24
24
8
30
16
28
15
30
92
N
N
HN
HN
Cl
B(OH)2
MeO
MeO
CuI (30)
B(OH)2
CuO NPs (30)
1421l
–
–
12 82
12 82
a Reaction conditions: phenylboronic acid (1 mmol), aniline (0.5 mmol),
K2CO3 (1.5 mmol), MeOH–H2O (1:1), r.t., stirring.
b Isolated yield.
N
N
HN
HN
B(OH)2
1521g
Table 3 Reaction of Different Arylboronic Acids with Different Ani-
a Conditions: aniline (0.5 mmol), arylboronic acid (1 mmol), nanocatalyst
(30 mol% w.r.t. aniline substrate), MeOH–H2O (1:1, v/v, 4 mL), K2CO3 (1.5
lines19 and Imidazoles20
mmol), r.t.
b Imidazole (1 mmol), arylboronic acid (1.2 mmol), nanocatalyst (15 mol%
w.r.t. imidazole substrate), MeOH–H2O (1:1, v/v, 4 mL), K2CO3 (1.5 mmol),
Entry Arylboronic acid
Anilinea
Imidazoleb Time Yield
(h) (%)c
r.t.
c Isolated yield.
B(OH)2
NH2
NH2
121a
–
–
8
8
92
90
From the substrate study, it was found that the cross-
coupling reaction of arylboronic acids with different ani-
lines gave good to excellent yields of the cross-coupling
products. Both electron-deficient and electron-rich anilines
coupled efficiently under the optimized reaction conditions
resulting in good to excellent yields of the products. How-
ever, phenylboronic acids with an electron-donating group
react more rapidly, giving higher yields (Table 3, entries 4,
7, 13, and 14) than those with electron-withdrawing
groups (Table 3, entries 3, 11, and 12). The compatibility of
the reaction with the functional groups (CN, CO2R, COR,
NO2) on the boronic acids were tested under the same reac-
tion conditions, but the results were not encouraging even
after heating and very low yields were observed (less than
25%). The presence of an alkyl substituent on the imidazole
ring has a slight effect in terms of the efficiency of the reac-
tion. For example, 4-methylimidazole requires a longer re-
action time (Table 3, entry 10) compared to imidazole (Ta-
ble 3, entry 9).
To assess the reusability of the CuO NPs, a series of six
consecutive runs was carried out using aniline and phenyl-
boronic acid under optimized conditions. After the end of
each reaction, the reaction mixture was filtered and the
catalyst was reused for the next batch. We find that the cat-
alyst was reusable up to a 5th cycle without any significant
loss of catalytic activity. After the 5th cycle, a notable loss of
catalytic activity was observed (Figure 3).
B(OH)2
221b
Et
NH2
B(OH)2
322
–
–
–
–
10 84
F
Br
B(OH)2
NH2
421c
521d
621e
8
93
MeO
NH2
NH2
B(OH)2
10 86
Br
Br
B(OH)2
9
8
8
90
90
92
B(OH)2
NH2
721f
–
–
Br
NH2
B(OH)2
B(OH)2
B(OH)2
821c
MeO
921g
–
10 88
11 84
N
N
HN
HN
1021h
–
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–F

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R. K. Borah et al.
Letter
Synlett
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Figure 3 Reusability of CuO NPs
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2012, 53, 6219.
In conclusion, we have developed an efficient process
for CuO NPs synthesis and these can effectively catalyze N-
arylation of imidazoles and anilines under mild conditions.
The reaction is carried out at room temperature and the
procedure works well with a variety of imidazole, aniline,
and boronic acid substrates having both electron-donating
and electron-withdrawing groups, affording good to excel-
lent yields and the catalyst can be recycled.
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Acknowledgement
The authors are thankful to Tezpur University for providing infra-
structure facilities to carry out the research work. In Sweden, the
Bio4Energy programme is acknowledged.
Supporting Information
(16) Goswami, A.; Raul, P. K.; Purkait, M. K. Chem. Eng. Res. Des. 2012,
90, 1387.
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0036-1588741.
S
u
p
p
ortioInfgrmoaitn
S
u
p
p
ortiInfogrmoaitn
Modified Procedure for Preparing CuO NPs
Copper(II) chloride dihydrate (6.8 g), NaOH (3.2 g), and capping
solvent (polyethylene glycol) were mixed in a 2:1:1.5 ratio with
EtOH–H2O (1:1, v/v, 200 mL) in a round-bottom flask fitted with
a reflux condenser. The mixture was heated to reflux for 12 h
and allowed to cool to r.t. Then the mixture was again heated to
reflux for 5 h. The dark brown precipitate was centrifuged and
washed with EtOH, acetone, and hot H2O sequentially. Finally,
the product was dried at r.t, heated to 120 °C in a vacuum oven
and allowed to cool to r.t..
References and Notes
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(19) Typical Procedure: N-Arylation of Aniline with Phenylbo-
ronic Acid
In a 50 mL round-bottomed flask, aniline (0.5 mmol), phenylbo-
ronic acid (1 mmol), K2CO3 (1.5 mmol), nanocatalyst (30 mol%
with respect to aniline substrate) were added and stirred in
MeOH–H2O (1:1) under air at r.t. for the required time, monitor-
ing by TLC. After completion, the mixture was diluted with H2O,
and the product was extracted with EtOAc (3×). The combined
(4) (a) Lindley, J. Tetrahedron 1984, 40, 1433. (b) Paine, A. J. J. Am.
Chem. Soc. 1987, 109, 1496.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–F

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R. K. Borah et al.
Letter
Synlett
extracts were washed with brine (3×) and dried over Na2SO4.
The product was purified by column chromatography (60–120
mesh silica gel, eluting with EtOAc–hexane solvent). The
product was a grey crystalline solid, mp 54 °C; isolated yield:
92%. 1H NMR (400 MHz, CDCl3): δ = 5.63 (br s, 1 H), 6.90 (t, J = 8
Hz, 2 H), 7.04–7.02 (m, 4 H), 7.25–7.21 (m, 4 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 143.2, 129.5, 121.1, 117.9 ppm.
(21) (a) Gogoi, A.; Sarmah, G.; Dewan, A.; Bora, U. Tetrahedron Lett.
2014, 55, 31. (b) Zhu, X.; Ma, Y.; Su, L.; Song, H.; Chen, G.; Liang,
D.; Wan, Y. Synthesis 2006, 3955. (c) Sun, W.-B.; Zhang, P.-Z.;
Jiang, T.; Li, C.-K.; An, L.-T.; Shoberu, A.; Zou, J.-P. Tetrahedron
2016, 72, 6477. (d) Sugimura, H.; Kondoh, A. CN 101587308,
2009. (e) Quach, T. D.; Batey, R. A. Org. Lett. 2003, 5, 4397.
(f) Qian, G.; Li, X.; Wang, Z. Y. J. Mater. Chem. 2009, 19, 522.
(g) Lan, J.-B.; Chen, L.; Yu, X.-Q.; You, J.-S.; Xie, R.-G. Chem.
Commun. 2004, 188. (h) Ueda, S.; Su, M.; Buchwald, S. L. J. Am.
Chem. Soc. 2012, 134, 700. (i) Garnier, T.; Sakly, R.; Danel, M.;
Chassaing, S.; Pale, P. Synthesis 2016, 49, 1223. (j) Li, Z.; Meng,
F.; Zhang, J.; Xie, J.; Dai, B. Org. Biomol. Chem. 2016, 14, 10861.
(k) Ge, X.; Chen, X.; Qian, C.; Zhou, S. RSC Adv. 2016, 6, 58898.
(l) Zhu, X.; Su, L.; Huang, L.; Chen, G.; Wang, J.; Song, H.; Wan, Y.
Eur. J. Org. Chem. 2009, 635.
(20) Typical Procedure: N-Arylation of Imidazole with Phenylbo-
ronic Acid
In a 50 mL round-bottomed flask, imidazole (1 mmol), phenyl-
boronic acid (1.2 mmol), K2CO3 (1.5 mmol), nanocatalyst (15
mol% with respect to imidazole substrate) were added and
stirred in MeOH–H2O (1:1) under air at r.t. for the required
time, monitoring by TLC. After completion, the mixture was
diluted with H2O and the product was extracted with EtOAc
(3×). The combined extracts were washed with brine (3×) and
dried over Na2SO4. The product was purified using column
chromatography (60–120 mesh silica gel, eluting with EtOAc–
hexane). The product was isolated as white solid. 1H NMR (400
MHz, CDCl3): δ = 7.87 (s, 1 H), 7.50–7.47 (m, 2 H), 7.40–7.35 (m,
3 H), 2.28 (m, 1 H), 7.21 (s, 1 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 137.4, 135.6, 130.3, 129.9, 127.6, 121.6, 118.3 ppm.
(22) Analytical Data for 4-Bromo-N-(4-fluorophenyl)benzenam-
ine (Table 3, Entry 3)
Colorless crystals; mp 43 °C. 1H NMR (400 MHz, CDCl3): δ = 5.54
(br s, 1 H), 7.32–7.25 (m, 2 H), 7.02–6.96 (m, 4 H), 6.82 (d, J = 8
Hz, 2 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 143.3, 132.2,
121.3, 121.2, 118.1, 116.2, 116.0, 112.2 ppm.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–F