Nano Copper(0)-Stabilized on Alumina: Efficient and Recyclable Heterogeneous Catalyst for Chemoselective Synthesis of 1,2-Disubstituted Benzimidazoles and Quinoxalines in Aqueous Medium

Article abstract of DOI:10.1007/s10562-017-2166-6

Abstract: The present communication elicits the use of copper nanoparticles on aluminium oxide derived from Cu–Al hydrotalcite as a heterogeneous catalyst in the green and operationally simple approach for the synthesis of selective 1,2-disubstituted benzimidazoles and quinoxaline. Wide ranges of substituted o-phenylenediamines and aldehydes or α-bromo ketones were used to achieve the desired products using water as the reaction medium. The recoverability and reusability of the catalyst are the significant features in this eco-friendly green protocol. Graphical Abstract: A simple and efficient process for the synthesis of benzimidazoles and quinoxaline in presence of Cu(0)/Al₂O₃ catalyst at room temperature is described.

Full text of DOI:10.1007/s10562-017-2166-6

Catal Lett
DOI 10.1007/s10562-017-2166-6
Nano Copper(0)-Stabilized on Alumina: Efficient and Recyclable
Heterogeneous Catalyst for Chemoselective Synthesis
of 1,2-Disubstituted Benzimidazoles and Quinoxalines in Aqueous
Medium
Jaya Pogula1 · Soumi Laha1 · Pravin R. Likhar1
Received: 25 May 2017 / Accepted: 7 August 2017
© Springer Science+Business Media, LLC 2017
Abstract The present communication elicits the use of
copper nanoparticles on aluminium oxide derived from
Cu–Al hydrotalcite as a heterogeneous catalyst in the green
and operationally simple approach for the synthesis of selec-
tive 1,2-disubstituted benzimidazoles and quinoxaline. Wide
ranges of substituted o-phenylenediamines and aldehydes or
α-bromo ketones were used to achieve the desired products
using water as the reaction medium. The recoverability and
reusability of the catalyst are the significant features in this
eco-friendly green protocol.
Graphical Abstract A simple and efficient process for the
synthesis of benzimidazoles and quinoxaline in presence of
Cu(0)/Al2O3 catalyst at room temperature is described.
Keywords Benzimidazole · Quinoxaline · Heterogeneous
catalysis · Copper nanoparticles on alumina ·
Recyclability · Aqueous medium
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-017-2166-6) contains supplementary
material, which is available to authorized users.
1 Introduction
Benzimidazole and quinoxaline are important N-heterocyclic
units in pharmaceutical industry due to their broad biologi-
cal functions such as antiviral, antiulcer, antihypertensive
and anticancer activities [1–5]. Additionally, benzimidazole
* Pravin R. Likhar
plikhar@iict.res.in
1
Organometallic Gp., I& PC Division, CSIR-IICT,
Hyderabad 500007, India
Vol.:(0123456789)
1 3

J. Pogula et al.
moieties frequently find applications in materials science,
especially in organic light-emitting diodes and membranes
for fuel cells [6, 7]. Similarly, quinoxalines are interesting
class of compounds due to the presence of its core structure
in many biocides [8], various biofunctional molecules [9]
and pharmaceuticals [10].
Further, the synthesis of quinoxaline is generally accom-
plished by the oxidative cascade reactions of o-phenylene-
diamine with α-hydroxy ketones [35, 36], epoxides [37] and
diols [38] in the presence of either noble metal or additional
oxidants. Reaction of o-phenylenediamine with dicarbo-
nyl compounds also produces quinoxaline as reported by
Lindsley and co-workers [39]. Additionally, multicomponent
reactions have recently been introduced for the formation of
quinoxaline [40]. Progress has also been made in the synthe-
sis of quinoxaline derivatives by using Brønsted acid [41],
and molecular iodine [42, 43]. However, It is intriguing to
note that rarely α-bromoketones have been used as reaction
partners of o-phenylenediamine for the formation of qui-
noxalines [33, 44] and most of the reported methodologies
suffer from serious draw backs such as use of stoichiometric
amount of catalyst, long reaction time, use of toxic solvents,
unsatisfactory yields and inability to recover and reuse the
catalyst.
In general, the synthesis of benzimidazoles moieties com-
prises the condensation of 1,2-phenylenediamines with alde-
hydes [11, 12] or carboxylic acid derivatives [13] in pres-
ence of strong acids such as polyphosphoric acid or mineral
acids. Recently, transition-metal-catalyzed C–N cross-cou-
pling reactions for the synthesis of benzimidazole deriva-
tives have been reported [14–21]. Palladium- and copper-
catalyzed aryl amination/condensation protocols have been
utilized for the synthesis of 1,2-disubstituted benzimidazoles
[22]. Brasche and Buchwald reported the copper catalyzed
synthesis of benzimidazoles from amidines through C–H
functionalization/C–N bond formation reaction [23]. Shi and
co-workers have used the palladium catalyzed C–H activa-
tion reaction for the formation of benzimidazoles [24]. The
CuI/N,N′-dimethylethylenediamine (DMEDA)-mediated
regiospecific reaction of 1,2-dihaloarenes with N-substituted
amidines or guanidines for the formation of benzimidazoles
was reported by Deng and co-workers [25]. Later, You and
co-workers have described the regio-specific synthesis of
1,2-disubstituted benzimidazoles [26].
Recently, we have focused our efforts on the synthesis of
mono dispersed and highly stable Cu(0) from copper–alu-
minium hydrotalcite [45–47]. One of the important charac-
teristics of nano-Cu(0) on alumina is that it is prepared from
single precursor, Cu–Al HT Brucite like structures and, upon
reduction, Cu(II) gets selectively reduced to Cu(0) with high
dispersion and stability. Herein, we report the activity of
this nano-Cu(0) on alumina catalyst towards the synthesis
of benzimidazole and quinoxaline derivatives (Scheme 1).
Other synthetic methodologies, such as reductive cycli-
zation reaction of o-nitroaniline with aldehydes [27], rho-
dium catalyzed hydroformylation of N-alkenyl-1,2-diamin-
obenzenes [28], tandem carbonylation–cyclization reaction
of o-phenylenediamines [29] and tandem dehydration–cou-
pling reaction of 2-bromoaniline catalyzed by palladium
have been reported [30]. However, the main limitation of
these methodologies is the formation of both 2-substituted
and 1,2-disubstituted benzimidazoles. The selective synthe-
sis of 1,2-disubstituted benzimidazoles have been reported
by very few research groups [31–34]. Among the methods
reported, some of the processes suffer from long reaction
times, use of toxic catalysts/solvent and a tedious work-up
process.
2 Results and Discussion
Copper–aluminum hydrotalcites [(Cu–Al HT) Cu:Al 3:1]
was prepared by co-precipitation of Cu- and Al nitrates.
The thermal reduction of copper aluminiumhydrotalcite
to nano copper(0) on alumina catalyst in presence of
hydrogen flow is performed according to the procedure
reported earlier [45]. Copper:aluminium ratio (Cu:Al)
in nano Cu(0)/Al2O3 catalysts was found to be 2.49:1 by
inductively coupled plasma-atomic emission spectroscopy
(ICPMS). The Cu content of Cu(0)/Al2O3 was obtained
Scheme 1 Synthesis of 1,2-dis-
ubstituted benzimidazole and
quinoxaline by using nano cop-
per supported alumina at room
temperature in aqueous medium
1 3

Nano Copper(0)-Stabilized on Alumina: Efficient and Recyclable Heterogeneous Catalyst for…
8 wt% by atomic absorption analysis (AAS). The aver-
age particle size of Cu(0)/Al2O3 was estimated to be
8.0 nm which is well matched with transmission electron
microscopy.
The X-ray photoelectron spectroscopic (XPS) investiga-
tion of Cu–Al HT, Fig. 3a and fresh Cu(0)/Al2O3, Fig. 3b
shows that there is a change in oxidation state of copper
from +2 to 0. The XPS analysis of Cu(0)/Al2O3 and Cu–Al
HT catalyst at the Cu 2p level shows 2p3/2 line at 932.3,
and 934.7 eV and 952.2 and 954.6 eV for 2p1/2, which cor-
responds to Cu in 0 and +2 oxidation states, respectively.
The XRD pattern was used to investigate both the crys-
tal structure and the average crystalline size of Cu nano
particles.
As shown in Fig. 1, the clear and strong peaks corre-
sponding to (111), (200), and (220) indicate the formation
of highly crystalline Cu nanoparticles [48].
2.1 Optimization Conditions
There is no change in the crystal structure of Cu nano
particles phases in both fresh and used form of catalysts
(Fig. 1a, c). Transmission electron microscopic (TEM)
studies of both fresh and used catalysts were carried out
to understand the shape and size of the particles. Figure 2
a and b shows the TEM images of the fresh and the used
catalyst after the fifth cycle. Interestingly it is observed that
the shape and size of the particles remain unchanged. This
supports that the morphology of the catalyst remains the
same even after recycling.
In the optimization study for the benzimidazole synthe-
sis, various solvents such as dichloromethane, acetonitrile,
acetone, methanol, toluene and DMF were tested to check
their activity and selectivity (Table 1). Surprisingly, dichlo-
romethane produced poor yield of the product (Table 1, entry
1), while acetonitrile gave good yields. However, increase
in reaction temperature had no effect on the isolated yield
of the product (Table 1, entries 2 and 3). Only moderate
yields of the product was obtained at room temperature and
in 60–110 °C with the other organic solvents used (Table 1,
Fig. 1 XRD profiles of a fresh Cu(0)/Al2O3, b Cu(II)–Al HT and (c) used catalyst
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J. Pogula et al.
Fig. 2 TEM images of the
Cu(0)/Al2O3: a fresh and b used
catalyst
product 3a and 4 were obtained (Table 2, entry 1). The Cu(0)/
Al2O3 catalyst was found to be more active and selective than
the other copper catalysts tested (Table 2, entries 2–8).
The excellent yield and chemoselectivity mediated by
Cu(0)/Al2O3 catalyst in water inspired us to further investi-
gate this transformation. Under the optimized reaction con-
ditions, the scope of this reaction was extended by subjecting
different diamines and aldehydes. The aromatic aldehydes
carrying either electron-donating or electron-withdrawing
substituents reacted efficiently to produce the corresponding
products in good to excellent yields.
Methyl, isopropyl and tertiary butyl groups on the alde-
hydes were well tolerated and the corresponding products
were obtained in good yields (Table 3, products 3b–3d). The
aromatic mono-, di- and tri-sustituted benzaldehydes con-
taining electron donating groups (4-methoxy, 2,3-dimethoxy
and 3,4,5-trimethoxy benzaldehyde) produced good yields of
the corresponding products (Table 3, products 3e–3h). Fur-
thermore, the reactivity of the substituent groups varies with
their position on the aromatic ring of the aldehyde. Intro-
duction of an electron-withdrawing group at the para and
meta-position of benzaldehyde compared to ortho-position
gave better product yields (Table 3, products 3i–3k). This
variation in product yields with position of the substituent
group may be due to resonating, inductive or steric effects.
Substituted benzaldehydes containing electron-withdrawing
fluoro and trifluoromethyl group also furnished good yields
of product (Table 3, products 3l and 3m) Interestingly, het-
eroaromatic aldehydes such as pyridine-2-aldehyde, pyri-
dine-3-aldehyde and furfuraldehyde produced moderate to
good yields of the products under similar reaction conditions
(Table 3, products 3n-3r).
Fig. 3 a The XPS spectra of Cu(II)–Al HT and b fresh Cu(0)/Al2O3
entries 4–7). Importantly, Cu(0)/Al2O3 catalyst showed
excellent yield and chemoselectivity by furnishing 3a as the
sole product in water at room temperature (Table 1, entry 8).
The model reaction was also optimized for catalyst con-
centration and reaction time (Table 1, entries 9–11). 8 wt%
of nano Cu(0)/Al2O3 was sufficient for maximum product
yields (Table 1, entry 8) while 4 wt% catalyst loading gave
moderate yields of the product (Table 1, entry 9). Interest-
ingly, an increment in catalyst concentration more than
8 wt% did not show effective increase in product yields
(Table 1, entry 11).
The proposed mechanism of the Cu(0)/Al2O3 catalyzed
synthesis of 1,2-disubstituted benzimidazoles, may tenta-
tively be visualized to occur via the formation of an N,N′-
dibenzylidene-o-phenylenediamine. The aldehyde firstly
reacted with diamine to form the intermediate 1 (Scheme2).
Then in the presence of electrophilic catalyst, the intramo-
lecular 1,3-hydride migration was induced to produce the
1,2-disubstituted benzimidazole. This kind of mechanism has
We also screened a variety of other copper catalysts such
as commercially available CuO, Cu2O, copper bronze, CuCl,
CuSO4 and Cu–Al HT with the model substrates and water as
solvent (Table 2). The reaction proceeded even without the
presence of any catalyst. However, nearly equal quantities of
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Nano Copper(0)-Stabilized on Alumina: Efficient and Recyclable Heterogeneous Catalyst for…
Table 1 Screening of reaction parameters
Entry
Solvent
Temp (°C)
Time (h)
Yield (%)
3a
4
1
2
3
4
5
6
7
8
9
10
11
DCM
Rt
Rt
1.0
2.0
2.0
2.5
1.0
1.5
2.5
1.0
1.0
0.5
1.0
40
85
85
50
65
78
70
96
62a
78b
89c
35
–
–
35
15
05
10
–
Acetonitrile
Acetonitrile
Acetone
Methanol
Toluene
DMF
Water
Water
Water
Water
60
70
60
100
110
Rt
Rt
–
–
–
Rt
Rt
Reaction conditions: 1,2-phenylenediamine (1 mmol), benzaldehyde (2 mmol), 25 mg Cu(0)/Al2O3 (8 wt% of Cu), solvent (2 mL), room tem-
perature
The bold row shows copper(0) on alumina catalyst activity more prominantly
a12.5 mg catalyst (4 wt% Cu)
b25 mg catalyst (8 wt% Cu) in 0.5 h
c50 mg catalyst (16 wt% Cu)
thought that functional ketones may also react with o-phe-
nylenediamine to give functional heterocyclic compounds.
As illustrated in Table 4, the catalyst, Cu(0)/Al2O3 (Cu
8 wt%, 25 mg, 2.69 mol%) and water as solvent at room
temperature with a variety of α-bromoketones and o-phe-
nylenediamines proved to be efficient catalytic protocol to
produce functionalized quinoxalines. The minimal effect
of the substituent groups on the course of the reaction was
observed when both substituted ketones as well as amines
were used (Table 4, entries 4b–4e).
Table 2 Screening of different catalysts
Entry
Catalyst
Time (h)
Yield (%)
3a
4
1
2
3
4
5
6
7
8
None
1.0
1
45
80
96
84
82
75
65
–
39
Cu–Al HT
Cu(0)/Al2O3
CuCl
CuO
Cu–Bronze
Cu2O
1.0
1.0
1.0
1.0
2.0
2.0
–
–
10
22
18
–
The recyclability study of recovered catalyst Cu(0)/
Al2O3 in the quinoxaline synthesis was also performed.
Figure 4 presents the results of recycling of the cata-
lyst using 1,2-phenylenediamine and α-bromoketone as
the model substrate. Cu(0)/Al2O3 catalyst shows nearly
consistent activity up to five reaction cycles. After each
cycle, the Cu(0)/Al2O3 catalyst was recovered by simple
centrifugation, washed with water and diethyl ether, oven-
dried and used directly for the next cycle without any fur-
ther purifications. Only a slight loss of catalytic activity
was observed. Moreover, the leaching of the copper metal
after each cycle was determined by AAS and was found
to be negligible. Interestingly, the recovered catalyst’s
CuSO4
Reaction conditions: 1,2-phenylenediamine (1 mmol), benzaldehyde
(2 mmol), copper catalyst (8 wt% of Cu), water (2 mL), room tem-
perature
The bold row shows copper(0) on alumina catalyst activity more
prominantly
been previously proposed in literature [12, 49] while Yuan-
jiang Pan and co-workers where the first ones to validate it
with isotope labelling reaction using benzaldehyde-d1 [33].
Encouraged by the high catalytic activity of Cu(0)/Al2O3
in the reaction of o-phenylenediamines with aldehydes, we
1 3

J. Pogula et al.
Table 3 Investigation of substrate scope
Reaction conditions: Diamine (1 mmol), aldehyde (2 mmol), 25 mg Cu(0)/Al2O3, water (2 mL), room temperature
aafter 3rd cycle
diminished catalytic activity could be regenerated using
thermal reduction in presence of hydrogen flow (Fig. 4
cycle 6).
We envisaged a plausible reaction mechanism of the for-
mation of quinoxaline derivatives from o-phenylenediamine
and phenacyl bromide (Scheme 3). Initially the electrophilic
catalyst activates the phenacyl bromide making it susceptible
1 3

Nano Copper(0)-Stabilized on Alumina: Efficient and Recyclable Heterogeneous Catalyst for…
The broad ranges of functional heterocyclic compounds have
been synthesized in water in the presence of copper nano-
particles stabilised on alumina without using any additional
surfactant or oxidant. The catalyst can be used for five cycles
with almost consistent activity.
4 Experimental Section
4.1 General Experimental Procedure for the Synthesis
of 1,2-Disubstituted Benzimidazoles in Water
A 10 mL Schlenk flask was charged with aldehyde
(2 mmol), 1,2-diamine (1 mmol), Cu(0)/Al2O3 catalyst
(Cu 8 wt%, 25 mg) in water (2 mL). The reaction mixture
was stirred at room temperature under vigorous stirring for
appropriate time. After the completion of the reaction, as
Scheme 2 Plausible mechanism of 1,2-disubstituted benzimidazole
synthesis
for a nucleophilic attack by the amino group of phenylen-
ediamine to afford the intermediate (1) intermediate 1 then
cyclises to form the intermediate (2) the charged water group
in intermediate 3 is expelled out by the nucleophilic attack
from the adjacent amino group which results in the forma-
tion of intermediate 4 which readily undergoes aromatiza-
tion under air oxidation to afford 2-phenylquinoxaline as the
final product.
3 Conclusion
In summary, a practical and green catalytic synthetic method
has been developed for the facile synthesis of 1,2-disubsti-
tuted benzimidazoles and quinoxalines with excellent yields.
Fig. 4 Recyclability of the catalyst in the quinoxaline reaction
Table 4 Substrate scope for
the synthesis of quinoxalines
derivatives
Reaction conditions: 1,2-phenylenediamine (1 mmol), α-bromoketone
(1 mmol), copper catalyst (8 wt% of Cu), water (2 mL), room temperature
1 3

J. Pogula et al.
Scheme 3 Plausible mechanism of quinoaline synthesis
monitored by TLC, 5 mL of ethyl acetate was added in the
reaction mixture. The catalyst was separated by simple cen-
trifugation and the reaction mixture was treated with brine
(10 mL). The organic layer was separated and the aqueous
layer was back extracted with ethyl acetate (3×5 mL). The
combined ethyl acetate extract was dried with anhydrous
Na2SO4 (50 g) and was concentrated under reduced pres-
sure. The pure product was isolated by flash column chro-
matography on silica gel using ethyl acetate/hexane (10%)
as an eluent.
4.1.3 1‑(4‑Isopropylbenzyl)‑2‑(4‑isopropylphenyl)‑1H‑ben
zo[d] imidazole (3c) [50]
1
White solid; H NMR (500 MHz, CDCl3) δ 7.86 (d,
J = 7.9 Hz, 1H), 7.64 (d, J = 8.2 Hz, 2H), 7.32–7.26 (m,
3H), 7.22–7.14 (m, 4H), 7.02 (d, J =8.1 Hz, 2H), 5.42 (s,
2H), 2.99–2.83 (m, 2H), 1.27 (s, 3H), 1.26 (s, 3H), 1.23 (d,
J=3.8 Hz, 3H), 1.22 (s, 3H). 13C NMR (101 MHz, CDCl3)
δ 154.3, 150.9, 148.4, 143.2, 136.1, 133.8, 129.3, 127.5,
127.1, 126.8, 125.9, 122.8, 122.5, 119.8, 110.6, 48.2, 34.1,
33.7, 23.9. ESI-MS [M + H]+: m/z = 369; HRMS (ESI
orbitrap): calc. for C26H28N2 [M + H]+: 369.23064 found:
369.23064.
4.1.1 1‑Benzyl‑2‑phenyl‑1H‑benzo[d]imidazole (3a) [49]
1
White solid; H NMR (400 MHz, CDCl3) δ 7.87 (d, 1H),
7.71–7.65 (m, 2H), 7.48–7.40 (m, 3H), 7.36–7.25 (m,
4H), 7.22 (q, 2H), 7.09 (d, 2H), 5.44 (s, 2H); 13C NMR
(101 MHz, CDCl3) δ 154.2, 143.2, 136.4, 136.1, 130.0,
129.3, 129.2, 128.7, 127.8, 126.0, 123.0, 122.7, 120.0, 48.4.
ESI-MS [M+H]+: m/z=285; HRMS (ESI orbitrap): calc.
for C20H16N2 [M+H]+: 285.1386, found: 285.1386.
4.1.4 1‑(4‑(Tert‑butyl)benzyl)‑2‑(4‑(tert‑butyl)
phenyl)‑1H‑benzo[d]imidazole (3d) [51]
1
White solid; H NMR (400 MHz, CDCl3) δ 7.87 (d,
J = 7.9 Hz, 1H), 7.66 (d, J = 8.3 Hz, 2H), 7.47 (d,
J = 8.3 Hz, 2H), 7.34 (d, J = 8.3 Hz, 2H), 7.32–7.27 (m,
1H), 7.21 (dd, J = 6.4, 3.5 Hz, 2H), 7.05 (d, J = 8.3 Hz,
2H), 5.44 (s, 2H), 1.34 (s, 8H), 1.30 (s, 8H); 13C NMR
(101 MHz, CDCl3) δ 154.3, 153.2, 150.7, 143.1, 136.0,
133.4, 129, 127.0, 126.0, 126.0, 122.8, 122.7, 119.8,
110.6, 48.2, 34.8, 34.5, 31.3; ESI-MS [M+H]+: m/z=397;
HRMS (ESI orbitrap): calc. for C28H32N2 [M + H]+:
397.2638, found: 397.2637.
4.1.2 1‑(2‑Methylbenzyl)‑2‑(o‑tolyl)‑1H‑benzo[d]imida‑
zole (3b) [32]
1
White solid; H NMR (400 MHz, CDCl3) δ 7.87 (d,
J = 7.9 Hz, 1H), 7.38–7.26 (m, 3H), 7.24–7.16 (m, 2H),
7.16–7.10 (m, 2H), 7.05–6.99 (m, 1H), 6.64 (d, J=7.8 Hz,
2H), 5.18 (s, 2H), 2.23 (s, 3H), 2.15 (s, 3H). 13C NMR
(126 MHz, CDCl3) δ 153.9, 143.1, 138.3, 134.9, 134.0,
130.4 25.7, 130.3, 129.8, 127.5, 126.3, 126.0, 125.6, 122.8,
122.3, 120.0, 110.5, 45.7, 19.8, 19.0. ESI-MS [M + H]+:
m/z = 313; HRMS (ESI orbitrap): calc. for C22H20N2
[M+H]+: 313.16993, found: 313.16824.
4.1.5 1‑(4‑Methoxybenzyl)‑2‑(4‑methoxyphenyl)‑1H‑benz
o[d]imidazole (3e) [29]
1
White solid; H NMR (400 MHz, CDCl3) δ 7.84 (d,
J=7.9 Hz, 1H), 7.64 (d, J=8.8 Hz, 2H), 7.29 (ddd, J=7.3,
5.3, 2.6 Hz, 1H), 7.21 (dd, J = 6.1, 3.1 Hz, 2H), 7.03 (d,
1 3

Nano Copper(0)-Stabilized on Alumina: Efficient and Recyclable Heterogeneous Catalyst for…
J=8.7 Hz, 2H), 6.97 (d, J=8.8 Hz, 2H), 6.85 (d, J=8.7 Hz,
2H), 5.38 (s, 2H), 3.85 (s, 3H), 3.78 (s, 3H); 13C NMR
(101 MHz, CDCl3) δ 160.9, 159.1, 154.1, 143.1, 136.1,
130.7, 128.5, 127.2, 122.7, 122.5, 119.7, 114.33, 114.0,
110.4, 55.3, 47.9 ; ESI-MS [M+H+]+: m/z=345; HRMS
(ESI orbitrap): calc. for C22H20N2O2 [M+H]+: 345.1597,
found: 345.1611.
4.1.9 1‑(2‑Nitrobenzyl)‑2‑(2‑nitrophenyl)‑1H‑benzo[d]
imidazole (3i) [49]
1
Yellow solid; H NMR (400 MHz, CDCl3) δ 8.15 (ddd,
J = 13.0, 7.2, 2.6 Hz, 2H), 7.86 (d, J = 7.9 Hz, 1H),
7.74–7.60 (m, 2H), 7.56–7.42 (m, 3H), 7.39–7.33 (m,
1H), 7.33–7.27 (m, 1H), 7.15 (d, J=7.9 Hz, 1H), 6.95 (d,
J=7.1 Hz, 1H), 5.69 (s, 2H); 13C NMR (126 MHz, CDCl3)
δ 149.8, 149.0, 147.0, 143.0, 134.8, 134.3, 133.1, 132.1,
131.5, 128.9, 128.4, 125.5, 125.27, 123.9, 123.2, 110.2,
45.7. ESI-MS [M + H]+: m/z = 375; HRMS (ESI orbit-
rap): calc. for C20H14N4O4 [M + H]+: 375.10878, found:
375.10951.
4.1.6 1‑(2,3‑Dimethoxybenzyl)‑2‑(2,3‑dimethoxyphenyl)‑1
H‑benzo[d]imidazole (3f)
Yellow solid (MP: 86–90 °C); 1H NMR (400 MHz, CDCl3)
δ 7.83 (d, J = 7.8 Hz, 2H), 7.30–7.23 (m, 5H), 7.22–7.17
(m, 2H), 7.16–7.13 (m, 4H), 7.07–7.02 (m, 2H), 6.82 (t,
J = 7.9 Hz, 2H), 6.76 (dd, J = 8.2, 1.3 Hz, 2H), 6.28 (dd,
J=7.6, 1.3 Hz, 2H), 5.34 (s, 4H), 3.91 (s, 6H), 3.82 (s, 6H),
3.65 (t, J = 5.7 Hz, 12H); 13C NMR (126 MHz, CDCl3)
δ 152.7, 152.5, 151.7, 147.5, 146.4, 143.2, 135.3, 130.0,
125.4, 124.5, 124.1, 123.8, 122.6, 122.0, 119.8, 114.0, 111.7,
110.9, 61.4, 60.4, 55.9, 55.72, 43.4; ESI-MS [M + H]+:
m/z = 405; HRMS (ESI orbitrap): calc. for C24H24N2O4
[M+H]+: 405.1808, found: 405.1809.
4.1.10 1‑(3‑Nitrobenzyl)‑2‑(3‑nitrophenyl)‑1H‑benzo[d]
imidazole (3j) [33]
1
Pale yellow solid; H NMR(300 MHz, CDCl3 + DMSO)
δ 8.52 (d, J = 1.9 Hz, 1H), 8.39–8.32 (m, 1H), 8.19 (d,
J=8.1 Hz, 1H), 8.11–8.00 (m, 2H), 7.89 (d, J=6.8 Hz, 1H),
7.72 (t, J=8.0 Hz, 1H), 7.58 (t, J=7.9 Hz, 1H), 7.45–7.29
(m, 4H), 5.65 (s, 2H); 13C NMR (75 MHz, CDCl3 +DMSO)
δ 151.0, 148.4, 142.8, 138.0, 135.8, 134.8, 131.8, 131.4,
130.3, 130.0, 124.6, 124.1, 124.0, 123.5, 123.2, 121.0,
120.4, 110.2, 47.6; ESI-MS [M + H]+: m/z = 375; HRMS
(ESI orbitrap): calc. for C20H14N4O4 [M+H]+: 375.10878,
found: 375.10954.
4.1.7 1‑(3,4,5‑Trimethoxybenzyl)‑2‑(3,4,5‑trimethoxypheny
l)‑1H‑benzo[d]imidazole (3g) [49]
White solid; 1H NMR (400 MHz, CDCl3) δ 7.91–7.84 (m,
1H), 7.37–7.34 (m, 1H), 7.34–7.28 (m, 2H), 6.92 (s, 2H),
6.36 (s, 2H), 5.40 (s, 2H), 3.89 (s, 3H), 3.82 (s, 3H), 3.72 (d,
J=1.2 Hz, 6H), 3.71 (s, 6H); 13C NMR (101 MHz, CDCl3)
δ 154.1, 154.0, 153.3, 142.8, 139.4, 137.4, 136.4, 132.5,
125.1, 123.2, 122.8, 119.9, 110.0, 106.3, 102.7, 60.89,
56.2, 56.0, 48.5; ESI-MS [M+H]+: m/z=465; HRMS (ESI
orbitrap): calc. for C26H28N2O6 [M+H]+: 465.2020, found:
465.2033, calc. for C26H28N2O6 [M+Na]+: 487.1839, found:
487.1852.
4.1.11 1‑(4‑Nitrobenzyl)‑2‑(4‑nitrophenyl)‑1H‑benzo[d]
imidazole (3k) [49]
1
White solid; H NMR (400 MHz, CDCl3) δ 8.33 (d,
J=8.9 Hz, 2H), 8.24 (d, J=8.8 Hz, 2H), 7.93 (d, J=7.9 Hz,
1H), 7.85 (d, J=8.8 Hz, 2H), 7.44–7.38 (m, 1H), 7.38–7.32
(m, 1H), 7.27 (d, J=9.3 Hz, 3H), 7.21 (d, J=7.9 Hz, 1H),
5.58 (s, 2H); 13C NMR (101 MHz, CDCl3) δ 148.5, 147.9,
143.1, 142.8, 135.7, 130.0, 126.7, 124.6, 124.1, 123.8,
120.8, 110.2, 48.0; ESI-MS [M + H]+: m/z = 375; HRMS
(ESI orbitrap): calc. for C20H14N4O4 [M+H]+: 375.10878,
found: 375.10947.
4.1.8 6‑Nitro‑1‑(3,4,5‑trimethoxybenzyl)‑2‑(3,4,5‑trimetho
xyphenyl)‑1H‑benzo[d]imidazole (3h)
1
Orange Solid (MP: 144–148 °C); H NMR (500 MHz,
CDCl3) δ 8.33–8.26 (m, 2H), 7.92 (d, J=9.0 Hz, 1H), 6.96
(s, 2H), 6.32 (s, 2H), 5.47 (d, J=17.1 Hz, 2H), 3.91 (s, 3H),
3.83 (s, 3H), 3.74 (s, 6H), 3.73 (s, 6H). 13C NMR (126 MHz,
CDCl3) δ 158.8, 154.1, 153.6, 147.4, 143.8, 140.4, 137.9,
135.8, 131.2, 123.8, 120.0, 118.9, 106.9, 106.5, 102.8, 60.9,
56.3, 56.0, 48.9. ESI-MS [M+H]+: m/z=510; HRMS (ESI
orbitrap): calc. for C26H27N3O8 [M + H]+ : 310.08223,
found: 310.08375.
4.1.12 1‑(4‑Fluorobenzyl)‑2‑(4‑fluorophenyl)‑1H‑benzo[d]
imidazole (3l) [33]
1
White solid; H NMR (500 MHz, CDCl3) δ 7.87 (d,
J = 7.9 Hz, 1H), 7.64 (dd, J = 7.9, 5.4 Hz, 2H), 7.32 (t,
J=7.5 Hz, 1H), 7.26 (t, J=7.4 Hz, 1H), 7.21 (d, J=7.9 Hz,
1H), 7.14 (t, J = 8.4 Hz, 2H), 7.02 (dd, J = 15.7, 7.2 Hz,
4H), 5.39 (s, 2H).; 13C NMR (101 MHz, CDCl3) δ 152.4,
143.1, 140.0, 135.9, 133.3, 131.8, 130.6, 129.5, 126.2,
125.9, 125.1, 123.9, 123.3, 122.4, 120.5, 110.3, 48.0; ESI-
MS [M+H]+: m/z=321; HRMS (ESI orbitrap): calc. for
C20H14F2N2 [M+H]+: 321.1197, found: 320.1198.
1 3

J. Pogula et al.
4.1.13 1‑(4‑(Trifluoromethyl)benzyl)‑2‑(4‑(trifluoromethyl)
4.1.17 2‑(Furan‑2‑yl)‑1‑(furan‑2‑ylmethyl)‑5, 6‑dime‑
phenyl)‑1H‑benzo[d]imidazole (3m) [52]
thyl‑1H‑benzo[d]imidazole (3q) [53]
1
Red solid; 1H NMR (500 MHz, CDCl3) δ 7.91 (d, J=8.1 Hz,
1H), 7.78 (d, J=8.2 Hz, 2H), 7.73 (d, J=8.4 Hz, 2H), 7.62
(d, J=8.2 Hz, 2H), 7.40–7.35 (m, 1H), 7.33–7.28 (m, 1H),
7.21 (t, J=7.1 Hz, 3H), 5.51 (s, 2H). 13C NMR (101 MHz,
CDCl3) δ 152.4, 143.17, 140.0, 135.9. 133.3, 129.5, 126.2,
125.8, 123.9, 123.3, 120.5, 110.3, 48.06. ESI-MS [M+H]+:
m/z = 421; HRMS (ESI orbitrap): calc. for C22H14F6N2
[M+H]+: 286.1133, found: 286.1128.
Brown Solid; H NMR (400 MHz, CDCl3) δ 7.57 (d,
J = 31.2 Hz, 2H), 7.33 (s, 1H), 7.24 (d, J = 11.4 Hz, 1H),
7.14 (s, 1H), 6.58 (s, 1H), 6.23 (d, J = 30.9 Hz, 2H), 5.57
(s, 2H), 2.39 (s, 3H), 2.37 (s, 3H). 13C NMR (101 MHz,
CDCl3) δ 149.9, 145.6, 143.6, 143.2, 142.5, 141.6, 134.0,
132.5, 131.8, 119.8, 112.2, 111.9, 110.5, 110.1, 108.1, 41.6,
20.7, 20.3. ESI-MS [M+H]+: m/z=293; HRMS (ESI orbit-
rap): calc. for C18H16N2O2 [M + H]+: 293.12845, found:
293.12657.
4.1.14 2‑(Pyridin‑2‑yl)‑1‑(pyridin‑2‑ylmethyl)‑1H‑benz
o[d]imidazole (3n ) [49]
4.1.18 2‑(Furan‑2‑yl)‑1‑(furan‑2‑ylmethyl)‑6‑ni‑
tro‑1H‑benzo[d]imidazole (3r)
1
White solid; H NMR (400 MHz, CDCl3) δ 8.58 (tdd,
1
J=2.6, 1.7, 0.8 Hz, 2H), 8.49 (d, J=8.1 Hz, 1H), 7.89–7.86
(m, 1H), 7.84 (dd, J = 7.9, 1.8 Hz, 1H), 7.49 (td, J = 7.8,
1.8 Hz, 1H), 7.38 (dd, J=7.2, 0.9 Hz, 1H), 7.35–7.29 (m,
2H), 7.29–7.25 (m, 2H), 7.15 (dd, J = 6.7, 5.0 Hz, 1H),
6.91 (d, J=7.9 Hz, 1H), 6.30 (s, 2H).13C NMR (101 MHz,
CDCl3) δ 157.4, 149.8, 149.1, 148.7, 137.2, 124.6, 123.9,
123.1, 122.3, 120.0, 110.8, 51.1. ESI-MS [M + H]+:
m/z = 286; HRMS (ESI orbitrap): calc. for C18H14N4
[M+H]+: 287.12912 found: 287.12764.
Brown solid; (MP: 146–150 °C) H NMR (500 MHz,
CDCl3) δ 8.49 (d, J=2.0 Hz, 1H), 8.23 (dd, J=8.9, 2.1 Hz,
1H), 7.80 (d, J=8.9 Hz, 1H), 7.74 (d, J=1.1 Hz, 1H), 7.39
(d, J=3.5 Hz, 1H), 7.36 (t, J=3.1 Hz, 1H), 6.69 (dd, J=3.5,
1.7 Hz, 1H), 6.37 (d, J = 3.1 Hz, 1H), 6.33 (dd, J = 3.1,
1.8 Hz, 1H), 5.76 (s, 2H); 13C NMR (126 MHz, CDCl3)
δ 155.6, 145.1, 143.2, 119.6, 119.0, 115.2, 112.6, 110.7,
109.2, 107.1, 42.1; ESI-MS [M + H]+: m/z = 310; HRMS
(ESI orbitrap): calc. for C16H11N3O4 [M+H]+: 310.0837,
found: 310.0822.
4.1.15 3‑(Pyridin‑3‑yl)‑1‑(pyridin‑3‑ylmethyl)‑1H‑benz
o[d]imidazole (3o) [32]
4.2 General Experimental Procedure for the Synthesis
of Quinoxalines from Substituted 1, 2-Diamines
and α-Bromo Ketones
1
Yellow gummy liquid; H NMR (400 MHz, CDCl3) δ
8.91 (s, 1H), 8.72 (t, J= 12.7 Hz, 1H), 8.60 (dd, J= 25.7,
10.1 Hz, 1H), 8.47 (s, 1H), 8.01 (d, J=7.9 Hz, 1H), 7.90 (d,
J = 7.9 Hz, 1H), 7.43 (dd, J = 7.7, 4.9 Hz, 1H), 7.40–7.33
(m, 1H), 7.30 (t, J=6.2 Hz, 2H), 7.28–7.22 (m, 2H), 5.50
(s, 2H); 13C NMR (126 MHz, CDCl3) δ 150.9, 149.5, 147.8,
143.2, 136.7, 135.7, 133.6, 131.5, 126.3, 123.9, 123.8, 123.3,
120.4, 110.2, 46.16; ESI-MS [M+H]+: m/z=286; HRMS
(ESI orbitrap): calc. for C18H14N4[M + H]+: 286.1291,
found: 286.1292.
A 10 mL Schlenk flask was charged with diamines
(1 mmol), α-bromoketone (1 mmol), Cu(0)/Al2O3 catalyst
(Cu 8 wt%, 25 mg) in water (2 mL). The reaction mixture
was stirred at room temperature for appropriate time. After
the completion of the reaction, as monitored by TLC, 5 mL
of diethyl ether was added and the catalyst was separated
by simple centrifugation. The reaction mixture was treated
with brine (10 mL) and the organic layer was separated.
The aqueous layer was back extracted with diethyl ether
(3 × 5 mL). The combined organic extract was dried with
anhydrous Na2SO4 (50 g) and was concentrated under
reduced pressure. The pure product was isolated by flash
column chromatography on silica gel using ethyl acetate/
hexane (10%) as an eluent.
4.1.16 2‑(Furan‑2‑yl)‑1‑(furan‑2‑ylmethyl)‑1H‑benzo[d]
imidazole (3p) [49]
Brown solid; 1H NMR (400 MHz, CDCl3) δ 7.83–7.74 (m,
1H), 7.62 (d, J=19.4 Hz, 1H), 7.49 (dd, J=5.9, 2.9 Hz, 1H),
7.35–7.27 (m, 3H), 7.22 (d, J=3.3 Hz, 1H), 6.63–6.59 (m,
1H), 6.31–6.20 (m, 2H), 5.64 (s, 2H).13C NMR (126 MHz,
CDCl3) δ 149.6, 145.4, 143.9, 142.9, 142.6, 135.5, 123.2,
122.9, 119.8, 112.9, 112.1, 110.5, 110.0, 108.3, 41.7; ESI-
MS [M + H]+: m/z = 265; HRMS (ESI orbitrap): calc. for
C16H12N2O2[M+H]+: 265.0971, found: 265.0979.
4.2.1 2‑Phenylquinoxaline (4a) [51]
1
White solid; H NMR (400 MHz, CDCl3) δ 9.34 (s, 1H),
8.23–8.18 (m, 2H), 8.18–8.10 (m, 2H), 7.82–7.72 (m, 2H),
7.61–7.50 (m, 3H). 13C NMR (101 MHz, CDCl3) δ 151.90
(s), 143.40 (s), 142.35 (s), 141.63 (s), 136.83 (s), 130.27 (d,
1 3

Nano Copper(0)-Stabilized on Alumina: Efficient and Recyclable Heterogeneous Catalyst for…
6. Meuthen B, Jandel AS (2008) Coil Coating, Auflage, Friedr.
J=9.7 Hz), 129.62 (d, J=9.8 Hz), 129.19 (s), 127.59 (s).
ESI-MS [M+H]+: m/z=207.
Vieweg & Sohn Verlag, Wiesbaden, p 65
7. Scherer GG (2008) Fuell cells II. Springer, Berlin, pp 65–120
8. Sarges R, Howard HR, Browne RG, Lebel LA, Seymour PA, Koe
BK (1990) J Med Chem 33:2240–2254
4.2.2 6‑Nitro‑2‑phenylquinoxaline (4b) [53]
9. Gazit A, App H, McMahon G, Chen J, Levitzki A, Bohmer FD
(1996) J Med Chem 39:2170–2177
Yellow solid; 1H NMR (400 MHz, CDCl3) δ 9.50 (s, 1H),
9.03 (d, J = 2.4 Hz, 1H), 8.56 (dd, J = 9.2, 2.4 Hz, 1H),
8.33–8.23 (m, 3H), 7.66–7.56 (m, 3H). 13C NMR (101 MHz,
CDCl3) δ 154.3, 147.4, 145.5, 144.9, 140.3, 135.6, 131.4,
131.2, 129.4, 127.9, 125.6, 123.8.ESI-MS [M + H]+:
m/z=252.
10. Seitz LE, Suling WJ, Reynolds RC (2002) J Med Chem
45:5604–5606
11. Preston PN (1981) Chemistry of heterocyclic compounds. Wiley,
New York
12. Kokare ND, Sangshetti JN, Shinde DB (2007) Synthesis
18:2829–2834
13. Dudd LM, Venardou E, Garcia-Verdugo E, Licence P, Blake AJ,
Wilson C, Poliakoff M (2003) Green Chem 5:187–192
14. Saha P, Ali MA, Ghosh P, Punniyamurthy T (2010) Org Biomol
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4.2.3 6‑Chloro‑2‑phenylquinoxaline (4c) [54]
15. Mahesh D, Sadhu P, Punniyamurthy T (2015) J Org Chem
80:1644–1650
1
White solid; H NMR (400 MHz, CDCl3) δ 9.32 (s, 1H),
8.25–8.13 (m, 3H), 8.05 (d, J = 8.9 Hz, 1H), 7.68 (dd,
J = 17.2, 10.1 Hz, 1H), 7.62–7.50 (m, 3H). 13C NMR
(101 MHz, CDCl3) δ 150.6, 142.8, 142.2, 141.6, 136.6,
135.2, 130.5, 129.8, 129.5, 129.1, 128.8. ESI-MS [M+H]+:
m/z=241.
16. Song XR, Qiu YF, Song B, Hao XH, Han YP, Gao P, Liu XY,
Liang YM (2015) J Org Chem 80:2263–2271
17. Rasheed Sk, Rao DN, Das P (2015) J Org Chem 80:9321–9327
18. Huang J, Chan J, Chen Y, Borths CJ, Baucom KD, Larsen RD,
Faul MM (2010) J Am Chem Soc 132:3674–3675
19. Ueda S, Buchwald SL (2012) Angew Chem Int Ed
51:10364–10367
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4.2.4 5‑Methyl‑2‑phenylquinoxaline (4d) [55]
21. Shargi H, Sarvari MH, Moeini F (2008) Can J Chem
86:1044–1051
1
White solid; H NMR (400 MHz, CDCl3) δ 9.31 (s, 1H),
22. Zou B, Yuan Q, Ma D (2007) Angew Chem Int Ed 46:2598–2601
23. Brasche G, Buchwald SL (2008) Angew Chem Int Ed
47:1932–1934
8.16 (t, J = 14.6 Hz, 2H), 8.10–7.82 (m, 2H), 7.66–7.45
(m, 4H), 2.61 (s, 3H). 13C NMR (75 MHz, CDCl3) δ 140.8,
140.1, 137.0, 132.6, 131.8, 130.0, 129.1, 127.4, 21.8. ESI-
MS [M+H]+: m/z=221.
24. Xiao Q, Wang W-H, Liu G, Meng F-K, Chen J-H, Yang Z, Shi Z-J
(2009) Chem Eur J 15:7292–7296
25. Deng X, McAllister H, Mani NS (2009) J Org Chem
74:5742–5745
4.2.5 2‑(4‑Chlorophenyl)quinoxaline (4e) [51]
26. Zhao D, Hu J, Wu N, Huang X, Qin X, Lan J, You J (2011) Org
Lett 13:6516–6519
1
White solid; H NMR (400 MHz, CDCl3) δ 9.31 (s, 1H),
27. Yang D, Fokas D, Li J, Yu L, Baldino CM (2005) Synthesis
1:47–56
8.20–8.10 (m, 4H), 7.84–7.72 (m, 2H), 7.55 (d, J=8.6 Hz,
2H).13C NMR (101 MHz, CDCl3) δ 150.6, 142.8, 142.2,
141.6, 136.6, 135.2, 130.5, 129.8, 129.5, 129.1, 128.8. ESI-
MS [M+H]+: m/z=241.
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31. Chebolu R, Kommi DN, Kumar D, Bollineni N, Chakraborti AK
(2012) J Org Chem 77:10158–10167
Acknowledgements The authors wish to thank the CSIR and indus-
try sponsored project SSP-0670 for financial support. S.L thanks DST,
govt of India for financial support (GAP-0566). The authors are grate-
ful to the Director, CSIR-IICT for providing the necessary infrastruc-
ture. J.P is grateful to Dr. Dharmendra Kumar Tiwari for his valuable
suggestions.
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54:6986–6990
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