One-pot synthesis of symmetrical and asymmetrical diphenylamines from guanidines with aryl iodide using Cu/Cu2O nanocatalyst

Article abstract of DOI:10.1016/j.mcat.2020.110998

This work reports the selective one-pot synthesis of symmetrical and asymmetrical amines from guanidines as ammonia surrogate. The use of guanidine as ammonia source will eliminate the need of handling liquid ammonia. The reaction was performed using Cu/Cu₂O nanocatalyst under ligand-free condition. The synthesized catalyst was characterized by a various technique like XRD, FEG-SEM, HRTEM and XPS. The different diphenylamines are produced in good to very good yields. Recyclability study of catalyst shows that up to five cycles there is no significant loss in its activity.

Full text of DOI:10.1016/j.mcat.2020.110998

MolecularCatalysis492(2020)110998
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
One-pot synthesis of symmetrical and asymmetrical diphenylamines from
guanidines with aryl iodide using Cu/Cu2O nanocatalyst
Shivkumar R. Chaurasia, B.M. Bhanage*
Institute of Chemical Technology, Mumbai, 400019, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Cu/Cu2O
Biphenyl amine
Nano-catalysis
Guanidine
This work reports the selective one-pot synthesis of symmetrical and asymmetrical amines from guanidines as
ammonia surrogate. The use of guanidine as ammonia source will eliminate the need of handling liquid am-
monia. The reaction was performed using Cu/Cu2O nanocatalyst under ligand-free condition. The synthesized
catalyst was characterized by a various technique like XRD, FEG-SEM, HRTEM and XPS. The different diphe-
nylamines are produced in good to very good yields. Recyclability study of catalyst shows that up to five cycles
there is no significant loss in its activity.
Recyclable
Introduction
nanospheres and shows that interface is highly active for the hydro-
amination of aromatic terminal alkynes with aniline derivatives [10].
The utilization of copper based nanoparticles for organic synthesis
has generated substantial interest in the field of catalysis due to their
high catalytic activity, low cost and easy availability [1]. Although Cu
and Cu2O nanoparticles are showing a higher catalytic activity for
various cross-coupling reaction than copper oxide (CuO) nanoparticles,
they get easily oxidized by air or under the environment of reaction
condition [2]. Several reports available prove that controlled synthesis
of core-shell inorganic hybrid nanostructures such as Cu/Cu2O leads to
enhance catalytic activity compared to its component [3]. Su and co-
worker have reported a Cu@Cu2O core–shell microspheres for di-
methyldichlorosilane synthesis [4]. Recently, Yuan et al. has reported a
Nb2O5 supported Cu/Cu2O nanaoparticle for direct synthesis of alky-
nylphosphonates from alkynes and phosphite esters [5]. Meanwhile,
Cu/Cu2O has also been utilized as efficient visible light photo-catalysts
due to efficient electron-hole pairs separation having application in
photocatalysis dye degradation and hydrogen generation, etc. [6].
To date, various methods have been reported to synthesize Cu/Cu2O
core-shell nanostructures with well-defined morphologies such as
cubes, octahedrons, tetrahexahedron, stars, and hollow structures [7].
Kamazani et al. has disclosed a solvent-free synthesis of Cu–Cu2O na-
nocomposites via green thermal decomposition route using novel pre-
cursor and investigation of its photocatalytic activity for degradation of
MB under visible-light irradiation [8]. Ohiienko et. al has reported a
polyol method for synthesis of uniform size Cu–CuO core–shell nano-
particle by double salt reduction [9]. Manideepa et al. has prepared a
Cu/Cu2O nanoparticle on nitrogen rich mesoporous carbon
However, in most of these methods, either a template or capping agents
are employed to control the growth and shape of nanoparticles. Also,
sometimes it is difficult to completely remove the template or capping
agent to get highly pure single-phase nanoparticles. On the other hand,
the microwave assisted route for the synthesis of a nanoparticle is
considered to be advantageous as it provides simple, fast and energy
efficient way for the synthesis of a nanoparticle [11].
Diphenylamine and its derivatives constitute an extensive family of
active compounds in pharmaceuticals, natural products, agrochemicals
and materials science [12]. They have usually been prepared from the
N-arylation of arylamines with aryl halides in the presence of transition
metals such as Pd, Cu, Ni and Fe as a catalyst [13]. Efforts have been
made to prepare diphenyl-amines selectively from ammonia with aryl
halides [14]. However, the handling of ammonia is a tedious task and it
causes high pressure inside the reaction vessel, obviously making the
procedures not to be operationally safe and simple from the applica-
tion’s point of view. In 2008, Gao and co-workers have shown the use of
amidine hydrochlorides as a new ammonia surrogate for the synthesis
of a primary amine with various aryl halide in the presence of copper
catalyst with suitable ligand [15]. In 2012, Tlili et al. have reported a
method for the synthesis of both, symmetrical and asymmetrical ary-
lated amines using LiNH2 source of nitrogen [16]. Alternatively,
copper-catalyzed N,N’-diarylation of guanidine has been documented
by Antilla et al. and Ma et al. [17]. However, no reports are available
for the synthesis of biaryl amine using guanidine as a nitrogen source.
This method provides a one-pot selective synthesis of symmetrical or
⁎ Corresponding author.
E-mail address: bm.bhanage@gmail.com (B.M. Bhanage).
https://doi.org/10.1016/j.mcat.2020.110998
Received 3 March 2020; Received in revised form 21 April 2020; Accepted 28 April 2020
2468-8231/©2020ElsevierB.V.Allrightsreserved.

S.R. Chaurasia and B.M. Bhanage
MolecularCatalysis492(2020)110998
Preparation of Cu2O catalyst
To a 100 mL aqueous solution of sodium thiosulfate (Na2S2O3),
25 mL of copper sulphate (CuSO4⋅5H2O) solution was added with
continuous stirring. The molar ratio of copper sulphate to sodium
thiosulfate and was maintained at 1:4. The resultant solution was then
added to the 125 mL of 5 M NaOH solution with vigorous stirring. The
obtained Cu2O nanoparticles were then washed with deionized water
and vacuum dried at 60 °C [18].
Preparation of Cu/Cu2O nanocatalyst
Cu/Cu2O nanoparticles were prepared by the method reported by
Bhosale et al. [11]. To a glass beaker containing 10 mL of 1,3-propa-
nediol, 0.4 g of Cu(CH3COO)2·H2O was added. The resultant mixture
was then kept inside a domestic microwave oven at an electric power of
600 W for 4 min with the on/off mode having a time interval of 30 s,
color of reaction mixture changes from blue to brick-red indicating
formation of Cu/Cu2O nanoparticles.
Scheme 1. Reaction of guanidine with aryl halides.
Preparation of Cu° catalyst
asymmetrical aryl amine using Cu/Cu2O as a heterogeneous recyclable
catalyst Scheme 1.
0.4 g of CuSO4⋅5H2O was taken in a glass beaker containing 10 mL
of 1,3-propanediol and 0.4 g of glucose. Then, other steps are same as
that of Cu/CuO preparation [19].
During the initial study, attempts were taken for the development of
amidine based chemistry. Considering the principle of green and sus-
tainable chemistry, we attempted this reaction in glycerol as a green
solvent, 3 equiv. of inexpensive and easily available KOH base and
prepared nano Cu/Cu2O as a catalyst without using any additional li-
gand at 130 °C for 36 h. Surprisingly, no formation of N,N-diaryl gua-
nidine could be noted, instead, there was a formation of aniline and
diphenylamine. This inspired us to use guanidine nitrate as a new
ammonia surrogate for the synthesis of symmetrical and asymmetrical
diphenyl amines as there was no such report in the literature. Herein,
we report simple one-pot synthesis of symmetrical diphenylamines and
asymmetrical diphenylamines from guanidines in the presence of Cu/
Cu2O-nanocatalyst.
Preparation of Cu/Cu2O@Fe3O4 nanocatalyst
For the preparation of Cu/Cu2O@Fe3O4 catalyst, 1.0 g of Fe3O4 was
first dispersed in 10 mL of 1,3-propanediol using sonication for 30 min.
To the resultant mixture 0.4 g of Cu(CH3COO)2·H2O was added. Then,
other steps are same as that of Cu/Cu2O preparation.
General procedure for the nano Cu/Cu2O catalyzed synthesis of
diphenylamine
To
a mixture of iodobenzene (1.0 mmol), guanidine nitrate
(0.5 mmol), and KOH (3.0 mmol) in DMSO (2 mL), Cu/Cu2O (25 mg)
was added. The resulting mixture was then sealed and stirred for 36 h at
130 °C. After completion of the reaction, the reaction mixture was
cooled to room temperature, diluted with water followed by extraction
using ethyl acetate. The organic phase was dried over anhydrous
Na2SO4. The crude residue was obtained after evaporation of the sol-
vent on rotavapor. The residue was purified by column chromatography
with ethyl acetate and pet-ether as eluent to give the pure product.
Experimental
Chemical
All the materials were obtained from commercial suppliers and used
without further purification. Thin-layer chromatography (TLC) was
performed using silica gel 60 F254 and visualized using UV light.
Column chromatography was performed with
a silica gel (mesh
60–120).
Results and discussion
Crystallinity and phase information of the synthesize Cu/Cu2O NP
was examined by the X-ray diffraction (XRD) pattern. Fig. 1 depicts the
XRD pattern of the prepared Cu/Cu2O nanocatalyst. The peaks at 2θ
values of 43.30°, 50.43°, 74.13° correspond to the reflections of (111),
(200) and (220) planes of cubic Cuo with lattice constant of a = 3.615 Å
(d-spacing = 0.2090 nm) and diffraction peaks at 2θ values 29.5°,
36.4°, 42.3°, 61.3°, 73.5°, 77.3° can be ascribed to the (110), (111),
Characterization
XRD pattern of synthesized materials was recorded using (Shimadzu
XRD-6100 using CuKα-1.54 Å) with scanning rate 2θ per min and 2
theta (θ) angle ranging from 20 (θ) to 80 (θ) with current 30 mA and
voltage 40 kW. The FT-IR spectra were recorded on Brucker Perkin
Elmer-100 spectrometer in the wavelength range from 400 to
4000 cm−1. Tescan MIRA 3 model with secondary electron (SE) de-
tector was used for Field emission gun scanning electron microscopy
analysis using 10.0 kV. The energy dispersive X-ray spectrum (EDS) was
recorded by Oxford instrument (model 51-ADD0007). The GC–MS-QP
(200), (220), (311), (222) planes of cubic Cu2O (d-spacing
=
0.2469 nm), respectively. The sharp peaks in the pattern indicate that
the Cu/Cu2O NPs are highly crystallized. The absence of peak for CuO
indicates that the prepared catalyst is Cu/Cu2O. SEM images of syn-
thesized nanoparticles were taken to examine the surface morphology.
From Fig. 2 we can see that Cu/Cu2O NP is having an irregular shape
morphology and particle in size ranges of 60−120 nm. The chemical
composition of the synthesized Cu/Cu2O NPs was examined by energy
dispersive spectroscopy (EDS). Spectra at different position were re-
corded to find the average composition of the Cu/Cu2O NPs. The pre-
sence of only oxygen and copper peak indicates the prepared
2010 instrument (Rtx-17, 30 m
×
25 mm ID, film thickness
100 °C–240 °C at
(df) = 0.25 μm) (column flow 2 mL min−1
,
10 °C min−1 rise) was used for the mass analysis of the organic pro-
ducts. The yields of synthesize compounds were confirmed by Perkin
Elmer Clarus 400 gas chromatography equipped with a flame ionization
detector and a capillary column (elite-1, 30 m × 0.32 μm × 0.25 μm).
2

S.R. Chaurasia and B.M. Bhanage
MolecularCatalysis492(2020)110998
differentiate between them (Fig. 4b).
The core-level O1 s spectrum is shown in Fig. 4(c). Oxygen 1 s peak
observed at 530.1 eV represent lattice oxygen in Cu2O. Thermal stabi-
lity of Cu/Cu2O nanocatalyst was determined by DSC-TGA analysis. A
weight loss was observed near 100 °C which can be due to presence of
moisture absorbed on the surface. A weight gain was observed near
250 °C which can be due to oxidation of metallic copper to copper oxide
(see ESI Fig. 2(a)).
After thoroughly characterizing the catalyst it was tested for its
catalytic activity. From Scheme 2 we can see that three different pro-
duct are possible on reaction of iodobenzene with guanidine nitrate.
Initially, iodobenzene (1a, 2.0 mmol) with guanidine nitrate (2a,
0.75 mmol) were chosen as model substrates. The reaction conditions
were optimized using various reaction parameters and include catalyst,
base, solvent, temperature and time. Only 35% of diphenylamine (3aa)
was formed when the reaction was performed using Cu2O catalyst in
glycerol at 130 °C for 36 h (Table 1, entry 1). Interestingly, addition of
DMSO led to a noticeable increase in the yield of 3aa to 44% (Table 1,
entry 2). A further increase in the yield of product 3aa was noted when
reaction was performed in only DMSO however, addition of ligand L-
proline led to a slight decrease in the yield this could because of change
in the solvent media system (Table 1, entry 3). Other Cu-catalyst such
as CuI, CuBr and CuO were also screened but they were not found to be
active to increase in the yield of 3aa (Table 1, entries 4–6). Next, an
attempt was made to carry out this reaction using nano-Cu/Cu2O
(Table 1, entry 7), a significant increase in the yield was noted pro-
ducing highest 88% yield of 3aa. Further, to check whether only Cu° or
Cu2O is sufficient to catalyze the reaction. Both Cu° and Cu2O nano-
catalyst were prepared and employed for this reaction individually
(Table 1, entries 8, 9). As per our expectation both the catalysts furn-
ished the desired product however a relatively lower yield of 3aa was
Fig. 1. XRD pattern of Cu/Cu2O NPs.
nanomaterial is pure and free from other metal impurities (see ESI
Fig. 2(b)).
To distinguish two different Cu phases HRTEM image was taken.
From Fig. 3, we can see that two different lattice fringes are having an
interlayer spacing of 0.21 nm and 0.25 nm which can be ascribed to the
(311) lattice plane of Cu0 and (111) lattice plane of Cu2O respectively.
XPS was carried out to study the surface chemical nature of syn-
thesized Cu/Cu2O nanoparticles. Fig. 4a shows the typical XPS spectra
of Cu/Cu2O NP ranging from 0 to 1400 eV. The presence of respective
peak for Cu, C and O in the survey spectrum confirms that the prepared
sample contains Cu, C and O only. The copper 2p peaks at binding
energies of 953.1 and 932.6 eV can be attributed to the binding energy
of Cu 2p1/2 and Cu 2p3/2 respectively. The satellite peak at 942.9 eV
and 963.7 eV was also observed. However, due to very small differences
in the binding energy of Cu1+ (Cu2O) and Cu0 by XPS it is difficult to
Fig. 2. SEM images of Cu/Cu2O NPs.
3

S.R. Chaurasia and B.M. Bhanage
MolecularCatalysis492(2020)110998
Fig. 3. (a, b) TEM images of Cu/Cu2O NPs, (c) HR-TEM image of Cu/Cu2O with lattice spacing, (d) HR-TEM image of Cu2O nanoparticle with lattice spacing.
Fig. 4. XPS mapping of Cu/Cu2O NPs (a) survey spectra of Cu/Cu2O; (b) high-resolution spectra of Cu2p3/2; (c) high-resolution spectra of O1 s.
4

S.R. Chaurasia and B.M. Bhanage
MolecularCatalysis492(2020)110998
nitro (NO2) only 4-nitroaniline was observed as a major product and
along with the trace of coupling product. A derivative of guanidine was
also tested to get asymmetrical diphenylamine (Table 2).
While using the substituted guanidine it was expected to get a
mixture of symmetrical and asymmetrical diphenylamine due to lib-
eration of ammonia by hydrolysis along with arylamine as shown in
Scheme 3. However, only corresponding diphenylamine was observed
as a major product. Different asymmetrical diphenylamines were syn-
thesized in good to excellent yields. using diphenyl guanidine as a
coupling partner along with substituted iodobenzene (Table 2). Simi-
larly substituted urea was also used for the synthesis of asymmetrical
amine. A good yield of product was observed which also indicate that
decomposition of guanidine nitrate results in the formation of urea
which on further decomposition gives another molecule of corre-
sponding ammonia or arylamine. Although a detailed mechanism for
this transformation is still unclear, we proposed a possible catalytic
cycle as outlined in Fig. 5 Fig. 6. Initially, iodobenzene and Cu/Cu2O
give intermediate-I via oxidative addition. At higher temperature, the
presence of base leads to the thermal decomposition of guanidine to
give ammonia and one molecule of urea as reported by Limatibul et al.,
Homer et al. and Lewis et al. [20] The ammonia liberated is then in
presence of intermediate I to gives respective aniline liberating 1 mo-
lecule of hydrogen halide. Aniline formed is then attacked by another
molecule of iodobenzene and catalyst via oxidative addition to give
intermediate-II with the release of another molecule of hydrogen ha-
lide. The product diphenylamine is then liberated from the catalytic
system by reductive elimination and catalyst is regenerated [21].
However, in the case of substituted guanidine, its decomposition gives
corresponding amine with urea as a decomposition product which then
reacts with one molecule of iodobenzene to give diphenylamine. The
formation of aniline was also confirmed by GCeMS by carrying out the
reaction for the shorter duration.
Scheme 2. Synthesis of diphenylamine with other possible side product.
observed with nano-Cu2O catalyst. To further improve the catalyst se-
parability Cu/Cu2O nanoparticles loaded on Fe3O4 were prepared and
used as a catalyst, however lower yield was obtained (Table 1, entry
10). While studying the effect of catalyst loading on yield it was ob-
served that decrease in the catalyst loading to 15 mol% led to decrease
in the yield of product 3aa and on the other hand increase in the cat-
alyst loading did not have any considerable effect on the reaction.
(Table 1, entry 1112) Three mmol of KOH was found to be crucial for
this reaction providing 88% yield of 3aa on other hand performing the
reaction with two mmol and one mmol of KOH resulted into formation
of only 74 and 44% yield of 3aa respectively. (Table 1, entry 13, 14)
Other inorganic bases such as K2CO3 and Na2CO3 were also screened
providing 3aa in poor yields (Table 1, entries 15, 16). The reaction also
resulted in the formation of a 66% yield in solvent DMF (Table 1, entry
17). While studying the effect of time on product yield it was observed
that cessation of reaction for the shorter duration gives aniline as a
major product. However, prolonging the reaction duration does not
have any effect on the yield of diphenylamine (Table 1, entry 18–21)
Changing reaction environment from N2 to air leads to a lower yield of
the product (Table 1, entry 22)
The optimized reaction conditions are, 10 mol% Cu/Cu2O, (con-
firm) 20 mol, 3 equiv. of KOH, 1 mmol of guanidine nitrate (1a) and
2 mmol of iodobenzene (2), DMSO as the solvent at 130 °C under a
nitrogen atmosphere for 36 h. To demonstrate the advantages of de-
veloped catalytical system different derivatives of iodobenzene were
allowed to react with guanidine nitrate to gives symmetrical amine. The
electron-donating group methyl, methoxy were furnishing the desired
product in good yield however with electron-withdrawing group like
To validate the heterogeneous nature of the Cu/Cu2O catalyst hot
filtration experiment was carried out. In the first set of experiment,
catalyst was filtered off ; after 12 h at 130 °C, allowing the remaining
Table 1
Optimization of reaction parameter for the synthesis of diphenylaminea.
Entry
Catalyst (mol%)
Base (mmol)
Time (hr)
Solvent
Yield[b] (%)
1
Cu2O (20)
Cu2O (20)
Cu2O (20)
CuI (20)
CuBr (20)
CuO bulk
KOH (3)
KOH (3)
KOH (3)
KOH (3)
KOH (3)
KOH (3)
KOH (3)
KOH (3)
KOH (3)
KOH (3)
KOH (3)
KOH (3)
KOH (2)
KOH (1)
K2CO3 (2)
Na2CO3 (2)
KOH (3)
KOH (3)
KOH (3)
KOH (3)
KOH (3)
KOH (3)
36
36
36
36
36
36
36
36
36
36
36
36
36
36
36
36
36
12
24
30
40
36
Glycerol
Glycerol : DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
35
44
55
38
32
43
88
65
50
67
76
89
74
44
42
39
66
20
50
80
87
75
2
3c
4
5
6
7
8
Nano-Cu/Cu2O (20)
Nano-Cuo (20)
Nano-Cu2O (20)
Nano-Cu/Cu2O @Fe3O4
Nano-Cu/Cu2O (15)
Nano-Cu/Cu2O (30)
Nano-Cu/Cu2O
Nano-Cu/Cu2O (20)
Nano-Cu/Cu2O (20)
Nano-Cu/Cu2O
Nano-Cu/Cu2O
Nano-Cu/Cu2O
Nano-Cu/Cu2O
Nano-Cu/Cu2O
Nano-Cu/Cu2O
Nano-Cu/Cu2O
9
10
11
12
13
14
15
16
17
18
19
20
21
22d
DMSO
DMSO
DMSO
DMSO
DMSO
a Reaction condition: (1a) (2 mmol), (2a) (0.75 mmol), catalyst (10 mol %), solvent (2 mL) temp 130 °C, time 36 h. under N2 atmosphere. b GC yield based on area.
d
cUse of proline as ligand. Reaction carried out under air atmosphere.
5

S.R. Chaurasia and B.M. Bhanage
MolecularCatalysis492(2020)110998
Table 2
Substrate study of diphenylamine derivative.
1
2
3
4
5
Fig. 5. Plausible reaction mechanism for biarylamine synthesis.
Synthesis of unsymmetrical diphenylamine from diphenyl guanidine
6
7
8
9
10
11
12
Fig. 6. Recyclability study of Cu/Cu2O. (a) Reaction conditions: (1a) (2 mmol),
(2a) (1 mmol), Cu/Cu2O catalyst (30 mg), base (3 equiv.), DMSO (2 mL) at
130 °C for 36 h.
Synthesis of unsymmetrical diphenylamine from phenyl urea
13
and washed with water-ethanol mixture several times followed by
drying at 80 °C. The resultant catalyst is and then used for further re-
action. To compensate the handling loss while recovering the catalyst
from reaction mixture fresh catalyst was added to during each cycle.
From the graph, we can see that there is no significant drop in the
significant drop in the catalyst activity up to five cycles.
14
15
16
Conclusions
In summary, a simple method showing the use of guanidine as an
ammonia surrogate was reported for one-pot synthesis of various
symmetrical or asymmetrical di- using aryl halides. The reaction was
performed using Cu/Cu2O nanocatalyst under ligand-free condition and
eliminates the need for expensive catalysts and ligands. The synthesized
catalyst was characterized by various techniques like XRD, FEG- SEM,
HRTEM, XPS. The different diphenylamines are produced in good to
very good yields. The catalyst was recycled up to 5 cycles.
Reaction condition: (1a) (2 mmol), (2a) (1 mmol), Cu/Cu2O catalyst (20 mg),
base (3 equiv.), DMSO (2 mL), 130 °C for 36 h.
Scheme 3. Synthesis of asymmetrical diphenylamine and related product.
Declaration of Competing Interest
solution to react further under identical conditions whereas in another
vessel reaction was continued without taking out catalyst. No progress
was observed in the first reaction vessel however complete yield was
obtained in the second reaction vessel.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Catalyst recycling
CRediT authorship contribution statement
To established the recyclability of catalytic system. After completion
of reaction, catalyst was separated from reaction mixture by filtration
Shivkumar R. Chaurasia: Conceptualization, Investigation,
6

S.R. Chaurasia and B.M. Bhanage
MolecularCatalysis492(2020)110998
Methodology, Data curation, Formal analysis, Writing - original draft.
B.M. Bhanage: Conceptualization, Methodology, Writing - review &
editing, Supervision, Validation, Resources, Funding acquisition.
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