Pd/AlO(OH): A Heterogeneous, Stable and Recyclable Catalyst for N-Arylation of Aniline Under Ligand-Free Aerobic Condition

Article abstract of DOI:10.1007/s10562-017-2163-9

Abstract: Many synthetic methods have been reported to construct an aryl-nitrogen bond. But they mainly suffer from the usage of expensive ligands. Herein, we report highly effective, heterogeneous, reusable and ligand-free nanocatalyst for the Buchwald-Hartwig coupling reaction under aerobic condition. Wafer-like structure with nano-porous nature of AlO(OH) was identified with TEM and BET surface area analyses. Zero valent Pd nanoparticles (with less than 10?nm crystallite size which strongly evident from TEM analysis) were observed in XRD pattern, which was further confirmed by XPS studies. The reaction conditions were optimized and the scope of the reaction was extended with various aryl halides and aniline using low amount of Pd (0.31?mol%) based heterogeneous catalyst. Heterogeneity, reusability and stability (confirmed by XRD) were found to be reasonable. Use of meagre amount of Pd and the ligand-free aerobic condition make this system economically as well as environmentally feasible. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-017-2163-9

Catal Lett
DOI 10.1007/s10562-017-2163-9
Pd/AlO(OH): A Heterogeneous, Stable and Recyclable Catalyst
for N-Arylation of Aniline Under Ligand-Free Aerobic Condition
Sundaram Ganesh Babu1,2 · Balakumar Emayavaramban1 · Peter Jerome1 ·
Ramasamy Karvembu1
Received: 14 March 2017 / Accepted: 2 August 2017
© Springer Science+Business Media, LLC 2017
Abstract Many synthetic methods have been reported to
construct an aryl-nitrogen bond. But they mainly suffer from
the usage of expensive ligands. Herein, we report highly
effective, heterogeneous, reusable and ligand-free nanocata-
lyst for the Buchwald-Hartwig coupling reaction under aero-
bic condition. Wafer-like structure with nano-porous nature
of AlO(OH) was identified with TEM and BET surface area
analyses. Zero valent Pd nanoparticles (with less than 10 nm
crystallite size which strongly evident from TEM analysis)
were observed in XRD pattern, which was further confirmed
by XPS studies. The reaction conditions were optimized and
the scope of the reaction was extended with various aryl
halides and aniline using low amount of Pd (0.31 mol%)
based heterogeneous catalyst. Heterogeneity, reusability and
stability (confirmed by XRD) were found to be reasonable.
Use of meagre amount of Pd and the ligand-free aerobic
condition make this system economically as well as envi-
ronmentally feasible.
Graphical Abstract
Keywords Pd/AlO(OH) · Buchwald–Hartwig coupling ·
Ligand-free · Reusable · Heterogeneous · Aerobic condition
1 Introduction
Amines are ubiquitous in biology. Many neurotransmitters
are amines, including epinephrine, norepinephrine, dopa-
mine, serotonin and histamine. Many amine-based drugs are
designed to mimic or to interfere with the action of natural
amine neurotransmitters. Ephedrine and phenylephrine, as
amine hydrochlorides, are used as decongestants. Due to
their interesting physiological activities, secondary amines
are extremely important pharmacophores in numerous bio-
logically active compounds, which have greatly been touted
in drug discovery where the secondary amine can be utilized
as important scaffolding for further manipulations [1]. Sec-
ondary amines are also a vital class of intermediates in the
preparation of natural products. With the growing repertoire
* Sundaram Ganesh Babu
Sundaram.Babu@uct.ac.za
* Ramasamy Karvembu
kar@nitt.edu
1
Department of Chemistry, National Institute of Technology,
Tiruchirappalli 620015, India
2
Catalysis Institute, Department of Chemical Engineering,
University of Cape Town, Cape Town 7701, South Africa
Vol.:(0123456789)
1 3

S. Ganesh Babu et al.
of biologically relevant nitrogenous molecules, there is the
need for efficient synthetic methods to produce secondary
amines as it is the more useful intermediates [2–4].
times without any activity loss [14]. Recently, copper on
boehmite was reported as an effective catalyst for the oxi-
dation of alcohols in water at room temperature [15] and
also for the formation of β-enamino ketones/esters under
solvent, ligand and base free conditions [16]. Pd/AlO(OH)
was found to be an efficient nanocatalyst for some of the
industrially important organic transformations such as the
conversion of alkene to alkane [17], aryl halide to benzene
[18], oxime to amine [19], silane to sillyl alcohol [20], and
nitro benzene to aniline [21], and one spot synthesis of
imines [22] and selective alkylation of ketones with alco-
hols [23]. Herein, we report an effective procedure for the
N-arylation of aniline using aryl halides in presence of recy-
clable heterogeneous Pd/AlO(OH) catalyst under ligand-
free aerobic condition.
Several methods have been published for the synthe-
sis of secondary amines. Palladium-based homogeneous
catalytic systems developed using a palladium salt with
organic ligands were found to possess superior catalytic
performance for the N-arylation of amines. For instance,
aryl triflates undergo N-arylation of amine (with aliphatic
as well as aromatic amines) in the presence of palladium-
allyl complex [5]. A general and efficient method for the
N-arylation of amine catalyzed by palladium-dinapthol
complex was reported by Dai et al. [6]. Similarly, a variety
of aryl amines was synthesized from the reaction of aryl
halides with amines in the presence of a catalytic amount of
palladium acetate [7]. All these systems are homogeneous
which possesses some disadvantages such as use of expen-
sive and less-stable ligands, and toxic solvents, besides the
lack of reusability.
2 Experimental
Heterogenization of homogeneous inorganic complexes
is one of the smart techniques to overcome the drawbacks
of homogeneous catalytic systems [8–10]. In this regard,
an efficient Buchwald–Hartwig reaction catalyzed by SPI-
ONs-Bis(NHC)-Pd(II) catalyst was reported by Ghotbine-
jad et al. in which N-heterocyclic dicarbene palladium(II)
complex was anchored on superparamagnetic iron oxide
nanoparticles [11]. However, developing a ligand-free Pd-
catalyzed Buchwald–Hartwig coupling remains a great
challenge. Recently, Arisawa and co-workers developed a
ligand-free Buchwald–Hartwig aromatic amination of aryl
halides catalyzed by low-leaching and highly recyclable
sulfur-modified gold-supported palladium (SAPd) material
in xylene at 130 °C [12]. To the best of their knowledge
SAPd-catalyzed aromatic amination is the first ever ligand-
free Buchwald–Hartwig C–N coupling system. Hence, we
aspire to develop heterogeneous nanocatalytic system for
the preparation of the secondary amines under ligand-free
condition. Nanocatalytic system combines the advantages of
both the homogenous and the heterogeneous catalysis. Nano-
catalysts have high surface to volume ratio which increases
the contact between the active site and reactant molecules
resulting in increase in the yield of the product. Moreover,
nanocatalysts are insoluble in common solvents and hence
the catalyst can be recovered easily by simple filtration/
centrifugation.
2.1 Materials and Methods
All the reagents used were of chemically pure and/or analar
grade and all the glassware were dried overnight in an oven
before use. Commercial grade solvents were distilled as per
standard procedures and dried over molecular sieves before
use. All other chemicals were purchased from Aldrich and
were used without further purification.
Powder X-ray diffraction (XRD) measurements were per-
formed on a Rigaku Ultima-III X-ray diffractometer using
Cu Kα radiation. Scanning electron microscopy-Energy dis-
persive spectroscopy (SEM-EDS) and transmission electron
microscopy (TEM) analyses were done in Hitachi instru-
ments. The BET surface area and pore diameter distribu-
tions were analyzed on Gemini V surface area and pore size
analyzer. X-ray photoelectron spectra (XPS) (Kratos Axis-
Ultra DLD, Kratos Analytical Ltd, Japan) were recorded to
confirm the chemical state of Pd in Pd/AlO(OH). During
the XPS analysis, the sample was irradiated with Mg Kα
ray source. ICP analysis was performed using Perkin Elmer
Optima 5300 DV ICP-OES instrument.
2.2 Pd/AlO(OH) Catalyst Preparation
PdCl2 (200 mg, 2.3 mmol), pluronic P123 (4.0 g) and abso-
lute ethanol (10 mL) were added in 100 mL round bottom
flask equipped with a condenser. The mixture was stirred
for 30 min at 27 °C. Then, Al(O-sec-Bu)3 (9.1 g, 37 mmol)
was added carefully. After being stirred at 160 °C for 3 h,
3 mL of water was added. The reaction mixture was stirred
further for 30 min at 160 °C, cooled down and kept at room
temperature for 3 h. The resulting solid was filtered, washed
Boehmite, AlO(OH) is one of the potential supporting
materials for metal nanoparticles (MNPs). For example,
aerobic oxidation of a wide range of alcohols to correspond-
ing carbonyl compounds was successfully achieved using
Au/AlO(OH) nanocatalysts [13]. Similarly, Rh/AlO(OH)
can be used as a catalyst for the hydrogenation of arenes and
ketones to produce the corresponding alkanes and alcohols,
respectively. Importantly, the catalyst can be reused for ten
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Pd/AlO(OH): A Heterogeneous, Stable and Recyclable Catalyst for N-Arylation of Aniline Under…
3 Results and Discussion
3.1 Characterizations
Powder XRD pattern of Pd/AlO(OH) catalyst is illustrated
in Fig. 1. The relatively broad peaks at Bragg’s angle of
14.78°, 28.06°, 49.82°, 56.36°, 64.92°, 68.20° and 72.14°
correspond to the (020), (120), (200), (151), (231), (260)
and (251) planes of the orthorhombic γ-AlO(OH) solid
matrix, respectively (JCPDS card # 21-1307) [24, 25].
The additional two peaks centred at 40.08° and 46.58°
represent the Bragg reflections from the (111) and (200)
planes of zero valent and cubic structured palladium
nanoparticles (Pd NPs) (JCPDS card # 88-2335); show-
ing clear immobilization of Pd(0) NPs onto the surface of
γ-AlO(OH) support [26, 27].
Scheme 1 Schematic representation for the preparation of Pd/
AlO(OH)
with acetone, and dried at 120 °C for 2 h to give an ash col-
our powder (3 g, 3.2 wt% of Pd) (Scheme 1) [23].
2.3 N-arylation of Aniline
Figure 2a represents the SEM image of Pd/AlO(OH),
which clearly shows tiny Pd NPs. The present SEM image
is similar to the previously reported SEM images of Pd/
AlO(OH) [28, 29]. Additionally, the elemental composi-
tion of Pd/AlO(OH) catalyst was identified by EDS analy-
sis (Fig. 2b). The EDS spectra showed all the expected
elements Pd, Al, C and O. The weight percentage of Pd
was found to be 3.32 (inset of Fig. 2b). Furthermore,
the weight percent of Pd content in the catalyst was also
analyzed with ICP and the value was found to be 3.46
which is in good agreement with the EDS analysis. TEM
images of pristine AlO(OH) (Fig. 3a) and Pd/AlO(OH)
(Fig. 3b, c) are illustrated. It is evident from Fig. 3a that
pure AlO(OH) has a wafer structure. It is worth to men-
tion here that AlO(OH) is highly transparent as can be
seen from Fig. 3b also. Aggregated Pd NPs were identified
in some places over AlO(OH) support (Fig. 3b). Never-
theless, the separated individual Pd NPs were found in
A mixture of aryl halide (1 mmol), aniline (1.2 mmol), Pd/
AlO(OH) (0.31 mol%), potassium phosphate (1.5 mmol) and
DMSO (3 mL) was stirred at 120 °C for the appropriate
time (Scheme 2). After completion of the reaction as indi-
cated by TLC or GC, the reaction mixture was centrifuged
to recover the catalyst. The centrifugate was diluted with
3 mL of ethyl acetate and treated with 3 mL of HCl (2 M)
to separate the product. Then, the organic layers were dried
over anhydrous sodium sulphate, filtered, concentrated, and
the resulting product was purified by column chromatogra-
phy using hexane:ethyl acetate (80:20) mixture.
2.4 GC Conditions
GC (Shimadzu-2010) was equipped with 5% diphenyl and
95% dimethyl siloxane Restek-5 capillary column (60 m
length, 0.32 mm dia) and a flame ionization detector (FID).
The initial column temperature was increased from 100 to
230 °C at the rate of 20 °C/min and hold for 11 min. Nitro-
gen gas was used as a carrier gas. The temperatures of both
injection port and FID were kept constant at 250 °C during
the product analysis.
Scheme 2 Schematic illustration for the Pd/AlO(OH)-catalyzed
N-arylation of aniline
Fig. 1 Powder XRD pattern of Pd/AlO(OH)
1 3

S. Ganesh Babu et al.
Fig. 2 a SEM and b EDS spectrum (inset elemental weight percentage) of Pd/AlO(OH)
Fig. 3 TEM images of a pristine AlO(OH) and b and c Pd/AlO(OH)
many places (Fig. 3c). The higher magnification image
illustrated in Fig. 3c revealed the smaller crystallite size
of Pd NPs (10 nm), which is important for achieving good
catalytic activity.
surface area, nano-porous structure and less than 10 nm size
Pd NPs played a vital role in excellent catalytic performance
(with meagre amount of Pd NPs) of Pd/AlO(OH) towards
the Buchwald–Hartwig coupling of amines with aryl halides
under ligand-free aerobic condition.
Because of the wafer structure and porous nature of
AlO(OH), the surface area was expected to be high [30]. As
expected, the BET surface area of Pd/AlO(OH) was esti-
mated to be 317.09 m2/g from the BET adsorption–desorp-
tion graph, given in Fig. 4a. Furthermore, Pd/AlO(OH) cata-
lyst possesses narrow pore size distribution and the mean
pore diameter was found to be 3.5 nm (Fig. 4b). This clearly
supports the presence of nano-porosity in Pd/AlO(OH). Con-
versely, the already reported crystalline boehmite nanotubes
exhibited a broad peak in the pore size distribution graph
corresponding to the pore diameter value of 10–50 nm with
a maximum at 24.3 nm [31]. Hence, the preparation proce-
dure adopted for the synthesis of the present catalyst made
a noteworthy influence in attaining the nano-porous wafer
AlO(OH) support. All the significant properties such as high
XPS is a very handy tool to estimate the elements/com-
ponents present in a sample [32–34]. The wide angle XPS
spectra shown in Fig. 5a manifest that only Al, O and Pd
are present in Pd/AlO(OH) catalyst. In other words, Pd/
AlO(OH) catalyst is pure and free from other impurities.
This obviously acknowledged the merit of the prepara-
tion procedure followed for the synthesis of Pd/AlO(OH)
catalyst. A broad peak was observed in the binding energy
range of 532–540 eV, which can be further deconvoluated
into two peaks centred at 535 and 537.5 eV correspond-
ing to O1s and Pd3p, respectively (Fig. 5b) [35, 36]. Simi-
larly, in the high-resolution Al2p spectrum, the Al2p peak
was located at 78 eV that exactly matches with the litera-
ture report (Fig. 5c) [37]. Two more peaks were identified
1 3

Pd/AlO(OH): A Heterogeneous, Stable and Recyclable Catalyst for N-Arylation of Aniline Under…
Fig. 4 a N2 adsorption and desorption graph and b pore diameter distribution graph for Pd/AlO(OH)
Fig. 5 a Wide angle, b high resolution O1s and Pd3p, c high resolution Al2p and d high resolution Pd3d XPS spectra of Pd/AlO(OH)
at 341.5 and 345.8 eV, which were evolved from Pd3d5/2
and Pd3d3/2 (Fig. 5d). Alike in previous reports, narrow
peaks correspond to Pd3d5/2 and Pd3d3/2 were observed in
the high-resolution Pd3d spectrum, which confirmed the
absence of Pd with higher chemical oxidation states [38].
Prominently, the XPS spectra supported the presence of zero
1 3

S. Ganesh Babu et al.
valent Pd NPs in Pd/AlO(OH) catalyst, which is in good
agreement with the powder XRD results.
3.2 Optimization of Reaction Conditions
To get effective results, the reaction conditions were opti-
mized. For this purpose, 4-chlorobenzonitrile (1 mmol)
and aniline (1.2 mmol) were chosen as model substrates
(Scheme 2). The completion of the reaction is monitored
by gas chromatograph (GC). The process conditions such
as solvent, base, catalyst amount and temperature were
optimized to get a good yield and better catalytic system.
Base plays a vital role in the C–N cross coupling reactions
[39, 40]. Sunesson et al., studied the role of bases in Buch-
wald–Hartwig amination using density functional theory
(DFT) [41]. Therefore, to find out the suitable base for the
N-arylation of aniline, the reaction was carried out with
different bases like potassium tertiary butoxide, potassium
carbonate, potassium phosphate or potassium hydroxide
(Fig. 6). The reaction with potassium phosphate showed a
better yield; therefore, potassium phosphate was used as a
base in the present system. Similarly, the influence of sol-
vent in the reaction was studied as it played a major role in
C–N cross coupling reactions [42]. The reaction was per-
formed in different solvents like toluene, dimethyl forma-
mide (DMF), dimethyl acetamide (DMAc) or dimethyl sul-
foxide (DMSO) (Fig. 7). The results showed that DMSO
was an appropriate solvent for this reaction. Alike, the
reaction was conducted in the presence of varying amount
of the catalyst (0, 0.15, 0.31 or 0.45 mol%) (Fig. 8). No
progress in the reaction was observed in the absence of
Fig. 7 Solvent optimization [Reaction conditions: 4-chloroben-
zonitirle (1 mmol), aniline (1.2 mmol), Pd/AlO(OH) (0.31 mol%),
K3PO4 (1.5 mmol), solvent (3 mL), 120 °C, 24 h]
Pd/AlO(OH) (0 mol%), which indicated the necessity of
the catalyst in this reaction. However, the utmost product
yield was achieved when the catalyst amount was 10 mg
(0.31 mol%). Hence, it was taken as the optimized cat-
alyst amount that was used for rest of the experiments.
Besides, the reaction temperature optimization was done
by carrying out the model reaction in the presence of Pd/
AlO(OH) at different temperatures (Fig. 9). Generally, the
Buchwald-Hartwig amination was performed at the tem-
perature range of 100–150 °C [43, 44]. In our case, the
highest yield was achieved at 120 °C; hence it was noted
as the optimum reaction temperature.
Fig. 6 Base optimization [Reaction conditions: 4-chlorobenzo-
nitirle (1 mmol), aniline (1.2 mmol), Pd/AlO(OH) (0.31 mol%), base
(1.5 mmol), DMSO (3 mL), 120 °C, 24 h]
Fig. 8 Catalyst amount optimization [Reaction conditions: 4-chlo-
robenzonitirle (1 mmol), aniline (1.2 mmol), Pd/AlO(OH), K3PO4
(1.5 mmol), DMSO (3 mL), 120 °C, 24 h]
1 3

Pd/AlO(OH): A Heterogeneous, Stable and Recyclable Catalyst for N-Arylation of Aniline Under…
aryl chlorides. For example, 2-bromobenzonitrile gave 67%
of the coupling product with aniline but 2-chlorobenzonitrile
gave only 55% of the product (Table 1; entries 2–3). This
might be owing to the better leaving group ability of bromide
relative to chloride. It is well known that fluoride is the poor-
est leaving group in halides family [48]. Hence, the yield of
coupling product of aniline with 4-fluorobenzaldehyde was
much lesser (60%) than that of the coupling product evolved
from 4-chlorobenzaldehyde (83%) (Table 1; entries 7–8). It
is noted that 4-bromobenzaldehyde produced higher yield
(96%) of the product compared to 4-chlorobenzaldehyde
and 4-fluorobenzaldehyde (Table 1; entries 6–8). Further,
the coupling of aniline with 2-bromoacetophenone yielded
85% of 1-(4-(phenylamino)phenyl)ethanone with the TON
and TOF of 274 and 11.4 h−1, respectively (Table 1; entry 9).
But only 63% of the same product was produced by the cou-
pling of iodobenzene with LiNH2 and 4-iodoacetophenone
in the presence of CuI (10 mol%) catalyst and K3PO4 base
at 130 °C after 24 h [46]. In the same way, 4-bromobenzoic
acid gave 70% of the desired coupling product with the TON
and TOF of 226 and 9.4 h−1, respectively (Table 1; entry 10).
Scope of the present system was extended to unsubsti-
tuted aryl halides and aryl halides with electron donating
group to explore the versatility of Pd/AlO(OH). The present
catalyst showed excellent activity for the coupling of ani-
line with bromobenzene, which gave 83% diphenyl amine
(Table 1; entry 11). On the other hand, the same product
(diphenyl amine) was obtained in only 78% yield after 36 h
in presence of 5 mol% Pd(OAc)2 catalyst using 1,10-phen-
anthroline (10 mol%) as a ligand under O2 saturated toluene
(0.5 mL) at 135 °C by aerobic amination of cyclohexanones
with anilines [49]. Likewise, diphenyl amine was yielded
by the coupling of phenyl tosylate and aniline in presence
of 2 mol% Pd(OAc)2, 5 mol% PhB(OH)2, and a hindered
and electron-rich MOP-type ligand (3 mol%) [50]. Largely,
diphenyl amine is commercially manufactured by the ther-
mal deamination of aniline using metal oxide catalyst.
Hartwig and co-workers have utilized 5 mol% of (DPPF)
PdCl2 as a homogeneous catalyst for the coupling of 4-iodo-
toluene and aniline to produce the C–N coupling product in
THF at 100 °C [51]. Nevertheless, only 0.31 mol% of Pd/
AlO(OH) was used for the coupling of 4-bromotoluene and
aniline to give 80% of the same product (Table 1; entry 12).
The above results clearly revealed the wider employability
of the catalyst.
Fig. 9 Reaction temperature optimization [Reaction conditions:
4-chlorobenzonitirle (1 mmol), aniline (1.2 mmol), Pd/AlO(OH)
(0.31 mol%), K3PO4 (1.5 mmol), DMSO (3 mL), 24 h]
3.3 Extension of Scope
The present protocol was extended to different aryl halides
under optimized reaction conditions and the results are sum-
marized in Table 1. Aniline reacted with 4-chlorobenzoni-
trile under ambient reaction conditions to yield 94% of the
coupling product with very high turnover number (TON)
of 303 (Table 1; entry 1). However, Hamann et al., demon-
strated the utility of 2 mol% Pd(dba)2 with 1,1′-bis(di-tert-
butylphosphino)ferrocene as a ligand to yield only 79% of
the same product at 110 °C after 16 h [45]. Likewise, CuI-
catalyzed (10 mol%) homogeneous catalytic system gave
only 81% of the same product after 24 h at 130 °C [46]. In
general, the para-substituted aryl halides react much faster
than the ortho-substituted ones. For illustration, the cou-
pling of aniline with 4-chlorobenzonitrile gave 94% of the
product (Table 1; entry 1), but the coupling of 2-chloroben-
zonitrile yielded only 55% of 2-phenylamino benzonitrile
with the TON and TOF of 177 and 7.4 h−1, respectively
(Table 1; entry 3). This can be justified by the steric hin-
drance in the case of ortho-substituted halides. On the other
hand, [RhCp*Cl2]2/AgSbF6 catalyzed the reaction homoge-
neously to afford 2-phenylamino benzonitrile in only 31%
after 24 h at 120 °C under N2 atmosphere [47]. Nonetheless,
a similar trend was not observed in some of the bromo aryl
systems. Both 4-bromobenzaldehyde and 2-bromobenzalde-
hyde exhibited similar reactivity with aniline to form 96%
of the respective products with TON of 310 and TOF of
12.9 h−1 (Table 1; entries 5–6). Likewise, 4-chlorobenzotri-
fluoride yielded 54% of the coupling product with the TON
of 174 when it underwent reaction with aniline (Table 1;
entry 4). As expected, aryl bromides are more reactive than
3.4 Heterogeneity
Hot-filtration test was followed to analyze the heteroge-
neous nature of the catalyst and to prove the absence of
leaching of Pd from boehmite [52, 53]. Pd/AlO(OH) was
separated from the mixture of 4-chlorobenzonitirle, aniline
and K3PO4 at 120 °C after attaining the product yield of
1 3

S. Ganesh Babu et al.
Table 1 Scope extension of Buchwald–Hartwig coupling reaction
TONb/TOF
(h―1)c
Entry
Aryl halide
Product
Yielda (%)
94
Cl
H
N
1
303/12.6
NC
CN
CN
H
N
2
3
4
67
55
54
216/9.0
177/7.4
174/7.2
Br
CN
CN
H
N
Cl
CN
Cl
H
N
F3C
CF3
CHO
H
N
5
6
96
96
310/12.9
310/12.9
OHC
Br
CHO
Br
CHO
H
N
CHO
Cl
H
N
7
83
268/11.2
OHC
1 3

Pd/AlO(OH): A Heterogeneous, Stable and Recyclable Catalyst for N-Arylation of Aniline Under…
Table 1 (continued)
TONb/TOF
(h―1)c
Entry
8
Aryl halide
Product
Yielda (%)
CHO
H
N
60
194 / 8.1
OHC
F
O
Br
O
H
N
9
85
70
83
80
274 / 11.4
226 / 9.4
O
O
H
O
H
O
10
11
12
N
r
B
H
268 / 11.2
258 / 10.7
N
H
r
B
H
N
r
B
H
3C
H
3C
Reaction conditions: Aryl halide (1 mmol), aniline (1.2 mmol), Pd/AlO(OH) (0.31 mol%), K3PO4 (1.5 mmol), DMSO (3 mL), 120 °C, 24 h
aDetermined by GC analysis
bTurnover number (TON)=product (mol)/catalyst (mol)
cTurnover frequency (TOF) (h−1)=TON/Time in h
32% (after 12 h of reaction time), by centrifugation and
allowed the reaction to proceed further without the cata-
lyst. Then, the reaction was monitored in different time
intervals (15, 18, 21 and 24 h). GC analysis showed that
there was no increase in the yield of the product (Fig. 10)
after the removal of the catalyst. The filtrate was ana-
lyzed by ICP-OES and a trace amount of Pd content of
about 7 ppm was identified. Hence it is confirmed that the
catalyst is of heterogeneous nature and stable during the
reaction.
3.5 Reusability
Reusability is one of the major advantages of heterogene-
ous catalytic systems [54, 55]. After the completion of
the reaction, the catalyst was recovered from the reaction
mixture by centrifugation and washed with diethyl ether.
Then, the recovered catalyst was used for the next cycles.
The reaction was carried out for four times under identi-
cal conditions with recycled Pd/AlO(OH). The yield of
product was 94% at the 1st run. No significant decrease
was observed in the 2nd, 3rd and 4th runs as the yield of
1 3

S. Ganesh Babu et al.
Fig. 12 Powder XRD pattern of the used Pd/AlO(OH)
Fig. 10 Heterogeneity test [Reaction conditions: 4-chlorobenzonitirle
(1 mmol), aniline (1.2 mmol), Pd/AlO(OH) (0.31 mol%), K3PO4
(1.5 mmol), DMSO (3 mL), 120 °C, 24 h]
4 Conclusions
A simple synthetic method for recyclable AlO(OH)-sup-
ported Pd NPs catalyst was developed from readily avail-
able reagents and its high activity towards the N-arylation
of aniline was demonstrated. The smaller size (10 nm) of
metallic Pd NPs, and wafer structure, nano porous nature
and high surface area of AlO(OH) made this catalytic proto-
col efficient. Essentially, the Buchwald–Hartwig cross cou-
pling was performed using less amount of Pd (0.31 mol%)
with high TON and TOF. Coupling of aniline with different
kind of aryl halides was successfully performed under sim-
ple operating conditions. This catalytic system is expected
to be applicable for preparing different substituted aromatic
amines with very good yield. The catalyst is truly heteroge-
neous, reusable and stable. Notably, this amination protocol
can be performed under ligand-free and aerobic conditions.
Acknowledgements R.K. acknowledges DAE-BRNS for the financial
support.
Fig. 11 Reusability test [Reaction conditions: 4-chlorobenzonitirle
(1 mmol), aniline (1.2 mmol), Pd/AlO(OH) (0.31 mol%), K3PO4
(1.5 mmol), DMSO (3 mL), 120 °C, 24 h]
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product was constantly maintained at 93% (Fig. 11). Fur-
thermore, the recycled catalyst was examined by powder
XRD. As can be seen from Fig. 12, the XRD pattern of
the used catalyst is like that of fresh Pd/AlO(OH) (Fig. 1).
Only difference observed between the two XRD patterns
is the intensity. The used Pd/AlO(OH) showed less intense
XRD peaks as compared to that of the fresh catalyst. These
results indicated the excellent stability and reusability of
Pd/AlO(OH).
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