Cross-linked cyclodextrins bimetallic nanocatalysts: Applications in microwave-assisted reductive aminations

Article abstract of DOI:10.3390/molecules25020410

The optimization of sustainable protocols for reductive amination has been a lingering challenge in green synthesis. In this context, a comparative study of different metal-loaded cross-linked cyclodextrins (CDs) were examined for the microwave (MW)-assisted reductive amination of aldehydes and ketones using either H₂ or formic acid as a hydrogen source. The Pd/Cu heterogeneous nanocatalyst based on Pd (II) and Cu (I) salts embedded in a β-CD network was the most efficient in terms of yield and selectivity attained. In addition, the polymeric cross-linking avoided metal leaching, thus enhancing the process sustainability; good yields were realized using benzylamine under H₂. These interesting findings were then applied to the MW-assisted one-pot synthesis of secondary amines via a tandem reductive amination of benzaldehyde with nitroaromatics under H₂ pressure. The formation of a Cu_xPd_y alloy under reaction conditions was discerned, and a synergic effect due to the cooperation between Cu and Pd has been hypothesized. During the reaction, the system worked as a bifunctional nanocatalyst wherein the Pd sites facilitate the reduction of nitro compounds, while the Cu species promote the subsequent imine hydrogenation affording structurally diverse secondary amines with high yields.

Full text of DOI:10.3390/molecules25020410

molecules
Article
Cross-Linked Cyclodextrins Bimetallic Nanocatalysts:
Applications in Microwave-Assisted
Reductive Aminations
Emanuela Calcio Gaudino 1 , Elisa Acciardo 1 , Silvia Tabasso 2 , Maela Manzoli 1
,
Giancarlo Cravotto 1,
*
and Rajender S. Varma 3
1
Dipartimento di Scienza e Tecnologia del Farmaco and NIS—Centre for Nanostructured Interfaces and
Surfaces, University of Turin, Via Giuria 9, 10125 Turin, Italy; emanuela.calcio@unito.it (E.C.G.);
elisa.acciardo@gmail.com (E.A.); maela.manzoli@unito.it (M.M.)
Dipartimento di Chimica, University of Turin, Via P. Giuria 7, 10125 Turin, Italy; silvia.tabasso@unito.it
Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of
Science, Palacky University, Šlechtitelu˚ 27, 783 71 Olomouc, Czech Republic; varma.rajender@epa.gov
Correspondence: giancarlo.cravotto@unito.it; Tel.: +39-011-670-7183
2
3
*
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Academic Editor: Derek J. McPhee
Received: 14 December 2019; Accepted: 14 January 2020; Published: 19 January 2020
Abstract: The optimization of sustainable protocols for reductive amination has been a lingering
challenge in green synthesis. In this context, a comparative study of different metal-loaded cross-linked
cyclodextrins (CDs) were examined for the microwave (MW)-assisted reductive amination of
aldehydes and ketones using either H2 or formic acid as a hydrogen source. The Pd/Cu heterogeneous
nanocatalyst based on Pd (II) and Cu (I) salts embedded in a β-CD network was the most efficient
in terms of yield and selectivity attained. In addition, the polymeric cross-linking avoided metal
leaching, thus enhancing the process sustainability; good yields were realized using benzylamine
under H2. These interesting findings were then applied to the MW-assisted one-pot synthesis of
secondary amines via a tandem reductive amination of benzaldehyde with nitroaromatics under H2
pressure. The formation of a CuxPdy alloy under reaction conditions was discerned, and a synergic
effect due to the cooperation between Cu and Pd has been hypothesized. During the reaction,
the system worked as a bifunctional nanocatalyst wherein the Pd sites facilitate the reduction of nitro
compounds, while the Cu species promote the subsequent imine hydrogenation affording structurally
diverse secondary amines with high yields.
Keywords: heterogeneous catalysis; bimetallic nanocatalyst; microwaves; one-pot reductive
amination; sustainable protocols
1. Introduction
The synthesis of amines and their derivatives represents one of the most active themes in organic
chemistry. Nitro compounds, as easily accessible starting materials, have been extensively explored
for the synthesis of primary amines with productive results [1]. The as-formed primary amines
can be further upgraded into diverse and highly valuable chemicals (such as secondary or tertiary
amines, imines, azo compounds, and other nitrogen-containing compounds), which are useful building
blocks in the synthesis of polymers, dyes, surfactants, pharmaceuticals, and agrochemicals [2–7].
Recently, the synthesis of primary amines through direct reductive amination of aldehydes using
ammonia as a nitrogen source has been explored. However, the transition metal-catalyzed reductive
aminations have frequently been accompanied by unwanted side product(s) due to, for example,
the reduction of aldehyde to an alcohol [8–10].
Molecules 2020, 25, 410; doi:10.3390/molecules25020410
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The catalyst plays a key role in the reductive amination protocols, in which noble metals comprising
Pd, Pt, and Rh are metals of choice due to their excellent hydrogenation and dehydrogenation
effectiveness [11]. In this regard, a variety of reductive amination methods catalyzed by heterogeneous
Pd or several other transition metals have been developed [12,13].
However, to improve the sustainability of the process, the development of alternative catalysts has
recently received growing attention. Of late, nanocatalysis have become an emerging field of science
due to its high activity, selectivity, and productivity. The nanoscale size, shape, and an exceptionally
large surface area to volume ratio impart unique properties to nanocatalysts because of the structural
and electronic changes, which differentiate them from the bulk materials [14].
Heterogeneous catalysis represents one of the oldest commercial practices of nanoscience;
nanoparticles of metals, semiconductors, oxides, and other compounds have been widely used
for important chemical reactions. Among diverse materials deployed, especially supported
nanoparticles-based heterogeneous catalysts, are of interest due to their easy separation,
potential activity, and selectivity [15]. Nickel nanoparticles prepared via pyrolysis of in situ generated
Ni-tartaric acid complex have been recently described as efficient catalysts for the synthesis of
functionalized amines starting from carbonyl compounds and ammonia in the presence of molecular
hydrogen [16].
As the recyclability of the catalysts is a stringent issue in terms of sustainability,
magnetic nanoparticles could represent a breakthrough in the field of green synthesis [17,18].
Among these, supported iron oxide nanocatalysts have been the focus of assorted catalytic
applications because of their low cost and toxicity, ready availability, and environmentally benign
nature [19]. Recently, an AuPd alloy nanocatalyst supported on Fe3O4 was employed for a one-pot
reduction of a nitro compound followed by reductive amination of the resulting amine [20]. In
this framework, we recently reported an efficient and heterogeneous catalytic system based on
nickel silica eggshell iron-based magnetic nanoparticles (Fe3O4@SiO2-Ni) for a fast and quantitative
microwave (MW)-assisted reductive amination of aryl aldehydes and ketones in aqueous ammonia [21].
The relentless demand for more sustainable processes requires an efficient and safer energy transfer
that can be nicely achieved by MW dielectric heating; advantages related to the use of MW in organic
synthesis are well documented in the literature [22]. MW promotes the remarkable reduction of
reaction time, yields enhancement, and cleaner reactions compared to conventional thermal heating.
MW irradiation is, therefore, a suitable tool to overcome the limitations encountered in challenging
organic reactions, such as direct reductive amination in the presence of heterogeneous catalysts and
gaseous reagents. Moreover, from the viewpoints of green and sustainable chemistry, it is highly
attractive to adopt one-pot reaction strategies. Indeed, compared to multi-step protocols, the one-pot
synthesis demonstrates higher atom efficiency by avoiding tedious purification steps and reducing the
amount of discharged wastes [23].
Reductive amination, the coupling of carbonyl compounds with primary amines in the presence
of a reducing reagent, has been considered one of the most efficient methods for the synthesis of
secondary amines. Although the reductive amination of carbonyl compounds with primary amines
has enjoyed huge success, the one-pot reductive amination directly from nitro compounds has been
less developed [24,25].
Some noble metals have achieved success as active catalysts for the one-pot reductive amination
under mild conditions; Pd/Ag bimetallic nanocatalyst can accomplish amination under mild conditions
(room temperature) and atmospheric hydrogen pressure [26], while formic acid (FA) could be exploited
as a hydrogen donor in one-pot secondary amines production utilizing Ag/Pd nanoparticles supported
on mesoporous graphitic carbon nitride (mpg-C3N4/AgPd) [27]. Another one-pot reductive amination
of carbonyl compounds was recently reported over N-doped carbon-supported cobalt nanoparticles
using CO/H2O as an efficient hydrogen donor [28].
Herein, we report a comparative study on the use of heterogeneous nanocatalysts based on Pd (II)
and Cu (I) salts embedded in a β-CD network, using either FA or H2 as hydrogen sources. The effect

Molecules 2020, 25, 410
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of the solvent has been illustrated for the reductive amination of benzaldehyde and acetophenone
with aniline and benzylamine. Aiming for MW-assisted one-pot amination reactions, the bimetallic
PdCu/CβCAT catalyst proved more efficient than the monometallic ones, providing high yields under
milder reaction conditions using H2 and ethanol as the solvent.
Transmission electron microscopy (TEM), powder X-Ray diffraction (PXRD), and diffuse reflectance
UV-Visible-near infrared (DR UV–Vis-NIR) were employed to investigate the structural and electronic
properties of mono- and bi-metallic nanocatalysts. The characterization revealed the presence of Cu (II)
as well as Pd (0) species in the form of small copper oxide nanoparticles and metallic Pd nanoparticles
in the bimetallic PdCu/CβCAT catalyst. During the reaction, these species are transformed to produce
a CuxPdy alloy, wherein both metallic sites cooperated to promote the catalytic activity enhancement.
2. Results
Catalytic reductive amination of carbonyl compounds with primary amines in the presence of
a reducing reagent is considered one of the most efficient methods for the synthesis of secondary
amines [29,30]. However, these reactions are often non-selective due to over alkylation and the
reduction of carbonyl compounds to the corresponding alcohols. To circumvent these problems,
the development of suitable catalytic systems that enables a fast amination reaction under milder
conditions has been a challenge. In this area, a variety of reductive amination methods, catalyzed by
heterogeneous Pd or several other transition metals, have been developed in recent years [31]. In view
of our prior experience, we report herein the development of distinctive nanocatalysts (C
βCAT) based
on palladium and copper salts embedded in a
carbonyl compounds to secondary amines.
β-CD network for the selective reductive amination of
2.1. Catalyst Preparation and Characterization
In the search for more sustainable processes, nanostructured catalysts have been the subject
of considerable academic and industrial research attention in recent times due to the numerous
potential benefits related to their use (minimal chemical waste, and energy efficiency, among others).
Despite exhibiting high catalytic activity and selectivity like homogeneous catalysis, the encapsulation
of metal within a polymeric network renders the nanocatalyst recoverable, thus avoiding metal
leaching [32
and PdCu/
procedure [34,35].
,
β
33]. Several metals loaded C
-CD with hexamethylenediisocyanate (HDI), according to a previously documented
βCAT were prepared by the reticulation of Pd/β-CD, Cu/β-CD,
The structure of Pd/CβCAT has been previously investigated by using powder X-ray diffraction
(PXRD), underlining the re-organization of the polymer crystalline phase into a less ordered material
upon metal addition and the presence of highly dispersed Pd nanoparticles of ~3 nm that were oxidized
at the surface (owing to exposure to air) [36]. A representative TEM image of the as-prepared Pd/CβCAT
catalyst is depicted in Figure 1a. Round Pd nanoparticles well contrasted with respect to the polymeric
support and of size ranging between 3 and 5 nm were clearly detected, in agreement with the previous
PXRD results. Notably, the Pd/
electron beam of the instrument, since neither metal coalescence nor polymer degradation was detected.
The comparative PXRD patterns for the Cu/C CAT and PdCu/C CAT are presented in Figure 1b.
β-CD fresh sample proved to be stable with prolonged exposure to the
β
β
In particular, the patterns displayed a broad peak that was related to the presence of crosslinked
hexamethylene diisocyanate amorphous moieties [36], along with with less intense peaks in the 15 to
50 2Theta range that revealed the presence of highly dispersed metal nanoparticles.
As for Cu/C
51.1◦ could be related to Cu-rich orthorhombic copper oxide (Cu64O, JCPDS file number 01-077-1898).
β
CAT (light green line), the peaks centered at 16.9◦, 18.9◦, 32.6◦, 33.4◦, 40.9◦, and
These peaks were observed with lower intensity in the case of as-prepared PdCu/CβCAT (green line)
along with the presence of components at 2Theta 40.4◦ and 46.1◦, due to metallic Pd in the cubic phase
(JCPDS file number 00-001-1201). The latter features were indicative of the presence of a reduced Pd
phase on the bimetallic catalyst, otherwise observed for the monometallic Pd/CβCAT [36].

Molecules 2020, 25, 410
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(a)
(b)
Figure 1.
(
a
) TEM image of as-prepared Pd/C
β
CAT. Instrumental magnification 150,000
×
. (
b
) Powder
CAT
CAT after reaction (dark green line). The peak positions are signaled by
dashed red lines. (Triangles: Copper oxide; asterisks: Metallic palladium; circles: PdCu alloy).
X-ray diffraction (XRD) patterns of as-prepared Cu/C
(green line) and of PdCu/C
βCAT (light green line), as prepared PdCu/Cβ
β
DR UV–Vis-NIR characterization was carried out on Cu/C
βCAT (light green line), as-prepared
PdCu/C CAT (green line), and the results are reported in Figure 2.
β
Diffuse reflectance
UV-Vis spectroscopy was employed to gain information about the Pd and Cu oxidation states.
Especially, a band at 24,000 cm−1, related to the d-d transition of Pd (II) species, was found on
PdCu/CβCAT (data not shown). This indicates that this catalyst contained highly dispersed PdO
particles, in agreement with previous PXRD findings [36].
Figure 2. DR UV-Vis_NIR spectra of as-prepared Cu/C
β
CAT (light green line), as prepared PdCu/C
βCAT
(green line) and of PdCu/CβCAT after reaction (dark green line). The spectrum of crosslinked βCD (red
dashed line) is also reported for comparison. Inset: Spectra shown in the 30,000 to 5000 cm−1 range.
A rather intense absorption band at 35,000 cm−1 attributed to the π−π * electronic transition
from the HOMO to LUMO of crosslinked
βCD [37] (red dashed line) was depleted upon metal
insertion, a weak absorption, assigned to the d-d transition in Cu (II) species [38
,
39] was observed at
about 13,300 cm−1 on both Cu/C
βCAT (inset, light green line) and as-prepared PdCu/C CAT (inset,
β
green line).

Molecules 2020, 25, 410
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2.2. Reductive Amination
Aiming for the development of a sustainable reductive amination protocol, we combined the
advantages of heterogeneous catalysts, such as the potential recyclability and negligible metal leaching
and those emanating from non-conventional technologies. The multimode MW reactor used for
the reductive amination was equipped with a pressure control system, and a multiple position rack,
suitable for multiple gas loading (both inert and reactive one) and parallel reaction runs. Easy reaction
scale-up (up to 10 mol scale) could also be achieved in the same reactor using different reaction vessels
(maximum 1 L capability). The use of two reducing agents was exploited and compared for this
purpose: H2 as an environmentally friendly reducing agent (water was the only generated oxidation
by-product), and FA. This latter H-donor has been regarded as a promising hydrogen storage material
owing to its advantageous features (e.g., high volumetric hydrogen capacity (53 g L−1); liquid at room
temperature, non-toxicity, and low cost) [40].
Benzaldehyde and acetophenone were used as model substrates to test the MW-assisted reductive
aminations with aniline or benzylamine over three different nano-C
βCATs: Cu/C
βCAT, Pd/CβCAT,
and PdCu/CβCAT. (Scheme 1, Table 1).
R1
O
CβCATs
R2
reducing agent
N
H
R1
R2 NH2
MW,solvent
R1= H, CH3
R2= phenyl, benzyl
Scheme 1. CβCATs reductive amination.
Table 1. MW-Assisted Reductive Amination (CβCAT comparison).
Yield (%) c
d
H2 (15 bar
)
FA (4 equiv.
)
Substrates
100 ◦
EtOH
C
180 ◦
C
100 ◦
EtOH
C
180 ◦
C
Entry a
Catalyst
b
R1
H2O
EtOH
H2O
H2O
EtOH
H2O
R2
1
2
3
Cu/CβCAT
Pd/CβCAT
PdCu/CβCAT
0
65
73
0
57
62
0
45
54
4
50
67
45
47
53
46
48
53
28
39
54
25
32
42
H
Phenyl
Benzyl
4
5
6
Cu/CβCAT
Pd/CβCAT
0
0
24
0
0
17
0
3
13
0
0
19
0
15
37
0
12
34
2
14
22
0
11
20
CH3
PdCu/C
βCAT
7
8
9
Cu/CβCAT
Pd/CβCAT
0
70
87
0
64
78
0
0
48
19
71
80
55
53
75
46
52
76
33
47
59
32
40
64
H
PdCu/C
βCAT
10
11
12
a
Cu/CβCAT
Pd/CβCAT
0
0
80
0
24
69
0
0
58
0
0
63
4
38
61
0
30
53
2
23
39
8
21
37
CH3
PdCu/C
βCAT
Reactions performed under MW irradiation on a 0.5 mmol scale of aldehyde or ketone, 1 equiv. of amine in
the presence of CβCAT (5 wt % metal loading) under H2 pressure (15 bar) into 2 mL solvent (ethanol or water);
b
4 equiv. of amine was used in the presence of FA (4 equiv.); c Determined by GC/MS d N2 back pressure (15 bar).
◦
The MW-assisted reductive aminations were performed first at 100 C on a 0.5 mmol scale
adopting a catalyst loading of 5 wt % (referred to the metal content of C
(15 bar) or FA (4 eq); either ethanol or water were exploited as sustainable solvents for this purpose.
Using PdCu/C CAT under H2 pressure, practically good results were obtained for the reductive
βCATs) in the presence of H2
β
amination of benzaldehyde with either aniline or benzylamine. In particular, benzylphenyl amine and
dibenzylamine were obtained in excellent (73% and 87%) and good (62% and 78%) yields in ethanol

Molecules 2020, 25, 410
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and water, respectively, as the only reaction products (Table 1, entries 3 and 9), thus avoiding the direct
benzaldehyde hydrogenation over PdCu/C CAT. Gratifyingly, when reactions were performed under
H2 pressure, the PdCu/C CAT exhibited fairly good activity and selectivity towards the reductive
β
β
amination of ketones as well (Table 1, entries 6 and 12). Especially, the aliphatic amine afforded up to
80% of N-benzyl-1-phenylethanamine in ethanol, whereas only 49% was achieved in water (Table 1,
entry 12). The monometallic catalysts were not so effective; for example, Cu/CβCAT proved to be
completely inactive for the reductive amination of both benzaldehyde and acetophenone at 100 ◦C under
H2 either in ethanol or in aqueous solutions (Table 1, entries 1, 4, 7, and 10). Conversely, Pd/CβCAT
afforded the secondary amines in good yields only when benzaldehyde was reacted under MW for 2 h,
with better results in ethanol for both benzyl phenyl amine (65% vs. 57%) and dibenzylamine (70%
vs. 64%) (Table 1, entries 2 and 8). Yet, this catalyst showed no activity in the reductive amination
of acetophenone. The influence of the co-metal on the catalyst performance in bimetallic palladium
catalysts (the alloying effect) is known [41]. Importantly, the introduction of another metal (Cu) into
the intrinsically active Pd/CβCAT led to a catalyst of fine-tuned activity, improving the selectivity
of the reaction [26]. In contrast, benzyl alcohol, polyalkylated compounds were detected as main
by-products when using the monometallic Pd nanocatalyst (Figure 3).
Figure 3. Influence of the co-metal on the bimetallic catalyst performance and selectivity.
To further improve the product yields, we studied the reductive amination at elevated temperature
(180 ◦C). The same catalyst activity trend was maintained (PdCu/C
βCAT > Pd/CβCAT >>> Cu/C
βCAT),
but unfortunately, no better results were recorded. However, the noticeable fact was that PdCu/C
βCAT
enabled higher product yields in water rather than in ethanol, reversing the previously encountered
behavior at 100 ◦C (Table 1).
FA was subsequently tested as a hydrogen donor for MW assisted reductive aminations because
it represents an economical, safe, and straightforward way for the in-situ generation of hydrogen.
The catalytic activity of the three different C
βCATs in the presence of 4 equiv. of FA (lower amounts
of FA could lead to reduced yields [29]) were compared using the same benchmark reaction and
◦
◦
conditions (2 h MW irradiation at 100 C or 180 C) adopted for reductive amination under H2
pressure (Table 1). However, as part of the amine was consumed by FA to form the corresponding
formamide as a by-product, more amine (4 equiv.) was added as a sacrificial component to improve
the yields of the desired products. As is well documented in the literature, Pd-based catalysts suffer
from deactivation problems during FA dehydrogenation owing to the strong adsorption of formate
anions [27]. To overcome this deactivation difficulty, Pd has been recently combined with various
metals including Au, Ag, and Ni, in a bimetallic or a multimetallic alloy or a core–shell structure [42].
Herein, we combined Pd with Cu into a β-CD-network to generate a more active PdCu/CβCAT. Actually,

Molecules 2020, 25, 410
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this bimetallic catalytic system enabled higher reductive amination yields than a monometallic one in
the presence of FA (Table 1).
However, although the increased activity of PdCu/CAT was maintained while using FA rather
than H2 gas, lower yields were attained for benzaldehyde and acetophenone reductive amination over
both Pd/CβCAT and PdCu/Cβ
CAT at 100 ◦C (Table 1). Moreover, using FA, no significant differences
in reaction yields were observed, changing the reaction solvent (water or ethanol), and a large amount
of formamide was detected as the by-product. An attempt at a higher temperature (180 ◦C) failed to
increase the yield. However, it must be emphasized that unlike what was observed under external
hydrogen pressure, the present reductive amination protocol enabled benzaldehyde conversion in the
presence of both aniline and benzylamine when Cu/CβCAT was used. As is well-known, the surface
structure and oxidation state of the metal may play a role in selectivity [43].
Therefore, according to the characterization results, such behavior can be due to the presence of
Cu-rich copper oxide species within the crosslinked βCD matrix. In order to be effective, the Cu (II)
species would have to be converted, possibly through the formation of Cu (I) species, into active
Cu (0) species to enable the activation of the hydrogen molecule. This could explain the lack of
activity under H2 pressure because Cu (II) species did not activate hydrogen. Conversely, the catalytic
transfer hydrogenation occurred when the reaction mixture containing Cu/CβCAT was irradiated in
the presence of FA.
As a result of the optimization experiments, it was found that the best reductive amination yields
for both aldehyde and ketone moieties can be acquired under MW irradiation by using the described
bimetallic PdCu/CβCAT in ethanol at 100 ◦C for 120 min. under H2 pressure (15 bar).
The general applicability of this bimetallic catalyst for the synthesis of secondary amines under
the optimum reaction conditions (100 ◦C, 2 h, H2 pressure, EtOH) (Scheme 2) was then evaluated.
The results about the MW-assisted reductive amination of benzaldehyde with assorted aryl- and
alkyl-amines are summarized in Table 2.
Based on the reported results, it is worth noting that all varieties of deployed substrates successfully
underwent reductive aminations; good and comparable yields (>64%) were achieved from the reductive
amination of aromatic aniline and 4-methylaniline with benzaldehyde to corresponding secondary
amines (Table 2, entries 1 and 2). Although the longer reaction time was required (up to 2.5 h,
yield 82%), the efficient reductive amination of 4-methoxyaniline (Table 2, entry 3) was observed.
The electron-donating effect of 4-methoxy- substituent on aniline could stabilize the imine intermediate
requiring a longer time for the reduction. Analogous behavior was observed for 4-iodoaniline to
reach 61% yield of desired secondary amine (Table 2, entry 4); a slightly lower yield was detected
for 4-trifluoromethylaniline (57%) (Table 2, entry 5). Superior results (87% yield) were recorded for
benzylamine (Table 2, entry 6), and even more hindered β-methyl substituted benzylamine afforded
good yields (76%) (Table 2, entry 7). Ultimately, the reactions of alkyl amines such as octylamine
and 2-methylpropan-1-amine proceeded quickly to produce the corresponding alkyl benzylamines in
excellent yields (Table 2, entries 8 and 9).
O
H2 (15 bar)
R
PdCu/CβCAT
N
H
H
R NH2
MW, 100°C
2 h, EtOH
Scheme 2. CβCATs reductive amination reaction.

Molecules 2020, 25, 410
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Table 2. PdCu/CβCAT catalyzed the reductive amination of different amines with benzaldehyde.
Entry a
R-NH2
Yield (%) b
1
73
2
3
4
64
82 c
61 c
5
6
57
87
7
76
8
9
84
68
a
Reaction conditions: 0.5 mmol benzaldehyde, 0.5 mmol amines, 5 wt % PdCu/CβCAT catalyst (based on Pd), 2 mL
ethanol, MW irradiation at 100 ◦C for 2 h, H2 (15 bar); b Determined by GC-MS (yields are based on area percentage);
c
Reaction time 2.5 h.
2.3. Reduction of Nitroaromatics
The reduction of nitrobenzene was used as a benchmark reaction (Scheme 3) to optimize the
conditions for the reduction of the nitro group in the following one-pot reactions. The reaction was
performed in EtOH at 100 ◦C, as these were the optimized conditions for reductive amination.
Scheme 3. MW assisted reduction of nitroaromatics in the presence of CβCATs.
Quite surprisingly, using the bimetallic nanocatalyst at 100 ◦C, the nitrobenzene was recovered
almost unreacted (1% of aniline was detected, Table 3), while the highest yield was realized using
Pd/C
β
CAT (87.5%). As PdCu/C
β
CAT was the best performing catalyst for the reductive amination,
◦
it was tested for the reduction of nitrobenzene at higher temperatures. An experiment at 150 C

Molecules 2020, 25, 410
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afforded 68% of aniline; a further increase of temperature was not taken into consideration since it
could negatively impact the reductive amination step, as described earlier (see the previous section).
On the contrary, dicyclohexylamine was the main product (58%) formed when Pd/C
β
CAT was used at
150 ◦C. As already observed in the reductive aminations, the monometallic Cu/C
βCAT was completely
inactive, owing to the presence of Cu(II) species.
Table 3. MW assisted Nitroarene reduction tests in the presence of CβCATs a.
Yield (%) b
Catalyst
100 ◦C
150 ◦C
Cu/CβCAT
Pd/CβCAT
PdCu/CβCAT
0
87.5
1
0
17
68
a
Reaction conditions: 0.5 mmol nitrobenzene, 5 wt % CβCAT (based on metal), 2 mL ethanol, MW irradiation at
100 ◦C or 150 ◦C for 1 h, H2 (15 bar); b Determined by GC-MS (yields are based on area percentage).
2.4. One-Pot Reductive Amination
Once the optimum conditions have been determined, we set out to evaluate the catalytic activity
of the bimetallic catalyst using benzaldehyde and nitrobenzene as the model system for the one-pot
reductive aminations (Scheme 4).
Scheme 4. One-pot reductive amination.
The best yield of N-benzylaniline was achieved at 150 ◦C, while working at 100 ◦C there was no
product formation detected. Product yields were dramatically reduced when the temperature was
enhanced to 180 ◦C, as only 34% N-benzylaniline was obtained. Interestingly, the position of the peaks
observed on the PdCu/CβCAT after the one-pot amination reaction (see Figure 1, dark green line)
was shifted toward high angles with respect to that of the as-prepared catalyst (green line). Peaks at
2Theta 17.8◦, 21.0◦, 43.0◦, and 52.0◦ may be ascribed to a copper-rich CuxPdy alloy in the tetragonal
phase (Cu2.991Pd1.009, JCPDS file number: 03-065-9675). DR UV–Vis-NIR characterization (Figure 2)
revealed quite big differences between the spectra of the bimetallic catalyst before (inset, green line)
and after (inset, dark green line) the reaction. In particular, an exponentially increasing absorbance
towards higher energy was observed as a consequence of interband transitions [44]. Such spectroscopic
feature can be reasonably related to the formation of bimetallic particles under reaction conditions,
in agreement with PXRD analysis. Therefore, we can hypothesize a synergic effect due to the formation
of a PdxCuy alloy, where the Pd sites facilitate the reduction of the nitro compound, whereas the Cu
species could promote the imine hydrogenation to secondary amine.
Negligible metal leaching was observed after completion of the reaction; indeed, after ICP-OES
analysis, the metal content of 87.10
PdCu/CβCAT.
±
1.35 g/kg of Cu and 84.98
±
1.57 g/kg of Pd were detected in the
3. Materials and Methods
All starting organic reagents and solvents were purchased from Sigma Aldrich (Sigma Aldrich,
Darmstadt, Germany) and used without further purification. The β-CD was purchased from Wacker
Chemie, Munich, Germany, and MW-assisted reactions were carried out in a SynthWAVE reactor

Molecules 2020, 25, 410
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(MLS GmbH, Leutkirch im Allgäu, Germany; Milestone Srl, Bergamo, Italy), which houses a closed
MW-cavity. The GC-MS analyses were performed with an Agilent 6890 system (Agilent Technologies,
Santa Clara, CA, USA) fitted with a mass detector Agilent Network 5973 using a capillary column
that was 30 m long and had an i.d. of 0.25 mm and a film thickness of 0.25 mm. GC conditions were:
Injection split of 1:20, injector temperature of 250 ◦C, and detector temperature of 280 ◦C. The gas
carrier was helium (1.2 mL min−1), and the temperature program was from 70 ◦C (2 min) to 300 ◦C at
5 ◦C min−1. The C
βCAT metals content was determined using ICP-OES with a Perkin Elmer Optima
7000 (Perkin Elmer, Norwalk, CT, USA) spectrometer via the addition of a standard.
Transmission electron microscopy (TEM) measurements were carried out by employing a side
entry Jeol JEM 3010 (300 kV) microscope equipped with a LaB6 filament and fitted with X-ray EDS
analysis by a Link ISIS 200 detector. The powdered sample was deposited on a Cu grid, coated with
a porous carbon film, and the digital micrographs were acquired using an Ultrascan 1000 camera.
The images were processed by Gatan digital micrograph.
Powder X-ray diffraction (PXRD) patterns were collected with a PW3050/60 X’Pert PRO MPD
diffractometer from PANalytical working in Bragg–Brentano geometry, using as a source the
high-powered ceramic tube PW3373/10 LFF with a Cu anode (λ = 0.541 Å) equipped with a Ni
filter to attenuate Kβ. Scattered photons were collected by a real-time multiple strip (RTMS) X’celerator
detector. Data were collected in the 5◦ ≤ 2θ ≤ 100◦ angular range, with 0.02◦ 2θ steps.
Diffuse reflectance (DR) UV–Vis-NIR spectra were recorded at room temperature on a Varian Cary
5000 spectrophotometer equipped with an integrating sphere attachment (BaSO4 as internal reference),
in the 50,000 to 4000 cm−1 range. DRUV–Vis-NIR spectra of the as-prepared samples were reported in
the Kubelka-Munk function [f (R ) = (1
the sample.
−
R )2/2R ; R = reflectance of an “infinitely thick” layer of
∞
∞
∞
∞
3.1. PdCu/β-CD Catalyst Preparation
An easy and reproducible one-pot sonochemical
hexamethylene diisocyanate (HDI) and Pd(II) or Cu(I) salts has been employed to obtain Pd/C
Cu/C CAT, and PdCu/C CAT [34]. To this end, the HDI crosslinking agent (2.8 mL, 17.4 mmol) was
slowly added under intense sonication to a -CD (1 g, 0.78 mmol) solution in DMF (4 mL) together
β-CDs reticulation in the presence of
βCAT,
β
β
β
with Pd(OAc)2 (200 mg, 0.90 mmol) or CuCl (90 mg, 0.9 mmol). For the preparation of the bimetallic
catalyst, 0.45 mmol (100 mg) of Pd(OAc)2 and 0.45 mmol (45 mg) of CuCl were added. The reaction
mixture was thermostated at 60 ◦C, and after 30 min sonication (20 kHz), a compact gel was obtained.
This reticulated product was crushed and washed twice with water, methanol, and acetone (50 mL
each). The final C
vacuum at 75 ◦C, to produce after 20 min grinding at 650 rpm in ball mill (Retsch PM100 High Speed)
a dark green powder (PdCu/C CAT), a brownish powder (Pd/C CAT), and a light green powder
(Cu/C CAT), respectively. The particle size of the catalyst powder, measured by photon correlation
spectroscopy (Coulter) after dispersion in water, was found to be in the range of 700 to 1100 nm.
The C CATs were then analyzed by ICP-OES, revealing an average metal content of
CAT) and 88.90 1.58 g/kg (Pd/C CAT). In the bimetallic catalyst
βCAT was filtered on a sintered glass Buchner funnel and dried overnight under
β
β
β
β
89.10 ± 1.43 g/kg (Cu/C
β
±
β
87.32 ± 1.55 g/kg of Cu and 85.13 ± 1.71 g/kg of Pd were detected.
3.2. Microwave-Assisted Reductive Amination
MW-promoted reactions were carried out in the SynthWAVE reactor, a multimode system that
enables multiple gas inlet. This instrument equipped with a high-pressure stainless-steel reaction
chamber, which can work up to a maximum of 300 ◦C temperature and 199 bar, enables MW reactions
both in small-scale (mL) and large-scale (L). Moreover, integrated reactor sensors continuously
monitored the internal pressure, temperature, and power applied inside the reactor cavity during all
reaction runs, adjusting the applied MW power in real-time to follow a predefined temperature profile.
A sampling valve on the bottom of the reactor was directly connected to the cooling chamber.

Molecules 2020, 25, 410
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In a typical experiment, benzaldehyde or acetophenone (0.5 mmol) were reacted with amine
in 2 mL of solvent (EtOH or H2O) in the presence of 94 mg of PdCu/C CAT, corresponding to the
β
5 wt % of metal (Pd content = wt %, Cu content = wt % determined by ICP-OES). The reactions were
performed under magnetic stirring (450 rpm) at fixed temperature and time (see Table 1) in an MW
SynthWAVE reactor that enables multiple gas inlet.
3.2.1. Reductive Amination under H2 Pressure
When H2 was used for reductive amination, 0.5 mL amine was added to benzaldehyde or
acetophenone and, after flushing the MW reactor 3 times with N2, a pressure of H2 (15 bar) was loaded
at room temperature before starting the heating program.
3.2.2. Reductive Amination in the Presence of Formic Acid
When the reductive amination was performed in the presence of an H-donor, FA (76 µL, 2 mmol),
the amine (2 mmol) was added to the reaction mixture before starting the heating program and loading
the MW reactor chamber with N2 (15 bar).
Upon completion of the heating stage, the reaction chamber was cooled down to room temperature.
The catalyst PdCu/CβCAT was removed by filtration and washed twice with ethanol. Then, 100 µL
of the reaction mixture was diluted with CHCl3 (1 mL), and the resulting solution was analyzed
by GC-MS.
When water was used as a reaction solvent, the products were extracted with CHCl3 (2
×
2 mL)
and, after drying over Na2SO4, the organic phase was injected into GC-MS. The GC analyses were
performed with an Agilent 6890 system (Agilent Technologies, Santa Clara, CA, USA) fitted with
a mass detector Agilent Network 5973 using a capillary column that was 30 m long and had an i.d.
of 0.25 mm and a film thickness of 0.25 mm. GC conditions were: Injection split of 1:20, injector
temperature of 250 ◦C, and detector temperature of 280 ◦C. The gas carrier was helium (1.2 mL min−1),
and the temperature program was from 70 ◦C (2 min) to 300 ◦C at 5 ◦C min−1
.
3.3. Microwave-Assisted Reduction of Nitrobenzene
In a typical experiment, nitrobenzene (0.5 mmol) was dissolved in 2 mL of ethanol in a flask.
After the addition of the catalyst (5 mol% of metal), the mixture was stirred (450 rpm) at the selected
temperature under H2 atmosphere (15 bar) for 1 h in SynthWave. After cooling, the crude reaction was
filtered, and 100 µL of the reaction mixture was diluted with CHCl3 (1 mL) and analyzed using GC-MS
(Agilent Technologies, Santa Clara, CA, USA).
4. Conclusions
The development of new heterogeneous catalysts for organic transformations is a significant task
for modern synthetic chemistry. In this respect, herein, we report the use of new nanocatalytic systems
for the synthesis of secondary amines from nitroarenes via a one-pot MW-assisted reductive amination
of carbonyl compounds under H2 pressure. The bimetallic PdCu/CβCAT catalyst showed fine-tuned
activity, due to the formation under reaction conditions, of a CuxPdy alloy, in which the metallic sites
can synergistically afford the best results in terms of yields and selectivity, without any metal leaching.
Thus, it can be suggested that the PdCu/CβCAT system could work as a bifunctional nanocatalyst,
in which the Pd sites could facilitate the reduction of nitro compounds, affording structurally diverse
secondary amines in high yields. The role of copper is still under investigation to shed light on the
imine reduction step. The synthesis of secondary amines directly starting from nitroaromatics as
precursors is advantageous for synthetic organic chemistry in terms of both step economy and cost
of the substrates. The present catalytic reductive amination protocol has several advantages over
the existing methodologies in the literature, such as providing the atom economy by the elimination
of extra reaction steps along with the high product yields in reduced times. The use of a bimetallic

Molecules 2020, 25, 410
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nanocatalyst combines the advantages coming from both homogeneous (for selectivity and high yields)
and heterogeneous catalysis (for easy catalyst recovery and no metal leaching).
Author Contributions: Methodology experimental design, E.C.G., S.T., and G.C.; Investigation, E.A., E.C.G.; S.T.;
data curation, S.T., M.M.; catalyst analysis M.M.; writing—original draft preparation, E.C.G., S.T.; review and
Editing, G.C. and R.S.V. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by University of Turin (Ricerca locale 2018 and 2019).
Conflicts of Interest: The authors declare no conflict of interest.
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©
2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).