One-pot oxidant-free dehydrogenation-Knoevenagel tandem reaction catalyzed by a recyclable magnetic base-metal bifunctional catalyst

Article abstract of DOI:10.1002/aoc.5897

A new base-metal bifunctional catalyst NH-Pd(0)@MNP was prepared via a facile procedure and fully characterized. The as-prepared catalyst was used as an efficient relay catalyst for the one-pot oxidant-free dehydrogenation-Knoevenagel condensation tandem reaction from benzyl alcohol in H₂O under mild conditions and generated benzalmalononitriles with yield up to 96%. Meanwhile, the catalyst could be easily recovered from the reaction system by an external magnetic field, and is reusable with little loss of activity up to 6 runs (<5%).

Full text of DOI:10.1002/aoc.5897

Received: 31 March 2020
DOI: 10.1002/aoc.5897
Revised: 2 June 2020
Accepted: 4 June 2020
F U L L P A P E R
One-pot oxidant-free dehydrogenation-Knoevenagel tandem
reaction catalyzed by a recyclable magnetic base-metal
bifunctional catalyst
Xiaofeng Yuan1 | Zijuan Wan1 | Jinfeng Ning1 | Qiang Zhang2 | Jun Luo1
1School of Chemical Engineering, Nanjing
A new base-metal bifunctional catalyst NH-Pd(0)@MNP was prepared via a
facile procedure and fully characterized. The as-prepared catalyst was used as
an efficient relay catalyst for the one-pot oxidant-free dehydrogenation-
Knoevenagel condensation tandem reaction from benzyl alcohol in H2O under
mild conditions and generated benzalmalononitriles with yield up to 96%.
Meanwhile, the catalyst could be easily recovered from the reaction system by
an external magnetic field, and is reusable with little loss of activity up to
6 runs (<5%).
University of Science and Technology,
Nanjing, 210094, P. R. China
2Jiangsu Key Laboratory of
Environmental Functional Materials,
School of Chemistry, Biology and Material
Engineering, Suzhou University of
Science and Technology, Suzhou, 215009,
China
Correspondence
Jun Luo, School of Chemical Engineering,
Nanjing University of Science and
Technology, Nanjing 210094, China.
Email: luojun@njust.edu.cn
K E Y W O R D S
bifunctionalization, Knoevenagel condensation, magnetic nanoparticles, oxidant-free
dehydrogenation, relay catalysis
Funding information
Natural Science Foundation of Jiangsu
Province of China, Grant/Award
Numbers: no. BK2010485, BK2010485;
Qing Lan Project of Jiangsu Province, P. R.
China
1 | INTRODUCTION
solvents have the risk of burning or even explosion.[7]
Anaerobic dehydrogenation of alcohols using olefins,
Dehydrogenation of primary and secondary alcohols to
carbonyl compounds is of great value in organic synthesis
and the products are also versatile organic
compounds.[1–6] The metal-based oxidants such as potas-
sium permanganate, chromium reagents, osmium
reagents, or ruthenium reagents are traditionally used in
most of the oxidative dehydrogenations. However, these
methods have the disadvantages of high cost, low selec-
tivity and large amount of waste. Although O2 and/or
H2O2 are also widely utilized in this kind of transforma-
tion as green oxidants, overoxidation is usually inevitable
during the oxidative dehydrogenation. Moreover, when
oxygen is chosen as oxidant, volatile alcohols or organic
ketones and N-oxides as hydrogen acceptors is an alterna-
tive method.[8] In view of environmental, economic and
safety factors, catalytic dehydrogenation of alcohols to
prepare carbonyl compounds in the absence of oxidants
and hydrogen acceptors is an ideal and atom-economic
way.[9–11] The hydrogen generated is one of the clean
energy source when it is utilized in industry-scale.
Besides, it's worth noting that this process prevents alco-
hols from being overoxidized. In recent years, some cata-
lysts have been developed for the dehydrogenation of
alcohols without oxidants or hydrogen receptors.[12–27]
However, most of them need to be carried out in organic
solvents or in the presence of acid or alkali additives and
Appl Organomet Chem. 2020;e5897.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5897

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YUAN ET AL.
have some unavoidable deficiencies including long reac-
tion time, high temperature and inconvenient recovery of
catalyst. Therefore, development of an efficient, atom-
economic and green method for the preparation of car-
bonyl compounds from the corresponding alcohols is still
demanded.
recyclable relay catalyst in one-pot oxidant-free and base-
free dehydrogenation-Knoevenagel condensation tandem
reaction in H2O under mild conditions.
2 | RESULTS AND DISCUSSION
Knoevenagel condensation is a well-known and
important method for the formation of carbon–carbon
double bonds.[28,29] It can provide a large number of use-
ful compounds with a wide range of applications in phar-
maceuticals, agriculture, biology and many other
fields.[30–34] Traditionally, this reaction is catalyzed by
various homogeneous catalysts like organic or inorganic
bases and Lewis acids,[35] which has the disadvantage of
difficult catalyst recovery. To solve this problem, various
heterogeneous catalysts such as carbon nanotubes,
metal–organic frameworks (MOFs), hydrotalcite, meso-
porous zirconia, and zeolites have been developed for the
Knoevenagel condensations, however, tedious or compli-
cated preparation of catalysts and weakened catalytic
activity are often inevitable.[35–37]
Tandem reaction has attracting intensive interests in
catalytic reactions owing to some merits such as high
atomic utilization rate, simpler operation, less waste dis-
charge and low cost.[38–43] The application of
multifunctional catalysts can make some multi-step syn-
thesis be carried out smoothly in one pot and provides
diverse products with high yields.[44] In recent years,
some bifunctional catalysts have been developed and
exhibit high catalytic activity in a series of tandem
reactions.[45–61] For example, Dong et al. prepared a
bifunctional heterogeneous catalyst Pd(0)@UiO-68-AP,
which was applied to catalyze oxidation–Knoevenagel
condensation tandem reaction from benzyl alcohols.[62]
In 2017, Gu et al. synthesized a hollow spherical-
structured HCP − A − B catalyst with acidic and basic
sites for catalyzing one-pot hydrolysis-Henry and
hydrolysis-Knoevenagel reactions.[63]
The preparation procedure of the secondary amine-
palladium bifunctional catalyst NH-Pd(0)@MNP is
shown in Scheme 1. The secondary amine functionalized
precursor NH-L@MNP (2) was prepared according to our
previous wꢀork. It was coordinated with Pd (OAc)2 in ace-
tone at 30 C for 24 hr. Then the bivalent palladium was
reduced to zerovalent state by NaBH4 to afford the
desired catalyst NH-Pd(0)@MNP (4).
Figure 1 shows the Fourier transform infrared (FT-
IR) spectra of Fe3O4, Fe3O4@SiO2 and NH-Pd(0)@MNP
compounds. The FT-IR absorption spectrum of
Fe3O4@SiO2 showed three peaks at 794, 1058, and
3,292 cm−1, corresponding to the Fe-O, Si-O-Si and O-H
stretching vibration, respectively. However, these signals
do not exist in that of Fe3O4, which indicates the com-
plete coating of silica shell on Fe3O4 core. The three char-
acteristic peaks at 794, 1058, and 3,292 cm−1 were also
observed in the FI-IR spectrum of NH-Pd(0)@MNP. In
addition, Figure 1(c) shows that the organic moiety was
successfully anchored on Fe3O4@SiO2 because intensive
signals of C=N and C=C bonds were observed at 1549
and 1,411 cm−1, respectively.
The magnetization curves of Fe3O4, Fe3O4@SiO2 and
NH-Pd(0)@MNP were measured by vibrating sample
magnetometer (VSM) at room temperature. As shown in
Figure 2, the VSM curves of all samples passed through
zero point without the presence of a hysteresis loop, indi-
cating that all samples were superparamagnetic. The sat-
uration magnetization values of Fe3O4, Fe3O4@SiO2 and
NH-Pd(0)@MNP were 60.9, 41.7 and 25.5 emuÁg−1
,
respectively. The magnetization value of Fe3O4@SiO2
was lower than that of Fe3O4, which confirmed the for-
mation of silica coating. Similarly, NH-Pd(0)@MNP has
the lowest magnetization value owing to the further func-
tionalization of the catalyst moiety on the surface.
The XRD patterns of the nanoparticles were depicted
in the Figure 3. The characteristic peaks of the standard
Fe3O4 (2θ = 30.1o, 35.4o, 43.1o, 53.4o, 57o and 62.6o)
(PDF#88–0866, reference JCPDS card no. 19–629)[73]
were observed in all nanoparticles. The broad peak at
2θ = 20-30o was attributed to silica shell. As shown in the
figure, the intensity of the characteristic peaks of
Fe3O4@SiO2 were the strongest, and the patterns of other
nanoparticles were similar to that of Fe3O4@SiO2, indi-
cating that the structure remained unchanged after being
functionalized.
On the other hand, magnetic nanoparticles (MNPs), as a
kind of material that can be evenly dispersed and easily
recovered, have attracted more and more attention in
organic catalysis in recent years.[64–71] It is a promising way
to design bifunctional catalysts supported by magnetic
nanoparticles for one-pot tandem reactions. Previously, we
have prepared a magnetic nanoparticles supported second-
ary amine/Cu (II) bifunctional catalyst and used it in
Knoevenagel condensation and 1,3-dipolar cycloaddition
domino reaction. This bifunctional catalyst exhibited relay
catalysis behavior and showed higher catalytic activity than
parallelly bifunctionalized analogue.[72]
In this work, a new magnetic nanoparticles supported
base-palladium bifunctional catalyst NH-Pd(0)@MNP
was prepared and applied as efficient and simply

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SCHEME 1 Synthesis of bifunctional catalyst NH-Pd(0)@MNP
ꢀ
The thermal stability of Fe3O4@SiO2, NH@MNP,
NH-L@MNP, NH-Pd(0)@MNP and Pd(0)@MNP was
evaluated by thermogravimetric analysis (TGA). As
shown in Figure 4, the weight loss before 170 ꢀC is attrib-
uted to the desorption of physically adsorbed water.
Sillica coated Fe3O4 microspheres are relatively staꢀble
scaffoldꢀwas seen in the temperature range from 300 C
to 650 C, and the weight loss of NH@MNP and NH-
L@MNP were 4.8% and 5.2%, respectively. Moreover, the
weight loss of NH-Pd(0)@MNP is greater than that of its
precursor NH-L@MNP. On this basis, it is verified that
the organic group containing palladium complex is well
[74]
ꢀ
with a weight loss of 3.8% between 170 C and 800 C.
grafted on the Fe3O4@SiO2.
The TGA curve of NH-
The thermogravimetric curve of NH-L@MNP is similar
to that of NH@MNP. The decomposition of organic
Pd(0)@MNP shows three steps of weight loss with 75.6%
remaining. The loss of physically adsorbed water, decom-
position of partial organic chain, and the decomposition
of the organic Pd complex respectively account for the
three weight losses.[4] Furthermore, there is less weight
loss for Pd(0)@MNP in comparison to NH-Pd(0)@MNP,
indicating that the loading capacity of NH-Pd(0)@MNP
is greater than Pd(0)@MNP.
The X-ray photoelectron spectroscopy (XPS) was per-
formed to investigate the surface composition of NH-
Pd(0)@MNP. The XPS survey spectrum shows peaks due
to O 2 s (25.1 eV), Si 2p (103.1 eV), Si 2 s (153.1 eV), C 1 s
(285.1 eV), Pd 3d (336.1 eV and 341.1 eV), N 1 s
(339.1 eV), and O 1 s (531.1 eV). As shown in Figure 5(b),
the peaks at 340.6 eV and 335.3 eV were assigned to
Pd(0) (3d3/2) and Pd(0) (3d5/2), respectively. At the same
time, the peaks at 342.8 eV and 337.6 eV were attributed
to Pd (II) (3d3/2) and Pd (II) (3d5/2), respectively. The
amount of Pd(0) was about 3.5 times of Pd (II), indicating
that most of Pd (II) was reduced. Simultaneously, there
were 399.6 eV and 398.4 eV peaks in the spectrum of N
FIGURE 1 FT-IR spectra of (a) Fe3O4, (b) Fe3O4@SiO2 and
(c) NH-Pd(0)@MNP

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FIGURE 2 VSM magnetization curve of
(a) Fe3O4, (b) Fe3O4@SiO2 and (c) NH-Pd(0)
@MNP
FIGURE 4 Thermogravimetric (TG) curves of
(a) Fe3O4@SiO2, (b) NH@MNP, (c) NH-L@MNP, (d) NH-Pd(0)
@MNP and (e) Pd(0)@MNP
FIGURE 3 XRD patterns of (a) Fe3O4@SiO2, (b) NH@MNP,
(c) NH-L@MNP, (d) NH-Pd(0)@MNP and (e) Pd(0)@MNP
1 s (Figure 5(c)), which correspond to –NH– and –N=,
respectively. Therefore, the organic chain and Pd were
successfully anchored on the carrier. Based on the ele-
mental analysis, the loading content of the organic moi-
ety was calculated to be 0.16 mmolÁg−1. The amount of
Pd was 3.6% as measured by inductively coupled plasma
atomic emission spectrometry (ICP-AES).
attached to the NH-L@MNP (Figure 6(b)). The average
size of the support particles was about 22 nm.
Then the catalytic activity of the as-prepared catalyst
NH-Pd(0)@MNP was first evaluated in the dehydrogena-
tion of benzyl alcohol in water. The effects of catalyst
amount, temperature and solvent on the reaction were
investigated to optimize the reaction conditions and the
results were listed in Table 1.
The SEM image of NH-Pd(0)@MNP showed the for-
mation
of
uniformly
dispersed
nanoparticles
The amount of the NH-Pd(0)@MNP has a significant
effect on the reaction (Table 1, entries 1–4). Benzalde-
hyde was obtained in a low yield of 36% as 2.0 mol%
(Figure 6(a)). Additionally, the TEM images of NH-Pd(0)
@MNP showed that Pd nanoparticles (about 3 nm) were

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FIGURE 5 The XPS
spectra of NH-Pd(0)@MNP.
(a) Survey scan; (b) Pd 3d and
(c) N 1 s
FIGURE 6 (a) SEM image
and (b) TEM image of NH-Pd(0)
@MNP
(30 mg) of catalyst was utilized (Table 1, entry 2). When
the catalyst amount was increased to 6.8 mol% (100 mg),
almost quantitative yield was achieved (Table 1, entry 4).
Further increase of the catalyst was not necessary
(Table 1, entry 5). Therefore, 6.8 mol% of catalyst was
employed in subsequent reactions. Afterwards, the influ-
ence of the reaction temperature was studied, and the
yield decreased along with the decline of temperature
(Table 1, entries 4, 6–7). In addition, the effect of solvents
was investigated. For comparison, solvents used in
oxidant-free dehydrogenation of alcohols in other litera-
tures were used to replace water (Table 1, entries 9–14).
The results showed that toluene, DMSO, DMF and
xylene were inferior to water while 1,4-dioxane gave a
similar result. Consequently, water was selected as the
optimal reaction solvent.
To show the superiority of the as-prepared catalyst,
the control experiments using its precursors and ana-
logue were studied and the results were listed in Table 2
accompanying with some reported results. Replacing
NH-Pd(0)@MNP (4) with its precursors Fe3O4@SiO2 or
NH-L@MNP (2) resulted in no reaction (Table 2, entries
2, 3). NH-Pd (II)@MNP (3) gave very low yield (Table 2,
entries 4). These results indicate that the embedded
zerovalent palladium was the active site. It's worth noting
that another zerovalent palladium catalyst Pd(0)

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TABLE 1 Optimization of oxidant-free dehydrogenation reaction of benzyl alcohol [a]
Entry
Catalyst mg (mol%)
0
Temp. (ꢀC)
Solvent
H2O
Yield (%)
0
1
80
80
2
30 (2.0)
H2O
36
3
60 (4.1)
80
H2O
64
4
100 (6.8)
150 (10.2)
100 (6.8)
100 (6.8)
100 (6.8)
100 (6.8)
100 (6.8)
100 (6.8)
100 (6.8)
100 (6.8)
100 (6.8)
80
H2O
>99
>99
88
5
80
H2O
6
70
H2O
7
50
H2O
68
8
25
H2O
23
9
Reflux
80
EtOH
toluene
DMF
26
10
11
12
13
14
91
80
65
80
DMSO
xylene
1,4-dioxane
58
80
78
80
>99
aReaction conditions: benzyl alcohol (0.5 mmol, 0.054 g), NH-Pd(0)@NMP, solvent (10 ml), N2, 12 hr. GC yield.
@MNP(5) without secondary amine moiety (see
Supporting Information for preparation) exhibited obvi-
ously lower activity (Table 2, entry 5), which indicates
that the secondary amine moiety promotes the catalytic
activity. In addition, the catalytic activity of NH-Pd(0)
@MNP was compared with other catalysts in the dehy-
drogenation condensation of benzyl alcohol reported in
literature.[2,8,9,22–27] Base was needed when Ag/ZnO and
PVP-Ru were used as catalysts (Table 2, entries 6, 7).
Other catalysts required either longer time or higher tem-
perature (Table 2, entries 8–14). Besides the high catalytic
activity, NH-Pd(0)@MNP owned other advantages such
as green water as using water as green solvent and easy
recovery of catalyst.
The heterogeneity of the oxidant-free dehydrogena-
tion was evaluated by a filtration test (Figure 7). After
reacting for 5 hours under the optimized conditions, NH-
Pd(0)@MNP was removed and the reaction liquid was
further let reacting for 23 hr under the same conditions.
The yield remained unchanged throughout the process.
Therefore, it can be concluded that there was no leakage
of active species and NH-Pd(0)@MNP belongs to a het-
erogeneous catalyst.
Table 3, a series of benzyl alcohols 6a-6i showed excellent
reactivity and delivered the desired products in good to
excellent yields. Ortho-Substituted alcohols 6d afforded
lower yields than para- and meta-isomers (6b and 6c),
indicating the presence of some steric effect. 4-Cyano and
4-nitrobenzyl alcohols (6 g and 6 h) gave lower yields
than other aromatic alcohols, however, it's worth noting
that no reduction products of cyano and/or nitro groups
were observed. Besides, cinnamyl alcohol (6j),
1-naphthalenemethanol (6 k) and 1-phenylethanol (6 l)
were compatible with this reaction system. Hetero-
aromatic alcohols 6 m-6o exhibited lower reactivity and
required longer time to ensure the completion of conver-
sion, and relatively lower yields were obtained. The rea-
son might be ascribed to the coordination ability of
hetero atoms in the aromatic rings. Unfortunately, ali-
phatic alcohols showed poor reactivity. For example, the
dehydrogenation of cyclohexylmethanol (6p) provided
cyclohexanecarbaldehyde (7p) in extremely low yield
(10%). It was reported that dehydrogenation of linear
alcohols was a rather difficult reaction.[75] Moreover,
1-hexanol (6q) did not show any reactivity to this
transformation.
With the optimized reaction conditions in hand, the
versatility of this approach was tested. As shown in
On the other hand, the secondary amine moiety in
the NH-Pd(0)@MNP is active catalytic site for

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TABLE 2 Oxidant-free dehydrogenation of benzyl alcohol to benzaldehyde with different catalysts
Entry
Catalyst
Temp. (ꢀC)
80
Solvent
H2O
base
time (hr)
Yield (%)
1
4
-
12
12
12
12
12
8
>99
0
2
Fe3O4@SiO2
80
H2O
-
3
2
80
H2O
-
0
4
3
80
H2O
-
21
86
98
99
82
98
91
99
98
99
76
5
5
80
H2O
-
6
Ag/ZnO[22]
PVP-Ru[8]
100
Toluene
H2O
KOH
7
Reflux
100
[BMMIM]OAc
24
24
24
6
[23]
8
Ag/Al2O3
Toluene
Toluene
p-xylene
p-xylene
p-xylene
Toluene
Toluene
-
-
-
-
-
-
-
9
Fe3O4@SiO2-Ag[9]
0.32%Pd/HT[24]
Ag/HT[25]
Reflux
120
10
11
12
13
14
130
10
12
10
6
Ag@IPN[26]
130
SMNP-PVP-Ag[2]
[27]
Reflux
120
NiFe2O4
Knoevenagel reaction. Therefore, NH-Pd(0)@MNP was
also used to catalyze the Knoevenagel condensation of
benzaldehyde with malononitrile in water for one hour
and benzalmalononitrile was obtained with an isolated
yield of 93%. In our previous study, it was proposed that
two adjacent active components could relay catalyze two
different reactions.[72] Since NH-Pd(0)@MNP displayed
catalytic activities in both dehydrogenation and
Knoevenagel condensation, it must be a bifunctional cat-
alyst to continuously catalyze the dehydrogenation of
FIGURE 7 Filtration test to identity the heterogeneity of the catalyst

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TABLE 3 Oxidant-free dehydrogenation of alcohols catalyzed by NH-Pd(0)@MNP (4)[a]
aReaction conditions: alcohol (0.5 mmol, 0.054 g), NH-Pd(0)@MNP (6.8 mol%), H2O (10 ml), N2, 80 ꢀC. GC yield.
benzyl alcohol and subsequent Knoevenagel condensa-
tion reaction. Table 4 summarizes the performance of
several catalysts for the synthesis of benzalmalononitrile
from benzyl alcohol.[35,44,76–80] As can be seen from
Table 4, except for the as-prepared catalyst, other cata-
lysts require oxidants to complete the first step of this
procedure. Cu3TATAT-3, Zr-MOF-NH2 and NH2-MIL-
101(Fe) even needed other conditions (TEMPO or light)
to facilitate the reaction (Table 4, entries 6–8). At the
same time, Zr-MOF-NH2 and NH2-MIL-101(Fe) took a
longer time (Table 4, entries 7–8). In a word, the one-pot
reaction catalyzed by NH-Pd(0)@MNP has the character-
istics of no requirement of oxidant, using water as clean
solvent and easy recovery and recycle of catalyst.
1-naphthalenemethanol (6 k) exhibited high reactivity as
well. However, 1-phenylethanol (6 l) yielded the desired
product in only 29% yield (Table 5, 8 l) although it gave
more than 99% of the acetophenone in the dehydrogena-
tion reaction (Table 3, 7 l). The reason might be attrib-
uted to the steric and electronic effect derived from
methyl group. 2-Piconol (6 m) did not deliver the desired
product. This observations could be partly attributable to
the further intramolecular cyclization of Knoevenagel
product to form 3-aminoindohzine-2-carbonitrile, even
the subsequent generation of Schiff base with
picolinaldehyde.[81] Meanwhile, furfuryl alcohol (6n) pro-
vided the product in 50% yield while thenyl alcohol (6o)
gave a higher yield of 83%. Since the dehydrogenation of
aliphatic alcohol 6p and 6q was very difficult, no desired
products were discovered in the one-pot tandem reac-
tions. In addition, when the corresponding aliphatic alde-
hydes were subjected to the Knoevenagel condensation
under the standard reaction conditions, no desired con-
densation product was obtained either.
Meanwhile, results of one-pot dehydrogenation-
Knoevenagel reaction catalyzed by NH-Pd(0)@MNP are
summarized in Table 5. Benzyl alcohols 6a-6i containing
both electron-withdrawing and electron-donating groups
proceeded smoothly and gave benzalmalononitriles in
excellent
yields.
Cinnamyl
alcohol
(6j)
and

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TABLE 4 Comparison of NH-Pd(0)@MNP with reported catalysts in the one-pot tandem reactions
time
(hr)
Temp. (ꢀC)
T1 T2
80 80
Yield
Oxidant (%)
Entry catalyst
t1
t2
Solvent
1[a]
NH-Pd(0)@MNP
12
1
H2O
-
93
2
Fe3O4@SiO2@PEI@Ru (OH)
x[35]
110
r.t. 10
12 Toluene
O2
90.2
3
Fe3O4@SiO2-Ag[76]
Ag/ZnO[77]
110
80
r.t. 15
80 1.5
7
1
5
Toluene
H2O
Air
O2
O2
O2
Air
O2
99
4
98.0
89.3
95
5
UiO-66-Ru[78]
Cu3TATAT-3[44]
[79]
100
75
100
75
1
Toluene
CH3CN
p-xylene
6[b]
7[c]
8[d]
12
48
Zr-MOF-NH2
90
90
91
NH2-MIL-101(Fe)[80]
r.t.
r.t. 40
trifluorotoluene/acetonitrile (10:
1)
72
Other condition:
aN2.
bTEMPO (0.5 equiv.).
cUV-light irradiation.
dlight irradiation (λ ≥ 420 nm).
Recovery and reusability are important properties of
heterogeneous catalysts. In this respect, the recyclability
of NH-Pd(0)@MNP was investigated in one-pot
dehydrogenation-Knoevenagel tandem reaction. After
being magnetically separated from the reaction mixture,
the recovered NH-Pd(0)@MNP was washed with water
and ethanol, and dried in vacuum overnight for the next
run. As shown in Figure 8, the catalyst can be recycled
up to 6 runs without any significant loss of its catalytic
activity.
4 | EXPERIMENTAL
4.1 | General
Fourier transform infrared (FT-IR) spectra were recorded
on a Nicolet spectrometer in KBr pellets. The X-ray dif-
fraction (XRD) images were obtained by a Bruker D8
Advance instrument with copper radiation (Cu Kα,
λ = 0.15418 nm). The magnetization curves were
obtained by a vibrating sample magnetometry (VSM;
Lakeshore7400-s) at room temperature. The ther-
mogravimetric (TG) curves were obtained from a
Shimadzu TGA-50 spectrometer. The XPS spectra were
recorded using an ESCALABTM 250Xi instrument. The
transmission electron microscopy (TEM) was performed
on a TECNAI 12 instrument. The scanning electron
microscope (SEM) image was obtained by FEI Nova
NanoSEM 450, USA. Elemental analysis was performed
with an Elementar Vario EL β recorder. Palladium con-
tent of the catalyst was measured using inductively
coupled plasma (ICP) with a L-PAD analyzer (Prodigy).
1H NMR and 13C NMR spectra were recorded with a
Bruker DRX500 spectrometer at 500 and 126 MHz
respectively. Toluene was strictly dried according to
3 | CONCLUSION
In summary, a magnetic nanoparticles supported second-
ary amine/Pd(0) bifunctional catalyst was prepared and
fully characterized. It was used as an efficient and recy-
clable relay catalyst for the oxidant- and base-free dehy-
drogenation of a number of benzyl alcohols in H2O and
dehydrogenation-Knoevenagel tandem reaction. It was
easy to separate the catalyst from reaction mixture by an
external magnetic field. The catalyst can be recycled up
to 6 runs without any significant loss of its catalytic
activity.

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TABLE 5 Tandem oxidant-free dehydrogenation-Knoevenagel condensation of alcohols with malononitrile catalyzed by NH-Pd(0)
@MNP (4)[a]
aReaction conditions: alcohol (0.5 mmol, 0.054 g), NH-Pd(0)@MNP (6.8 mol%), H2O (10 ml), N2, 80 ꢀC. Then, malononitrile (0.75 mmol,
0.050 g), 80 ꢀC, 1 hr. Isolated yield.
FIGURE 8 Recyclability of NH-Pd(0)@MNP

YUAN ET AL.
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standard operation and stored on 4 Å molecular sieves.
All other chemicals were commercially available and uti-
lized without further purification. Nano-Fe3O4@SiO2 was
synthesized by referring our previous work.[69]
4.6 | One-pot tandem reaction
Benzyl alcohol (0.5 mmol, 0.054 g), NH-Pd(0)@MNP
(6.8 mol%), and H2O (10 ml) were added to a 25 ml
round-bottomed flask. The mixture was reacted at 80 C
ꢀ
for 12 hr under nitrogen atmosphere. Then malononitrile
(0.75 mmol, 0.050 g) was added. After 1 hr, the catalyst
was separated by an external magnetic field, washed with
water and ethanol successively, and then dried overnight
in vacuum for the next run. The reaction mixture was
extracted with ethyl acetate. The organic phase was dried
by anhydrous sodium sulfate, filtered and evaporated
under reduced pressure to obtain crude products. The
crude product was purified by column chromatography
on silica gel (ethyl acetate/petroleum ether 1:20).
4.2 | Preparation of NH@MNP (1)
The Fe3O4@SiO2 (1.0 g) was dispersed in 50 ml anhy-
drous toluene using an ultrasonic bath for 1 hr. Then
[3-(2-aminoethyl)aminopropyl]triethoxysilane (0.529 g,
2.0 mmol) and pyridine (0.5 ml) were added, and the
ꢀ
mixture was stirred at 110 C for 24 hr under nitrogen
atmosphere. The solid was recovered through an external
magnet, and washed repeatedly with anhydrous toluene
and acetone. The obtained solid was dried overnight in
vacuum to give NH@MNP (1).
ACKNOWLEDGMENTS
This work was sponsored by the Natural Science Founda-
tion of Jiangsu Province of China (no. BK2010485) and
Qing Lan Project of Jiangsu Province, P. R. China (2014).
4.3 | Preparation of NH-L@MNP (2)
1.0 g of NH@MNP (1) was poured into 40 ml anhydrous
ethanol followed by ultrasonication for 30 min. Pyridine-
2-carbaldehyde (0.214 g, 2.0 mmol) was then added to the
mixture. The reaction was refluxed for 4 hr in nitrogen
atmosphere. After that, the reaction mixture was cooled
to room temperature. The solid was separated by an
external magnetic field and washed repeatedly with dry
ethanol. Subsequently, NH-L@MNP (2) was obtained by
drying overnight in vacuum.
CONFLICT OF INTEREST
There are no conflicts to declare.
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How to cite this article: Yuan X, Wan Z, Ning J,
Zhang Q, Luo J. One-pot oxidant-free
dehydrogenation-Knoevenagel tandem reaction
catalyzed by a recyclable magnetic base-metal
bifunctional catalyst. Appl Organomet Chem. 2020;
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