Chemoselective hydrogenation of heteroarenes and arenes by Pd-Ru-PVP under mild conditions

Article abstract of DOI:10.1039/d0ra09981c

Monometallic (Pd, Ru or Rh) and bimetallic (Pd0.5-Ru0.5) alloy NPs catalysts were examined for the hydrogenation of quinoline. Pd-Ru alloy catalyst showed superior catalytic activity to the traditional Rh catalyst. The characterization of Pd0.5-Ru0.5 catalysts, HAADF-EDX mapping and XPS analysis suggested that the alloy state of PdRu catalysts remained unchanged in the recovered catalyst. Furthermore, the catalyst was highly selective for the hydrogenation of different arenes. This journal is

Full text of DOI:10.1039/d0ra09981c

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Chemoselective hydrogenation of heteroarenes
and arenes by Pd–Ru–PVP under mild conditions†
Cite this: RSC Adv., 2020, 10, 44191
a
ab
a
Chandan Chaudhari,* Katsutoshi Sato,
Yoshihide Nishida,
c
d
cd
e
Tomokazu Yamamoto, Takaaki Toriyama, Syo Matsumura, Yasuyuki Ikeda,
Kenji Terada, Naoya Abe, Kohei Kusuda,
and Katsutoshi Nagaoka
e
e
f
f
Hiroshi Kitagawa
a
*
Monometallic (Pd, Ru or Rh) and bimetallic (Pd0.5–Ru0.5) alloy NPs catalysts were examined for the
hydrogenation of quinoline. Pd–Ru alloy catalyst showed superior catalytic activity to the traditional Rh
catalyst. The characterization of Pd0.5–Ru0.5 catalysts, HAADF-EDX mapping and XPS analysis suggested
that the alloy state of PdRu catalysts remained unchanged in the recovered catalyst. Furthermore, the
catalyst was highly selective for the hydrogenation of different arenes.
Received 25th November 2020
Accepted 2nd December 2020
DOI: 10.1039/d0ra09981c
rsc.li/rsc-advances
1,2,3,4-Tetrahydroquinolines are important building blocks for and activity of Rh catalysts, the high cost of Rh make it unfav-
the synthesis of ne chemicals, pharmaceuticals and petro- ourable for the industrial purposes. From the economical and
1–3
chemicals.
Traditionally, 1,2,3,4-tetrahydroquinolines have environmental point of view, the cost-effective catalyst is
been synthesized by catalytic cyclization or the Beckman rear- necessary for the hydrogenation of quinolines under mild
3–6
rangement method. Direct hydrogenation of readily available condition.
quinoline by molecular hydrogen is a simple and atom-
Solid-solution alloy method is a promising way for the
economical route for the hydrogenated quinoline compounds. synthesis of alloy NPs which offers an opportunity to control the
Nevertheless, the hydrogenation of quinolines reaction suffers electronic state of alloy by changing the composition ratio and/
from catalyst deactivation caused by the strong interaction or different combinations of alloy in this method. In search of
between the nitrogen atom of quinoline and active sites of the a cheap alternative for Rh catalyst, Pd and Ru are important
catalyst. Recently, several heterogeneous (Co, Au, Ru, Pt, Pd and metals because both metals are relatively cheaper than Rh metal
Rh) catalysts have been developed to overcome the catalyst and their well-known activity for hydrogenation reactions. We
7–24
deactivation problem (Table S1†).
Specically, Rh catalysts believe that Pd0.5–Ru0.5 solid solution alloy NPs could be alter-
exhibits remarkably excellent activity in comparison with other native for the traditional Rh catalyst. However, a solid-solution
noble metal catalysts but some of them require high tempera- alloy of Pd and Ru is a challenging task due to the immiscibility
ꢀ
13,21
14–21
25,26
ture (>80 C)
and/or high pressure (>10 bar).
Later, Rh/ of Pd and Ru in bulk state.
Recently, our research group led
AlOOH and Rh nanoparticles (NPs) catalysts have been re- by H. Kitagawa has reported the rst successful example of
ported for efficient hydrogenation of quinolines under ambient Pd0.5–Ru0.5 NPs via a chemical reduction method using a nano-
ꢀ
22–24
conditions (1 bar H
2
and 25 C).
Although high selectivity size effect which showed almost similar catalytic activity than
those of Rh for CO oxidation and automobile exhaust purica-
27,28
tion.
As part of our continuing interest in the applications of
a
Department of Chemical Systems Engineering, Graduate School of Engineering,
Nagoya University, Furo-cho, Chikusa-ku, Nagoya-464-8603, Japan. E-mail:
Pd–Ru as pseudo Rh catalyst, we herein report that chemo-
nagaoka.katsutoshi@material.nagoya-u.ac.jp;
chaudhari.chandan.subhash@d. selective hydrogenation of heteroarenes and arenes under mild
mbox.nagoya-u.ac.jp
condition.
b
Elements Strategy Initiative for Catalysts and Batteries (ESCIB), Kyoto University,
Katsura, Kyoto 615-8520, Japan
In preliminary experiments, we carried out the hydrogena-
tion of quinoline to 1,2,3,4-tetrahydroquinoline as model reac-
tion (Table 1). Monometallic Pd and Ru catalysts were inactive
for hydrogenation of quinoline (entries 1 and 2). In previous
reports, our group has shown that the electronic state and
consequently, catalytic activity of monometallic Pd and Ru
c
Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University,
Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan
d
The Ultramicroscopy Research Center, Kyushu University, 744 Motooka, Nishi-ku,
Fukuoka 819-0395, Japan
e
Furuya Metal Co., Ltd, Higashitsukuba Niihari Kogyodanchi, 57-4, Sawabe, Tsuchiura
27,28
City, Ibaraki 300-4104, Japan
could be modied by making Pd0.5–Ru0.5 alloy catalyst.
We
f
Division of Chemistry, Graduate School of Science Kyoto University Kitashirakawa-
found the similar trend using Pd0.5Ru0.5 catalyst and it showed
full conversion with 95% yield of 1,2,3,4-tetrahydroquinolines
Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra09981c
(entry 3). The effect of the solvent showed that the reactivity of
1
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a
Table 1 Optimization of reaction conditions for hydrogenation of Table 2 Hydrogenation of heteroarenes by Pd0.5Ru0.5PVP catalyst
quinoline
a
Entry
1
Heteroarene
Product
Conv. (%)
99
GC yield (%)
96
b
Entry Catalyst
Particle size (nm) Conv. (%) GC yield (%)
2
3
4
5
6
7
98
99
64
99
99
94
99
90
92
60
76
99
1
2
3
4
5
6
7
Ru–PVP
Pd–PVP
8.4 Æ 2.4
4.8 Æ 0.8
13
41
99
63
45
21
66
13
32
95
60
40
13
66
Pd0.5Ru0.5–PVP 5.6 Æ 1.6
Pd0.5Ru0.5–PVP 5.6 Æ 1.6
Pd0.5Ru0.5–PVP 5.6 Æ 1.6
Pd0.5Ru0.5–PVP 5.6 Æ 1.6
c
d
e
f
Rh–PVP
5.4 Æ 1.0
a
Reaction conditions: quinoline (1 mmol), catalyst (2 mol%), CH
3
OH (1
ꢀ
b
mL), 5 bar H
2
, 25e C, 6 h, CH
3
OH. Determined by STEM analysis.
c
d
f
THF. Toluene. 1,4-Dioxane. Ref. 38.
Pd0.5Ru0.5 was improved using methanol as solvent. Probably,
methanol provides the better dispersion of Pd0.5–Ru0.5 NPs in
the reactions and/or high solubility of hydrogen gas in meth-
anol. Next, we examined the activity of previously reported Rh
catalyst. Although, the particle size of Rh and Pd0.5–Ru0.5 was
similar, the moderate yield of tetrahydroquinoline was obtained
in the presence of Rh–PVP (entry 7). Due to the high amount of
PVP, the catalytic activity of Rh was slightly declined via less
b
45
8
80
a
2
Reaction conditions: substrate (1 mmol), catalyst (2 mol%), 5 bar H ,
ꢀ
b
25 C, 24 h. 3,4-Dihydroquinazoline was obtained.
29
interaction between quinoline and metal sites.
The substrate scope of Pd0.5–Ru0.5PVP catalyst was explored
ꢀ
under mild reaction conditions (5 bar H
2
and 25 C). As hand, Pd0.5–Ru0.5–PVP catalyst showed the high activity in
summarized in Table 2, quinolines with different functional solvent-free conditions. Hydrogenation of benzene and its
groups converted to corresponding tetrahydroquinolines. The derivative were also studied under optimized conditions. The
catalyst showed reduction of the heteroarene ring only. Pd0.5
–
high yields of the corresponding hydrogenated products was
Ru0.5 was also active for the hydrogenation of biologically obtained in the presence of the Pd0.5–Ru0.5 catalyst.
important heterocyclic compounds such as 1,8-naphthyridine
and quinazoline. O-Containing heterocycle, benzofuran was
also transformed to 1,2-dihydrobenzofuran with high yield
a
Table 3 Hydrogenation of arenes by Pd0.5Ru0.5PVP catalyst
(80%).
Hydrogenation of arenes to saturated compounds is an
Entry
Arene
Product
Conv. (%)
GC yield (%)
important transformation for petrochemical and pharmaceu-
tical industries. For instance, hydrogenation of benzoic acid to
cyclohexanecarboxylic acid, it is used for the synthesis of
1
2
3
4
5
99
99
99
95
99
94
92
99
95
92
30
pharmaceutical drugs such as ansatrienin. Hydrogenation of
benzoic acid has been reported by using supported and polymer
31–38
stabilized metal nanoparticles.
Among them, Rh catalyst
demonstrated the excellent activity for the hydrogenation of
benzoic acid. The scope of Pd0.5–R0.5 catalyst was examined for
different arene hydrogenation for the hydrogenation of benzoic
acid. The scope of Pd0.5–R0.5 catalyst was examined for different
arene hydrogenation under solvent-free condition (Table 3).
Pd0.5–Ru0.5–PVP catalyst was able to hydrogenate benzoic acid
with high yield. In previous reports, the role of solvent was
explained for the selective hydrogenation of benzoic acid. The
selectivity for cyclohexanecarboxylic acid was controlled using
polar solvent (water). The byproduct (benzyl alcohol) was ob-
tained using less polar solvent (cyclohexane). On the other
6
80
60
a
2
Reaction conditions: substrate (1 mmol), catalyst (2 mol%), 10 bar H ,
ꢀ
50 C, 24 h.
1
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Fig. 1 Recycle experiment for the hydrogenation of quinoline over
Pd0.5Ru0.5–PVP catalyst.
The reusability of Pd0.5–Ru0.5 catalyst was investigated using
ꢀ
optimized condition (5 bar H
2
and 25 C) for the hydrogenation
of quinoline. The catalyst was washed by acetone multiple times
aer completion of reaction and separated by centrifugation.
ꢀ
The recovered catalyst was dried under vacuum at 40 C for 12 h
and reused without further purication. The catalyst was reused
three times without loss in activity (Fig. 1).
Fig. 3 XPS spectra for Pd0.5–Ru0.5fresh (blue) and Pd0.5–Ru0.5used
(red) with Pd 3d (A) and Ru 3p (B).
To study morphological changes, the fresh and recovered
catalyst aer the 1st cycle was analysed by high-angle annular
dark eld. The reconstructed overlaying image of fresh Pd0.5
–
Ru0.5 catalyst revealed that Pd and Ru atom was distributed
equally in the particle which indicated the formation of
Pd0.5Ru0.5 alloy via homogeneous mixing at atomic-level. Deal-
loying and/or aggregation were not observed in Pd0.5–Ru0.5
(Fig. 2). Next, the electronic state of fresh and used Pd0.5–Ru0.5
catalyst was examined by XPS (Fig. 3). In our previous reports,
the binding energy of Pd0.5–Ru0.5 was shied positively in 3d5/2
(334.5 eV) from 3d5/2 (334.30 eV) of monontellic Pd and nega-
tively in 3P (460.6 eV) from 3P3/2 (461.4 eV) of monometallic
3
/2
39
Ru. Such shiing was also observed in fresh and used Pd0.5
–
Ru0.5 catalyst. From these results, we concluded that the elec-
tronic state of used catalysts was not changed signicantly aer
Scheme 1 Plausible transition state for the hydrogenation of quinoline
by Pd0.5–Ru0.5–PVP.
1st cycle.
The superiority of Pd0.5–Ru0.5 alloy over Pd and Ru can be
explained by unique electronic state. Our group has used rst-
principles methods to study the electronic states of Pd, Ru, parental Pd or Ru metal. This result has indicated that a new
Rh, and the Pd0.5–Ru0.5 alloy. Interestingly, we have found that electronic state is produce by alloying Pd and Ru. The density of
the electronic state of the Pd–Ru alloy differs from those of the state of the Pd0.5–Ru0.5 alloy is similar to that of Rh.
Furthermore, Pd0.5–Ru0.5 alloy shows the electron transfer
from Pd to Ru and produced the partial positive (d+) charge at
Pd and partial negative charge (dÀ) at Ru. This unique elec-
tronic state with bifunctional sites of Pd0.5–Ru0.5 alloy made it
favorable for chemoselective for hydrogenation of quinoline. In
case of monometallic Pd or Ru catalyst, the strong adsorption of
quinoline might be responsible for low conversion of quinoline.
It is well-known that the hydrogenation of organic functional
group such as C]C and C]N proceeded via heterolytic
+
À
cleavage of H . Such cleavage of H yielded to H and H using
2
2
8,37
metal and support/ligand. In colloidal Pd0.5Ru0.5 catalyst, the
heterolytic cleavage of hydrogen proceeded using bifunctional
2
sites of Pd0.5–Ru0.5 catalysis (Scheme 1). The better H cleavage
Fig. 2 HAADF-STEM images Pd-L and Ru-L STEM-EDX maps and
over bifunctional sites of Pd0.5Ru0.5 catalyst may have been
reconstructed overlay images of fresh (A) and recovered (B) Pd0.5
–
Ru0.5–PVP catalyst.
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