Pr2O3 Supported Nano-layered Ruthenium Catalyzed Acceptorless Dehydrogenative Synthesis of 2-Substituted Quinolines and 1,8-Naphthyridines from 2-Aminoaryl Alcohols and Ketones

Article abstract of DOI:10.1002/cctc.201902311

Pr₂O₃ supported Ru nanolayers and Ru nanoparticles catalysts were examined for the synthesis of quinolines. The Ru nanolayer was most active catalyst and showed a broad substrate scope. Structure-activity relationship demonstrated that the metallic state and morphology of Ru as well as the basic site of Pr₂O₃ were indispensable factors of this catalytic system.

Full text of DOI:10.1002/cctc.201902311

Accepted Article
Title: Pr2O3 Supported Nano-layered Ruthenium Catalyzed
Acceptorless Dehydrogenative Synthesis of 2-Substituted
Quinolines and 1,8-Naphthyridines from 2-Aminoaryl Alcohols
and Ketones
Authors: Katsutoshi Nagaoka, Chandan Chaudhari, Katsutoshi Sato,
Yuta Ogura, and Shin-ichiro Miayahara
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To be cited as: ChemCatChem 10.1002/cctc.201902311
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10.1002/cctc.201902311
ChemCatChem
COMMUNICATION
Pr2O3 Supported Nano-layered Ruthenium Catalyzed Acceptorless
Dehydrogenative Synthesis of 2-Substituted Quinolines and 1,8-
Naphthyridines from 2-Aminoaryl Alcohols and Ketones
Chandan Chaudhari,*[a] Katsutoshi Sato,[a] Yuta Ogura,[a] Shin-ichiro Miayahara,[a] and Katsutoshi
Nagaoka*[a]
Abstract: Pr2O3 supported Ru nanolayers and Ru nanoparticles
catalysts were examined for the synthesis of quinolines. The Ru
nanolayer was most active catalyst and showed a broad substrate
scope. Structure-activity relationship demonstrated that the metallic
state and morphology of Ru as well as the basic site of Pr2O3 were
indispensable factors of this catalytic system.
Acceptorless dehydrogenation of alcohols has gained
much of attention of researchers for the direct synthesis of
chemicals.[8] The only byproducts of this atom-economical and
environmentally friendly reaction are water and hydrogen.
Several catalytic acceptorless dehydrogenative syntheses of
quinolines from 2-aminobenzyl alcohols and alcohols and/or
ketones have been reported.[9] For example, homogeneous Ru,
Pd, Ir, Cu, Ni, and Co catalysts have been explored for similar
transformations under basic conditions,[9a–l] and the use of
recyclable Pd and Pd-Ag catalysts in the presence of KOH has
also been investigated.[9m,n] However, these methods require
high catalyst loading and a basic additive (Table S1). To the
best of our knowledge, acceptorless dehydrogenative synthesis
of 2-substituted 1,8-naphthyridines has not been reported.
Praseodymium oxide is an important rare earth oxide as
adsorbent, catalysts, promoter and high electricity materials.[10]
Despite praseodymium oxides have interesting unique structure
and phases, it is much less explored as support for noble metal
loading.[11] Recently, we developed Pr2O3 supported low-
crystalline Ru nanolayers catalysts by using Ru3(CO)12
precursor.[12] Interestingly, this catalyst showed excellent activity
for ammonia synthesis, better than that of MgO supported Ru
nanoparticles catalyst. The synergistic combination of strong
basic support (Pr2O3) and unique Ru structure could play a vital
role in acceptorless dehydrogenative synthesis of heterocycles
in absence of basic additives. We herein report that we have
successfully developed the first general method for acceptorless
dehydrogenative synthesis of 2-substituted quinolines and 1,8-
naphthyridines under neutral conditions.
To optimize reaction conditions for the synthesis of 2-
phenylquinoline, we performed the reaction of 2-aminobenzyl
alcohol (1) and acetophenone (2) as a model reactions (Table 1).
RuCO/Pr2O3 and RuAC/Pr2O3 catalysts were prepared by using
Ru3(CO)12 and Ru(acac)3 respectively. Among the tested Pr2O3
supported metals (Ru, Rh, Ir and Re), Ru was found to be the
most active metal for the synthesis of 2-phenylquinoline (entries
1-6). Interestingly, RuCO/Pr2O3 catalyst demonstrated higher
activity than RuAC/Pr2O3 catalyst. 13 % yield of byproduct
(Dibenzo[b,f] [1,5]diazocine) was obtained when RuAC/Pr2O3 was
used as catalyst (entry 3). Pr2O3 was found to be optimal
support leading to excellent yield (entries 7-10). Low yield of 2-
sustituted quinoline was obtained in the presence of Pr2O3
Quinolines and 1,8-naphthyridines are important nitrogen-
containing heterocyclic compounds which exist in numerous
natural products.[1] Both classes exhibit a wide range of
biological activities such as antibacterial,[2] anti-inflammatory,[3]
antihypertensive,[4] and antiplatelet activities.[5]Although these
compounds are structurally similar to each other, the additional
nitrogen atom in 1,8-naphthyridines can impart different
chemical properties and require different synthetic methods.
Traditionally, quinolines and 1,8-naphthyridines were
synthesized by using Friedländer method which involves the
condensation of 2–aminoaromaticaldehydes and ketones.[6]
However, the limited availability and poor stability of 2-
aminoaromaticaldehydes, makes them unfavorable as reactant
for the synthesis of quinolines and 1,8-naphthyridines.
Quinolines have also been synthesized from 2-aminobenzyl
alcohols, which are relatively stable. Using 2-aminobenzyl
alcohol, Ru and Cu catalysts were reported for the synthesis of
quinolines in the presence of a sacrificial hydrogen acceptor.[7]
However, dehydrogenation of 2-aminobenzyl alcohol in the
absence of an acceptor would be more desirable in terms of
atom economy.
[a]
[b]
Dr. C. Chaudhari, Dr. K. Sato, Dr. Y. Ogura, S. Miyahara, Prof. Dr. K.
Nagaoka
Department of Chemical Systems Engineering,
Graduate school of Engineering, Nagoya University
Furo-cho, Chikusa-ku, Nagoya-464-8603, Japan
E-mail: nagaoka.katsutoshi@material.nagoya-u.ac.jp
E-mail: chaudhari.chandan.subhash@d.mbox.nagoya-u.ac.jp
Dr. K. Sato
support (entry 12).
To elucidate the dependence of the catalytic activity on the
Ru structure, we measured the X-ray absorption near-edge
structure (XANES) and X-ray absorption fine structure (EXAFS)
spectra of RuCO/Pr2O3-red, RuCO/Pr2O3-unred, and RuAC/Pr2O3-
red, as well as Ru powder as a reference compound (Fig. 1; the
results of EXAFS curve-fitting analysis are listed in Table S2).
Elements Strategy Initiative for Catalysts and Batteries,
Kyoto University, 1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245,
Japan
Supporting information for this article is given via a link at the end of the
document.
1
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10.1002/cctc.201902311
ChemCatChem
COMMUNICATION
Table 1. Optimization of reaction conditions for the synthesis of 2-phenyl
Table 2. Synthesis of 2-substituted quinoline from 2-aminobenzyl alcohol and
ketonea
a
quinolines
Entry
1
Ketones
Products
Yieldb
(%)
Entry
Catalyst
Conv.
(%)
99
71
99
99
99
50
88
99
58
66
65
Yieldb (%)
Yieldb (%)
3
4
0
0
0
0
2
2
0
0
1
2
0
0
1
2c
3d
4
5
6
7
8
9
RuCO/Pr2O3
RuCO/Pr2O3
RuAC/Pr2O3
Ir/Pr2O3
Rh/Pr3O3
Re/Pr3O3
Ru/MgO
Ru/CeO2
Ru/La2O3
Ru/γ-Al2O3
Ru3CO12
Pr2O3
95
57
83
76
80
48
67
36
52
66
5
95
90
96
2
3
10
11
12
36
15
a] Reaction conditions: 1 (1 mmol), 2 (1.5 mmol), mesitylene (1.2 mL),
catalyst (2 mol% of Ru ), 170°C oC, 24 h, Ar. [b] GC yield. [c] Without reduction.
[d] Dibenzo[b,f][1,5]diazocine was obtained as a byproduct.
4
5
87
80
The designation “red” indicates that the catalyst was reduced at
500 °C for 0.5 h prior to analysis. The EXAFS spectrum of
species with a Ru–Ru bond length of 2.66 Å and a coordination
number of 10.5. Similarly, the EXAFS of spectrum of
RuAC/Pr2O3-red indicated a Ru–Ru bond length of 2.64 Å and a
coordination number of 7.9. The coordination numbers for
RuCO/Pr2O3-red and RuAC/Pr2O3-red were smaller than that of the
Ru powder (12). The XANES spectra of RuCO/Pr2O3-red and
RuAC/Pr2O3-red catalyst were identical to that of Ru powder
which indicated the state of Ru was made of metallic state. The
EXAFS spectrum of the unreduced catalyst (RuCO/Pr2O3-unred)
indicated the presence of a Ru–O species (coordination number,
5.5;bond length, 1.99Å). The XANES spectrum of this catalyst
had a slightly higher white line intensity, indicating that the Ru
had been partially oxidized by exposure to air after catalyst
synthesis from Ru3(CO)12.
We also examined the morphology of Ru using scanning
transmission electron micrograph (STEM) and energy dispersive
X-ray (EDX) (Fig.S1-S4). The reconstructed overlaying image of
RuCO/Pr2O3-red catalyst showed that metallic Ru was highly
dispersed on the surface of Pr2O3 in the form of nanolayers (Fig.
S1). In contrast, Ru nanoparticles were observed in
reconstructed overlaying image when Ru precursor was
changed from Ru3(CO)12 to ruthenium (III) acetylacetonate (Fig.
S3 & S4).
6
7
92
51
8
9
86
71
85
10
11c
62
[a] Reaction conditions: 1 (1 mmol), ketones (1.5 mmol), mesitylene (1.2 mL),
RuCO/Pr2O3 (2 mol% of Ru ), 170 oC, 24 h, Ar. [b] Isolated yield. [c] 48.
B
A
With a combination of structural results and Table 1, it is
concluded that metallic and morphology of Ru were key factors
for the synthesis of quinolines.
5
Ru powder
Next, we studied the general applicability of RuCO/Pr2O3
catalyst for the synthesis of quinoline from 2-aminobenzyl
alcohol and various ketones under optimized conditions
(Table 2). Acetophenone bearing electron-donating group
(OCH3 andCH3) were converted to corresponding
quinolines with excellent yield (Table 3, entry 2 and 3). The
catalyst was tolerable to electron-withdrawing group and
transformed acetophenenone bearing electron-withdrawing
to quinolines. Heteroaryl (methyl) ketone, cyclic ketone,
methyl ketone and nonmethyl ketone proceeded smoothly
to desired products (entries 7-10). Interestingly, this method
was applicable not only to ketones but also alcohol. The
RuCO/Pr2O3-red
Ru powder
RuAC/Pr2O3-red
RuCO/Pr2O3-unred
RuCO/Pr2O3-red
RuAC/Pr2O3-red
RuCO/Pr2O3-unred
22100
22150
22200
0
1
2
3
4
5
6
X-ray energy/eV
R / Å
Figure 1. (A) XANES spectra and (B) EXAFS Fourier transforms at Ru K-edge
for Ru catalysts and a reference compound (Ru powder)
2
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10.1002/cctc.201902311
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reaction of 2-aminobenzyl alcohol and phenyl ethanol
resulted in moderate yield of quinoline (entry 11).
condensation over relatively more exposed Pr2O3 surface of
Ru nanoparticles catalyst and formed (dibenzo[b,f]
[1,5]diazocine) as byproduct.
Encouraged by quinoline results, we examined the
synthesis of 1,8-naphthyridines from (2-aminopyridin-3-
yl)methanol (5) and acetophenone (Table 3). The catalyst
was also tolerable N-containing heterocyclic alcohol and
converted electron-rich acetophenone to corresponding
naphthyridines products with moderate-good yield (Table 4,
entries 2 and 3).
Table 3. Synthesis of 2-substituted 1,8-naphthyridine from and ketonesa
Entry
1
Ketones
Products
Yieldb (%)
95
Scheme 1. Plausible mechanism for the synthesis of quinoline from 2-
aminobenzyl alcohol and acetophenone.
2
3
90
In conclusion, we have developed a general method for the
synthesis of 2-substituted quinolines and 1,8-naphthyridines
by Ru nanolayers catalyst under acceptor-free and additive-
free conditions. Pr2O3 supported Ru nanolayers catalyst
showed superiority over conventional Pr2O3 supported Ru
nanoparticles catayst for synthesis of 2-substituted
quinolines from 2-aminobenzyl alcohol and ketones. Ru
nanolayers catalyst was reused and tolerant to different
functional groups of acetopheneone.
96
[a] Reaction conditions: 5 (1 mmol), ketones (1.5 mmol), mesitylene (1.2 mL),
RuCO/Pr2O3 (2 mol% of Ru ), 170 °C, 24 h, Ar. [b] Isolated yield.
The recyclability of RuCO/Pr2O3 was investigated for the
synthesis of quinolines from 2-aminobenzyl alcohol and
acetophenone (Fig. S5). After the first cycle, the catalyst
was washed by acetone and centrifuged followed by drying
at 50°C for 12 h under vacuum. The recovered catalyst was
reduced at 500 °C for 0.5 h and used for the second cycle.
Without significant change in yield of product, the catalyst
was reused for cycle number 2 and 3.
Experimental Section
General
Commercially available chemicals (from Tokyo Chemical Industry,
Wako Pure Chemical Industries, FURUYA METAL Co., Ltd.) were
used without further purifications.
In mechanistic studies, we carried out time-yield profile
for the reaction of
1 with acetophenone which showed a
consecutive reaction mechanism via intermediate (2-
aminobenzaldehyde). The yield of the intermediate was
increased with time and then decreased accompanying
increase in yield of the final product (Fig S6). Furthermore,
we confirmed the hydrogen evolution by using gas
chromatography in absence of ketones or in presence of
ketone (eq. S1 and S2). From these results, we proposed a
plausible mechanism for the synthesis of 2-substituted
quinolines from 1 and ketones (Scheme 1). The reaction
begins with Ru catalyzed acceptorless dehydrogenation of
2-aminobenzyl alcohol to 2-aminobenzaldehyde and
hydrogen. Then, Pr2O3-promoted condensation of 2-
aminobenzaldehyde and ketone leads to cyclised final
product. The stability of 2-aminobenzaldehyde over strongly
basic Pr2O3 support was a crucial factor. In comparison to
Ru nanoparticles, Ru nanolayers contain less exposed
Pr2O3 surface which was confirmed by H2 chemisorptions
(Table S3). It provides more stability for 2-
aminobenzaldehyde and converted to quinoline selectively.
Catalyst preparation
Pr2O3 support was prepared by precipitating Pr(NO3)3.6H2O with 25
wt% NH3 aqueous solution. The obtained light green suspension
was kept in solution overnight with constant stirring, followed by
filtration of precipitate, washing with distilled water, drying at 70°C
for 12 h. Then, the solid was calcined at 700°C in static air for 5 h.
MgO, CeO2 and La2O3 supports were prepared by precipitation
method using Mg(NO3)2·6H2O (Wako) La(NO3)3·6H2O (Wako) and
Ce(NO3)3·6H2O (Kanto Chemical, Japan) precursor respectively
and calcined at 700°C for 5 h. γ-Al2O3 was supplied by Sumitomo
Chemical Company Ltd. Pr2O3 supported Ru nanolayers (named,
RuCO/Pr2O3) catalyst with 5 wt% metal loading was prepared by an
impregnation method; a mixture of Pr6O11 and Ru3(CO)12 dissolved
in tetrahydrofuran (THF) was evaporated at 50°C, followed by
drying at 70 °C for 12h, and heated at 350°C for 5 h under a flow of
He flow to remove CO ligand from Ru3(CO)12 precursor. Similarly,
Pr2O3 supported Ru nanoparticles (named, RuAC/Pr2O3) catalyst
was prepared by using ruthenium (III) acetylacetonate precursor. A
mixture of support and precursor dissolved in THF was evaporated
In
contrast,
2-aminobezaldehyde
undergo
self-
3
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and dried at 70 °C for 12h. Then, the catalyst was calcined at 350°C
for 5 h in air. M/Pr2O3 (M = Ir, Re, Rh) catalysts were prepared
using corresponding metal carbonyl precursor method. Ru/MxOy
(MxOy= MgO, CeO2, La2O3, Al2O3) catalysts were prepared by using
Ru3(CO)12 precursor.12,13 Before each catalytic experiment, a pre-
reduced catalyst was prepared by in situ pre-reduction of the
precursor in a Pyrex tube under a flow of H2 (30 cm3 min-1) at 500°C
for 0.5 h.
Acknowledgements
This research was supported by the ACCEL program, Japan
Science and Technology Agency (JST), JPMJAC1501. The
authors thank T. Yamamoto, T. Toriyama, and Prof. S.
Matsumura for HAADF-STEM analysis. The X-ray absorption
experiments were performed with the approval of the Japan
Synchrotron Radiation Research Institute (JASRI) (Proposal No.
2016A1358, 2017B1349).
Catalyst characterization
Keywords: heterogeneous catalyst • nanolayers • nanoparticles
• quinolines • naphthyridines
High-angle annular dark field scanning transmission electron
microscopy (HAADF-STEM) images and EDX elemental mappings
were obtained on a JEM-ARM200F electron microscope (JEOL,
Japan) operated at 120 kV. The samples were prepared by
dispersing in ethanol followed by dropping onto a carbon-coated
copper grid, and drying under vacuum at ambient temperature for
24 h. X-ray absorption near-edge structures (XANES) and X-ray
absorption fine structure (EXAFS) at Ru K-edge were measured at
the BL01B1 in the SPring-8 with the approval of Japan Synchrotron
Radiation Research institute. A Si (111) two-crystal was used to
obtain both absorption edges. The EXAFS analysis was performed
using the Athena. The parameters for the Ru–O and Ru–Ru shells
were provided by Artemis programs (ver. 0.9.25) included in the
Demeter package. The specific surface areas of RuCO/Pr2O3 and
RuAC/Pr2O3 were calculated after N2 treatment at 300°C by using
Brunauer–Emmett–Teller method and BEL-mini instrument (BEL
Japan Inc., Japan). H2 chemisorption of RuCO/Pr2O3 and RuAC/Pr2O3
catalysts was measured to estimate number of exposed Ru atoms.
The sample kept in H2 flow (60 NmL min–1), followed by increasing
the temperature from room temperature to 500 °C. The temperature
was maintained for 1 h and then sample was purged by Ar flow for
30 min, cooled to −78 °C, and flushed with Ar for 60 min. After this
pretreatment, H2 chemisorption analysis was carried out at −78 °C
in an Ar stream (60 NmL min–1) by a pulsed-chemisorption
technique. Energy dispersive X-ray fluorescence analyses were
carried out using an EDXL 300 (Rigaku).
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10.1002/cctc.201902311
ChemCatChem
COMMUNICATION
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Ru
Ru
Nanolayers
Highly selective
Nanoparticles
Less selective
Acceptorless dehydrogenative synthesis and Ru nanolayers: Ru nanolayers were efficient catalyst for acceptorless
dehydrogenative synthesis of quinolines. This catalyst was reused and applicable for the synthesis of 1,8-naphthyridine.
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