Heterogeneously palladium-catalyzed acceptorless dehydrogenative aromatization of cyclic amines

Article abstract of DOI:10.1246/cl.190080

In this manuscript, we report an efficient heterogeneously catalyzed acceptorless dehydrogenative aromatization of cyclic amines under relatively mild conditions. In the presence of a supported catalyst Pd/LDH (LDH = layered double hydroxide), various kinds of structurally diverse cyclic amines including piperidines, tetrahydro(iso)quinolines, and indolines could be converted into the corresponding heteroarenes. Pd/LDH could be reused several times though its catalytic activity gradually declined due to the increase in the palladium particle size.

Full text of DOI:10.1246/cl.190080

Advance Publication Cover Page
Heterogeneously Palladium-catalyzed Acceptorless DehydrogenativeAromatization
of CyclicAmines
Takashi Oyama, Takafumi Yatabe, Xiongjie Jin, Noritaka Mizuno, and Kazuya Yamaguchi*
Advance Publication on the web March 12, 2019
doi:10.1246/cl.190080
©
2019 The Chemical Society of Japan
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1
Heterogeneously Palladium-catalyzedAcceptorless DehydrogenativeAromatization
of CyclicAmines
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Takashi Oyama, Takafumi Yatabe, Xiongjie Jin, Noritaka Mizuno, and Kazuya Yamaguchi*
Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656
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Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656
E-mail: kyama@appchem.t.u-tokyo.ac.jp
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In this manuscript, we report an efficient
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heterogeneously catalyzed acceptorless dehydrogenative
aromatization of cyclic amines under relatively mild
conditions. In the presence of a supported catalyst Pd/LDH
(LDH = layered double hydroxide), various kinds of
structurally diverse cyclic amines including piperidines,
tetrahydro(iso)quinolines, and indolines could be converted
into the corresponding heteroarenes. Pd/LDH could be
reused several times though its catalytic activity gradually
declined due to the increase in the palladium particle size.
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Keywords: Dehydrogenative Aromatization, Cyclic
amines, Palladium
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Catalytic dehydrogenation is one of the most
fundamentally important reactions in both bulk and fine
chemical productions.1 In industrial alkene productions,
dehydrogenation has frequently been performed using
2
heterogeneous catalysts under harsh reaction conditions.
Several homogeneous complexes have been utilized for
laboratory scale dehydrogenation-based fine chemical
synthesis in the presence of appropriate hydrogen
Scheme 1. (a) Our recently developed acceptorless
dehydrogenative aromatization reactions and (b) acceptorless
dehydrogenative aromatization of cyclic amines (this work).
LTH = Mg–Al–Ni layered triple hydroxide, LDH = Mg–Al
layered double hydroxide.
3
acceptors. In recent years, development of functional group
transformation methods with low environmental load and
high atom efficiency under mild reaction conditions is
highly demanded, especially attention is focused on
development of reactions based on catalytic acceptorless
dehydrogenation.4 For example, considering alcohol
oxidation, if the reaction can be realized as a highly efficient
and many efficient catalytic systems using various kinds of
oxidants (hydrogen acceptors) have been recently
catalytic acceptorless dehydrogenation, H
2
gas can be taken
9
developed. Also, many efficient catalytic systems for the
out at the same time in addition to useful carbonyl
acceptorless dehydrogenative aromatization of benzo-fused
5
compounds.
Therefore, development of catalytic
N-heterocycles, e.g., tetrahydro(iso)quinolines and indolines,
dehydrogenation systems plays an important role not only
10
and piperazines have also been reported until now.
from a synthetic point of view but also in H
transportation.
2
production and
However, with respect to simple non-activated piperidines,
6
there are almost no reports so far, including systems using
From the above-mentioned viewpoint, we have
focused on developing several acceptorless dehydrogenative
aromatization systems using supported palladium-
nanoparticles-based catalysts; namely, dehydrogenation of
11
hydrogen acceptors;
development.
thus there is still room for
12
Herein, we report an efficient acceptorless
dehydrogenative aromatization of cyclic amines by
supported palladium nanoparticles on layered double
7a
cyclohexanols/cyclohexanones to phenols, dehydrogenation
of cyclohexylamines to anilines,7a transformation of
hydroxide Mg
6
Al
2
(OH)16CO
3
·4H O (Pd/LDH) (Scheme 1b).
2
7b
cyclohexanone oximes to anilines, and synthesis of
In the presence of Pd/LDH, dehydrogenative aromatization
of various kinds of structurally diverse cyclic amines
including piperidines, tetrahydro(iso)quinolines, and
indolines efficiently proceeded to afford the corresponding
heteroarenes in moderate to high yields under relatively
mild conditions (80–125 ºC). The choice of solvents was
very important to allow the reaction to proceed under mild
conditions, and aliphatic hydrocarbon solvents such as octane
7
c
7d
symmetrically
and
unsymmetrically
substituted
diarylamines (Scheme 1a). Based on these findings, we here
aimed to develop a highly efficient catalytic system for
acceptorless dehydrogenation of cyclic amines to produce
heteroarenes. Heteroarenes are very important compounds
8
which are frequently seen in many pharmaceuticals.
Dehydrogenative aromatization of cyclic amines is drawing
attention as one of the methods for synthesizing heteroarenes,

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gave good results. The observed catalysis was truly
heterogeneous in nature, and the Pd/LDH catalyst could be
reused at least four times for the reaction of a model
substrate (4-isopropylpiperidine, 1a) though its catalytic
activity gradually declined due to the increase in the
palladium particle size.
60 Table 1. Acceptorless dehydrogenative aromatization of 1a to
61 2a under various conditionsa
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63
64
65
In our previous studies, we have found that only
Entry
1
2
Catalyst
Pd/LDH
Pd/LDH
Pd/LDH
Pd/LDH
Pd/LDH
Pd/LDH
Pd/LDH
Solvent
Octane
Cyclooctane
p-Xylene
Mesitylene
DMA
Time/ h
Yield/ %
95
19
palladium-based catalysts exhibited high performance for
dehydrogenative aromatization reactions.7 Therefore, we
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prepared supported palladium catalysts using various
3
24
supports by the following deposition-precipitation method
7
4
20
(Pd/support, support = LDH, Al
2
O
3
, CeO
2
, or TiO ).
2
5
17
Support (1.0 g) was added to a 30 mL aqueous solution of
6
Diglyme
DMF
3
PdCl
PdCl
2
(8.3 mM) and KCl (two equivalents with respect to
, 16.7 mM). The mixture was vigorously stirred at
7
1
2
room temperature for 15 min. Then, the pH of the mixture
was adjusted to 10 by using an aqueous solution of NaOH
(1.0 M), and the resulting slurry was further stirred for 18 h
at room temperature. The solid was then filtered off, washed
with water (ca. 3 L), and dried in vacuo overnight to afford
Pd/support as light brown powder.
The X-ray photoelectron spectroscopy (XPS)
spectrum of the freshly prepared Pd/LDH catalyst showed
that the binding energies of Pd 3d3/2 and Pd 3d5/2 were
341.1 eV and 335.9 eV, respectively (Figure S1a),
indicating that the oxidation state of the palladium species
was +2. The X-ray diffraction (XRD) pattern of Pd/LDH
was almost the same as that of the parent LDH support, and
no obvious signals due to palladium oxides and hydroxides
were detected (Figure S2a, b). Thus, Pd(II) hydroxide
species are likely highly dispersed on LDH. To begin with,
8
Pd/Al
2
O
3
Octane
88
9
Pd/CeO
2
Octane
73
10
11
12
Pd/TiO
Pd/C
2
Octane
58
Octane
82
LDH
Octane
<1
66 aReaction conditions: 1a (0.5 mmol), catalyst (metal: 3 mol%),
67 solvent (2 mL), 120 °C, Ar atmosphere (1 atm). Yields were
68 determined by GC analysis using decane as the internal standard.
69
70 particle size (dav) = 3.9 nm, standard deviation (σ) = 1.1 nm,
71 Figure S3a). Based on the above experimental results, it was
72 revealed that in situ generated Pd(0) nanoparticles were the
73 active species for the present acceptorless dehydrogenative
74 aromatization.
75
The effect of solvents on the reaction of 1a appeared
76 markedly. Among the solvents examined such as octane,
77 cyclooctane, p-xylene, mesitylene, N,N-dimethylacetamide
78 (DMA), diethyleneglycol dimethyl ether (diglyme), N,N-
79 dimethylformamide (DMF), a linear aliphatic hydrocarbon
80 solvent octane showed the best result (Table 1, entry 1). The
81 reaction in a cyclic alkane solvent cyclooctane gave 2a in
82 only 19% yield (Table 1, entry 2). When the reaction of 1a
83 was carried out under the conditions described in Table 1
84 using aromatic solvents such as p-xylene and mesitylene,
85 the yields of 2a were 24% and 20%, respectively (Table 1,
86 entries 3 and 4). For several reported functional group
87 transformations by palladium nanoparticle catalysts,
88 aromatic solvents have been utilized so far, and our research
89 group has also often used aromatic solvents previously.
90 However, we recognized that aromatic solvents are not
91 suitable for reactions by palladium nanoparticle catalysts
92 from the results of the above-mentioned solvent effect and
93 the fact that the addition of an aromatic compound such as
94 naphthalene completely inhibited the reaction of 1a.13 Also,
95 polar solvents such as DMA, diglyme, and DMF were not
96 effective for the reaction of 1a (Table 1, entries 5–7).
we
conducted
the
acceptorless
dehydrogenative
aromatization of 1a to 4-isopropylpyridine (2a) in the
presence of Pd/LDH. The Pd/LDH-catalyzed reaction in
octane efficiently proceeded under relatively mild
conditions (120 ºC) in comparison with previously reported
dehydrogenation systems10–12 including our systems, giving
the desired 2a in 95% yield for 2 h without detectable by-
products (Table 1, entry 1). It was confirmed by volumetric
measurement and MS analysis of the evolved gas that three
7
equivalents of H gas with respect to 2a was produced
2
during the reaction. Under the present conditions, the
reaction hardly proceeded in the absence of the catalysts or
the presence of just LDH (Table 1, entry 16).
The color of Pd/LDH was changed from initial light
brown to black within a few minutes after the start of the
reaction. After completion of the reaction, the catalyst was
recovered, and its XPS spectrum was measured. The peaks
due to the binding energies of Pd 3d3/2 and Pd 3d5/2 were
observed at 339.9 eV and 334.5 eV, respectively
(Figure S1b). Therefore, the Pd(II) species in the freshly
prepared catalyst was reduced to Pd(0) species during the
reaction. The XRD pattern of the used Pd/LDH indicated
that the structure of LDH was maintained and no large
palladium particles were formed during the reaction 100 available Pd/C also gave the desired 2a in moderate to high
(Figure S2c). The transmission electron microscopy (TEM) 101 yields (Table 1, entries 7–10). The average particle sizes of
analysis of the used catalyst revealed that palladium 102 the catalysts used were almost the same (Table S1), thus
nanoparticles were formed on the surface of LDH (average 103 indicating that the effect of supports was not so significant.
97
98 prepared by the similar procedure as above, such as
99 Pd/Al Pd/CeO and Pd/TiO and commercially
For the reaction of 1a, supported palladium catalysts
2
O
3
,
2
,
2
,

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We also prepared Ru/LDH, Rh/LDH, Ir/LDH, and Au/LDH
according to the similar procedure as above and performed
the reaction of 1a using these catalysts; however, 2a did not
produce at all (Table S2). To date, Pt(0) nanoparticle
catalysts have been often utilized for acceptorless
dehydrogenative aromatization of organic hydrides such as
methylcyclohexane under harsh conditions (typically
59 Table 2. Scope of the proposed Pd/LDH-catalyzed acceptorless
60 dehydrogenative aromatization of cyclic aminesa
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>300 ºC). Thus, we examined the reaction of 1a in the
presence of a commercially available supported Pt(0)
1
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catalyst Pt/Al
2
O
3
; however, 2a was not produced at all
(Table S2). In addition, commercially available Ru/Al
2
O
3
was not effective (Table S2). Therefore, it is due to the
specific catalysis of Pd(0) nanoparticles that acceptorless
dehydrogenative aromatization of cyclic amines could be
realized under relatively mild conditions like this system.
To establish whether the observed catalysis for the
acceptorless dehydrogenative aromatization of 1a occurred
heterogeneously on Pd/LDH or was a result of the presence
of leached palladium species in the reaction solution, the
catalyst was removed by hot filtration 1.5 h after the
reaction was started (at ca. 60% conversion of 1a) under the
conditions described in Figure S4a, and the reaction was
restarted with the filtrate under the same conditions. The
production of 2a immediately ceased upon the removal of
Pd/LDH (Figure S4a). Additionally, after the reaction, the
filtrate was analyzed using inductively coupled plasma
atomic emission spectroscopy (ICP-AES), and it was
observed that the palladium species was barely detectable
(below 0.01%). Consequently, the observed catalysis for the
reaction was determined to be truly heterogeneous.1 After
the reaction was completed, Pd/LDH could easily be
retrieved from the reaction mixture by simple filtration with
more than 90% recovery. We performed the repeated reuse
experiments using the retrieved catalyst. Although Pd/LDH
could be reused at least four times for the reaction of 1a, its
catalytic activity unfortunately gradually declined; 92%
yield of 2a with the fresh catalyst and 72% yield for the
fourth reuse under the conditions described in Figure S4b.
We confirmed by the TEM analyses that the average
palladium particle size gradually increased during the
repeated reuse experiments; the average particle size after
5
93
94
95
96
97 aReaction conditions: substrate (0.5 mmol), Pd/LDH (Pd: 3 mol%),
98 octane (2 mL), Ar atmosphere (1 atm). Yields were determined by
b
99 GC analysis using decane as the internal standard. Heptane (2 mL)
c
the first run was 3.9 nm and increased up to 6.7 nm 100 was used as the solvent. DMA (2 mL) was used as the solvent, and
(σ = 2.6 nm) after the third reuse experiment (Figure S3b). 101 the yield was determined using biphenyl added to the solution after
Therefore, the deactivation is likely caused by the increase 102 the reaction as the internal standard. Aminoindole was formed as
in the palladium particle size.
d
103 the by-product (ca. 20%).
With the optimized reaction conditions, the substrate 104
scope for the proposed Pd/LDH-catalyzed acceptorless 105 in a good yield (Table 2, entry 5). The reaction of 2,6-
dehydrogenative aromatization was investigated. As shown 106 dimethylpiperidine (1f) also proceeded efficiently to afford a
in Table 2, various cyclic amines including piperidine, 107 quantitative yield of 2,6-dimethylpyridine (Table 2, entry 6),
tetrahydro(iso)quinoline, and indoline derivatives could be 108 indicating that the influence of the steric crowding around
converted into the corresponding heteroarenes under 109 the nitrogen atom is not considered significant. It is
relatively mild conditions (80–125 ºC). When piperidines 110 noteworthy that this reaction system was also applicable to
substituted with an alkyl group at the 2-, 3-, or 4-position 111 the reaction of non-substituted piperidine (1g) (Table 2,
(1a‒1d) were used as the substrates, the reaction proceeded 112 entry 7). The reaction of benzo-fused piperidines such as
efficiently to give the corresponding monoalkylated 113 tetrahydroisoquinoline (1h) and tetrahydroquinoline (1i)
pyridines in moderate to high yields (Table 2, entries 1–4). 114 gave isoquinoline and quinoline, respectively (Table 1,
2-Phenylpiperidine (1e) also gave the corresponding pyridine
115 entries 8 and 9). The reaction of indolines (1j and 1k)
116 smoothly proceeded even at 80 ºC (Table 1, entries 10 and
17 11). In the case of 6-nitroindoline (1k), 6-nitroindole was
1

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obtained as the major product with concomitant formation of
6-aminoindole as the by-product (Table 2, entry 11).
57
This work was financially supported by ENEOS
58 Hydrogen Trust Fund and JSPS KAKENHI Grant No.
59 15H05797 in Precisely Designed Catalysts with Customized
60 Scaffolding.
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63 http://dx.doi.org/10.1246/cl.#####.
64
Finally, the reaction pathway for the Pd/LDH-
catalyzed acceptorless dehydrogenative aromatization of
piperidines was investigated. The reaction profile for the
Pd/LDH-catalyzed reaction of 1e exhibited that an imine
2,3,4,5-tetrahydro-6-phenylpyridine (3e) was detected
during the reaction albeit in only small amounts (Figure S5),
suggesting that the reaction proceeds through the imine
intermediate. In our previous reports of palladium-
nanoparticles-catalyzed dehydrogenative aromatization of
cyclohexanones or cyclohexylamines, the reactions were
composed of the rate-limiting substrate dehydrogenation
Supporting
Information
is
available
on
6
5 References and Notes
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G. E. Dobereiner, R. H. Crabtree, Chem. Rev. 2010, 110, 681.
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Wiley, New York, 2003, vol. 3.
a) J. Choi, A. H. R. MacArthur, M. Brookhart, A. S. Goldman,
Chem. Rev. 2011, 111, 1761. b) C. M. Jensen, Chem. Commun.
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C. Gunanathan. D. Milstein, Science, 2013, 341, 1229712.
a) J. Zhang, E. Balaraman, G. Leitus, D. Milstein,
organometallics, 2011, 30, 5716. b) K. Fujita, T. Yoshida, Y.
Imori, R. Yamaguchi, Org. Lett. 2011, 13, 2278.
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followed by the fast disproportionation. We considered that
the proposed Pd/LDH-catalyzed aromatization of cyclic
amines also proceeds through such a dehydrogenation-
disproportionation sequence. Thus, we prepared 3e
according to the reported procedure, and the reaction of 3e
was carried out under the standard conditions. As shown in
Figure S6, it was confirmed that the corresponding pyridine
2e was formed by the imine disproportionation. However,
the production rate of 2e starting from 3e was significantly
smaller than that starting from 1e (Figure S6 vs Figure S5).
In a separate experiment, we confirmed that the presence of
an excess of an imine relative to the catalyst strongly
inhibited the amine dehydrogenation; the addition of 3e (1.3
equivalents to 1a) in the reaction of 1a totally inhibited the
dehydrogenation while the disproportionation of 3e
preferentially occurred (Table S3). We interpret these
experimental results as follows. Firstly, acceptorless
dehydrogenation of an amine proceeds to afford an imine
intermediate (Scheme 2a). Then, the corresponding pyridine
is immediately formed through disproportionation before
the imine accumulates (Scheme 2b-1). However, since the
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R. H. Crabtree, Chem. Rev. 2017, 117, 9228.
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possibility of
a
direct dehydrogenation pathway
95
96
(Scheme 2b-2) cannot be completely denied, we consider
that further detailed investigations are necessary.
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04 11
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W. Yao, Y. Zhang, X. Jia, Z. Huang, Angew. Chem., Int. Ed.
2014, 53, 1390.
06 12
During the preparation of this manuscript, acceptorless amine
dehydrogenation and transamination using pallaium-doped
hydrotalcites have been reported: D. Ainembabazi, N. An, J. C.
Manayil, K. Wilson, A. F. Lee, A. M. Voutchkova-Kostal, ACS
Catal. 2019, 9, 1055.
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11 13
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In our previously reported palladium-nanoparticles-catalyzed
7
acceptorless dehydrogenative aromatization, we confirmed that
13
the addition of an aromatic compound strongly inhibited the
reactions (unpublished findings).
114
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15 14
a) F. Alhumaidan, D. Cresswell, A. Garforth, Energy Fuels, 2011,
25, 4217. b) C. Zhang, X. Liang, S. Liu, Int. J. Hydrogen Energy,
2011, 36, 8902. c) Y. Wang, N. Shah, G. P. Huffman, Energy
Fuels, 2004, 18, 1429. d) N. Kariya, A. Fukuoka, M. Ichikawa,
Appl. Catal. A, 2002, 233, 91.
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Scheme 2. Plausible reaction path for the Pd/LDH-catalyzed
acceptorless dehydrogenative aromatization of piperidines.
120 15 R. A. Sheldon, M. Wallau, I. W. C. E. Arends, U. Schuchardt,
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Acc. Chem. Res. 1998, 31, 485.
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