Ti?Pd Alloys as Heterogeneous Catalysts for the Hydrogen Autotransfer Reaction and Catalytic Improvement by Hydrogenation Effects

Article abstract of DOI:10.1002/cctc.201900318

Ti?Pd alloys were investigated as heterogeneous catalysts for hydrogen autotransfer reactions. This is the first reported study of alloys as catalysts for hydrogen-borrowing reactions using alcohols. We improved the catalytic activities of alloys by increasing their specific surface areas via a hydrogenation?powdering process. The reactivities and selectivities of hydrogenated Ti?Pd alloys [Ti?Pd(Hy)] were higher than those of non-hydrogenated alloy catalysts in N-alkylation by hydrogen autotransfer using alcohols. A plausible catalytic cycle is proposed based on control studies and deuterium labelling experiments.

Full text of DOI:10.1002/cctc.201900318

Accepted Article
Title: Ti–Pd Alloys as Heterogeneous Catalysts for Hydrogen
Autotransfer Reaction and Catalytic Improvement by
Hydrogenation Effects
Authors: Yuya Takahashi, Ryota Kondo, Masayoshi Utsunomiya,
Takeyuki Suzuki, Hiroyuki T. Takeshita, and Yasushi Obora
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10.1002/cctc.201900318
ChemCatChem
COMMUNICATION
Ti–Pd Alloys as Heterogeneous Catalysts for Hydrogen
Autotransfer Reaction and Catalytic Improvement by
Hydrogenation Effects
Yuya Takahashi, Ryota Kondo,* Masayoshi Utsunomiya, Takeyuki Suzuki, Hiroyuki T. Takeshita,
Yasushi Obora*
Dedication ((optional))
Abstract: Ti–Pd alloys were investigated as heterogeneous catalysts
for hydrogen autotransfer reactions. This is the first reported study of
alloys as catalysts for hydrogen-borrowing reactions using alcohols.
We improved the catalytic activities of alloys by increasing their
specific surface areas via a hydrogenation–powdering process. The
reactivities and selectivities of hydrogenated Ti–Pd alloys [Ti–Pd(Hy)]
were higher than those of non-hydrogenated alloy catalysts in N-
alkylation by hydrogen autotransfer using alcohols. A plausible
catalytic cycle is proposed based on control studies and deuterium
labelling experiments.
materials.7 Ti-based alloys containing Pd have improved
hydrogen absorption abilities because of the catalytic effect of Pd,
which accelerates hydrogen molecule dissociation, and promotes
easy, rapid hydrogen absorption.4 These findings suggest that Ti–
Pd alloys could catalyse hydrogen autotransfer reactions.
We investigated Ti–Pd alloys as catalysts for N-alkylation of
amines via hydrogen autotransfer reactions using alcohols. The
C–N bond is an important building blocks and is present in
medicines, pesticides, surfactants, and biological materials.8 N-
Alkylation is a useful reaction for forming C–N bonds. In particular,
hydrogen autotransfer using alcohols has attracted much
attention as a green reaction with high atom economy.9
Novel catalyst identification is important for the development
of modern society, in terms of efficient energy use to reduce
costs.1 In organic chemistry, catalysts with new characteristics
enable the development of new routes for the synthesis of useful
materials and functional groups and allow reactions to be
performed under mild conditions.2 Our groups have focused on
investigating highly active catalysts such as nanoparticles and
have reported their use in various organic reactions.3
Ti–xPd alloy catalysts (x = 0.2, 1.0 mol%) were prepared by
an arc melting method.4 We also prepared hydrogenated Ti–Pd
alloys, i.e. [Ti–Pd(Hy)], and compared their catalytic activities with
those of non-hydrogenated Ti–Pd alloy catalysts. Hydrogenation
decreases the alloy strength by hydrogen embrittlement. Ti–
Pd(Hy) alloys can therefore be easily powderized, whereas this is
difficult for bare Ti–Pd alloys (Figure 1). The Ti–Pd(Hy) alloys was
prepared by Sievert’s type apparatus with 3.0 MPa H2 at 673 K.
Then the alloys were powdered by stirring with magnetic stirrer for
5 min in toluene under Ar. Powdering gives a large specific
surface area therefore it was thought that the catalytic activity of
Ti–Pd(Hy) would be higher than that of Ti–Pd.
We have reported bulk Ti–Pd alloys as novel heterogeneous
Pd catalysts; these have high catalytic activity in Suzuki–Miyaura
cross-coupling and Mizoroki–Heck reactions.4,5 We used X-ray
photoelectron spectroscopy (XPS) to examine the surfaces of Ti–
Pd alloys and detected zero-valent Pd on/in a titanium oxide film.
The surface characteristics of alloy catalysts differ from those of
supported catalysts, which are commonly used as heterogeneous
catalysts. Supported catalysts consist of metal particles and
carriers such as inorganic oxides, carbon, and polymers.,6
Ti-based alloys are well known as hydrogen-storage materials
and have various applications, e.g. in hydrogen storage materials,
biomaterials, aircraft materials, and marine facility structural
Figure 1. Photographs of Ti–Pd turnings (left) and Ti–Pd(Hy) powder (right).
First, we investigated the characteristics of the alloys before
and after hydrogenation. Ti–Pd and Ti–Pd(Hy) were analysed in
the form of turnings and a powder, respectively. X-ray diffraction
showed that the crystal system changed from hexagonal close-
packed to face-centred cubic, and the peaks in the Ti–Pd(Hy)
pattern matched those in the TiH1.924 pattern (Figure 2). These
results suggest that the Ti–Pd(Hy) alloy consists of a hydride with
a metal–hydrogen bond. XPS showed that the Pd valence in Ti–
Pd(Hy) was higher than that in Ti–Pd, and zero-valent Ti was
present (Figure 3). This is the result of electronic interactions
between Pd and Ti. The surface and internal valence state of Ti
and Ti hydride were reported as following; on the surface, these
[*]
Y. Takahashi, Prof. Dr. R. Kondo, M. Utsunomiya, Prof. Dr. H. T.
Takeshita, Prof. Dr. Y. Obora
Department of Chemistry and Materials Engineering
Faculty of Chemistry, Materials and Bioengineering
Kansai University
Suita, Osaka 564-8680 (Japan)
E-mail: obora@kansai-u.ac.jp, rkondo@kansai-u.ac.jp
Prof. Dr. T. Suzuki
Comprehensive Analysis Center
The Institute of Science and Industrial research (ISIR)
Osaka University
8-1 Mihogaoka, Ibaraki, Osaka 567-0057 (Japan)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201900318
ChemCatChem
COMMUNICATION
Ti and hydride were covered with Ti oxide and in the internal side,
the valence state of Ti hydride was shift approximately 0.4 eV in
high binding energy state comparing that of Ti.12 Focused on
alloyed Pd, the valence state was metallic state on the surface
and in the internal Pd.4 Therefore, the new face which was
fabricated by powderizing on the Pd should be metallic state. We
also determined the Brunauer–Emmett–Teller surface areas of
the alloys. The specific surface area of Ti–Pd(Hy) was
approximately ten times that of Ti–Pd [Ti–Pd(Hy): 0.283 m2 g−1,
Ti–Pd: 0.0354 m2 g−1]. Scanning electron microscopy (SEM)
showed that the Ti–Pd(Hy) particle size was 20–30 μm (Figure
S1). The measurement of surface Pd atoms by CO-uptaking
measurement can hardly be elucidated since the number of Pd
atoms specific surface area of alloys were too small.
The scope and limitation of this reaction were investigated by
testing various amines and alcohols (Table 2). In these tests,
imine intermediate 4 was hardly obtained. First, amines 3b–3g
were tested. Aniline derivatives bearing electron-donating groups,
i.e. p-toluidine and p-anisidine, and an electron-withdrawing
group, i.e. 4-chloroaniline, were tolerated and gave the
corresponding alkylated amines in good yields. This catalyst was
suitable for reactions using N-heterocyclic or sterically hindered
amines. 2-Aminopyridine reacted with 2a to give the desired
product 3e in 82% yield. The reactions using o-toluidine and 1-
naptylamine gave the corresponding products 3f and 3g in 66%
and 77% yields, respectively. However, the reactions of aliphatic
amines were sluggish. No reaction occurred between secondary
amines and 2a.
Table 1. Optimization of alloy-catalysed N-alkylation of aniline (1a) with
benzyl alcohol (2a).[a]
Entry
Alloy catalyst
Base
Conversion
(%)[b]
Yield (%)[b]
Figure 2. X-ray diffraction patterns of Ti–Pd (above) and Ti–Pd(Hy) (below).
1a
80
87
>99
>99
19
5
3a
4a
12
8
1
Ti–0.2Pd
KOH
KOH
KOH
KOtBu
K3PO4
none
KOH
KOH
KOH
KOH
64
2
Ti–1.0Pd
76
3
Ti–0.2Pd(Hy)
Ti–0.2Pd(Hy)
Ti–0.2Pd(Hy)
Ti–0.2Pd(Hy)
Ti–0.2Pd(Hy)
Ti–0.2Pd(Hy)
Ti–0.2Pd(Hy)
none
97 (85)
>99
<1
<1
<1
8
4
5
6
n.d.[c]
6
<1
<1
5
7[d]
8[e]
9[f]
10
13
32
71
21
Figure 3. X-ray photoelectron spectra for (left) Pd 3d and (right) Ti 2p.
24
Ti–Pd-alloy-catalysed N-alkylation reactions of amines with
alcohols via hydrogen autotransfer were investigated. Aniline (1a)
and benzyl alcohol (2a) were selected as model reactants (Table
1). Ti–0.2Pd catalysed the reaction to give the desired alkylated
amine, N-benzyl aniline (3a), in 64% yield and the imine
intermediate, benzylideneaniline (4a), in 12% yield (entry 1). The
yield of 3a increased with increasing Pd content in the alloy, up to
a maximum yield of 80% (entry 2). Ti–0.2Pd(Hy) was a more
effective catalyst than Ti–0.2Pd and gave 3a in 97% yield, with
excellent selectivity, and in an isolated yield of 85% (entry 3).
Strong bases were necessary for the reaction; reactions with
weak bases or under base-free conditions were sluggish (entries
4–6 and Table S1). Excess alcohol accelerated the reaction
smoothly; the optimum amount of 2a was 1.5 mmol (3 molar
equivalents with respect to 1a) (entry 7). A reaction temperature
of 135 °C and reaction time of 48 h were needed for good yields
(entries 8 and 9). Polar solvents were not suitable for this reaction
system (Table S2). Little reaction occurred without an alloy
catalyst (entry 10).
68
<1
4
17
[a] Reaction conditions: 1a (0.50 mmol), 2a (1.5 mmol), alloy catalyst (0.50
mmol), and base (1.0 mmol) were stirred in toluene (2 mL) at 135 °C for 48
h under Ar. [b] Conversions and yields were determined by GC based on 1a
used. Numbers in parentheses showed isolated yields. [c] Not detected by
GC. [d] 2a (0.50 mmol) was used. [e] At 120 °C. [f] Reaction time was 16 h.
Next, the reactions of aniline (1a) with various alcohols were
investigated. The reactions of 4-methylbenzyl alcohol and 4-
methoxybenzyl alcohol with 1a gave the corresponding products
3h and 3i in excellent yields. When the substituent was an
electron-withdrawing group, the reaction was slow; the reaction of
4-chlorobenzyl alcohol with 1a for 72 h gave the desired product
in moderate yield. Aliphatic alcohols reacted with 1a to give N-
alkylation products. N-heterocyclic alcohols were tolerated, and
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10.1002/cctc.201900318
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COMMUNICATION
cat. Ti-1.0Pd(Hy) (0.50 mmol)
NH2
+
OH
sterically hindered alcohols were also suitable. Secondary
alcohols did not react with 1a; the reaction with 1-phenylethyl
alcohol gave styrene.
KOH (1.0 mmol)
Toluene (2 mL)
135 C, 48 h
H2N
0.50 mmol
1.5 mmol
(2)
H
N
N
Ph
Ph
N
Ph
+
Ph
+
Table 2. N-alkylation of various amines with alcohols.[a]
N
H
Ph
N
H
Ph
N
4o
44%
3o
13%
5o
17%
H
cat. Ti-0.2Pd(Hy) (0.50 mmol)
N
R2
R1 NH2
+
OH
R2
R1
The reusability of the Ti–Pd(Hy) alloy catalysts was tested by
KOH (1.0 mmol)
Toluene
135 C, 48 h
performing multiple reactions under the optimum conditions
(Table 1, entry 3). Similarly to recycling of the Ti–Pd alloy catalyst
turnings,5 recycling of the Ti–Pd(Hy) alloy catalysts was simple;
preactivation was not needed, and only supernatant separation
and catalyst washing were required. Figure 4 shows that the Ti–
Pd(Hy) catalysts could be reused at least five times, and gave
good product yields and excellent selectivities. The crystal
structure of Ti–Pd(Hy) was unchanged by the reaction (Figure 5
and Figure S2). The alloy catalysts therefore retained their
hydride structures and hydrogen in the alloys was not used for the
reaction.
1
2
3
Isolated yield
97
>99 >99
>99
87
91
100
80
60
40
20
0
4a
3a
0
1
2
3
4
5
Recycle
Figure 4. Recycling of Ti–Pd(Hy) alloy catalysts for N-alkylation of aniline (1a)
with benzyl alcohol (2a).
[a] Conditions are the same as those for Table 1, entry 3, unless otherwise
noted. [b] At 150 °C. [c] Reaction time was 72 h.
We further investigated the scope of the reaction by using
diols or diamines. 1,4-Benzenedimethanol reacted with 1a to
produce the mono-alkylated compound 3n in 45% yield and with
high selectivity [Eq. (1)]. The reactions of diamines with alcohols
gave dialkylation products. 1,4-Phenylenediamine reacted with 2a
to give three dialkylated compounds, namely the desired
alkylation amine 3o, the partial reduction product 4o, and the non-
reduced imine intermediate 5o. These products were isolated in
13%, 44%, and 17% yields, respectively [Eq. (2)].
Figure 5. X-ray diffraction pattern of Ti–Pd(Hy) before (upper) after (bottom)
reaction.
Next, we investigated the heterogeneous properties of the
alloy catalysts by performing filter tests and inductively coupled
plasma atomic emission spectroscopy (ICP-AES) analysis. Figure
6 shows the filter test results. When the alloy catalysts were
removed after reaction for 8 h under the model conditions, the
reaction stopped and the product yield did not increase further. If
potassium hydroxide was added at the same time as the alloy
catalyst was removed by filtration, the reaction degree was only
that caused by the base effect. Additionally, ICP-AES analysis of
the reaction mixture showed that the amount of leached Pd was
OH
H
cat. Ti-0.2Pd(Hy) (0.50 mmol)
NH2
OH
N
(1)
+
HO
KOH (2.0 mmol)
Toluene (2 mL)
135 C, 48 h
3n
45%
0.50 mmol
1.5 mmol
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10.1002/cctc.201900318
ChemCatChem
COMMUNICATION
<0.08 ppm (less than the detection limit), i.e. little Pd leaching
from the alloy catalysts occurred during the reaction (Table S3
and Figure S3).
reaction intermediate and transfer hydrogenation was achieved
by the alloy catalysts. The reaction of benzaldehyde instead of
benzyl alcohol with aniline under the optimum conditions gave 4a
in 73% yield (Scheme 2a). The reaction in the absence of an alloy
catalyst also gave 4a (Scheme 2b). These results indicate that
benzaldehyde served as an intermediate and the alloy catalyst
was not involved in this step.
Tests with Hg(0) (2.5 mmol) were conducted under the model
conditions (Table 1, entry 3). In the presence of Hg, the catalytic
activity of a metal-supported catalyst is lost because of amalgam
formation.10 The alloy catalysts retained their catalytic activities
and the reaction proceeded to give the alkylated product in 37%
yield.
Next, we investigated hydrogen sources. The reaction of
benzylideneaniline (4a) with benzyl alcohol (2a) under the above
conditions gave the hydrogenated amine 3a in 73% yield
(Scheme 2c). No reaction occurred between 4a and aniline (1a)
(Scheme 2d). These results show that the hydrogen source was
the alcohol, not the alloy or amine.
The proposed reaction mechanism was clarified by
performing deuterium labelling experiments with ,-
dideuteriobenzyl alcohol (2a-d7, C6D5CD2OH), benzyl alcohol-OD
(2a-Od1, C6H5CH2OD), and deuterated aniline (1a-d7, C6D5ND2).
In the first test, deuteration occurred only at the methylene
position of the product (Scheme 2e). However, in the second and
third tests, the hydrogen on the imino group was replaced by
deuterium but that on the methylene group was not (Scheme 2f
and g).
Figure 6. Filter tests on Ti–Pd(Hy) alloy catalysts for N-alkylation under the
model conditions (entry3, Table 1).
Control experiments and deuterium labelling experiments
were performed to explore possible reaction mechanisms. The
proposed general process involved in hydrogen autotransfer for
alkylation of amines using alcohols is shown in Scheme 1.11 First,
metal-catalysed dehydrogenation of an alcohol produces an
aldehyde and a metal hydride. The aldehyde then undergoes
condensation reaction with the amine to generate an imine
intermediate. Finally, hydrogenation of the imine by the metal
hydride gives the desired N-alkylamine, with regeneration of the
metal catalyst.
Scheme 1. Proposed general reaction mechanism for N-alkylation of amines
using alcohols.
First, we checked the reaction intermediate. Aldehydes are
often observed in hydrogen autotransfer reactions with alcohols,11
and are thought to be formed by alcohol dehydrogenation. In our
reaction system, a small amount of benzaldehyde was often
detected by GC, which suggests that benzaldehyde was a
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10.1002/cctc.201900318
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COMMUNICATION
catalysed N-alkylation of amines by alcohols via hydrogen
autotransfer. The catalysts had high activities and good
selectivities. The alloy catalysts were stable during the reaction
and metal was not leached into the reaction solution. The
catalysts were recycled without the need for complicated
activation processes. We have proposed a reaction mechanism
for this system on the basis of control experiments.
Acknowledgements
We thank the members of the Comprehensive Analysis Center,
SANKEN (ISIR), Osaka University for ICP-AES analysis. High-
resolution mass spectra were performed at Global Facility Center,
Hokkaido University.
Scheme 2. Mechanistic studies of N-alkylation reaction.
This is a product of research which was financially supported by
JSPS KAKENHI Grant Number 17K14837, the Environment
Research and Technology Development Fund (2RF-1801) of the
Environmental Restoration and Conservation Agency of Japan,
and the Kansai University Fund for Supporting Young Scholars,
2017.
Based on the results of these control experiments and
deuterium labelling experiments, we propose a plausible catalytic
cycle (Scheme 3). Initially, benzyl alcohol (2a) coordinates with
Ti–Pd(Hy). Abstraction of a methylene proton by Ti–Pd(Hy) forms
benzaldehyde 2a’ with simultaneous generation of a [Ti–Pd(Hy)–
H] species. Benzaldehyde then reacts with aniline (1a) to give
imine intermediate 4a. The imine is hydrogenated by [Ti–Pd(Hy)–
H] and benzyl alcohol to generate the desired N-alkylamine
product 3a. Hydrogen source was from alcohols during the course
of the reaction and hydrogen adsorption of alloys were not
involved in the reaction.
Keywords: Alloy catalysts • Hydrogen transfer • Alkylation•
Alcohols • Heterogeneous catalysis
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aniline (1a) with benzyl alcohol (2a).
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10.1002/cctc.201900318
ChemCatChem
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Ti–Pd alloys were served as
Yuya Takahashi, Ryota Kondo,*
hydrogen borrowing catalysts. Their
catalytic activity was improved by
hydrogenation-powdering process to
give desired alkylated product in
excellent yield with high selectivity.
Masayoshi Utsunomiya, Takeyuki
Suzuki, Hiroyuki T. Takeshita, Yasushi
Obora*
Page No. – Page No.
Ti–Pd Alloys as Heterogeneous
Catalysts for Hydrogen Autotransfer
Reaction and Catalytic Improvement
by Hydrogenation Effects
This article is protected by copyright. All rights reserved.