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Chemistry Letters Vol.37, No.9 (2008)
Efficient Reductive Alkylation of Aniline with Acetone over Pt Nanoparticles Encapsulated
in Hollow Porous Carbon
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Ã1
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Takashi Harada, Shigeru Ikeda, Natsumi Okamoto, Yun Hau Ng, Suguru Higashida,
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Tsukasa Torimoto, and Michio Matsumura
Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka 560-8531
Department of Industrial Chemistry, Osaka Prefectural College of Technology, 26-12 Saiwai, Neyagawa 572-8572
Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603
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(Received May 15, 2008; CL-080500; E-mail: sikeda@chem.es.osaka-u.ac.jp)
Reductive alkylation of aniline with acetone over various Pt
Recently, we have reported the fabrication of a Pt nanopar-
ticle encapsulated in a hollow porous carbon (Pt@hmC) through
a templating technique. The rattle-like nanostructure is effec-
catalysts was studied under atmospheric pressure of hydrogen
at room temperature. It was found that our newly designed Pt
catalyst, i.e., Pt nanoparticles encapsulated in hollow porous
carbon, showed excellent catalytic activity in comparison with
activities of other Pt catalysts. Moreover, one-pot reductive
alkylation of nitrobenzene with acetone proceeded smoothly
over the catalyst.
6
tive for preventing aggregation of Pt core particles. Moreover,
the medial void space is suitable as a catalytic reaction site.
Due to these specific properties, this material was proved to
act as a highly active catalyst for hydrogenation of nitrobenzene
and various olefins in comparison with several Pt catalysts. In
this study, we investigated the applicability of Pt@hmC for the
reductive alkylation of aniline, with a carbonyl compound of
acetone using H2 as a reductant. Moreover, one-pot reductive
alkylation of nitrobenzene with acetone, i.e., hydrogenation
of nitrobenzene followed by reductive alkylation of aniline,
was also examined.
Reductive alkylation of aromatic amine with carbonyl com-
pounds (i.e., aldehyde or ketone) is a useful reaction for produc-
ing important amine compounds as intermediate chemicals for
various industries. In general, the reaction is assumed to be start-
ed by addition of a carbonyl compound to aromatic amine. Then
subsequent dehydration of an intermediate hemiaminal produces
an imine. Finally, the C=N bond of the imine is hydrogenated
with molecular hydrogen (H2) or hydride compounds to yield
the corresponding amine product (Scheme 1).
Pt@hmC was synthesized by a procedure similar to that re-
ported previously using poly(N-vinyl-2-pyrrolidone)-protected
7
Pt (Pt-PVP) as a starting material. As determined from induc-
tively coupled plasma (ICP) analysis, the content of Pt in
Pt@hmC was 0.49 wt % in the present conditions. For compari-
son, Pt catalysts containing the same amount of Pt as that in
Pt@hmC supported on activated carbon (AC) and alumina
Stoichiometric reductants such as hydride compounds are
the most widely employed reagents for this reaction. For exam-
ple, a well-known Borch reduction which uses NaBH3CN as a
(
Al2O3), labeled Pt/AC and Pt/Al2O3, respectively, were pre-
1
reductant exhibits efficient activity. However, the production
of large amounts of toxic cyanide as a by-product greatly dimin-
ishes its applicability. Although other borohydride derivatives,
7
pared by a conventional impregnation/H2 reduction technique.
For the evaluation of catalytic activity, room temperature re-
ductive alkylation of aniline with acetone and atmospheric pres-
sure of H2 was performed as a model reaction. Table 1 summa-
rizes the conversion of aniline and yield of the corresponding
product, N-isopropylaniline (N-IPAN), using various Pt cata-
2
a
2b
including NaBH4(OAc)3, pyridine–borane, and Ti(Oi-Pr)4/
NaBH4,2 have been developed to overcome the problem of
toxic waste production, these systems often suffer from low
atom efficiency, leading to the requirement of excessive amounts
of hydrides.
c
Table 1. Reductive alkylation of aniline with acetone catalyzed
a
An attractive alternative for reductive alkylation of amine is
the use of a catalytic system based on molecular hydrogen (H2)
as a reductant because this only produces water as by-product.
by various Pt catalysts
Conversion Yield Pt size
b
Entry Catalysts
3
/%
/%
/nm
Actually, various efficient homogeneous metal complexes and
heterogeneously supported metals based on Pt, Pd, Ru, Rh, Ni,
1
2
3
4
5
6
7
8
9
Pt@hmC
Pt@hmC
Pt-PVP
Pt/AC
Pt/Al2O3
Pt/AC (Wako)
93
99
52
33
30
31
40
92
96
75
99
23
3
18
7
23
72
77
3.1
3.1
2.2
4.5
3.1
4.2
3.4
3.1
4
,5
c
and Co have been reported. However, the major drawback
of these catalytic systems is the requirement of high pressure
of H2 and high temperature. Hence, the construction of an envi-
ronmentally benign catalytic system having both high level of
catalytic activity and sufficient reusability under mild conditions
is strongly desired.
d
d
Pt/AC (N. E. Chem)
e
Pt@hmC
Pt@hmCf
g
3.3
a
Reaction conditions: Pt (0.25 mmol), aniline (1 mmol), acetone
1 cm ), r.t., H2 (balloon), 1 h. Average Pt particles
3
b
(
size. 2-h reaction time. Commercial Pt/AC (Pt: 5 wt %) pur-
chased from Wako pure chemical and N. E. Chemcat. 2nd run
of Entry 1. 3rd run of Entry 1. After reaction.
c
d
e
Scheme 1. Reductive alkylation of amine and carbonyl com-
pound.
f
g
Copyright ꢀ 2008 The Chemical Society of Japan