Chemoselective hydrogenation of functionalized nitroarenes and imines by using carbon nanofiber-supported iridium nanoparticles

Article abstract of DOI:10.1002/asia.201301184

The reaction of three types of carbon nanofibers (CNFs; platelet: CNF-P, tubular: CNF-T, herringbone: CNF-H) with Ir₄(CO)₁₂ in mesitylene at 165 °C provided the corresponding CNF-supported iridium nanoparticles, Ir/CNFs (Ir content=2.3-2.6 wt. %). Transmission electron microscopy (TEM) studies of these Ir/CNF samples revealed that size-controlled Ir nanoparticles (average particle size of 1.1-1.5 nm) existed on the CNFs. Among the three Ir/CNF samples, Ir/CNF-T showed an excellent catalytic activity and chemoselectivity towards hydrogenation of functionalized nitroarenes and imines; the corresponding aniline derivatives were obtained with high turnover numbers at ambient temperature under 10 atm of H₂, and the catalyst is reusable. Ir/CNF-T was also effective for the reductive N-alkylation of anilines with carbonyl compounds.

Full text of DOI:10.1002/asia.201301184

COMMUNICATION
DOI: 10.1002/asia.201301184
Chemoselective Hydrogenation of Functionalized Nitroarenes and Imines by
Using Carbon Nanofiber-Supported Iridium Nanoparticles
Yukihiro Motoyama,*[a] Masahiro Taguchi,[b] Nelfa Desmira,[a] Seong-Ho Yoon,[c]
Isao Mochida,[c] and Hideo Nagashima[c]
Abstract: The reaction of three types of carbon nanofibers
(CNFs; platelet: CNF-P, tubular: CNF-T, herringbone:
CNF-H) with Ir4(CO)12 in mesitylene at 165 8C provided the
corresponding CNF-supported iridium nanoparticles, Ir/
CNFs (Ir content=2.3–2.6 wt.%). Transmission electron mi-
croscopy (TEM) studies of these Ir/CNF samples revealed
that size-controlled Ir nanoparticles (average particle size of
1.1–1.5 nm) existed on the CNFs. Among the three Ir/CNF
samples, Ir/CNF-T showed an excellent catalytic activity and
chemoselectivity towards hydrogenation of functionalized
nitroarenes and imines; the corresponding aniline deriva-
tives were obtained with high turnover numbers at ambient
temperature under 10 atm of H2, and the catalyst is reusable.
Ir/CNF-T was also effective for the reductive N-alkylation
of anilines with carbonyl compounds.
more suitable solid supports for iridium nanoparticles to
provide highly active and selective catalysts is a challenging
problem.
In this context, carbon nanofibers (CNFs) that have con-
trolled nanostructures consisting of stacked graphene sheets
are attractive supports for metal nanoparticles. CNFs are
classified into three types, where graphite layers are either
perpendicular (platelet, CNF-P), parallel (tubular, CNF-T),
or stacked obliquely (herringbone, CNF-H) to the fiber
axis.[4] We have recently established selective synthetic
methods for the preparation of carbon nanofibers
(CNFs)[4c,d] and explored methods to immobilize ruthenium,
rhodium, palladium, and platinum nanoparticles on their
surface by thermal decomposition of organometallic precur-
sors.[5] The Ru, Rh, Pd, and Pt/CNFs thus produced act as
efficient catalysts for arene hydrogenation (Ru- and Rh/
CNFs)[5a,b,d,e] and the reduction of aromatic nitro compounds
(Pd- and Pt/CNFs)[5c,f] with high turnover numbers. It is im-
portant that neither sintering nor leaching of metal nanopar-
ticles is observed after the reaction; this results in possible
reuse of the catalyst without loss of the catalytic activity. In
this paper, we report that iridium nanoparticles can be im-
mobilized on the surface of CNFs by a procedure similar to
that used for Ru- and Rh/CNFs (see Scheme 1), which are
prepared by pyrolysis of metal carbonyl clusters, and that
the resulting Ir nanoparticles (see Figure S1 in the Support-
ing Information for TEM images of the obtained samples
and their size distribution), Ir/CNF-P and Ir/CNF-T, behave
as an efficient catalyst for nitro- and imino-selective hydro-
genation of functionalized nitroarenes and imines.
Recently, much attention has been paid to the develop-
ment of transition metal nanoparticles for technological ap-
plications in various areas such as chemical sensors, electro-
catalysts for fuel cells, and heterogeneous catalysts for or-
ganic transformations.[1] Although a number of solid-sup-
ported transition metal nanoparticles (NPs), such as Ru, Rh,
Ni, Pd, and Pt NPs, have been widely used as heterogeneous
catalysts for various organic transformations,[1,2] the use of
supported iridium catalysts is limited to the hydrogenation
of alkenes, arenes, and nitro compounds.[3] Since the catalyt-
ic properties of these heterogeneous catalysts are highly de-
pendent on the surface design of the support along with the
size and shape of the metal nanoparticles,[1,2] the search for
[a] Prof. Dr. Y. Motoyama, Dr. N. Desmira
Toyota Technological Institute
Nagoya, Aichi 468-8511 (Japan)
Fax : (+81)52-809-1721
E-mail: motoyama@toyota-ti.ac.jp
[b] M. Taguchi
Graduate School of Engineering Sciences
Kyushu University
Kasuga, Fukuoka 816-8580 (Japan)
Scheme 1. Preparation of Ir/CNFs and Ir/AC.
[c] Prof. Dr. S.-H. Yoon, Prof. Dr. I. Mochida, Prof. Dr. H. Nagashima
Institute for Materials Chemistry and Engineering
Kyushu University
The catalytic activity of Ir/CNFs is compared with that of
iridium nanoparticles supported on activated carbon (Ir/AC)
using various organic compounds as substrate. The reaction
was carried out using 1 mL of substrate and 5 mg of Ir cata-
Kasuga, Fukuoka 816-8580 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201301184.
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Yukihiro Motoyama et al.
Table 1. Hydrogenation of various substrates using Ir/CNFs and Ir/AC as
a catalyst.[a]
Table 2. Hydrogenation of various nitroarenes 1a–i to anilines 2a–i using
Ir/CNF-T as a catalyst.[a]
Entry
Substrate
Catalyst
Product
TOF[b]
À
1
2
3
4
Ir/AC
Ph NH2
65
82
180
150
À
Ir/CNF-H
Ir/CNF-P
Ir/CNF-T
Ph NH2
À
Ph NH2
À
Entry
1
Substrate
t [h]
Product
Yield [%]
98
Ph NH2
5[c]
6
Ir/CNF-T
Ir/CNF-T
160
130
60
2a
2
11
11
17
2b
2c
2d
99
99
99
7[d]
8
Ir/CNF-T
Ir/CNF-T
–
no reaction
no reaction
3
4[b,c]
30
5[b]
11
11
2e
2 f
98
97
–
9
Ir/CNF-P
Ir/CNF-T
10
20
6
11[d]
12
Ir/CNF-T
Ir/CNF-T
no reaction
no reaction
–
–
7[c]
8[b,c]
9
36
30
53
2g
2h
2i
99
À
Ph CN
92[d]
83[e]
[a] Unless stated otherwise, all reactions were performed using substrate
(1 mL) and Ir catalyst (5 mg) at 258C for 12 h under H2 (initial pres-
sure=10 atm). [b] TOF=mol (product)/mol (Ir)·h. [c] 9 mmol of imine
and 3 mL of ethyl acetate was used. [d] 5 mol% of Et3N was added.
[a] Unless stated otherwise, all reactions were performed using
1 (1 mmol) and Ir/CNF-T catalyst [S/C=1700 mol (1)/mol (Ir)] in ethyl
acetate (3 mL) at 258C under H2 (initial pressure=10 atm). [b] At 508C.
[c] S/C=500 mol (1)/mol (Ir). [d] 3% of di(4-aminobenzyl)amine was
formed. [e] 16% of ethyl 4-aminodihydrocinnamate was formed.
lyst for 12 h at 258C under H2 (10 atm), except in the case
of a solid substrate, N-phenylbenzaldimine. As shown in
Table 1, entries 1–4, all iridium nanoparticles acted as cata-
lysts for the hydrogenation of nitrobenzene, and Ir/CNF-P
and Ir/CNF-T showed good catalytic activity;[6] the turnover
frequency (TOF) of the reaction over these catalysts were
180 [mol (product)/mol (Ir)·h] for Ir/CNF-P and 150 for Ir/
CNF-T, respectively (entries 3 and 4). Ir/CNF-T also acted
as a good catalyst for the reduction of imines and ketones;
the hydrogenation of N-phenylbenzaldimine and acetophe-
none over Ir/CNF-T proceeded with TOFs of 160 and 130 to
afford N-phenylbenzylamine and 1-phenyl ethanol, respec-
tively, as the sole product (entries 5 and 6). Phenyl glycidyl
ether was converted into cyclohexyl glycidyl ether (TOF=
30) through arene hydrogenation, without promoting the
ring-opening hydrogenation of the epoxide group; however,
no reaction proceeded using Ir/CNF-P as a catalyst (entry 8
vs. entry 9). The reaction of chlorobenzene gave cyclohex-
ane (TOF=20), which was obtained by hydrogenolytic
genation of halogenated nitrobenzenes over conventional
heterogeneous catalysts affords a mixture of aniline and
halogenated aniline; therefore, sulfur-, nitrogen-, phospho-
rus-, or halogen-containing compounds are often added as
a catalyst poison to improve the chemoselectivity.[5d,f,8] We
previously reported that the hydrogenation of 4-chloro- and
4-bromonitrobenzene (1b and 1c, respectively) over Pd-
and Pt/CNF produced aniline in 4–6% yield.[5d] However,
complete suppression of the reductive dehalogenation was
achieved in the reaction of halonitrobenzenes over Ir/CNF-
T under H2 (initial pressure=10 atm); 4-chloro-, 4-bromo-,
and 4-iodonitrobenzene (1b–d) were converted into the cor-
responding halogenated anilines 2b–d in almost quantitative
yields without contamination by the dehalogenated aniline
(entries 2–4). Furthermore, the nitro group was selectively
reduced to the amino moiety in the presence of benzyloxy
and carbonyl groups; the reduction of the keto group and
À
cleavage of the C Cl bond of chlorobenzene followed by
arene hydrogenation (entry 10). By contrast, in the case of
nitrile compounds, no reaction took place and the starting
material was recovered quantitatively (entry 12). It is note-
worthy that the addition of a small amount of amine to the
reaction mixture eliminated the catalytic activity of Ir/CNF-
T towards the reduction of the keto group, hydrogenolytic
À
the hydrogenolytic cleavage of the C O bond in the benzy-
loxy group, which are often seen in the Pd/C-catalyzed hy-
drogenation, were not observed (entries 5 and 6). It is also
well known that the nitro-selective reduction of nitro com-
pounds bearing an epoxide group is difficult to achieve be-
cause the ring-opening reaction of the highly strained epoxy
function proceeds easily under reductive conditions.[9] In
fact, Sajiki and co-workers reported that the reaction of (ni-
trophenyl)methyl glycidyl ethers over Pd/C(en) catalyst af-
À
cleavage of the C Cl bond, and hydrogenation of the aro-
matic ring (entries 7 and 10).[7]
The usefulness of Ir/CNF-T was demonstrated by the hy-
drogenation of nitroarenes that contain other reducible
functional groups (Table 2). It is well known that the hydro-
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Yukihiro Motoyama et al.
forded a mixture of (aminophenyl)methyl glycidyl ethers
and its ring-opening products (93–95% selectivity).[10] How-
ever, in our study (4-aminophenyl)methyl glycidyl ether
(2g) was obtained as the sole product in the reaction with
an increased amount of catalyst (substrate/catalyst molar
ratio S/C=500, entry 7).[11] Reduction of the nitro group was
favored with 4-cyanonitrobenzene (1h), affording 4-cyanoa-
niline (2h) in 92% yield, but small amounts of di(4-amino-
benzyl)amine, which forms by reductive alkylation via elimi-
nating NH3,[12,13] and other unidentified compounds were ob-
tained as by-products (entry 8). The attempted nitro-selec-
tive reduction of ethyl 4-nitrocinnamate (1i) gave the de-
sired ethyl 4-aminocinnamate (2i) in 83% yield along with
16% of ethyl 4-aminodihydrocinnamate (entry 9).
Ir/CNF-T could be reused without loss of catalyt perfor-
mance. After the hydrogenation of 4-chloronitrobenzene
(1b) (S/C=2300) was performed in ethyl acetate at 258C
for 12 h under H2 (initial pressure=10 atm), the catalyst was
recovered by filtration and subjected to subsequent hydro-
genation reactions. Even after the fifth run, 1b was com-
pletely consumed and 4-chloroaniline (2b) was obtained as
the sole product in 99% yield.
Ir/CNF-T is also a useful catalyst for the chemoselective
reduction of functionalized imines to the corresponding
amines (Table 3). Similar to the reduction of halogenated ni-
trobenzenes, imines having aromatic carbon–halogen bonds
(3b: R=Cl, 3c: R=Br) were converted into the corre-
sponding secondary amines 4b and 4c in quantitative yields
without contamination by the dehalogenated aniline deriva-
tive (entries 2 and 3). Furthermore, the imino group was se-
lectively reduced to the amino moiety in the reaction of N-
(4-acetylphenyl)-4-methylbenzaldimine (3d); the reduction
of the keto group did not proceed and the amino ketone 4d
was obtained in quantitative yield (entry 4).
Scheme 2. Reductive N-alkylation of 4-chloroaniline (2b) and 4-chloroni-
trobenzene (1b) with acetone over Ir/CNF-T under H2 (10 atm).
N-alkylation of anilines with carbonyl compounds
(Scheme 2).[14] For example, the reaction of 4-chloroaniline
(2b) with acetone in ethyl acetate in the presence of Ir/
CNF-T (S/C=2200) at 258C for 12 h under H2 (initial pres-
sure=10 atm) afforded N-isopropyl-4-chloroaniline (5) in
96% yield. Furthermore, direct and one-pot reductive N-al-
kylation can be achieved by using nitrobenzenes as an
amine source; 4-chloronitrobenzene (1b) was treated with
acetone in the presence of Ir/CNF-T (S/C=2200) at 258C
for 13 h under H2 (initial pressure=10 atm) to afford N-iso-
propyl-4-chloroaniline (5) in 98% yield. In both cases, re-
ductive dechlorination did not proceed and the alkylated
product 5 was obtained as the sole product.
In conclusion, we have successfully synthesized iridium
nanoparticles supported on carbon nanofibers (Ir/CNFs)
and activated carbon (Ir/AC) with a narrow size distribution
by thermal decomposition of Ir4(CO)12 in the presence of
the carbon supports. Among them, Ir/CNF-T has been
found to be an efficient catalyst for the hydrogenation of ni-
troarenes and imines to the corresponding aniline deriva-
tives. These reactions proceed under mild conditions with
high TONs with other functional groups remaining intact.
Since the average size of the Ir particles is independent of
the type of CNFs, we consider that the catalytic activity
seen in the hydrogenation reactions can be mainly attributed
to steric and/or electronic effects derived from interactions
between Ir nanoparticles and the CNF support rather than
the size of iridium particles. A detailed mechanistic study is
now under investigation.
Table 3. Hydrogenation of imines 3a–d using Ir/CNF-T as a catalyst.[a]
Experimental Section
Entry
Substrate
t [h]
Product
Yield [%]
Preparation of Ir/CNF-T
1
2
3
4
3a: R=H
12
12
12
21
4a
4b
4c
4d
99
99
99
99
3b: R=Cl
3c: R=Br
3d: R=COMe
To a suspension of CNF-T (100 mg) in mesitylene (17 mL) was added
Ir4(CO)12 (7.2 mg, [Ir]=5.0 mg) under an argon atmosphere. After stir-
ring at 1658C for 24 h, the insoluble carbon materials were isolated by fil-
tration using membrane filters (Durapore; 0.45 mm HV). The solids were
washed with toluene (50 mL) and ether (50 mL) and then dried under
vacuum (0.04 Torr) at room temperature for 3 h to afford Ir/CNF-T. To
prevent oxidation of Ir species from air, Ir/CNF-T was stored under an
inert atmosphere.
[a] All reactions were performed using 3 (1 mmol) and Ir/CNF-T catalyst
[S/C=1200 mol (3)/mol (Ir)] in ethyl acetate (3 mL) at 258C under H2
(initial pressure=10 atm).
The above described results clearly demonstrate the high
chemoselectivity, efficiency, and reusability of the Ir/CNF-T
catalyst in the hydrogenation of functionalized nitroarenes
and imines to the corresponding amines. Another applica-
tion of Ir/CNF-T as a hydrogenation catalyst is the reductive
Hydrogenation of Nitroarenes
The hydrogenation of nitroarenes was performed in a 100 mL stainless
steel autoclave fitted with a glass inner tube in the presence of nitroarene
(1, 1 mmol), ethyl acetate (3 mL), and Ir/CNF-T [S/C=1700 or 500 mol
(1)/mol (Ir)] at 25 or 508C under H2 (initial pressure=10 atm). After
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Yukihiro Motoyama et al.
completion of the reaction, the insoluble catalyst was removed by filtra-
tion, and the filtrate was concentrated under reduced pressure to give
aniline 2 as the product. The purity of the product was determined by ca-
pillary gas–liquid chromatography (GLC) and/or 1H NMR spectroscopic
analysis.
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Hydrogenation of Imines
[6] The catalytic activity of Ir/CNF-T decreased remarkably at an H2
pressure below 5 atm, and the hydroxylamine derivative, which is
the intermediary species in the hydrogenation of nitrobenzene to
aniline, was obtained as the major product.
[7] The TOFs of the hydrogenation of nitrobenzene and N-phenylben-
zaldimine over Ir/CNF-T in the presence of Et3N (5 mol%) were
110 and 92 [mol (product)/mol (Ir)·h], respectively.
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The hydrogenation of imines was performed in a 100 mL stainless steel
autoclave fitted with a glass inner tube in the presence of nitroarene (3,
1 mmol), ethyl acetate (3 mL), and Ir/CNF-T [S/C=1200 mol (3)/mol
(Ir)] at 258C under H2 (initial pressure=10 atm). After completion of
the reaction, the insoluble catalyst was removed by filtration, and the fil-
trate was concentrated under reduced pressure to give amine 4 as the
product. The purity of the product was determined by capillary GLC
and/or 1H NMR spectroscopic analysis.
Acknowledgements
The authors are grateful to Dr. J. Miyawaki and Dr. M. H. Seo (Kyushu
University) for help with the synthesis of CNFs. A part of this work was
supported by Japan Science and Technology Corporation (Advanced
Low Carbon Technology Research and Development program).
Z.-Y. Xu, J. Mol. Catal. A 2005, 235, 137–142; PdACTHNURGTNEUNG(OAc)2-hydrosi-
lanes: h) R. J. Rahaim, Jr., R. E. Maleczka, Jr., Org. Lett. 2005, 7,
5087–5090; Mo(CO)6/DBU under microwave irradiation: i) J.
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334, and references therein.
Keywords: carbon nanofibers · heterogeneous catalysis ·
hydrogenation · imines · nitroarenes
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Received: August 31, 2013
Published online: November 4, 2013
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