Chemoselective hydrogenation of nitroarenes with carbon nanofiber-supported platinum and palladium nanopraticles

Article abstract of DOI:10.1021/ol800277a

Platinum and palladium nanoparticles supported on three types of carbon nanofibers (CNFs) are synthesized and used as catalysts in the hydrogenation of nitroarenes. Nanosized platinum particles dispersed on platelet-type CNF efficiently catalyze the reduction of functionalized nitroarenes to the corresponding substituted anilines in high turnover numbers with other functional groups remaining intact.

Full text of DOI:10.1021/ol800277a

ORGANIC
LETTERS
2008
Vol. 10, No. 8
1601-1604
Chemoselective Hydrogenation of
Nitroarenes with Carbon
Nanofiber-Supported Platinum and
Palladium Nanoparticles
Mikihiro Takasaki, Yukihiro Motoyama,* Kenji Higashi, Seong-Ho Yoon,
Isao Mochida, and Hideo Nagashima
Graduate School of Engineering Sciences, Institute for Materials Chemistry and
Engineering, Kyushu UniVersity, Kasuga, Fukuoka 816-8580, Japan
motoyama@cm.kyushu-u.ac.jp
Received February 6, 2008
ABSTRACT
Platinum and palladium nanoparticles supported on three types of carbon nanofibers (CNFs) are synthesized and used as catalysts in the
hydrogenation of nitroarenes. Nanosized platinum particles dispersed on platelet-type CNF efficiently catalyze the reduction of functionalized
nitroarenes to the corresponding substituted anilines in high turnover numbers with other functional groups remaining intact.
Functionalized aniline derivatives are important intermedi-
ates in the preparation of dyes, herbicides, pesticides, and
pharmaceuticals.1 Catalytic hydrogenation of nitroarenes over
heterogeneous catalysts is a simple method for the production
of aromatic amines. However, the application of conventional
catalytic systems to the reduction of nitroarenes containing
other reducible functional groups, e.g., halogen, carbonyl,
cyano, benzyloxy, and alkenic groups, is problematic because
the reaction is often accompanied by the reduction of these
functional groups.2 A typical example is the preparation of
haloanilines from halonitrobenzenes, where the hydrogena-
tion over the conventional heterogeneous catalysts affords a
mixture of haloaniline and dehalogenated aniline. This is due
to the fact that reduction of a halogen atom (k2 and k2′)
competes with the reduction of a nitro group (k1 and k1′), as
shown in Scheme 1.
Scheme 1. Reduction of Halonitrobenzene with Hydrogen
(1) Reviews: (a) Tafesh, A. M.; Weiguny, J. Chem. ReV. 1996, 96, 2035.
(b) Downing, R. S.; Kunkeler, P. J.; van Bekkum, H. Catal. Today 1997,
37, 121. (c) Ono, N. The Nitro Group in Organic Synthesis; Wiley-VCH:
New York, 2001. (d) Adams, J. P. J. Chem. Soc., Perkin Trans. 1 2002,
2586.
(2) (a) Rylander, P. In Catalytic Hydrogenation in Organic Synthesis;
Academic Press: New York, 1979; p 112. (b) Baumeister, P.; Studer, M.;
Roessler, F. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger,
H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, 1997; pp 2186-2209.
(c) Blaser, H.-U.; Siegrist, U.; Steiner, H.; Studer, M. In Fine Chemicals
through Heterogeneous Catalysis; Sheldon, R. A., van Bekkum, H., Eds.;
Wiley-VCH: Weinheim, 2001; p 389.
Although poisoning of the catalyst by sulfur-, nitrogen-,
phosphorus-, or halogen-containing compounds sometimes
suppresses the reductive dehalogenation,2-4 the catalytic
activity toward the reduction of the nitro group is also
10.1021/ol800277a CCC: $40.75
© 2008 American Chemical Society
Published on Web 03/14/2008

significantly decreased. Thus, the catalytic hydrogenation of
p-chloro- and p-bromonitrobenzene using Pd and Pt/C-
ZnBr2 systems occurs below 65 °C under a hydrogen
atmosphere (1 atm) to result in the exclusive formation of
the corresponding haloanilines. However, the catalyst effi-
ciency is low (substrate/catalyst mole ratio; S/C ) 100 for
Pd, and 20 for Pt).3b Use of sulfide-modified Co, Rh, Pd,
and Pt/C as catalysts reportedly leads to high turnover
numbers (TONs) [up to 2400 mol (substrate)/mol (metal)],
but the reaction requires high temperatures and high H2
pressures (100-175 °C, 30 atm) for a complete conversion
to haloanilines.3a Thus, the development of a new catalyst
system, which realizes an efficient conversion of halonitro-
arenes under mild conditions and provides a selective
reduction of the nitro group with other functional groups
remaining intact, is an important target in organic synthesis.
It is well known that the catalytic properties of heteroge-
neous catalysts are dependent on the particle size of the metal
and the surface structure of the supports.5 We have recently
developed ruthenium nanoparticles supported on carbon
nanofibers (CNFs) [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)],6 which showed a high efficiency
in arene hydrogenation.7 This is attributed to the high
catalytic activity of nanosized ruthenium particles, which are
effectively supported by nanolevel-controlled surface struc-
tures of CNF. We were interested in the application of
nanometal particles supported on CNFs to the hydrogenation
of nitroarenes in the hope that these may exhibit high
catalytic activity with high selectivity toward the reduction
of the nitro group. Attempted hydrogenation of halonitro-
arenes over Ru/CNFs led to problems in both the activity
and the selectivity of the catalyst. However, Pt nanoparticles
on CNFs (Pt/CNFs) were found to act as efficient hydroge-
nation catalysts for the conversion of nitroarenes to anilines
in high TONs without affecting the reduction of halogen or
other functional groups.
Preparation of Pt and Pd/CNFs was achieved successfully
by the reaction of Pt(dba)2 or Pd2(dba)3‚CHCl3 with CNFs
in toluene under an argon atmosphere (Scheme 2). Three
types of CNF were used for the immobilization of platinum
and palladium, where the metal particles could be highly
Scheme 2. Preparation of Pt and Pd/CNFs
dispersed on the surface in all cases with particle size range
of 1-3 nm for Pt and 3-7 nm for Pd (see Supporting
Information).
The Pt and Pd/CNFs synthesized were used in the catalytic
hydrogenation of p-chloronitrobenzene (1a). The reaction of
1a (157 mg, 1 mmol) was carried out in the presence of the
catalyst (5 mg; S/C ) 710-1630) in ethyl acetate (3 mL)
under H2 pressure (initial pressure: PH2 ) 10 atm) at room
temperature (Table 1). The reaction over a commercially
Table 1. Hydrogenation of 1a with Pt and Pd Catalysts
(3) Representative papers: (a) Greenfield, H.; Dovell, F. S. J. Org. Chem.
1967, 32, 3670. (b) Baumeister, P.; Blaser, H. U.; Siegrist, U.; Studer, M.
Catal. Org. React. 1998, 75, 207. (c) Wu, G.; Huang, M.; Richards, M.;
Poirier, M.; Wen, X.; Draper, R. W. Synthesis 2003, 1657. (d) Lee, H.-Y.;
An, M. Bull. Kor. Chem. Soc. 2004, 25, 1717. (e) Akao, A.; Sato, K.;
Nonoyama, N.; Mase, T.; Yasuda, N. Tetrahedron Lett. 2006, 47, 969. Other
catalyst systems; Cp2TiCl2/Sm system: (f) Huang, Y.; Liao, P.; Zhang, Y.;
Wang, Y. Synth. Commun. 1997, 27, 1059. Pt/C in ScCO2: (g) Ichikawa,
S.; Tada, M.; Iwasawa, Y.; Ikariya, T. Chem. Commun. 2005, 924. Pt/C in
ionic liquid: (h) Xu, D.-Q.; Hu, Z.-Y.; Li, W.-W.; Luo, S.-P.; Xu, Z.-Y. J.
Mol. Catal. A: Chem. 2005, 235, 137. Pd(OAc)2-hydrosilanes: (i) Rahaim,
R. J., Jr.; Maleczka, R. E., Jr. Org. Lett. 2005, 7, 5087. Mo(CO)6/DBU
under microwave irradiation: (j) Spencer, J.; Anjum, N.; Patel, H.; Rathnam,
R. P.; Verma, J. Synlett 2007, 2557. Au/TiO2: (k) Corma, A.; Serna, P.
Science 2006, 313, 332, and references cited therein.
(4) Blaser, H.-U. Science 2006, 313, 312, and references cited therein.
(5) (a) Feldheim, D. L.; Foss, C. A., Jr. Metal Nanoparticles: Synthesis,
Characterization, and Application; Marcel Dekker: New York, 2002. (b)
Catalyst Supports and Supported Catalysts; Stiles, A. B., Ed.; Butter-
worths: Boston, 1987. (c) Handbook of Heterogeneous Catalysis; Ertl, G.,
Kno¨zinger, H., Weitkamp, J., Eds.; VCH: Weinheim, 1997.
entry
catalyst
solvent 2a (%)a 3a (%)a 4a (%)a TOF
1
2
3
4
5
6
7
8
Pt/CNF-T AcOEt
Pt/CNF-H AcOEt
Pt/CNF-P AcOEt
Pd/CNF-T AcOEt
Pd/CNF-H AcOEt
Pd/CNF-P AcOEt
Pt/CNF-P hexane
Pt/CNF-P EtOH
Pt/CNF-P AcOEt
Pt/CNF-H AcOEt
86
5
9
750
815
610
355
445
410
610
610
610
815
610
165
96
3
NDb
NDb
NDb
NDb
NDb
NDb
7
96.3c
91.0c
95.9c
96.1c
93
3.7c
9.0c
4.1c
3.9c
6
86
97.0c
97.1c
7
9d
10e
11e
1.5c
1.3c
NDb
NDb
NDb
NDb
NDb
NDb
Pt/CNF-P AcOEt >99.9c
12e,f Pd/CNF-P AcOEt >99.9c
a Determined by 1H NMR analysis. b ND ) not detected. c Determined
by GLC analysis. d H2 (3 atm). e 20 µL of n-octylamine (10 mol % to 1a)
was added. f For 5 h.
(6) The CNFs are classified into three types: graphite layers are
perpendicular (platelet: CNF-P), parallel (tubular: CNF-T), and stacked
obliquely (herringbone: CNF-H). These three CNFs can be synthesized
selectively in large scales; see: (a) Rodriguez, N. M. J. Mater. Res. 1993,
8, 3233. (b) Tanaka, A.; Yoon, S.-H.; Mochida, I. Carbon 2004, 42, 591;
1291.
(7) (a) Motoyama, Y.; Takasaki, M.; Higashi, K.; Yoon, S.-H.; Mochida,
I.; Nagashima, H. Chem. Lett. 2006, 35, 876. (b) Takasaki, M.; Motoyama,
Y.; Higashi, K.; Yoon, S.-H.; Mochida, I.; Nagashima, H. Chem. Asian J.
2007, 2, 1524. (c) Takasaki, M.; Motoyama, Y.; Yoon, S.-H.; Mochida, I.;
Nagashima, H. J. Org. Chem. 2007, 72, 10291.
available Pt/C catalyst under similar conditions proceeded a
turnover frequency (TOF: TON/h) of 650 to give a mixture
of desired chloroaniline 2a (75%), dechlorinated aniline 3a
(10%), and cyclohexylamine 4a (12%). All of the Pt/CNFs
used exhibited good to high catalytic efficiency in the
reduction of the nitro group. 1a was consumed within 2 h
1602
Org. Lett., Vol. 10, No. 8, 2008

and turnover frequencies reached 610-815 mol (1a)/mol
(metal)‚h (entries 1-3). The selectivity of the reduction of
p-chloroaniline 2a was highly dependent on the CNF support.
The product ratio obtained in the reaction with Pt/CNF-T
was similar to that of commercially available Pt/C in
producing large amounts of 3a and 4a as byproducts (entry
1). In sharp contrast, the reaction over Pt/CNF-H and Pt/
CNF-P improved the selectivity of 2a up to 96%, in which
the arene hydrogenation was suppressed effectively (entries
2 and 3). Pd/CNFs showed catalytic properties similar to
the Pt/CNFs. The selectivity of the reaction of 2a is about
96% when using Pd/CNF-H and Pd/CNF-P, but the
activities of the catalysts are slightly lower (TOF ) 355-
445) than that found when using Pt/CNF-H or Pt/CNF-P
(entries 4-6). The solvent strongly affects the chemoselec-
tivity of the reaction, and a more selective formation of 2a
was attained in ethyl acetate than in hexane or EtOH (entries
3 vs 7 and 8). Lower H2 pressure (3 atm) led to a higher
selectivity toward 2a (98.4%) as shown in entry 9. Complete
suppression of the side reactions leading to 3a and 4a was
achieved with Pt/CNF-P by adding n-octylamine (10 mol
% for 1a); p-chloroaniline 2a was obtained as a single
product in quantitative yield (entry 11). It is noteworthy that
no retardation was observed on the addition of n-octylamine
to the reaction when using the Pt/CNFs as catalysts. It is
known that addition of heteroatom-containing compounds
to the reaction medium in catalytic hydrogenation reactions
often decreases the reaction rate. In fact, addition of
n-octylamine to the reaction with the Pd/CNF-P catalyst,
while resulting in exclusive formation of 2a, led to a marked
decrease of the reaction rate; TOF decreased from 410 to
165 [mol (1a)/mol (Pd)‚h] (entries 6 vs 12).
dechlorinated aniline 3a were formed (entries 2 and 3). It is
known that the hydrogenation of p-bromo- (1d) and p-
iodonitrobenzene (1e) with high chemoselectivity is difficult
to achieve with commercially available catalysts. In fact, we
have found that Pt/C catalyst produced a 3:2 mixture of
desired product 2d (57%) and debrominated aniline 3a
(42%). Interestingly, the reaction with Pt/CNF-P under the
same conditions afforded p-bromoaniline 2d in 94% yield
in the absence of an additive (entry 4), whereas addition of
n-octylamine resulted in formation of 2d in >99% yield with
99.8% selectivity (entry 5). Nitro-selective hydrogenation of
p-iodonitrobenzene 1e was also achieved with 97% selectiv-
ity by running the reaction with the Pt/CNF-P-amine
system at 50 °C for 4 h (entry 6).
Pt/CNF-P was also effective in the hydrogenation of other
nitroarenes even in the absence of n-octylamine (Table 3).8
Hydrogenation of p-nitrophenol (5a) gave p-aminophenol
Table 3. Hydrogenation of Other Nitroarenes 5a-ha
Catalytic hydrogenation of other halogenated nitrobenzene
derivatives (1b-e) were also achieved with high chemose-
lectivity when usig the Pt/CNF-P-n-octylamine system
(Table 2). Both ortho- and meta-chloronitrobenzenes 1b and
1c were converted to the corresponding chloroanilines 2b
and 2c in almost quantitative yields; only trace amounts of
Table 2. Hydrogenation of Various Halogenated Nitroarenes
1a-ea
a All reactions were carried out with 5 (1 mmol) and Pt/CNF-P (5 mg;
S/C ) 1220) in ethyl acetate (3 mL) at rt for 4 h under H2 (10 atm). b In
THF/EtOH ) 2:1 at 100 °C for 12 h under 25 atm of H2. c 5 mmol of 5d
(S/C ) 6100) and 15 mL of solvent were used. d Determined by 1H NMR
analysis. e 18% of substrate 5g was recovered. f 9% of ethyl p-aminodihy-
drocinnamate 7 was formed. g 32% of di(p-aminobenzyl)amine was formed.
entry
substrate
time (h)
2 (%)b
3 (%)b
1
2
3
1a
1b
1c
1d
1d
1e
2
2
4
4
4
4
>99.9
99.9
98d
NDc
0.1
<2d
6d
0.2
3d
(6a) as a single product within 4 h. Aromatic hydrogenation
leading to the corresponding cyclohexanone or cyclohexanol
derivatives, which often seen in the hydrogenation with
conventional heterogeneous catalysts, did not occur (entry
1). The nitro group was selectively reduced to the amino
moiety with carbonyl groups present in the molecule remain-
ing intact; no reduction of amide, ester, or ketone groups
4e
5
94d
99.7
96d
6f
a All reactions were carried out with 1 (1 mmol), Pt/CNF-P (5 mg),
and n-octylamine (20 µL) in ethyl acetate at room temperature under H2
(10 atm). b Determined by GLC analysis. c ND ) not detected. d Determined
1
by H NMR analysis. e In the absence of n-octylamine. f At 50 °C.
(8) No reduction took place in the reaction of aliphatic nitro compounds.
Org. Lett., Vol. 10, No. 8, 2008
1603

was observed as shown in entries 3-5. Although the catalytic
hydrogenation over heterogeneous catalysts often induces
hydrogenolytic cleavage of the C-O bond in benzyl alcohols
or benzyl ethers, nitroarenes containing the benzyloxy group
(5f) or the benzyl alcohol moiety (5g) were completely
converted to the corresponding aniline derivatives 6f and 6g
without cleavage of the benzyl group (entries 6 and 7). Re-
duction of the nitro group was favored with ethyl m-
nitrocinnamate (5h) affording ethyl m-aminocinnamate (6h)
with 91% selectivity (entry 8). In the reaction of p-nitro-
styrene (5i), however, nitro- and alkenyl groups were both
reduced by the present system to afford p-ethylaniline (6i)
as a single product in quantitative yield (entry 9). Attempted
nitro-selective reduction of p-cyanonitrobenzene (5j) gave
the desired p-cyanoaniline (6j) in 64% yield along with 32%
of di(p-aminobenzyl)amine (entry 10). We also examine the
reaction of 3-nitropyridine as a nitro-substituted heteroarene.
However, the desired 3-aminopyridine was formed in only
13% yield; the main product was the intermediary hydroxyl
amine derivative.
The results shown in Tables 2 and 3 suggest that the
hydrogenation of nitroarenes over Pt/CNF-P is tolerant
toward halogen and other functional groups. This is clearly
demonstrated by the selective hydrogenation of the nitro
group in trisubstituted benzene derivatives (5k and 5l) over
the Pt/CNF-P catalyst. In these cases, nitro-selective reduc-
tion proceeded without addition of n-octylamine to give
3-chloro-4-benzyloxyaniline (6k) and 3-carbomethoxy-4-
chloroaniline (6l), respectively, as a single product in quan-
titative yields (Scheme 3).
of the nitro group in various nitroarenes with other functional
groups remaining intact. In the hydrogenation of haloni-
troarenes, addition of n-octylamine contributes to raising the
selectivity without retarding the reduction of the nitro group.
Besides the efficiency and selectivity in the reaction, the
hydrogenation over Pt/CNF-P has two more advantages;
reusability without leaching of platinum species and possible
application to production of anilines in gram quantities,
which is exemplified by the following experiments: (1) after
the reaction with 5b (Table 3, entry 2), the catalyst was
recovered by filtration and reused in the same reaction.
p-Methoxyaniline 6b was obtained in a yield of over 99%
with repeated use of the catalyst (5×). No Pt species was
detected in the product (<30 ppb, determined by ICP-MS).
(2) The hydrogenation of 5b (1.53 g, 10 mmol) over Pt/
CNF-P (5 mg, 3.2 wt %; 160 µg of Pt) at room temperature
for 4 h under H2 (10 atm) afforded 6b in quantitative yield
(1.23 g). The TON of the reaction was calculated to be
12 200 (TOF ) 3050/h).
In conclusion, we have developed a catalytic system
consisting of nanoplatinum particles dispersed on nanocarbon
fiber support, Pt/CNF-P. Pt/CNF-P has been found to be
an efficient and reusable catalyst for the hydrogenation of
nitroarenes to aromatic amines. The reaction proceeds under
mild conditions in high TONs with other functional groups
remaining intact. Since the average particle size of the Pt is
independent of the type of CNF, we consider that the high
chemoselectivity seen in the nitro-group selective hydrogena-
tion of halonitrobenzenes accompanied by efficient catalyst
poisoning by n-octylamine with Pt/CNF-P are rather at-
tributed to the steric and/or electronic effects derived from
interaction between Pt particles and the CNF support than
the size of platinum particles. A detailed mechanistic study
is now under investigation.
Scheme 3. Nitro-selective Reduction of Trisubstituted
Benzene Derivatives 5k and 5l
Acknowledgment. This work was partially supported by
the CREST-JST (Japan Science and Technology Corpora-
tion) and by a Grant-in Aid for Scientific Research from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan. We are grateful to Dr. Thies Thiemann (Kyushu
University) for his helpful discussion.
Supporting Information Available: Experimental pro-
cedures and copies of NMR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
Above-described results clearly demonstrate that Pt/
CNF-P is an effective catalyst for selective hydrogenation
OL800277A
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Org. Lett., Vol. 10, No. 8, 2008