Highly efficient solvent-free catalytic hydrogenation of solid alkenes and nitro-aromatics using Pd nanoparticles entrapped in aluminum oxy-hydroxide

Article abstract of DOI:10.1016/j.tetlet.2010.06.024

Solid alkenes and aromatic nitro compounds are readily hydrogenated to the corresponding alkanes without further reduction of other functional group and amino compounds in nearly quantitative yields in the presence of Pd nanoparticles entrapped in aluminum oxy-hydroxide under the solvent-free condition.

Full text of DOI:10.1016/j.tetlet.2010.06.024

Tetrahedron Letters 51 (2010) 4250–4252
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Highly efficient solvent-free catalytic hydrogenation of solid alkenes and
nitro-aromatics using Pd nanoparticles entrapped in aluminum oxy-hydroxide
b
c
c
Fei Chang a, Hakwon Kim b, , Byeongno Lee , Sungho Park , Jaiwook Park
*
a The School of Environment and Architecture, University of Shanghai for Science and Technology, Shanghai 200093, PR China
b Department of Applied Chemistry, Kyung Hee University, Gyeonggi-do 446-701, Republic of Korea
c Department of Chemistry, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Solid alkenes and aromatic nitro compounds are readily hydrogenated to the corresponding alkanes
without further reduction of other functional group and amino compounds in nearly quantitative yields
in the presence of Pd nanoparticles entrapped in aluminum oxy-hydroxide under the solvent-free
condition.
Received 15 April 2010
Revised 3 June 2010
Accepted 7 June 2010
Available online 11 June 2010
Ó 2010 Published by Elsevier Ltd.
Hydrogenation of alkenes and nitro-aromatics is synthetically
important in organic transformation,1 which provides alkanes
and aromatic amines, widely utilized as intermediates for dyes,
photographic materials, pharmaceutical chemicals, and antioxi-
dants, respectively. However, conventional reduction procedures
require organic solvents and work-up process that could cause
environmental pollution. Therefore, many environmentally benign
chemical processes have been developed to eliminate organic sol-
vents from synthesis.2 Solvent-free (solventless) reactions have at-
tracted much attention because of the several advantages they
have over traditional reactions in organic solvents: smaller reac-
tion vessels, no organic medium, higher efficiency, easier opera-
tion, simpler purification, and different product distribution.3
Palladium(0) catalysts are popular and important for many hydro-
genation reactions of organic compounds.4 In general, commercial
Pd sources, such as Pd/C and Pd/Al2O3, show unsatisfactory results
in selectivity and/or yield from the hydrogenation reactions that
require high selectivity and low structure sensitivity probably
due to Pd particles of variable size unevenly distributed upon the
support surfaces.5
sponding alkanes and amines in high selectivities and reactivities,
respectively, which may afford an effective and environmentally
benign methodology.
A preliminary study on the catalytic utility of Pd/AlO(OH) for
solid substrates in the hydrogenation of methyl-cinnamate was
performed with and without solvents.7 Solvent-free hydrogenation
of methyl-cinnamate was also examined with some commercially
available Pd catalysts. Table 1 shows the results of the hydrogena-
tion of solid methyl-cinnamate to methyl 3-phenylpropionate un-
der various conditions. It is seen that a solid mixture of substrate
and Pd/AlO(OH) catalyst (2 mol % of Pd) under atmospheric pres-
sure of hydrogen at room temperature reacted within 5 s to give
the saturated product in a satisfactory yield (entry 1). When the
reaction time was extended to 30 s, an almost quantitative yield
was observed (entries 2 and 3). Furthermore, quantitative conver-
sions were still retained in the presence of smaller amounts
(0.5 mol %) of Pd with a longer reaction time (10 min). Interest-
ingly, solvent-free hydrogenations by commercially available Pd
catalysts, such as Pd/C, Pd/Al2O3, and Pd/CaCO3, in even longer
reaction time (60 s) did not afford the corresponding product in
good yields (16%, 15%, and 4%, respectively, entries 4–6). We attrib-
uted this inefficiency to the bulky size and poor dispersion of Pd
particles on the support.5
Hence, Pd nanoparticles have been investigated to improve
selectivity and/or reactivity in organic reactions, especially hydro-
genation.4 Recently, Pd nanoparticles embedded in aluminum oxy-
hydroxide, [Pd/AlO(OH)], as a robust and recyclable catalyst, were
reported as a powerful catalyst to catalyze hydrogenation in organ-
ic mediums.6 Herein, we report the catalytic hydrogenation of solid
alkenes and aromatic nitro compounds using Pd/AlO(OH) under
the solvent-free conditions at room temperature to give the corre-
ð1Þ
Pd/AlO(OH)-catalyzed hydrogenations of methyl-cinnamate in
various organic solvents, such as methanol, acetone, tetrahydrofu-
ran (THF), and hexane, were also examined and showed much low-
er efficiencies than those in solvent-free reactions (entries 7–10).
* Corresponding author. Tel.: +82 31 201 2459; fax: +82 31 202 7337.
E-mail address: hwkim@khu.ac.kr (H. Kim).
0040-4039/$ - see front matter Ó 2010 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2010.06.024

F. Chang et al. / Tetrahedron Letters 51 (2010) 4250–4252
4251
Table 1
Hydrogenation of methyl-cinnamate using Pd catalysts under various conditionsa
Table 2
Hydrogenation of various solid alkenes and aromatic nitro compoundsa
Entry
Catalysts
Solvents
Time (s)
Yieldb (%)
Entry
1
Substrate (mp, °C)
Product (mp, °C)
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
9
Pd/AlO(OH)
Pd/AlO(OH)
Pd/AlO(OH)
Pd/C
Pd/Al2O3
Pd/CaCO3
Pd/AlO(OH)
Pd/AlO(OH)
Pd/AlO(OH)
Pd/AlO(OH)
No
No
No
No
No
No
Methanol
Acetone
THF
5
10
30
60
60
60
60
60
60
60
81
90
99
16
15
4
3
6
5
2
1
98
99
2
3
1
3
10
Hexane
95
99
a
All reactions were performed at room temperature under atmospheric pressure
of hydrogen gas. Methyl-cinnamate (0.20 mmol) was hydrogenated by the Pd cat-
alyst (2 mol % of Pd) indicated in the table.
4
1
b
Determined by GC analysis.
The rapid solvent-free hydrogenation observed can be rationalized
with a higher concentration of the substrate under the solvent-free
condition compared to that in organic solvent.8
5
6
2
5
99
96
Various solid alkenes containing the other functional group
were conducted for this solvent-free reaction in the presence of
Pd/AlO(OH) with hydrogen in order to confirm the reactivity and
the chemoselectivity. The results are summarized in Table 2 (en-
tries 1–9). Only double bond moiety of substrates was hydroge-
nated quantitatively using Pd/Al(O)OH catalyst in the absence of
solvent without any further hydrogenation of the carbonyl, sulfo-
nyl, or nitrile group. Most substrates were hydrogenated within
5 min. However, trans-stilbene (entry 8) and maleic acid (entry
9) showed lower reactivities under the same conditions: their cor-
responding products, bibenzyl and succinic acid, were obtained in
only 89% and 82% yields, even after 2 h. According to the literature,
it can be proposed that there is a close relationship between the
yields obtained and the melting points of the substrates involved
in the solvent-free hydrogenation.9
7
5
99
8
9
120
120
89
82
10
11
12
5
5
5
99
99c
95
To extend the scope of the solvent-free hydrogenation, a series
of solid aromatic nitro compounds containing diverse functional
groups have been treated with Pd/AlO(OH). As shown in Table 2
(entries 10–14), all nitro substrates underwent nearly complete
conversions to their corresponding aromatic amines in a short
time.
To the best of our knowledge, this is the first Letter of a solvent-
free hydrogenation of solid nitro-aromatics, though the solvent-
free hydrogenation of liquid nitro compounds has been reported.10
Although we did not obtain valuable microscopic evidence for the
reaction mechanism, we observed macroscopically that the color of
the solid mixture was changed quickly as the hydrogen gas was
introduced into the reactor. Subsequently we did a comparative
experiment using a simple mixture of blocky-shaped substrate
and catalyst with and without grounding. It was observed that
the blocky substrate melted immediately after introducing hydro-
gen gas. Also, as shown in Table 2, it was found that the efficiencies
of solvent-free hydrogenations of solid substrates by Pd/AlO(OH)
catalyst were closely depended on the melting points of the reac-
tants. It seems that our solvent-free hydrogenations proceeded in
a fused state based on this observation and the literature explana-
tion,9,11 although we may not exclude the possibility of the hydro-
genation in the solid-to-solid state. Accordingly, we can suggest
that substrates with low melting points can be easily melted into
the fused state by the heat liberated from the exothermic hydroge-
nation, which can enhance the molecular mobility and the cata-
lyst-substrate collisions to result in fast reaction. From this point
of view, we may conclude that substrates with low melting points
proceed very quickly for the solvent-free hydrogenation by Pd/
AlO(OH) catalyst, and substrates with high melting points are
13
14
30
10
92
99
a
Without otherwise stated, substrate (0.2 mmol) was used in every entry with
Pd/AlO(OH) (2 mol % of Pd) under atmospheric pressure of hydrogen gas at room
temperature, without controlling the vessel temperature during the reaction.
b
Determined by GC analysis.
Determined by 1H NMR analysis.
c
hydrogenated slowly under the same conditions. We also found
that the aluminum oxy-hydroxide matrix in the Pd catalyst would
be crucial for the remarkable catalytic activities in the solvent-free
conditions.6a Also, it should be mentioned that the carbonyl group
in the nitro-aromatic compound (entry 12) was not hydrogenated
at all. Additionally, recyclability of Pd/AlO(OH) catalyst after the
solvent-free reaction was examined in the hydrogenation of
methyl-cinnamate and 2-fluoro-4-nitrotoluene. Methylene chlo-
ride or acetone was employed in order to recover Pd/AlO(OH) after

4252
F. Chang et al. / Tetrahedron Letters 51 (2010) 4250–4252
2. Tanaka, K.; Toda, F. Chem. Rev. 2000, 100, 1025.
Table 3
3. (a) Cave, G. W. V.; Raston, C. L.; Scott, J. L. Chem. Commun. 2001, 2159; (b)
Hermans, S.; Raja, R.; Thomas, J. M.; Johnson, B. F. G.; Sankar, G.; Gleeson, D.
Angew. Chem., Int. Ed. 2001, 40, 1211.
Reusability of Pd/AlO(OH)
Recovery
1st
2nd
3rd
4. Tsuji, J. Palladium Reagents and Catalysts: New Perspectives for the 21st Century;
John Wiley & Sons: Chichester, 2004.
5. Lee, S.-S.; Park, B.-K.; Byeon, S.-H.; Chang, F.; Kim, H. Chem. Mater. 2006, 18,
5631.
Methyl-cinnamate
2-Fluoro-4-nitrotoluene
>99
>99
>99
>99
>99
95
6. (a) Kwon, M. S.; Kim, N.; Park, C. M.; Lee, J. S.; Kang, K. Y.; Park, J. Org. Lett. 2005,
7, 1077; (b) Kwon, M. S.; Kim, N.; Seo, S. H.; Park, I. S.; Cheedrala, R. K.; Park, J.
Angew. Chem., Int. Ed. 2005, 44, 6913; (c) Choi, Y. K.; Kim, Y.; Han, K.; Park, J.;
Kim, M.-J. J. Org. Chem. 2009, 74, 9543.
7. All reactions were carried out in small and dried glassware (10 mL).
Hydrogenation of methyl-cinnamate under solvent-free condition: a mixture
of substrate and Pd/AlO(OH) (2.0 mol % of Pd) was ground finely with a mortar
and pestle at room temperature before use, and then 0.081 g of the mixture
solvent-free hydrogenation by simply washing and drying under
vacuum. The Pd catalyst did not show any significant decrement
of hydrogenation activity at least three times (see Table 3). Now
we are trying to improve the recycling of the Pd catalyst using
supercritical CO2 extraction. More detailed results will be pub-
lished in the near future.
(including 0.2 mmol methyl-cinnamate and
a
corresponding amount of
rubber septum,
catalyst) was loaded into glassware equipped with
a
a
In summary, we developed a highly efficient and selective sol-
vent-free hydrogenation of solid alkenes and nitro-aromatic com-
pounds using Pd/AlO(OH) to yield the corresponding alkanes and
aromatic amines.
followed by sealing with parafilm. N2 gas was charged into the reactor for
several times to remove air and then was expelled by an atmosphere of
hydrogen (0.1 Mpa). The mixture was then allowed to stand for a desired time
without controlling the temperature in the vessel. After filtering the catalyst,
the product analyses were performed using GC and 1H NMR by comparing to
the authentic samples. Hydrogenation of methyl-cinnamate in organic
solvents: a mixture (0.081 g) of methyl-cinnamate and Pd/AlO(OH) in 1 mL
Acknowledgment
organic solvent was hydrogenated at room temperature in
a hydrogen
atmosphere. After reaction, the analysis procedure was the same as
described above.
8. (a) Rothenberg, G.; Downie, A. P.; Raston, C. L.; Scott, J. L. J. Am. Chem. Soc. 2001,
123, 8701; (b) Toda, F.; Yagi, M.; Kiyoshige, K. Chem. Commun. 1988, 958.
9. Nozoe, T.; Tanimoto, K.; Takemitsu, T.; Kitamura, T.; Harada, T.; Osawa, T.;
Takayasu, O. Solid State Ionics 2001, 141, 695.
We acknowledge a Manpower Development Program for En-
ergy & Resources supported by the Ministry of Knowledge and
Economy (MKE).
10. Vass, A.; Dudas, J.; Toth, J.; Varma, R. S. Tetrahedron Lett. 2001, 42, 5347.
11. Kitamura, T.; Harada, T. Green Chem. 2001, 3, 252.
References and notes
1. Larock, R. C., 2nd ed.; In Comprehensive Organic Transformations; Wiley-VCH:
New York, 1999; Vol. 41. pp 7–11.