Angewandte
Chemie
DOI: 10.1002/anie.200803850
Catalysis
Hydrogenation of Arenes by Dual Activation: Reduction of Substrates
Ranging from Benzene to C60 Fullerene under Ambient Conditions**
Ravindra R. Deshmukh, Ji Woong Lee, Ueon Sang Shin,* Jin Yong Lee, and Choong Eui Song*
Catalytic hydrogenation of aromatic compounds is an impor-
tant reaction which is useful for making key intermediates in
organic chemistry and for the production of aromatic-
content-free fuels. More recently, there has been much
interest in the potential for storage of molecular hydrogen,
as an energy source, by catalytic hydrogenation of carboneous
materials (e.g., fullerene and carbon nanotubes).[1,2] Although
the science of metal and metal-oxide-catalyzed arene hydro-
Scheme 1. Concept for a new cooperative catalytic system for the
genations have been significantly advanced since the original
findings of Sabatier and Senderens,[3] harsh conditions (high
temperatures or pressures) are still required for the catalytic
hydrogenation of aromatic compounds. The reduction of
benzene requires harsher conditions compared to that of most
other aromatic compounds because of its high stabilization
energy resulting from aromaticity. The catalytic activity of
heterogeneous noble metal catalysts for the hydrogenation of
benzene decreases in the order of Rh > Ru > Pt > Ni > Pd.[4]
At low temperature palladium usually does not reduce a
benzene ring, and palladium nanoparticles[5] have been shown
to be nearly inactive for the hydrogenation of benzene. It is
well known that Lewis acids can activate aromatic com-
pounds[6] and that Pd/C can be used to activate molecular
hydrogen. We wondered if these two types of activation could
work cooperatively for the hydrogenation of arenes by the
novel ionic mechanism depicted in Scheme 1. Herein we
report the highly efficient catalytic hydrogenation of benzene
and other hydrocarbon-based aromatic compounds, including
C60 fullerene, under ambient conditions (1 bar of H2 and RT)
by simultaneous activation of molecular hydrogen and the
aromatic substrate with Pd/C and a Lewis acidic ionic liquid
[bmim]Cl-AlCl3 (1, bmim = 1-butyl-3-methylimidazolium; x
of AlCl3 = 0.67 where x is the mole fraction of AlCl3),
respectively.
double activation of an arene and molecular hydrogen.
Table 1: Catalytic hydrogenation of benzene to cyclohexane.[a]
Entry Metal cat.
(equiv)
Lewis acid
(equiv)
H2 [bar] Time [h] Conversion [%][b]
1
2
Pd/C (0.02)
PdNP
–
–
3
3
6
6
n.r.
n.r.
[c]
(0.02)
PdNP/SiO2
–
Pd/C (0.02) AlCl3 (0.5)
Pd/C (0.02) 1 (0.5)
Pd/C (0.1) 1 (1)
3[d]
4
5
6
7
–
30
3
3
3
1
2
6
6
6
4
1 (0.5)
n.r.
n.r.
>99
>99
24
[a] Reaction conditions: Unless otherwise indicated, reactions were
carried out with benzene (1 mmol), Pd/C (2 mol%) in the presence or
absence of a Lewis acid in 1,2-dichloroethane (2 mL) at room temper-
ature. [b] Conversions were determined by GC methods. [c] PdNP =Pd
nanoparticles; PdNP was prepared from [Pd(acac)2] according to a
literature procedure.[10] [d] Data from reference [5]. n.r.=no reaction;
acac=acetylacetonate.
also ineffective for the hydrogenation reaction (Table 1,
entry 4).[7] However, when Lewis acidic ionic liquid 1 and
Pd/C (0.5 and 0.02 equiv, respectively) were combined, the
hydrogenation reaction went to completion (Table 1, entry 6).
When the amounts of 1 and Pd/C were increased to 1 and
0.1 equivalents, respectively, benzene was hydrogenated at
1 bar of H2 (Table 1, entry 7). When 1 was replaced by AlCl3
(Table 1, entry 5), a mixture of unidentified condensation
products were formed by the Scholl reaction.[8,9]
We initially examined the hydrogenation of benzene to
cyclohexane in the presence of Lewis acids and palladium
(Table 1). Pd/C (Table 1, entry 1) and palladium nanoparti-
cles (Table 1, entries 2 and 3) were ineffective as catalysts for
the hydrogenation reaction. Lewis acidic ionic liquid 1 was
Subsequent to our encouraging results, we focused on the
hydrogenation of bicyclic and polycyclic aromatic compounds
(Table 2). These aromatic substrates are more reactive to
hydrogenation than benzene, therefore all of the compounds
were smoothly hydrogenated to completion under ambient
conditions when both 1 and Pd/C were used in combination
(Table 2, entries 3, 4, 7, 10, and 12). Here again, the hydro-
genation reactions were ineffective in the absence of 1, even
under higher pressures of H2 (Table 2, entries 1, 5, 8, and 11).
When 1 was replaced with AlCl3, various inseparable
condensation products (> 99%) were formed by the Scholl
[*] Dr. R. R. Deshmukh, J. W. Lee, Dr. U. S. Shin, Prof. J. Y. Lee,
Prof. C. E. Song
Department of Chemistry, Sungkyunkwan University
300 Cheoncheon, Jangan, Suwon, Gyeonggi, 440-746 (Korea)
Fax: (+82)31-290-7075
E-mail: s1673@skku.edu
[**] This work was supported by the following grants: KRF-2005-
005J11091 (MOEHRD), R01-2006-000-10426-0 (KOSEF), and R11-
2005-008-00000-0 (SRC program of MOST/KOSEF).
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2008, 47, 8615 –8617
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8615