Hydrogenation of Arenes by Dual Activation: Reduction of Substrates Ranging from Benzene to C60 Fullerene under Ambient ConditionsThis 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)

Catalytic hydrogenation of aromatic compounds is an important reaction which is useful for making key intermediates in organic chemistry and for the production of aromaticcontent-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). Although the science of metal and metal-oxide-catalyzed arene hydrogenations have been significantly advanced since the original findings of Sabatier and Senderens, 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. At low temperature palladium usually does not reduce a benzene ring, and palladium nanoparticles have been shown to be nearly inactive for the hydrogenation of benzene. It is well known that Lewis acids can activate aromatic compounds 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. 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 nanoparticles (Table 1, entries 2 and 3) were ineffective as catalysts for the hydrogenation reaction. Lewis acidic ionic liquid 1 was also ineffective for the hydrogenation reaction (Table 1, entry 4). 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. 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 hydrogenation 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 Scheme 1. Concept for a new cooperative catalytic system for the double activation of an arene and molecular hydrogen.