Hydrogenation of aromatic compounds

Adams and Marshall216 hydrogenated benzene (15.6 g) quantitatively to cyclohexane in glacial acetic acid at room temperature and 2-3 atm hydrogen pressure within 2 h in the presence of Pt02 (0.2 g). 

Only if absolutely free from sulfur can aromatic compounds be hydrogenated with platinum metals at low temperatures. Willstatter and Hatt217 hydrogenated twelve times recrystallized naphthalene in glacial acetic acid quantitatively to decalin in presence of platinum black at room temperature and atmospheric pressure. Schroeter218 hydrogenated naphthalene to decalin in precence of nickel catalysts at 40 atm and about 200°, and to tetralin at 15 atm and about 200°  

Like benzoic acid, phenol, and other benzene derivatives, naphthalene can be hydrogenated more easily than benzene. 

Durland and Adkins219 were able to hydrogenate phenanthrene to 9,10-dihydrophen-anthrene by using copper-chromium oxide  

The preparation of 4-aminocyclohexanecarboxylic acid220 will be described as an example of hydrogenation of aromatic compounds. The technique has the advantage that an aqueous suspension is used, without an organic solvent. 

A similar water-soluble colloidal system has been described by James and coworkers [19]. Rhodium colloids were classically produced by reducing RhCk 3H2O with ethanol in the presence of PVP and triethylamine. The mono-phasic hydrogenation of various substrates such as benzyl acetone, propyl-phenol and benzene derivatives was performed under mild conditions (25 C and 1 bar H2). The nanoparticles were jxjorly characterized but benzyl acetone is reduced with 50 TTO (total number of turnovers) in 43 h. 

The formation and stabilization of noble metal colloids in the aqueous phase are widely known. Platinum and palladium are most widely used in hydrogenation of C=C bonds but some results have been described with rhodium. Generally, surfactants are investigated as stabilizers for the preparation of rhodium nanoparticles for biphasic catalysis in water. In many cases, ionic surfactants, such as ammonium salts, which provide sufficiently hydrophilic character to maintain the catalytic species within the aqueous phase, are used. The obtained micelles constitute interesting nanoreactors for the synthesis of controlled size nanoparticles due to the confinement of the particles inside the micelle cores. Aqueous colloidal solutions are then obtained and can be easily used as catalysts. 

Fluorinated surfactants can also serve as miceUar stabilizers for nanoparticles in water-in-supercritical CO2 (SCCO2) microemulsions. Recently, Tsang described Ru nanoparticles as catalysts in the presence of ammonium perfluorotetradecano- 

Catalyst Conversion (%) CIAL (%) DHAL (%) DMOL (%) CIOL (%) 

The influence of the Ru environment on the activily and product distribution was investigated. The main products were the 2,3-conjugated C=C (citronellal, CIAL), the fully saturated aldehyde (dihydrodtronellal, DHAL), the unsaturated alcohol (dtronellol, CIOL), and the fully saturated alcohol (3,7-dimethyloctanol, DMOL) (Table 11.3). 

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The catalyst is inactive for the hydrogenation of the (isolated) benzene nucleus and so may bo used for the hydrogenation of aromatic compounds containing aldehyde, keto, carbalkoxy or amide groups to the corresponding alcohols, amines, etc., e.g., ethyl benzoate to benzyl alcohol methyl p-toluate to p-methylbenzyl alcohol ethyl cinnamate to 3 phenyl 1-propanol. 

Rhodium- and cobalt-catalyzed hydrogenation of butadiene and 1-hexene [47, 48] and the Ru-catalyzed hydrogenation of aromatic compounds [49] and acrylonitrile-butadiene copolymers [50] have also been reported to be successful in ionic liquids. 

Silica-supported metal (e.g., Pd/Si02) catalysts also have surface silanol groups that can react with the alkoxysilane groups of the complexes. These combination catalysts consist of a tethered complex on a supported metal. A Rh complex was tethered to the surface of a Pd/Si02 catalyst, and the tethered catalyst was more active for the hydrogenation of aromatic compounds than the free complex or the supported catalyst separately.33 It is possible that the H2 is activated on the supported metal and the hydrogen atoms migrate to the silica, where they react with the reactant molecules coordinated by the tethered complex. 

As of now no details of the synthesis of optically active tritiated compounds produced under microwave-enhanced conditions have been published. Another area of considerable interest would be the study of solvent effects on the hydrogenation of aromatic compounds using noble-metal catalysts as considerable data on the thermal reactions is available [52]. Comparison between the microwave and thermal results could then provide useful information on the role of the solvent, not readily available by other means. 

Catalytic hydrogenation of aromatic compounds proceeds exothermically under atmospheric pressure, without addendum required for keeping the reaction conditions. In contrast to the other media such as compressed hydrogen and liquefied hydrogen, external heat should be provided toward organic chemical hydrides at the site of hydrogen utilization. 

Schiith, C., and M. Reinhard, Hydrodechlorination and hydrogenation of aromatic compounds over palladium on alumina in hydrogen-saturated water , Appl. Catal. B Environ., 18, 215-221 (1998). 

Figure 5. Transformation pathway of lindane to benzene. Reprinted from Applied Catalysis B Environmental, Vol. 18, Schtlth and Reinhard, Hydrodechlorination and Hydrogenation of Aromatic Compounds over Palladium on Alumina in Hydrogen-Saturated Water, pp. 219, Copyright 1998, with permission from Elsevier Science.

Cobalt catalysts are generally known to be less active than nickel catalysts for the hydrogenation of aromatic compounds (see, e.g., Table 11.2).4,5 However, properly prepared reduced cobalt or Raney Co have been reported to be more active than the corresponding nickel catalysts in the hydrogenation of benzene13-15 and naphthalene.15... 

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