Hydrogenation of aromatic rings

Hydrogenation (Section 11.16) Hydrogenation of aromatic rings is somewhat slower than hydrogenation of alkenes, and it is a simple matter to reduce the double bond of an unsaturated side chain in an arene while leaving the ring intact. 

Finally, hydrogenation of aromatic rings in synthetic or natural polymers such as polystyrene or lignin, respectively, is also investigated for various applications. The polystyrene hydrogenation process developed by Dow Plastics for media applications is an interesting example [7,8]. 

Several ruthenium systems catalyze the hydrogenation of aromatic rings, and this topic is detailed in Chapter 16. An early example reported by Bennett and coworkers was that of RuHCl( 76-C6Me6)(PPh3), which catalyzed the hydrogenation of benzene to cyclohexane at 25 °C, 1 bar H2 [69]. Since ruthenium colloids are very active for this reaction under certain conditions, there is evidence that at least some of the reported catalysts are heterogeneous [70]. 

Acidity or alkalinity of the medium plays a very important role. Hydrogenation of aromatic rings over platinum catalysts requires acid medium. Best results are obtained when acetic acid is used as the solvent. Addition of... 

Hydrogenation and hydrocracking activity of iron catalysts has been extensively investigated using coal and model compounds (93 -95). Iron catalysts can hydrogenerate olefinic unsaturated bonds, while they are known to be less active for the hydrogenation of aromatic rings compared with molybdenum-based catalysts. 

Hydrogenation of aromatic rings will not be described in detail in this review since it is mostly concerned with isolated double bonds. This is however an area of active research, as recently demonstrated by the use of catalytic niobium complexes for the selective hydrogenation of aryl phosphines (Scheme 3)56. 

Up to now, our discussion has focused on interactions involving the 8+ hydrogens of aromatic rings as donor atoms in enthalpically favorable interactions. We shall now turn our attention to the role of the 8 ir-electron cloud as an acceptor. 

To conclude, in a manner complementary to the interactions of oxygens and sulfurs with the hydrogens of aromatic rings, amino group hydrogens can make enthalpically favorable, weak, polar interactions with the tt-electrons of aromatic rings. These interactions are of sufficient importance to cause a marked anisotropy in the distribution of N—H and NH3 groups in the vicinity of aromatic side chains. 

Dovell and Greenfield have shown that base metal sulfides as well as noble metal sulfides are excellent catalysts for the preparation of secondary amines by the reductive alkylation of primary amines (or their nitro precursors) with ketones.36,37 There was little or no hydrogenation of aromatic rings for arylamines, an important side re-... 

Several catalytic hydrogenations of aromatic rings in compounds containing free carboxyl groups are described (cf. method 4). Low-pressure hydrogenation over platinum oxide catalyst has been used. p-Toluic acid in acetic acid at 60° gives 4-methylcyclohexanecarboxylic acid (S>5%). 

Phosphorus has positive effects on isomerization and hydrogenolysis reactions, which require acidic properties. Phosphorus also improves activity for hydrogenation reactions of aromatic rings, especially on NiMoP/Al catalysts. Phosphorus shows minor effects in thiophene HDS, and it may improve the HDS activity of catalysts for heavier sulfur-containing molecules such as those in VGO, in which hydrogenation of aromatic rings may... 

Partial hydrogenation of aromatic rings can also be accomplished with catalysts such as [Co2(CO)g] under an atmosphere of carbon monoxide and hydrogen, but the isolated benzene nucleus cannot be re-duced. ° Anthracene (57), naphthacene, perylene (61) and pyrene (63) are converted to 9,10-dihydroan-thracene (60), 5,12-dihydronaphthacene, 1,2,3,10,11,12-hexahydroperylene (62) and 4,5-dihydropyrene (64), respectively. 

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