Hydrogenation of Simple Alkenes and Arenes

The hydrogenation of simple alkenes, such as hexene, cyclohexene, cyclo-hexadiene and benzene, has been extensively studied using biphasic, alternative solvent protocols. These hydrocarbon substrates are more difficult to hydrogenate compared to substrates with electron withdrawing groups. Benzene and alkyl substituted aromatic compounds are considerably more difficult to hydrogenate 

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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. 

Hydrocarbons contain only hydrogen and carbon. The hydrocarbon functional groups include alkanes, alkenes, alkynes, and arenes (aromatic compounds). Simple hydrocarbons have few medicinal applications, but are the feedstock of the petrochemical industry to produce plastics, dyes, solvents, detergents, and adhesives (to name just a few). Therefore, hydrocarbons are essential to the medical field. Additionally, all hydrocarbons are flammable and, therefore, find application as fuels. For example, gasoline is a mixture of hydrocarbons. 

The transition-metal NPs dispersed in imidazolium ILs are active catalysts for the hydrogenation of alkenes, arenes and ketones (Table 6.3). Moreover, Pd(0) NPs are active catalyst precursors for C—C coupling reactions, serving as reservoirs of mononuclear catalytically active species. In most cases, the catalytic reactions are typically multiphase systems in which the NPs dispersed in the ILs form the denser phase and the substrate and product remain in the upper-phase. In these cases the ionic catalytic solution is easily recovered by simple decantation and can be reused several times without any significant loss in catalytic activity. 

Second, despite the generalization above, specihc conditions required for reduction may well vary from one alkene to the next. It is particularly worthy to note that most arenes (in accord with their extraordinary stability and as thus expected) can only be reduced with difficulty. In contrast to simple alkenes, which can often be reduced at or near 1 atm of hydrogen in the presence of a Pt or Pd catalyst, benzene requires high pressures and temperatures (e.g., powdered Ni, 40 atm H2, 200°C) (Equation 3.3). Further, the reduction of alkynes to (Z)-alkenes (addition of hydrogen [H2] being suprafacial) frequently cannot be stopped at the alkene stage and complete reduction to alkane occurs. Special poisoned catalysts have been developed, which frequently succeed in stopping the reaction. (Equation 3.4). 

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