Aromatic rings metal-catalyzed hydrogenations

Iridium nanopartides also catalyze the hydrogenation of benzyhnethylketone, with high selectivity in reduction of the aromatic ring (92% selectivity in saturated ketone, 8% in saturated alcohol at 97% benzylmethylketone conversion). This preferential coordination of the aromatic ring can be attributed to steric effects that make carbonyl coordination difficult. Therefore, metallic iridium nanoparticles prepared in ILs may serve as active catalysts for the hydrogenation of carbonyl compounds in both solventless and biphasic conditions. 

The Birch reduction comprises a means for adding two hydrogen atoms to an aromatic ring by means of a metal, most often lithium, and an alcohol in liquid ammonia as solvent. A co-solvent, often tetrahydrofuran (THF), is often added due to the very poor solubility of steroids in ammonia. The use of the more expensive sodium was at one time precluded because traces of iron in that metal catalyzed the conversion of the metal to the strong base sodium amide. Very pure sodium, free of iron impurities, is now used for commercial-scale reductions. 

PEI-RhCl3 and PEI-RUCI3 complexes exhibit different catalytic activity with regard to a mixture of the cis- and trans isomers of pentadiene [61]. The former complex is more selective for pentene (0.94) than the latter. A quantitative aniline yield results from the reduction of nitrobenzene in the presence of PEI complexes with Ni(II), Co(ll), Sn(II), Pd(II) and Rh(III) [61]. The reduction proceeds rapidly at 20- 70°C and at 1-25 atm pressure of H2 both in a solvent and without. Besides aniline, cyclohex-ylamine is produced by further hydrogenation of the aromatic ring in the presence of the PEI-Rh(III) complex. Polymer-metal catalysts do not lose their catalytic stability after repeated application. For example, when eight hydrogenation reactions were catalyzed with PEI-Pd(II), in each case a 100% product yield of aniline was attained. 

In the frames of this chapter we are going to focus on the features, scope, and limitations of the C-H functionalizations which are associated with activation of an aromatic ring for a direct nucleophilic attack and are free of any metal catalysis. This relatively new synthetic methodology is based on direct displacement of hydrogen (Sn ) in ur-deficient aromatic compounds by action of nucleophilic reagents. Cross-coupling C-H functionalizations of aromatic compounds dealing with metal-catalyzed activation of the C-H bond are well reflected in the literature [8, 9, 50, 51], and, therefore, they are beyond of consideration in this book. 

Acid-catalyzed reactions of aromatics with monoolefins result in nuclear alkylation. But the base-catalyzed reactions of aromatics with olefins do not result in nuclear alkylation as long as benzylic hydrogens are available. This is true even with aromatics, such as cumene, which have deactivated benzylic hydrogens resulting in facile metalation of the ring. Apparently phenyl carbanions do not readily add to olefins. Pines and Mark (20) found that in the presence of sodium and promoters only small yields of alkylate were produced at 300° in reactions of benzene with ethylene and isobutylene and of t-butylbenzene with ethylene. With potassium, larger yields may be obtained at 190° (24)-... 

Aromatic hydrocarbons with two or more rings are hydrogenated in a stepwise manner permitting regioselective saturation of one of the rings. Individual metals have a marked influence on selectivity. Biphenyl, for example, is transformed to cyclohexylbenzene with 97% selectivity on palladium, which is the least active to catalyze saturation of benzene under mild conditions.11 12 Even better selectiv-ities are achieved in transfer hydrogenation.105... 

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