Carbocyclic aromatics, reduction

Pyrroles are hydrogenated with more difficulty than are carbocyclic aromatics. In compounds containing both rings, hydrogenation will proceed nonselectively or with preference for the carbocyclic ring (/9), unless reduction of the carbocyclic ring is impeded by substituents. Acidic solvents are frequently used but are not necessary. 

Other Carbocyclic Aromatic Compounds Reduction of Carbocyclic Rings... 

Although hydrogenation of carbocyclic aromatics usually leads to the fully saturated derivative, there is abundant evidence that many reductions proceed through intermediate olefins. Hydrogenation of cre-sols was assumed to proceed through formation of all kinds of 1,2-dihydro-cresols produced in equal amounts (38) and hydrogenation of xylenes to afford equal quantities of all tetrahydroxylenes as intermediates (36). 

Hydrogenation of carbocyclic aromatic compounds requires only mild conditions over Rh catalysts. Rhodium is an outstandingly active catalyst for reduction of benzene. Catalyst efficiency is influenced by trace materials that act as inhibitors or promoters. Hydrogen halides are strong inhibitor for reductions in MeOH over Rh-on-carbon or on alumina. Small amounts of acetic acid promote reduction of aromatics over Rh-on-alumina. In a clean medium, 5% Rh-on-carbon or Rh-on-alumina in MeOH reduces alkyl benzenes at room temperature, under 500 kPa . Reaction is facilitated at higher T or by adding glacial acetic acid. 

Intramolecular arylation of G-H bonds gives cyclic aromatic compounds. In this intramolecular arylation, the carbon-palladium cr-bond is first formed by the oxidative addition of Pd(0) species and then the resulting electrophilic Pd(n) species undergoes the intramolecular G-H bond activation leading to the formation of the palladacycle, which finally affords the cyclic aromatic compounds via reductive elimination.87 For example, the fluoroanthene derivative is formed by the palladium-catalyzed reaction of the binaphthyl triflate, as shown in Scheme 8.88 This type of intramolecular arylation is applied to the construction of five- and six-membered carbocyclic and heterocyclic systems.89 89 89 ... 

Aromatic carbocycles, 9 267-283 Aromatic carboxylic acids, microbial reduction of, 16 402... 

The discovery of the metal-like properties of conducting polymers has once again focused attention on the oxidation and reduction characteristics of aromatic systems. It turns out that most of these conducting materials consist of chainlike connected carbocyclic or heterocyclic aromatics [94-97]. 

Reduction of carbocyclic rings in aromatic ketones can be accomplished by catalytic hydrogenation over platinum oxide or rhodium-platinum oxide and takes place only after the reduction of the carbonyl group, either to the alcoholic group, or to a methylene group [5S]. 

The aromaticity of the imidazole nucleus ensures stability towards reduction, and when benzimidazole (27) is hydrogenated over Adams catalyst in acetic acid the carbocyclic ring is reduced first to give the tetrahydrobenzimidazole (28). However, if the solvent is changed to acetic anhydride, A(-acylation promotes the reduction of the heterocycle and the 1,3-diacetylbenzimidazoline (29) is then formed (Scheme 1). Imidazole (30) under these conditions gives 1,3-diacetylimidazoline (31). Imidazolium salts (32) are easily reduced and treatment with excess sodium borohydride in 95% aqueous ethanol culminates in the formation of 1,2-diamines, (33) or (34). Either N—C bond may cleave, although if the substituent R is benzyl the major products are benzylamines (33 R = Bn). ... 

Internal alkynes will also readily undergo palladium-catalyzed annulation by functionally substituted aromatic or vinylic halides to afford a wide range of heterocycles and carbocycles. However, the mechanism here appears to be quite different from the mechanism for the annulation of terminal alkynes. In this case, it appears that the reaction usually involves (1) oxidative addition of the organic halide to Pd(0) to produce an organopalladium(II) intermediate, (2) subsequent insertion of the alkyne to produce a vinylic palladium intermediate, (3) cyclization to afford a palladacycle, and (4) reductive elimination to produce the cyclic product and regenerate the Pd(0) catalyst (Eq. 28). 

The partial or total hydrogenation (or reduction) of substituted aromatic compounds is an important transformation in synthetic organic chemistry due to the abundance of aromatic molecules found in Nature. In the first case, a diene or an olefin is produced, which is much more reactive than the aromatic precursor, so a new wide range of possible reactivities can be explored. The total saturation of the aromatic ring is of special interest because this is a new entry to functionalized saturated carbocycles that generally show a different reactivity than the parent aromatics. 

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