Ring hydrogenation preferential

Alkyl Side Chains of Aromatic Rings. The preferential position of attack on a side chain is usually the one a to the ring. Both for active radicals such as chlorine and phenyl and for more selective ones such as bromine such attack is faster than that at a primary carbon, but for the active radicals benzylic attack is slower than for tertiary positions, while for the selective ones it is faster. Two or three aryl groups on a carbon activate its hydrogens even more, as would be expected from the resonance involved. These statements can be illustrated by the following abstraction ratios ... 

In pyridylpyrrole derivatives the pyridine ring was hydrogenated preferentially, giving piperidylpyrroles, which were further hydrogenated to piper-idylpyrrolidines over platinum oxide in acetic acid at room temperature and 2 atm [453]. 

Pyrazole was hydrogenated over palladium on barium sulfate in acetic acid to 4,5-dihydropyrazole (A -pyrazoline), and 1-phenylpyrazole at 70-80° to 1-phenylpyrazolidine [478]. In benzopyrazole (indazole) and its homologs and derivatives the six-membered ring is hydrogenated preferentially to give... 

The effect of hydrogen on naphthenic hydrocarbons is mainly that of ring scission followed by immediate saturation of each end of the fragment produced. The ring is preferentially broken at favored positions, although generally all the carbon-carbon bond positions are attacked to some extent. For example, methyl-cyclopentane is converted (over a platinum-carbon catalyst) to 2-methylpentane, 3-methylpentane, and n-hexane. 

The selectivity observed in the hydrogenation of pyrroles is strongly dependent on the catalyst and conditions used for the reactions. The pyrrole ring in nicotyrine, 42, was hydrogenated preferentially over palladium in acetic acid, 7 but over platinum the pyridine ring was saturated first (Eqn. 17.42), 48,149 K.eto and ester groups on an N-unsubstituted pyrrole were... 

When the methosulfate of l-(2-pyridyl)-2-propanone was hydrogenated the pyridine ring was preferentially reduced to give A-methyl-isopelletierine (130). 

Ring hydrogenation of furans—Reductive furan ring opening—Preferential hydrogenation of carbon-carbon double bonds s. 16, 86... 

Selective ring hydrogenation of nitroarenes Partial ring hydrogenation s. 18, 80 Hydrogenation of cyclimmonium salts selective and preferential s. 16, 53 ... 

Palladous chloride/sodium tetrahydridoborate Preferential stereospecific ring hydrogenation with catalyst prepared in situ... 

Preferential replacement of methoxyl by hydrogen with partial benzene ring hydrogenation... 

Ring hydrogenation of isocyclic amines preferential s. 12, 106 Ring hydrogenation of steroids... 

Excited states of enones such as 100 can abstract hydrogen preferentially from the a position of alkyl ether or alkyl amines leading to the biradical intermediates V. Intramolecular addition to the benzene ring leads to dihydropyran or dihydropyridin derivatives 101 (Scheme 17). Dibenzofuran derivatives 103 were obtained from polyfluorinated aryloxodihydronaphthalene compounds such as 102. Probably, in such cases, a photochemical electron transfer from the nonfluorinated aryl substituent to the fluori-nated moiety takes place. When compound 104 was irradiated in the presence of HCIO4, the isoquinoline derivative 105 was isolated in good yields. ... 

Hydrogenation with Raney catalysts Ring hydrogenation of furans Reductive furan ring opening Preferential hydrogenation of carbon-carbon doubie bonds... 

Hydrogenation of 2-naphthol can proceed at either ring with the general tendency to reduce the unsubstituted ring preferentially. The ratio R of... 

Interestingly, when R1 and R2 are hydrogens, the -configurated amino nitriles 1 arc obtained, whereas one or two methoxy substituents on the aromatic ring leads to (S)-diastereomers. This surprising effect is caused by the preferential crystallization of the (R)- or the (.S )-diastereomers, respectively. If the pure diastereomers of 1 are dissolved in methanol, equilibration occurs. On concentration, the optically pure diastereomer again crystallizes from the solution45. 

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