Ring hydrogenolysis

Heterocyclic rings do not undergo hydrogenolysis readily unless they are strained, are activated by unsaturation, or contain a weak bond, e.g., N—O, (38,61,114), but these exceptions constitute a wide variety of compounds, whose reductions have proved very useful in syntheses. 

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Aromatic rings are hydrogenated with a variety of catalysts. However, aromatic alkoxy and hydroxyl substituents are susceptible to hydrogenolysis under most conditions used to saturate the ring. Hydrogenolysis does not occur to any appreciable extent with ruthenium catalysts even though high temperatures and pressures are required. Thus, substituted phenols are... 

The influence of alkyl substituents on the ease of ring hydrogenolysis depends on ring size. Thus, over a platinum/pumice catalyst at 100°C,... 

It was replaced later by the sextet-doublet mechanism (106), in which physically adsorbed cyclopentane lies parallel to the surface (Fig. lb) (107, 108). In the transition state, the five carbon atoms of the ring are located over the interstices of the (111) plane of platinum. The distance between two contiguous interstices allows them to accommodate two consecutive carbon atoms, but one C-C bond in cyclopentane is necessarily stretched, and this favors the hydrogenolysis of the ring. In contrast, all the carbon atoms of cyclohexane or paraffins may be brought in contact with the metal surface, which would explain the selectivity of platinum for Cj-ring hydrogenolysis. In the first presentation of the model, the relative inertness of the tertiary-secondary C-C bonds in substituted cyclopentanes was explained by some kind of steric hindrance. 

Hydrogenation of N-heterocycles -Hydrogenation of arene rings -Hydrogenolysis and cleavage of C-N bonds... 

The main reaction in the conversion of toluene and ethylbenzene is hydrogenolysis of the side chain producing benzene from the former and a mixture of benzene and toluene from the latter. However, in addition hydrogenation of the benzene nucleus, isomerization of the six- to the five-membered ring, hydrogenolysis of the latter, and isomerization of the alkanes formed takes place. 

The relative content in the catalysates of the products of partial and complete isopropylbenzene dealkylation is illustrated by the experiment with 20 % Ni-AUOs at 465° 25 atmospheres and 0.8 space velocity. In this case the catalysate contained 32% benzene, 20.2% toluene, and 12.8% ethylbenzene. Besides this main reaction, other conversions also take place under these conditions namely, hydrogenation of the benzene ring to the hexamethylene cycle, contraction of the latter to a five-membered ring, hydrogenolysis of the five-membered ring with the formation of alkanes, partial isomerization of isopropylbenzene to w-propylbenzene and methyla-tion of the benzene nucleus by methylene radicals arising from partial decomposition of the cyclanes. 

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