Regioselective Catalytic Hydrogenation of Polycyclic Aromatic Hydrocarbons under Mild Conditions

Article abstract of DOI:10.1021/jo01302a010

Hydrogenation of polynuclear hydrocarbons over a palladium catalyst at low pressure and ambient temperature affords regiospecifically the corresponding K-region dihydroarenes, while analogous reactions over a platinum catalyst take place regioselectively on terminal rings to provide the related tetrahydroarenes.Hydrogenation over palladium of phenanthrene, benzanthracene, 7,12-dimethylbenzanthracene, benzpyrene, 3-methylcholanthrene, dibenzanthracene, and chrysene gave 9,10-dihydrophenanthrene, 5,6-dihydrobenzanthracene, 5,6-dihydro-7,12-dimethylbenzanthracene, 4,5-dihydrobenzopyrene, 11,12-dihydro-3-methylcholanthrene, 5,6-dihydrodibenzanthracene, and 5,6-dihydrochrysene, respectively (Table I).Hydrogenation over platinum of benzanthracene, 7-methylbenzanthracene, 12-methylbenzanthracene, 7,12-dimethylbenzanthracene, benzopyrene, chrysene, dibenzanthracene, 5,6-dihydrobenzanthracene, 5,6-dihydro-7,12-dimethylbenzanthracene, and 4,5-dihydrobenzopyrene furnished the corresponding terminal ring tetrahydroarenes (Table II).Partial hydrogenation beyond the dihydro stage in the presence of palladium was exhibited only by hydrocarbons with more than one K region.In these cases, the second stage of hydrogen addition was generally slower than the first, so that the extent of reaction was readily controllable.Hydrogenation was blocked by alkyl substitution in an otherwise susceptible ring, and regioselectivity was diminished or abolished by increased hydrogen pressure, prolonged reaction, or acidity.The mechanism of reaction over palladium is proposed to involve concerted hydrogen addition to the K region, the region of minimum bond delocalization energy, preceded by localized ? and/or ? complexes.Evidence is presented that hydrogenations over platinum do not involve initial addition to the K region followed by isomerization into the terminal ring.Instead, these reactions are suggested to involve addition of 2 mol of hydrogen via an intermediate ? complex to the terminal ring which affords the most thermodynamically favored tetrahydroarene product, i.e., that which requires the minimum amount of energy for its formation.