Efficient Oxidative Aromatization of 9,10-Dihydroanthracenes with Molecular Oxygen Catalyzed by Ruthenium Porphyrin Complex

Abstract

In the presence of a catalytic amount of a ruthenium porphyrin complex, various 9,10-dihydroanthracene derivatives were aromatized with molecular oxygen to the corresponding anthracenes. It was found that the addition of sulfuric acid accelerated the aromatization at room temperature under atmospheric pressure of oxygen to afford various anthracenes in high yields.

Key words

catalysis - complexes - oxidations - oxygen - ruthenium

References

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8

Typical procedure: To a suspension of sulfuric acid (136 mg, 1.4 mmol) in benzene (1 mL) was added a benzene (6 mL) solution of Ru(tmp)(O)₂ (0.6 mg, 0.007 mmol) under an oxygen atmosphere. A benzene (7 mL) solution of 9,10-dihydroanthracene (25 mg, 0.14 mmol) was then added and the mixture was stirred for 83 hours at room temperature. The reaction mixture was directly evaporated and purified by silica gel column chromatography (hexane) to give the aromatized anthracene in quantitative yield.

12

By FAB-Mass measurement of the dioxoruthenium(VI)porphyrin complex and the mixture of the complex and sulfuric acid, the peak at MW 783, assigned as tetramesitylporphyrin ligand, was not observed. Several ruthenium salts, such as 5 mol% of ruthenium(III) chloride, ruthenium(II) tris(triphenylphosphine)dichloride, or tetrapropylammonium perruthenate were employed for the reaction at 50 °C for 5 hours in benzene in the presence of sulfuric acid (the same conditions as entry 7 in Table [1] ), and the aromatized product was obtained in 1%, 14%, and 1% yields, respectively. Therefore, it is reasonable to consider that the ruthenium salt released from porphyrin complex could scarcely contribute to the present aromatization reaction.