Catalytic Hydroxylation of Benzene to Phenol by Dioxygen with an NADH Analogue

Hydroxylation of benzene by molecular oxygen (O₂) occurs efficiently with 10-methyl-9,10-dihydroacridine (AcrH₂) as an NADH analogue in the presence of a catalytic amount of Fe(ClO₄)₃or Fe(ClO₄)₂with excess trifluoroacetic acid in a solvent mixture of benzene and acetonitrile (1:1 v/v) to produce phenol, 10-methylacridinium ion and hydrogen peroxide (H₂O₂) at 298 K. The catalytic oxidation of benzene by O₂with AcrH₂in the presence of a catalytic amount of Fe(ClO₄)₃is started by the formation of H₂O₂from AcrH₂, O₂, and H⁺. Hydroperoxyl radical (HO₂^.) is produced from H₂O₂with the redox pair of Fe³⁺/Fe²⁺by a Fenton type reaction. The rate-determining step in the initiation is the proton-coupled electron transfer from Fe²⁺to H₂O₂to produce HO^.and H₂O. HO^.abstracts hydrogen rapidly from H₂O₂to produce HO₂^.and H₂O. The Fe³⁺produced was reduced back to Fe²⁺by H₂O₂. HO₂^.reacts with benzene to produce the radical adduct, which abstracts hydrogen from AcrH₂to give the corresponding hydroperoxide, accompanied by generation of acridinyl radical (AcrH^.) to constitute the radical chain reaction. Hydroperoxyl radical (HO₂^.), which was detected by using the spin trap method with EPR analysis, acts as a chain carrier for the two radical chain pathways: one is the benzene hydroxylation with O₂and the second is oxidation of an NADH analogue with O₂to produce H₂O₂.