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

Abstract

Hydroxylation of benzene by molecular oxygen (O-2) occurs efficiently with 10-methyl-9,10-dihydroacridine (AcrH(2)) as an NADH analogue in the presence of a catalytic amount of Fe(ClO4)(3) or Fe(ClO4)(2) with 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 O-2 with AcrH(2) in the presence of a catalytic amount of Fe(ClO4)(3) is started by the formation of H2O2 from AcrH(2), O-2, and H+. Hydroperoxyl radical (HO2 center dot) is produced from H2O2 with 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 H2O2 to produce HOC and H2O. HOC abstracts hydrogen rapidly from H2O2 to produce HO2 center dot and H2O. The Fe3+ produced was reduced back to Fe2+ by H2O2. HO2 center dot reacts with benzene to produce the radical adduct, which abstracts hydrogen from AcrH(2) to give the corresponding hydroperoxide, accompanied by generation of acridinyl radical (AcrH(center dot)) to constitute the radical chain reaction. Hydroperoxyl radical (HO2 center dot), 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 O-2 and the second is oxidation of an NADH analogue with O-2 to produce H2O2.