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

Abstract

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 )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 (H2 O2 ) at 298 K. The catalytic oxidation of benzene by O2 with AcrH2 in the presence of a catalytic amount of Fe(ClO4 )3 is started by the formation of H2 O2 from AcrH2 , O2 , and H(+) . Hydroperoxyl radical (HO2 (.) ) is produced from H2 O2 with the redox pair of Fe(3+) /Fe(2+) by a Fenton type reaction. The rate-determining step in the initiation is the proton-coupled electron transfer from Fe(2+) to H2 O2 to produce HO(.) and H2 O. HO(.) abstracts hydrogen rapidly from H2 O2 to produce HO2 (.) and H2 O. The Fe(3+) produced was reduced back to Fe(2+) by H2 O2 . HO2 (.) reacts with benzene to produce the radical adduct, which abstracts hydrogen from AcrH2 to 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 O2 and the second is oxidation of an NADH analogue with O2 to produce H2 O2 .