One-Step Selective Hydroxylation of Benzene to Phenol

Recent developments in the one-step thermal and photochemical catalytic hydroxylation of benzene to phenol using various oxidants are described, focusing on the catalytic mechanisms. Hydroxylation of benzene with hydrogen peroxide is catalyzed by homogeneous metal complexes and heterogeneous metal oxides to produce phenol when high-valent metal-oxo complexes are reactive species The selective hydroxylation of benzene to phenol is achieved by using mesoporous silica-alumina as the catalyst support, which prohibits further oxidation of phenol due to the strongly acidic sites. Photocatalytic oxidation of benzene with dioxygen and water is made possible by using organic photocatalysts via photoinduced electron transfer from benzene to the excited states of the photocatalysts, followed by the hydroxylation of the benzene radical cation with water to yield phenol selectively. The further oxidation of phenol is prohibited because of the fast back electron transfer from the one-electron reduced species of the photocatalyst to the phenol radical cation, whereas back electron transfer from the one-electron reduced species to the benzene radical cation is slowed down because of the large driving force of the back electron transfer, which is in the Marcus inverted region.