Solvent‐Free One‐Step Photochemical Hydroxylation of Benzene Derivatives by the Singlet Excited State of 2,3‐Dichloro‐5,6‐dicyano‐p‐benzoquinone Acting as a Super Oxidant

Abstract

Photoinduced hydroxylation of neat deaerated benzene to phenol occurred under visible-light irradiation of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), which acts as a super photooxidant in the presence of water. Photocatalytic solvent-free hydroxylation of benzene derivatives with electron-withdrawing substituents such as benzonitrile, nitrobenzene, and trifluoromethylbenzene used as neat solvents has been achieved for the first time by using DDQ as a super photooxidant to yield the corresponding phenol derivatives and 2,3-dichloro-5,6-dicyanohydroquinone (DDQH2 ) in the presence of water under deaerated conditions. In the presence of dioxygen and tert-butyl nitrite, the photocatalytic hydroxylation of neat benzene occurred with DDQ as a photocatalyst to produce phenol. The photocatalytic reactions are initiated by oxidation of benzene derivatives with the singlet and triplet excited states of DDQ to form the corresponding radical cations, which associate with benzene derivatives to produce the dimer radical cations, which were detected by the femto- and nanosecond laser flash photolysis measurements to clarify the photocatalytic reaction mechanisms. Radical cations of benzene derivatives react with water to yield the OH-adduct radicals. On the other hand, DDQ(.) (-) produced by the photoinduced electron transfer from benzene derivatives reacts with the OH-adduct radicals to yield the corresponding phenol derivatives and DDQH2 . DDQ is recovered by the reaction of DDQH2 with tert-butyl nitrite when DDQ acts as a photocatalyst for the hydroxylation of benzene derivatives by dioxygen.

Photocatalytic oxidation of benzene to phenol using dioxygen as an oxygen source and water as an electron source in the presence of a cobalt catalyst

“…16 In addition, the catalytic aerobic oxidation of benzene without sacrificial reducing agents has so far required harsh conditions, such as high temperatures or UV light photoirradiation. 14,15,17,18 Moreover, although the photocatalytic hydroxylation of benzene to phenol using organic photocatalysts without overoxidation has been demonstrated, 19–22 the turnover numbers (TONs) of the catalytic hydroxylation of benzene to phenol are still low ( e.g. , 13).…”

One-step selective hydroxylation of benzene to phenol with hydrogen peroxide catalysed by copper complexes incorporated into mesoporous silica–alumina

“…1,2 Thus, much effort has been devoted to develop the one-step hydroxylation of benzene to phenol using different oxidants, such as N 2 O, 3 O 2 with reductants 4–7 and H 2 O 2 . 8–22 Among these oxidants, H 2 O 2 has a clear advantage from the viewpoint of an environmentally benign green process and economical efficiency because of relatively mild reaction conditions for catalytic hydroxylation of benzene.…”

Teaching Old Compounds New Tricks: DDQ‐Photocatalyzed C−H Amination of Arenes with Carbamates, Urea, and N‐Heterocycles

“…Electron‐deficient arenes with very high oxidation potentials (e.g. ethyl benzoate, benzonitrile) are beyond the substrate scope. The electron transfer rate to triplet DDQ is known to be small for these substrates, which makes the nucleophilic trapping inefficient.…”