Phenol is an very important bulk organic compound, having a variety of applications in industry. Phenol is commercially produced through three-step cumene process via cumene hydroperoxide, which is the key intermediate. In addition acetone is produced as a co-product and a-methylstyrene is produced as a byproduct together with phenol. Thus, phenol synthesis from benzene in a one-step reaction with high benzene conversion and high phenol selectivity is highly desired from the viewpoints of an environmentally benign green process and economical efficiency. Extensive efforts have so far been devoted to develop the one-step direct hydroxylation of benzene using various oxidants. we would like to focus on the catalytic mechanisms of selective hydroxylation of benzene to phenol. First, catalytic mechanisms of liquid phase hydroxylation of benzene with H₂O₂ using metal complexes are discussed by identifying the reactive intermediates, which act as strong electrophiles for the benzene hydroxylation. Next, photochemical hydroxylation of benzene with an organic oxidant via benzene radical cation produced in photoinduced electron transfer from benzene to the excited state of the organic oxidant is discussed to clarify the reason of the selective hydroxylation to phenol. The reason why phenol is not further oxidized is discussed based on the Marcus theory of electron transfer. Finally, the photocatalytic hydroxylation of benzene with O₂ to phenol is made possible by using an appropriate organic photocatalyst. The photochemical hydroxylation of benzene with an organic oxidant to phenol is also combined with the catalytic reoxidation of the reduced organic oxidant by O₂ to make the selective hydroxylation of benzene with O₂ to phenol catalytic. How overoxidation of phenol can be avoided is discussed in both the thermal and photochemical catalytic hydroxylation of benzene.

Photochemical hydroxylation of benzene with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and water occurs under visible light irradiation to yield phenol and DDQH₂. The yield of phenol was 99% with 99% conversion (>99% selectivity) [1]. The mechanism of the photochemical hydroxylation of benzene with DDQ was clarified by time-resolved transient absorption spectroscopy. The photochemical reaction was initiated by the photoinduced electron transfer from benzene to ³DDQ^* to produce benzene p-dimer radical cation and DDQ^•−. The free energy change for electron transfer from benzene to ³DDQ^* is largely negative (DG_et = −0.70 eV), as determined from the one-electron oxidation potential of benzene (E_ox = 2.48 V vs. SCE) and the one-electron reduction potential of ³DDQ^* (E_red = 3.18 V vs. SCE). Then, benzene radical cation reacts with H₂O to produce an OH-adduct radical of benzene. Finally, phenol was produced by hydrogen atom transfer from the OH-adduct radical to DDQ^•–.

Photoinduced oxygenation of neat cyclohexane in the presence of O₂ also occurred under visible light irradiation of DDQ which acts as a super photooxidant. The products detected by GC and NMR analyses were cyclohexanol, cyclohexanone and cyclohexane hydroperoxide with 70, 40, 210% yields, respectively. When cyclohexane was replaced by n-butane, n-pentane and 3-methylpentane, the oxygenation reactions also took place to form the corresponding oxygenated products under the otherwise same photochemical reaction conditions [2-5].

[1] Ohkubo, K.; Fujimoto, A.; Fukuzumi, S. J. Am. Chem. Soc. 2013, 135, 5368.

[2] Ohkubo, K.; Hirose, K.; Fukuzumi, S. Chem.–Eur. J. 2015, 21, 2855.

[3] Ohkubo, K.; Hirose, K.; Fukuzumi, S. Photochem. Photobiol. Sci. 2016, 15, 731.

[4] Ohkubo, K.; Hirose, K.; Fukuzumi, S. Chem. Asia J. 2016, 11, 2218.

[5] Fukuzumi, S.; Ohkubo, K. Asian J. Org. Chem. 2015, 4, 836 (Focus Review).