Photocatalytic hydroxylation of benzene to phenol with dioxygen using sodium decatungstate

Abstract

A one-pot hydroxylation reaction, in which benzene is converted to phenol using oxygen under mild conditions has attracted attention as an environmentally benign alternative to the cumene method. Herein, we report on the photocatalytic hydroxylation of benzene to phenol with O2 by using sodium decatungstate (W10O324−) in air atmosphere (1 atm) and at low temperature (10 °C). W10O324− was active for the benzene hydroxylation process in pure water, which achieved a ca. 16–23% phenol yield with a ca. 61–70% selectivity within 60 min. The phenol yield and the selectivity were improved through the addition of acetic acid to the reaction solution. This enhancement was due to the improved benzene solubility and suppressed benzene vaporization and phenol overoxidation. The effect of the reaction conditions on the phenol formation was investigated in detail. Under anaerobic conditions, the reduced form of W10O324− was generated, with the simultaneous formation of phenol, whereas phenol was not formed in the reoxidation of the reduced decatungstate with O2. According to a kinetic isotopic effect, the C−H bond activation process in benzene hydroxylation was dominated by electron transfer (KIE, kH/kD = 1.0). The reaction mechanism for hydroxylation of benzene to phenol was also discussed.