Reactivity of semiquinone radical anions of hydroxyperylenequinone with oxygen

Abstract

Hyprocrellin B (HB) could be regarded as a 4,9-perylenequinone derivative bearing two hydroxy group at the 3- and 10- two carbonyl groups. The product of demethylation of the 11-methoxy of HB, HB(11-OH), bears three hydroxy groups at the 3-, 10-positions peri to positions. Illumination of the HB (11-OH) in the presence of an electron donor led to accumulation of the semiquinone radical anion HB − (11-OH). The ESR hyperfine structure was recorded and the UV-visible spectum was investigated. HB − (11-OH) reacted with oxygen with generation of the conjugated mono base HB(11-O −) and hydrogen peroxide. The effect of the position of the hydroxy groups on the reactivity of the semiquinone radical anion with oxygen was investigated by redox potential, acid dissociation constant and NMR measurements. It was indicated that the acidity of the 11-hydroxy group in HB (11-OH) was stronger than that of the groups at 3- and 10-positions. The reaction of HB − (11-OH) with oxygen yields at first the original quinone HB(11-OH) and O 2 −, and then HB (11-OH) transfers the phenolic hydroxy proton at the 11-position to O 2 to yield the conjugated base HB (11-O −) and HO 2 which disproportionates rapidly to H 2O 2 and O 2.