Reactivities of 1,2-, 1,3-, and 1,4-dihydroxynaphthalenes toward electrogenerated superoxide in N,N-dimethylformamide through proton-coupled electron transfer

We have carried out an electrochemical and theoretical study on the reactivity of 1,2-, 1,3-, and 1,4-dihydroxynaphthalenes (1nH(2)NQ, n = 2, 3, 4) toward electrogenerated superoxide radical anion (O-2(center dot-)) in N,N-dimethylformamide. Cyclicvoltammetry and in situ electrolytic electron spin resonance measurements revealed that the quinone-hydroquinone pi-conjugation plays an important role in a successful O-2(center dot-) scavenging by 12H(2)NQ and 14H(2)NQ through proton-coupled electron transfer (PCET) reaction. The reactivities of 12H(2)NQ and 14H(2)NQ toward O-2(center dot-) were mediated by the ortho- (catechol) or para-diphenol (hydroquinone) moieties, as experimentally confirmed in comparative analyses with catechol, hydroquinone, and 13H(2)NQ, aided by density functional theory (DFT) calculations. The electrochemical and DFT results suggested that a concerted PCET mechanism involving two-proton transfers and one-electron transfer proceeds, demonstrating a successful O-2(center dot-) scavenging by 12H(2)NQ and 14H(2)NQ. Furthermore, a subsequent electron transfer between molecular dioxygen and product-naphthoquinone-radicals was observed, where O-2(center dot-) was regenerated. The DFT analysis suggested that the spin distribution on the planar naphthalene ring embodies the superior kinetics of the PCET and the subsequent generation of O-2(center dot-) from dioxygen demonstrated in the electrochemical results.

Automated summary

In this study, the reactivity of 1,2-, 1,3-, and 1,4-dihydroxynaphthalenes towards electrogenerated superoxide radical anion was investigated. The results showed that proton-coupled electron transfer reaction played an important role in scavenging the superoxide radical anion. The position of the quinone moiety mediated the reactivity. Additionally, a subsequent electron transfer between molecular dioxygen and product-naphthoquinone-radicals was observed, which regenerated the superoxide radical anion.