Long-range intramolecular photoredox reaction via coupled charge and proton transfer of triplet excited anthraquinones mediated by water

Abstract

The formal intramolecular photoredox reaction initially discovered for the parent 2-(hydroxymethyl)anthraquinone (1) has been extended to include analogs 3–6 in which the oxidizable benzyl alcohol group is significantly further away from the anthraquinone moiety. All of 3–6 undergo a clean and efficient formal intramolecular photoredox reaction in water catalyzed by acid (Φ = 0.1–0.6), in which the alcohol is oxidized to aldehyde and the anthraquinone is reduced to dihydroxyanthracene. The need for water, observation of acid catalysis, unimolecularity of reaction in anthraquinone, AM1 calculations and LFP studies support a mechanism involving a highly polarized triplet excited state in which the electron density of the distal phenyl moieties is transferred to the central anthraquinone ring, which is subsequently trapped adiabatically by protonation at the anthraquinone carbonyl oxygen and deprotonation at the benzyl C–H.