Hydroquinone/Quinone Cycle for Reductive Photocatalytic Transformations

Phenanthrenehydroquinones (H₂PQs), generated in situ from phenanthrenequinones under visible light using alcohols as hydrogen sources, act as catalytic organic photoreductants. They deoxygenate efficiently diverse N-heterocyclic N-oxides, including substrates having more negative reduction potentials than the H₂PQ excited-state reduction potential ($E_{H_2{PQ}^{+·}/H_2{PQ}^{*}}$ = -2.10 V vs SCE), suggesting hydrogen bonding assisted electron transfer. TD-DFT calculations indicate H₂PQs act as both photoacids and photoreductants. This work demonstrates the harnessing of the hydroquinone/quinone cycle in visible-light-driven reductions.