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 (EH2PQ+·/H2PQ* = −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.