Dedicated to Professor František Liška on the occasion of his 80th birthday. Flavinium salts are frequently used in organocatalysis but their application in photoredox catalysis has not been systematically investigated to date. We synthesized a series of 5-ethyl-1,3dimethylalloxazinium salts with different substituents in the positions 7 and 8 and investigated their application in lightdependent oxidative cycloelimination of cyclobutanes. Detailed mechanistic investigations with a coumarin dimer as a model substrate reveal that the reaction preferentially occurs via the triplet-born radical pair after electron transfer from the substrate to the triplet state of an alloxazinium salt. The very photostable 7,8-dimethoxy derivative is a superior catalyst with a sufficiently high oxidation power (E* = 2.26 V) allowing the conversion of various cyclobutanes (with E ox up to 2.05 V) in high yields. Even compounds such as all-trans dimethyl 3,4-bis (4-methoxyphenyl)cyclobutane-1,2-dicarboxylate can be converted, whose opening requires a high activation energy due to a missing pre-activation caused by bulky adjacent substituents in cis-position.
The Cover Feature shows the photocatalytic oxidative cycloelimination of a coumarin dimer by a flavinium salt inspired by the light‐induced photolyase DNA repair reaction in nature. Light absorption and subsequent intersystem crossing (isc) enable diffusion‐controlled electron transfer (eT) from the coumarin dimer to the excited triplet flavinium salt. The oxidized coumarin dimer dissociates and back electron transfer (beT) completes the photocatalytic cycle. More information can be found in the Full Paper by R. Cibulka, R. Kutta, and co‐workers.
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