Site‐Selective Radical Aromatic C−H Functionalization of Alloxazine and Flavin through Ground‐State Single Electron Transfer

Flavins and their alloxazine isomers are key chemical scaffolds for bioinspired electron transfer strategies. Their properties can be fine‐tuned by functional groups, which must be introduced at an early stage of the synthesis as their aromatic ring is inert towards post‐functionalization. We show that the introduction of a remote metal‐binding redox site on alloxazine and flavin activates their aromatic ring towards direct C−H functionalization. Mechanistic studies are consistent with a synthetic sequence involving ground‐state single electron transfer (SET) with an electrophilic source followed by radical‐radical coupling. This unprecedented reactivity opens new opportunities in molecular editing of flavins by direct aromatic post‐functionalization and the utility of the method is demonstrated with the site‐selective C6 functionalization of alloxazine and flavin with a CF3 group, Br or Cl, that can be further elaborated into OH and aryl for chemical diversification. Selective functionalization of flavin analogues is desirable for property tuning but requires de novo synthesis. A synthetic strategy involving the introduction of a remote redox site followed by copper complexation turns an alloxazine and a flavin into redox‐active structures and activates their aromatic ring for site‐selective aromatic C−H functionalization. This unprecedented mechanism opens new vistas in the chemistry of flavins and analogues.