Flavin Photocatalysts for Visible‐Light [2+2] Cycloadditions: Structure, Reactivity and Reaction Mechanism

New photocatalysts from the flavin family were found to mediate the [2+2] photocycloaddition reaction. 3‐Butyl‐10‐methyl‐5‐deazaflavin (3 a) and 1‐butyl‐7,8‐dimethoxy‐3‐methylalloxazine (2 e), if irradiated by visible light, were shown to allow an efficient (Φ≈3–10 %) intramolecular cyclisation of various types of substrates including substituted styrene dienes and bis(aryl enones), considered as electron‐rich and electron‐poor substrates, respectively, without any additional reagent. The versatility of the procedure was demonstrated by the cyclisation of photosensitive cinnamyl (E)‐3‐iodoallyl ether. Structure–activity studies found alloxazine 2 e was more active than 7‐monosubstituted (R=Cl, Br and MeO) alloxazines. The introduction of chlorine and bromine atom on the deazaflavin skeleton did not enhance the catalytic efficiency of 3 a. A detailed electrochemical and spectroscopic study explains the reaction mechanism proceeding through energy transfer from the flavin excited triplet state to the diene followed by its cyclisation. New flavins useful for photocatalysis! New flavin‐type photocatalysts, alloxazines and deazaflavins, which are no riboflavin derivatives, are demonstrated to be suitable dyes for [2+2] cycloadditions enabling reactions both of electron‐poor and electron rich‐substrates without any additives through an energy‐transfer mechanism.