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

Abstract

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.

Robust Photocatalytic Method Using Ethylene‐Bridged Flavinium Salts for the Aerobic Oxidation of Unactivated Benzylic Substrates

“…However, methoxy functionalities have a positive effect on the properties of the flavin photocatalyst causing its high stability, while maintaining its highly positive excited state reduction potential. A similar effect has already been described among the neutral alloxazines used in energy transfer cycloadditions, [28] 5‐ethylalloxazinium salts used in [2+2] cycloelimination, [29] and acridinium salts used in various photooxidative processes [30] . We plan to further investigate this effect among flavin derivatives and to evaluate the performance of stable and powerful salt 1 a in other photooxidative processes.…”

Visible‐Light‐Induced Di‐π‐Methane Rearrangement of Dibenzobarrelene Derivatives

“…But based on our results with the representative substrate 1 d , an electron‐transfer reaction can be excluded because the DPM rearrangement of this compound, like the parent compound, is not induced by those catalysts that are known to generate intermediate radical ions upon photoinduced electron transfer process, such as DDQ [E 0 (DDQ*/DDQ − −)≈3.1 V], 3 [E 0 ( 3 + */ 3 − )≈2.9 V], Eosin Y [E 0 (EY*/EY − −)≈0.8 V], and [Ru(bpy) 3 ]Cl 2 [E 0 (Ru II*/Ru III)≈−0.8 V; E 0 (Ru II*/Ru I)=+0.77 V] . In contrast, the catalysts Ir(ppy) 3 [E 0 (Ir III*/Ir IV)≈−1.7 V], Ir(dFppy) 3 [E 0 (Ir III*/Ir IV)≈−1.4 V] and flavin 4 [E 0 ( 4 */ 4 − −)≈1.0 V, E 0 ( 4 */ 4 − + )=−0.6 V], that have been shown already to be efficient photosensitizers for olefins, e. g. in [2+2] photocycloadditions or E – Z isomerizations, also turned out to sensitize the DPM rearrangement of dibenzobarrelene 1 d . Although the catalysts Ir(ppy) 3 ( E T =243 kJ/mol), 4 ( E T =220 kJ/mol) and Ir(dFppy) 3 ( E T =266 kJ/mol; Figure ), have somewhat lower triplet energies, E T , than those sensitizers that are commonly applied for UV‐light induced DPM rearrangements, i. e. benzophenone ( E T =287 kJ/mol) or thioxanthen‐9‐one ( E T =265 kJ/mol), they are apparently still able to act as triplet sensitizer for this reaction, indicating a triplet energy of 1 d in this energy range.…”

Molecular Editing of Flavins for Catalysis

“…63 They have also been applied as catalysts in [2+2]-photocycloadditions. 64 In 2020, König, Cibulka, Kutta, and co-workers reported deazaflavin 24, which was successfully applied in one-electron reductions under irradiation resulting in dehalogenation of aryl halide substrates (Scheme 18A). 65 These results are remarkable since their benchmark substrate 4-bromoanisole is very resistant towards reduction: E red = -2.75 V (vs. SCE).…”