Flavin Dibromide as an Efficient Sensitizer for Photooxidation of Sulfides

Abstract

Flavin derivatives (FLs) are the building blocks and functional groups within many enzymes that absorb strongly in the visible light region and are redox cofactors in a large number of biological processes. We directly attached Br atoms into the conjugated framework of FL to afford FL dibromide (DBFL) and expected the heavy atom effect of Br to facilitate the intersystem crossing of excited FLs to reach the triplet states for efficient sensitization of O2. Compared with FL (ε = 1.01 × 104 M–1 cm–1 at 441 nm), DBFL shows stronger absorption in the visible range (ε = 1.90 × 104 M–1 cm–1 at 450 nm). The singlet oxygen quantum yield of DBFL is enhanced from 55.3% in FL to 92.2% at the expense of decreased luminance quantum yield from 37.7% in FL to 5.5%, confirming that a large portion of the excited DBFL molecules evolves into triplet excited states. Both FL and DBFL were used in photosensitized oxidation of various sulfides to afford corresponding sulfoxides. DBFL exhibits a twofivefold performance enhancement with respect to FL in sensitizing O2 for photocatalytic oxidation. In addition, the oxidation of sulfides with DBFL was found efficient and led exclusively to sulfoxides, with no secondary oxidation products observed. Mechanistic investigations showed that both singlet oxygen and superoxide anion radical are formed as reactive oxygen species. The findings pave the way for design and application of novel organic sensitizers for photocatalytic oxidation.

Electrochemistry Broadens the Scope of Flavin Photocatalysis: Photoelectrocatalytic Oxidation of Unactivated Alcohols

“…[11] Fore xamples,r iboflavin tetraacetate (RFT) is ar eadily available compound with an oxidation potential of + 1.67 V( vs.S CE) upon photoirradiation (n max = 440 nm). This characteristic has allowed RFT and its derivatives to function as photocatalysts in the aerobic oxidation of benzylic C À Hb onds, [12] benzyl alcohols, [13] benzyl amines, [14] sulfides, [15] and carboxylic acids [16] using O 2 as the terminal oxidant. Nonetheless,t he scope of flavin catalysis is limited by the oxidation potential of the photoexcited state.T he substrates suitable for this reaction system are thus confined primarily to those with electron-releasing substituents that can stabilize the key radical or radical cation intermediates (Scheme 2).…”

Electrochemistry Broadens the Scope of Flavin Photocatalysis: Photoelectrocatalytic Oxidation of Unactivated Alcohols

“…[11] Fore xamples,r iboflavin tetraacetate (RFT) is ar eadily available compound with an oxidation potential of + 1.67 V( vs.S CE) upon photoirradiation (n max = 440 nm). This characteristic has allowed RFT and its derivatives to function as photocatalysts in the aerobic oxidation of benzylic C À Hb onds, [12] benzyl alcohols, [13] benzyl amines, [14] sulfides, [15] and carboxylic acids [16] using O 2 as the terminal oxidant. Nonetheless,t he scope of flavin catalysis is limited by the oxidation potential of the photoexcited state.T he substrates suitable for this reaction system are thus confined primarily to those with electron-releasing substituents that can stabilize the key radical or radical cation intermediates (Scheme 2).…”

Molecular Editing of Flavins for Catalysis

“…Similar observations were made by Zhao et al who reported the C7,C8-dibrominated flavin 30, again with the intention to benefit from the heavy atom effect (Scheme 25B). 70 The authors could show the superior catalytic activity of Scheme 25 Designed flavin catalysts with iodo (A) and bromo (B) substitution, which both benefit from the heavy atom effect.…”

“…Similar observations were made by Zhao, Guo, and co-workers who reported the C7,C8-dibrominated flavin 30, again with the intention to benefit from the heavy atom effect (Figure 7B). 70 They showed that the superior catalytic…”