Aromatic hydroxylations by flavins: evidence on direct attack of phenylalanine by flavin radical species

Facile Preparation of Flavinium Organocatalysts

We developed a safe, simple, inexpensive, and environmentally benign method for preparing N(5)-ethylated flavinium organocatalysts without using hazardous reagents or inert conditions as previously required. 5-Ethyl-3-methyllumiflavinium cation was prepared from its reduced form by NaNO₂ -free aerobic oxidation, which was subsequently extracted onto commercial cation-exchange resins under NaClO₄ -free conditions. The resulting resin-immobilized flavinium salts were found to be effective organocatalysts for aerobic oxidation reactions.

Flavin-Catalyzed Oxidation of Amines and Sulfides with Molecular Oxygen: Biomimetic Green Oxidation

Flavin-catalyzed green oxidation of heteroatom compounds such as sulfides and amines with molecular oxygen and even air in the presence of hydrazine monohydrate in a fluorous solvent such as 2,2,2-trifluoroethanol at room temperature gives the corresponding oxidation products highly efficiently and selectively along with water and molecular nitrogen, which are environmentally benign by-products. The proposed reaction mechanism is based on the kinetics, solvent effect, and redox properties of flavin catalysts.

Preparation and Redox Properties ofN,N,N-1,3,5-Trialkylated Flavin Derivatives and Their Activity as Redox Catalysts

Eight different flavin derivatives have been synthesized and the electronic effects of substituents in various positions on the flavin redox chemistry were investigated. The redox potentials of the flavins, determined by cyclic voltammetry, correlated with their efficiency as catalysts in the H2O2 oxidation of methyl p-tolyl sulfide. Introduction of electron-withdrawing groups increased the stability of the reduced catalyst precursor.