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.