3‐Halochromones Through Oxidative α‐Halogenation of Enaminones and its Photophysical Investigation: Another Case of Photo‐induced Partially Aromatised Intramolecular Charge Transfer?

• Published in: Chemistry, an Asian journal an Asian journal, 2024-01, Vol.19 (2), p.e202300852-n/a
• Main Authors: Neto, José S. S., Coelho, Filipe T., Doerner, Carlos V., Braga, Antonio L., Lião, Luciano M., Coelho, Felipe L.
• Format: Article
• Language: English
• Subjects: Aromatic compounds; Charge transfer; Chemistry; Emission; enaminones; Fluorescence; Functional groups; green chemistry; halochromones; Halogenation; intramolecular charge transfer; Oxidation; Oxone; Synthesis
• ISSN: 1861-4728; 1861-471X
• DOI: 10.1002/asia.202300852

A versatile synthesis strategy for fluorescent 3‐halo‐4H‐chromen‐4‐one derivatives is reported. The method involves the oxidative α‐halogenation of enaminones performed by an efficient and sustainable oxidation system. The use of Oxone® in combination with KCl, KBr, or KI enables the preparation of 3‐chloro‐, 3‐bromo‐, or 3‐iodo‐4H‐chromen‐4‐one in good to excellent yields, with great functional group tolerance where the protocol is amenable to gram‐scale synthesis. The analysis of the photophysical properties of the presented 4H‐chromen‐4‐one showed absorption in the UV region and fluorescence emission in the violet‐to‐cyan region with a relatively large Stokes shift. In solution, all compounds present a dual fluorescence emission, regardless of the solvent, assigned to a partially aromatised intramolecular charge transfer mechanism, considering the presence of a pseudo‐aromatic ring in the chromone scaffold and the absence of the influence of substituent electronic features in optical behaviour. The work reports a versatile method to access 3‐chloro‐, 3‐bromo‐, or 3‐iodo‐4H‐chromen‐4‐ones from oxidative α‐halogenation of enaminones. The Oxone®/KX system provides a wide range of 3‐halo‐4H‐chromen‐4‐ones in good to excellent yields with great functional group tolerance. The analysis of the photophysical properties revealed a dual fluorescence emission in the violet‐to‐cyan assigned to a partially aromatised intramolecular charge transfer mechanism.