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

Abstract

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.

Photocatalytic Pyridine Synthesis with Enaminones and TMEDA under Metal-Free Conditions

“…On the other hand, TMEDA is an easily available and low-cost chemical with wide-spread application in organic synthesis by acting as an organic base or a ligand . Recent results also proved that the feasible C–N bond cleavage in TMEDA enables this chemical in the synthesis of different organic products by playing the role of a C1 donor. , The novel function of TMEDA together with our long-standing interest in the exploration of enaminone-based organic synthesis have enlighten us to develop new methods for pyridine synthesis with enhanced sustainability by a catalytic approach such as photocatalysis . Here, we report the visible light photocatalytic synthesis of 2,3,4,6-tetrasubstituted pyridines via the cascade reactions of enaminones and TMEDA.…”