Construction of highly fluorescent N–O seven-membered heterocycles via thermo-oxidation of oxazolidines

Abstract

Nitrogen heterocycles are significant structural components for organic compounds and pharmaceuticals. A new class of photooxidation products of oxazolidines with seven-membered heterocycles were reported in our previous work. These photosynthesized molecules have great potential for attractive fluorescent materials due to their unusual conjugated structure and rigidity. However, the violent UV irradiation during synthesis results in many undesirable side-products, making the purification and scale-up of desirable products difficult. Herein, the desired N–O seven-membered heterocycles were quantitatively synthesized via one-step thermo-oxidation using O₂/CH₃I, I₂ or N-iodosuccinimide (NIS) as an oxidant. The structures of these compounds were confirmed by single-crystal X-ray diffraction. During the thermal-oxidation process, intramolecular ionization generated a flexible –CH₂CH₂O⁻ group, which attacked the CC bond with the assistance of oxidants and subsequently led to the formation of the products. The formed seven-membered heterocycles exhibited remarkably enhanced fluorescence compared to the corresponding ring-open forms of oxazolidines, which possess more flexible conjugated substructures. In particular, the fluorescence quantum yields of some products reach 0.92 and 0.62 in the red emission region. Theoretical calculations suggest that the strong fluorescence originates from their unique structural rigidity and planarity and thus highly conjugated electron system. This work provides a new synthesis method for demanding medium-sized heterocycles with notably strong fluorescence. These new dyes have potential use as visible and fluorescent dual-mode colour inks, nucleophilic reagent response materials, and bio-label materials.