关键词: 可见光催化, 苯并噁嗪, 氧气, 异硫氰酸酯, 脱硫环化

2-Amino-1,3-benzoxazines, as privileged heterocyclic scaffolds, exhibit broad applications in pharmaceuticals and functional materials. This study presents a metal-free visible-light photocatalytic strategy using rhodamine B (Rh.B) as the photocatalyst and molecular oxygen as a green oxidant, enabling the efficient one-step synthesis of 2-amino-1,3-benzoxazine derivatives via desulfurative cyclization of 2-aminobenzyl alcohols and isothiocyanates. Systematic optimization revealed acetonitrile as the optimal solvent and 4-dimethylaminopyridine as the preferred base, achieving an 85% yield of the target product under ambient air and blue LED irradiation. Substrate scope evaluation demonstrated excellent compatibility with halogen substituents (F, Cl, Br, I), electron-donating (-CH₃, -OCH₃), electron-withdrawing (-NO₂, -CN), naphthyl, and pyridyl groups, affording products in 45%~90% yields. Mechanistic investigations, including radical trapping experiments and atmosphere-controlled studies, confirmed a radical-mediated pathway. The reaction initiates with thiourea intermediate formation, followed by Rh.B^*-triggered single-electron transfer to generate a sulfur-centered radical, which undergoes oxygen-assisted desulfurization and subsequent intramolecular C—N/C—O bond formation. This protocol eliminates the need for metal catalysts, stoichiometric oxidants, or harsh conditions, offering a sustainable and operationally simple route for benzoxazine synthesis. The typical operational steps are as follows: In a test tube, 2-aminobenzyl alcohol (24.6 mg, 0.2 mmol), rhodamine B (9.6 mg, 0.02 mmol), p-tolyl isothiocyanate (35.8 mg, 0.24 mmol), and acetonitrile (2 mL) were sequentially added. The mixture was stirred under irradiation with a 10 W blue LED (λ_max=455 nm) at room temperature under air. Reaction progress was monitored by thin-layer chromatography (TLC). After completion, the solvent was removed under reduced pressure. The crude product was purified by silica gel column chromatography using petroleum ether/ethyl acetate (10∶1, V/V) as the eluent to afford 3aa as a white solid (40.5 mg, 85% yield).

Key words: visible-light photocatalysis, benzoxazine, oxygen, isothiocyanates, desulfurative cyclization