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 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, molecular oxygen, isothiocyanates, desulfurative cyclization