关键词: 汉斯酯, 自由基环化, 2-吲哚酮, 无金属, 可见光

The 3,3-dialkyl-2-oxindole scaffold is a privileged structure found in numerous bioactive compounds, yet its synthesis often relies on precious metal catalysts or toxic radical initiators. Addressing these limitations, we report a sustainable, metal-free photochemical strategy for constructing 3,3-disubstituted oxindoles via visible-light-induced radical cyclization. This method employs an economical Hantzsch ester (HEH) as a dual-function reagent, serving simultaneously as a stoichiometric photoreductant and a hydrogen atom donor, eliminating the need for external photocatalysts or transition metals. The reaction proceeded with 2-iodo-N-methylmethacrylamides using HEH (2.0 equiv) and tetramethylguanidine (TMG, 1.0 equiv.) in acetonitrile under argon atmosphere. Irradiation with a 20 W blue LED at room temperature for 21 h afforded the target oxindoles in excellent yields (up to 98%). Control experiments confirmed the essential roles of HEH, TMG, light, and inert conditions. The protocol showed strong preference for aryl iodides, with bromo- and chloro-analogs giving significantly lower yields. Substrate scope investigation demonstrated broad functional group tolerance. Various substituents on the aryl ring, including electron-donating (-OMe, -Me) and electron-withdrawing groups (-F, -Cl, -Br, -COOMe), were well accommodated. Modifications on the acrylamide moiety, such as different N-alkyl groups (ethyl, cyclopropyl, benzyl) and variations at the α- and β-positions of the acryloyl group, also proceeded successfully, though efficiency depended on the alkene's electronic properties. A gram-scale reaction delivered the product in 87% yield, demonstrating practical scalability. Post-synthetic derivatizations, including reduction, thionation, and N-deprotection, highlighted the method's synthetic utility. Mechanistic studies revealed a photoinduced electron transfer pathway. Radical trapping experiments confirmed aryl radical intermediate, while deuterium labeling identified HEH as the hydrogen source. Fluorescence quenching studies supported PET from photoexcited HEH to the substrate. Based on this evidence, we propose a mechanism involving photoexcitation of HEH/TMG complex to generate a potent reductant, single electron transfer (SET) to cleave the C—I bond, forming an aryl radical, 5-exo-trig cyclization, and hydrogen atom transfer (HAT) from HEH-derived species to furnish the product.

Key words: Hantzsch ester, radical cyclization, 2-indolinone, metal-free, visible-light