Light-Activated BF3·Et2O-Promoted Generation of Singlet Oxygen and Cascade Reaction of Unsaturated Amides.

Herein, BF3·Et2O-promoted O-insertion/spirocyclization/fluorination/ring-expansion of unsaturated amides to synthesis of spiro[benzo[b]-[1,4]dioxepine-3,5'-oxazole] skeletons in the presence of natural light and O2 (3Σg-) was reported. Air was the oxygen source of the 1O2-generation and O-insertion reaction under metal-free and mild conditions. BF3·Et2O played multiple roles, such as Lewis acid, activating reagent, and fluorine source in the reported cascade. A mechanism involving 1O2 generation/activation of double bond/O-insertion/spirocyclization/fluorination/ring expansion was supposed.

Different Performances of BF3, BCl3, and BBr3 in Hypervalent Iodine-Catalyzed Halogenations.

Herein, hypervalent iodine-catalyzed halogenation of aryl-activated alkenes using BX3 (X = Cl, Br) as the halogen source and activating reagents was reported. Various halogenated 1,3-oxazine/2-oxazoline derivatives were obtained in good-to-high yields. Using BF3 resulted in different substitute sites from BBr3 and BCl3 of the products, indicating different reactive intermediates and reaction pathways. The reaction underwent a "ligand coupling/oxidative addition/intermolecular nucleophilic attack/1,2-aryl migration/reductive elimination/intramolecular nucleophilic attack" cascade when BF3 was applied as the halogen source, while 1,2-aryl migration has "disappeared" when the halogen source was BBr3 or BCl3. Possible catalytic cycles were proposed, and DFT calculations were conducted to demonstrate the differences among BX3 (X = F, Cl, Br) in the hypervalent iodine-catalyzed halogenations.

Catalytic Synthesis of 5-Fluoro-2-oxazolines: Using BF3·Et2O as the Fluorine Source and Activating Reagent.

Hypervalent iodine catalyst-catalyzed nucleophilic fluorination of unsaturated amides using BF3·Et2O as the fluorine source and activating reagent was reported. Various 5-fluoro-2-oxazoline derivatives were synthesized in good to excellent yields (up to 95% isolated yield) within 10 min. The process was efficient and metal-free under mild conditions. A mechanism involving a fluorination/1,2-aryl migration/cyclization cascade was proposed on the basis of previous work and experimental results.

Catalytic asymmetric nucleophilic fluorination using BF3·Et2O as fluorine source and activating reagent.

Fluorination using chiral catalytic methods could result in a direct access to asymmetric fluorine chemistry. However, challenges in catalytic asymmetric fluorinations, especially the longstanding stereochemical challenges existed in BF3·Et2O-based fluorinations, have not yet been addressed. Here we report the catalytic asymmetric nucleophilic fluorination using BF3·Et2O as the fluorine reagent in the presence of chiral iodine catalyst. Various chiral fluorinated oxazine products were obtained with good to excellent enantioselectivities (up to >99% ee) and diastereoselectivities (up to >20:1 dr). Control experiments (the desired fluoro-oxazines could not be obtained when Py·HF or Et3N·3HF were employed as the fluorine source) indicated that BF3·Et2O acted not only as a fluorine reagent but also as the activating reagent for activation of iodosylbenzene.