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.