Synthesis and Characterization of β-Haloalkenyl-λ³-bromanes:  Stereoselective Markovnikov Addition of Difluoro(aryl)-λ³-bromane to Terminal Acetylenes

Reported here for the first time are the synthesis, isolation, and characterization of hypervalent β-haloalkenyl-λ³-bromanes. Exposure of terminal alkynes to p-trifluoromethylphenyl(difluoro)-λ³-bromane activated by BF₃-i-Pr₂O resulted in fluoro-λ³-bromanation of the triple bonds in a Markovnikov fashion, yielding (E)-β-fluoroalkenyl-λ³-bromanes stereoselectively in good yields. 5-Chloro-1-pentynes undergo domino λ³-bromanation−chlorine shift−fluorination or λ³-bromanation−chlorine shift−alkyl shift−fluorination reaction, depending on the substituents and afford (E)-β-chloroalkenyl-λ³-bromanes stereoselectively in high yields. The β-chloroalkenyl-λ³-bromanes contain three kinds of halogen atoms, F, Cl, and Br, in the molecule.