Difluoro-λ3-Bromane-Induced Oxidative Carbon−Carbon Bond-Forming Reactions:  Ethanol as an Electrophilic Partner and Alkynes as Nucleophiles

Reported here for the first time are the oxidative couplings of alkynes and primary alcohols yielding conjugated enones. Although the BF3-catalyzed reaction of terminal alkynes with p-trifluoromethylphenyl(difluoro)-λ3-bromane results in the fluoro-λ3-bromanation of triple bonds to afford (E)-β-fluorovinyl-λ3-bromanes, reaction of an alkyne with the difluoro-λ3-bromane in the presence of an alcohol and BF3−Et2O affords directly conjugated enones in good yields. The reaction proceeds in a highly stereo- and regioselective manner under metal-free conditions. Interestingly, no formation of enones was detected, when difluoro-λ3-iodane p-CF3C6H4IF2 was used instead of the λ3-bromane. A mechanism involving a λ3-bromane-induced oxidation of an alcohol to an aldehyde, [2 + 2] cyclization with alkynes yielding 2H-oxetes, and finally the electrocyclic ring opening is discussed.