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.