Gold(I)-Catalyzed [2 + 2] and [3 + 2] Cycloadditions of 1,1-Difluoroallenes with Aldehydes: Switchable Syntheses of Fluorinated Oxetanes and Furans

1,1-Difluoroallenes underwent regioselective [2 + 2] and [3 + 2] cycloadditions with aldehydes using Au(I) catalysts. An AuCl catalyst enabled an α,β-selective [2 + 2] cycloaddition of 1,1-difluoroallenes, yielding (E)-3-alkylidene-2,2-difluorooxetanes. Conversely, an AuCl(IPr)-AgSbF₆ catalyst facilitated an α,γ-selective [3 + 2] cycloaddition, followed by dehydrofluorination to produce aromatized 2-fluorofurans. DFT calculations (B3LYP level) suggested that the former reaction proceeded through Au(III)-containing metallacycles, while the latter involved cyclic Au(I) carbenoids.