Selective Generation of Carbocations and Construction of Fused Ring Systems by Using α-Cation-Stabilizing Effect of Fluorine

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Fluorine possesses (i) α-cation-stabilizing effect and (ii) leaving-group ability as fluoride ion (F^-). Using these properties, we have recently developed fluorine-directed Nazarov cyclizations via α-fluorocarbocations generated from 2,2-difluorovinyl vinyl ketones. Our interest on fluorine-containing carbocations prompted us to investigate their generation from simple 1,1-difluoroolefins without a carbonyl group, which would allow ring constructions by intramolecular trapping of the cations with unsaturated groups. Furthermore, to expand the scope of α-fluorocarbocation chemistry, we also explored the generation of fluoroallyl cations from conjugated 1,1-difluorodienes by regioselective protonation of the non-fluorinated double bond. Thus, we have accomplished intramolecular Friedel-Crafts cyclizations of 1,1-difluoro-1-alkenes and 1,1-difluoro-1,3-alkadienes bearing one or two aryl groups. These reactions efficiently provide a variety of fused polycyclic compounds.Treatment of 1,1-difluoro-1-alkenes or 1,1-difluoro-1,3-alkadienes bearing an aryl group with FSO_3H SbF_5 or p-toluenesulfonic acid, respectively, promotes intramolecular Friedel-Crafts reactions in (CF_3)_2CHOH (HFIP). The cyclizations proceed via terminal α-fluorocarbocations generated by regioselective protonation of the double bond. Bicyclic ketones are eventually obtained after spontaneous hydrolysis of the C-F bonds.In the case of difluoroolefins bearing two aryl groups, tandem cyclizations successfully occur to afford fused polycyclic compounds, whose dehydrogenation with palladium on activated carbon leads to [4]helicenes in high yields.