Construction of Fused Polycyclic System via Tandem Cationic Cyclization by Using the Properties of Fluorine

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Fluorine possesses (i) leaving-group ability as fluoride ion(F^-) and (ii) α-cation-stabilizing effect due to its lone pairs. Our interest in using these two properties prompted us to investigate the generation of fluorine-containing carbocations from 1,1-difluoroalkenes, which would allow ring constructions by intramolecular trapping of the cations with unsaturated groups, followed by F^-elinination. Thus, we have accomplished fluorine-promoted Friedel-Crafts cyclization of 1,1difluoro-1-dalkenes bearing two aryl groups via α-fluorocarbocations, generated by the regioselective protonation of the fluorinated double bonds, leading to the construction of fused polycyclic systems. Moreover, this cyclization has proved to be effected eve by transition metal complexes instead of strong protic acids.The SN"' reaction of 4-ary-2-trifluoromethyl-q-butenes and benzyl anions affords 1,1-difluoro-1-alkenes with two aryl groups at appropriate positions. Treatment of thus obtained difluoroalkenes with FSO_3H/SbF_5 or Pd(BF_4)_2(MeCN)_4 in (CF_3)_CHOH (HFIP) readily promotes intramolecular Friedel-Crafts reactions. The tandem cyclizations successfully proceed to afford fused polycyclic compounds, whose dehydrogenation with palladium hydroxide on activated carbon leads to [4]helicenes in high yield.The sequence of reactions: the SN2' reaction, the tandem cationic cyclizations, and the dehydrogenation affords fused polycyclic compounds such as helicenes in only three steps from the 1-trifluoromethyl vinyl compounds. Since most of helicenes have been prepared via photo cyclization of stilbene moieties, developing a methodology for supplying large amount of helicenes have been a desirable goal. The results disclosed herein open a new approach to helicenes including functionalized ones.