Synthesis of Helicenes and Related Compounds Based on the Domino Cationic Cyclization of gem- Difluoroalkenes

Fluorine possesses (1) α-carbocation stabilizing effect caused by donation of its unshared electronpair to the vacant p-orbital of the α-carbon，despite (2) its leaving group ability as a fluoride ion (F^-) due to its high electronegativity. Wehave accomplished the synthesis of fused polyaromatic compounds starting from gem-difluoroalkenes（1,1-difluoro-1-alkenes） via the reactions where both of the two properties were fullyutilized.1,1-Difluoro-1-alkenes bearing a phenyl group at the c - 3,4or 5 position were treated with fso_3 H / sbf_5 to undergo Friedel-Crafts cyclization viaα- fluorocarbocations,followed by spontaneous hydrolysis of the C-F bond to afford bicyclic ketones including a five，six,or seven-membered ring.The key step in the above mentioned cyclization was the Friedel-Crafts-typereaction,where Magic acid was required to activate the difluoroalkene moiety. to avoid such strong acidicconditions, we have succeeded to conduct this type of cyclization by transition met…al complexes, such as cationic Pd, Ru, and Au complexes. Especially，[Pd(MeCN)_4](bf_4)_2 effectively promoted the cyclization to give polycyclic products.On treatmentwith Magic acid，the difluoroalkenes bearing two aryl groups underwent the domino cyclization via successivelygenerated two kinds of α-fluorocarbocations这是由intramolecular aryl groups to construct two 6 membered rings. the cyclizedproducts were readily dehydrogenated to afford substituted helicenes. Using this strategy we haveaccomplished the syntheses of [4] to [6]helicenes from easy available compounds in only threesteps:(i) the preparation of 1,1-difluoro-1-alkenes; (ii) the domino Friedel-Crafts-type cyclizationand (iii) dehydrogenation.treatment of 2 - trifluoromethyl-1-alkenes bearing an aryl group withet_2 AlCl induced Friedel-Crafts-type cyclization via attack of the aryl group on the carbon γ to thefluorine to give exo- diflooromethylene -substituted cyclic compounds, the helicene precursors. Less

Fluorine possesses (1) α-carbocation stabilizing effect caused by donation of its unshared electron pair to the vacant p-orbital of the α-carbon, despite (2) its leaving group ability as a fluoride ion (F^-) due to its high electronegativity. We have accomplished the synthesis of fused polyaromatic compounds starting from gem-difluoroalkenes (1,1-difluoro-1-alkenes) via the reactions where both of the two properties were fully utilized.1,1-Difluoro-1-alkenes bearing a phenyl group at the C-3, 4, or 5 position were treated with FSO_3H・SbF_5 to undergo Friedel-Crafts cyclization via α-fluorocarbocations, followed by spontaneous hydrolysis of the C-F bond to afford bicyclic ketones including a five, six, or seven-membered ring.The key step in the above mentioned cyclization was the Friedel-Crafts-type reaction, where Magic acid was required to activate the difluoroalkene moiety. To avoid such strong acidic conditions, we have succeeded to conduct this type of cyclization by transition met … More al complexes, such as cationic Pd, Ru, and Au complexes. Especially, [Pd(MeCN)_4](BF_4)_2 effectively promoted the cyclization to give polycyclic products.On treatment with Magic acid, the difluoroalkenes bearing two aryl groups underwent the domino cyclization via successively generated two kinds of α-fluorocarbocations, which were trapped by the intramolecular aryl groups to construct two 6-membered rings. The cyclized products were readily dehydrogenated to afford substituted helicenes. Using this strategy we have accomplished the syntheses of [4] to [6]helicenes from easily available compounds in only three steps: (i) the preparation of 1,1-difluoro-1-alkenes, (ii) the domino Friedel-Crafts-type cyclization, and (iii) dehydrogenation.Treatment of 2-trifluoromethyl-1-alkenes bearing an aryl group with Et_2AlCl induced Friedel-Crafts-type cyclization via attack of the aryl group on the carbon γ to the fluorine to give exo-difluoromethylene-substituted cyclic compounds, the helicene precursors. Less