2006 Fiscal Year Final Research Report Summary
Synthesis of Helicenes and Related Compounds Based on the Domino Cationic Cyclization of gem- Difluoroalkenes
| Project/Area Number |
17550031
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Organic chemistry
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| Research Institution | The University of Tokyo |
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Principal Investigator |
ICHIKAWA Junji The University of Tokyo, Graduate School of Science, Associate Professor, 大学院理学系研究科, 助教授 (70184611)
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| Project Period (FY) |
2005 – 2006
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| Keywords | carbocation / fluoroalkene / Friedel-Crafts reaction / fused ring / S_N2' reaction / helicene / intramolecular cyclization / catalysis |
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| Research Abstract |
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
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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
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Research Products
(4 results)
