The Impact of Antiaromatic Subunits in [4n+2] 蟺-Systems: Bispentalenes with [4n+2] 蟺-Electron Perimeters and Antiaromatic Character

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• 作者：Jing Cao ; G谩bor London ; Oliver Dumele ; Margarete von Wantoch Rekowski ; Nils Trapp ; Laurent Ruhlmann ; Corinne Boudon ; Amnon Stanger ; Fran莽ois Diederich
• 刊名：Journal of the American Chemical Society
• 出版年：2015
• 出版时间：June 10, 2015
• 年：2015
• 卷：137
• 期：22
• 页码：7178-7188
• 全文大小：704K
• ISSN：1520-5126

文摘

Three series of stable, neutral, 蟺-extended bispentalene derivatives, with two pentalenes fused to a central benzene or naphthalene moiety, have been prepared through a modified double carbopalladation cascade reaction. While these chromophores feature skeletons with [4n+2] 蟺-electron perimeters, the two 8 蟺-electron pentalene subunits strongly influence bonding and spectral properties. ¹H NMR spectra showed large upfield shifts of the protons in the pentalene moieties, comparable to antiaromatic monobenzopentalenes. Further investigations on magnetic ring currents through NICS-XY-scans suggest a global paratropic current and a local diatropic current at the central benzene ring in two of the series, while the third series, with a central naphthalene ring, showed more localized ring currents, with stronger paratropic ring currents on the pentalene moieties. X-ray diffraction analyses revealed planar bispentalene cores with large double- and single-bond alternation in the pentalene units, characteristic for antiaromaticity, and small alternation in the central aromatic rings. In agreement with TD-DFT calculations, both optical and electrochemical data showed much smaller HOMO鈥揕UMO energy gaps compared to other neutral, acene-like hydrocarbons with the same number of fused rings. Both experimental and computational results suggest that the molecular properties of the presented bispentalenes are dominated by the antiaromatic pentalene-subunits despite the [4n+2] 蟺-electron perimeter of the skeletons.