From Homoconjugated Push–Pull Chromophores to Donor–Acceptor‐Substituted Spiro Systems by Thermal Rearrangement |
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Authors: | Cagatay Dengiz Oliver Dumele Dr Shin‐ichiro Kato Dr Michal Zalibera Dr Pawel Cias Dr W Bernd Schweizer Prof?Dr Corinne Boudon Dr Jean‐Paul Gisselbrecht Prof?Dr Georg Gescheidt Prof?Dr François Diederich |
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Institution: | 1. Laboratorium für Organische Chemie, ETH Zürich, H?nggerberg, HCI, 8093 Zürich (Switzerland), Fax: (+41)?44‐632‐1109;2. Institute of Physical and Theoretical Chemistry, Graz University of Technology, Stremayrgasse 9, 8010 Graz (Austria);3. Laboratoire d'Electrochimie et de Chimie Physique du Corps Solide, Institut de Chimie‐UMR 7177, CNRS, Université de Strasbourg, 4, rue Blaise Pascal, 67000 Strasbourg (France) |
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Abstract: | Series of homoconjugated push–pull chromophores and donor–acceptor (D–A)‐functionalized spiro compounds were synthesized, in which the electron‐donating strength of the anilino donor groups was systematically varied. The structural and optoelectronic properties of the compounds were investigated by X‐ray analysis, UV/Vis spectroscopy, electrochemistry, and computational analysis. The homoconjugated push–pull chromophores with a central bicyclo4.2.0]octane scaffold were obtained in high yield by 2+2] cycloaddition of 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ) to N,N‐dialkylanilino‐ or N,N‐diarylanilino‐substituted activated alkynes. The spirocyclic compounds were formed by thermal rearrangement of the homoconjugated adducts. They also can be prepared in a one‐pot reaction starting from DDQ and anilino‐substituted alkynes. Spiro products with N,N‐diphenylanilino and N,N‐diisopropylanilino groups were isolated in high yields whereas compounds with pyrrolidino, didodecylamino, and dimethylamino substituents gave poor yields, with formation of insoluble side products. It was shown by in situ trapping experiments with TCNE that cycloreversion is possible during the thermal rearrangement, thereby liberating DDQ. In the low‐yielding transformations, DDQ oxidizes the anilino species present, presumably via an intermediate iminium ion pathway. Such a pathway is not available for the N,N‐diphenylanilino derivative and, in the case of the N,N‐diisopropylanilino derivative, would generate a strained iminium ion (A1,3 strain). The mechanism of the thermal rearrangement was investigated by EPR spectroscopy, which provides good evidence for a proposed biradical pathway starting with the homolytic cleavage of the most strained (CN)C?C(CN) bond between the fused four‐ and six‐membered rings in the homoconjugated adducts. |
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Keywords: | charge transfer EPR spectroscopy homoconjugation push– pull chromophores spiro compounds thermal rearrangement mechanism |
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