The Missing C1鈥揅5 Cycloaromatization Reaction: Triplet State Antiaromaticity Relief and Self-Terminating Photorelease of Formaldehyde for Synthesis of Fulvenes from Enynes

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• 作者：Rana K. Mohamed ; Sayantan Mondal ; Kjell Jorner ; Thais Faria Delgado ; Vladislav V. Lobodin ; Henrik Ottosson ; Igor V. Alabugin
• 刊名：Journal of the American Chemical Society
• 出版年：2015
• 出版时间：December 16, 2015
• 年：2015
• 卷：137
• 期：49
• 页码：15441-15450
• 全文大小：874K
• ISSN：1520-5126

文摘

The last missing example of the four archetypical cycloaromatizations of enediynes and enynes was discovered by combining a twisted alkene excited state with a new self-terminating path for intramolecular conversion of diradicals into closed-shell products. Photoexcitation of aromatic enynes to a twisted alkene triplet state creates a unique stereoelectronic situation, which is facilitated by the relief of excited state antiaromaticity of the benzene ring. This enables the usually unfavorable 5-endo-trig cyclization and merges it with 5-exo-dig closure. The 1,4-diradical product of the C₁鈥揅₅ cyclization undergoes internal H atom transfer that is coupled with the fragmentation of an exocyclic C鈥揅 bond. This sequence provides efficient access to benzofulvenes from enynes and expands the utility of self-terminating aromatizing enyne cascades to photochemical reactions. The key feature of this self-terminating reaction is that, despite the involvement of radical species in the key cyclization step, no external radical sources or quenchers are needed to provide the products. In these cascades, both radical centers are formed transiently and converted to the closed-shell products via intramolecular H-transfer and C鈥揅 bond fragmentation. Furthermore, incorporating C鈥揅 bond cleavage into the photochemical self-terminating cyclizations of enynes opens a new way for the use of alkenes as alkyne equivalents in organic synthesis.