title: The Missing C1-C5 Cycloaromatization Reaction: Triplet State Antiaromaticity Relief and Self-Terminating Photorelease of Formaldehyde for Synthesis of Fulvenes from Enynes.
abstract: 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-C5 cyclization undergoes internal H atom transfer that is coupled with the fragmentation of an exocyclic C-C 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-C bond fragmentation. Furthermore, incorporating C-C 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.
authors.forename: Rana K
authors.initials: RK
authors.lastname: Mohamed
authors.forename: Sayantan
authors.initials: S
authors.lastname: Mondal
authors.affiliation: Department of Chemistry - BMC, Uppsala University , Box 576, 751 23 Uppsala, Sweden.
authors.forename: Kjell
authors.initials: K
authors.lastname: Jorner
authors.forename: Thais Faria
authors.initials: TF
authors.lastname: Delgado
authors.affiliation: National High Magnetic Field Laboratory, Tallahassee, Florida 32310, United States.|Future Fuels Institute, Florida State University , Tallahassee, Florida 32310, United States.
authors.forename: Vladislav V
authors.initials: VV
authors.lastname: Lobodin
authors.affiliation: Department of Chemistry - BMC, Uppsala University , Box 576, 751 23 Uppsala, Sweden.
authors.forename: Henrik
authors.initials: H
authors.lastname: Ottosson
authors.forename: Igor V
authors.initials: IV
authors.lastname: Alabugin