title: A Puckered Singlet Cyclopentane-1,3-diyl: Detection of the Third Isomer in Homolysis.
abstract: In the photochemical denitrogenation of 1,4-diaryl-2,3-diazabicyclo[2.2.1]heptane (AZ6) bearing sterically hindered substituents, a curious new absorption band at about 450 nm was observed under low-temperature matrix conditions, together with the previously well-characterized planar singlet diradical pl-(1) DR6 with λmax =≈580 nm. The 450 nm species was electron paramagnetic resonance (EPR)-silent. Instead of generating the planar diradical pl-(1) DR6 and the precursor azoalkane AZ6 upon warming, the ring-closed bicyclo[2.1.0]pentane derivative SB6, that is, the AZ6 denitrogenation product was identified. Based on product analysis, low-temperature spectroscopic observations, high-level quantum-mechanical computations, viscosity effect, and laser-flash photolysis, the puckered singlet diradicaloid puc-(1) DR6 was assigned to the new 450 nm absorption. The latter was detected experimentally at the same time as the planar singlet diradical pl-(1) DR6. Sterically demanding substituents as well as viscosity impediments were essential for the detection of the experimentally hitherto unknown puckered singlet cyclopentane-1,3-diyl diradicaloid puc-(1) DR6, that is, the third isomer in homolysis. The present findings should stimulate future work on the mechanistically fascinating stereoselectivity documented in the formation of bicyclo[2.1.0]pentanes during the 2,3-diazabicyclo[2.2.1]heptane denitrogenation.
authors.affiliation: Department of Chemistry, Graduate School of Science, Hiroshima University (HIRODAI), 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8526, Japan.
authors.forename: Jianhuai
authors.initials: J
authors.lastname: Ye
authors.affiliation: Department of Chemistry, Graduate School of Science, Hiroshima University (HIRODAI), 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8526, Japan.
authors.forename: Sayaka
authors.initials: S
authors.lastname: Hatano
authors.affiliation: Department of Chemistry, Graduate School of Science, Hiroshima University (HIRODAI), 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8526, Japan. mabe@hiroshima-u.ac.jp.|Research Center for Future Science, Hiroshima University (HIRODAI), 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8526, Japan. mabe@hiroshima-u.ac.jp.
authors.forename: Manabu
authors.identifier: http://orcid.org/0000-0002-2013-4394
authors.initials: M
authors.lastname: Abe
authors.affiliation: Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka, 560-8531, Japan. rkishi@cheng.es.osaka-u.ac.jp.
authors.forename: Ryohei
authors.initials: R
authors.lastname: Kishi
authors.affiliation: Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka, 560-8531, Japan.
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Murata
authors.affiliation: Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka, 560-8531, Japan. mnaka@cheng.es.osaka-u.ac.jp.
authors.forename: Masayoshi
authors.initials: M
authors.lastname: Nakano
authors.affiliation: Institute of Organic Chemistry, University of Würzburg, Am Hubland, 97074, Würzburg, Germany.|Department of Chemistry, Facundo Bueso 110, University of Puerto Rico, Rio Piedras, Puerto Rico, 00931, USA.
authors.forename: Waldemar
authors.initials: W
authors.lastname: Adam
keywords: UV/Vis spectroscopy; mass spectrometry; photochemistry; radical reactions; reactive intermediates