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A Puckered Singlet Cyclopentane‐1,3‐diyl: Detection of the Third Isomer in Homolysis
Author(s) -
Ye Jianhuai,
Hatano Sayaka,
Abe Manabu,
Kishi Ryohei,
Murata Yusuke,
Nakano Masayoshi,
Adam Waldemar
Publication year - 2016
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201503975
Subject(s) - diradical , chemistry , homolysis , photochemistry , cyclopentane , steric effects , singlet state , singlet oxygen , stereochemistry , organic chemistry , excited state , radical , oxygen , nuclear physics , physics
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 λ max =≈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.