Detailed studies on singlet diradicals are crucial to understand the bond-breaking process. However, in strained C–C homolysis, only σ-bonded isomers and planar singlet diradicals have been identified to date. The elusive third isomer has evaded detection, as it was too short-lived to identify.
Manabu Abe and colleagues, Hiroshima University and Osaka University, Japan, have “frozen” this previously unknown chemical curiosity by the careful selection of bulky substituents and the use of low-temperature reaction conditions. The team looked at the photochemical denitrogenation of 1,4-diaryl-2,3-diazabicyclo[2.2.1]heptane bearing bulky substituents. They observed a curious absorption band under low-temperature matrix conditions at about 450 nm, together with the well-known planar singlet diradical at about 580 nm.
Based on product analysis, low-temperature spectroscopic observations, high-level quantum-mechanical computations, viscosity effects, and laser-flash photolysis, the researchers assigned the new 450 nm absorption to a puckered singlet cyclopentane-1,3-diyl. These findings may encourage future work on the mechanistically interesting stereoselectivity documented in the formation of bicyclo[2.1.0]pentanes during the 2,3-diazabicyclo[2.2.1]heptane denitrogenation.
- A Puckered Singlet Cyclopentane-1,3-diyl: Detection of the Third Isomer in Homolysis,
Jianhuai Ye, Sayaka Hatano, Manabu Abe, Ryohei Kishi, Yusuke Murata, Masayoshi Nakano, Waldemar Adam,
Chem. Eur. J. 2015.