On the electronic character of localized singlet 2,2-dimethoxycyclopentane-1,3-diyl diradicals: Substituent effects on the lifetime

Photodenitrogenation of the diazenes 4 affords exclusively the housanes 5 through intramolecular cyclization of the spectrally detected and characterized singlet diradicals 3. The lifetime of singlet diradical 3, determined by transient absorption measurements, depends on the Y and Z substituents at the para position of the phenyl ring and has the following order: Y, Z = OMe, OMe ≫ OMe, CN ≥ CN, CN ≥ OMe, H ≥ Cl, Cl ∼ CN, H ∼ Me, Me ≥ H, H. This unprecedented substituent effect reveals stabilization of the singlet 2,2-dimethoxycyclopentane-1,3-diyl diradicals 3 through radical, zwitterionic, π-bonding, and hyperconjugative structures. Copyright © 2002 American Chemical Society.