The Radical Anion of 1,2‐Diphenylcyclopentene

ESR. and ENDOR. studies are reported for the radical anions of 1,2‐diphenylcyclopentene ( 3 ) and its di(pe+deuteriophenyl)‐derivative (3‐D 10 ). Comparison of the coupling constants of the phenyl protons in 3 \documentclass{article}\pagestyle{empty}\begin{document}$ ^{\ominus \atop \dot{}} $\end{document} . with the analogous values for the radical anions of 1,2‐diphenyl substituted cyclopropene ( 1 ) and cyclobutene ( 2 ) reveals regular changes in the sequence 1 \documentclass{article}\pagestyle{empty}\begin{document}$ ^{\ominus \atop \dot{}} $\end{document} , 2 \documentclass{article}\pagestyle{empty}\begin{document}$ ^{\ominus \atop \dot{}} $\end{document} , 3 \documentclass{article}\pagestyle{empty}\begin{document}$ ^{\ominus \atop \dot{}} $\end{document} , which are caused by an increasing twist of the phenyl groups about the C(1), C(1′)‐ and C(2), C(1″)‐bonds linking them to the ethylene fragment. Such a twist is shown to be also responsible for the large difference in the coupling constants of the methylene β‐protons in 3 \documentclass{article}\pagestyle{empty}\begin{document}$ ^{\ominus \atop \dot{}} $\end{document} . (0.659 and 0.293 mT). It is suggested that ‐ in order to minimize the losses caused by this twist in the π‐delocalization energy ‐ the 2 p z ‐axes at the centres 1 and 2 deviate from a perpendicular orientation to the mean plane of the cyclopentene ring. A deviation by 19° from such an orientation is required to account for the observed β‐proton coupling constants in terms of their conventional cos 2 ‐dependence on the dihedral angles θ.