Rotational isomerism in 1,4-diarylbut-2-ene-1,4-dione radical anions: an electron paramagnetic resonance study

Abstract

The radical anions of (Z)- and (E)-1,4-diarylbut-2-ene-1,4-diones 1–8 have been obtained by electrolytic reduction in dimethylformamide. Identical EPR spectra are recorded at room temperature for both the (Z)- and (E)-isomers evidencing rapid (Z)–(E) interconversion. Individual conformers are not observed for radical anions derived from either diphenyl, bis(p-tolyl) or bis(p-anisyl) derivatives 1^˙––6^˙– in the temperature range 223–298 K. Rapid interconversion between the possible conformers has been theoretically supported by INDO calculations. For the highly strained radical anion from the dimesityl compound 8^˙– three isomers with different g-values are distinguishable. The different isomers have also been assigned by means of INDO calculations.