Taming the stilbene radical anion

Abstract

Radical anions appear as intermediates in a variety of organic reductions and have recently garnered interest for their role as mediators for electron-driven catalysis as well as for organic electron conductor materials. Due to their unstable nature, the isolation of such organic radical anions is usually only possible by using extended aromatic systems, whereas non-aromatic unsaturated hydrocarbons have so far only been observed in situ. We herein report the first isolation, structure and spectroscopic characterization of a simple aryl substituted alkene radical anion, namely that of stilbene (1,2-diphenyl ethylene), achieved by encapsulation between two [K{18c6}] cations. The formation of the radical anion is accompanied by Z → E isomerization of the involved double bond, also on a catalytic scale. Employing the linear iron(I) complex [Fe(NR₂)₂]⁻ as a reductant and coordination site also allows for this transformation, via formation of an iron(II) bound radical anion. The use of the iron complex now also allows for Z → E isomerization of electron richer, simple alkenes bearing either mixed alkyl/aryl or even bis(alkyl) substitution.