Stereochemical effects in the electrochemistry of some hindered stilbenes in dimethylformamide

Abstract

A series of progressively hindered stilbenes shows a decreasing rate of electrode reaction for radical-ion formation in dimethylformamide, which is mainly a reflection of the Arrhenius prexponential term. The effect is discussed in terms of entropy effects and geometry changes. Increasing hindrance leads to shifts in reduction and oxidation potential, which are discussed in terms of the effects of non-planarity on the electronic energy levels. For the geometrical isomers cis and trans stilbene, electron transfer to the former is slower than to the latter and isomerization of the anion radical to the planar trans configuration is slow. Reaction of the anions in their isomeric forms with carbon dioxide is discussed in terms of non-bonded interactions and rotation about the central carbon-carbon bond in the reaction intermediates, whose lifetimes are less than 1 msec. Results for heterogeneous electron transfer are paralleled by homogeneous electron exchange rates.