Voltammetric studies of the reduction of cis- and trans-α-nitrostilbene

The reduction of cis-α-nitrostilbene (cis-1) in acetonitrile occurs by initial formation of the radical anion followed by irreversible dimerization to give the dinitronate dianion (3), of 1,4-dinitro-1,2,3,4-tetraphenylbutane (2). The standard potential for the neutral/radical anion couple is −1.44 V vs. ferrocenium/ferrocene. The dimerization rate constant is 5.7 × 104 M−1 s−1 at 298 K and the dimerization equilibrium constant exceeds 106 M−1. The reduction of trans-α-nitrostilbene (trans-1) is similar to that of cis-1 except for a marked deviation between simulation and experiment that occurs near the foot of the reduction peak. However, voltammograms of trans-1 can be accounted for by a different mechanism in which the initially formed radical anion of trans-1 rapidly isomerizes (kf,1=105 s−1) to the cis form of the radical anion followed by its partial oxidation to cis-1 thus explaining the marked similarity between the voltammograms of the two isomers. In separate experiments, partial electrolysis of trans-1 resulted in redox-catalyzed equilibration of the neutral isomers. Chromatographic analysis of the partially electrolyzed solutions showed that the ratio of cis-1 to trans-1 approached 12 and this was taken as a good estimate of the isomerization equilibrium constant. When this value was used in the simulations, it was found that the analogous equilibrium constant for the radical anions is 170. The observed preference for the cis isomers is discussed on the basis of calculations of the structures and energies of the isomers of the neutral compounds and the radical anions.