Electrochemically induced isomerization of 1,1,4,4-tetra-phenyl-1,2-butadiene to 1,1,4,4-tetraphenyl-1,3-butadiene: An indirect self-protonation mechanism

At a hanging mercury drop electrode in dimethylformamide containing tetra- n -butylammonium perchlorate, a cyclic voltammogram for 1,1,4,4-tetraphenyl-1,2-butadiene shows two irreversible waves on the first negative scan; we attribute the first wave to reduction of 1,1,4,4-tetraphenyl-1,2-butadiene, whereas the second wave is due to reduction of a product (1,1,4,4-tetraphenyl-1-butene). At a sufficiently slow scan rate (200 mV s −1 ), the current trace on the positive scan crosses that for the negative scan, and an oxidation wave appears at more positive potentials. If a second negative scan is initiated without any interruption, one sees a pair of new cathodic waves corresponding to stepwise reduction of 1,1,4,4-tetraphenyl-1,3-butadiene, along with the waves recorded on the first scan. At a scan rate of 1 V s −1 , no cyclic voltammetric evidence is observed for production of 1,1,4,4-tetraphenyl-1,3-butadiene. Analysis of the cyclic voltammetric data indicates that transformation of 1,1,4,4-tetra-phenyl-1,2-butadiene to 1,1,4,4-tetraphenyl-1,3-butadiene occurs via an indirect self-protonation mechanism; hydroxide ion derived from residual water in the system acts as a base toward unreduced starting material to initiate the isomerization, and the rate-determining step in the process is heterogeneous electron transfer to 1,1,4,4-tetraphenyl-1,2-butadiene.