Electrochemical reduction of 1,1-diaryl-substituted ethenes in dimethylformamide

The cathodic behaviour of 1,1-diaryl-substituted ethenes (Ar)PhC[double bond, length half m-dash]CH2(where Ar = phenyl-, 2-naphthyl-, 2-pyridyl-, 2-thienyl-, 2-furyl-) at a Hg cathode in N,N-dimethylformamide (DMF) and 0.1 mol dm–3 tetraethylammonium tetrafluoroborate (TEAF), was investigated by cyclic voltammetry and controlled potential electrolysis experiments. In cyclic voltammetry, at a potential scan rate of 0.2 V s –1, all substrates showed an irreversible reduction peak [(Ep,c) between –2.27 and –2.54 V vs. saturated calomel electrode (SCE)]. Controlled potential coulometries indicated that the apparent charge number of the process involved ranges from 1 to 2. The reduction products were characterised as (Ar)PhCH–Me (1-aryl-1-phenylethanes) and (Ar)PhCH–[CH2]2–CHPh(Ar)(1,4-diaryl-1,4-diphenylbutanes). The charge number of the process and the ratio between the two different reduction products depended strongly on the structure and concentration of the substrate. Exhaustive electrolyses performed in the presence of variable amounts of D2O or in [2H7]DMF allowed us to suggest that two distinct pathways are involved in the reduction process, leading to the monomeric and dimeric product, respectively. Hypotheses on the nature of the two reduction mechanisms are discussed.