Anodic Dimerization of Enamines, 2-Cyano-2-phenylvinylamines

The effects of substituents (R1, R2, and R3) on the electrochemical oxidation of the title enamines, R1-C6H4-C (CN)=CH-NR2R3 (1) were investigated in acetonitrile at a glassy carbon electrode together with the oxidation of a related amine, Ph-CH (CN)-CH2-NMe2 (5). Cyclic voltammetry of 1 showed two or more anodic peaks depending upon the substituents. On controlled potential electrolysis at the potential of the first anodic wave, 1 with R1=H, R2=alkyl, and R3=alkyl or H gave diphenylmethane derivatives in which two molecules of the starting enamine are coupled between the β-carbon atom and the phenyl ring para to the enamino group. Essentially the same results were obtained for 1 with R1=m-Me or m-Cl and R2=R3=Me. A mechanism involving the dimerization of radical cations from 1 is proposed. Electrolysis of 1 with R1=p-Cl or p-Me and R2=R3=Me, on the other hand, gave diphenylethane derivatives via coupling between the β-carbon atoms. No stable product was obtained from 1 with a phenyl group on the nitrogen atom. Results on electrolyses of the amine 5 and 1 with R1=H and R2=R3=Me at the voltammetric peak potential of 5 argue against the possibility that an enamine is formed as an intermediate in the electrochemical dealkylation of aliphatic amines.