Electrochemical oxidation of organic compounds containing C[dbnd]N double bonds

Abstract

Hydrazones from aromatic aldehydes (ArCH=NNH2, I–III) and acetophenones (ArC(Me)=NNH2, IV–X), oxime (ArC(Me)=NOH, XI) and oxime ether ArC(Me)=NOCH3, XII), were synthesized according to known procedures. An azine derivative (ArC(Me)=NN=C(Me)Ar, XIII) was prepared by anodic oxidation of the corresponding hydrazone. Their redox properties were studied in acetonitrile by cyclic voltammetry. They all show 1–3 irreversible oxidation waves and 1–2 irreversible reduction waves. Also, as expected, electron-donating groups attached to a phenyl ring at the para position decrease the anodic peak potentials with respect to the non-substituted molecule, whereas electron-withdrawing substituents increase the anodic peak potentials.Controlled potential electrolyses (CPE) at the onset of the measured oxidation potentials of hydrazones of acetophenones were carried out in acetonitrile (or methanol) and gave mainly azines and acetophenones. Anodic oxidation of hydrazones derived from aldehydes in different media did not form the desired nitriles but instead, gave the corresponding azines. CPE of oxime XI in acetonitrile (or methanol) gave the corresponding acetophenone whereas the oxime ether derivative XII gave both acetophenone and azine in acetonitrile. Electrolysis of azine XIII under similar conditions afforded mostly acetophenone (and unreacted substrate).Constant current electrolysis of hydrazones of acetophenones (IV–X) in methanol yielded four types of products: α-methoxyarylethane (1), acetophenone dimethylacetal (2), acetophenone (3) and azine (4). Their relative ratio was highly dependent on the nature of the aryl group.