The electrochemical reduction of 1,4-dichloroazoethanes : reductive elimination of chloride to form aryl azines

Abstract

A series of 1,4-dichloroazoethanes (1-X/Y, X and Y = 4-NO2, 4-CN, 4-CH3 or 4-H) were studied in N,N-dimethylformamide using cyclic voltammetry, constant potential sweep voltammetry (CPSW) and constant potential electrolysis. The voltammograms of 1-X/Y exhibit an irreversible two-electron wave corresponding to dissociative electron transfer (DET) reduction of the carbon–chlorine bond resulting in formation of the azines 2-X/Y in quantitative yield. Additional redox waves correspond to the reversible reduction of the azines to the 2-X/Y•− radical anion and 2-X/Y2− dianion consecutively, with the exception of 1-NO2/NO2 where both NO2 groups are reduced simultaneously in a two-electron reversible wave. Thermodynamic and kinetic parameters were determined from CPSW: the standard reduction potentials (Eo) vary between −0.7 and −1.3 V versus SCE as a function of electron-withdrawing substituent; the heterogeneous rate constants (khet) are consistent with a slow heterogeneous electron transfer with values ranging from 10−3 to 10−5 cm s−1; the transfer coefficients (˛) for 1-NO2/NO2 and 1-NO2/H are greater than 0.5, indicative of a stepwise DET mechanism for the C–Cl bond cleavage while the remaining 1-X/Y compounds have ˛ values between 0.35 and 0.5, and the intrinsic barriers are all significantly lower than predicted for a concerted DET, thereby also suggesting a stepwise DET mechanism.