The relationship between redox potentials and substituent constants in biologically active arylazoxy compounds

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Seven compounds containing the cyanoazoxy group were studied by cyclic voltammetry, in protic (aqueous/ethanolic phosphate buffer, pH 6.9) and aprotic (DMF + TBAP 0.1 mol L −1) media. In protic medium, all the arylazoxycyanides containing electrowithdrawing groups behaved similarly. A 4e −/4H + reduction wave generating the hydrazo derivative is evident, as well as its counterpart for oxidation, in the reverse scan, to the azo derivative, which undergoes further reduction, in a successive scan. For the compounds with electrodonating substituents, slightly different reduction behaviour is evident, due to the instability of the electrogenerated azo compound. In aprotic medium, all the arylazoxycyanides follow a similar and standard electrochemical reduction pathway, represented by two main waves, the first one reversible, corresponding to the uptake of one electron to form a stable anion-radical and a second one, much more intense and irreversible. In the case of the electrowithdrawing nitro and cyano derivatives, additional waves are observed, due to the reducibility of those groups. In both media, linear relationships between E pIc (potential of first cathodic wave), E redox (for aprotic) and Hammett substituent constants ( σ p, σ −) were observed. The reduction potentials correlate considerably better with ( σ p, σ −), showing the strong influence of resonance factors in the mechanism of reduction.