Kinetics and thermodynamics of reversible disproportionation–comproportionation in redox triad oxoammonium cations – nitroxyl radicals – hydroxylamines

Kinetics and equilibrium of the acid‐catalyzed disproportionation of cyclic nitroxyl radicals R₂NO^? to oxoammonium cations R₂NO⁺ and hydroxylamines R₂NOH is defined by redox and acid–base properties of these compounds. In a recent work (J. Phys. Org. Chem. 2014, 27, 114‐120), we showed that the kinetic stability of R₂NO^? in acidic media depends on the basicity of the nitroxyl group. Here, we examined the kinetics of the reverse comproportionation reaction of R₂NO⁺ and R₂NOH to R₂NO^? and found that increasing in –I‐effects of substituents greatly reduces the overall equilibrium constant of the reaction K₄. This occurs because of both the increase of acidity constants of hydroxyammonium cations K_3H+ and the difference between the reduction potentials of oxoammonium cations E_R2NO+/R2NO? and nitroxyl radicals E_R2NO?/R2NOH. pH dependences of reduction potentials of nitroxyl radicals to hydroxylamines E_1/3Σ and bond dissociation energies D(O–H) for hydroxylamines R₂NOH in water were determined. For a wide variety of piperidine‐ and pyrrolidine‐1‐oxyls values of pK_3H+ and E_R2NO+/R2NO? correlate with each other, as well as with the equilibrium constants K₄ and the inductive substituent constants σ_I. The correlations obtained allow prediction of the acid–base and redox characteristics of redox triads R₂NO^?–R₂NO⁺–R₂NOH. Copyright © 2014 John Wiley & Sons, Ltd.