Kinetics and thermodynamics of reversible disproportionation-comproportionation in redox triad oxoammonium cations - Nitroxyl radicals - Hydroxylamines

Abstract

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 K3H+ and the difference between the reduction potentials of oxoammonium cations ER2NO+/R2NO· and nitroxyl radicals ER2NO·/R2NOH. pH dependences of reduction potentials of nitroxyl radicals to hydroxylamines E1/3σ and bond dissociation energies D(O-H) for hydroxylamines R2NOH inwater were determined. For a wide variety of piperidine- and pyrrolidine-1-oxyls values of pK3H+ and ER2NO+/R2NO· correlate with each other, as well aswith the equilibriumconstants K4 and the inductive substituent constants ωI. The correlations obtained allowprediction of the acid-base and redox characteristics of redox triads R2NO·-R2NO+-R2NOH.