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 R 2 NO• to oxoammonium cations R 2 NO + and hydroxylamines R 2 NOH 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 R 2 NO• in acidic media depends on the basicity of the nitroxyl group. Here, we examined the kinetics of the reverse comproportionation reaction of R 2 NO + and R 2 NOH to R 2 NO• and found that increasing in -I-effects of substituents greatly reduces the overall equilibrium constant of the reaction K 4 . 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 R 2 NOH 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 K 4 and the inductive substituent constants σ I . The correlations obtained allow prediction of the acid-base and redox characteristics of redox triads R 2 NO• -R 2 NO + -R 2 NOH.

Oxygen Atom Transfer in the Oxidation of Dimethyl Sulfoxide by Oxoammonium Cations

“…The studies of the redox characteristics of NR by the method of cyclic voltammetry had demonstrated that, in many solvents (H 2 O, MeCN, DMF), electron transfer in the pair of NR–OC is characterized by a high degree of reversibility. , However, our experiments and literature data showed the presence of an irreversible process in DMSO, as evidenced by a decrease in the amplitude of the current–voltage curves during repeated oxidation–reduction cycles. Since the NR solutions in DMSO are stored unchanged for long time periods exceeding weeks, the irreversibility mentioned can only be caused by the reaction of DMSO with OC.…”

“…The synthesis of oxoammonium perchlorates 1 was carried out according to the methods described elsewhere. 18 The purity of salts 1, checked by iodometric titration and/or by the 1 H NMR, was 98 ± 2%. MeCN was purified by distillation from P 2 O 5 and stored over 3 Å molecular sieves.…”

Impact of Group 10 Metals on the Solvent‐Induced Disproportionation of o‐Semiquinonato Complexes

“…To gain further insight into the observed trends regarding the solvent‐, metal‐ion‐, and temperature‐dependent disproportionation behavior of {[M](PF 6 )} n (M=Ni ( n =2), Pd ( n =1), and Pt ( n =1)), we conducted electrochemical studies as the redox potential serves as an indicator of the stability of each species . Figure shows the cyclic voltammograms of [M] (M=Ni, Pd, and Pt) recorded in CH 2 Cl 2 and in DMF containing 0.1 m (TBA)(PF 6 ) as the supporting electrolyte (see Figures S22 and S23 in the Supporting Information).…”

Protein Spin Labeling with a Photocaged Nitroxide Using Diels–Alder Chemistry

“…For the synthesis of PaNDA label 1 , carboxy TEMPO 5 was quantitatively transformed into hydroxylammonium salt 6 by making use of the acid‐dependent redox triad of a nitroxyl radical, an oxoammonium cation, and hydroxylamine (Scheme A) . Conveniently, the carboxylic acid is simultaneously protected under these conditions.…”