Hydrazones are easily oxidised by molecular oxygen, yielding α-azohydroperoxides [1, 2]. The rate determining reaction in a propagation step appears to be the abstraction of hydrogen atom from the NH group [1]. Our measurements show that it was of comparable magnitude to those of hydrogen atom abstraction from 1,3- and 1,4-dihydrobenzene with rate constants of about 10³ M^-1s^-1 [3].

In order to measure the rate of binding of molecular oxygen to the hydrazonyl radical, cyclopropylcarbinyl radical clocks [4] were synthesized. We found out that the rate constant of the addition of oxygen is of the order of 10⁹ M^-1s^-1.

The overall rates of autoxidation were measured by means of ¹H NMR and UV-Vis spectroscopy in solutions, saturated with oxygen (pseudo-first order conditions). Good correlation between Hammett s and logk_rel was obtained, giving r values of -2.0 and -1.0 for substituents on the arylhydrazone and arylketone moiety, respectively. Pseudo-first order rate constants were about 10^-3 s^-1for the autoxidation of arylhydrazones of aliphatic ketones, about 10^-5 s^-1 for aromatic aldehydes and ketones and 10^-7 s^-1 for nitro-substituted substrates and phenylhydrazones of conjugated ketones and aldehydes. The latter also inhibit the autoxidation of the more reactive hydrazones.

Quantumchemical DFT calculations demonstrated that the energies of hydrogen abstraction with hydroperoxy radicals are in the range of +4 to -7 kcal/mol, decrease with the number of alkyl substituents and are the lowest for hydrazones with aryl subtituents. The energies of activation appear to be relatively low, below 10 kcal/mol, also dependent on the number and structure of the substituents.

[1] K. H Pausacker, J. Chem. Soc. 1950, 3478-3481.

[2] R. Criegee, G. Lohaus, Chem. Ber. 1951, 84, 219-224.

[3] L. Valgimigli, K. U. Ingold, J. Lusztyk, J. Org. Chem. 1996, 61, 7947-7950.

[4] D. C. Nonhebel, Chem. Soc. Rev. 1993, 22(5), 347-359.

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