Ricerc@Sapienza

The oxidation of a series of hydrocarbons by the nonheme iron(IV)-oxo complex [(N4Py)FeIV═O]2+ is efficiently mediated by N-hydroxyphthalimide. The increase of reactivity is associated to the oxidation of the mediator to the phthalimide N-oxyl radical, which efficiently abstracts a hydrogen atom from the substrates, regenerating the mediator in its reduced form.

A kinetic study of the hydrogen atom transfer (HAT)reaction from a series of N-Boc- or N-Acetyl-protected amino acids to the phthalimide N-oxyl radical (PINO)was carried out to obtain information about reactivity and selectivity patterns. With amino acids containing aliphatic side chains, the 2nd order rate constants are of the same order of magnitude, in agreement with a HAT process involving the Cα−H bond. Proline is the most reactive substrate suggesting that HAT process involves the Cδ−H bond instead of Cα−H bond.

Evaluation of polar effects in hydrogen atom transfer (HAT) processes is made difficult by the fact that in most cases substrates characterized by lower bond dissociation energies (BDEs), activated from an enthalpic point of view, are also more activated by polar effects. In search of an exception to this general rule, we found that the introduction of a methoxy substituent in the 3-position of 2,6-dimethylphenol results in a small increase in the O-H BDE and a decrease of the ionization potential of the phenol.

1,8-Naphthalenediol (dihydroxynaphthalene, 1,8-DHN) has been shown to be a potent hydrogen atom transfer (HAT) antioxidant compound because of the strong stabilization of the resulting free radical by intramolecular hydrogen bonding. However, the properties, reactivity, and fate of the 1,8-DHN phenoxyl radical have remained so far uncharted. Herein, we report an integrated experimental and computational characterization of the early intermediates and dimer products that arise by the oxidation of 1,8-DHN.