Pierluigi Stipa

Synthesis and thermal stability of alkoxyamines

N-alkoxyamines are prepared by oxidizing Grignard reagents with lead dioxide in the presence of aminoxyls and a kinetic study of their thermal decomposition is described. The results obtained confirm that decomposition mainly occurs through carbon-oxygen bond cleavage.

Chemical and Electrochemical Study on the Interactions of Aminoxyls with Superoxide Anion

The electrochemical behaviour of tetramethyl-pyrrolinic, -piperidinic, indolinonic and quinolinic aminoxyls and of oxygen was studied in DMFAI20 in order to evaluate the feasibility of an electron transfer process between aminoxyls and superoxide anion. The rate constants of these reactions were calculated by applying the Marcus theory. A new mechanism for the reaction, which operates in competition with those already reported in the literature involving an electron transfer process, is proposed. Copyright

Unexpected Deoxygenation of 2,2,6,6-Tetramethylpiperidine-1-Oxyl (TEMPO) by Thiyl Radicals through the Formation of Arylsulphinyl Radicals

Abstract 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO), upon reaction with thiophenols, undergoes deoxygenation leading mainly to the formation of tetramethylpiperidinium arylsulphinates and arylsulphonates. Other identified products of the reaction are aryldisulphides, 2,2,6,6-tetramethylpiperidine, S-arylarylthiosulphonates, N-arylsulphinyl- and N-arylsulphonyl-2,2,6,6-tetramethyIpiperidine. The formation of the reaction products is discussed on the basis of the interaction of arylsulphinyl and arylsulphonyl radicals with TEMPO as well as on the basis of the evolution of the arylsulphinyl radical itself.

Reactivity of an indolinonic aminoxyl with superoxide anion and hydroxyl radicals

The increasing knowledge on the participation of free radicals in many diverse clinical and pathological conditions, has consequently expanded the search for new and versatile antioxidants aimed at combating oxidative stress. Our interest in this field concerns aromatic indolinonic aminoxyls (nitroxides) which efficiently react with alkoxyl, peroxyl, aminyl, arylthiyl and alkyl radicals to give non-paramagnetic species. This prompted us to test their antioxidant activity on different biological systems exposed to free radical-induced oxidative stress and the results obtained so far have been very promising. However little is known about their behaviour towards superoxide and hydroxyl radicals. Here, we report on the reactivity of an indolinonic aminoxyl, with the two above mentioned radicals using hypoxanthine/xanthine oxidase and potassium superoxide for generating the former and the Fenton reagent for the latter. Besides performing the deoxyribose assay for studying the reaction of the aminoxyl with hydroxyl radical and monitoring spectral changes of the aminoxyl in the presence of superoxide radical, macroscale reactions were performed in both cases and the products of the reactions isolated and identified. The EPR technique was used in this study to help elucidate the data obtained. The results show that this compound efficiently reacts with both hydroxyl and superoxide radicals and furthermore, it is capable of maintaining iron ions in its oxidized form. The results thus contribute to increasing the knowledge on the reactivity of indolinonic aminoxyls towards free radical species and as a consequence, these compounds and/or other aminoxyl derivatives, may be considered as complementary, and sometimes alternative sources for combating oxidative damage.

Homolytic substiution in indolinone nitroxides- IV. Reactions with aminyl radicals. A spectroscopic and crystallographic study

Abstract The reactions between 2,2-diphenyl-indolinone-N-oxyl (3) and a series of aminyl radicals generated in situ by oxidation of secondary aromatic amines with PbO2 lead to animated hydroxylamines (4), which in the presence of PbO2 are converted to the aminated nitroxides (5). When starting from primary amines, these are further oxidated to quinonimine-N-oxides (6) which then undergo nucleophilic substitution by another amine molecule to give the diaminated hydroxylamines (7). These are further oxidated to nitroxides (8), the final products of the reaction being the aminated quinone-diimine-N-oxides (9). The paramagnetic species (5) and (8) have been characterized using ESR spectroscopy; in addition, the molecular geometry of one of the N-oxides (9) was elucidated by means of X-ray structure analysis.

Reactivity of Sulfur‐Centered Radicals with Indolinonic and Quinolinic Aminoxyls

Indolinonic, phenylimino-indolinonic and quinolinic aromatic aminoxyls readily react with sulfur-centered radicals, generated upon reaction with p-methylthiophenol at room temperature. The main product is the deoxygenated derivative i.e. the corresponding amine. The other compounds, obtained in low yields, are N-substituted amines and amines substituted in a conjugated position with respect to the amino group by arylthiyl, arylsulphinyl, arylsulphonyl and arylsulphonyloxy radicals. The formation of the products are explained by the initial attack of the thiophenol radical onto the NO· function to give an unstable adduct which decomposes to aminyl and arylsulphinyl radicals. From here the reaction can take two different routes to give the products obtained.

Indolinic nitroxides: evaluation of their potential as universal control agents for nitroxide mediated polymerization

Indolinic nitroxides derived from 2,2-diphenyl-3-phenylimino-2,3-dihydroindol-1-yloxyl (DPAIO) nitroxide were designed to improve the bulk polymerization of methacrylate derivatives. The corresponding alkoxyamines were prepared by reacting alkyl halide and nitroxide in the presence of tin hydride and PbO2. The replacement of one phenyl ring at the C-2 position on the nitroxide by either a neopentyl or an isopropyl group led to compounds with faster decomposition kinetics and better selectivity for the C–ON bond homolysis. The alkoxyamines derived from these two nitroxides were used to control the polymerization of methyl methacrylate and were also tested for the polymerization of styrene as a model for mono-substituted monomers. The shift of the molar mass distribution during the NMP of styrene showed for the first time that pure homopolymerization of styrene and methacrylate derivatives could be controlled by the same nitroxide, thus opening the way to a universal nitroxide for NMP.

Reactivity of Ubiquinones and Ubiquinols with Free Radicals

The reactivity of quinones 1–4 and of the corresponding quinols 5–8 towards carbon- and oxygen-centred radicals were studied. All quinones bearing at least one nuclear position free, readily react with alkyl and phenyl radicals to afford the alkylated quinones 12–24; however, quinones 1 and 3 reacted with 2-cyano-2-propyl radical to yield products (the mono- and di-ethers 9–11) derived from the attack on the carbonylic oxygen. The reactions carried out on quinones with the benzoyloxy radical led to no reaction products and in the case of Q10, the isoprenic chain also remained unchanged. Quinols 5–8 reacted only with oxygencentred radicals (benzoyloxy and 2-cyano-2-propylperoxy radicals) to give the corresponding quinones. The isoprenic chain of Q10 did not undergo attack even with peroxy radicals. Carbon-centred radicals resulted unable to abstract hydrogen from the studied quinols.

Chemical and electrochemical synthesis of quinoneimine n-oxides from indolinone-3-arylimino nitroxide radicals

Abstract 2,2-Diphenyl-3-arylimino-indoline-1-oxyls are efficiently converted to the corresponding isomeric 5- and 7-quinoneimine N-oxides either by chemical (cerium ammonium nitrate) or electrochemical oxidation in aqueous acetonitrile. The electrochemical studies indicate that mono-electronic oxidation of the starting compounds leads to the corresponding oxoammonium ions. These are stable in anhydrous acetonitrile, as indicated by cyclic voltammetry, but in non-anhydrous conditions undergo rapid nucleophilic attack by water molecule followed by further oxidation to the final products. The relative economic and practical advantages of the electrochemical unvs the chemical process are discussed. The ESR parameters for the new nitroxide radicals involved in the present investigation are also reported.

Aromatic secondary amines as antioxidants for polyolefins. Part 2: phenothiazines

Abstract The thermoxidation of polypropylene (PP) at 200 °C in the presence of phenothiazine ( 1 ) and 3,7-di- tert -butylphenothiazine ( 2 ) was studied by measuring the oxygen consumption. The rate of oxygen consumption during the induction period passes through a minimum when the antioxidant concentration increases. Both compounds studied have the same critical concentrations but at high concentrations 2 is more efficient than 1 . The results obtained may be explained by the differences in the evaporation rate of 1 and 2 during oxidation. ESR signals of radicals originating from 1 and 2 after polymer oxidation at 160 °C were observed.