Tullio Caronna

Oxidative degradation of dyes by ultraviolet radiation in the presence of hydrogen peroxide

The decolorization and mineralization of some azo and anthraquinone dyes by photoactivated hydrogen peroxide has been studied. The degradation process seems to occur according to a similar mechanism for all the selected dyes. Decolorization is complete in a relatively short time and follows apparent first order kinetics, whereas mineralization requires longer irradiation times. Initially fluorescent intermediates are generated in all cases by hydroxylation of the studied compounds. A simple kinetic model, describing adequately the process, has been proposed; pH does not influence significantly the process in the range going from 3 to 9.

Nucleophilic character of alkyl radicals—XII : Mechanism and new syntheses in the oxidation of alcohols by peroxydisulphate

Abstract The debated mechanism of the oxidation of alcohols by peroxydisulphate was investigated by trapping the nucleophilic carbon-centered free-radical intermediates by protonated quinoline. The oxidation of t-butanol, 1-hexanol, 2,3-dimethyl-2-butanol and ethylene glycol in the presence of silver salt takes place via alkoxy radical intermediates arising from the interaction R-OH + Ag 2+ → R-O · + Ag + + H + ; it gives rise to new interesting selective synthetic processes. In the absence of silver salt the hydrogen abstraction from CH bonds is the main reaction.

Photoinduced molecular rearrangements. The photochemistry of 1,2,4-oxadiazoles in the presence of sulphur nucleophiles. Synthesis of 1,2,4-thiadiazoles

Abstract The photochemistry of some 1,2,4-oxadiazoles in the presence of sulphur nucleophiles has been investigated. Irradiation of the 5-amino-3-phenyl- and 3,5-diphenyl-1,2,4-oxadiazole at γ = 254 nm in methanol in the presence of sodium hydrogen sulphide or thiols gave a photo-induced redox reaction at the ring ON bond, leading to the corresponding N -substituted benzamidines. By contrast, irradiation of the 5-amino-3-phenyl-1,2,4-oxadiazole in the presence of thioureas or thiocarbamates, essentially gave 3-phenyl-5-substituted 1,2,4-thiadiazoles, which presume an NS bond formation between the ring-photolytic species and the sulphur nucleophile. In turn, irradiation of the same 5-amino-3-phenyl-1,2,4-oxadiazole in the presence of thioamides again afforded the redox reaction; in addition, amounts of 3-phenyl-5-substituted-1,2,4-thiadiazoles were also formed. Some mechanistic considerations are reported, and synthetic methodologies leading to 1,2,4-thiadiazoles are emphasized.

Comparative evaluation of photodynamic efficiency of some natural quinonoid fungal toxins

Abstract A scale of photodynamic efficiency of some quinonoid fungal metabolises measured in vitro as amount of hydroperoxides produced by methyl oleate est

Photodecomposition of substituted 4-diethylaminoazobenzenes under visible light irradiation in different solvents

Abstract The visible light-induced decomposition of two series of 4-diethylaminoazobenzenes has been studied in solvents of different polarity, in the presence and absence of O2, in order to clarify the reaction mechanism. Two different paths have been recognised, whose importance depends on the nature of the solvent; both involve the formation of organic free radicals. Decomposition is much faster in acetone than in the other solvents. A strong inhibiting effect of molecular oxygen has been observed in all cases.

An easy photochemical approach to the synthesis of the food-borne carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine

Abstract Mutations induced by substances formed during food cooking are a field of growing interest; for a better comprehension of the mechanism of action of these carcinogens, simple routes to their synthesis are needed. In this letter we describe an easy method for 2-amino-1-methyl-6-phenylimidazo[4,5- b ]pyridine (PhIP) synthesis, starting from the commercially available 2,3-diaminopyridine 1 via the 2-amino-3-methylamino-5-phenylpyridine 5 formation. The key step of this approach is the one pot synthesis of 5 performed by photolysis of 2-amino-5-iodo-3-( N -methyl- N -tosylamino)pyridine 4 to obtain simultaneous phenylation and tosyl group removal. Compound 5 was then used as an intermediate to obtain the 1-methyl-6-phenylimidazo[4,5- b ]pyridine 6 which was aminated to afford PhIP in good overall yields.

Photochemistry of dicyanopyridines

Abstract The photochemistry of a variety of dicyanopyridines (2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-dicyanopyridine) in solution at room temperature was investigated. Pulsed UV (308 nm) laser irradiation in deoxygenated acetonitrile yields the triplet state with lifetimes between 4 and 10 μs and absorption bands in the 400 and 320 nm regions. In the presence of added HCl an air-insensitive transient ( τ ≈ 10–12 μ s, λ max ≈ 360–380 nm) was observed, suggesting the formation of a protonated excited state. Irradiation in the presence of amines resulted in the production of the pyridyl radical anion ( τ ≈ 40–80 μ s, air sensitive, λ max ≈ 360–380 nm) formed by electron transfer from the amine to the pyridine triplet excited state. Stern-Volmer analysis gave electron transfer rate constants in the range (1–8) × 10 −8 M −1 s −1 . In methanol solvent, irradiation yielded an air-insensitive transient assigned as the neutral pyridyl radical ( τ ≈ 30–200 μ s, λ max ≈ 370–385 nm). The formation of these transients is discussed in the context of previous photochemical electron spin resonance and product studies.

Free Radical Synthesis of Pyridylderivatives: Influence of the Solvent

The photochemically induced reactions of the azaheteroaromatic bases are strongly affected by the medium. For example, changing the acidity of the solution results in a change of mechanism of the reaction from that of hydrogen abstraction to the electron-transfer. 2, 4-Dicyanopyridine (1) is a substrate that works only by electrontransfer, but the two charged species, radical anion and radical cation, created by photochemical excitation give rise to very different reactions depending on the polarity of the solvent. In non polar medium, if (1) is photochemically excited in the presence of an alkene, substitution of a cyano group with an allyl group occurs, resulting in the formation of 2-cyano-4-allyl pyridine and 4-allyl-2-cyano pyridine. In polar solvents, however, there is the formation of a new ring between position 5 of the heterocyclic ring and the carbon atom of the cyano group in position 4, resulting in a cyclic imino derivative. An explanation of the two different paths of the reaction are given in terms of the stabilization of the intermediate ionic couple.