Gian Franco Bettinetti

On 1,3-beonzoxazepine and 3,1-benzoxazepine

The photochemistry of quinoline and isoquinoline N-oxides has been reexamined. 3, 1-Benzoxazepine, which had not been isolated as yet, was obtained from the former, while the formation of 1, 3-Benzoxazepine from the latter was demonstrated.

Heteropentalenes. On 5H-pyrazolo[1′,2′:1,2]-1,2,3-triazolo[5,4-a]-phenazinyliumide

Some properties of the heteropentalenes 5H-pyrazolo[1′,2′:1,2]-1,2,3-triazolo[5,4-a]phenazinyliumide (1) and 1,3-dimethyl-5H-pyrazolo[1′,2′:1,2]-1,2,3-triazolo [5,4-a]phenazinyliumide (2) are discussed. The u.v. absorption spectra of the free and of the protonated from are reported. Catalytic hydrogenation at room temperature reduces the C-6–C-7 double bond so that the loss of aromaticity is minimized. Compound (2) is oxidized by peracids at room temperature, with attack at the carbon atom in position 12b (negatively charged in the ‘azomethinylide’ structure of this heteropentalene) and formation of a quinone imine.

Photo-oxidation of 1,3-dimethylpyrazolo[1,2-α]benzotriazole

The dye-sensitized photo-oxidation of the title mesomeric betaine yields two pairs of epoxy-ketones and α,β-unsaturated ketones, formally arising from addition of singlet oxygen to its two 1,3-dipolar mesomeric forms (2a) and (2b); in methanol stable methoxy-hydroperoxides are obtained.

Singlet oxygen photo-oxidation of some triazapentalenes

The self-sensitized photo-oxidation of 1,3-dimethyl-5H-pyrazolo[1′,2′:1,2]-1,2,3-triazolo[3,2-a]phenazin-4-ium inner salt (1) leads to the cleavage of the triazapentalene moiety, yielding an epoxy-ketone. If the two methyl groups are absent, as in compound (2), the photo-oxidation takes place much better in the presence of dyes and yields an αβ unsaturated aldehyde. The first reaction is shown to involve singlet oxygen, the formation of which requires oxygen-promoted intersystem crossing from singlet to triplet (1), followed by energy transfer from the latter state. The rate of the singlet oxygen addition onto (1) was measured in some solvents, and showed no dependence on the polarity of the medium. The mechanism of the addition is discussed, taking into account also the results from the photo-oxidation of the 6,7-dihydro-derivatives of compounds (1) and (2).

On 1-(3,5-dimethylpyrazolyl)phenazinyl-2-nitrene

Nucleophilic intramolecular attack leading to 1,3-dimethyl-5H-pyrazolo[1′,2′:1,2][1,2,3]triazolo[3,2-a]-phenazin-4-ium inner salt (3) is the fate of the title nitrene with little intermolecular hydrogen abstraction from the solvent and no intramolecular radical attack. The same behaviour is observed from both the singlet and triplet states of this nitrene.

Photoreaction of 2-nitrophenazine 10-oxide with amines

The photodeoxygenation and the photosubstitution of the nitro-group of 2-nitrophenazine 10-oxide has been investigated by kinetics methods supplemented by quenching measurements. Photodeoxygenation by triethylamine occurs quite efficiently (Φrca. 0, 2) in acetonitrile and methanol and is little affected by triethylamine concentration. The other amines employed are much less efficient; their efficiency strongly depends on the amine concentrations. Examination of the fluorescence quenching data with the quantum yields of reduction allows the conclusion that in the case of triethylamine, the main pathway of reduction involves an exciplex derived from the singlet state. The same pathway is also proposed for the reduction by concentrated solutions of other amines in acetonitrile. In different conditions a pathway from a triplet state seems to be more important. Both the pathways lead to the same intermediate. Quenching of the reduction by oxygen and by tetramethyldiazetine dioxide supports this point. The mechanism of photosubstitution of the nitro group is also discussed.

Photochemistry of some azaphenanthrene N-oxides

1- and 4-Azaphenanthrene N-oxides undergo solvent-dependent photoisomerization, yielding naphtho-1,3-oxazepines in aprotic solvents and benzoquinolin-1 (2H)-ones in water. No ring enlargement but only isomerization to the corresponding lactam is observed to take place in the case of 9-azaphenanthrene N-oxide. Measurements of the quantum yield of two photoprocesses (lactam formation and ring enlargement) confirm the difference between the photochemical behaviour of 9-azaphenanthrene N-oxide and that of the other two N-oxides examined

Solvent effects in the photoisomerization of phenazine 5-oxide

The photoisomerization of phenazine 5-oxide in acetonitrile–water mixtures has been investigated. The quantum yields of 10H-azepino[1,2-a]benzimidazol-10-one (3) and 2-(4-cyanobuta-1,3-dienyl)benzoxazole (4) are closely related to solvent composition in that they decrease strongly with increasing content of water. In acetonitrile ϕ3 and ϕ4 are temperature independent. The quantum yield of oxepino[2,3-b]quinoxaline (5) is almost independent of the composition of the solvent mixtures. A mechanism is proposed which relates the quantum yield variations to the properties of the excited state.

Photochemical reactions of amines on nitrophenazines. Part 2. Photoreduction versus photosubstitution

The (n–π*) singlet excited state of 2-nitrophenazine reacts efficiently with amines. With tertiary amines extraction of a hydrogen atom from the α-carbon of the amine takes place to yield the 2-nitrophenazinyl radical (limiting quantum yield 0.64). The reaction occurs through the intermediate formation of a non-emitting exciplex, while a ground state complex formed only at high amine concentration is unreactive. The triplet excited state is similarly reduced by triethylamine. On the other hand, primary amines do not reduce but add to the triplet excited state of 2-nitrophenazine yielding addition products which are then dehydrogenated by oxygen and/or other ground state molecules of 2-nitrophenazine to give alkylaminonitrophenazines.

PHOTOCHEMICAL DECOMPOSITION OF 2‐AZIDOPHENAZINE IN THE PRESENCE OF AMINES

Die Photozersetzung der Titelverbindung (I) in 0,1 M-Losungen der Amine (II) in Acetonitril liefert neben dem Aminophenazin (III) die Alkylamino-aminophenazine (IV).