Matrix isolation and computational study of the photochemistry of p-azidoaniline

The photochemistry of p-azidoaniline was studied in argon matrices in the absence and presence of oxygen. With the help of quantum chemical calculations we were able to characterize the triplet p-aminophenylnitrene as well as the cis- and trans-p-aminophenylnitroso oxides. It was found that the latter two isomers can be interconverted by selective irradiation and that they are ultimately converted into p-nitroaniline. Although restricted wavefunctions of the nitroso oxides are unstable, CASSCF calculations turned up no evidence for the claimed diradical character of these compounds. Also we found no evidence for dioxaziridines as intermediates of the conversion of the nitroso oxides to p-nitroaniline

Matrix isolation and computational study of the photochemistry of 1,3,2,4-Benzodithiadiazine

Photolysis of 1,3,2,4-benzodithiadiazine (1) at ambient temperature yields stable 1,2,3-benzodithiazolyl radicals. In order to reveal the mechanism of this unusual transformation, the photochemistry of 1 was studied in argon matrices using IR and UV-vis spectroscopy. A series of intermediates, including four- and five-membered heterocyclic and o-quinoid acyclic species, were characterized spectroscopically with the help of quantum chemical calculations. With selective irradiation, these intermediates can be mutually interconverted as well as converted back to the starting compound

Photochemical study on the reactivity of tetrasulfur tetranitride, S₄N₄

To elucidate the multifaceted but poorly understood chemistry of the pivotal polysulfur−nitrogen heterocycle, tetrasulfur tetranitride (S₄N₄, 1), its photochemistry was studied in Ar matrices. Thereby two primary intermediates and a secondary one (2−4) were detected, and their UV−vis and IR spectra were identified through specific interconversions of 1−4 that can be induced by selective irradiations. The structures associated with these spectra were assigned with the help of DFT calculations. From these assignments it follows that, under the conditions of the present experiments, the cage structure of 1 transforms into isomeric structures 2−4, one of which is a boat-shaped 8-membered cycle (2), and the two other are novel 6-membered S₃N₃ cycles carrying exocyclic (N)SN (3) or (S)N=S (4) groups, respectively, which have not been previously described. These three intermediates probably play a pivotal role in the formation of the diverse products that are observed in the reactions of S₄N₄ even under mild reaction conditions

Radical anions, radical-anion salts, and anionic complexes of 2,1,3-benzochalcogenadiazoles

By means of cyclic voltammetry (CV) and DFT calculations, it was found that the electron‐acceptor ability of 2,1,3‐benzochalcogenadiazoles 1–3 (chalcogen: S, Se, and Te, respectively) increases with increasing atomic number of the chalcogen. This trend is nontrivial, since it contradicts the electronegativity and atomic electron affinity of the chalcogens. In contrast to radical anions (RAs) [1].− and [2].−, RA [3].− was not detected by EPR spectroscopy under CV conditions. Chemical reduction of 1–3 was performed and new thermally stable RA salts [K(THF)]+[2].− (8) and [K(18‐crown‐6)]+[2].− (9) were isolated in addition to known salt [K(THF)]+[1].− (7). On contact with air, RAs [1].− and [2].− underwent fast decomposition in solution with the formation of anions [ECN]−, which were isolated in the form of salts [K(18‐crown‐6)]+[ECN]− (10, E=S; 11, E=Se). In the case of 3, RA [3].− was detected by EPR spectroscopy as the first representative of tellurium–nitrogen π‐heterocyclic RAs but not isolated. Instead, salt [K(18‐crown‐6)]+2[3‐Te2]2− (12) featuring a new anionic complex with coordinate Te−Te bond was obtained. On contact with air, salt 12 transformed into salt [K(18‐crown‐6)]+2[3‐Te4‐3]2− (13) containing an anionic complex with two coordinate Te−Te bonds. The structures of 8–13 were confirmed by XRD, and the nature of the Te−Te coordinate bond in [3‐Te2]2− and [3‐Te4‐3]2− was studied by DFT calculations and QTAIM analysis

Radical Anions, Radical‐Anion Salts, and Anionic Complexes of 2,1,3‐Benzochalcogenadiazoles

By means of cyclic voltammetry (CV) and DFT calculations, it was found that the electron‐acceptor ability of 2,1,3‐benzochalcogenadiazoles 1–3 (chalcogen: S, Se, and Te, respectively) increases with increasing atomic number of the chalcogen. This trend is nontrivial, since it contradicts the electronegativity and atomic electron affinity of the chalcogens. In contrast to radical anions (RAs) [1].− and [2].−, RA [3].− was not detected by EPR spectroscopy under CV conditions. Chemical reduction of 1–3 was performed and new thermally stable RA salts [K(THF)]+[2].− (8) and [K(18‐crown‐6)]+[2].− (9) were isolated in addition to known salt [K(THF)]+[1].− (7). On contact with air, RAs [1].− and [2].− underwent fast decomposition in solution with the formation of anions [ECN]−, which were isolated in the form of salts [K(18‐crown‐6)]+[ECN]− (10, E=S; 11, E=Se). In the case of 3, RA [3].− was detected by EPR spectroscopy as the first representative of tellurium–nitrogen π‐heterocyclic RAs but not isolated. Instead, salt [K(18‐crown‐6)]+2[3‐Te2]2− (12) featuring a new anionic complex with coordinate Te−Te bond was obtained. On contact with air, salt 12 transformed into salt [K(18‐crown‐6)]+2[3‐Te4‐3]2− (13) containing an anionic complex with two coordinate Te−Te bonds. The structures of 8–13 were confirmed by XRD, and the nature of the Te−Te coordinate bond in [3‐Te2]2− and [3‐Te4‐3]2− was studied by DFT calculations and QTAIM analysis

Carbocyclic functionalization of quinoxalines, their chalcogen congeners 2,1,3-benzothia/selenadiazoles, and related 1,2-diaminobenzenes based on nucleophilic substitution of fluorine

Previously unknown mono-, di- and in some cases tri- and tetra- carbocycle-substituted quinoxalines (2–8), 2,1,3-benzothiadiazoles (11, 12, 14–17) and 2,1,3-benzoselenadiazoles (20-25) were synthesized by nucleophilic substitution of fluorine in 5,6,7,8-tetrafluoroquinoxaline (1) and 4,5,6,7-tetrafluoro-2,1,3-benzothia/selenadiazoles (10 and 19, respectively) with methoxide and dimethylamine. In the 1:1 reactions, the nucleophiles attacked selectively the position 6 of 1 or the position 5 of 10 and 19. The regioselective nature of the 1:1 reactions was confirmed by the DFT calculations at the M06-2X/6-31+G(d,p) level of theory. Disubstituted quinoxaline (28), thia- (29) and selena- (30) diazoles bearing two different substituents, i.e. MeO- and Me2N-, were synthesized in a similar way. New substituted 1,2-diaminobenzenes (31–33) were prepared by reduction of corresponding thiadiazoles (12, 14, 15) and isolated in the form of hydrochlorides. Compound 33 was converted into a new quinoxaline (34) by reaction with (PhCO)2. Compounds 5, 7 and 14 were studied for cytotoxicity toward the human cancer cells and effects on the cytochrome P450 mRNA expression. They did not cause any significant modulations in the expression of several cytochrome P450 genes, and 7 was weakly toxic for the Hep2 (carcinoma) and U937 (leukemia) cells, particularly, apoptosis was observed

Carbocyclic functionalization of quinoxalines, their chalcogen congeners 2,1,3-benzothia/selenadiazoles, and related 1,2-diaminobenzenes based on nucleophilic substitution of fluorine

Previously unknown mono-, di- and in some cases tri- and tetra- carbocycle-substituted quinoxalines (2–8), 2,1,3-benzothiadiazoles (11, 12, 14–17) and 2,1,3-benzoselenadiazoles (20-25) were synthesized by nucleophilic substitution of fluorine in 5,6,7,8-tetrafluoroquinoxaline (1) and 4,5,6,7-tetrafluoro-2,1,3-benzothia/selenadiazoles (10 and 19, respectively) with methoxide and dimethylamine. In the 1:1 reactions, the nucleophiles attacked selectively the position 6 of 1 or the position 5 of 10 and 19. The regioselective nature of the 1:1 reactions was confirmed by the DFT calculations at the M06-2X/6-31+G(d,p) level of theory. Disubstituted quinoxaline (28), thia- (29) and selena- (30) diazoles bearing two different substituents, i.e. MeO- and Me2N-, were synthesized in a similar way. New substituted 1,2-diaminobenzenes (31–33) were prepared by reduction of corresponding thiadiazoles (12, 14, 15) and isolated in the form of hydrochlorides. Compound 33 was converted into a new quinoxaline (34) by reaction with (PhCO)2. Compounds 5, 7 and 14 were studied for cytotoxicity toward the human cancer cells and effects on the cytochrome P450 mRNA expression. They did not cause any significant modulations in the expression of several cytochrome P450 genes, and 7 was weakly toxic for the Hep2 (carcinoma) and U937 (leukemia) cells, particularly, apoptosis was observed