Synthesis and Photochemistry of Phenyl Subtituted-1,2,4-Thiadiazoles; 15N-Labeling Studies

Changtong, Chuchawin

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Synthesis and Photochemistry of Phenyl Subtituted-1,2,4-Thiadiazoles; 15N-Labeling Studies

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Photochemistry studies of phenyl substituted-1,2,4-thiadiazoles have revealed that 5-phenyl-1,2,4-thiadiazoles 31, 90, 98, 54 and 47 undergo a variety of photochemical reactions including photofragmentation, phototransposition, and photo-ring expansion while irradiation of 3-phenyl-1,2,4-thiadiazoles 46, 105 and 106 leads mainly to the formation of photofragmentation products. The formation of the phototransposition products has been suggested to arise from a mechanism involving electrocyclic ring closure and sigmatropic sulfur migration via a bicyclic intermediate: phenyl-1,3-diaza-5-thiabicyclo[2.1.0]pentene (BC). 15N-Labeling experiments confirm that sulfur undergoes sigmatropic shifts around all four sides of the diazetine ring. Thus, irradiation of 31-4-15N or 54-4-15N leads to the formation of 31-2-15N or 54-2-15N and to an equimolar mixture of 46-2-15N and 46-4-15N or 57-2-15N and 57-4-15N. Work in this laboratory on 15N-labeling of 46-2-15N also shows that 46 does not undergo electrocyclic ring closure but reacts exclusively by photofragmentation of the thiadiazole ring. 15N-Scrambling in the photofragmentation products observed after irradiation of 31-4-15N or 54-4-15N is greater than 15N-scrambling in the starting thiadiazoles suggesting that these products cannot arise only from direct fragmentation of the thiadiazole rings. An additional pathway for the formation of these products is required. The formation of phenyltriazines, the photo-ring expansion products 39 and 40 or 65 and 66 from photolysis of 31 or 54 is proposed to arise via phenyldiazacyclobutadienes (CB), generated from elimination of atomic sulfur from the bicyclic intermediates. It is suggested that phenyldiazacyclobutadienes then undergo [4+2] cycloaddition self-coupling resulting in the formation of unstable tricyclic intermediates which finally cleave to give phenyltriazines and nitriles. The observed 15N distribution in the phenyltriazine photoproducts formed after photolysis of 31-4-15N or 54-4-15N and the formation triazine 72 after irradiation of a mixture of 31+54 are consistent with this mechanism. The formation of nitriles by this pathway would account for the additional 15N-scrambling in the photofragmentation products. The photochemically generated phenyl-1,3-diaza-5-thiabicyclo[2.1.0]pentenes are the key intermediates in this suggested mechanism. In the presence of furan, these intermediates are expected to be trapped as Diels-Alder adducts. Irradiation of phenylthiadiazoles 31, 54 and 47 in furan solvent lead to increased consumption of these thiadiazoles, to quenching of the known photoproducts, and to the formation of new products suggested to result from furan trapping of the thiadiazoles followed by elimination of sulfur. Irradiation of 46 in furan solvent leads only to the formation of the photofragmentation product; no furan trapping adduct is observed. This result is consistent with the 15N-labeling experiment indicating that 46 does not undergo electrocyclic ring closure after irradiation. The photoreactivity of these phenylthiadiazoles in acetonitrile is substantially decreased when the phenyl ring at position 4 is substituted with an electron donating or withdrawing group. However, they are more photoreactive in cyclohexane solvent than in acetonitrile. The fluorescence emission spectra of these (4Â¢-substituted)phenyl-1,2,4-thiadiazoles exhibit moderate - large Stokesâ€™ shifts in acetonitrile. The magnitudes of these Stokesâ€™ shifts decrease in cyclohexane. This suggests a charge transfer character associated with the excited states of these thiadiazoles. In acetonitrile, these charge transfer excited states would be stabilized and become the lowest energy excited state. These charge transfer excited states would not be photoreactive and, thus, fluorescence emission becomes an effective deactivation process. In cyclohexane solvent, the charge transfer excited states would be less stabilized and, thus, the relaxed S1 would, then, become the lowest excited state. The relaxed S1 would be the state from which the observed photoproducts originate and the observed fluorescence with the smaller Stokesâ€™ shifts compared with the Stokesâ€™ shifts observed in acetonitrile.

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