Thesis (M.Sc.)--Chulalongkorn University, 2001

This research involves the exploration in the photochemical reactions of phenyl substituted-1,2,4-thiadiazoles. 3- And 5-phenyl-1,2,4-thiadiazole were initially synthesized and their photochemical behaviors were investigated. Since the structure of 1,2,4-thiadiazole system could be viewed as a combination of isothiazole and thiazole, thus, the photoproduct formation could be predicted by analogy with thiazole and isothiazole. Therefore, 3-pheny-1,2,4-thiadiazole and 2-phenyl-1,3,4-thiadiaozle were anticipated to observe upon the reaction of 5-phenyl-1,2,4-thiadiazole. In contrast, 3-phenyl-1,2,4-thiadiazole should undergo phototransposition reaction to 5-phenyl-1,2,4-thiadiazole and 2-phenyl-1,3,4-thiadiazole. The photoreactions were carried out with appropriate light sources and monitored by GC-FID and GC-MS. The product identification was performed by the comparison of their chromatographic and mass spectroscopic data with the authentic samples. Upon irradiation of 5-phenyl-1,2,4-thiadiazole, the results indicated the generation of four major and four minor products while only one quantitatively detectable product was observed on the reaction of 3-phenyl-1,2,4-thiadiazole. According to the mass spectral analysis, the reaction of 5-phenyl-1,2,4-thiadiazole gave benzonitrile, 3-phenyl-1,2,4-thiadiazole, 3,5-diphenyl-1,2,4-thiadiazole, phenyl- and diphenyl-sym-triazine while the other three products were observed in trace amount and, thus, unidentifiable. Benzonitrile was observed in quantitative yield upon irradiation of 3-phenyl-1,2,4-thiadiazole while the predicted transposition product was undetectable. The formation of these photoproducts can be explained by a mechanism involving either electrocyclic ring closure followed by heteroatom migration or the cleavage of the S-N bond and loss of sulfur. Benzonitrile sulfide was suspected to serve as an intermediate for the formation of 3,5-diphenyl-1,2,4-thiadiazole. The presence of benzonitrile sulfide wasproved by allowing it to react with ethyl cyanoformate to yield ethyl 3-phenyl-1,2,4-thiadiazole-5-carboxylate.