1,2,3-Thiadiazole photolysis

Photolysis of 1,2,3-thiadiazole (35) gives thiirene (36) which can be trapped by an alkyne (70AHC(ll)l). 4,5-Diphenyl-l,2,3-thiadiazole (37) is photolyzed at low temperatures to the thiobenzoylphenylcarbene triplet (38). Diphenylthioketene (39) is formed on warming (81AHC(28)231). 

Phenyl-l,2,3-thiadiazole (513) as well as the 5-phenyl isomer (514) both lose N2 on photolysis. The 1,3-dithiole derivative shown was formed from both thiadiazoles (58LA(614)4). 

Thiirenes have been isolated in argon matrices at 8 K by photolysis of 1,2,3-thiadiazoles or vinylene trithiocarbonates (Scheme 151) (80PAC1623, 8UA486). They are highly reactive and decompose to thioketenes and alkynes (Scheme 22). Electron withdrawing substituents stabilize thiirenes somewhat, but no known thiirene is stable at room temperature unlike the relatively stable thiirene 1-oxides and thiirene 1,1-dioxides. 

Unlike simple 1,2,3-thiadiazoles, 1,2,3-benzothiadiazoles do not form thioketenes on thermolysis or photolysis. Instead, they yield many products, depending on the conditions (Equation 3). The mechanisms of these reactions have been extensively studied <1984CB107>. 

Already in the first experiment of this series (1990) we found136 that photolysis or pyrolysis of benzotris(thiadiazole) 98 leads to pentatetraenedithione (99), the third carbondisulfide beyond the already known low members CS2 and C3S2. The long-chain derivative 99 can be kept in a solution even at room temperature for some time without polymerization. 

CASRN 51707-55-2 molecular formula C9H8N4OS FW 220.20 Soil. The reported half-life in soil is approximately 26-144 d (Hartley and Kidd, 1987). Photolytic. Klehr et al. (1983) studied the photolysis of thiadiazuron on adsorbed soil surfaces. Irradiation was conducted using artificial sunlight having a wavelength <290 nm. The amount of thiadiazuron remaining after irradiation times of 0.25, 0.5, 1, 2, 3.75, and 18.0 h were 56.4, 42.8, 35.7, 23.8, 25.0, and 67.2%, respectively. The primary photoproduct identified was l-phenyl-3-(l,2,5-thiadiazol-3-yl)urea and five unknown polar compounds. The unknown com pounds could not be identified because the quantities were too small to be detected. 

Photolysis (2537 A) in methyl cyanide solution of the compound 243, R = Me, = Ph, gives iV-methylthiobenzamide. This reaction has been interpreted in terms of the homolysis of the valence tautomer 244, which has been detected spectroscopically ( % 2260 cm N=C=0). The l,3,4-thiadiazol-2-ones have dipole moments (243, R = R = Ph,/U = 7.75 D 243, R = Me, R = Ph, /U = 7.7 D) consistent with their meso-ionic formulation. They react with triethyloxonium tetra-fluoroborate, yielding the salts 245. The compound 243, R = Me, R = Ph, and aniline gives the semicarbazide 246. ... 

Thiirene intermediates in the photolysis of 1,2,3-thiadiazoles readily undergo ring opening to a diradical which can be trapped by reaction with an alkyne (Scheme 25) (79CB1769). Significant polymerization is not observed. 

Nitrogen extrusion has been used to make fragile molecules 2-thiirene (99) has been obtained by matrix photolysis of 1,2,3-thiadiazole (98), and azirines from 4//-triazoles (100 — 101). 

The photolysis of 1,2,3-thiadiazoles also leads to the evolution of nitrogen, but the unstable thiirene is not obtained. 1,2,3-Thiadiazole itself yields nitrogen, acetylene, and polymer.96 In the presence of perfluorobut-2-yne, however, 2,3-bis(trifluoromethyl)thiophene is... 

The photolysis of 1,2,4-oxadiazoles in the presence of sulfur nucleophiles has been shown to afford 1,2,4-thiadiazoles. N—S bond formation between the ring species and the sulfur nucleophile is thought to account for the observed products.96 A review has appeared which includes an account of the rearrangement of 1,2,3-thiadiazoles to other heterocycles such as 1,2,3-triazoles and 1,2,3,4-thiatriazoles.97... 

The most convenient sources of species 89 are 1,2,3-thiadiazoles 91, which are valence tautomers of the unknown a-diazothioketones. Loss of nitrogen from 91 may be achieved by irradiation or by thermolysis. Larsen and coworkers383 examined the irradiation of 91 in the presence of diethylamine and isolated N, IV-diethylthioacetamide in high yield, which implies trapping of thioketenes during photolysis. Mechanistic studies excluded thiirene as the intermediate in the photolysis at 150 K (equation 95). 

This explains the formation of various products like benzonitrile, 3-phenyl-l,2,4-thiadiazole 103, 2-phenyl-l,3,5 triazine 105a/b, and diphenyl 1,3,5-triazine 104a/b in the photolysis of 5-phenyl-l,2,4-thiadiazole 97. 

Dihydro-4,4,6,6-tetramethylfuro[3,4-rf]-l,2,3-thiadiazole (37) has been used to prepare some highly reactive intermediates. Photolysis of compound (37) in the presence of furan gave the adduct (38) in 11% yield a similar reaction in the presence of 2,5-dimethylfuran gave the adduct (39) in 12% yield. The authors claim that formation of adducts (38) and (39) unequivocally proves the intermediacy of the antiaromatic thiirene (40) <87TL2867>. 

In an attempt to prepare the 1,2-dithione (41), 4,6-dihydro-4,4,6,6-tetramethylfuro[3,4-d]-l,2,3-thiadiazole (37) was treated with molten sulfur, yielding a mixture of cyclopolysulfides. Subsequent photolysis of this mixture in the presence of an alkene (e.g., norbornene) gave adducts such as compound (42), indicating the intermediacy of the dithione (41) (87TL4833). 

Evidence for the formation of diradical (14 equation 6) from the photolysis of 1,2,3-thiadiazoles has been gained by ESR work (79JA3976). Photolysis of thiadiazoles at low temperature led to the characterization of thioketenes by IR and a thiirene was postulated as a reaction intermediate (74JA6768). 

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