1.2.3- Thiadiazole, pyrolysis

High-vacuum pyrolysis of 2,5-dimercapto-l,3,4-thiadiazole 34 and 2-mercapto-5-methyl-l,3,4-thiadiazole 9 performed between ambient and 800 °C gave products that were trapped by matrix-isolation techniques and characterized by IR spectroscopy. Pyrolysis of the dimercaptothiadiazole 34 gave HNCS, CS2, and HCN (Equation 2), whereas the thiadiazolethione 9 showed a more complex fragmentation pattern forming HNCS, CH3NCS, HCN, and CS2 (Equation 3) <2002J(P2)1620>. 

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. 

Thioketenes can be prepared in several ways, from carboxylic acid chlorides by thionation with phosphorus pentasulfide [1314-80-3], P2S5, from ketene dithioacetals by p-elimination, from 1,2,3-thiadiazoles with flash pyrolysis, and from alkynyl sulfides (thioacetylenes). The dimerization of thioketenes to 2,4-bis(alkylidene)-l,3-dithietane compounds occurs quickly. They can be cleaved back pyrolytically (63). For a review see Reference 18. 

Brown and coworkers105 used microwave spectroscopy to determine the structure of propadienethione H2C=C=C=S (16) through the analysis of the rotational constants for several of its isotopomers (obtained by pyrolysis of cyclopenteno-l,2,3-thiadiazole and deuterated derivatives). The main structural parameters are shown in Scheme 3a. A most remarkable (and yet unexplained) feature is the fact that this molecule has a C2V geometry, while propadienone, H2C=C=C=0 (17) is kinked 105, as shown in Scheme 3b. 

BSF1149) and a plot of the tt densities against the log rates of pyrolysis of l-(thiazolyl)ethyl acetates is linear. The reactivity order for 1,2,5-diazoles from ir-density measurements is oxadiazole > selenadia-zole > thiadiazole (73CHE1331) the effects of substituents in 1,2,4-tri-azole have also been calculated (71CHE377). 

In the study of novel fluoroionophores as receptors for transition metal cations, 1,2,5-thiadiazole-1,1-dioxide 322 is used as an intermediate to produce the 4,5-dicyano analog 323. Upon treatment of compound 321 wth sulfamide and HCl in anhydrous ethanol, 1,2,5-thiadiazole-1,1-dioxide 322 is obtained. Vacuum pyrolysis of compound 322 yields 1-nitro-4,5-dicyanonaphthalene 323 <03TL2087>. 

Flash-vacuum pyrolysis at 550 °C/0.08 Torr of 2-(dimethylamino)-5-phenyl-6//-l,3,4-thiadi-azin-6-ones results in thermal fragmentation with formation of the corresponding N,N-dimethyl-5-phenyl-l,3,4-thiadiazol-2-amine together with dimethylcyanamide and benzo-nitrile.92... 

Without additional reagents Thioketenes from 1,2,3-thiadiazoles Flash vacuum pyrolysis... 

So far 1,2,3-thiadiazoles have found little application in organic synthesis. Flash pyrolysis to give thioketenes and ring opening to produce acetylenes can occur. The latter are preferably prepared by thermolysis of 1,2,3-selenadiazoles ... 

The next higher member of this series, C5OS 8, was generated by pyrolysis ofbenzo-[l,2-d 4,5-d]bis[l,2,3]thiadiazole-4,7-dione 7 or matrix photolysis of thiophenetetracarboxylic acid bis-thioanhydride 9. ... 

The Diels-Alder reaction of thioketenes with cyclopentadiene and 2,3-dimethylbutadiene proceeds across the C=S bond of the thioketene to give the adducts in high yields. For example, trapping of dichlorothioketene, obtained in the flash vacuum pyrolysis of its dimer, with cyclopentadiene in an argon matrix at -196 °C, affords the [4-1-2] cycloadduct ". Also, pyrolysis of 5-isopropyl-4-phenyl-1,2,3-thiadiazole at 530 C and trapping of the thioketene 83 with cyclopentadiene gives a quantitative yield of the [4-1-2] cycloadduct 84 . ... 

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