Thioketens from 1,2,3-Thiadiazoles

Flash thermolysis is a convenient way of producing thioketenes from 1,2,3-thiadiazoles.134,138,139 Thiirenes are formed prior to thioketenes in the gas phase140 or matrix127 photolysis of 1,2,3-thiadiazoles. 

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

Thioketens from, l, i-Thiadiazoles. The synthesis of thioketens from 1,2,3-thiadiazoles, projected by Staudinger in 1916, has now been achieved on a convenient preparative scale (1—10 g), in good yield, by the flash thermolysis of 1,2,3-thiadiazoles at 580 °C and 10 Torr in a specially designed apparatus. The thermolysis of 1,2,3-thiadiazoles, preferably in boiling diglycol, also provides a practical source of thioketens for immediate further synthetic use, e.g. in the production of thioesters, as shown in Scheme 1. The use of 4- and 5-phenyl-l,2,3-... 

A -1,3,4-Thiadiazoline-1 -oxides (146) (Equation (18)) are formed by addition of diazo compounds (R2C=N2) to sulfines (R R C =S=0). The adducts from diazomethane and aryl substituted sulfines are unstable and give the thiadiazole (147) via a Pummerer-type aromatization <84CHEC-I(4)545>. The A -thiadiazoline-1,1-dioxide (149) has been produced by oxidation of the hydrazone (148) (Equation (19)) and treatment with sulfur dioxide <84CHEC-i(4)545>. 2-Alkylidenethiadiazolines can be obtained from the dipolar addition of diazo compounds to thioketenes <83CB66, 90TL3571, 92HCA1825>. 

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. 

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). 

As early as 1916, Staudinger and Siegwart predicted that nitrogen extrusion from thermally excited 4,5-diphenyl-l,2,3-thiadiazole would be facile leading to thioketenes (B-61MI42400). Their experiments only led to the isolation of tetraphenylthiophene. 

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). 

Thiadiazoles extrude nitrogen and thiirenes can be formed, but their lifetime is fleeting as they rearrange to thioketenes Scheme 3 shows the eventual formation of propadienethione 33 from 32 <1988JA789, 1990GPL (168)1>. 

Laureni and co-workers have studied the photochemical decomposition of 1,2,3-selena and 1,2,3-thiadiazoles in argon or nitrogen matrix. In each case they could identify the products as the ethynylselenol or thiol (136) and the seleno-or thioketene (137). In addition, in the case of 1,2,3-selen-adiazole acetylene was also detected. Using isotopically labelled substrates, they demonstrated that a major portion of the ethynylthiol formed from 1,2,3-thiadiazoles must have undergone an equilibriation of the carbons, probably through the symmetrical thiirene intermediate (138). In the case of the selenium compound, however, their results showed that the selenirene is not on the route to the ethynylselenol (136, X = Se). 

Photochemical elimination of nitrogen from a bicyclic 1,2,3-thiadiazole results in the formation of a thioketen, e.g. (79), instead of a thiiren (see Scheme 4), which is in contrast to the reaction of monocyclic thiadiazoles. When 1,2,3-selenadiazoles are subjected to thermolysis at 500—600 C, they give the selenoketens (80)— (82). The selenoketens have been trapped and characterized at —196 C. 

Photolysis of 1,2,3-thiadiazole (15a) in argon or nitrogen matrices gave thioketene (16a) and ethynyl mercaptan (17a) as the main products (Scheme 4). When the singly labeled deuterium isotopomers 15b and 15c were similarly photolyzed, the two isotopomers of ethynyl mercaptan, 17b and 17c, were formed in the same ratio from each precursor. Since the isomeric deuterium labeled mercaptans were not interconverted under the matrix photolysis conditions, it was concluded that an intermediate was involved in their formation — probably thiirene (18b,c) — in which the deuterium label became scrambled. The same conclusion was reached when labeled isotopomers of 15a were also photolyzed in matrices. In this second study, a new species was detected that arose on initial photolysis of the thiadiazole at 300 nm and decomposed on further photolysis, with concomitant production of ethynyl mercaptan. It seemed likely that this species was thiirene (18a), but an unequivocal identification could not be made on the basis of the few IR bands observed. 

---