Thioketenes 1,2,3-thiadiazoles

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>. 

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). 

In their mass spectra 1,2,3-thiadiazoles commonly fragment initially by expulsion of nitrogen, and the fragmentation then proceeds to form thiirene ion radical (8 Scheme 1). These studies have been reviewed (73SST(2)717) and additional studies have shown, depending on substituents, that thioketene radical (9) can be formed in the mass spectrum (72T1353). 

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. 

For similar systems, results have been reported (77JOC575) which do not require a thiirene intermediate (equation 2). Further, researchers monitoring the decomposition of 1,2,3-thiadiazoles by PE spectroscopy saw no evidence of thiirenes (77JA1663), only thioketenes (equation 3). 

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>. 

A general synthesis of benzofuran-2-thiolates utilizes 1,2,3-thiadiazoles as cyclization precursors. Upon treatment with base, the heterocycle is deprotonated and extrudes nitrogen to generate an alkynethiolate which can be spectroscopically monitored. This species can take up a proton to form a thioketene that undergoes cyclization to a benzofuranthiolate which is finally alkylated with an alkyl halide (Equation 125) <1997CC1753>. 

Thioketen Allyl-trimethylsilyl- Ell, 247 (4-SiR3-l,2,3-thiadiazol + Allyl-Br)... 

Furthermore, transient thioketenes with a wide variety of substituents R and R, including hydrogen, can be generated by flash vacuum thermolysis of l,2,3-thiadiazoles. 4 By in situ reactions thioxoesters are formed with alcohols (equation 47). 

Irradiation (2 = 235.0-280.0 nm) of either [4-l3C]-l,2,3-thiadiazole or [5-l3C]-l,2,3-thiadiazole gives [l3C]-thiirene with bands shifted to 3198 cm-1, 3163 cm-1, 3158 cm-1, 1634 cm-1, 910 cm-1 and 558 cm-1. [l3C]-Thiirene is transformed with light of wavelength = 33-37 nm to ethynyl mercaptane and thioketene, with the C-13 label scrambled in both products (Equation (4)). Irradiation of either 4-methyl-5-carboethoxy- or 5-methyl-4-carboethoxy-l,2,3-thiadiazole using a 265 nm interference filter resulted in the appearance of a spectrum with significant absorptions at... 

A formation of thiirene by secondary photolysis of thioketene, which is generated by photolysis of 1,2,3-thiadiazole, has been reported <92ACS482>. Irradiation of [4-13C]-l,2,3-thiadiazole together with diethylamine in an EPA glass (diethyl ether-isopentane-ethanol (5 5 2)) at 77 K gave N,N-diethylthioacetamide with 37% carbon randomization in the thioketene (Scheme 9). Experiments with [4-13C]-l,2,3-thiadiazole at room temperature demonstrate lack of carbon randomization in the thioacetyl group of the trapping product. Therefore, the reaction mechanism of photolysis of... 

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). 

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. 

The formation of thioketene has been confirmed by direct observation during matrix photolysis of 1,2,3-thiadiazole In addition, ethynyl mercaptan was observed in this work. The simultaneous formation of two deuterated ethynyl mercaptans in the photolysis of 4- or 5-deuterio-1,2,3-thiadiazole implies the involvement of a S3unmetrical intermediate, thiirene... 

---