Thiirens

Flash thermolysis of the thiadiazoles (334 R = Me, Pr, Bu , Ph, Cl, or CN R = Me, Pr, Bu , or Ph) gave the thioketens R R C=C=S, which could be detected spectroscopically.From irregularities in the migratory aptitudes of R it has been suggested that thiirens (335) participate in the formation of the thioketens. 

Capozzi, L. DaCol, V. Lucchini, G. Modena, and G. Valle, J. Chem. Soc., Perkin Trans. 2, 1980, 68. 

Photolysis of the argon-matrix-isolated thiadiazole (332) at 265 nm gave the thiiren (334) via the intermediate (333).  

Electrochemical reduction of (335) at — 1.8 V, followed by methylation, affords /raws-stilbene (40%) and (336) (27%). Similar reduction of (337) produces mainly diphenylacetylene (80%) with benzil (10%). 

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For 4,5-dialkylthiazoles, the molecular ion decomposes by two competitive pathways, either loss of HCN followed by elimination of the radical R in the position /3 to the double bond of the resulting substituted thiirene, or by p cleavage followed by elimination of HCN (119). 

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). 

The isothiazole ring is very stable to moderate heating, but quaternized compounds are dequaternized by this means (see Section 4.02.3.12). Photolysis of isothiazole itself causes loss of hydrogen cyanide, forming thiirene, but substituted compounds are converted into mixtures of thiazoles and isothiazoles isomeric with the starting compound (Section 4.1.1.2). 

For the antiaromatic three-membered heterocycles, experimental data are available only for thiirenes (and there is some doubt about the true antiaromaticity of thiirenes). Bond lengths have been calculated, however, for these antiaromatic 47r-systems (80PAC1623). In comparison with the corresponding saturated heterocycles, the C—X bond lengths are increased by 0.05 to 0.17 A and the C—C bond length is decreased by 0.2 A. 

Heterocyclics of all sizes, as long as they are unsaturated, can serve as dipolarophiles and add to external 1,3-dipoles. Examples involving small rings are not numerous. Thiirene oxides add 1,3-dipoles, such as di azomethane, with subsequent loss of the sulfur moiety (Section 5.06.3.8). As one would expect, unsaturated large heterocyclics readily provide the two-atom component for 1,3-dipolar cycloadditions. Examples are found in the monograph chapters, such as those on azepines and thiepines (Sections 5.16.3.8.1 and 5.17.2.4.4). 

Three-membered rings containing one sulfur atom are named as thiiranes (1) or thiirenes... 

Table 1 Bond Lengths and Angles for some Thiiranes and Thiirenes...

Chemical shifts ( H, C) and coupling constants for some thiiranes and thiirenes are given in Table 2. The shifts of substituted derivatives may be calculated by the use of additivity relationships found in textbooks on NMR. 

Diphenylthiirene 1-oxide and several thiirene 1,1-dioxides show very weak molecular ions by electron impact mass spectrometry, but the molecular ions are much more abundant in chemical ionization mass spectrometry (75JHC21). The major fragmentation pathway is loss of sulfur monoxide or sulfur dioxide to give the alkynic ion. High resolution mass measurements identified minor fragment ions from 2,3-diphenylthiirene 1-oxide at mje 105 and 121 as PhCO" and PhCS, which are probably derived via rearrangement of the thiirene sulfoxide to monothiobenzil (Scheme 2). 

The vertical ionization potentials from the photoelectron spectra of some thiirane and thiirene derivatives are given in Table 3. A Walsh localized scheme of bonding is generally preferred. There is a strong hyperconjugative interaction in thiirene 1,1-dioxides between the occupied C=C tt-MO and the occupied SO2 pure sulfur d-AO. Thiirene oxides are suggested to be less aromatic than cyclopropenones and tropone. 

Thiirenes are photochemically converted to thioketenes or to alkynylthiols (Scheme 22) (80PAC1623). 2-Acetyl-3-methylthiirene rearranges on irradiation at 310 nm to a mixture of acetylmethylthioketene and thioacetylmethylketene, identified by their IR spectra (80NJC703). 2,3-Diphenylthiirene 1-oxide at 130 °C is believed to be isomerized to monothiobenzil which is air-oxidized to benzil (Scheme 23) (79JA390). 

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

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