Thiiren Derivatives

The thiiren 1,1-dioxide system has been investigated theoretically and by u.v. photoelectron spectroscopy. Charge transfer from the carbon-carbon double bond to the SO.2 unit is about one-third that predicted for the cyclopropenone system. Aromaticity decreases in the order cyclopropenone tropone  

Thiirenium ions (24), stable at low temperatures, have been obtained by addition of methyl bis(methylthio)sulphonium hexafluoroantimonate (11) to excess but-2-yne or hex-3-yne at — SC C. The H n.m.r. spectrum is unchanged after several hours at — 50 to — 70 °C. Addition of A-methylpyridinium 

The reaction of 2,3-diphenyl- or 2,3-bis-(4-chlorophenyl)-thiiren 1,1-dioxide with enamines R R C=CR NR 2 yields a variety of heterocyclic compounds, including medium- and large-ring sulphur heterocycles, via decomposition of a bicyclic thiiran dioxide intermediate (26). Treatment of these thiiran dioxides with 2-pyrrolidinyl- or 2-piperidinyl-2-norbornene gave adducts which possessed antifertility activity. The reaction of 2,3-diphenylthiiren 1,1-dioxide with several meso-ionic compounds, e.g. (27), gave 2,3,5,6-tetraphenyl-4 -l,4-thiazine 1,1-dioxides by loss of carbon dioxide from the intermediate adduct (28).  

A variety of nucleophiles add to the double bond of 2,3-diphenylthiiren 1,1-dioxide to give mainly ring-opened products. The principal mode of fragmentation of thiiren oxides and dioxides in mass spectrometry involves elimination of SO or SOj. Fragmentation of 2,3-diphenylthiiren 1-oxide is also believed to involve rearrangement to monothiobenzil.  

6 Three-membered Rii containii Sulj ur and One or Two Other Heteroatoms 

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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. 

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). 

Methylene thiirane is obtained by thermolysis of several spirothiirane derivatives which are formally Diels-Alder adducts of methylenethiirane and cyclopentadiene or anthracene <78JA7436). They were prepared via lithio-2-(methylthio)-l,3-oxazolines (c/. Scheme 121). A novel synthesis of the allene episulfide derivatives, 2-isopropylidene-3,3- dimethylthiirane (good yield) or its 5-oxide (poor yield), involves irradiation of 2,2,3,3-tetramethyl-cyclopropanethione or its 5-oxide (81AG293). Substituents on the thiirane ring may be modified to give new thiiranes (Section 5.06.3.9). The synthesis of thiirane 1-oxides and thiirane 1,1-dioxides by oxidation is discussed in Section 5.06.3.3.8 and the synthesis of 5-alkylthiiranium salts by alkylation of thiiranes is discussed in Section 5.06.3.3.4. Thiirene 1-oxides and 1,1-dioxides may be obtained by dehydrohalogenation of 2-halothiirane 1-oxides and 1,1-dioxides (Section 5.06.4.1.2). 

Acyclic sulphones with a-chlorine substituents also produce sulphonic acid derivatives in good yields, although in these cases rearrangement occurs via a thiiren dioxide intermediate (equation 91)208. 

The suggestion that three-membered heterocycles (362) are involved is illustrated by the postulated generation of derivatives of IH-diazirine (315 and 345), thiiren (328), thiazirin (337), and lif-triazirine (358). These three-membered heterocycles belong to the class of 4n-antiaromatic heterocycles their possible role as reaction intermediates is of general interest. There is, however, an interesting difference between the photochemistry of sydnones (Fig. 4) and meso-ionic l,2,4-triazol-3-ones (Fig. 6). In both cases IH- azirines (315 and... 

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