Thermal decomposition of thiiranes

It has been generally assumed that thermal decomposition of thiirane oxides proceeds stereospecifically to the corresponding olefins by elimination of sulfur monoxide, possibly through a concerted nonlinear chelatropic reaction96 with retention of configuration of the liberated olefin. 

The thermal desulfonylation of episulfones is highly stereospecific e.g., cis-2,3-dimethyl-thiirane-1,1-dioxide gives cw-2-butene , and trans- and cw-2,3-diphenyl thiirane 1,1-dioxides give trans and cis stilbenes, respectively . This result is remarkable in that the Woodward-Hoffman symmetry selection rules for inter-molecular cycloadditions (and their reversals) appear to exclude a concerted thermal decomposition of thiirane-l,l-dioxides . ... 

The homogeneous, gas phase, thermal decomposition of thiiranes, ethylene episulfide, propylene episulfide, and 2-butene episulfide sheds additional light on the mechanism of the S + olefin reactions. Below ca, 250 °C the decomposition products are sulfur and the olefin. The rate of olefin formation is first order in episulfide concentration, and the reaction features an activation energy which is considerably lower than the... 

An analogous sulfur transfer reaction has been proposed for the thermal decomposition of neutral thiirane to give ethylene and elemental sulfur, on the basis of ah initio calculations at the G3(MP2) level of theory [135]. 

Thermal decomposition of each thiirane-1-oxide 531 <1997JOC8366> and trisulfide 2-oxide 533 <20010L3565> delivers sulfur monoxide to 1,3-dienes giving 2,5-dihydrothiophene-l-oxides 532 (Scheme 84). 

Cyclic sulhdes (cyclic thioethers), such as thiirane (which is less stable than oxirane), can also be made by such displacement reactions. However, it is often more convenient to prepare thiirane and substituted thiiranes by thermal decomposition of thiocarbonates, as shown in Equation 8.66. 

Since a similarity between the rates of decomposition of thiirene dioxide complexes and those of thiirane dioxides was found, it was suggested103 that upon coordination the carbon-carbon bond order of thiirene dioxides decreases and the ligand becomes thiirane dioxide-like. The role of the metal is thus to saturate the carbon-carbon double bond so that the reactivity of the coordinated thiirene dioxide approaches that of the thermally less stable thiirane dioxide. 

The symmetric diarylthiirene oxides (18) are much more thermally stable than the corresponding saturated thiiranes and unsaturated thiirene dioxides. Thus, the thiirene oxide 18a shows only slight decomposition after 24 hours of reflux in benzene, whereas the analogous sulfone 19b fragments completely to SO2 and diphenylacetylene after less than six hours under the same conditions Irradiation of the oxide 18a, however, does result in the elimination of sulfur monoxide and formation of diphenylacetylene. Its thermolysis at 130 °C afforded benzil as the only isolable product, implying that SO is not being eliminated in this thermolytic process. 

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