Propadienethione

Brown and coworkers105 used microwave spectroscopy to determine the structure of propadienethione H2C=C=C=S (16) through the analysis of the rotational constants for several of its isotopomers (obtained by pyrolysis of cyclopenteno-l,2,3-thiadiazole and deuterated derivatives). The main structural parameters are shown in Scheme 3a. A most remarkable (and yet unexplained) feature is the fact that this molecule has a C2V geometry, while propadienone, H2C=C=C=0 (17) is kinked 105, as shown in Scheme 3b. 

As mentioned earlier, flash vacuum thermolysis (FVT) is a very useful technique for the synthesis of reactive species20. It has been applied to the synthesis of thioketenes288 (equation 56), such as propadienethione, which were trapped by reaction with dimethylamine. 

Ripoll and coworkers288 have reported the cleavage of silylated ketene dithioacetals under flash vacuum thermolysis (FVT) conditions20, which allowed the synthesis of propadienethione by combining this process along with the retro-Diels-Alder reaction (see Section III.F.5). Propadienethione was not isolated, but it underwent addition of the nucleophiles present in the reaction mixture288 (equation 65). 

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>. 

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