Electrons Electrophilic rearrangements

As formal a, /i-unsaturated sulfones and sulfoxides, respectively, both thiirene dioxides (19) and thiirene oxides (18) should be capable, in principle, of undergoing cycloaddition reactions with either electron-rich olefins or serving as electrophilic dipolarophiles in 2 + 3 cycloadditions. The ultimate products in such cycloadditions are expected to be a consequence of rearrangements of the initially formed cycloadducts, and/or loss of sulfur dioxide (or sulfur monoxide) following the cycloaddition step, depending on the particular reaction conditions. The relative ease of the cycloaddition should provide some indication concerning the extent of the aromaticity in these systems2. 

The attachment of an electron to an organic acceptor generates an umpolung anion radical that undergoes a variety of rapid unimolecular decompositions such as fragmentation, cyclization, rearrangement, etc., as well as bimolecular reactions with acids, electrophiles, electron acceptors, radicals, etc., as demonstrated by the following examples.135"137... 

Electrophilic substitution reactions on the carbon atoms of 1,3,4-thiadiazoles are rare due to the low electron density of ring carbons. G-Acylation can be accomplished via rearrangement of intermediate W-acylthiadiazolium salts while radical halogenation can give chlorinated or brominated 2-halo-5-substituted thiadiazoles. Examples can be found in CHEC(1984) <1984CHEC(6)545> and in Houben-Weyls Science of Synthesis <2004HOU(13)349>. 

Intramolecular electrophilic aromatic substitution to give tricyclic products 142 is also a viable process, with trapping efficiency related to the electron density of the arene trap (equation 3)67. With a simple phenyl group pendant, rearrangement to the 2-pyrone was... 

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