Thietanes, 2-methylene-, formation

Base-induced rearrangement of quaternized thiazoles results in the formation of formamido-methylene thietanes (128) (Eq. 18). 

Both the 2-thiahex-5-enyl and the 2-sulfonylhex-5-enyl radicals exhibit a significant preference for 5-exo cyclisa-tion to the thiophene over formation of tetrahydrothiopyran by 6-endo ring closure (Equation 144) <2000TL7987> and only at higher I2 concentration does the formation of tetrahydrothiopyrans compete successfully with thietane and tetrahydrothiophene production when methylene-intermpted dienoates 455 react with dimethyl disulfide (Equation 145) <1994TL5575>. 

Intramolecular alkylation of the sulfur atom of thietanes also is invoked in reactions of a-haloalkylthietanes, for example, the formation of 75 in reactions of alcohol 74. The salt 75 has been isolated. The ion 76 is attacked at carbon by acetate ion, but at sulfur by hydride ion. The reactions of sulfur dichloride with 5-exo-methylene-2-norbornene to give 53 is believed to involve an intramolecularly S-alkylated thietane. ... 

Higher conversions in thiirane polymerizations, however, proceed with chain scission transfer mechanism under the influence of BF3 (C2H5)20 [192]. This is indicated by a change in the molecular weight distribution, a bimodal character. When the reaction is complete there is a marked decrease in the average molecular weight of the polymer. When thietane polymerizes with triethyl-oxonium tetrafluoroborate initiation in methylene chloride, the reaction terminates after only limited conversion [193]. This results from reactions between the reactive chain ends (cyclic sulfonium salts) and the sulfur atoms on the polymer backbone. In propylene sulfide polymerization, however, terminations are mainly due to formations of 12-membered ring sulfonium salts from intramolecular reactions [193]. 

Similar results have been obtained by combining the living cationic polymerization of vinyl ethers and the ROP of thietane. In this case, the thietane acts as growing species stabilizer for the vinyl ether polymerization by reversible formation of an a-alkoxy-thietanium ion. This ion can, however, also be attacked at an endocyclic methylene by another thietane molecule leading to an alkyl-thietanium ion. This ion is incapable of reacting with a vinyl ether but is the active species for the thietane polymerization. Also in this case the end product is a star-shaped segmented polymer. 

The mechanism of thietan-3-one formation by oxidation of 4-arylbutan-2-ones with thionyl chloride involves oxidation exclusively at the a-methylene... 

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