Methylene thietanes

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

The same equatorial preference is also manifested in the 3,3-disubstituted thietane oxides66,194. Thus, the NMR spectra of 5e,f contain two Me singlets at 1.23 and 1.30 ppm and two methylene multiplets at 3.03 and 3.53 ppm (in CDC13). The large difference in the chemical shifts of the axial and equatorial a-methylene hydrogens is characteristic of an axial nonbonded electron pair on sulfur (conformation 5e equation 73). This conformational preference is corroborated by the small differences in the chemical shifts of the two methyl groups, and fits the contention that 1,3-diaxial interactions are responsible for this ultimate result. Certainly, these interactions are greater in the conformer 5f. 

Flash vacuum pyrolysis (FVP) represents a special method for inducing decomposition of thietane compounds. Block et al. succeeded for the first time in the preparation of methylene sulfine in the gas phase, applying this method to thietane 1-oxide. The reaction starting at 600°C was followed by mass and microwave spectroscopy. 

Hydroxy-3,4-dihydro-2H-pyrido[2,l-b][l,3]thiazin-5-ium chloride was formed when 2-hydroxy-3-(2-pyridylthio)propyl chloride was stirred in methylene chloride overnight (92JOC6335). When the reaction was carried out in methanolic sodium methylate, 3-(2-oxo-l,2-dihydro-l-pyridyl) thietane was obtained in 58% yield. 

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>. 

The total barrier to planarity of thietane is 274 cm" (784 cal/mole) so that at room temperature only about 25% of the molecules occupy vibrational levels above the barrier. The barrier is greater than that in oxetane but less than that in cyclobutane. Ring-strain favors planarity whereas eclipsing of methylene protons favors torsion and nonplanarity, a-methylthietan possesses equatorial and axial con-formers observable at room temperature with an inversion barrier of 341 cm (975 cal/mole). 3,3-dimethylthietane has a barrier of 300cm (857 cal/mole). °... 

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. ... 

Platinum and palladium complexes of thietane and 3,3-dimethylthietane have been prepared as illustrated for 90. The platinum complexes exist in cis and trans configurations, but no cis-trans isomerization of the palladium complexes in the solid state was observed. Stability constants of thietane with Mn(ll), Co(II), and Ni(II) chelates have been determined. Proton nmr studies show that the absorption of the a-methylene protons, which are syn to the metal, is shifted downfield (about 0.7 ppm) more than the absorption of the protons anti to the metal (about 0.4 ppm downfield). Energies of activation for pyramidal inversion were determined. Bis-ruthenium complexes of di-, tri- and tetraspirothietanes (e.g., 90a) show rapid electron transfer between the ruthenium ions long-range electron tunneling was proposed. ... 

The presence of several 2-methylene-3-thietanones and 2-methylene-3-hydroxy-thietanes in African plants of the tribe Arctotideae (e.g., 356) has been estab-... 

Methylene derivatives of thietanes have been obtained by the Wittig reaction of 3-thietanones, as in the preparation of 365 and the phosphonium ylide 345 with p-nitrobenzaldehyde (to give 354). A Wittig reaction also has been applied to a 2-thietanone. ... 

The similarity in the reactivities of free ions and corresponding ion pairs derived from the same cyclic monomer is more intriguing. Whereas ion pair reactivities are about 10 times smaller than corresponding free ion values in the anionic polymerization of vinyl monomers [39], and probably of the same relative proportions in cationic systems, the difference in cationic ring opening polymerizations is considerably less. For polymerization of THF in methylene chloride the factor is only 7, and for polymerization of 3,3-dimethylthietan, 40 in methylene chloride and 1 in nitrobenzene. Because the overall reactivity in cyclic monomer reactions is lower than for olefinic polymerizations, it might be expected that difference between free ion and ion pair reactivities, within one system, would also be less. However, this does not seem to be the whole answer. Plesch [44] has pointed out that in the polymerization of cyclic ethers and thietans (and presumably, therefore, other cyclic monomers)... 

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... 

Thiobarbiturates (59) undergo (2-i-2)-cycloaddition with alkenes to give the thietanes (60) and the methylene derivatives (61) by secondary photolysis involving ehmination of either thioacetone or thio-formaldehyde (Scheme 8). > Dithiobarbiturate and trithiobarbiturate undergo analogous reactions. 

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